Gold Extraction Plants Research Articles

Membrane Contactors as a Cost-Effective Cyanide Recovery Technology for Sustainable Gold Mining

Cyanide is the ligand of choice for large-scale gold extraction plants and is increasingly replacing amalgamation for small-scale gold extraction processes. Although cyanide is less recalcitrant than mercury, it is still acutely toxic. Current absorption column-based recovery and oxidation processes have high capital and chemical costs, respectively, and are therefore rarely implemented and only at large-scale operations. Membrane contactors with a pH gradient can be used for selective separation of cyanide into a distillate solution. This study evaluated cyanide recovery using polypropylene (PP) and polytetrafluoroethylene (PTFE) membranes and the impact of operating conditions with synthetic and real mining wastewater. The results show the successful recovery of cyanide with the PTFE and PP membranes, as well as a significant impact of distillate pH, temperature, and feed cyanide concentration on process efficiency. Conversely, distillate cyanide concentrations had a very limited impact on process performance, suggesting limited freshwater resources are required for cyanide recovery. The results of a techno-economic analysis indicate that membrane contactors are cost-effective on large and small scales. While the PTFE membrane system is slightly less expensive at a small scale and the PP membrane system is more advantageous at a large scale, both membrane systems can be considered cost-competitive with current cyanide recovery methods.

Динамическое моделирование подвижности минеральной постели в центробежных концентраторах

The article discusses a method for determining the effective parameters of centrifugal concentrators with fluidization of a mineral bed with water through holes in the walls of a rotating cone. The device and the principle of operation of a laboratory setup for studying the constrained movement of mineral particles in a centrifugal field are described. Also, the optimization of the degree of loosening of the mineral bed in the centrifugal concentrator is considered. It was established experimentally that with existing water flow rates the condition of the mineral bed was close to the pressed state. Using the developed method of optimizing the ratio of water pressure in the fluidization system and the separation factor, the optimum water pressure and its flow rate were calculated in laboratory conditions. As a result, it was possible to raise the extraction during the first stage of gravity concentration by 2%. Therefore, it was determined that the degree of loosening depends on the correlation of the separation factor and the water pressure in the fluidization system. Based on the proposed approach, it is possible to develop an automation system to adjust the parameters of the centrifugal concentrators. Based on the results of calculating the balance of the concentration process in the gold extraction plant at the Kedrovskoe deposit, it was established that the application of the proposed methodology helped to improve the efficiency of the centrifugal free-flow concentrator.

Natural rehabilitation of arsenic-rich historical tailings at the Alexander mine, Reefton, New Zealand

ABSTRACT Mine tailings resulting from orogenic gold processing commonly form arsenic (As)-enriched environments. A result of this is that they draw environmental attention in modern gold mining settings and significant effort in long-term rehabilitation. The Alexander River mine processing site in the Reefton area of Westland provides an opportunity to examine the natural rehabilitation of As-enriched tailings that have been left undisturbed for >70 years in a humid environment. These tailings comprise a ∼1 m thick package of finely laminated (mm-scale) sediments that were manually accumulated on a terrace downstream of the gold extraction plant. The tailings sediments are As-rich (up to 5000 mg/kg) due to relict arsenopyrite and As-bearing pyrite. Diagenetic Ca–Fe–arsenate (tentatively identified as yukonite) and As-bearing iron oxyhydroxide have formed during localised oxidation. There was sufficient calcite in the tailings to maintain circumneutral pH in the tailings despite sulphide oxidation. The historical tailing impoundment is now covered by grass, ferns and shrubs, with beech and rimu forest covering several metres at the margins. Leaf analyses indicate that the vegetation is absorbing only minor As. Therefore, the tailings substrate has not halted natural vegetation re-colonisation of this As-rich historical site.

