Gold Complexes and Activated Carbon A LITERATURE REVIEW Gloria J. McDougall Klipfontein Organic Products, Kempton Park, South Africa and Robert D. Hancock Sentrachem Research Group, Chemistry Department, University of the Witwatersrand, Johannesburg, South Africa Exciting new developments are taking place in the extractive metallurgy of gold, which are based upon the adsorption of the metal or its corn- plexes on carbon and subsequent elution. Despite the efforts of in- vestigators over a period of almost 70 years, however, the mechanisme of the adsorption and elution processes have not yet been established unequivocally. The literature relating to them is reviewed here. Metallurgical interest in activated carbon, notably and pulps in which granular or powdered activated in Australia, can be traced back as far as, 1880 when carbon is used as an adsorbent for the gold. Un- Davis (1) patented a process in which wood charcoal doubtedly, that which enjoys the greatest prominence was used for the recovery of gold from chlorination at present is the carbon-in-pulp (CIP) process. In this, leach liquors. Soon after the discovery by MacArthur carbon granules are added directly to the cyanided and the Forrest brothers in 1890 that cyanide was a pulp and moved counter-current to it. The gold- good solvent for gold (2), Johnson in 1894, patented loaded carbon is later recovered by screening. Much the use of wood charcoal for the recovery of gold from of the technology on which the CIP process is based cyanide solutions (3). Cyanidation gradually replaced was developed by Zadra et al. (6, 7) in the laboratories chlorination and, although charcoal was never exten- of the United States Bureau of Mines, and it was this sively used for the recovery of gold from cyanide technology that paved the way for the installation in leach liquors, the process, due largely to the efforts of 1973 of the first large-scale CIP plant by the Moore and Edmands (4), reached its highest state of Homestake Mining Company in the U.S.A. (8). The efficiency at the Yuanmi Mine, Australia, in 1917. In process, which has four adsorption stages, incor- this process, the pregnant solution was pumped porates a procedure for the simultaneous elution and through three successive filters that contained fine electrowinning of the gold, and the carbon is returned wood charcoal. Since the charcoal was prepared simp- to the adsorption circuit after suitable re-activation. A ly by heating wood to red heat, quenching it in water simplified flow-sheet for the Homestake adsorption and then grinding the charcoal fine, this product circuit is shown in Figure 1. This plant has operated could not have developed the high surface area and successfully from its inception and has effectively porosity typical of modern activated carbons, and demonstrated the viability of the CIP process at therefore possessed a very low gold capacity. Another commercial level. serious disadvantage of this earlier process was the In recent years, much interest in the CIP process lack of a suitable procedure for elution or desorption has been generated in South Africa. Excellent results of the gold values from the loaded charcoal, so that obtained on a pilot-plant scale (9) at various gold the gold had to be recovered by smelting after burn- mines on the Witwatersrand, for example Durban ing the charcoal to ashes. Very low loadings were Roodepoort Deep and Grootvlei, and also at the Fair- achieved and a large amount of ash was obtained that view Mine in the Barberton area, are responsible for contained only about 10 to 20 per cent gold. Since the fact that at least five South African gold mines are these problems remained unsolved and the planning to install large-scale fully integrated CIP technology of zinc cementation advanced during the plants by the end of 1981, and a further five are com- early years of the cyanidation process, interest in car- mitted to do so by the end of 1982. Two main factors bon for gold recovery declined rapidly. are responsible for the rapid development of CIP and In recent years, many processes (5) have been pro- related processes in recent years. In the first place, posed for the recovery of gold from cyanide solutions carbon products that combine high gold activity with 138 Gold Bull., 1981, 14, (4) Fig. 1 Carbon-in-pulp adsorption cir • FEED PULP cuit as used at the Homestake Mine • After (9) SCREEN (24 MESH) OVERSIZE RECYCLE FRESH AND REACTIVATED UNDERSIZE FEED CARBON CYANIDE PULP 4 VIBRATING SCREENS 20 MESH 20 MESH 20 MESH 1 20 MESH I 1ST STAGE 2ND STAGE 3RD ST/ LOADED CARBON TO L — — -- — — — — DESORPTION AND O^ UNDERSIZE —PULP REACTIVATION TAILING SM TO WASTE — —CARBON the superior abrasion resistance necessary in order to constituents of cyanide leach liquors, some of minimize