Merrill-Crowe Research Articles

Stabilization of residual mercury from gold mining as metacinnabar and cinnabar in ball mills on a pilot scale

Waste liquid mercury generated as a by-product of the Merrill–Crowe process in gold mining and recovered from mercury-containing waste must be stabilized for secure storage or disposal. This study developed a procedure for mercury stabilization. A ball mill with a 0.5 m3 capacity and a rotational speed of 43 rpm was used to stabilize the residual mercury with sulfur. The treatments were conducted for 30, 60, and 90 min at mercury: sulfur molar ratios of 1.0, 0.8, and 0.67. The ball loading ratio was 7.0 with residual mercury, and the temperature was below 40 °C. The treatment efficiency was evaluated by measuring the concentrations of mercury and other metals using the Toxicity Characteristic Leaching Procedure (TCLP), examining the stabilized residual mercury by X-ray diffraction, and conducting bioassays on Daphnia magna and Lactuca sativa. Principal component analysis (PCA) was performed on the aforementioned variables. The 90-min treatment, with a mercury-to-sulfur molar ratio of 0.67, stabilized mercury mainly as cinnabar compared with the other treatments and presented leachate mercury values below the detection limit <0.003. The leachate from the treatments also showed values of 21.28–38.44 toxic units, classified as very toxic, and generated toxicity, particularly for D. magna, because of the presence of other metals such as Al, Ba, B, Ca, Cu, Cr, Fe, Mn, Ni, and Zn. The variability of the residues in the PCA analysis was explained by the treatment effect and the presence of other metals in the residual mercury. The stabilized residual mercury obtained was classified as non-hazardous and could be stored or disposed of as ordinary waste in a security landfill.

Comparing cyanidation with amalgamation of a Colombian artisanal gold mining sample: Suggestion of a simplified zinc precipitation process

We highlight the inefficiency of whole ore amalgamation practices used by artisanal gold miners. Cyanide is an alternative to mercury, but cleaner (cyanidation) plants are expensive and demand certain skills to operate. The main costs associated with operating a standard cyanidation plant using gold precipitation with zinc are linked to filtration, which can be costly for artisanal producers to deploy. To overcome the cost barrier, we call for the adoption of a Modified Merrill-Crowe Process (MMCP), which simplifies gold precipitation from cyanide solutions because it eliminates the need for filtration and vacuum system. Under this setup, zinc shavings in a fabric bag are introduced into the pulp of the leaching tanks with very low agitation. Lab results from a Colombian ore sample revealed that 84% of the gold was leached in 24 h with 1 g/L of CN, and over 99% of gold was precipitated in two hours. By comparison, lab amalgamation tests on the same ore using 28 times more mercury used by artisanal miners yielded less than 19% gold recovery.

Precipitation of zinc from cyanide leach solutions using Trimercapto-s-triazine (TMT)

Cyanide leach solutions produced from gold/silver ores can be contaminated with zinc due to the gold/silver cementation in Merrill-Crowe process and/or the dissolution of zinc minerals present in the ore. Zinc levels in cyanide solutions should be controlled to avoid potential problems in leaching and downstream processes associated with the accumulation of zinc. In this study, the application of a novel method was demonstrated for the precipitation of zinc from synthetic/real cyanide solutions using Trimercapto-s-triazine (TMT), a non-toxic organic reagent. Experimental studies have shown that precipitation of zinc with TMT is a fast process occurring in a short period of time i.e., ≤15 min. Increasing the concentration of TMT (33–90 mM) and pH (10–12.5) was found to improve the precipitation of zinc, which was unaffected by increasing the temperature (20–60 °C). Response surface methodology was adopted for detailed investigation of the effect of [TMT]/[Zn] ratio (2.70–16.25), [CN−]/[Zn] ratio (3.00–8.00) and initial concentration of Zn (50–950 mg/L) on the precipitation of zinc. High recoveries (≤95.4%) were obtained under suitable conditions. Precipitation tests were also performed using synthetic multi-metal and real leach solutions. The complete recovery/dissolution of zinc from the precipitates was also demonstrated using different acids (HCl and H2SO4). The findings have shown that TMT can be effectively used as an environmentally friendly reagent for removal of zinc from cyanide leach solutions.

Recovery of silver from slags generated by melting precipitates from the Merrill–Crowe process

ABSTRACTSlag that is produced from induction furnaces that use the Merrill–Crowe process still contains precious metals. In this study, a pyrometallurgical treatment was performed for their recovery in order to realise economic savings and reduce environmental concerns. An analysis was performed to determine the effect of two fluxes (sodium carbonate and anhydrous borax) on the recovery of silver at different temperatures (1373, 1423 and 1473 K). Experiments that used 20% by weight of anhydrous borax at 1473 K showed recoveries up to 99.68% of silver in the metal phase. However, it also showed an increment of impurities in the doré. On the other hand, the silver recovery was 97.2% when 20% of sodium carbonate was used at the same temperature. Additionally, the sodium carbonate flux produced a better quality doré.

