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Studies have been carried out to assess the ability of Phanerochaete chrysosporium to degrade cyanide in gold and silver effluents under different conditions, including changes in cyanide concentration, culture mass, time, closed system and open system. Analytical grade sodium cyanide, sodium hydroxide, and silver nitrate were used along with fungal spores of Phanerochaete chrysosporium grown on a millet and corn cob medium. At the end of 24 hour contact in an open agitated system with initial pH of 11.5, a control experiment using 100 mg/L cyanide revealed a natural attenuation of 15% with pH decreasing to 9.88, while the best myco-detoxification of 85% was achieved by contacting 100 mg/L cyanide with 0.5 g culture mass, translating into degradation capacity of 17.2 mg/g (milligram of cyanide per gram of culture) with pH reducing to 8.4 in 24 hours. The degradation could be based on a number of mechanisms including hydrolysis to HCN, oxidation to cyanyl radical and cyanate due to natural attenuation through atmospheric contact, and secretion of organic acid, oxidative enzymes, and hydrogen peroxide by the fungus., Studies have been carried out to assess the ability of Phanerochaete chrysosporium to degrade cyanide in gold and silver effluents under different conditions, including changes in cyanide concentration, culture mass, time, closed system and open system. Analytical grade sodium cyanide, sodium hydroxide, and silver nitrate were used along with fungal spores of Phanerochaete chrysosporium grown on a millet and corn cob medium. At the end of 24 hour contact in an open agitated system with initial pH of 11.5, a control experiment using 100 mg/L cyanide revealed a natural attenuation of 15% with pH decreasing to 9.88, while the best myco-detoxification of 85% was achieved by contacting 100 mg/L cyanide with 0.5 g culture mass, translating into degradation capacity of 17.2 mg/g (milligram of cyanide per gram of culture) with pH reducing to 8.4 in 24 hours. The degradation could be based on a number of mechanisms including hydrolysis to HCN, oxidation to cyanyl radical and cyanate due to natural attenuation through atmospheric contact, and secretion of organic acid, oxidative enzymes, and hydrogen peroxide by the fungus.

The Ghanaian State has jurisdiction over commercial hydrocarbon discoveries in the Deepwater Tano-Cape Three Point Jubilee field and the responsibility to maintain an attractive and efficient fiscal system to generate maximum economic benefits. Different field reserves and oil prices have been used to develop a discounted cash flow model to investigate the effect of various fiscal terms on the end benefits to the contractor and the Government. Rigorous sensitivity analysis of different price scenarios and field sizes suggested that for all field sizes considered at oil prices above, The Ghanaian State has jurisdiction over commercial hydrocarbon discoveries in the Deepwater Tano-Cape Three Point Jubilee field and the responsibility to maintain an attractive and efficient fiscal system to generate maximum economic benefits. Different field reserves and oil prices have been used to develop a discounted cash flow model to investigate the effect of various fiscal terms on the end benefits to the contractor and the Government. Rigorous sensitivity analysis of different price scenarios and field sizes suggested that for all field sizes considered at oil prices above

A study has been carried out of the geological characteristics of ore from four active pits at a gold mine in Ghana to ascertain the influence of ore blends on the performances of comminution and leaching circuits. Mineralogical, comminution, gravity recoverable gold and leaching investigations were conducted on fresh and weathered ore samples and their blends. The main rock types present were dolerite, phyllites, conglomerates and sandstone with the dominant minerals being quartz and plagioclase, along with traces of pyrite. The Crushability Work Indices of the rocks were between 30 and 37 kwh/t, which are generally higher than the maximum design value of 31.9 kwh/t, which will pose throughput challenges. The Bond Ball Mill Work Indices of the blends tested were between 16.4 kWh/t and 9.6 kWh/t and a blend ratio of 85% fresh and 15% weathered was found to have a Bond Ball Work Index almost equal to the design value of 14 kWh/t. With gold assays of 2.5 g/t for dolerite, 2.1 g/t for phyllite, 3.7 g/t for sandstone and 3.4 g/t for conglomerate, the gravity recovery of gold was 36% for sandstone, phyllite 31.5%, dolerite 29.5%, and conglomerate 