Recovery of gold from cyanide leach solutions can be carried out by zinc precipitation, carbon-in-pulp (CIP) and resin-in-pulp methods. CIP has been generally favored as the method in recently constructed plants in the Western world for ores containing less than 15 g gold per tonne. Resin-in-pulp is reportedly being used in Russian gold plants for gold recovery. CIP technology, which has developed rapidly since 1974, is now accepted as an economically and technically viable process for the efficient recovery of gold. The CIP technique was originally developed to bypass the expensive liquid/solid separation steps involved in the zinc precipitation method of recovering gold.
A recent development is the introduction of the carbon-in-leach (CIL) process, wherein leaching and adsorption are carried out simultaneously. The CIL process offers the advantage of lower capital costs, since a separate adsorption section is not necessary. Disadvantages in CIL result from decreased adsorption efficiency, since pulp conditions must be maintained for favorable leaching performance rather than for carbon adsorption. There is little information available comparing CIL with CIP for identical ores, and so actual performance results are inconclusive as to the cost advantages in either process.
It is believed that gold in cyanide leach solutions exists as cyanide complex. The adsorption of gold from such leach solutions by carbon is claimed to take place either in a form chemically distinct from Auro cyanide, reduction to metallic state during adsorption, or typically as an ion exchange mechanism. The work carried out by the National Institute of Metallurgy (NIM), South Africa, and Anglo American Research Laboratories (AARL) tends to favor the former two mechanisms.
The adsorptive properties are due to high surface areas, reactive surface and pore size distribution. Activated carbon is commercially produced from coconut shells, fruit pits, bituminous coal and wood. For the CIP process, it is necessary to use an activated carbon that is hard and abrasion- resistant in order to minimize the production of carbon fines that can result in gold losses. Also, the carbon has to be relatively coarse to allow easy separation from pulp by screening but, at the same time, have acceptable adsorption kinetics. Consequently, the hard, abrasion-resistant product from cocounut shell at 6 to 16 mesh is preferred. TYPICAL SPECIFICATION LIST FOR CARBON Surface Area 1050 to 1150 sq m per g Apparent Density 0.48 g per cu cm Particle Density 0.85 g per cu cm Voids in dense packed column about 40%
The flowsheet of a typical CIP circuit consists primarily of four steps:
* Adsorption of the dissolved gold by activated carbon in adsorption contactors;
* Elution of gold from loaded carbon;
* Recovery of gold from pregnant strip solutions; and
* Regeneration of eluted carbon before recycle to adsorption section.
During the flowsheet development, a useful rule of thumb for gold and silver loading is: L = 3000 C0.7 for gold, and C = 1500 C0.5 for silver where L = the gold or silver loading on carbon from the first CIP stage g/t dry carbon. C = the gold or silver in the feed pulp g/t of solution.
Pulp state residence times are initially one hour for gold and two hours for silver. Carbon concentrations are set within ranges constrained by practical consideration. At levels of less than 10 g per l of pulp, carbon inventory becomes difficult to measure and considerably more pulp needs to be shifted to advance the carbon. At levels greater than 40 g per l, attrition and carbon losses become significant. Process Design
During the conventional mill flowsheet development, areas pertinent to CIP would generally include areas downstream to crushing and grinding. Furthermore, it is assumed the flowsheet will be of normal dissolution, CIP, desorption, recovery and melt.
The information required to establish design criteria can be obtained from metallurgical test work, mine life calculations and plant data. Occasionally, pilot plant data may also be available.
The pertinent design parameters are hence obtained for thickening, cyanidation, carbon adsorption, desorption, gold recovery and melt. Typical carbon consumption can be 0.03 to 0.05 lb of carbon per ton of ore milled. Normal tail solution analyses are 0.001 oz gold per ton of solution.
Charcoal inventory in the first CIP tanks can be 15 g per l rising to 40 g per l in the last tank. The loaded charcoal from the first tank, when sent to stripping, will contain 150 to 200 oz gold per ton of charcoal. The charcoal in the last tank will be loaded to less than 10 oz gold per ton. The first tank produces highly loaded charcoal for bullion production; whereas the last tank produces low-grade tail solution for discard.