Use of Electrolytic Zinc Powder for Cementation of Gold from Cyanide Solutions

Features of gold cementation from cyanide solutions with electrolytic zinc powder are studied. The test powders are prepared by electro-extraction of alkaline solutions after leaching zinc-containing dust. Cementation is conducted in two regimes: under pressure (Merrill–Crowe process) and a fluidized bed regime. It is established that with respect to degree of gold extraction and specific consumption of test dendritic powder it is no worse or surpasses powders prepared by vapor condensation used traditionally in the gold extraction industry. The features established may be explained by physical features of powders prepared by electro-extraction from alkaline solutions.

Recovery of silver from cyanide leach solutions by precipitation using Trimercapto-s-triazine (TMT)

Cyanide leaching is the most common method used for production of silver from gold/silver ores. Silver (and gold) can be recovered from pregnant leach solutions (PLSs) using Merrill-Crowe process (zinc cementation) and activated carbon adsorption. Due to their inherent limitations and paucity of selectivity, development of alternative processes for selective recovery of silver from PLSs is of great interest. In the current study, selective recovery of silver by precipitation from cyanide solutions using an environmentally friendly organic reagent i.e. Trimercapto-s-triazine (C3N3S3; TMT) was investigated. The results have shown that precipitation of silver by TMT is a rapid process in that 87.9% of silver could be precipitated at the initial period of 5min. Response surface methodology (i.e. central composite design) was adopted to examine the main and interaction effects of [TMT]/[Ag] ratio (0.09–36.52), [CN]/[Ag] ratio (3−10) and initial concentration of silver (1–40mg/L Ag) on the precipitation of silver in five levels. The statistical analysis of the experimental data revealed that ratio of [TMT]/[Ag] (i.e. concentration of TMT) was the key parameter exerting a profound effect on the precipitation of silver. However, effects of [CN]/[Ag] ratio and Ag concentration appeared to be statistically insignificant. Complete precipitation of silver was found to be readily achieved under suitable conditions. The precipitation process was proved to be highly selective for silver over gold and copper as also confirmed by the tests on real pregnant leach solutions. It was found that the presence of TMT had no discernible effect on adsorption of gold by activated carbon, which was observed to even improve from copper containing solutions. Silver-TMT precipitates were characterised using SEM/EDS, FT-IR and other techniques. The current findings demonstrate that this precipitation process can be exploited as a potent alternative for selective recovery of silver from cyanide leach solutions of silver and gold/silver ores or purified/concentrated solutions ahead of gold electro-winning.

The Eh-pH Diagram and Its Advances

Since Pourbaix presented Eh versus pH diagrams in his “Atlas of Electrochemical Equilibria in Aqueous Solution”, diagrams have become extremely popular and are now used in almost every scientific area related to aqueous chemistry. Due to advances in personal computers, such diagrams can now show effects not only of Eh and pH, but also of variables, including ligand(s), temperature and pressure. Examples from various fields are illustrated in this paper. Examples include geochemical formation, corrosion and passivation, precipitation and adsorption for water treatment and leaching and metal recovery for hydrometallurgy. Two basic methods were developed to construct an Eh-pH diagram concerning the ligand component(s). The first method calculates and draws a line between two adjacent species based on their given activities. The second method performs equilibrium calculations over an array of points (500 × 800 or higher are preferred), each representing one Eh and one pH value for the whole system, then combines areas of each dominant species for the diagram. These two methods may produce different diagrams. The fundamental theories, illustrated results, comparison and required conditions behind these two methods are presented and discussed in this paper. The Gibbs phase rule equation for an Eh-pH diagram was derived and verified from actual plots. Besides indicating the stability area of water, an Eh-pH diagram normally shows only half of an overall reaction. However, merging two or more related diagrams together reveals more clearly the possibility of the reactions involved. For instance, leaching of Au with cyanide followed by cementing Au with Zn (Merrill-Crowe process) can be illustrated by combining Au-CN and Zn-CN diagrams together. A second example of the galvanic conversion of chalcopyrite can be explained by merging S, Fe–S and Cu–Fe–S diagrams. The calculation of an Eh-pH diagram can be extended easily into another dimension, such as the concentration of a given ligand, temperature or showing the solubility of stable solids. A personal computer is capable of drawing the diagram by utilizing a 3D program, such as ParaView, or VisIt, or MATLAB. Two 3D wireframe volume plots of a Uranium-carbonate system from Garrels and Christ were used to verify the Eh-pH calculation and the presentation from ParaView. Although a two-dimensional drawing is still much clearer to read, a 3D graph can allow one to visualize an entire system by executing rotation, clipping, slicing and making a movie.

