W.T. Yen

Improved grindability and gold liberation by microwave pretreatment of a free-milling gold ore

Abstract The gravity concentration of gold is very efficient if the metal is fully liberated and the particle size of the gold is relatively coarse. Liberation is usually achieved by comminution, but due to the association of the gold with the other minerals in the ore, overgrinding occurs in conventional comminution circuits and slime generation leads to inefficient recovery. The liberation of minerals can be improved by adding grinding aids, which modify the mechanical properties of the ore and allow breakage at lower stress levels. In this research, microwave pretreatment was used to augment the grinding of a free-milling gold ore containing quartz, silicates and iron oxides. Under microwave irradiation, selective heating of the different mineral components resulted in thermal stresses that caused cracking. These intergranular and transgranular fractures were confirmed by scanning electron microscopy. After microwave processing, the grindability of the ore was improved and the crushing strength and the Bond Work Index were reduced by 31˙2% and 18˙5%, respectively. In addition to the enhanced grindability, gold was released from the matrix of the host minerals at a coarser size, resulting in a significant increase in free gold recovery by gravity concentration. For a gold ore with a head grade of 6˙4 gt−1, the gold recovery improved from about 28% to 40% after microwave pretreatment.

Effects of sulphide minerals and dissolved oxygen on the gold and silver dissolution in cyanide solution

Abstract To better understand how various sulphide minerals influence the dissolution kinetics of gold and silver in air-saturated and oxygen-enriched cyanide solutions, an extensive study has been carried out using the gold and silver rotating disc method. Special emphasis has been focused on the effects of gold-associated minerals on gold and silver dissolution in atmospheric and oxygen-enriched cyanide solutions. The results demonstrated that the leaching behaviours of gold and silver in the presence of various sulphide minerals depend strongly on both the solubility of the sulphides and the oxygen concentration in solution. Since the degree of sulphide decomposition varied with oxygen concentration, gold and silver dissolution during cyanidation was enhanced or diminished, depending on the type and amount of dissolved constituents. In the oxygen-enriched cyanide solution, the presence of some sulphide minerals, such as pentlandite, chalcopyrite, pyrrhotite, sphalerite, molybdenite, arsenopyrite and pyrite, increased the gold dissolution rate while others, such as galena, stibnite and chalcocite caused the reverse to occur under the experimental conditions. Silver dissolution was also significantly improved by leaching with oxygen-enriched cyanide solution, but the rate of silver dissolution with the association of sulphide minerals was still lower than that without sulphide minerals.

The influence of chalcopyrite, galena and pyrite on the selective extraction of zinc from base metal sulphide concentrates☆

Abstract Multi-mineral complex sulphide ores would, typically, not respond well to conventional single mineral flotation due to their fine grained nature. Thus, many complex ore deposits remain undeveloped due to a lack of alternative economic treatment scenarios. The selective leaching process that has been explored is capable of selectively extracting zinc from concentrates containing both sphalerite and chalcopyrite. Unfortunately, many complex sulphide concentrates may also contain appreciable levels of pyrite and galena. Understanding the influence of these mineral on the leaching system is critical to developing a selective leaching process that is easily adapted to a wide variety of concentrate materials. Based on these considerations the selective extraction of zinc using pressure oxidation from multi-mineral concentrates was investigated. Concentrates were artificially produced from pure minerals and ranged in composition of: 25–100% by weight sphalerite, 5–75% chalcopyrite, 0–75% pyrite and 0–75% galena. Zinc was selectively extracted from several Cu/Zn concentrates by utilizing various combinations of temperature and oxygen concentration in the pressure leaching process and the influence of the additions of pyrite and galena was defined. It was discovered that addition of only 5% by weight galena to the selective leaching system retarded the dissolution of sphalerite by up to 13%. Alternatively, a 10% pyrite addition increased zinc extractions while simultaneously decreasing the copper extractions, however, increased concentrations resulted in increased copper extraction and thus, decreased zinc selectivity.

Pulp potential and floatability of chalcopyrite

Abstract Contact angle measurements, ultraviolet (UV) spectroscopy and microflotation studies were carried out in order to investigate the effect of externally applied potential on the wettability of both synthetic and natural chalcopyrites in potassium amyl xanthate (PAX) solutions. The contact angle on the mineral electrode surface was measured after the mineral electrodes were conditioned at a fixed potential for 10 min. The effects of pH, concentration of PAX and depressants such as sodium cyanide, sodium sulfide and MAA (a mixture of magnesium chloride, ammonium chloride and ammonium hydroxide) on the contact angle of synthetic and natural chalcopyrites were also investigated. The hydrophobic film on the chalcopyrite surface was extracted using hexane and examined by UV spectroscopy. Collector and collectorless microflotation of natural chalcopyrite under controlled potential at pH 10 was performed in order to determine the floatable potential range of chalcopyrite. The contact angle results indicated that synthetic and natural chalcopyrites have similar wettability. Their floatable potential range in 7×10 −4 M PAX solution was −0.2 to 0.2 V vs. standard calomel electrode at pH 10 and was −0.25 to 0.3 V at pH 7. Sodium sulfide showed the strongest inhibiting effect among the three depressants investigated. The flotation results indicated that natural chalcopyrite floated in the pulp potential range of −0.087 to +0.2 V (vs. SCE and monitored by a platinum electrode) without collector and in the potential range of −0.2 to +0.205 V in the 7×10 −4 M PAX solution.

