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Experiments were carried out to test the amenabilities of mineral deposits that contained cobalt deported in arseno-sulfide (cobaltite) and arsenide (skutterudite) minerals, to oxidative bioleaching at mesophilic temperatures and low pH. An ore sample from the Iron Mask deposit (Canada) and a mineral concentrate from a working mine (Bou Azzer, Morocco) were thoroughly characterised, both prior to and following bio-processing. A “top down” approach, using microbial consortia including (initially) 13 species of mineral-degrading acidophiles was used to bioleach the ore and concentrate in shake flasks and bioreactors. Cobalt was successfully liberated from both materials tested (up to 93% from the ore, and 49% from the concentrate), though the chemistries of the leach liquors were very different, with redox potentials being >200 mV lower, and concentrations of soluble arsenic about 7-fold greater, with the concentrate. Addition of pyrite to the arsenide concentrate was found to promote the biomineralisation of scorodite (ferric arsenate), which was detected by both XRD and SEM-EDX, but was not found in bioleached residues of the arseno-sulfide ore. A model was proposed wherein pyrite had three critical roles in facilitating the genesis of scorodite: (i) providing the catalytic surface to promote the oxidation of As (III) to As (V); (ii) acting as a putative “seed” for scorodite crystallisation; (iii) being a secondary source of iron, since the molar ratios of iron:arsenic in the concentrate itself (0.19:1) was well below that required for effective removal of soluble arsenic as scorodite (1:1). This work provided proof of concept that cobalt arseno-sulfide and arsenide ores and concentrates are amenable to bio-processing, and also that it is possible to induce concurrent solubilisation of arsenic from primary minerals and immobilisation in a secondary mineral, scorodite., Copyright: © 2020 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by/4.0/) The attached file is the published version of the article., NHM Repository

type:Original Research Paper, Objective To enable removal of highly toxic As(III) from acidic waters by inducing indirect microbial As(III) oxidation by Fe-oxidizing bacteria via carbon-assisted redox-coupling between As(III) oxidation and Fe<3+> reduction. / Results Carbon-fiber (CF) was shown to function as an electron-mediator to catalyze chemical (abiotic) redox-coupling between As(III) oxidation and Fe<3+> reduction. Accordingly, by taking advantage of Fe<3+> regeneration by Fe-oxidizing bacteria, it was possible to promote oxidative removal of As(III) as ferric arsenate at moderate temperature. This reaction can be of use under the situation where a high-temperature treatment is not immediately available. Arsenic once concentrated as ferric arsenate on carbon-fibers can be collected to undergo phase-transformation to crystalline scorodite as the next re-solubilization/re-crystallization step at a higher temperature (70°C). / Conclusions While extremely acidophilic Fe-oxidizing bacteria are widely found in nature, the As-oxidizing counterparts, especially those grown on moderately-thermophilic and mesophilic temperatures, are hardly known. In this regard, the finding of this study could make a possible introduction of the semi-passive, low-temperature As-treatment using readily available Fe-oxidizing bacteria.

type:Environment, The necessity of arsenic (As) removal from metallurgical wastewaters is increasing. Despite its wide recognition as a natural oxidant, the utility of Mn oxide for scorodite production is mostly unknown. In acidic solutions containing both As(III) and ^^^<2+>, simultaneous oxidation of the two progressed by MnO_2 and the resultant As(V) and ^^^<3+> triggered the formation of crystalline scorodite (FeAsO_4·2H_2O). At 0.5% or 0.25% MnO_2, 98% or 91% As was immobilized by day 8. The resultant scorodite was sufficiently stable according to the TCLP test, compared to the regulatory level in US and Chile (5 mg/L): 0.11 « 0.01mg/L at 0.5% MnO_2, 0.78 « 0.05mg/L at 0.25% MnO_2. For the oxidation of As(III) and ^^^<2+>, 54% (at 0.5% MnO_2) or 14% (at 0.25% MnO_2) of initially added MnO_2 remained undissolved and the rest dissolved in the post As(III) treatment solution. For the Mn recycling purpose, the combination of ^^^<2+>-oxidizing bacteria and biogenic birnessite (as homogeneous seed crystal) was used to recover up to 99% of dissolved ^^^<2+> as biogenic birnessite ((Na, Ca)0.5(^^^,^^^III)_2O_4·1.5H_2O), which can be utilized for the oxidation treatment of more dilute As(III) solutions at neutral pH. Although further optimization is necessary, the overall finding in this study indicated that Mn oxide could be utilized as a recyclable oxidant source for different As(III) treatment systems.

Herein we report the findings of an integrated green process for the continuous oxidation and biomineralization of the most toxic As(III) from acidic streams using a laboratory-scale airlift bioreactor operated at thermoacidophilic conditions and fed with Fe(II) as electron donor. As(III) oxidation catalyzed by granular activated carbon (GAC), biological Fe(II) oxidation and scorodite crystallization took place simultaneously in the reactor, allowing the treatment of influent solutions containing 0.65 g·L−1 As(III). At a hydraulic retention time of 2.2 days, a stable arsenite oxidation efficiency of 99% was achieved, while the removal of total arsenic was 93%. Scorodite was yielded as the main solid product whose physical characteristics such as average size (250 µm) and the developed crystalline structure allowed the easy harvesting from the reactor and reflected the high stability by the low arsenic release of 0.4 mg.L−1 after 60 days of leaching. The analysis of the microbial composition in the reactor suspension and the precipitates indicated the dominance of thermoacidophilic archaeon of the genus Acidianus. Similarly, the attachment of microorganisms to the precipitates observed by scanning electron microscopy, suggested that the precipitation in our system was biologically mediated. The simultaneous arsenic oxidation and removal through catalyzed oxidation and biological processes provide the basis for a new and cost effective green methodology for arsenic fixation from acid As(III)-containing wastewaters.

The synthesis of biogenic scorodite combined with the oxidation of As(III) catalysed by granular activated carbon (GAC) was previously demonstrated. However, the colloidal size of the formed scorodite particles is still a bottleneck, as it would hinder the easy separation of the precipitates in a full-scale application. Here, we studied the effect of GAC concentration on biological scorodite precipitation at thermoacidophilic conditions in batch experiments. Higher arsenic removal efficiency and precipitation of larger and more stable scorodite particles were found only in biotic tests and at low catalyst concentration of 4 g L-1. Furthermore, with 4 g L-1 GAC, the Fe and As predominantly precipitated in solution while with 20 g L-1 GAC the Fe and As predominantly precipitated on the GAC. For experiments with 4 and 20 g L-1 of GAC, the average particle size was 66 and 2.6 μm, respectively. This could be explained by the lower saturation level of the solution at the lower GAC level. This study shows for the first time that the oxidative catalytic capacity of GAC can be used to influence crystallization of scorodite.

Scorodite can be used as a final product for the storage of arsenic removed from circuits in copper smelters. It is characteried by very low solubility and high stability over a broad pH range and is the most stable combination of iron and arsenic. The effects were investigated of temperature, reaction time, scorodite seeding, initial arsenic concentration and initial copper concentration on the precipitation of scorodite at atmospheric pressure. The results showed that a solid product containing scorodite with particle size of 15-35 micrometres and low arsenic leaching rate of 1.0 mg/kg) was obtained at temperatures above 95 degrees C, Fe(III)/As(V) molar ratio in the range 1:1 to 1.3 :1, reaction time slightly longer than 6 h, scorodite seeding of more than 40 g/l and As(V) concentration of 5-15 g/l. The Cu concentration in the feed solution had no effect on product quality., Scorodite can be used as a final product for the storage of arsenic removed from circuits in copper smelters. It is characteried by very low solubility and high stability over a broad pH range and is the most stable combination of iron and arsenic. The effects were investigated of temperature, reaction time, scorodite seeding, initial arsenic concentration and initial copper concentration on the precipitation of scorodite at atmospheric pressure. The results showed that a solid product containing scorodite with particle size of 15-35 micrometres and low arsenic leaching rate of 1.0 mg/kg) was obtained at temperatures above 95 degrees C, Fe(III)/As(V) molar ratio in the range 1:1 to 1.3 :1, reaction time slightly longer than 6 h, scorodite seeding of more than 40 g/l and As(V) concentration of 5-15 g/l. The Cu concentration in the feed solution had no effect on product quality.

Detection and sizing of natural colloids involved in the release and transport of toxic metals and metalloids is essential to understand and model their environmental effects. Single-particle inductively coupled plasma mass spectrometry (SP-ICP-MS) was applied for the detection of arsenic-bearing particles released from mine wastes. Arsenic-bearing particles were detected in leachates from mine wastes, with a mass-perparticle detection limit of 0.64 ng of arsenic. Conversion of the mass-per-particle information provided by SP-ICP-MS into size information requires knowledge of the nature of the particles; therefore, synchrotron-based X-ray absorption spectroscopy (XAS) was used to identify scorodite (FeAsO4·2H2O) as the main species in the colloidal particles isolated by ultrafiltration. The size of the scorodite particles detected in the leachates was below 300¿350 nm, in good agreement with the values obtained by TEM. The size of the particles detected by SP-ICP-MS was determined as the average edge of scorodite crystals, which show a rhombic dipyramidal form, achieving a size detection limit of 117 nm. The combined use of SP-ICP-MS and XAS allowed detection, identification, and size determination of scorodite particles released from mine wastes, suggesting their potential to transport arsenic.

