Processing Of Refractory Gold Ores Research Articles

Use of cavitation to enhance the leaching kinetics of refractory gold ores

ABSTRACT The cyanidation process for refractory gold ores has been the subject of numerous investigations aimed at improving gold recovery and leaching kinetics. From recent literature investigations, hydrodynamic cavitation has been found to be a promising new approach which may modify the cyanidation processes. In this study, this approach results in the enhancement of mass transfer kinetics in multiphase fluid streams due to impacting two pulps streams against one another in a vessel called the Jetleach reactor. In this work, the Jetleach reactor was applied to a flotation concentrate from a South African gold tailings processing plant (Ergo) where an improvement of almost 10% in gold recovery was obtained while decreasing cyanide and oxygen consumption by almost 8% and 50%, respectively. The main reason for the improvement is surmised to be due to the generation of micro-cavitation which improves the mass transfer of oxygen and cyanide within the slurry.

Изучение минерального состава отходов золотодобывающего производства

The experimental research and analysis of the various technological schemes for processing of refractory gold ores allowed to be considered one of the most promising proposed autoclave-thiourea and autoclave-hypochlorite technological schemes.

Arsenic Precipitation from Solutions in Autoclave-Hydrometallurgical Technology of Processing Sulphide Concentrates

Currently, the share of gold extracted from technologically simple gold ores is steadily decreasing, which determines the involvement in the processing of refractory gold ores containing finely disseminated gold and silver in sulfides, mainly in pyrites and arsenopyrites. Autoclave oxidation is a promising method of pretreatment of the refractory sulfide-arsenic gold-bearing raw materials before cyanidation. A serious problem of auriferous ores autoclave-hydrometallurgical processing is the removal of contained arsenic into relatively harmless and capable of being kept forms. This article shows the results of behavior of arsenic during the neutralization of solid after autoclave oxidative leaching of refractory gold-containing materials.

Analysis of Microbial Communities Associated with Bioremediation Systems for Thiocyanate-Laden Mine Water Effluents

During the processing of refractory gold ores, cyanide (CN-) and residual sulphur species react to form an effluent stream containing thiocyanate (SCN-) and residual CN-. The release of SCN- and CN- containing effluent water to the environment is prohibited, necessitating effective treatment prior to discharge and/or reuse of contaminated plant water. Biologically mediated effluent remediation processes have been developed for commercial use, to remediate SCN- containing effluents, with the aim of enabling recycling of process water and improving the quality of effluent water prior to disposal. Bioremediation processes to treat these effluents rely on a complex consortium of microorganisms to metabolise the SCN- resulting in the production of ammonium that is in turn removed by conversion to nitrite and subsequent denitrification. Increasingly, genomic methods are being used to investigate processes in wastewater treatment to identify key microbial species and, thereby, inform the rationale design and operation of these bioremediation systems. The microbial ecology of laboratory-based SCN- degrading bioprocesses have been investigated, using genome resolved metagenomics, to provide detailed information on the community composition and metabolic profile of abundant microbial community members. Our on-going research is focused on developing a greater understanding of the heterotrophic and autotrophic populations of microorganisms within the SCN- degrading community as well as the role of the component members in SCN- destruction. We are interested in the formation of microbial biofilm and the spatial distribution of key microorganisms within the resulting biofilm communities. This information is being used to inform further rational development of SCN- degradation processes for treatment of contaminated wastewater effluents.

Bio-modification of carbonaceous matter in gold ores: Model experiments using powdered activated carbon and cell-free spent medium of Phanerochaete chrysosporium

Carbonaceous matter in refractory gold ore is known to be one of the primary causes of gold recovery loss. Model experiments were conducted to simulate the bio-modification of carbonaceous matter using powdered activated carbon (PAC) as a surrogate and cell-free spent medium (CFSM) of Phanerochaete chrysosporium. The CFSM was used because of the lignin peroxidase and manganese peroxidase secreted by the microbe during its incubation. In the present work, an investigation was conducted to determine the physical and chemical alterations in PAC after enzymatic treatment and its effect on Au(CN)2− uptake. Characterization of the solid residues of PAC by 13C NMR and N2 adsorption after bio-modification revealed that the treatment had decomposed poly-aromatic carbons into aliphatic carbons and also reduced the specific surface area from 1430m2/g to 697m2/g in 14days. As a result, Au(CN)2− uptake decreased from 100% (0.048mmol/g) to 43% within 12h primarily due to the enzyme treatment and adsorption of CFSM components. It further decreased to 26% due to surface passivation by bio-chemicals derived from CFSM and/or decomposed aliphatic hydrocarbons from aromatic carbons between 7days and 14days. These findings may contribute to efforts to decrease preg-robbing in hydrometallurgical processing of refractory gold ores.

