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An electrochemical method (potential sweep method) was utilized to investigate the nature of the absorbed gold on magnetite and to determine the gold sorption mechanism. A magnetite electrode was prepared in the laboratory, made in contact with a gold chloride solution at different conditions, and was subjected to cyclic voltammetry. The cyclic voltammogram of the magnetite electrode pretreated in a gold chloride solution recorded an anodic current peak at around 1.0 V (vs. Ag/AgCl), which corresponds to the current generated for the anodic dissolution of metallic gold in a chloride medium. The results of the electrochemical experiments suggest that gold chloride complexes are reduced to metallic gold on the surface of magnetite. A four-stage gold uptake mechanism was proposed: (i) the transport of AuC14- from the bulk solution to the magnetite surface, (ii) adsorption of AuC14- ions on magnetite surface by electrostatic attraction, (iii) electrochemical reduction of AuC14- to metallic Au, and (iv) the transport of soluble species to the solution phase. The electrochemical investigation also revealed that Fe3+ ions released from magnetite into the solution, suppressed the gold uptake at the acidic pH region.

Batch-sorption tests were carried out on the recovery of gold and platinum from NaCl solution by commercial grade magnetite powder, looking at the effect pH, contact time, and precious metal and NaCl concentration had on recovery. Au and Pt had similar pH dependent recovery curves, with peaks at pH 6-7. At this pH range, a maximum of 4.4 micro mol Au/g Fe3O4and 3.0 micro mol Pt/g Fe3O4 were recovered after 24 h at an initial metal concentration of 0.05 mol/m3. Metal recovery increased with increasing initial precious metals concentration, and decreased with high NaCl concentration. SEM images showed that the gold had agglomerated and was deposited on the magnetite surface., Batch-sorption tests were carried out on the recovery of gold and platinum from NaCl solution by commercial grade magnetite powder, looking at the effect pH, contact time, and precious metal and NaCl concentration had on recovery. Au and Pt had similar pH dependent recovery curves, with peaks at pH 6-7. At this pH range, a maximum of 4.4 micro mol Au/g Fe3O4and 3.0 micro mol Pt/g Fe3O4 were recovered after 24 h at an initial metal concentration of 0.05 mol/m3. Metal recovery increased with increasing initial precious metals concentration, and decreased with high NaCl concentration. SEM images showed that the gold had agglomerated and was deposited on the magnetite surface.

The development of environmental-friendly lixiviants as alternatives to cyanide and the increasing recycling activities to recover precious metals from waste materials have prompted researchers to find efficient methods and alternative sorbents to recover precious metals, such as Au and Pt, from aqueous solutions. This study explored the use of magnetite (Fe3O4) as a sorbent to recover Au and Pt from chloride solutions. Magnetite is a semiconductor and has the capability of transferring electrons both within the solid state and across the solid-liquid interface and has the ability to reduce metal species on its surface. The sorption of AuCl4- and PtCl62- from NaCl solution on commercial grade magnetite powder was investigated by batch-sorption experiments. The effects of pH, contact time, NaCl concentration and precious metal concentration on the recovery were studied. Sorption experiments revealed that magnetite exhibited selectivity towards other metals with the strongest affinity for Au. Both Au and Pt recoveries showed similar pH dependence curves, with peaks at pH 6-7. At this pH range, a maximum of 4.4 µmol Au/g Fe3O4 and 3.0 µmol Pt/g Fe3O4 were recovered after 24 h at an initial metal concentration of 0.05 mol/m3. Increasing the initial precious metal concentration increased the Au and Pt uptake amount by magnetite. It was also observed that the recovery decreased with high NaCl concentrations. The SEM images of the magnetite particles after the treatment showed that gold, which was confirmed by the EDX analysis, agglomerated and were deposited on Fe3O4 surface.