The development is discussed of biooxidation processes employing stirred tank technologies for the pre-treatment of Au-bearing pyrite and arsenopyrite concentrates, which resulted in the commercialisation of technologies employing mesophilic or moderately thermophilic microorganisms such as the BIOX and BACOX processes. The technology was extended to the processing of base metal concentrates, and pilot-scale operations for the bioleaching of Cu, Ni and Zn sulphides using mesophiles, moderate thermophiles and thermophiles have been demonstrated. Conditions in primary, secondary and tertiary tanks differ but are relatively stable in the absence of significant changes in the feed material. A key difference through the reactor series is a progressive decrease in sulphide content, accompanied by the formation of reaction products including ferrous ions, elemental S, jarosite and scorodite. Microorganisms enhance the oxidation of ferrous ions and reduced inorganic sulphur compounds (RISCs). High concentrations of selected elements and sulphate anion could be experienced by microbial consortia depending on the mineral concentrate and the solids loading. The growth patterns of three archaea, Acidianus brierleyi, Metallosphaera hakonensis and Sulpholobus metallicus, in media containing two substrates were compared. Each species was presented simultaneously with ferrous and tetrathionate ions, with the latter chosen as the RISC because the solubility of elemental sulphur in water is practically zero. Individual archaea species exhibited different patterns of utilisation and growth. Specific growth rates differed for different substrate concentrations or when cells were pre-adapted to different substrates, and the preferences exhibited by a given species were not always as anticipated. Total cell numbers for A. brierleyi were about 60% of those for M. hakonensis and S. metallicus after complete substrate utilisation. In mixed culture at 60 degrees C both M. hakonensis and S. m, The development is discussed of biooxidation processes employing stirred tank technologies for the pre-treatment of Au-bearing pyrite and arsenopyrite concentrates, which resulted in the commercialisation of technologies employing mesophilic or moderately thermophilic microorganisms such as the BIOX and BACOX processes. The technology was extended to the processing of base metal concentrates, and pilot-scale operations for the bioleaching of Cu, Ni and Zn sulphides using mesophiles, moderate thermophiles and thermophiles have been demonstrated. Conditions in primary, secondary and tertiary tanks differ but are relatively stable in the absence of significant changes in the feed material. A key difference through the reactor series is a progressive decrease in sulphide content, accompanied by the formation of reaction products including ferrous ions, elemental S, jarosite and scorodite. Microorganisms enhance the oxidation of ferrous ions and reduced inorganic sulphur compounds (RISCs). High concentrations of selected elements and sulphate anion could be experienced by microbial consortia depending on the mineral concentrate and the solids loading. The growth patterns of three archaea, Acidianus brierleyi, Metallosphaera hakonensis and Sulpholobus metallicus, in media containing two substrates were compared. Each species was presented simultaneously with ferrous and tetrathionate ions, with the latter chosen as the RISC because the solubility of elemental sulphur in water is practically zero. Individual archaea species exhibited different patterns of utilisation and growth. Specific growth rates differed for different substrate concentrations or when cells were pre-adapted to different substrates, and the preferences exhibited by a given species were not always as anticipated. Total cell numbers for A. brierleyi were about 60% of those for M. hakonensis and S. metallicus after complete substrate utilisation. In mixed culture at 60 degrees C both M. hakonensis and S. m

Batch testing of the Galvanox process was conducted in stirred-tank reactors to identify alternative catalysts for leaching an enargite concentrate grading 28.3% Cu, 9.4% As and 15.0% Fe. Tests with an activated carbon catalyst (Norit GCA 612) reached completion in about 1 day, whereas without a catalyst in the same time only about 55% of Cu was extracted. A pyrite catalyst was found to be ineffective. The effects on enargite leaching of carbon recycling, stirring speed and carbon catalyst breakage are reported. In pilot-plant tests using the activated carbon catalyst, near-complete copper extraction was achieved in about 30 h. A modified Galvanox flowsheet is described in which activated carbon is used to catalyse Cu-As minerals and pyrite oxidation is employed to heat the reactors and provide Fe to precipitate the leached As as scorodite. A prediction has been developed of the capital and operating costs for a 60 000 t/a refinery treating 29% Cu concentrate containing 9% As, using the METSIM model of the Galvanox process., Batch testing of the Galvanox process was conducted in stirred-tank reactors to identify alternative catalysts for leaching an enargite concentrate grading 28.3% Cu, 9.4% As and 15.0% Fe. Tests with an activated carbon catalyst (Norit GCA 612) reached completion in about 1 day, whereas without a catalyst in the same time only about 55% of Cu was extracted. A pyrite catalyst was found to be ineffective. The effects on enargite leaching of carbon recycling, stirring speed and carbon catalyst breakage are reported. In pilot-plant tests using the activated carbon catalyst, near-complete copper extraction was achieved in about 30 h. A modified Galvanox flowsheet is described in which activated carbon is used to catalyse Cu-As minerals and pyrite oxidation is employed to heat the reactors and provide Fe to precipitate the leached As as scorodite. A prediction has been developed of the capital and operating costs for a 60 000 t/a refinery treating 29% Cu concentrate containing 9% As, using the METSIM model of the Galvanox process.

The stability of atmospheric scorodite in concentrated aqueous solutions under reducing conditions was investigated in a laboratory. Chemical analysis, XRD, Raman, AT-IR, particle size, BET, SEM and TGA was used to characterise the scorodite. The results showed that As release was increased as the redox potential was reduced, and that the dissolution was inhibited by the presence of gypsum., The stability of atmospheric scorodite in concentrated aqueous solutions under reducing conditions was investigated in a laboratory. Chemical analysis, XRD, Raman, AT-IR, particle size, BET, SEM and TGA was used to characterise the scorodite. The results showed that As release was increased as the redox potential was reduced, and that the dissolution was inhibited by the presence of gypsum.

Waste rock materials with a diverse sulphide mineralogy from an abandoned mine site in northern Queensland were selected for a thirty-week column leach kinetic test following the recommendations of the geochemistry-mineralogy-texture (GMT) approach. Ten mesotextural groups were defined on mineralogical and textural characteristics. Twelve columns were established, with two size fractions (-10 mm and -4 mm) prepared from each sample to investigate the effects of grain size on pH, metal elution and secondary mineral precipitation. The mineralogy and microtexture of the column feed material were examined every five weeks through quantitative X-ray diffractometry, scanning electron microscopy and laser ablation ICP-MS. The data were directly compared with leachate chemistry (pH, electrical conductivity, SO4 and cation contents) to identify the controls on sulphide oxidation and trace element liberation. These experiments showed that the mineralogical and chemical evolution of some of the mesostructural groups are quite different and that they should be treated as individual entities when undertaking site rehabilitation works. This study indicates that through routine mineralogical and textural characterisation, sulphide oxidation processes can be more effectively characterised and better site rehabilitation strategies proposed., Waste rock materials with a diverse sulphide mineralogy from an abandoned mine site in northern Queensland were selected for a thirty-week column leach kinetic test following the recommendations of the geochemistry-mineralogy-texture (GMT) approach. Ten mesotextural groups were defined on mineralogical and textural characteristics. Twelve columns were established, with two size fractions (-10 mm and -4 mm) prepared from each sample to investigate the effects of grain size on pH, metal elution and secondary mineral precipitation. The mineralogy and microtexture of the column feed material were examined every five weeks through quantitative X-ray diffractometry, scanning electron microscopy and laser ablation ICP-MS. The data were directly compared with leachate chemistry (pH, electrical conductivity, SO4 and cation contents) to identify the controls on sulphide oxidation and trace element liberation. These experiments showed that the mineralogical and chemical evolution of some of the mesostructural groups are quite different and that they should be treated as individual entities when undertaking site rehabilitation works. This study indicates that through routine mineralogical and textural characterisation, sulphide oxidation processes can be more effectively characterised and better site rehabilitation strategies proposed.

A description is given of the DMSP scorodite synthesis technology developed by Dowa Metals and Mining for the immobilisation of As, which involves the precipitation of scorodite by the oxidation of Fe(II) ions in a high concentration As(V) solution. Two methods are used for producing As(V) by selectively separating and removing As from Cu smelting intermediates, acid leaching and alkaline leaching with NaOH. The NaOH process has advantages in the processing of liberator slimes for Cu removal. Cu2O and CuO are fixed in the residue and can be readily treated using a pyrometallurgical process, and As can be effectively leached from stable compounds such as ferrite. The NaOH can be recycled and only the low-cost chemicals lime and sulphuric acid are used. Acid leaching can be used for sulphide precipitates. The Cu is fixed in the residue as CuS which is easily recovered by pyrometallurgical processing. Cu arsenide in liberator slimes can be fixed in the residue as CuS by adding elemental S. As in solution following scorodite synthesis can be recovered by sulphidisation. The alkaline leaching process can be employed for the recycling of GaAs scrap at temperatures of 60 degrees C or above and an O2 partial pressure of 0.1 MPa. The As is removed using CaO/Ca(OH)2 and the remaining Ga in solution is recovered by electrolysis. As is precipitated as Ca-As and fixed by synthesising scorodite and gypsum. The DMSP process can also be used to simultaneously fix As and Th by treatment in an autoclave at 175 degrees C. The main component of the precipitate obtained was iron arsenate hydrate. Scorodite readily releases As in a strongly alkaline NaOH solution and a process was developed to produce a porous iron oxide which can be used for the separation and removal of heavy metals from contaminated solutions., A description is given of the DMSP scorodite synthesis technology developed by Dowa Metals and Mining for the immobilisation of As, which involves the precipitation of scorodite by the oxidation of Fe(II) ions in a high concentration As(V) solution. Two methods are used for producing As(V) by selectively separating and removing As from Cu smelting intermediates, acid leaching and alkaline leaching with NaOH. The NaOH process has advantages in the processing of liberator slimes for Cu removal. Cu2O and CuO are fixed in the residue and can be readily treated using a pyrometallurgical process, and As can be effectively leached from stable compounds such as ferrite. The NaOH can be recycled and only the low-cost chemicals lime and sulphuric acid are used. Acid leaching can be used for sulphide precipitates. The Cu is fixed in the residue as CuS which is easily recovered by pyrometallurgical processing. Cu arsenide in liberator slimes can be fixed in the residue as CuS by adding elemental S. As in solution following scorodite synthesis can be recovered by sulphidisation. The alkaline leaching process can be employed for the recycling of GaAs scrap at temperatures of 60 degrees C or above and an O2 partial pressure of 0.1 MPa. The As is removed using CaO/Ca(OH)2 and the remaining Ga in solution is recovered by electrolysis. As is precipitated as Ca-As and fixed by synthesising scorodite and gypsum. The DMSP process can also be used to simultaneously fix As and Th by treatment in an autoclave at 175 degrees C. The main component of the precipitate obtained was iron arsenate hydrate. Scorodite readily releases As in a strongly alkaline NaOH solution and a process was developed to produce a porous iron oxide which can be used for the separation and removal of heavy metals from contaminated solutions.

