The simplest method of treating a carbonaceous ore is the method described by R. W. Nice (1971) at the McIntyre Porcupine Mine where the carbon, which contains very little gold, is floated off and discarded. The remaining ore is then responsive to cyanidation. This does not work at Carlin because the carbonaceous compounds have a high gold content. A considerable amount of flotation work was done on Carlin ores, however, to produce a high grade concentrate for possible shipment to a smelter and a tailing which could either be discarded or directly cyanided. All of the concentrates contained both carbonaceous material and pyrite and showed low recoveries of gold and poor selectivity. This was because of insufficient liberation of carbonaceous material and sulfide from the gangue, even at 100 per cent minus 325 mesh.
In the treatment of an auriferous sulfide associated with carbonaceous shale from South Africa by American Cyanamid Company (1939), up to 77 per cent of the carbon was eliminated by depressing it with Cyanamid reagent 637 while floating the sulfides for 90.5 per cent gold recovery, and 20.4 to 1 ratio of concentration. Fortunately, the carbon in the Witwatersrand ores is in a form which does not interfere with normal cyanidation.
Cyanide Leaching with Carbon or Resin in Pulp
The addition of activated carbon to adsorb gold preferentially to the acivated carbon component in the ore was proposed by T. G. Chapman (1939 and 1950) who recovered the activated carbon by flotation. A modification of this procedure, using granular activated carbon instead of finely divided Carbon, was tried on mildly refractory Carlin ores with partial success. Extractions of about seventy per cent of the gold were obtained by adding granular carbon to a standard cyanide leach, compared to fifteen per cent extraction without the addition of carbon. For purposes of this presentation mildly refractory ores are defined as those requiring only ten to twenty Kg of chlorine per tonne in a cyanidation pretreatment to be described later, in order to yield 83 or more per cent gold in cyanidation. Similar results were obtained with Rohm and Haas anion exchange resin IRA 425.
These methods have the advantage of low capital and operating costs, and for ores whose only deleterious constituent is carbonaceous material in concentrations of less than 0.5 per cent organic carbon, may be the best way to go. For the Carlin ores, however, these methods were abandoned in favor of chemical oxidation which had the potential for much higher gold extractions.
Since the U. S. Bureau of Mines had shown that humic acid could be extracted by a hot solution of sodium hydroxide and since the use of a hot solution containing one per cent NaOH and 0.2 per cent NaCN was in commercial use to strip the gold cyanide complex from activated carbon at Homestake, K. B. Hall (1974), Newmont looked into using a solution of that composition to leach gold from highly refractory Carlin ores. These ores contained from 0.8 to 1.0 per-cent organic carbon and from 1.5 to 2.5 per cent pyrite. Highly refractory ores are defined as those requiring over 100 Kg of chlorine per tonne in a pretreatment process in order to yield 83 or more per cent gold in cyanidation. The hot caustic leach used carbon in pulp, the theory being the same as with the previous carbon in pulp work with milder ores at ordinary temperatures. The gold would be desorbed from the natural activated carbon in the ore and transferred to the more strongly adsorbent added activated carbon until the concentration of gold on the added carbon was in chemical equilibrium with the concentration of gold on the natural carbon.
Extensive research was done on the method. With ores containing up to 0.8 per cent organic carbon and 1.5 per cent pyrite, the method worked quite well giving 83.0 per cent extraction of gold and a carbon loading of 1000 grams of gold per tonne in locked batch tests from an ore containing ten grams of gold per tonne. However, on the most refractory ores which contained 1.0 per cent organic carbon and 2.0 to 2.5 per cent pyrite the method gave only 78.3 per cent extraction of gold and a loading of only 333 grams of gold per tonne from an ore containing 10.3 grams per tonne, so the method was finally abandoned.
With Carlin ores, roasting drives off carbonaceous materials as CO and CO2, and decomposes pyrite to iron oxides from which the gold can be leached. Extensive research at Newmont Exploration has shown that roasting requires a fine feed of minus ten mesh, at least four hours retention, and precise temperature and atmosphere in the furnace. Below about 500C or with the use of a slightly reducing atmosphere, roasting is incomplete, resulting in a drastic reduction in gold extraction. At 550C and above the pyrite is converted to a refractory form of hematite from which the gold cannot be leached with cyanide. There is also the need for expensive drying of the feed to the furnace, and the need for rigid emission controls on the furnace gases. Extractions of 85 to 87 per cent of the gold were the best that could be obtained from ores containing thirteen grams of gold per tonne. For these reasons and because of the very high capital cost requirements, roasting was abandoned as a suitable alternative at Carlin.
In a research program which began in 1967, the Reno, Nevada station of the U. S. Bureau of Mines showed that the deleterious effects of the carbonaceous materials in Carlin type carbonaceous ores could be overcome by oxidizing agents in aqueous pulps, B. J. Scheiner (1971). After extensive bench scale work by the U. S. Bureau of Mines, Newmont Exploration Limited, and Carlin Gold Mining Company, the choice of oxidation agents for the Carlin ores was narrowed to either chlorine or sodium hypochlorite generated in situ by electrolysis of an ore pulp containing brine, W. J. Guay (1973). After investigating these two methods on a small batch pilot plant scale over a period of several months in 1969, the chlorination method was selected. Electrolysis in pulp featured high capital and low operating costs while the use of chlorine featured low capital and high operating costs, chiefly for chlorine. The limited tonnage of ore available at that time dictated the use of chlorine. Since its startup in 1971, the chlorine plant has consistently outperformed its original expectations, giving gold extractions in the 83 to 90 per cent range at tonnages of 455 tonnes per day or more. Freeport Gold, Nevada Mining Association (1980), has announced that it will use chlorine to treat carbonaceous ores at Jerritt Canyon in Nevada.
Initially, the plant created ores which required about 15 Kg of chlorine per tonne of ore to oxidize the carbonaceous material and the pyrite. In mid 1975 additional chlorination capacity was installed to treat ores which required up to 40 Kg of chlorine per tonne. Estimates in 1975 showed over 1,000,000 tonnes of refractory ore reserves which required over 50 Kg of chlorine per tonne. These larger quantities of chlorine are quite expensive and thus led to a search for alternatives.
A process was developed, W. J. Guay (1960), in which air is dispersed in an aqueous slurry or ground ore at forty to fifty per cent solids at temperatures of 80 to 86° C, until a considerable portion of the pyrite is oxidized to iron oxides.
Some decomposition of carbonaceous materials also occurs, but it is necessary to follow the air oxidation with chlorination in order to complete the oxidation of the carbonaceous materials and pyrite. The process of aeration followed by chlorination has been named “Double Oxidation”. The commercial plant was started up in December 1977. Extractions of gold of over 85 per cent at acceptable economics and under practical operating conditions have been shown to be feasible.
The key to the successful use of air to oxidize pyrite at Carlin is the porosity and large surface area of the spheroidal pyrite. The cubic pyrite is not completely oxidized, but fortunately it has a lower gold content. Other Carlin type ores from Nevada that we have seen have only the cubic pyrite and are not amenable to air oxidation of the pyrite. We have also seen Carlin type ores from Nevada that contain arsenopyrite, which is definitely not amenable to air oxidation. Other Carlin type ores have shown locking of pyrite in quartz. Thus, many Carlin type Nevada ores which are similar in gross composition and in megascopic properties are quite different when viewed in the submicron size range.