18%. The overall gold recoveries were in the sequence of conglomerate (95%), sandstone (94%), phyllite (92%) and dolerite (87%)., A study has been carried out of the geological characteristics of ore from four active pits at a gold mine in Ghana to ascertain the influence of ore blends on the performances of comminution and leaching circuits. Mineralogical, comminution, gravity recoverable gold and leaching investigations were conducted on fresh and weathered ore samples and their blends. The main rock types present were dolerite, phyllites, conglomerates and sandstone with the dominant minerals being quartz and plagioclase, along with traces of pyrite. The Crushability Work Indices of the rocks were between 30 and 37 kwh/t, which are generally higher than the maximum design value of 31.9 kwh/t, which will pose throughput challenges. The Bond Ball Mill Work Indices of the blends tested were between 16.4 kWh/t and 9.6 kWh/t and a blend ratio of 85% fresh and 15% weathered was found to have a Bond Ball Work Index almost equal to the design value of 14 kWh/t. With gold assays of 2.5 g/t for dolerite, 2.1 g/t for phyllite, 3.7 g/t for sandstone and 3.4 g/t for conglomerate, the gravity recovery of gold was 36% for sandstone, phyllite 31.5%, dolerite 29.5%, and conglomerate 18%. The overall gold recoveries were in the sequence of conglomerate (95%), sandstone (94%), phyllite (92%) and dolerite (87%).

The fungus Phanerochaete chrysosporium has been shown to biotransform carbonaceous matter, thus reducing its ability to rob pregnant gold solutions. However, the possible transfer of fungal biomass entrained into carbon-in-leach (CIL) circuits has been reported to decrease the activity of activated carbon, and a proposed solution to this was to wash the fungally-treated material thoroughly with water before CIL operation. This paper therefore sets out to assess the effect of water-washing on aurocyanide adsorption by activated carbon in CIL following fungal pretreatment of carbonaceous ores. To realise the objective, activated carbon was contacted with cell-free extract of P. chrysosporium under varying conditions of pH and time, after which it was washed with different volumes of water, and its gold-adsorption ability assessed. The results revealed a decrease in the activity of activated carbon as a function of increasing contact time with the cell-free extract. The percentage decrease was higher after treatment in the acidic medium (13%) than the basic medium (9%). After washing the carbon (treated in acidic medium), gold adsorption was found to increase directly with the volume of water used from 64% at 0 ml to 84% at 500 ml and 91% at 1 000 ml. Correspondingly, the carbon treated in basic medium recorded 69% at 0 ml to 87% at 500 ml and 93% at 1 000 ml. This paper thus concludes that, sufficient water washing should be employed after fungal-biotransformation of refractory ores before CIL operation to decrease the effect of entrained biomass on the adsorption capacity of activated carbon., The fungus Phanerochaete chrysosporium has been shown to biotransform carbonaceous matter, thus reducing its ability to rob pregnant gold solutions. However, the possible transfer of fungal biomass entrained into carbon-in-leach (CIL) circuits has been reported to decrease the activity of activated carbon, and a proposed solution to this was to wash the fungally-treated material thoroughly with water before CIL operation. This paper therefore sets out to assess the effect of water-washing on aurocyanide adsorption by activated carbon in CIL following fungal pretreatment of carbonaceous ores. To realise the objective, activated carbon was contacted with cell-free extract of P. chrysosporium under varying conditions of pH and time, after which it was washed with different volumes of water, and its gold-adsorption ability assessed. The results revealed a decrease in the activity of activated carbon as a function of increasing contact time with the cell-free extract. The percentage decrease was higher after treatment in the acidic medium (13%) than the basic medium (9%). After washing the carbon (treated in acidic medium), gold adsorption was found to increase directly with the volume of water used from 64% at 0 ml to 84% at 500 ml and 91% at 1 000 ml. Correspondingly, the carbon treated in basic medium recorded 69% at 0 ml to 87% at 500 ml and 93% at 1 000 ml. This paper thus concludes that, sufficient water washing should be employed after fungal-biotransformation of refractory ores before CIL operation to decrease the effect of entrained biomass on the adsorption capacity of activated carbon.