The long-term storage was investigated of scorodite produced by a process developed by Dowa Metals and Mining for fixing As by oxidation of Fe(II) ions in the presence of As(V) ions at atmospheric pressure. Leaching experiments were carried out using various solvents including CaO, Mg(OH)2, NaOH, H2SO4, HCl, HNO3 and NaCl. The results showed that As solubility depended more on solvent pH than the type of solvent. Fe solubility was relatively high in the pH range 3-5 but As solubility was very low. Long-term leaching behaviour tests according to the US Environmental Protection Agency’s Toxicity Characteristic Leaching Procedure showed very low As solubilities, and the low solubility in sea water indicated that storage in coastal areas was acceptable. The results of consecutive leaching tests showed that the addition of iron-rich compounds close to the surface of the scorodite would effectively prevent As release, and experiments were carried out involving the addition of goethite and the production of ferrihydrite-coated scorodite by the addition of FeSO4. Tests also showed that the toxicity of the scorodite was vey low. Waste storage methods are discussed and a description given of a storage vessel with a seepage control liner sheet., The long-term storage was investigated of scorodite produced by a process developed by Dowa Metals and Mining for fixing As by oxidation of Fe(II) ions in the presence of As(V) ions at atmospheric pressure. Leaching experiments were carried out using various solvents including CaO, Mg(OH)2, NaOH, H2SO4, HCl, HNO3 and NaCl. The results showed that As solubility depended more on solvent pH than the type of solvent. Fe solubility was relatively high in the pH range 3-5 but As solubility was very low. Long-term leaching behaviour tests according to the US Environmental Protection Agency’s Toxicity Characteristic Leaching Procedure showed very low As solubilities, and the low solubility in sea water indicated that storage in coastal areas was acceptable. The results of consecutive leaching tests showed that the addition of iron-rich compounds close to the surface of the scorodite would effectively prevent As release, and experiments were carried out involving the addition of goethite and the production of ferrihydrite-coated scorodite by the addition of FeSO4. Tests also showed that the toxicity of the scorodite was vey low. Waste storage methods are discussed and a description given of a storage vessel with a seepage control liner sheet.

Some minerals are colloidal and show no X-ray diffraction patterns. Vibrational spectroscopy offers one of the few methods for the determination of the structure of these minerals. Among this group of minerals is pitticite, simply described as (Fe, AsO4, SO4, H2O). In this work, the analogue of the mineral pitticite has been synthesised. The objective of this research is to determine the molecular structure of the mineral pitticite using vibrational spectroscopy. Raman and infrared bands are attributed to the AsO43−, SO42− and water stretching and bending vibrations. The Raman spectrum of the pitticite analogue shows intense peaks at 845 and 837 cm−1 assigned to the AsO43− stretching vibrations. Raman bands at 1096 and 1182 cm−1 are attributed to the SO42− antisymmetric stretching bands. Raman spectroscopy offers a useful method for the analysis of such colloidal minerals.

The 9 papers in the session comprised laboratory studies into the chemistry of the treatment of impurities in tailings. They are: The fate of entrained hydrocarbon materials in the McClean Lake operation mill tailings, by Hughes K.A., Rinas C., Frey R. and Rowson J., p.317-326, 6 refs., which describes the chemistry of the oxidation of carbon in entrained hydrocarbons to soluble bicarbonate; The potential arsenic retention role of Ca-Fe(III)-AsO4 compounds in lime-neutralised co-precipitation tailings, by Becze L., Gomez M.A., Petkov V., Cutler J.N. and Demopoulos G.P., p.327-336, 35 refs., reporting on laboratory studies on the potential use of synthetic yukonite for the control of arsenic in uranium tailings; The transformation of ferrihydrite under a range in pH (2-10) and temperature (25-100 degrees C), by Das S., Hendry M.J. and Essilfie-Dughan J., p.337-349, 51 refs., covering laboratory studies into the rate of crystallisation of ferrihydrate; Mineralogical characterisation of arsenic, iron and nickel in uranium mine tailings using XAS and EMPA, by Essilfie-Dughan J., Hendry M.J., Warner J. and Kotzer T., p.351-363, 16 refs.; Arsenic control in process tailings: continuous co-precipitation of As(V) with iron sulphate media, by Daenzer R., De Klerk R.J. and Demopoulos G.P., p.365-375, 16 refs., on the two- stage co-precipitation of arsenic with iron sulphate with lime neutralisation; Ageing of reduced arsenic minerals in uranium mill tailings at the McClean Lake operation, by Rinas C., Rowson J., Frey R. and Hughes K., p.377-387, 9 refs., identifying the causes of changing As concentrations in tailings sediments' pore water; Identification of poorly crystalline scorodite in uranium mill tailings, by Frey R., Rowson J., Hughes K., Rinas C. and Warner J., p.389-399, 12 refs., covering the mineralogical identification of As in McClean Lake tailings; Molybdenum and selenium speciation in uranium mine tailings using X-ray absorption spectroscopy, by Essilfie-Dughan, The 9 papers in the session comprised laboratory studies into the chemistry of the treatment of impurities in tailings. They are: The fate of entrained hydrocarbon materials in the McClean Lake operation mill tailings, by Hughes K.A., Rinas C., Frey R. and Rowson J., p.317-326, 6 refs., which describes the chemistry of the oxidation of carbon in entrained hydrocarbons to soluble bicarbonate; The potential arsenic retention role of Ca-Fe(III)-AsO4 compounds in lime-neutralised co-precipitation tailings, by Becze L., Gomez M.A., Petkov V., Cutler J.N. and Demopoulos G.P., p.327-336, 35 refs., reporting on laboratory studies on the potential use of synthetic yukonite for the control of arsenic in uranium tailings; The transformation of ferrihydrite under a range in pH (2-10) and temperature (25-100 degrees C), by Das S., Hendry M.J. and Essilfie-Dughan J., p.337-349, 51 refs., covering laboratory studies into the rate of crystallisation of ferrihydrate; Mineralogical characterisation of arsenic, iron and nickel in uranium mine tailings using XAS and EMPA, by Essilfie-Dughan J., Hendry M.J., Warner J. and Kotzer T., p.351-363, 16 refs.; Arsenic control in process tailings: continuous co-precipitation of As(V) with iron sulphate media, by Daenzer R., De Klerk R.J. and Demopoulos G.P., p.365-375, 16 refs., on the two- stage co-precipitation of arsenic with iron sulphate with lime neutralisation; Ageing of reduced arsenic minerals in uranium mill tailings at the McClean Lake operation, by Rinas C., Rowson J., Frey R. and Hughes K., p.377-387, 9 refs., identifying the causes of changing As concentrations in tailings sediments' pore water; Identification of poorly crystalline scorodite in uranium mill tailings, by Frey R., Rowson J., Hughes K., Rinas C. and Warner J., p.389-399, 12 refs., covering the mineralogical identification of As in McClean Lake tailings; Molybdenum and selenium speciation in uranium mine tailings using X-ray absorption spectroscopy, by Essilfie-Dughan

A process for immobilising As was developed involving the synthesis of scorodite by the oxidation of ferrous ions with oxygen gas in the presence of As(V) ions under atmospheric conditions. The scorodite was well crystallised in a short retention time of 1-7 h. The scorodite particles up to 15 micrometres in diameter had a moisture content of less than 10%, were readily washable and exhibited excellent packing properties. The synthesis method can be incorporated into a hydrometallurgical process. The solubility of the scorodite was investigated using batch leaching tests according to the US Environmental Protection Agency’s Toxicity Characteristic Leaching Procedure (TCLP) test and the Japanese Ministry of the Environment Notification No. 13 method, with minor modifications. In the TCLP test, the scorodite particles released very low, almost negligible, concentrations of As in the pH range 3-5, indicating long-term stability. Leach tests carried out using solutions with various pH values showed that scorodite solubility was significantly affected by outside environmental factors such as pH. High concentrations of As were released under certain leaching conditions, in particular in the presence of a combination of CaO and NaCl in alkaline solutions. Scorodite storage methods should be based on accurate evaluation of the conditions that can cause As leaching in order to minimise the risk of environmental contamination., A process for immobilising As was developed involving the synthesis of scorodite by the oxidation of ferrous ions with oxygen gas in the presence of As(V) ions under atmospheric conditions. The scorodite was well crystallised in a short retention time of 1-7 h. The scorodite particles up to 15 micrometres in diameter had a moisture content of less than 10%, were readily washable and exhibited excellent packing properties. The synthesis method can be incorporated into a hydrometallurgical process. The solubility of the scorodite was investigated using batch leaching tests according to the US Environmental Protection Agency’s Toxicity Characteristic Leaching Procedure (TCLP) test and the Japanese Ministry of the Environment Notification No. 13 method, with minor modifications. In the TCLP test, the scorodite particles released very low, almost negligible, concentrations of As in the pH range 3-5, indicating long-term stability. Leach tests carried out using solutions with various pH values showed that scorodite solubility was significantly affected by outside environmental factors such as pH. High concentrations of As were released under certain leaching conditions, in particular in the presence of a combination of CaO and NaCl in alkaline solutions. Scorodite storage methods should be based on accurate evaluation of the conditions that can cause As leaching in order to minimise the risk of environmental contamination.