the loss of gold that would result if softer which (for example, the cyanide complexes of carbons were employed, became available in large copper and nickel) are detrimental to the quantities only in the early sixties. Excellent such recovery of gold by zinc precipitation products can be manufactured from coconut shells (2) The carbon granules are added directly to the and coal. Secondly, efficient elution procedures cyanided pulp, which obviates the need for the which allow the carbon to be re-used after suitable re- expensive filtration and clarification stages that activation, were developed only in comparatively re- are required in the cementation process cent times. The gold cyanide is tenaciously held by (3) The soluble gold losser are usually significantly the carbon and, in the currently employed elution lower than on a conventional plant. procedures, the gold values are desorbed with hot Therefore, it is generally believed that CIP offers (about 90°C) water after pretreatment of the carbon economie advantages over zinc precipitation, not only with a solution of sodium hydroxide and sodium with respect to higher gold recoveries, but also in cyanide (5, 7, 10) or simply by circulating a hot terms of lower capital and operating costs. caustic cyanide solution (1 per cent NaOH and 0.2 As far as current and future developments are con- per cent NaCN) through a column of gold-loaded car- cerned, a number of interesting processes warrant bon (7). The gold is recovered from the eluant by mention. electrolysis. The rate at which the gold is desorbed First, it has been demonstrated that activated car- from the carbon can be substantially improved by bon is an excellent scavenger for small concentrations modification of the water or caustic cyanide eluant, of dissolved gold (0.2 mg/1 or less) (13, 14), and by the addition to it of 5 to 20 per cent methanol or therefore it is likely that activated carbon will find in- ethanol for example (11), or by conducting the elution creased use in finishing applications, such as in the under pressure at temperatures between 100 and removal of dissolved gold cyanide from gold plant ef- 180°C (10, 12). fluents and dam return water. It is convenient to con- The CIP process has several important advantages duct such operations using the column technique, over the conventional zinc cementation route (5, 9). with either a fixed or a fluidized bed of granular car- These are: bon, the choice of loading technique depending on (1) The ability of carbon to adsorb gold (and silver) the amount of solids present in the gold-containing cyanide is not affected by any of the common solution. Gold Bull., 1981, 14, (4) 139 URANIUM RECOVERY CIRCUIT H2SO, CuSO4 GOLD CARBON / PLANT POWDER I`/// 5g SOLIDS 0.16g Auït PH+4 FLOAT 1 / PLANT FILTER CAKE REPULPED TAIL TO SLIMES DAM CYANIDE PYRITESI LEACH I GALCINE CARBON FOR GOLD BEFORE ±4g Au/t WITH CARBON ± 6g Au/t GOLD RECOVERED ACID PLANT Fig. 2 Flow-sheet for a process in which the gold content of a calcine is upgraded by addition of activated carbon powder to the re-pulped filter cake Secondly, fine carbon can be added to a cyanided Moreover, the carbon could be re-used after elution of pulp to adsorb gold (9, 15) and then be collected by a the gold which could be recovered by electrowinning, flotation technique. The gold can subsequently be whereas much of the zinc is lost by dissolution as recovered by burning the carbon. Plant trials on a Zn(CN)4 -. Such an operation could also conveniently reverse leach plant have shown that carbon powder be conducted by a column technique. can be added directly to the re-pulped filter cake to Lastly, in cyanide heap leaching (17, 18) — a com- adsorb the soluble gold locs. The carbon powder is paratively recent hydrometallurgical development for subsequently floated together with the pyrites and exploiting low grade ores, mine-waste material or calcined. This results in a significant upgrading of the deposits too small to justify the construction of mill- gold content (from 3.5 to 6 g/ton) of the calcine ing facilities — activated carbon processes which offer material. In this case, the fine carbon is floated a viable means of recovering the dissolved gold values together with the pyrites and calcined with it. A have found application in the U.S.A. simplified flow-sheet for such a process is shown in In view of its high selectivity for gold (and silver) Figure 2. cyanide in the presence of large concentrations of base It is perhaps appropriate at this point to mention metals, and because it is relatively cheap, activated that it is improbable that carbon powder will ever be carbon is likely to play a significant future role in gold employed for the recovery of gold directly from recovery processes such as those outlined above.