Research of hydrometallurgical method of leaching gold from flotation tails with using bio-oxidation

This paper shows the results of the comparative study of the efficiency of gold extraction methods from technogenic flotation tails by agitation cyanidation and biooxidation followed by leaching. A representative sample of flotation tails was taken at gold extraction plant of Altyntau Kokshetau LLP. It was established that in the test sample contains 0.47 g / t Au and 0.62 g / t Ag. The gold extraction degree from flotation tailings with 80% content of 0.071 mm fraction was 50%, and that with 90% content of 0.071 mm fraction was 60%. Gold extraction plant's flotation tailings grinded to 0-0.044 mm fraction with sodium peroxide were subjected to pre-oxidation. The gold extraction degree from flotation tailings with 90% content of 0.044mm fraction at a solid-liquid ratio of 1:4 and a cyanide concentration of 1 g/dm3 is 65.2%. By using an active strain of thionic cultures the gold extraction degree was 72.1%

Additional recovery of gold from waste after processing of gold-containing ores of Kazakhstan

The one of the important activities of gold mining industry is the involvement of refractory hard-to-enrich gold-containing raw materials into retreatment, as well as the tailings dams, which occupy huge areas and worsen the ecological state of the regions.The studies are aimed at re-extraction of gold out of technogenic raw materials possess not only scientific and practical, but also social and environmental significance. The pre-oxidation procedures are applied in the retreatment of raw materials, one of which is bio-oxidation of gold-containing raw materials method using A.Ferrooxidans bio-leaching solution.This method makes it possible to intensify the process of bio-oxidation extraction of gold from gold-containing raw materials due to the deeper development of sulphides. Therefore, the studies aimed at increasing the efficiency of gold re-extraction out of technogenic raw materials are of current interest. The results of researches regarding the extraction of gold out of stale tailings of gold extraction plant Altyntau Kokshetau using A.Ferrooxidans bio-leaching solution are performed in the article. The chemical, mineral and X-ray phase matter of sample was studied. It has been shown that the sample has a silicate base, represented by quartz, albite-anorthite mixture, muscovite and kaolinite.It has been found that the sample contains 8.49 g / t Au and 2.4 g / t Ag. The different leaching options were carried out: 1) re-grinding is followed by cyanidation; 2) cyanide leaching with preliminary sulfuric acid washing; 3) bio-oxidation using A.Ferrooxidans bacterial solution. It has been shown that the use of preliminary bio-oxidation makes it possible to increase the gold extraction. As a result, it was found that re-extraction of gold out of the stale tailings of sorption by two-stage leaching – bacterial opening of the raw materials, and then cyanidation are the most effective one.The best indicator of gold extraction in solutions as regards the first option resulted in 62.7% of gold, at the second option with the use of sodium chloride and the subsequent cyanidation was 79%, and at the third option of biochemical technology with sodium cyanide was 82.8% of gold.

Characterization of As-polluted soils by laboratory X-ray-based techniques coupled with sequential extractions and electron microscopy: the case of Crocette gold mine in the Monte Rosa mining district (Italy).

Arsenic concentration and distribution were studied by combining laboratory X-ray-based techniques (wavelength dispersive X-ray fluorescence (WDXRF), micro X-ray fluorescence (μXRF), and X-ray powder diffraction (XRPD)), field emission scanning electron microscopy equipped with microanalysis (FE-SEM-EDX), and sequential extraction procedure (SEP) coupled to total reflection X-ray fluorescence (TXRF) analysis. This approach was applied to three contaminated soils and one mine tailing collected near the gold extraction plant at the Crocette gold mine (Macugnaga, VB) in the Monte Rosa mining district (Piedmont, Italy). Arsenic (As) concentration, measured with WDXRF, ranged from 145 to 40,200mg/kg. XRPD analysis evidenced the presence of jarosite and the absence of any As-bearing mineral, suggesting a high weathering grade and strong oxidative conditions. However, small domains of Fe arsenate were identified by combining μXRF with FE-SEM-EDX. SEP results revealed that As was mainly associated to amorphous Fe oxides/hydroxides or hydroxysulfates (50-80%) and the combination of XRPD and FE-SEM-EDX suggested that this phase could be attributed to schwertmannite. On the basis of the reported results, As is scarcely mobile, even if a consistent As fraction (1-3g As/kg of soil) is still potentially mobilizable. In general, the proposed combination of laboratory X-ray techniques could be successfully employed to unravel environmental issues related to metal(loid) pollution in soil and sediments.