The application was investigated of the CESL process to various enargite-bearing Cu concentrates. Initial tests showed that sufficient feed solution free acid was required to promote rapid leaching of the sulphide fraction, but excessive acid levels appeared to either slow the reaction or possibly cause copper sulphide precipitation under highly acidic conditions that may result in reducing conditions in localised zones. Optimal extraction occurred at a free acid concentration of 40-60 g/l. Cu extraction was maximised at a chloride level of about 12 g/l in the feed solution, similar to that achieved when processing chalcopyrite concentrates. There was a steady increase in Cu extraction with increasing retention time. Bench-scale tests under optimum conditions confirmed that the process can effectively treat the concentrates, achieving a Cu extraction of 97% with moderate sulphur oxidation. The majority of the As, about 99%, was precipitated in the pressure oxidation autoclave. XRD analysis confirmed the presence of more than 20 % S(0) and 50 % scorodite in the residue, with no other crystalline ferric arsenate phases being identified., The application was investigated of the CESL process to various enargite-bearing Cu concentrates. Initial tests showed that sufficient feed solution free acid was required to promote rapid leaching of the sulphide fraction, but excessive acid levels appeared to either slow the reaction or possibly cause copper sulphide precipitation under highly acidic conditions that may result in reducing conditions in localised zones. Optimal extraction occurred at a free acid concentration of 40-60 g/l. Cu extraction was maximised at a chloride level of about 12 g/l in the feed solution, similar to that achieved when processing chalcopyrite concentrates. There was a steady increase in Cu extraction with increasing retention time. Bench-scale tests under optimum conditions confirmed that the process can effectively treat the concentrates, achieving a Cu extraction of 97% with moderate sulphur oxidation. The majority of the As, about 99%, was precipitated in the pressure oxidation autoclave. XRD analysis confirmed the presence of more than 20 % S(0) and 50 % scorodite in the residue, with no other crystalline ferric arsenate phases being identified.

Dowa Metals and Mining, Japan, has developed a method for arsenic immobilisation by the synthesis of crystalline scorodite (FeAsO4.2H2O). In the first step arsenic is leached from intermediate materials including arsenic sulphide precipitated from the wastewater from sulphuric acid production and slime from copper electrolysis. After pressure filtration the residue is recycled to the copper smelting process. As(III) in the leach solution is oxidised to As(V) using H2O2 and scorodite is crystallised from the solution at 95 degrees C under atmospheric conditions. A plant producing scorodite has been operating at the Kosaka smelter since June 2008. The scorodite exhibits good crystallinity, large particle size averaging more than 20 micrometres, high X-ray diffraction intensities and filtering and handling attributes similar to those of laboratory synthesised samples. The concentration of As with respect to the solubility of scorodite is low enough to satisfy the environmental regulations in most countries., Dowa Metals and Mining, Japan, has developed a method for arsenic immobilisation by the synthesis of crystalline scorodite (FeAsO4.2H2O). In the first step arsenic is leached from intermediate materials including arsenic sulphide precipitated from the wastewater from sulphuric acid production and slime from copper electrolysis. After pressure filtration the residue is recycled to the copper smelting process. As(III) in the leach solution is oxidised to As(V) using H2O2 and scorodite is crystallised from the solution at 95 degrees C under atmospheric conditions. A plant producing scorodite has been operating at the Kosaka smelter since June 2008. The scorodite exhibits good crystallinity, large particle size averaging more than 20 micrometres, high X-ray diffraction intensities and filtering and handling attributes similar to those of laboratory synthesised samples. The concentration of As with respect to the solubility of scorodite is low enough to satisfy the environmental regulations in most countries.

A review is presented of the technologies and processing options currently available for the treatment of high-As Cu-Au ores and concentrates. Two flowsheets are described which may be used to selectively remove As from a Cu sulphide concentrate to produce a saleable Cu concentrate. Reductive roasting involves the selective removal of more than 90% of the As in fluidised-bed reactors operated at about 680 degrees C. Enargite is converted to covellite and any pyrite contained in the concentrate is also oxidised to provide part of the heating requirements, while chalcopyrite is left essentially unreacted. As is volatilised as As2S3 gas. The high temperature off-gas from the roaster passes through cyclones before dilution air is added in a combustion chamber to oxidise As2S3 to As2O3 with further formation of SO2. From the cyclones, the off-gas is cooled to 350-380 degrees C in an air-cooled heat exchanger before dust is removed in a dry electrostatic precipitator. Dust from the roaster and the electrostatic precipitator is routed to the calcine cooler to form a combined roaster calcine product which is sold to smelters. The As2O3 gas is condensed and scrubbed from the off-gas stream which leaves the remaining gas to be cleaned and SO2 to be recovered as sulphuric acid in a sulphuric acid plant. The solution from the gas cleaning circuit containing the As is then treated in a hydrometallurgical plant, where the As is first oxidised to As(V) using H2O2 and then precipitated with addition of ferric sulphate and limestone to produce crystalline scorodite in a waste product that is stored in a lined storage facility. Alkaline sulphide leaching as applied to enargite-containing concentrates involves the selective leaching of As with sodium sulphide as the active agent. A significant proportion of the Au also dissolves but is removed selectively from the solution using a resin-based process. The flowsheet includes a small high-temperature pressure oxidation autoclave using p, A review is presented of the technologies and processing options currently available for the treatment of high-As Cu-Au ores and concentrates. Two flowsheets are described which may be used to selectively remove As from a Cu sulphide concentrate to produce a saleable Cu concentrate. Reductive roasting involves the selective removal of more than 90% of the As in fluidised-bed reactors operated at about 680 degrees C. Enargite is converted to covellite and any pyrite contained in the concentrate is also oxidised to provide part of the heating requirements, while chalcopyrite is left essentially unreacted. As is volatilised as As2S3 gas. The high temperature off-gas from the roaster passes through cyclones before dilution air is added in a combustion chamber to oxidise As2S3 to As2O3 with further formation of SO2. From the cyclones, the off-gas is cooled to 350-380 degrees C in an air-cooled heat exchanger before dust is removed in a dry electrostatic precipitator. Dust from the roaster and the electrostatic precipitator is routed to the calcine cooler to form a combined roaster calcine product which is sold to smelters. The As2O3 gas is condensed and scrubbed from the off-gas stream which leaves the remaining gas to be cleaned and SO2 to be recovered as sulphuric acid in a sulphuric acid plant. The solution from the gas cleaning circuit containing the As is then treated in a hydrometallurgical plant, where the As is first oxidised to As(V) using H2O2 and then precipitated with addition of ferric sulphate and limestone to produce crystalline scorodite in a waste product that is stored in a lined storage facility. Alkaline sulphide leaching as applied to enargite-containing concentrates involves the selective leaching of As with sodium sulphide as the active agent. A significant proportion of the Au also dissolves but is removed selectively from the solution using a resin-based process. The flowsheet includes a small high-temperature pressure oxidation autoclave using p

Oxidation of arsenopyrite can result in the mobilisation and migration of As into the environment. This study considers the arsenopyrite-rich mine wastes that have undergone more than three decades of oxidation in the Terrubias mining area. In this area two Sn-W mineralization types are present: stratiform with calcosilicate bands, and in subvertical veins of NNE-NE and N100-130E directions, cutting the scheelite-rich calcosilicate bands. The paragenesis of these veins is dominated by quartz, mica, turmaline, wolframite, scheelite, casiterite and arsenopyrite. The aim of this work is to identify the arsenopyrite oxidation products in the Terrubias mining area in order to evaluate if arsenic is fixed by them or, on the contrary, is easily released, with the consequent risk of contamination of surrounding ecosystem compartments.

Mining and ore processing, where tungsten is associated with arsenopyrite (FeAsS), produce As-rich mine wastes. Oxidation of arsenopyrite can result in the mobilization and migration of As from these wastes into the environment. This study considers the arsenopyriterich mine wastes that have undergone more than two decades of oxidation in the Barruecopardo ore deposit (exploited during several decades). This deposit is one of the most important tungsten deposits in Spain. It is constituted by subvertical quartz veins disposed in parallel intragranitic bands. The main minerals present are: scheelite and wolframite, that constitute the ore, arsenopyrite, pyrite, chalcopyrite and ilmenite, as primary minerals, and tungstite, scorodite, melanterite and calcantite, as secondary minerals. These minerals appear in the veins and on the borders of granite (Pellitero, 1981). The aim of this work is to identify the arsenopyrite oxidation products in order to evaluate if this toxic element is fixed by them or, on the contrary, is easily released from them, likely causing the contamination of surrounding ecosystem compartments.

Arsenopyrite ore was processed at the Blackwater Au mine, New Zealand, between 1908 and 1951 and no rehabilitation was undertaken after mine closure. High As concentrations of up to 40 wt% in solid processing residues are due to secondary As minerals. Site pH values vary from 4.1 to about neutral. All processed As was originally present as arsenolite, a by-product of arsenopyrite roasting. Near the roaster, scorodite precipitated as a result of the high dissolved As concentration during arsenolite dissolution. The formation of scorodite has two major effects. The scorodite precipitate cements the ground in the vicinity of the roaster area, creating an impermeable surface crust containing up to 30 wt% As and encapsulating weathered arsenolite grains within the cement, and formation of scorodite temporarily immobilises some of the dissolved As generated during nearby arsenolite dissolution. Where all the available arsenolite has dissolved, scorodite becomes soluble and the dissolved As concentrations are controlled by scorodite solubility. Dissolved As contents downstream reach up to 52 mg/l, exceeding drinking water guidelines. The results show that dissolved As concentrations in discharge waters at historic mine sites are dependent on the processing technology and associated mineralogy., Arsenopyrite ore was processed at the Blackwater Au mine, New Zealand, between 1908 and 1951 and no rehabilitation was undertaken after mine closure. High As concentrations of up to 40 wt% in solid processing residues are due to secondary As minerals. Site pH values vary from 4.1 to about neutral. All processed As was originally present as arsenolite, a by-product of arsenopyrite roasting. Near the roaster, scorodite precipitated as a result of the high dissolved As concentration during arsenolite dissolution. The formation of scorodite has two major effects. The scorodite precipitate cements the ground in the vicinity of the roaster area, creating an impermeable surface crust containing up to 30 wt% As and encapsulating weathered arsenolite grains within the cement, and formation of scorodite temporarily immobilises some of the dissolved As generated during nearby arsenolite dissolution. Where all the available arsenolite has dissolved, scorodite becomes soluble and the dissolved As concentrations are controlled by scorodite solubility. Dissolved As contents downstream reach up to 52 mg/l, exceeding drinking water guidelines. The results show that dissolved As concentrations in discharge waters at historic mine sites are dependent on the processing technology and associated mineralogy.