Pretreatment of Refractory Gold Ores Using Cell-Free Extracts of <i>P. chrysosporium</i>: A Preliminary Study

The fungus Phanerochaete chrysosporium has been proven to biotransform refractory gold ores, leading to increase in gold recovery. This transformation has been attributed to enzymes secreted by the microbe. This paper reports the findings of preliminary investigations aimed at assessing the use of hydrogen peroxide and cell-free extracts from the fungus, P. chrysosporium, to effect biotransformation of sulphidic refractory gold ores. The investigations show that the total dissolved arsenic, iron and sulphur in solution were up to 5.2 wt%, 0.9 wt% and 6.0 wt% respectively from flotation concentrate after 72 hrs of treatment. Analysis for sulphide sulphur in the residual solids of the gold concentrate indicated about 25 wt% oxidation within 24 hours of treatment. In general, cell-free decomposition of the samples did not increase beyond 24 hours of contact time, possibly due to exhaustion of the active components. Gold extraction by cyanidation increased by 24% after 24-hr treatment with the cell-free extracts. Comparatively, cell-free (in vitro) treatment recorded 66% overall gold recovery as against 61% for whole cell (in vivo) after 72 hours of treatment. These initial results indicate clearly that in vitro processing is a promising alternative to in vivo processing of refractory gold ores using P. chrysosporium.

Characterization of ferric arsenate-sulfate compounds: Implications for arsenic control in refractory gold processing residues

A combination of techniques, including powder X-ray diffraction (XRD), electron microprobe analysis (EPMA), transmission electron microscopy (TEM), and X-ray absorption spectroscopy (XAFS), is used to characterize the common ferric-arsenate-sulfate compounds, which could result from the pressure oxidation of refractory gold ores at elevated temperatures. Three general types of precipitate are identified; namely, arsenate-bearing basic ferric sulfate [FeSO 4 (OH) and designated as BFS], ferric arsenate-sulfate [an extensive solid solution Fe(AsO 4 ) 0.2–0.7 (SO 4 ) 0.7–0.2 (OH) 0.7–0.2 and designated as FAS], and hydrated ferric orthoarsenate (FeAsO 4 ·0.75H 2 O). The crystal structure of FAS is solved by precession electron-diffraction experiments. The structures of BFS and FAS are constructed from octahedral Fe 3+ chains, which are cross-linked by sulfate and arsenate tetrahedra. Extensive substitution of arsenate for sulfate occurs in both types of compounds with charge neutrality being maintained by variations in the (OH) content. The XAFS spectra indicate that the local structures of both BFS and FAS are made of corner-linked single chains of FeO 6 octahedra where the chains are linked by AsO 4 or SO 4 tetrahedra forming alternating layers of FeO 6 octahedra and AsO 4 or SO 4 tetrahedra. Preliminary toxicity characteristics leaching procedure (TCLP) testing of the precipitates indicates that FAS with a molar ratio As/(As+S) ratio of ≤0.5 could be an acceptable material for disposal in a tailings impoundment, whereas more As-rich FAS and BFS may require further treatment. The results for the laboratory-prepared precipitates are compared with those obtained on three residues from the processing of refractory gold ores. The major As-carrier in one of the residues is FAS, whereas As-bearing goethite and hematite are the dominant As-carriers in the other two residues. Thus, the mineralogical characteristics of the residues dictate the appropriate arsenic management and disposal options in the processing of refractory gold ores.

Strategies for efficient start-up of continuous biooxidation process for refractory gold ores

Strategies for efficient start-up of a continuous process for biooxidation of refractory gold ore and concentrate obtained from Hutti Gold Mines Limited (HGML), India are discussed in this work. The biooxidation of the concentrate at high pulp density (10%) with wild strain of Thiobacillus ferrooxidans isolated from HGML mines is characterized by significant lag phase (20 days) and incomplete oxidation (35%) even after prolonged operation (60 days). Two strategies, biooxidation with concentrate adapted cells and a step leaching strategy, in which the pulp density is progressively increased from 2% to 10% were considered and the latter resulted in efficient biooxidation of concentrate. Conversion of such a process from batch to continuous operation is shown to result in complete biooxidation of the concentrate and gold extraction efficiency in excess of 90%.

Advances in the application of the BIOX® Process for refractory gold ores

The Biological Oxidation (BIOX®) Process for the treatment of refractory gold ores was commercialized in 1988. During the last decade, the technology has developed into a mature, commercially viable process, with five additional operations commissioned, including operations at Sao Bento in Brazil, Harbour Lights and Wiluna in Australia, Ashanti in Ghana and Tamboraque in Peru. With ongoing development, the efficiency and cost effectiveness of the process have both improved, making it an attractive alternative to conventional refractory treatment processes such as roasting and pressure oxidation. The bacterial culture is robust and can withstand the normal fluctuations experienced on an operating plant. The plants are simple to operate, making it ideal for remote areas. The scale-up potential of the process has been proven, and it can be applied to large refractory ore bodies. The process is environmentally friendly. Neutralization of plant effluents produces precipitates that comply with the most stringent environmental regulations.