Jig separation of light small particles of non-spherical shape and specific gravities close in value was conducted using a TACUB jig to investigate the relationship between separation condition and fluidisation of the particles. The frequency and amplitude of the vibrations was varied. It was found that the particle layer was fluidised without turbulence by the rising water pulse, resulting in separation, but that too strong a water pulse reduced the separation efficiency. The expansion ratios of the thickness of the particle layer were measured in a cylindrical column. Pressure losses by the particle layer were calculated from the expansion ratios and the velocity of the rising flows using Ergun's equation. The curve of pressure loss against velocity of rising flow showed a maximum corresponding to the point of fluidisation. The velocity of a rising flow near the point of fluidisation defined the optimum separation condition., Jig separation of light small particles of non-spherical shape and specific gravities close in value was conducted using a TACUB jig to investigate the relationship between separation condition and fluidisation of the particles. The frequency and amplitude of the vibrations was varied. It was found that the particle layer was fluidised without turbulence by the rising water pulse, resulting in separation, but that too strong a water pulse reduced the separation efficiency. The expansion ratios of the thickness of the particle layer were measured in a cylindrical column. Pressure losses by the particle layer were calculated from the expansion ratios and the velocity of the rising flows using Ergun's equation. The curve of pressure loss against velocity of rising flow showed a maximum corresponding to the point of fluidisation. The velocity of a rising flow near the point of fluidisation defined the optimum separation condition.

Experimental results from previous investigations on gravity separation of cobalt-containing manganese nodules and crusts are reviewed. Observations and analysis of specimens from 10 different locations show that the liberation ratio of crushed product is high even at the coarse 0.5 to 4 mm fraction. If the less dense substrate rock or nucleus on which the crust/nodules form is removed with high separation efficiency, the separation product conforms to the requirement of the subsequent smelting process. Air table separation was used as a dry method and jig separation as a wet method. High grades with high recoveries were obtained for both methods., Experimental results from previous investigations on gravity separation of cobalt-containing manganese nodules and crusts are reviewed. Observations and analysis of specimens from 10 different locations show that the liberation ratio of crushed product is high even at the coarse 0.5 to 4 mm fraction. If the less dense substrate rock or nucleus on which the crust/nodules form is removed with high separation efficiency, the separation product conforms to the requirement of the subsequent smelting process. Air table separation was used as a dry method and jig separation as a wet method. High grades with high recoveries were obtained for both methods.

A carrier-in-pulp method is proposed to extract and recover heavy metals from melted fly ash. A carrier material, which recovers extracted metals from the aqueous phase, is added simultaneously with fly ash to leaching solutions and is harvested from the pulp by physical separation methods such as magnetic separation or sieving. Shaking flask experiments were conducted under various conditions using NaCl solution as lixiviant and iron powder or granular activated carbon as carrier material. Using iron powder over 97 wt% of Pb and Zn were extracted to the solution phase, but only Pb was recovered by cementation from the iron powder harvested from the pulp by magnetic separation, leaving behind Zn ions in the solution phase. Single-metal adsorption experiments with granular activated carbon gave 85% Pb and 88% Zn recovery, and 60% Pb and 70% Zn in the multi-metal experiments. The leaching test conducted on the Fe- treated residue showed that the solubilisation of Pb was suppressed to a value well below that defined as safe for landfill disposal., A carrier-in-pulp method is proposed to extract and recover heavy metals from melted fly ash. A carrier material, which recovers extracted metals from the aqueous phase, is added simultaneously with fly ash to leaching solutions and is harvested from the pulp by physical separation methods such as magnetic separation or sieving. Shaking flask experiments were conducted under various conditions using NaCl solution as lixiviant and iron powder or granular activated carbon as carrier material. Using iron powder over 97 wt% of Pb and Zn were extracted to the solution phase, but only Pb was recovered by cementation from the iron powder harvested from the pulp by magnetic separation, leaving behind Zn ions in the solution phase. Single-metal adsorption experiments with granular activated carbon gave 85% Pb and 88% Zn recovery, and 60% Pb and 70% Zn in the multi-metal experiments. The leaching test conducted on the Fe- treated residue showed that the solubilisation of Pb was suppressed to a value well below that defined as safe for landfill disposal.