Many water treatment technologies for arsenic removal that are used today produce arsenic-bearing solid residuals (ABSR), which are disposed in mixed solid waste landfills. It is now well established that many of these residuals will release arsenic into the environment to a much greater extent than predicted by standard regulatory leaching tests and, consequently, require stabilization to ensure benign behaviour after disposal. Conventional waste stabilization technologies, such as cement encapsulation and vitrification, are not suitable for ABSR applications due to their lack of effectiveness or high cost, thus creating a need for a more effective and low-cost treatment technology for ABSR. Arsenic Crystallization Technology (ACT) is a proposed arsenic stabilization method that involves in converting the ABSR into arsenic-bearing minerals that resemble natural materials and have high arsenic capacity, long term stability, and low solubility compared to untreated ABSR. Three arsenic minerals, scorodite, arsenate apatite and ferrous arsenate, have been investigated in this research for their potential application as ACT for ABSR stabilization. Among the three minerals, ferrous arsenate is demonstrated to be the most suitable arsenate mineral for safe arsenic disposal due to its low arsenic solubility and ease of synthesis. An innovative treatment procedure has been developed in this research for stabilization of ABSR to a stable phase of ferrous arsenate using zero-valent iron (ZVI) as the reducing agent. The procedure works at ambient temperature and pressure, and neutral pH. In addition, a modified four-step sequential extraction method has been developed as a means to determine the proportions of various arsenic phases in the stabilized as well as untreated ABSR matrices. This extraction method, as well as traditional leach and solubility tests, show that arsenic stability in the solid phase is dramatically increased after formation of crystalline ferrous arsenat

This paper outlines low pressure and temperature NSC (nitrogen-species catalysed) pressure leaching and ASL (alkaline sulphide leaching) with mixed sodium sulphide and sodium hydroxide for the recovery from enargite of copper as cathode metal or concentrate and gold as concentrate, with precipitation of arsenic as scorodite and ferrihydrite. Test conditions and results are presented and flowsheets proposed for both processes., This paper outlines low pressure and temperature NSC (nitrogen-species catalysed) pressure leaching and ASL (alkaline sulphide leaching) with mixed sodium sulphide and sodium hydroxide for the recovery from enargite of copper as cathode metal or concentrate and gold as concentrate, with precipitation of arsenic as scorodite and ferrihydrite. Test conditions and results are presented and flowsheets proposed for both processes.

This paper reports on the five-year development of a new processing method for the arsenic-bearing smelter dust of the El Teniente smelter in Chile. The smelter dust is leached with sulphuric acid to dissolve arsenic, which is then precipitated as crystalline ferric arsenate, scorodite, at ambient pressure by seeding to promote crystal nucleation and growth. Cu and Zn are then recovered from the As-free solution. It is estimated that the total cost of consumables and wastes would be offset by the value of recovered products, leaving a net cost of US, This paper reports on the five-year development of a new processing method for the arsenic-bearing smelter dust of the El Teniente smelter in Chile. The smelter dust is leached with sulphuric acid to dissolve arsenic, which is then precipitated as crystalline ferric arsenate, scorodite, at ambient pressure by seeding to promote crystal nucleation and growth. Cu and Zn are then recovered from the As-free solution. It is estimated that the total cost of consumables and wastes would be offset by the value of recovered products, leaving a net cost of US

An ambient-pressure synthesis which yields a large particle size crystalline scorodite (ferric arsenate dihydrate) precipitated at 95 degrees C in a period of 1-7 h when ferrous ions are oxidised by oxygen gas in the presence of concentrations of up to 50 g/l of pentavalent arsenic ions in sulphate solution is reported. The crystalline scorodite particles obtained after 1 h had a mean particle size of about 15 micrometres, a BET surface area of 0.3 m2/g, a moisture content of 10% and an arsenic content of approximately 30 mass%. After a reaction time of 7 h, the arsenic leach value was 0.02 mg/l. This process is applicable to a primary smelter, which produces copper, zinc, lead and other secondary materials., An ambient-pressure synthesis which yields a large particle size crystalline scorodite (ferric arsenate dihydrate) precipitated at 95 degrees C in a period of 1-7 h when ferrous ions are oxidised by oxygen gas in the presence of concentrations of up to 50 g/l of pentavalent arsenic ions in sulphate solution is reported. The crystalline scorodite particles obtained after 1 h had a mean particle size of about 15 micrometres, a BET surface area of 0.3 m2/g, a moisture content of 10% and an arsenic content of approximately 30 mass%. After a reaction time of 7 h, the arsenic leach value was 0.02 mg/l. This process is applicable to a primary smelter, which produces copper, zinc, lead and other secondary materials.

A technology for decontamination of flue dust and recovery of valuable metals at copper smelters in Chile was developed as part of a Japanese- Chilean cooperative project. A process for immobilising arsenic in flue dust by leaching and forming crystalline ferric arsenic (scorodite) was established, with Cu recovered by flotation and solvent extraction fed to the production process at Las Ventanas., A technology for decontamination of flue dust and recovery of valuable metals at copper smelters in Chile was developed as part of a Japanese- Chilean cooperative project. A process for immobilising arsenic in flue dust by leaching and forming crystalline ferric arsenic (scorodite) was established, with Cu recovered by flotation and solvent extraction fed to the production process at Las Ventanas.

Papers are presented in ten sections: Overview of iron control technologies (6 papers); Zinc operations I, II and III (19 papers); Copper hydrometallurgy (6 papers); Nickel-cobalt processing (7 papers); Nickel laterite processing (6 papers); New technologies and residue disposal (7 papers); Pickle liquor treatment (6 papers, on recycling of acids and iron by pyrohydrolysis, biotechnology, fluidised beds, spray roasters, crystallisation or selective precipitation); and Iron for arsenic control (5 papers, on As co-precipitation with ferric ions, scorodite formation, co-precipitation with ferrihydrite or Al-modified ferrihydrite, arsenate removal with jarosite residues and Bauxsol, and treatment of enargite concentrates)., 2 ind., Papers are presented in ten sections: Overview of iron control technologies (6 papers); Zinc operations I, II and III (19 papers); Copper hydrometallurgy (6 papers); Nickel-cobalt processing (7 papers); Nickel laterite processing (6 papers); New technologies and residue disposal (7 papers); Pickle liquor treatment (6 papers, on recycling of acids and iron by pyrohydrolysis, biotechnology, fluidised beds, spray roasters, crystallisation or selective precipitation); and Iron for arsenic control (5 papers, on As co-precipitation with ferric ions, scorodite formation, co-precipitation with ferrihydrite or Al-modified ferrihydrite, arsenate removal with jarosite residues and Bauxsol, and treatment of enargite concentrates).

Three technologies are described that are based on high-rate engineered bioreactors to allow optimum control, efficient resource use and predictable and reliable process performance. The BIOSCORODITE process converts As to scorodite, which is most suitable for storage under oxidising conditions. The method can be applied to medium to high concentrations of As in acidic process streams. The THIOMETEQ and ASTRASAND THIOMETEQ processes are used for reducing conditions as they produce orpiment (As2S3). These methods are ideal for treating effluent containing up to 100 mg/l As., Three technologies are described that are based on high-rate engineered bioreactors to allow optimum control, efficient resource use and predictable and reliable process performance. The BIOSCORODITE process converts As to scorodite, which is most suitable for storage under oxidising conditions. The method can be applied to medium to high concentrations of As in acidic process streams. The THIOMETEQ and ASTRASAND THIOMETEQ processes are used for reducing conditions as they produce orpiment (As2S3). These methods are ideal for treating effluent containing up to 100 mg/l As.

It has been determined the thermal behaviour at high temperatures of an iron aluminum arsenate mineral first described in the location of La Serena Valley, Extremadura, Spain, which expands until 1173 K contracting later and softening at 1653 K and full melting at 1773 K. The DTA/TG experiment only detects some rearrangement with an endothermic at 1173 K due to the mica mixing. The mineral is a scorodite variety mixed with micaceous clay which has been found as aggregates of pyramidal crystals associated with quartz, wolframite, molibdenite, cassiterite and other secondary minerals in the pegmatite of the San Nicolas mine. The chemical composition, the physicochemical characteristics such as density and hardness, as well as the thermal behaviour has allowed to conclude that this mineral can be an Al-enriched scorodite (mansfieldite): (Fe, Al) AsO4·2H2O mixed with illite from the scorodite-mansfieldite series.

Recent developments in the field are reviewed, especially new data being generated at McGill University. It is concluded that the high-Fe arsenical ferrihydrites, together with the high-temperature arsenic minerals scorodite and Type II, remain the best and most viable vehicles for industrial arsenic control and disposal, although a new look is taken at the structure and As retention mechanism of ferrihydrite. It is also noted that chemical environments cannot be considered in isolation and that account must be taken of microbiological activity. A survey of industrial practices shows that the use of arsenical ferrihydrite is by far the most widely practised and accepted method worldwide., Recent developments in the field are reviewed, especially new data being generated at McGill University. It is concluded that the high-Fe arsenical ferrihydrites, together with the high-temperature arsenic minerals scorodite and Type II, remain the best and most viable vehicles for industrial arsenic control and disposal, although a new look is taken at the structure and As retention mechanism of ferrihydrite. It is also noted that chemical environments cannot be considered in isolation and that account must be taken of microbiological activity. A survey of industrial practices shows that the use of arsenical ferrihydrite is by far the most widely practised and accepted method worldwide.