New Approach for Lead, Zinc and Copper Ions Elimination in Cyanidation Process to Improve the Quality of the Precipitate

The Merrill-Crowe zinc-precipitation process has been applied worldwide on a large number of operations for recovering gold and silver from cyanide solutions. However, in some of the large plants this precipitate is of low quality, because copper and especially lead are precipitated along with gold and silver resulting in a higher consumption of zinc powder, fluxes in the smelting of the precipitate, formation of mate and short life for crucibles. In this research, a method is proposed to recover lead, zinc and copper cyanide ions from barren solutions, from the Merrill-Crowe process with hydroxyapatite before the filter press. This technology is based on inducing nucleated precipitation of zinc, copper and silver ions in a serpentine reactor, using sodium sulfide as the precipitator and sulfuric acid for pH control. This procedure was successfully applied at WILLIAM Mining and BACIS Mining Group in Mexico.

Recovery of Silver from Cyanide Solutions Using Electrochemical Process Like Alternative for Merrill-Crowe Process

The conventional processes for recovery of silver from cyanide leach solutions are the carbon adsorption, the Merrill-Crowe zinc dust cementation, the Ion Exchange, and Solvent Extraction processes; among other available options for recovery of precious metals from cyanide solutions, Electrocoagulation (EC) is a very promising electrochemical process that does not require high concentrations of silver in cyanide solutions to yield excellent results and neither pretreatment of cyanide solutions like Merrill-Crowe process (deoxygenating and clarification). The present study has been done for the recovery of silver contained in pregnant solution from the cyanidation process using the electrocoagulation technology with iron electrodes, and therefore develops an alternative technology for Merril-Crowe process. The average silver content in pregnant solution was of 52 ppm, recovery was obtained of 99% of silver, with this optimum operating parameters, pH = 8, residence time = 20 minutes and conductivity by addition of sodium chloride = 4 grs/L. Finally the characterization of the solid products formed during the EC process with X-ray Diffraction and Scanning Electronic Microscope was performed, results suggest that magnetite particles and amorphous iron oxyhydroxides are present (Lepidocrocite).

Evaluation of the Merrill–Crowe process for the simultaneous removal of platinum, palladium and gold from cyanide leach solutions

Studies were conducted to evaluate the Merrill–Crowe cementation method for the simultaneous extraction of platinum, palladium and gold associated with copper and nickel from cyanide solutions, particularly high temperature eluates from carbon adsorption and desorption process. The paper reviews the Merrill–Crowe method and its potential for platinum and palladium recovery in the presence of gold, copper and nickel. In a mechanically stirred tank reactor and in the presence of pure nitrogen gas, statistically designed experiments were performed at two levels of the five operating parameters which were the free cyanide concentration, temperature, cementation time, base and precious metal concentrations and their studied ranges were 50–150ppm, 60–90°C, 15–90min, 175–1350ppm and 220–880ppm, respectively.The optimum experimental conditions resulted in a solution with free cyanide concentration of 150ppm at 60°C and a reaction time of 90min, base metal concentration of 175ppm and precious metal concentration of 880ppm. Under these conditions, cementation yield obtained was 91; 54; 19% for Au(I), Pd(II) and Pt(II) respectively. Zn addition was 1.5 times the stoichiometric amount for complete cementation for the low level case, while it was 0.68 times less the stoichiometric for the high level, in order to monitor competitive aspects of cementation from a mixed solution on a time scale and at conditions that might be relevant to industrial operations.In contrast to Au cementation reaction, Pd and Pt cementation did not go to completion even though excess zinc powder (0.15g Zn i.e. 1.5 to 3.4 stoichiometric ratio) was used with their mono-ionic solutions, which gave 48 and 36% of Pd(II) and Pt(II) respectively. In preliminary kinetic studies, deviations from the first-order rate law were observed. The rate of cementation of Au(I) was about 5 times higher than for Pd(II) and 15 times higher than for Pt(II), based on the apparent rate constant, k′1. It was also found that the ratio k′1/k′2 was smallest for Pt, in other words the deviation from 1st order linear kinetics was greatest.