Kinetics of gold chloride adsorption onto activated carbon

Abstract The kinetics of activated carbon adsorption of gold from acidic chloride solutions has been studied. The adsorption mechanism was the reduction of the gold chloride complex to metallic gold on the surface of the activated carbon. The adsorption was a first-order reduction process, which was limited by mass transfer of gold chloride species from solution to carbon surface. A rate equation was developed for the adsorption process, in which activated carbon particle size and carbon concentration in solution determined the first-order rate constant. Although acidity of solution, concentration of chloride, zinc and copper ions had a limited effect on the gold adsorption, ferric ion was found to accelerate the removal of gold chloride from solution as a result of the reduction of Fe3+ to Fe2+ by the activated carbon. In contrast, excess aqueous chlorine impaired gold adsorption efficiency due to the redissolution of gold from the loaded carbon surface.

Surface potential and wettability of enargite in potassium amyl xanthate solution

Abstract The voltammetric, ultraviolet techniques and contact angle measurements were used to investigate the oxidation of xanthate and the reduction of dixanthogen on synthetic and natural enargite electrodes which were kept under a fixed potential. The effect of applied potential on the enargite electrode surface in potassium amyl xanthate (PAX) solutions at pH 10 and 7 was investigated by contact angle measurement after the electrodes were conditioned for 10 min. The effect of PAX concentration on enargite floatable potential range was also studied. The hydrophobic film on the enargite surface was extracted using hexane and examined by ultraviolet spectroscopy. The results indicate that the hydrophobic film on the enargite surface in PAX solution was dixanthogen, and it began to form on the enargite electrode surface in 7×10 −4 M PAX solution from −0.2 V vs. Standard Calomel Electrode at pH 10 and from −0.275 V vs. Standard Calomel Electrode at pH 7. The floatable potential range of enargite was 60–70 mV narrower in 7×10 −5 M PAX solution than in 7×10 −4 M PAX solution. The potential needed to reduce dixanthogen on enargite surface was as low as −0.6 V vs. Standard Calomel Electrode.

Selective zinc extraction from complex copper/zinc sulphide concentrates by pressure oxidation

Abstract Currently the world demand for non-ferrous metals can still be met by simple ores treated in a conventional manner. However, mineral resources are finite and as these reserves become depleted, attention must be focused on utilizing the more complex sulphide deposits. Frequently complex sulphide ores exhibit poor metallurgical response to conventional froth flotation. Since hydrometallurgical processes can tolerate lower metal grades and higher impurity levels than can conventional pyrometallurgical processes, treatment of bulk flotation concentrates is conducive to hydrometallurgical methods. The application of bulk flotation results in a simplified milling flowsheet and is capable of producing higher metal recoveries and revenues with respect to conventional smelter contracts. The primary objective was to develop hydrometallurgical process conditions which would enable selective pressure leaching of copper/zinc bulk flotation concentrates to extract zinc producing a zinc electrolytic solution. The upgraded leach residue containing the unreacted copper sulphide and precious metals would be treatable by conventional copper smelting. Zinc was selectively extracted from several complex Cu/Zn concentrates by utilizing various combinations of temperature and oxygen concentration in the pressure leaching process. A bulk concentrate grading 12.2% Cu, 3.8% Pb, 22.2% Zn, 1.4 g/t Au and 184 g/t Ag was pressure leached at 210°C with a 50% oxygen concentration at an over-pressure of 689 KPa with a retention time of 180 minutes. Extractions of zinc, and copper from the concentrate were 99%, and 0.32% respectively. The grade of the residue produced was approximately: 26% copper, 8.8% lead, 0.5% zinc, 435 g/t silver, 3.5 g/t gold and 42% iron. Similar results were also obtained from two bulk concentrates of different ore deposits.

Comparison of elution methods for carbon loaded with gold thiourea complex

Abstract Gold thiourea complex adsorbs easily onto activated carbon, but elution from the carbon is difficult. Several elution conditions and eluants have been studied, such as organic acidic thiourea solutions at ambient temperature, alkaline thiourea solution, alkaline cyanide solution, sodium sulphide and sodium thiosulphate solutions at an elevated temperature range. Effects of variables, such as the concentrations of solution constituents, retention time and solution temperature etc., were investigated. The elution rate constant and the activity of the eluted carbon were used to compare the merits of the processes. The results demonstrated that organic thiourea can elute the gold thiourea complex from carbon at ambient or low temperatures. At an elevated temperature range, alkaline thiourea and sodium sulphide solutions are more efficiency. The best eluted carbon assays less than 0.2 mg Au/g C.

Effect of cell design and electrode configuration on the efficiency of applied potential and sulfide mineral flotation

Theoretical and experimental work was carried out in this investigation to study the effect of the electrode configuration of an electrochemical flotation cell on sulfide mineral flotation and the optimum configuration of the electrodes. The factors that may affect the efficiency of the externally applied potential were discussed. The results of this study illustrate that both the value of the potential and the distribution of electrical current in the pulp are crucial for effective control of mineral flotation.