Carrier-microencapsulation (CME) suppresses both the floatability and oxidation of pyrite by coating the pyrite particles with a thin layer of metal oxide or hydroxide using catechol solution as a carrier combined with metal ions. The layer converts the pyrite surface from hydrophobic to hydrophilic and acts as a protective coating against oxidation. Studies have been carried out to demonstrate the effect of CME using Si-catechol complex to suppress the pyrite floatability. The bubble pick-up experiments showed that attachment of pyrite particles to air bubble is suppressed by the CME treatment at pH 4-10, Si-catechol complex concentration over 0.5 mol m3 and treatment time within 2 min. Hallimond tube flotation experiments showed that the pyrite floatability is suppressed by the CME treatment even in the presence of typical flotation collectors such as kerosene and xanthate. SEM-EDX analysis confirmed that Si was present on the pyrite surface treated by the Si-catechol complex, implying that the SiO2 or Si(OH4) layer formed by the CME treatment converting the pyrite surface from hydrophobic to hydrophilic., Carrier-microencapsulation (CME) suppresses both the floatability and oxidation of pyrite by coating the pyrite particles with a thin layer of metal oxide or hydroxide using catechol solution as a carrier combined with metal ions. The layer converts the pyrite surface from hydrophobic to hydrophilic and acts as a protective coating against oxidation. Studies have been carried out to demonstrate the effect of CME using Si-catechol complex to suppress the pyrite floatability. The bubble pick-up experiments showed that attachment of pyrite particles to air bubble is suppressed by the CME treatment at pH 4-10, Si-catechol complex concentration over 0.5 mol m3 and treatment time within 2 min. Hallimond tube flotation experiments showed that the pyrite floatability is suppressed by the CME treatment even in the presence of typical flotation collectors such as kerosene and xanthate. SEM-EDX analysis confirmed that Si was present on the pyrite surface treated by the Si-catechol complex, implying that the SiO2 or Si(OH4) layer formed by the CME treatment converting the pyrite surface from hydrophobic to hydrophilic.

Melting has become a popular pretreatment method for municipal solid waste (MSW) incineration residues in Japan. Although the method is effective in stabilizing the hazardous organic substances such as dioxin in the residue, it concentrates the heavy metals such as Zn, Pb, Cu, and Cd in a newly formed fly ash which is also known as melting fly ash. Melting fly ash is classified as a hazardous waste because of the leaching potential of these heavy metals in landfill environments. Extraction and recovery of these metals seem to be more environmentally and economically attractive than disposing the material to landfill after immobilization process. This study proposes a Carrier-in-Pulp (CIP) method as a new hydrometallurgical route to reject and recover heavy metals from fly ash before landfilling. In this method, a carrier material which recovers the extracted metals is added simultaneously with fly ash to leaching solutions and is harvested from the pulp by physical separation method like magnetic separation or sieving. To demonstrate the effect of CIP method, shaking flask experiments for melting fly ash were conducted under various conditions using NaCl solution as lixiviant and iron powder or granular activated carbon as carrier materials.In the CIP method using iron powder, more than 97 wt% of Pb, Zn, Cu, and Cd, were extracted from the ash to the solution phase. However, the iron powder which was harvested from the pulp by magnetic separation, recovered only Pb and Cu (more than 94%) through cementation, leaving behind Zn and Cd ions in the solution phase. Due to this problem, granular activated carbon was used as an alternative carrier material. The CIP method using activated carbon extracted more than 90% of Zn, Pb, Cu, and Cd from the ash. The carbon granules were separated from the pulp through sieving. The results showed that activated carbon was superior to the iron powder since it was able to recover the extracted heavy metals from the ash through adsorpt

Pyrite coexisting with copper sulphide minerals such as chalcopyrite is separated by flotation as tailings and is oxidised in tailings dams, resulting in the formation of acid mine drainage. To suppress both the flotability and the oxidation of pyrite, a carrier micro-encapsulation process has been developed in which an organic carrier combined with Ti is used to form a thin TiO2 layer on the surface of the pyrite. Ground pyrite samples were treated with a Ti-catechol complex solution under various conditions. Bubble pick-up and flotation experiments showed that the characteristics of the pyrite changed from hydrophobic to hydrophilic, even in the presence of xanthate as a flotation collector. Oxidation tests using 0.02 kmol/m3 HNO3 showed that Fe release was suppressed by the treatment due to the formation of a TiO2 coating., Pyrite coexisting with copper sulphide minerals such as chalcopyrite is separated by flotation as tailings and is oxidised in tailings dams, resulting in the formation of acid mine drainage. To suppress both the flotability and the oxidation of pyrite, a carrier micro-encapsulation process has been developed in which an organic carrier combined with Ti is used to form a thin TiO2 layer on the surface of the pyrite. Ground pyrite samples were treated with a Ti-catechol complex solution under various conditions. Bubble pick-up and flotation experiments showed that the characteristics of the pyrite changed from hydrophobic to hydrophilic, even in the presence of xanthate as a flotation collector. Oxidation tests using 0.02 kmol/m3 HNO3 showed that Fe release was suppressed by the treatment due to the formation of a TiO2 coating.

