Countercurrent carbon cyanidation with simultaneous dissolution of gold by cyanide and its adsorption by carbon offers several advantages over other carbon cyanidation processes as:
- the rate of dissolving the gold is faster,
- higher grade gold-bearing carbon is obtained,
- less carbon per ton of ore treated is required,
- separate dissolving and absorbing units are not needed,
- adsorption of dissolved gold by colloids or graphite is reduced, and
- capital and operating costs of plant are reduced.
Pilot-plant operations since 1940 which employed this method of cyanidation include one plant which operated without agitation and three plants which employed agitation but used different methods of separating the carbon from the ore pulp.
Carbon Cyanidation at the Harquahala Pilot Plant. The Eagle Picher Mining and Smelting Company in 1940-1941 erected and operated a 25-ton capacity pilot plant on Harqua Hala amalgamation tailing near Salome, Ariz. The tailing treated contained considerable colloidal material, and treatment of this material by standard cyanide and flotation methods had been tried without satisfactory results. The tailing was pulped with water, lime, cyanide, and activated carbon to a consistency of 70 per cent solids. The pulp was allowed to stand without agitation for a period of 15 to 24 hr., and then the carbon was separated from the pulp with a Denver flotation machine. The extraction of gold ranged from 70 to 75 per cent, and the concentrate contained from 25 to 50 oz. gold per ton. The explanation for the completeness of the reactions involved without the usual agitation of the pulp lies in the continuous migration of gold ions to points of low gold concentration which surround the particles of activated carbon.
Getchell Pilot Plant with Flotation. In 1946, the Getchell mine of Nevada installed a 100-ton capacity pilot plant employing flotation for the recovery of the carbon from the ore pulp. Lime, cyanide, and activated carbon were added to the ball mill, and the carbon was separated from the pulp with mechanical sub-A flotation machines. On heads assaying 0.10 oz. gold per ton, the extraction amounted to 85.7 per cent, the tailing assayed 0.014 oz. per ton, and the barren solution contained 0.0005 oz. gold per ton. The grade of concentrate, 2.0 oz. gold per ton, was low owing to the presence of a black mineral in certain parts of the Getchell ore body which floated with the activated carbon.
Getchell Pilot Plant with Carbon Containers. In 1947, Getchell mine installed a second pilot plant of 100-ton capacity to experiment with the elimination of the flotation step in recovering the carbon from the pulp. Originally, coarse activated carbon, minus 10 plus 30 mesh, was used, and the coarse carbon was confined in revolving screens which were partially submerged in the ore pulp. Three agitators were used in series with one 4- by 4-ft. screen, 30-mesh stainless-steel cloth, in each of the first two agitators and two screens in the third agitator. The coarse carbon was advanced countercurrent to the direction of pulp flow from agitator 3 to agitator 1. The carbon discharged from agitator 1 to the refinery assayed 82.6 oz. gold per ton.
In the latest (1949) modification of this process, five Dorr agitators are used in series and the revolving screens are suspended vertically in the pulp. In this new scheme the carbon-loaded pulp is on the outside of the screens and is transferred from one agitator to the next by air lifts, while the carbon-free pulp which passes through the revolving screens is passed by air lifts into the preceding agitator countercurrently to the carbon-pulp flow. Thus, pulp which is largely barren of gold leaves the system at one end, while the activated carbon with its load of gold (approximately 400 oz. per ton) is withdrawn from the other end, separated from entrained pulp in a small washing trommel and passed on to “desorption” or step of removal of the gold (see page 265). Barren solutions as low as 0.001 oz. per ton have been reported. In this particular adaptation of the process cyanidation is carried to completion before the carbon is added.
In 1948, American Cyanamid Company employing the pilot-plant unit of the Eagle Picher Mining and Smelting Company at Sahuarita, Ariz., conducted continuous tests employing magnetic activated carbon for the adsorption of gold. The carbon was prepared by mixing magnetite with activated carbon and sodium silicate binder. After drying, the magnetic portion was separated and introduced to the ore pulp. After the completion of dissolution and adsorption of the gold, the carbon was separated from the pulp with wet magnetic concentrators.