The treatment of arsenical copper ore from the Chilean mine was tested using a benchtop autoclave. Oxidation of concentrates typically extracted around 95% of Cu. Extraction was limited by the reprecipitation of a complex precipitate containing arsenic. Cyanidation of the oxidation residue recovered about 95% of Au, but less than 10% of Ag was extracted due to the formation of argentojarosite in the autoclave. Whole-ore oxidation at low pulp density was more successful, routinely achieving Cu extractions of 98-99% and extracting more than 95% of Au, while Ag extraction could be increased to as high as 95% by lime boiling. The results formed the basis for a proposed process flowsheet, in which the dilute solution from the autoclave discharge would be treated by direct solvent extraction and electrowinning of copper., The treatment of arsenical copper ore from the Chilean mine was tested using a benchtop autoclave. Oxidation of concentrates typically extracted around 95% of Cu. Extraction was limited by the reprecipitation of a complex precipitate containing arsenic. Cyanidation of the oxidation residue recovered about 95% of Au, but less than 10% of Ag was extracted due to the formation of argentojarosite in the autoclave. Whole-ore oxidation at low pulp density was more successful, routinely achieving Cu extractions of 98-99% and extracting more than 95% of Au, while Ag extraction could be increased to as high as 95% by lime boiling. The results formed the basis for a proposed process flowsheet, in which the dilute solution from the autoclave discharge would be treated by direct solvent extraction and electrowinning of copper.

Crystallisation has the advantage over conventional neutralisation and precipitation that it fixes contaminants in stable, compact and highly dense compounds. Two systems for effluent treatment at ambient pressure have been developed on a laboratory scale: precipitation of ferric oxyhydroxide from acid mine drainage solutions and fixation of arsenic as crystalline scorodite. In both cases, precipitates of superior quality are attained by controlled neutralisation. The sludges from the acid mine drainage solutions are of very high density (50-70% solids), while the production of amorphous ferric arsenate is avoided by the application of supersaturation control principles to sulphate solutions at 95 degrees C. The latter process may be extended to the treatment of As2O3 dust., Crystallisation has the advantage over conventional neutralisation and precipitation that it fixes contaminants in stable, compact and highly dense compounds. Two systems for effluent treatment at ambient pressure have been developed on a laboratory scale: precipitation of ferric oxyhydroxide from acid mine drainage solutions and fixation of arsenic as crystalline scorodite. In both cases, precipitates of superior quality are attained by controlled neutralisation. The sludges from the acid mine drainage solutions are of very high density (50-70% solids), while the production of amorphous ferric arsenate is avoided by the application of supersaturation control principles to sulphate solutions at 95 degrees C. The latter process may be extended to the treatment of As2O3 dust.

In this process, crystalline ferric arsenate (scorodite) is produced by means of a controlled precipitation technique in which the supersaturation of the arsenic-rich solution is carefully controlled. By such control in the presence of seed material, the formation of amorphous ferric arsenate is prevented. Scorodite is the most stable As(V) compound and is therefore preferred as host mineral for arsenic immobilisation. Discussion is also presented of the effects on precipitation behaviour and kinetics of: type of base, seed concentration, initial As, sulphate concentration, neutralisation rate, seed recycling and method of FeIII addition to Fe-deficient arsenic solutions., In this process, crystalline ferric arsenate (scorodite) is produced by means of a controlled precipitation technique in which the supersaturation of the arsenic-rich solution is carefully controlled. By such control in the presence of seed material, the formation of amorphous ferric arsenate is prevented. Scorodite is the most stable As(V) compound and is therefore preferred as host mineral for arsenic immobilisation. Discussion is also presented of the effects on precipitation behaviour and kinetics of: type of base, seed concentration, initial As, sulphate concentration, neutralisation rate, seed recycling and method of FeIII addition to Fe-deficient arsenic solutions.

The long-term stability of crystalline scorodite is discussed from the standpoint of thermodynamics and kinetics. At pH 5 and over, scorodite is expected to dissolve to release arsenic concentrations above permissible levels in a closed system, but examination of factors affecting the dissolution kinetics, such as crystallinity, particle size and solubility, indicates that in the normal environment of an open scorodite/water system the extremely slow rate of arsenic release should not cause problems. It is likely that even the high-Fe/As amorphous precipitates currently produced from hydrometallurgical solutions will be converted, at least in part, to scorodite during their ageing-crystallisation process. This makes scorodite an attractive carrier for arsenic in many respects. Two new methods of producing crystalline scorodite are outlined. The first involves precipitation of scorodite from hot chloride and sulphate solutions under ambient pressure and a controlled supersaturation regime. The second involves the conversion of baghouse arsenic trioxide dust to scorodite by reaction with HNO3 and scrap iron in an autoclave at 130-160 degrees C., The long-term stability of crystalline scorodite is discussed from the standpoint of thermodynamics and kinetics. At pH 5 and over, scorodite is expected to dissolve to release arsenic concentrations above permissible levels in a closed system, but examination of factors affecting the dissolution kinetics, such as crystallinity, particle size and solubility, indicates that in the normal environment of an open scorodite/water system the extremely slow rate of arsenic release should not cause problems. It is likely that even the high-Fe/As amorphous precipitates currently produced from hydrometallurgical solutions will be converted, at least in part, to scorodite during their ageing-crystallisation process. This makes scorodite an attractive carrier for arsenic in many respects. Two new methods of producing crystalline scorodite are outlined. The first involves precipitation of scorodite from hot chloride and sulphate solutions under ambient pressure and a controlled supersaturation regime. The second involves the conversion of baghouse arsenic trioxide dust to scorodite by reaction with HNO3 and scrap iron in an autoclave at 130-160 degrees C.

Very little work has been done on the characterisation of the arsenate compounds formed during acidic pressure oxidation of sulphidic gold ores, although they are believed to be environmentally stable and suitable for long-term disposal. It was found that at moderate temperatures (150-175 degrees C), low acid concentrations and 103 kPa oxygen partial pressure, the oxidation products of arsenopyrite are similar to the natural mineral scorodite and can be represented by the generic formula FeAsO4(SO4)v.wH2O, where v is 0.02-0.04 and w is 0.8-1.0. The compounds formed at higher temperatures (200-230 degrees C) are basic ferric arsenates with Fe/As molar ratios 1.2-1.3, and can be represented by the generic formula Fe(AsO4)x(OH)y(SO4)z where x is 0.8, y is 0.3-1.3 and z is 0.04-0.06. This behaviour is consistent with that previously reported for the hydrothermal precipitation of arsenate from ferric solutions., Very little work has been done on the characterisation of the arsenate compounds formed during acidic pressure oxidation of sulphidic gold ores, although they are believed to be environmentally stable and suitable for long-term disposal. It was found that at moderate temperatures (150-175 degrees C), low acid concentrations and 103 kPa oxygen partial pressure, the oxidation products of arsenopyrite are similar to the natural mineral scorodite and can be represented by the generic formula FeAsO4(SO4)v.wH2O, where v is 0.02-0.04 and w is 0.8-1.0. The compounds formed at higher temperatures (200-230 degrees C) are basic ferric arsenates with Fe/As molar ratios 1.2-1.3, and can be represented by the generic formula Fe(AsO4)x(OH)y(SO4)z where x is 0.8, y is 0.3-1.3 and z is 0.04-0.06. This behaviour is consistent with that previously reported for the hydrothermal precipitation of arsenate from ferric solutions.

A comparison was made between a number of arsenical end-products which could be produced by a typical non-ferrous smelting operation, including the most common hydrometallurgical precipitates. The maximum availability test used was that specified in the standard Dutch leaching test NVN2508. It was concluded that calcium arsenate should not be considered as a product for disposal. Iron arsenate, especially the crystalline scorodite, displays leaching characteristics which make it a candidate for disposal, though with some reservations. Immobilisation and encapsulation techniques can produce relatively stable products; the long-term leaching behaviour of silicate slags warrants further investigation to assess its suitability for eventual disposal., A comparison was made between a number of arsenical end-products which could be produced by a typical non-ferrous smelting operation, including the most common hydrometallurgical precipitates. The maximum availability test used was that specified in the standard Dutch leaching test NVN2508. It was concluded that calcium arsenate should not be considered as a product for disposal. Iron arsenate, especially the crystalline scorodite, displays leaching characteristics which make it a candidate for disposal, though with some reservations. Immobilisation and encapsulation techniques can produce relatively stable products; the long-term leaching behaviour of silicate slags warrants further investigation to assess its suitability for eventual disposal.

An investigation is presented, using arsenic and nickel K-edge X-ray absorption spectroscopy, X-ray absorption fine structure analysis and X-ray diffraction, of the coordination environments of arsenic and nickel in aluminium-magnesium phases in field samples of slaked lime-neutralised (approx. pH steps 4.0, 9.5 and 10.5) precipitates of raffinate acidic wastewater from Key Lake uranium mill, Saskatchewan, Canada and in synthetic samples of raffinate precipitates with or without iron. The following results are presented: arsenic and nickel are predominantly in the +V and +II oxidation states, respectively; at pH values of 4-6, arsenic(V) is precipitated as ferric arsenate and is adsorbed on to an amorphous hydrobasaluminite and ferrihydrite as bidentate-binuclear complexes, while nickel(II) is adsorbed on to amorphous aluminium hydroxide as edge-sharing bidentate-mononuclear complexes; at pH values of 9.5-9.9, arsenic(V) is adsorbed on to amorphous aluminium hydroxide, ferrihydrite and MgAlFe-hydrotalcite as a bidentate complex, while nickel(II) is adsorbed on to amorphous aluminium hydroxide with the probable formation of a nickel-aluminium layered double hydroxide surface precipitate on MgAlFe-hydrotalcite; and, at pH values of ca. 10.5, arsenic(V) is adsorbed on to amorphous aluminium hydroxide, ferrihydrite and MgAlFe-hydrotalcite, while nickel(II) is adsorbed on to amorphous aluminium hydroxide with the formation of a nickel-aluminium layered double hydroxide surface precipitate on MgAlFe-hydrotalcite., An investigation is presented, using arsenic and nickel K-edge X-ray absorption spectroscopy, X-ray absorption fine structure analysis and X-ray diffraction, of the coordination environments of arsenic and nickel in aluminium-magnesium phases in field samples of slaked lime-neutralised (approx. pH steps 4.0, 9.5 and 10.5) precipitates of raffinate acidic wastewater from Key Lake uranium mill, Saskatchewan, Canada and in synthetic samples of raffinate precipitates with or without iron. The following results are presented: arsenic and nickel are predominantly in the +V and +II oxidation states, respectively; at pH values of 4-6, arsenic(V) is precipitated as ferric arsenate and is adsorbed on to an amorphous hydrobasaluminite and ferrihydrite as bidentate-binuclear complexes, while nickel(II) is adsorbed on to amorphous aluminium hydroxide as edge-sharing bidentate-mononuclear complexes; at pH values of 9.5-9.9, arsenic(V) is adsorbed on to amorphous aluminium hydroxide, ferrihydrite and MgAlFe-hydrotalcite as a bidentate complex, while nickel(II) is adsorbed on to amorphous aluminium hydroxide with the probable formation of a nickel-aluminium layered double hydroxide surface precipitate on MgAlFe-hydrotalcite; and, at pH values of ca. 10.5, arsenic(V) is adsorbed on to amorphous aluminium hydroxide, ferrihydrite and MgAlFe-hydrotalcite, while nickel(II) is adsorbed on to amorphous aluminium hydroxide with the formation of a nickel-aluminium layered double hydroxide surface precipitate on MgAlFe-hydrotalcite.