Characterization of a gold extraction plant environment in assessing the hazardous nature of accumulated wastes (Kemerovo region, Russia)

Geochemical and geophysical investigations were performed in the area of the Komsomolsk tailings impoundment. Gold ore tailings produced with cyanidation have been generated by the Komsomolsk Gold Extraction Plant. The relatively low sulfide content in the Komsomolsk tailings and the presence of carbonates result in a low acid production potential (AP) for these tailings. The tailings pond is characterized by neutral to slightly alkaline conditions and metal concentrations, except for those of As and Sb, below the maximum permissible concentration (MPC). The situation is complicated by the fact that the displaced Berikul tailings are stored in the southern part of the Komsomolsk tailings impoundment. Sulfide concentrate cyanidation tailings were produced by the former Berikul Gold Extraction Plant. In the Berikul tailings, the sulfide content is approximately 25%. This high content has resulted in a high AP and the formation of acidic and ultra-acidic surface ponds (pH ∼2) with extremely high concentrations of metals and metalloids (As up to 4.1 g L−1). The estimated duration of acid drainage generated by the Berikul tailings is approximately 2400 years. Surface drainage from the tailings enters the Voskresenka River; as a result, the concentrations of As and Sb in the river water approach the MPCs established by the Russian Ministry of Health. Based on electrical resistivity tomography (ERT), the uncontrolled leakage of acidic and highly mineralized solutions through a natural geological fault into groundwater horizons was revealed. Groundwater contamination was confirmed via an analysis of drinking water from a well located near the fault.

Development and Introduction of Contemporary Technology of Gold Cyanide Leaching From Gravitational Concentrates

The problem of improving the efficiency of processing of rich gold-bearing concentrates is considered. A review and the critical analysis of existing ways of processing concentrates is provided. Special attention is paid to cyanide leaching that is a priority way of processing gold-bearing concentrates in gold-extraction plants using a cyanide cycle. As applied to the gravitational concentrate of Berezovsky Mine, measures are developed aimed at accelerating leaching of coarse gold. Features of gold dissolution using oxidizing agents as an alternative to oxygen are studied: hydrogen and calcium peroxides, and potassium bromate. Research and laboratory verification of technology of gold cyanide leaching with chemical activation of the process are performed. Based on research results, an operational section has been planned and introduced for processing of rich gold-bearing concentrates at the Berezovsky Mine enterprise. With the use of hydrogen peroxide compared with traditional conditions, the leaching rate for gold increases by a factor of 1.5–2. With an initial content in concentrates of 100–500 g/ton under comparable conditions, the content in tailings decreases from 10–15 to 1–3 g/ton. Commercial operation of the technology developed shows that gold extraction from concentrates reaches 98–99%, and at the same time loss of gold with dump tailings is reduced by 4 kg a year.

Improvements in gold ore cyanidation by pre-oxidation with hydrogen peroxide

It is known that the presence of sulphidic minerals in the cyanidation of gold ores may cause significant consumption of oxygen supplied in the injected air. This may result in dissolved oxygen starvation for the oxidative leaching of the gold, and ultimately it will reduce the maximum attainable recovery of gold from the ore. In addition, the presence of sulphides leads to extra consumption of the cyanide-leaching agent, NaCN, due to the formation of thiocyanate, therefore increasing costs. These types of gold sulphidic ores may be pre-treated prior to cyanidation by means of an oxidation step, converting the sulphides into oxides or sulphates. This treatment leads to a reduction in the consumption of dissolved oxygen and of cyanide in the cyanidation step and to an improvement in the metallurgical recovery. In the current work we present the results of a five month full-scale trial carried out in a gold extraction plant in Brazil, which normally operated with three tanks in series carrying out an alkaline pre-oxidation step using compressed air only, followed by a train of fourteen aerated and mechanically-agitated tanks for the cyanidation. The ore feeding the leaching circuit averages 1.70gAu/t, with about 2.5% of pyrrhotite (FeS) as the main sulphide constituent. The addition of 60L/h of concentrated hydrogen peroxide, H2O2, 50%w/w (density=1.19g/mL) for pre-oxidation of a slurry of 60% solids at a rate of 150t/h (dry ore) resulted in a marked increase in dissolved oxygen (DO). This addition corresponds to a dosing rate of 0.24kg 100%H2O2 per ton of dry ore and increased the dissolved oxygen level from an average of about 1.0 to 7.2mg/L in the pre-oxidation tanks. It also led to an overall reduction of NaCN consumption from an average of 0.52 to 0.40kg/t of ore, and an increase in metallurgical Au recovery from an average of 91.3% to 92.5%.