This paper, which is a revisit of an earlier one, presents an overview of on-going research aimed at assessing the capability of the fungus, Phanerochaete chrysosporium, to degrade sulphides and carbonaceous matter (CM). Surrogate carbonaceous materials (lignite, bituminous and anthracite coals) and pure sulphides (pyrite and arsenopyrite) were used to model the behaviour of CM and sulphides in refractory gold ores. To monitor the extent of biotransformation, preg-robbing test was conducted on the as-received and treated CM, and sulphide sulphur in the residual sulphidic materials was also determined. The ability of CM to preg-rob gold reduced by 70-95% in increasing order of lignite, bituminous, anthracite within 21 days of treatment, whereas there were 18% and 39% oxidation of sulphide sulphur in pyrite and arsenopyrite respectively. XRD examination of the treated anthracite confirmed reduction in the graphitic structure of carbon following fungal transformation. Similarly, there was a decline in the major sulphide peak after microbial pretreatment. The results indicate that the fungus biotransforms through destruction of the ordered structure, followed by introduction of oxygen groups. The amorphous nature thus generated inhibits the uptake of aurocyanide ions by CM, while enhancing the affinity of cyanide for the oxidised sulphide material in subsequent cyanidation treatment. The findings contribute to knowledge on novel and technically viable alternative methods for oxidative pretreatment of refractory gold ores., This paper, which is a revisit of an earlier one, presents an overview of on-going research aimed at assessing the capability of the fungus, Phanerochaete chrysosporium, to degrade sulphides and carbonaceous matter (CM). Surrogate carbonaceous materials (lignite, bituminous and anthracite coals) and pure sulphides (pyrite and arsenopyrite) were used to model the behaviour of CM and sulphides in refractory gold ores. To monitor the extent of biotransformation, preg-robbing test was conducted on the as-received and treated CM, and sulphide sulphur in the residual sulphidic materials was also determined. The ability of CM to preg-rob gold reduced by 70-95% in increasing order of lignite, bituminous, anthracite within 21 days of treatment, whereas there were 18% and 39% oxidation of sulphide sulphur in pyrite and arsenopyrite respectively. XRD examination of the treated anthracite confirmed reduction in the graphitic structure of carbon following fungal transformation. Similarly, there was a decline in the major sulphide peak after microbial pretreatment. The results indicate that the fungus biotransforms through destruction of the ordered structure, followed by introduction of oxygen groups. The amorphous nature thus generated inhibits the uptake of aurocyanide ions by CM, while enhancing the affinity of cyanide for the oxidised sulphide material in subsequent cyanidation treatment. The findings contribute to knowledge on novel and technically viable alternative methods for oxidative pretreatment of refractory gold ores.