Fundamental considerations on the mechanisms of silver cementation onto zinc particles in the Merril–Crowe process

Studies on the Merrill–Crowe process as applied to silver recovery have shown that one half of the used zinc powder is wasted in water reduction at high cyanide concentrations, while the other half reduces silver ions from the cyanide solution. However, the cementation mechanisms as an electrochemical process taking place on the zinc surface do not explain the split of the electric current resulting from the anodic dissolution of zinc into two equal values. This study demonstrates that the mechanism for silver precipitation at high and low cyanide concentrations differs considerably. At low cyanide concentrations cementation is essentially an electrochemically-controlled process following a shrinking-core behavior. At high cyanide concentrations, the process seems not to be electrochemically controlled. The areas for zinc dissolution and silver deposition are not connected by an electrical-conducting medium and reduction of silver–cyano complex ions takes place by hydrogen adsorbed onto silver growing outward from the cementing zinc particles. The results are based on scanning electron microscopy of solids recovered from cementations in stirred reactors and in situ observations by optical microscopy of the cementation process on the edge of thin zinc disks in cyanide solutions.

Removal of zinc cyanide from a leach solution by an anionic ion-exchange resin

The Merrill-Crowe process has been used for the precipitation of silver metal from cyanide solutions. Although the Merrill-Crowe system operates effectively most of the time, if the recycled solution contains a high zinc cyanide concentration, it leads to a decrease in efficiency of the solution as a solvent. In this study an anionic ion-exchange resin, Lewatit MP62, was used for the treatment of excess zinc cyanide from alkaline leach solutions. Ion-exchange tests were conducted in a laboratory-scale glass column using recycled water from the 100th Year Silver Cyanidation Plant in Turkey. It contains 474 mg/l Zn and 720 mg/l CN. The exchange capacity of the resin and the initial pH effect on the loading capacity were determined by breakthrough curves. It was found that zinc cyanide was completely removed from a recycled solution until it reached the breakthrough point.

Study of Merrill-Crowe processing. Part II: Regression analysis of plant operating data

The recovery of precious metals from alkaline cyanide solution is accomplished effectively utilizing the well-established Merrill-Crowe process. Plant data from three silver operations were analyzed in terms of zinc efficiency, and mathematical models were generated with multiple regression techniques. Projections of the mathematical model are in good agreement with actual plant performance. The results of the regression analysis showed that the silver concentration in the feed is the dominant operating variable in determining zinc efficiency in Merrill-Crowe processing, and this is especially true for operations with low-pregnant grades. Pregnant grade and flow rate are the two operating variables which are beneficial to zinc efficiency. Increases in sodium cyanide concentration and zinc addition result in reduced zinc efficiency. Finally, the mathematical models indicate that modest economic improvement in Merrill-Crowe processing can be realized with relatively minor modifications to the circuits.

Zinc-dust cementation of silver from alkaline cyanide solutions — analysis of Merrill-Crowe plant data

The direct recovery of precious metals from alkaline cyanide solution is frequently accomplished by the well-established Merrill-Crowe process involving zinc-dust cementation. This is particularly true for silver. Plant data from two operations are presented and analyzed in terms of intrinsic reaction features. In such Merrill-Crowe plants a plate-and-frame filter press acts as a semicontinuous fixed-bed reactor with a distribution of particle retention times. For the plants studied, 99% of the silver is recovered with a solution retention time of less than 2 min. Differences in plant operating conditions and product characteristics can be related to the nature of the surface deposit of silver which forms on the zinc particles. From laboratory experiments and plant samples it has been found that the presence of lead in the system activates a change in the silver deposit morphology to the dendritic state which affects the rate of silver deposition as well as the filter cake porosity. Finally the plate-and-frame press is considered from an engineering perspective as a semi-continuous fixed-bed reactor to describe the behavior of the system.

Au Recovery Using an Activated Carbon Column

The feasibility of a process involving the use of activated carbon in a multistage column (NIMCIX contactor) for the recovery of gold from unclarified pregnant solutions was investigated on pilot-plant scale. It was found that satisfactory extractions, corresponding to a recovery of 99.6 percent, can be achieved with an average gold concentration of 0.0077 p.p.m. in the barren solutions from a feed averaging 1.89 p.p.m. The results show that excessive loading of CaCO3 can be prevented by lowering of the pH of the feed to about 7.6. This obviates the need for the carbon to be treated with hydrochloric acid. The gold was eluted from the carbon by the Zadra procedure. The operating conditions of the elution and electrowinning circuits are reported, and the elution of the gold and other metals is discussed. The reactivation of the carbon, which was carried out in a vibrating-deck kiln, is described, and the effects of this reactivation on the activity of the carbon and its microstructure are also discussed. The consumption of reagents in the adsorption and elution circuits is presented in sufficient detail to allow the potential of the activated-carbon process as an alternative to the Merrill-Crowe process for the treatment of high-grade solutions to be evaluated. In addition, the results of parallel small-scale investigations on the effect of pH on the loading of CaCO3, gold, and other metals on activated carbon are presented and discussed briefly.