Oxidation of cyanide in effluents by Caro’s Acid

Caro’s Acid (peroxymonosulphuric acid: H2SO5) is a powerful liquid oxidant made from hydrogen peroxide that has been adopted for the detoxification of effluents containing cyanides in gold extraction plants in recent years.The present work reports the findings of a study on the kinetics of aqueous cyanide oxidation with Caro’s Acid. Experiments were conducted in batch mode using synthetic solutions of free cyanide. The experimental methodology employed involved a sequence of two 23 factorial designs using three factors: initial [CN−]: 100–400mg/L; H2SO5:CN− molar ratio: 1–1.5–3–4.5; pH: 9–11; each one conducted at one level of Caro’s Acid strength which is obtained with the H2SO4:H2O2 molar ratio used in Caro’s Acid preparation of 3:1 and 1:1. The objective was the evaluation of the effect of those factors on the reaction kinetics at room temperature. Statistical analysis showed that the three investigated variables were found to be significant, with the variables which affected the most being the initial [CN−] and the H2SO5:CN− molar ratio. The highest reaction rates were obtained for the following conditions: H2SO5:CN− molar ratio=4.5:1; pH=9; and Caro’s Acid strength produced from the mixture of 3mol of H2SO4 with 1mol of H2O2. These conditions led to a reduction of [CN−] from an initial value of 400mg/L to [CN−]=1.0mg/L after 10min of batch reaction time at room temperature. An empirical kinetic model incorporating the weight of the contributions and the interrelation of the relevant process variables has been derived as: −d[CN−]/dt=k [CN−]1.8 [H2SO5]1.1 [H+]0.06, with k=3.8 (±2.7)×10−6L/mgmin, at 25°C.

Assessment of the effluent quality from a gold mining industry in Ghana

The physical and chemical qualities of the process effluent and the tailings dam wastewater of AngloGold-Ashanti Limited, a gold mining company in Ghana, were studied from June to September, 2010. The process effluent from the gold extraction plant contains high amounts of suspended solids and is therefore highly turbid. Arsenic, copper and cyanide were identified as the major pollutants in the process effluent with average concentrations of 10.0, 3.1 and 21.6 mg L(-1), respectively. Arsenic, copper, iron and free cyanide (CN(-)) concentrations in the process effluent exceeded the Ghana EPA discharge limits; therefore, it is necessary to treat the process effluent before it can be discharged into the environment. Principal component analysis of the data indicated that the process effluent characteristics were influenced by the gold extraction process as well as the nature of the gold-bearing ore processed. No significant correlation was observed between the wastewater characteristics themselves, except for the dissolved oxygen and the biochemical oxygen demand. The process effluent is fed to the Sansu tailings dam, which removes 99.9 % of the total suspended solids and 99.7 % of the turbidity; but copper, arsenic and cyanide concentrations were still high. The effluent produced can be classified as inorganic with a high load of non-biodegradable compounds. It was noted that, though the Sansu tailings dam stores the polluted effluent from the gold extraction plant, there will still be serious environmental problems in the event of failure of the dam.

Thermodynamic estimation of the stability field of bukovskyite, an iron sulfoarsenate

Sulfide flotation concentrate of gold‐quartz ores was cyanated at the Berikul gold extraction plant in the Kemerovo district, southwestern Siberia. Tailing cakes were stored as a 3- to 6-m-high dump at the territory of the plant, along the Mokryi Berikul River. About 70 thousand tonnes of ore were accumulated in 1942‐ 1972. This type of ore preservation (above the ground water level) causes the intense oxidation of the residual sulfides, because atmospheric precipitates percolating through the rock are partially retained in the pore space, which is filled with air and solutions (pore waters) rich in oxidation products. However, the oxygen content sharply decreases downward owing to its consumption by oxidizing sulfides. This can be exemplified by reactions with pyrite and arsenopytite, the major sulfides of the Berikul cakes (1, 2). The oxidizing role of trivalent iron is typically constrained in these objects because of retardation of reaction (3) in highly acidic solutions. This explains the predominant development of sulfates of bi- rather than trivalent iron.