The separation of charcoal from pulp is a mechanical procedure using screens. The screens may be vibrating or stationary and located internally or externally to the CIP tanks. Desorption
Parameters to establish in this section: method of stripping (pressure, atmosphere), time, number and volume of strip tanks, temperature and chemistry of strip solution. Three principal stripping methods include pressure, atmospheric and solvent- based strip.
Atmospheric strip with caustic acid and cyanide solution conducted at about 95 C is slow (48 to 72 hours). Pressure strip using caustic and cyanide carried out at 120 to 130 C is faster (about eight hours). Alcohol strip with caustic and cyanide strip solution at 90 to 95 C and atmospheric pressure is also fast (eight to 12 hours). Gold Recovery
Recovery of gold from pregnant strip solution is accomplished either by electrowinning or zinc precipitation. The amount of zinc added to gold tenor in solution, reaction time and filter area are some of the parameters to be considered in zinc precipitation. When gold production is higher than 500 oz per day zinc, precipitation may be advantageous.
In electrowinning, steel wool is used as cathode material. Design criteria required for electrowinning include the ratio of gold per unit weight of steel wool and retention of solution in cathode volume. Reactivation
Reactivation is achieved by washing the carbon in dilute acid, neutralizing with sodium hydroxide, washing with water and calcining in an indirect fired kiln. The kiln product is screened to reject fine carbon, slurried with water and returned to the circuit as required. Construction material, acid requirements, time/temperature profile, screen area, re-attrition tank and agitator are some of the criteria to consider. Bullion Production
Production of dore bullion is achieved by melting loaded cathodes or precipitate with an oxidizing flux. Initial flux components can include silica, borax and nitrate. Density of the charge, melting schedule, feed grade and size of base should be considered for design.
In general, the CIP process for the recovery of gold is an accepted, established and proven technology. It offers significant reductions in capital and operating costs as compared to classic countercurrent decantation and Merrill Crowe precipitation. The metallurgical performance of the CIP process is at least equal to classical leaching procedures.
The key to successful implementation of the process is the establishment of design criteria which will function as a common base for the development of the project. Process Option
The precipitation is attractive where ratio of silver-to-gold is high (5:1). Labor requirement under steady state is lower. Disadvantages for a conventional mill, of course, are the costs of liquid/solid separation, de-aeration, effect of interfering ions and solution concentration. CIP can handle slurry and carbonaceous ores and generally offer efficient recoveries irrespective of solution concentrations. Disadvantages include effects due to alkaline earth salts and organic carbon. Ion Exchange for Gold Recovery
Although resin-in-pulp (RIP) technology is not new to the mining industry (it has been used for many years in the processing of uranium ores), its application in gold and silver circuits has been limited. As a result of the new MINTEK EPAC (equalized-pressure air collector), technology and the development of larger, ore-resistant resins, a greater interest in RIP technology is now evident. In Russia, RIP circuits seem to be more prominent than CIP operations for the recovery of gold and silver. In the Western world, however, such developments have only reached the pilot stage.
The RIP process has a number of advantages over CIP, including superior loading kinetics, higher adsorption capacity, simple elution and, most importantly, elimination of fouling by organic species or clay. On the other hand, the drawbacks in RIP vs. CIP techniques include decreased selectivity for gold, smaller particle size and low density. The performance of resins in gold plant operations is relatively unknown.
Both resins and carbon are likely to play key roles in future gold recovery processes. Although less is known about the performance of resins in gold ores, the ion exchange process is well developed for other metals. Some of the technology designed for the recovery of other elements will inevitably be applied to gold ore processing.
Future gold recovery processes are likely to utilize innovative technologies borrowed from other metal industries. In particular, the use of continuous fluid bed type adsorption processes for treating liquors and slurries will be more prominant in the gold industry. These have become relatively well developed over the years and offer the advantages of lower resin inventory as well as the ability to treat unclarified solutions.
Evaluation of ion exchange resins as an alternative to carbon to recover gold from leach liquors and waste effluents is being examined by researchers in South Africa and Canada. V. I. (Lucky) Lakshmanan is business development manager, mineral resources, for the Ontario Research Foundation. He recently received the 1987 Sherritt Hydrometallurgy Award from the Metallurgical Society of the Canadian Institute of Mining and Metallurgy.
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