Development by EcoMetales Ltd of a high-pressure/high-temperature oxidation process (PLCC project) for the leaching of complex Chilean copper concentrates and roaster dusts containing high levels of impurities, e.g. arsenic (more than 0.5%), is presented. The process produces sulphuric acid and soluble sulphates of metals such as copper, zinc and molybdenum while precipitating arsenic as ferric arsenate (scorodite). The latter is removed in thickeners and filtration units and the resulting acidic copper-rich pregnant leach solution is transported to existing solvent-extraction/electrowinning plants (CODELCO northern district) for subsequent production of high-purity copper cathodes. Some advantages of the process including high recoveries of copper, low make-up water requirements, potential use of process-generated heat, lack of sulphur dioxide emissions and formation of stable arsenic-bearing waste are discussed. A block diagram flowsheet for the PLCC process is provided., Development by EcoMetales Ltd of a high-pressure/high-temperature oxidation process (PLCC project) for the leaching of complex Chilean copper concentrates and roaster dusts containing high levels of impurities, e.g. arsenic (more than 0.5%), is presented. The process produces sulphuric acid and soluble sulphates of metals such as copper, zinc and molybdenum while precipitating arsenic as ferric arsenate (scorodite). The latter is removed in thickeners and filtration units and the resulting acidic copper-rich pregnant leach solution is transported to existing solvent-extraction/electrowinning plants (CODELCO northern district) for subsequent production of high-purity copper cathodes. Some advantages of the process including high recoveries of copper, low make-up water requirements, potential use of process-generated heat, lack of sulphur dioxide emissions and formation of stable arsenic-bearing waste are discussed. A block diagram flowsheet for the PLCC process is provided.

Complex redox and pH microenvironments have developed on arsenic-rich (%level As) mine wastes at two gold mine processing areas abandoned 70-80 years ago in southern New Zealand . Authigenic realgar is forming in low-redox microenvironments in a high-sulphur setting with decomposing primary pyrite and arsenopyrite, and in low-sulphur pre-oxidised As-rich mine tailings. Realgar has formed in close proximity (micron scale) to authigenic and/or detrital arsenolite at one low-sulphur site, and in close proximity (cm scale) to authigenic scorodite at the sulphur-rich site. The latter site also has minor accumulations of authigenic gold, which is nanoparticulate and microparticulate and dispersed through a coating of Fe oxyhydroxide and biofilm on authigenic scorodite. Gold was probably mobilised by ephemeral metastable thiosulphate complexes during oxidation of primary auriferous sulphides, and redeposited as those complexes decomposed. Detrital gold has not been noticeably dissolved and redeposited in the low-sulphur mine wastes. The dynamic redox gradients developed on these mine wastes are distinctly different from those that form on most mine wastes elsewhere, where an oxidation front progressively penetrates into primary sulphide residues. The dynamic redox gradients can persist at these sites until all the organic debris has decomposed, after which the more reduced components including authigenic realgar will become oxidised., Complex redox and pH microenvironments have developed on arsenic-rich (%level As) mine wastes at two gold mine processing areas abandoned 70-80 years ago in southern New Zealand . Authigenic realgar is forming in low-redox microenvironments in a high-sulphur setting with decomposing primary pyrite and arsenopyrite, and in low-sulphur pre-oxidised As-rich mine tailings. Realgar has formed in close proximity (micron scale) to authigenic and/or detrital arsenolite at one low-sulphur site, and in close proximity (cm scale) to authigenic scorodite at the sulphur-rich site. The latter site also has minor accumulations of authigenic gold, which is nanoparticulate and microparticulate and dispersed through a coating of Fe oxyhydroxide and biofilm on authigenic scorodite. Gold was probably mobilised by ephemeral metastable thiosulphate complexes during oxidation of primary auriferous sulphides, and redeposited as those complexes decomposed. Detrital gold has not been noticeably dissolved and redeposited in the low-sulphur mine wastes. The dynamic redox gradients developed on these mine wastes are distinctly different from those that form on most mine wastes elsewhere, where an oxidation front progressively penetrates into primary sulphide residues. The dynamic redox gradients can persist at these sites until all the organic debris has decomposed, after which the more reduced components including authigenic realgar will become oxidised.

A novel process has been proposed for the separation of Cu and As from Cu-smelting waste acid (CSWA). The process includes three steps, namely, the oxidation of As via the addition of H2O2, the selective separation of Cu through the use of Na2S, and the removal of As via the synthesis of scorodite. The CSWA was initially oxidised using H2O2 where the As(III) transformed into As(V). The oxidation of As(III) is believed to be the decisive step with regard to the selective separation of Cu and As via the sulphide precipitation process. The results showed that 99.4% of the Cu and 1.8% of the As were precipitated via the sulphidisation process and that 98.8% of the As in the Cu-removed CSWA was removed in the form of scorodite (FeAsO4·2H2O). This novel process exhibits promising application prospects for the treatment of CSWA. (Authors.), A novel process has been proposed for the separation of Cu and As from Cu-smelting waste acid (CSWA). The process includes three steps, namely, the oxidation of As via the addition of H2O2, the selective separation of Cu through the use of Na2S, and the removal of As via the synthesis of scorodite. The CSWA was initially oxidised using H2O2 where the As(III) transformed into As(V). The oxidation of As(III) is believed to be the decisive step with regard to the selective separation of Cu and As via the sulphide precipitation process. The results showed that 99.4% of the Cu and 1.8% of the As were precipitated via the sulphidisation process and that 98.8% of the As in the Cu-removed CSWA was removed in the form of scorodite (FeAsO4·2H2O). This novel process exhibits promising application prospects for the treatment of CSWA. (Authors.)

Fe(III)-As(V) precipitates were synthesised from the Fe(II)-As(V)-SO42--H2O system at a temperature of 90 degrees C and a constant pH maintained at 1.5±0.05. The precipitates obtained were characterised by X-ray diffraction, chemical composition analysis, scanning electron microscope and Raman spectrum, thermogravimetric analyser and electron probe micro-analyser. The precipitates were in irregular aggregation of about 1-4 micron in size, consisting of scorodite, ferric arsenate and an amorphous ferric hydroxide sulphate formulated as Fe(OH)x(SO4)y. The precipitates were stable in modified toxicity characteristic leaching procedure (TCLP) tests at pH 4.93 for 60 h. Arsenic concentrations in the leaching solutions of 0.27 mg/l and 0.59 mg/l were obtained for the precipitates prepared initial Fe(II)/As(V) molar ratios of 4.0 and 5.0, respectively. Significantly more Fe than As was dissolved with up to 280 mg/l of iron reporting to solution. Long-term stability tests of the precipitates were carried out by leaching them for 40 days at 25 degrees C under various media of pH between 9.50 and 10.57. The results showed that the precipitates tested in this study were more stable than those by previous researchers owing to a preferential dissolution of the amorphous ferric hydroxide sulphate., Fe(III)-As(V) precipitates were synthesised from the Fe(II)-As(V)-SO42--H2O system at a temperature of 90 degrees C and a constant pH maintained at 1.5±0.05. The precipitates obtained were characterised by X-ray diffraction, chemical composition analysis, scanning electron microscope and Raman spectrum, thermogravimetric analyser and electron probe micro-analyser. The precipitates were in irregular aggregation of about 1-4 micron in size, consisting of scorodite, ferric arsenate and an amorphous ferric hydroxide sulphate formulated as Fe(OH)x(SO4)y. The precipitates were stable in modified toxicity characteristic leaching procedure (TCLP) tests at pH 4.93 for 60 h. Arsenic concentrations in the leaching solutions of 0.27 mg/l and 0.59 mg/l were obtained for the precipitates prepared initial Fe(II)/As(V) molar ratios of 4.0 and 5.0, respectively. Significantly more Fe than As was dissolved with up to 280 mg/l of iron reporting to solution. Long-term stability tests of the precipitates were carried out by leaching them for 40 days at 25 degrees C under various media of pH between 9.50 and 10.57. The results showed that the precipitates tested in this study were more stable than those by previous researchers owing to a preferential dissolution of the amorphous ferric hydroxide sulphate.