Dynamic simulation of the carbon-in-pulp and carbon-in-leach processes

Carbon-in-leach and carbon-in-pulp are continuous processes that use activated carbon in a cascade of large agitated tanks, which have been widely used to recover or concentrate precious metals in gold extraction plants. In the carbon-in-pulp process adsorption occurs after the leaching cascade section of the plant, and in the carbon-in-leach process leaching and adsorption occur simultaneously. In both processes the activated carbon is moved from one tank to another in countercurrent with the ore pulp until the recovery of the loaded carbon in the first tank. This paper presents a dynamic model that describes, with minor changes, the carbon-in-leach, the carbon-in-pulp, and the gold leaching processes. The model is numerically solved and calibrated with experimental data from a plant and used to perform a study of the effect of the activated carbon transfer strategy on the performance of the adsorption section of the plant. Based on the calculated values of the gold loss in the liquid and of the gold recovered in the loaded activated carbon that leaves the circuit, the results indicate that strategies in which a significant amount of activated carbon is held in the first tank and the contact time between the carbon and the pulp is longer are the best carbon transfer strategies for these processes.

Prospects and problems of stack processing of waste dumps of gold-extraction plants

The long-lived waste dumps of gold-extraction plants, by the amount of gold concentrated in them, are an alternative source of raw materials for the gold-mining enterprises. During the long-term storage of waste dumps, difficult-to-extract gold under the effect of environmental factors is being opened up and can be extracted using the cyanide process. It is established by investigations that the extraction of Au during percolation cyanidation attains 75–82%, and waste dumps can be used for their heap leaching. However, because of a low fineness of mineral formations, percolation of working solutions through the stack bulk and its retention from static destruction are complicated. The technology of pelletizing the waste dumps with the use of predrying with warm air or by mixing with the ore in definite proportions is suggested. To prevent washing out, the stack height should be smaller than that with the use of the ore, and its edges should be protected by natural coatings. Therefore, processing of waste dumps by heap leaching can turn out to be a promising practical direction.

Nonlinear data reconciliation in gold processing plants

Data reconciliation in hydrometallurgy process is less common than in mineral processing, due to the more complex mathematical formulation of the problem to describe the process balances that can include multiphase and multi-component reactions and mass transfer. In this case, to avoid thermodynamic violations, the use of inequality constraints, in addition to the classical equality mass and energy balances, can be mandatory. This paper presents two case studies of data reconciliation for a gold extraction plant using the direct optimization of a nonlinear objective function and nonlinear constraints that includes equality and inequality equations. The problems are solved using a sequential quadratic programming algorithm and the results demonstrate the applicability of this approach to data reconciliation in hydrometallurgical processes generating reliable results, without mass balance or thermodynamic violations.

Cyanide recovery by ion exchange from gold ore waste effluents containing copper

The recovery of cyanide from waste effluents of the cyanidation process in gold extraction plants is important environmentally and economically. In this respect ion exchange is being tested as a possible economic and versatile method for gold ores containing copper minerals. The present study shows the potential of strong and weak base ion exchange resins, particularly the former, for separating copper cyanide complexes from these industrial effluents, and in particular the feasibility of elution with different eluants. Three possible reagents are proposed for use with these resins and the industrial effluents as load solutions: NaCN, NaSCN and NaNO3. The recovery of copper complexes from industrial effluents using a strong base resin, Lewatit MP-500, and NaSCN as eluant solution, is very high, with recoveries of over 90% in each step. Problems arise when several cycles of load and elution are carried out with the resin, and the performance of the resin decreases when working with unfiltered solutions after several operational cycles.