Depending on the type and maturity of carbonaceous materials (CM) encountered, preg-robbing of aurocyanide ion can lead to reduction in gold recovery ranging from a few percentages to more than 50%. This paper, as part of an on-going work, presents results on the degree of adsorption by the various carbonaceous materials encountered in gold processing and estimates the amount of aurocyanide complex preg-robbed by them. To do this, various weights of carbonaceous materials (wood chips, charcoal, barren carbon and fresh activated carbon) were contacted with gold solution for a given period of time. The percentages of gold adsorbed on the various carbons were determined, and the results showed an increasing trend of preg-robbing in favour of wood chips, charcoal, barren carbon, activated carbon. The adsorption capacities in grams of gold per tonne of carbon over a 24-hr period were 30-65 for wood chips, 320-370 for charcoal, 410-420 for barren carbon and 580-650 for activated carbon. The trend shows direct correlation with the activities and the degrees of graphitisation and maturity of the various carbonaceous materials. Of all the CMs, wood chips have not undergone any carbonisation and/or activation, and thus exhibited the lowest affinity. It is thus important to characterise carbonaceous gold ores to know the maturity of the CM present, which will inform on the pre-treatment processes required., Depending on the type and maturity of carbonaceous materials (CM) encountered, preg-robbing of aurocyanide ion can lead to reduction in gold recovery ranging from a few percentages to more than 50%. This paper, as part of an on-going work, presents results on the degree of adsorption by the various carbonaceous materials encountered in gold processing and estimates the amount of aurocyanide complex preg-robbed by them. To do this, various weights of carbonaceous materials (wood chips, charcoal, barren carbon and fresh activated carbon) were contacted with gold solution for a given period of time. The percentages of gold adsorbed on the various carbons were determined, and the results showed an increasing trend of preg-robbing in favour of wood chips, charcoal, barren carbon, activated carbon. The adsorption capacities in grams of gold per tonne of carbon over a 24-hr period were 30-65 for wood chips, 320-370 for charcoal, 410-420 for barren carbon and 580-650 for activated carbon. The trend shows direct correlation with the activities and the degrees of graphitisation and maturity of the various carbonaceous materials. Of all the CMs, wood chips have not undergone any carbonisation and/or activation, and thus exhibited the lowest affinity. It is thus important to characterise carbonaceous gold ores to know the maturity of the CM present, which will inform on the pre-treatment processes required.

Microwave energy was used for the treatment of manganese carbonate ore from Nsuta, Ghana, containing 34.43% Mn, 3.45% Si, 3.09% Mg, 1.64% Fe, 1.01% Al, 0.68% Ca, 0.05% P, 0.01% S and 2.70% moisture. Thermogravimetric analysis showed that most of the carbon dioxide present in the ore evolved over the temperature range 500-750 degrees C. The permittivities of both the ore and calcine were relatively low at low temperatures and increased slowly from room temperature to about 600 degrees C. At higher temperatures the real and imaginary permittivities increased more rapidly, reaching high values of 26 and 43, respectively, for the calcine. The temperature of the ore samples increased with processing time, sample mass and microwave power. The calcine exhibited excellent microwave coupling compared with the ore, attributed to the presence of manganese oxides in the calcine. Ore sample temperatures of more than 1 350 degrees C were obtained for a processing time of 20 minutes and an incident microwave power of 1 000 W, or 10 minutes at 1 500 W. The calcination rate was higher compared with the use of conventional resistance heating. The average crushing strength of the microwave sintered material was 2 550 kN and the tumbler index was about 90., Microwave energy was used for the treatment of manganese carbonate ore from Nsuta, Ghana, containing 34.43% Mn, 3.45% Si, 3.09% Mg, 1.64% Fe, 1.01% Al, 0.68% Ca, 0.05% P, 0.01% S and 2.70% moisture. Thermogravimetric analysis showed that most of the carbon dioxide present in the ore evolved over the temperature range 500-750 degrees C. The permittivities of both the ore and calcine were relatively low at low temperatures and increased slowly from room temperature to about 600 degrees C. At higher temperatures the real and imaginary permittivities increased more rapidly, reaching high values of 26 and 43, respectively, for the calcine. The temperature of the ore samples increased with processing time, sample mass and microwave power. The calcine exhibited excellent microwave coupling compared with the ore, attributed to the presence of manganese oxides in the calcine. Ore sample temperatures of more than 1 350 degrees C were obtained for a processing time of 20 minutes and an incident microwave power of 1 000 W, or 10 minutes at 1 500 W. The calcination rate was higher compared with the use of conventional resistance heating. The average crushing strength of the microwave sintered material was 2 550 kN and the tumbler index was about 90.