Recovery of Adsorbed Gold and Silver
The desorption of gold and silver from a carbon which has been in contact with an ore pulp apparently depends upon the shifting of the equilibrium encountered between adsorption and desorption. Four methods of shifting the equilibrium succeeded in desorbing the gold and silver. The four methods comprised (1) the use of a solvent in conjunction with a large excess of precipitant in order to remove the gold from the receiving solution as the desorption progressed; (2) the use of a solvent in conjunction with electrolysis to accomplish the same purpose; (3) the use of a solvent and large volumes of solutions, added in stages, to keep the concentrations of the gold and silver in the receiving solutions sufficiently low to accomplish desorption; and (4) the use of higher temperatures by employing hot solvent solutions in a pressure chamber. The authors believe that the last method given is the most feasible, as this method is very rapid, has the lowest reagent cost, and accomplishes the desorption with the minimum volume of desorbing solution.
The treatment involved consisted of placing the charge of gold- and silver-bearing carbon in a 10- by 12-in. horizontal batch-type pressure- filter press with filtrate valve closed, adding cyanide solution followed by hot water and subjecting the charge to full boiler steam pressure of 90 to 95 lb. per .sq. in. for a soaking period of 20 min. The filtrate valve was then opened, and the pregnant solution forced through a heat exchanger and into a storage tank. Five cycles as described were used for each charge of 2.22 lb. carbon, equal to about 26 lb. carbon per 24 hr. (approximately the amount used to treat 7.5 to 9 tons low-grade ore).
The amount of cyanide used was equivalent to 2 to 20 lb. per ton of carbon treated and the hot water to 50 tons per ton of carbon, which corresponds to about 10.65 tons pregnant solution per 100 tons of ore treated. Conventional cyanide practice would require the handling and precipitating of nearly thirty times as much pregnant solution.
Typical metallurgical data for the adsorbing and desorbing phases of this process are shown in Tables 37 and 38.
The barren solution given in the tabulation, viz., 0.0012 oz. gold per ton, was the solution after treatment in the scavenging screen placed in the tailing launder. The barren solution from agitator 3 is given in the second footnote as assaying 0.0025 oz. gold per ton. The scavenging screen, with a very short time of contact with the tailing pulp, therefore reduced the assay of the barren solution from 0.0025 to 0.0012 oz. gold per ton.
Although the data show that desorbed carbon which has not been reactivated has decreased adsorptive speeds, it is entirely possible that more effective contact with such carbon will make up to a considerable degree for the loss of adsorptive speed for such carbons. The data indicate that the reactivation of desorbed carbon restores its adsorptive speed to that of fresh carbon.
The authors believe that the loss of adsorptive speed of a desorbed carbon as compared with a fresh carbon is due to the partial filling of the interstitial spaces of the carbon by slime. The reactivation of desorbed carbon, which apparently restores its adsorptive speed, is believed to increase the interstitial spaces by oxidation with steam during reactivation.
Electrolytic Method
In the desorption scheme worked out at the Getchell mines the gold-bearing carbon is leached with a hot solution of sodium sulphide and caustic soda and the dissolved gold precipitated electrolytically on graphitized wood shavings using a stainless-steel screen anode. The spent electrolyte is returned continuously to the leaching step.
When treating 700 tons per day of minus 325-mesh slimes, it is expected that the consumption of activated carbon will not exceed about 60 lb. per day.
Electrolytic and Other Methods. Electrolysis of cyanide solutions results in regeneration of the cyanide combined with the metal complexes, but this method has been largely abandoned owing to the high cost of installation and operation. Other methods proposed for the precipitation of the metals dissolved in cyanide solution usually result is some degree of cyanide regeneration, though, few have passed beyond the experimental stage.