Cyanide recovery by ion exchange from gold ore waste effluents containing copper

The recovery of cyanide from waste effluents of the cyanidation process in gold extraction plants is important environmentally and economically. In this respect ion exchange is being tested as a possible economic and versatile method for gold ores containing copper minerals. The present study shows the potential of strong and weak base ion exchange resins, particularly the former, for separating copper cyanide complexes from these industrial effluents, and in particular the feasibility of elution with different eluants. Three possible reagents are proposed for use with these resins and the industrial effluents as load solutions: NaCN, NaSCN and NaNO 3. The recovery of copper complexes from industrial effluents using a strong base resin, Lewatit MP-500, and NaSCN as eluant solution, is very high, with recoveries of over 90% in each step. Problems arise when several cycles of load and elution are carried out with the resin, and the performance of the resin decreases when working with unfiltered solutions after several operational cycles.

Characterization and availability of cyanide in solid mine tailings from gold extraction plants

The aim of this study was to investigate the availability and fate of cyanide in gold mill solid tailings. For this purpose, aged (6–9 years) and recently discharged (3 months) tailings were sampled at various depths from two gold mining sites in Quebec (Canada). A physicochemical characterization of the tailings along with a bacterial enumeration was performed and batch-leaching tests with water and caustic solution were conducted to assess the soluble cyanide fraction. Following the standard acid reflux/distillation method, the total cyanide concentration (CN T) in fresh tailings was higher (average of 19.5±2.0 compared to 3.2±0.7 mg/kg) and the pH more alkaline (10.6±0.1 compared to 7.6±0.2) compared to aged ones. In aged tailings, the enumeration of total heterotrophic (10 4–10 6 cfu/g) and cyanide-resistant bacteria (10 2–10 5 cfu/g) showed a significant indigenous microflora, but no growth on agar plates was detected in fresh tailings, suggesting an evolution and a gradual acclimatization of bacterial populations with weathering. Elevated concentrations of CN T, weak acid dissociable cyanide (CN WAD), cyanate (CNO −), and thiocyanate (SCN −) were measured in the supernatant solution of the fresh tailings samples collected in-situ, highlighting their reactivity, whereas these species were not detectable in aged tailings. Accordingly, when batch-leaching tests were performed with fresh samples, 68–72% of the total cyanide was released into aqueous solution with more than 20% being CN WAD. Again, CN T and CN WAD were not detectable during leaching of aged tailings. In fresh tailings, approximately 50% of the cyanides were quantifiable with the standard acid reflux/distillation method (readily soluble cyanides, and weak to moderately strong complexes), whereas in aged tailings only 15 to 33% of the actual total cyanide content was measured by the standard method. These results stress the necessity of performing a preliminary alkaline extraction prior to total cyanide determination. In summary, this study revealed a difference in physicochemical properties, cyanide concentration and speciation, viable bacterial populations and cyanide leaching behavior between aged and fresh solid mine tailings. The findings indicate that the more reactive cyanide species initially associated with the solid tailings have naturally degraded within the mine tailings impoundment area, resulting primarily from volatilization (decrease in pH), leaching, and bacterial degradation.

Neural net analysis of the liberation of gold using diagnostic leaching data

The interrelationship between mineral liberation and leaching behaviour of a gold ore is ill defined, mainly due to the complexity of both leaching and mineral liberation. A better understanding of this relationship could result in lower operating costs on gold extraction plants, since an increase in the efficiency of gold dissolution and a decrease in costs related to the crushing and grinding operations could be expected. In this investigation artificial neural nets were used to analyse diagnostic leaching data of gold ores obtained from South African gold mines. A self-organising neural net with a Kohonen layer was used to generate order preserving topological maps of the characteristics of both the unmilled and milled ores. The arrangement and shapes of these clusters could then be used to develop simple neural net models which were capable of predicting the degree of liberation more accurately than previously proposed models. Moreover, the neural net models were also capable of providing direct estimates of the reliability of their predictions by comparing new inputs with the data in their training bases.

The combined influence of oxygen and cyanide on adsorption isotherms in carbon-based gold extraction systems

In carbon-based gold extraction plants the operating conditions are frequently such that the concentrations of cyanide and oxygen in the gold-bearing solution vary both with time and through the adsorption circuit. To estimate the effect of such variation on the performance of the circuit reference is made to various studies where the individual influence of oxygen or of cyanide has been investigated. Todate no work has been reported in which the combined influence of these two variables has been studied. Such a study — involving the determination of adsorption isotherms in synthetic systems — is reported in this paper.