Leaching tests using chloride media and ferric ions as oxidant were performed to investigate the relationship between the leaching rate of Cu from chalcopyrite and the oxidation-reduction potential. A zone of high leaching rates was found where Cu and Fe concentrations increased sharply; leaching reached a maximum at low oxidation-reduction potentials where ferrous ions were more concentrated than ferric ions., Leaching tests using chloride media and ferric ions as oxidant were performed to investigate the relationship between the leaching rate of Cu from chalcopyrite and the oxidation-reduction potential. A zone of high leaching rates was found where Cu and Fe concentrations increased sharply; leaching reached a maximum at low oxidation-reduction potentials where ferrous ions were more concentrated than ferric ions.

The results of shaking flask leaching experiments showed that leaching rate of chalcopyrite was determined by the normalised redox potential, a function of Cu2+, Fe2+ and Fe3+ concentrations. The results of the column leaching experiments showed that the control of the normalised redox potential improved copper extraction from chalcopyrite., The results of shaking flask leaching experiments showed that leaching rate of chalcopyrite was determined by the normalised redox potential, a function of Cu2+, Fe2+ and Fe3+ concentrations. The results of the column leaching experiments showed that the control of the normalised redox potential improved copper extraction from chalcopyrite.

The optimum conditions for Mn2+ removal from acid mine drainage were studied. Experiments were carried out to investigate the effects of pH, coexisting metal ions and S2- (as Na2S) concentration on MnS formation from Mn2+ solutions. The amount of Mn removed decreased with decreasing pH. Zn2+ or Fe2+ coexisting in the solutions consumed S2- by forming ZnS or FeS, and this inhibited Mn removal. A high molar ratio of S2- to Mn2+ concentrations was required to form manganese sulphides. Experiments using the sulphate-reducing bacteria Desulphovibrio desulphuricans (ATCC 7757) were carried out at 37 degrees C, pH 5-7 and in the absence of coexisting ions to investigate the effects of Mn2+ concentration on the activity of the bacteria in modified ATCC 1249 media containing 100-600 g/m3 Mn2+. In the solutions initially containing 100 and 200 g/m3 Mn2+, Mn concentration decreased to less than 10g/m3, the maximum contaminant level for environmental water quality in Japan. X-ray diffraction and an X-ray fluorescence spectroscopy analysis showed that the precipitate formed during Mn removal in the bioreactor was a metastable gamma-manganese sulphide., The optimum conditions for Mn2+ removal from acid mine drainage were studied. Experiments were carried out to investigate the effects of pH, coexisting metal ions and S2- (as Na2S) concentration on MnS formation from Mn2+ solutions. The amount of Mn removed decreased with decreasing pH. Zn2+ or Fe2+ coexisting in the solutions consumed S2- by forming ZnS or FeS, and this inhibited Mn removal. A high molar ratio of S2- to Mn2+ concentrations was required to form manganese sulphides. Experiments using the sulphate-reducing bacteria Desulphovibrio desulphuricans (ATCC 7757) were carried out at 37 degrees C, pH 5-7 and in the absence of coexisting ions to investigate the effects of Mn2+ concentration on the activity of the bacteria in modified ATCC 1249 media containing 100-600 g/m3 Mn2+. In the solutions initially containing 100 and 200 g/m3 Mn2+, Mn concentration decreased to less than 10g/m3, the maximum contaminant level for environmental water quality in Japan. X-ray diffraction and an X-ray fluorescence spectroscopy analysis showed that the precipitate formed during Mn removal in the bioreactor was a metastable gamma-manganese sulphide.