Refractory Au ores contain metal sulphides which encapsulate the Au, preventing its dissolution by cyanide, and carbonaceous matter which adsorbs, or preg-robs, the Au cyanide complex during cyanidation. The capability was investigated of the fungus Phanerochaete chrysosporium to degrade sulphides and carbonaceous matter during the pre-treatment of refractory Au ores. Pure pyrite and arsenopyrite with initial sulphide sulphur contents of 52% and 20%, respectively, were used as models for the metal sulphides and lignite and bituminous and anthracite coals were used to model the behaviour of carbonaceous matter. Experiments were conducted using a standard Au solution containing 50 mg/l Au, 1 g/l NaOH and 0.5 g/l NaCN. Within 21 days of treatment, 18% and 39% oxidation of sulphide sulphur in pyrite and arsenopyrite, respectively, was observed, while the preg-robbing effect of carbonaceous matter was reduced by 70%-95%, with the reduction increasing in the order lignite, bituminous coal and anthracite. X-ray diffraction characterisation of the treated anthracite showed disruption to the graphitic structure, reducing Au adsorption, and for pyrite there was a decline in the major sulphide peak after microbial pre-treatment., Refractory Au ores contain metal sulphides which encapsulate the Au, preventing its dissolution by cyanide, and carbonaceous matter which adsorbs, or preg-robs, the Au cyanide complex during cyanidation. The capability was investigated of the fungus Phanerochaete chrysosporium to degrade sulphides and carbonaceous matter during the pre-treatment of refractory Au ores. Pure pyrite and arsenopyrite with initial sulphide sulphur contents of 52% and 20%, respectively, were used as models for the metal sulphides and lignite and bituminous and anthracite coals were used to model the behaviour of carbonaceous matter. Experiments were conducted using a standard Au solution containing 50 mg/l Au, 1 g/l NaOH and 0.5 g/l NaCN. Within 21 days of treatment, 18% and 39% oxidation of sulphide sulphur in pyrite and arsenopyrite, respectively, was observed, while the preg-robbing effect of carbonaceous matter was reduced by 70%-95%, with the reduction increasing in the order lignite, bituminous coal and anthracite. X-ray diffraction characterisation of the treated anthracite showed disruption to the graphitic structure, reducing Au adsorption, and for pyrite there was a decline in the major sulphide peak after microbial pre-treatment.

An investigation is presented of causative factors leading to particle siltation in leaching tanks in the carbon-in-leach plant of Gold Fields Ghana Ltd at Tarkwa using plant tests (screen analyses, slurry settling rate determinations and agitation efficiency analyses) and of mitigation measures using laboratory tests (de-flocculation and grind analyses). Samples for the study obtained from the plant included semiautonomous grinding mill cyclone overflow product, ball mill feed, ball mill cyclone overflow product, pre-leach thickener feed, leach feed and silt material. A size analysis indicated a P67 value of 106 micrometres rather than a design size P80 value of 106 micrometres for the ball mill cyclone overflow product. The average settling velocity of the ore was estimated to be 0.000 005 67m/s. Extended grinding of the ball mill feed provided 94% passing 106 micrometres and the settling velocity showed a reduction of 71% after a caustic de-flocculant was added (1 000 ppm). The agitator efficiency was evaluated by measurement of its off-bottom distance (5.8 m), compared to an expected distance of 1.7 m. Overall, de-flocculation and finer grinding are proposed as potentially useful strategies for mitigating siltation in gold leaching tanks., An investigation is presented of causative factors leading to particle siltation in leaching tanks in the carbon-in-leach plant of Gold Fields Ghana Ltd at Tarkwa using plant tests (screen analyses, slurry settling rate determinations and agitation efficiency analyses) and of mitigation measures using laboratory tests (de-flocculation and grind analyses). Samples for the study obtained from the plant included semiautonomous grinding mill cyclone overflow product, ball mill feed, ball mill cyclone overflow product, pre-leach thickener feed, leach feed and silt material. A size analysis indicated a P67 value of 106 micrometres rather than a design size P80 value of 106 micrometres for the ball mill cyclone overflow product. The average settling velocity of the ore was estimated to be 0.000 005 67m/s. Extended grinding of the ball mill feed provided 94% passing 106 micrometres and the settling velocity showed a reduction of 71% after a caustic de-flocculant was added (1 000 ppm). The agitator efficiency was evaluated by measurement of its off-bottom distance (5.8 m), compared to an expected distance of 1.7 m. Overall, de-flocculation and finer grinding are proposed as potentially useful strategies for mitigating siltation in gold leaching tanks.