Alkaline and acidic autoclaving of refractory gold ores

American Barrick operates two autoclaves for the processing of refractory gold ores. Because of the mineralogy of the ores, one autoclave is alkaline and the other acidic. The pH of operation has a significant effect on materials of construction, as well as capital and operating costs. The following summary concentrates on the practical and economic aspects of such operations.

Processing of refractory gold ores

During the last ten years, increasing attention has been given by operating companies, research organizations, and engineering and consulting companies to the recovery of precious metals from refractory ores. More ore bodies have been found where the gold is not amenable to simple cyanidation alone, and low recoveries have resulted from metallurgical testwork. In the simplest of terms these are refractory ore bodies. Reasons for ores being refractory will be highlighted. In addition to deposits that are totally refractory, another increasingly common occurence is where the surface ore body is well oxidized and non-refractory and frequently overlays large tonnages of non-oxidized refractory ore. In these cases, the initial metallurgical operation will be conventional, with an ‘add-on’ refractory treatment process at a later stage. This paper will review some of the existing techniques and operations that are being used for the treatment of refractory gold ores, with particular emphasis on the techniques that are well established, such as Roasting and Pressure Oxidation, together with the developing technology of Biological Oxidation. Part of the paper will discuss costs. It is becoming increasingly apparent that pretreatment can be very expensive and as more plants are built and operated, better cost data is becoming available. Approximate and relative capital and operating costs will be discussed and their major components defined. Other important issues that will be addressed are the effect of preconcentration (such as flotation ahead of the pretreatment stage), on project economics and the importance and cost of neutralization and effluent control. The best choice of a processing route to treat a refractory ore will in most cases be site specific, due mainly to the metallurgical and mineralogical characteristics of the ore. The successful application of these processing techniques can result in mining operations that would otherwise have been impossible. However for an increasing number of low grade refractory ores, the estimated capital and operating costs mean that very careful project evaluation will be necessary, particularly at current gold prices.

Bacterial oxidation technology — A viable process for refractory gold ores and base metals recovery

Bacterial oxidation technology can be applied to refractory sulphide ores and concentrates to break down the mineral matrix and release the precious and base metals. The precious metals and some base metals remain in the oxidized residues and can be recovered in a conventional CIP or other chemical leaching step. Some base metals, such as copper, zinc and nickel, enter the acidic solution and may be recovered directly by conventional solvent extraction and electrowinning. A moderately thermophilic bacterial culture has been developed which can operate at high ambient temperatures without the need to cool the process reactors. The ability to operate at these higher ambient temperatures makes the culture more suitable for use in hot tropical or hot arid regions than the Thiobacillus species, which can only operate at temperatures up to 40 °C. The moderately thermophilic culture has been proven in the gold fields of Western Australia during the summer when ambient temperatures were as high as 49 °C. The maximum reactor temperature reached 55 °C and regularly exceeded 50 °C. This bacterial culture has been applied to a range of refractory gold ores and concentrates from Australia, South-East Asia and North America. Types of samples tested included pyrite ore and concentrate, arsenopyrite concentrate, arsenical bulk concentrate, copper — gold concentrate, carbonaceous pyrite concentrate and antimonial concentrate. The culture has shown resistance to high levels of arsenic present in some concentrates and has been applied at concentrations up to 25 g/L arsenic. Antimony has not affected the bacterial oxidation process as it was relatively insoluble in the sulphate environment and probably precipitated as antimony oxides. One refractory gold sample has been extensively tested at a mine site over a 6 month period to prove the technology and to obtain design data for a commercial plant. The bacterial culture proved extremely robust under typical operating conditions. The technology was simple to apply and the plant was operated by site personnel for the majority of the test period. A feasibility study showed the operating costs were very competitive and that the estimated capital cost could be recovered over a relatively short operating period. Base metals present in refractory gold concentrates were released as the bacterial oxidation progressed, often in the early stages of oxidation. One concentrate containing 0.5 to 1.0% nickel as a nickel arsenic sulphide gave nickel extractions greater than 95%. A second concentrate contained significant copper. More than 88% of this copper was removed during the oxidation which was necessary to improve gold recovery and reduce the cyanide consumption. The copper could have been recovered by solvent extraction and electrowinning. A third concentrate contained 7% zinc and 110 ppm silver. Both metals were released from the mineral matrix as the concentrate was oxidized and could be recovered, the silver with the gold in CIP leaching and the zinc by precipitation from the bacterial oxidation solution. The bacterial culture has been also applied for the extraction of base metals from sulphides not containing precious metals. A nickel - iron sulphide ore was oxidized using bacterial oxidation treatment and gave a nickel extraction of 93% compared with only 33% by conventional iron (III) sulphate leaching. The nickel did not prove toxic to the bacteria nor did it retard the progress of the oxidation. A second application was to use bacterial oxidation to boost the recovery of copper from a chalcocite -chalcopyrite ore. Recovery after oxidation was about 95%. The economics of these base metal extraction processes are being determined.