Pre-eminent as vehicles for the environmentally safe disposal of As are the high-Fe (Fe/As molar ratio greater than 4) ambient-temperature amorphous arsenical ferrihydrite precipitates and scorodite (Fe/As molar ratio 1) or other crystalline minerals from high-temperature processes. More recent work has shown that As can be precipitated as scorodite from both chloride and sulphate media at 80-95 degrees C, reducing iron and lime requirements and hence disposal costs. A brief review of trivalent As oxidation shows that the more exotic oxidants, such as ozone and elemental chlorine, are effective in acidic media at moderate to low temperatures. Hydrogen peroxide can achieve virtually quantitative oxidation at 95 degrees C at pH 1-3, but is expensive to use. Most recently and interestingly, SO2-O2 gas mixtures have been shown to be highly effective., Pre-eminent as vehicles for the environmentally safe disposal of As are the high-Fe (Fe/As molar ratio greater than 4) ambient-temperature amorphous arsenical ferrihydrite precipitates and scorodite (Fe/As molar ratio 1) or other crystalline minerals from high-temperature processes. More recent work has shown that As can be precipitated as scorodite from both chloride and sulphate media at 80-95 degrees C, reducing iron and lime requirements and hence disposal costs. A brief review of trivalent As oxidation shows that the more exotic oxidants, such as ozone and elemental chlorine, are effective in acidic media at moderate to low temperatures. Hydrogen peroxide can achieve virtually quantitative oxidation at 95 degrees C at pH 1-3, but is expensive to use. Most recently and interestingly, SO2-O2 gas mixtures have been shown to be highly effective.

An assessment of thermodynamic data shows that there are significant problems with the use of jarosite and scorodite as phases for the disposal of iron and/or arsenic. Neither is stable under typical atmospheric weathering conditions: scorodite is transformed to goethite at pH levels above 4 and jarosite at pH above 7. Currently permitted discharge levels for As are only achieved due to the transformation's slow kinetics. The amorphous iron III arsenate precipitate often formed in low-temperature processes is two orders of magnitude more soluble than crystalline scorodite, but even the latter would not meet discharge limits if they were reduced five-fold. It is essential that other crystalline phases be investigated to find a safe, long-term disposal method for arsenical wastes., An assessment of thermodynamic data shows that there are significant problems with the use of jarosite and scorodite as phases for the disposal of iron and/or arsenic. Neither is stable under typical atmospheric weathering conditions: scorodite is transformed to goethite at pH levels above 4 and jarosite at pH above 7. Currently permitted discharge levels for As are only achieved due to the transformation's slow kinetics. The amorphous iron III arsenate precipitate often formed in low-temperature processes is two orders of magnitude more soluble than crystalline scorodite, but even the latter would not meet discharge limits if they were reduced five-fold. It is essential that other crystalline phases be investigated to find a safe, long-term disposal method for arsenical wastes.

A review of the literature from a Canadian perspective is complemented by information obtained by visits to selected sites. Currently favoured practice is the formation of an insoluble ferric arsenate compound which is allowed to settle to the bottom of ponds. This poorly formed crystalline compound is similar to arsenical ferrihydrite, on which arsenate anions are strongly adsorbed. Despite thermodynamic concerns about its longterm stability, it appears stable for many years in a slightly acidic, oxidising environment, especially if the Fe/As ratio is high and heavy metals are present. High-temperature operations are conducive to the formation of scorodite, FeAsO4.2H2O and/or a series of ferric arseno-hydroxysulphate compounds. Scorodite has a lower iron demand, higher density and greater thermodynamic stability than arsenical ferrihydrite and new procedures have been developed to generate it at ambient pressure. Lime precipitation, widely used in the past, is being abandoned as there is strong evidence that calcium arsenate decomposes slowly in contact with CO2 to form soluble arsenic acid. In all cases, stability may also depend on site characteristics, particle crystallinity and size, and the presence of complexing agents or bacteria., A review of the literature from a Canadian perspective is complemented by information obtained by visits to selected sites. Currently favoured practice is the formation of an insoluble ferric arsenate compound which is allowed to settle to the bottom of ponds. This poorly formed crystalline compound is similar to arsenical ferrihydrite, on which arsenate anions are strongly adsorbed. Despite thermodynamic concerns about its longterm stability, it appears stable for many years in a slightly acidic, oxidising environment, especially if the Fe/As ratio is high and heavy metals are present. High-temperature operations are conducive to the formation of scorodite, FeAsO4.2H2O and/or a series of ferric arseno-hydroxysulphate compounds. Scorodite has a lower iron demand, higher density and greater thermodynamic stability than arsenical ferrihydrite and new procedures have been developed to generate it at ambient pressure. Lime precipitation, widely used in the past, is being abandoned as there is strong evidence that calcium arsenate decomposes slowly in contact with CO2 to form soluble arsenic acid. In all cases, stability may also depend on site characteristics, particle crystallinity and size, and the presence of complexing agents or bacteria.

Arsenic disposal is one of the most important environmental issues of the copper and metallurgical industries. An alternative to the formation in effluent streams of amorphous ferric arsenate using Fe(III) ions and lime is the production of crystalline scorodite, ferric arsenate. This has a high arsenic content and low solubility, with various operational advantages. It can be produced either by the traditional hydrothermal route, or by controlled crystallisation and neutralisation at atmospheric pressure. The latter operates at about 90 degrees C, has a considerably lower capital cost, does not require the simultaneous treatment of a concentrate, and is independent of plant size. Laboratory testing of the process is described, as well as its application to arsenic-containing plant effluents and arsenic flue dusts generated by copper smelters and other metallurgical plants., Arsenic disposal is one of the most important environmental issues of the copper and metallurgical industries. An alternative to the formation in effluent streams of amorphous ferric arsenate using Fe(III) ions and lime is the production of crystalline scorodite, ferric arsenate. This has a high arsenic content and low solubility, with various operational advantages. It can be produced either by the traditional hydrothermal route, or by controlled crystallisation and neutralisation at atmospheric pressure. The latter operates at about 90 degrees C, has a considerably lower capital cost, does not require the simultaneous treatment of a concentrate, and is independent of plant size. Laboratory testing of the process is described, as well as its application to arsenic-containing plant effluents and arsenic flue dusts generated by copper smelters and other metallurgical plants.

Pyrometallurgical treatment of copper arsenide concentrates is becoming unacceptable as the by-product arsenic trioxide has become difficult to market. A brief review of the hydrometallurgical treatment options for copper arsenide minerals is presented. The most promising method is the high-temperature pressure oxidation process, which dissolves more than 99% of the copper from low-grade concentrates and fixes most of the arsenic as the stable mineral scorodite. From high-grade concentrates, copper extractions are about 95%. Several examples of the laboratory application of pressure oxidation for various copper arsenide feeds are given, with discussion of the fate and stability of the arsenic in the various process streams., Pyrometallurgical treatment of copper arsenide concentrates is becoming unacceptable as the by-product arsenic trioxide has become difficult to market. A brief review of the hydrometallurgical treatment options for copper arsenide minerals is presented. The most promising method is the high-temperature pressure oxidation process, which dissolves more than 99% of the copper from low-grade concentrates and fixes most of the arsenic as the stable mineral scorodite. From high-grade concentrates, copper extractions are about 95%. Several examples of the laboratory application of pressure oxidation for various copper arsenide feeds are given, with discussion of the fate and stability of the arsenic in the various process streams.

There are two major novel steps involved in the proposed process: oxidation of As(III) to As(V) in acidic sulphate solutions using SO2/O2 gas mixtures and precipitation of As as scorodite at ambient pressures. An investigation of the kinetics of the two steps demonstrated that both take place at industrially viable rates in acidic sulphate solutions at temperatures above 90 degrees C. Various types of iron source for the production of scorodite were tested; the results suggest the potential for using any available iron, including ferrous or ferric salts and industrial Fe-containing residues. The only As-bearing product generated from the process is a compact, scorodite-based sludge which is stable for disposal. The process employs commonly available and inexpensive reagents and is operated under ambient pressure. From an economic point of view, it is attractive when compared with the two main existing industrial processes., There are two major novel steps involved in the proposed process: oxidation of As(III) to As(V) in acidic sulphate solutions using SO2/O2 gas mixtures and precipitation of As as scorodite at ambient pressures. An investigation of the kinetics of the two steps demonstrated that both take place at industrially viable rates in acidic sulphate solutions at temperatures above 90 degrees C. Various types of iron source for the production of scorodite were tested; the results suggest the potential for using any available iron, including ferrous or ferric salts and industrial Fe-containing residues. The only As-bearing product generated from the process is a compact, scorodite-based sludge which is stable for disposal. The process employs commonly available and inexpensive reagents and is operated under ambient pressure. From an economic point of view, it is attractive when compared with the two main existing industrial processes.

Discussion is presented of methods for the disposal of arsenic from metallurgical processes, with emphasis on hydrothermal precipitation of crystalline ferric arsenate. Precipitation of solids in the Fe-AsO4-SO4 system at a pH of less than 1 was carried out at 150-225 degrees C. Phases identified were haematite, scorodite, arsenical ferrihydrite and two unknown compounds (Type-1 and Phase X). The scorodite and Phase X compounds had very low solubilities. Natural scorodite is the most stable arsenate compound formed in nature and it was predicted that crystalline synthetic scorodite should also remain stable for prolonged periods of time. It may, therefore, be suitable for arsenic disposal purposes. The conceptualised processing of arsenical liquors by the hydrothermal route has been called the scorodite process and is considered to represent a realistic option for arsenic disposal., Discussion is presented of methods for the disposal of arsenic from metallurgical processes, with emphasis on hydrothermal precipitation of crystalline ferric arsenate. Precipitation of solids in the Fe-AsO4-SO4 system at a pH of less than 1 was carried out at 150-225 degrees C. Phases identified were haematite, scorodite, arsenical ferrihydrite and two unknown compounds (Type-1 and Phase X). The scorodite and Phase X compounds had very low solubilities. Natural scorodite is the most stable arsenate compound formed in nature and it was predicted that crystalline synthetic scorodite should also remain stable for prolonged periods of time. It may, therefore, be suitable for arsenic disposal purposes. The conceptualised processing of arsenical liquors by the hydrothermal route has been called the scorodite process and is considered to represent a realistic option for arsenic disposal.