THE CESL PROCESS: SUCCESSFUL REFINING OF A COMPLEX COPPER SULPHIDE CONCENTRATE

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1 THE CESL PROCESS: SUCCESSFUL REFINING OF A COMPLEX COPPER SULPHIDE CONCENTRATE GLENN BARR AND WILLY GRIEVE 1

2 INTRODUCTION Cominco Engineering Services Ltd. (CESL) is a wholly - owned subsidiary of Cominco Ltd., an integrated mining company producing zinc, copper, and lead, together with precious metals and various by-products. CESL formerly was a full service engineering company until 1994 (when sold to H.A. Simons Ltd.). It now specializes in research and development, focussed on a hydrometallurgical alternative to the conventional route of smelting and refining of copper sulphides. The project has since been expanded to include other base metals such as nickel, cobalt and zinc, as well as precious metals. CESL has built an integrated pilot plant facility in the Vancouver area, as well as a larger demonstration plant nearby in Richmond, B.C. The Demo Plant is designed to produce 1 tpd of copper. In comparison, the Pilot Plant has a design capacity of 36 kg/day copper cathode. A Precious Metals Recovery (PMR) Demo Plant was constructed adjacent to the Copper Demo in 1997 after testing of the PMR process at the pilot plant. The purposes of the two scales of plant are quite different, however, with the demo plant intended to solve equipment and engineering challenges before building a full-scale plant. The CESL Copper Process treats copper-bearing sulphide concentrates through a multistep extraction process. The final residue contains elemental sulphur, hematite, trace amounts of pyrite, and any precious metals (gold and silver) present in the original concentrate. Treatment of the final residue from the CESL Copper Plant for precious metals recovery is necessary for process economics with most concentrates. Direct cyanidation of the Copper Plant residue yields poor precious metals recovery due to refractory minerals that are not amenable to cyanidation and in addition consumes large amounts of cyanide. Therefore, pretreatment of the residue, before cyanidation, is necessary. Bench testwork indicated that both an oxidative leach and a sulphur removal step were required to economically recover the precious metals. The PMR process produces precious metals and the copper left in the residue from the Copper Plant. Results from the CESL Pilot & Demo Copper Plants have been reported at previous Alta conferences. DEMO PLANT PHILOSOPHY Traditionally, pilot plants are built to test out the process flowsheet and establish that the whole process works satisfactorily. The pilot plant must of course have the correct feed material, and the pilot should include all process steps, not just those anticipated to be troublesome. Typically, however, only certain elements of a new process are actually piloted, with the frequent assumption that the rest is "known technology". In CESL's view, it is imperative that all process steps be piloted simultaneously, and that all recycles be incorporated into the testing process. The intention is to ensure that the process as a whole works metallurgically, and the products and byproducts are satisfactory. However, even if the piloting is done thoroughly it is no guarantee that the full-scale plant will succeed. At the full scale there are significant challenges in the choice and design of the process equipment. Many new processes have failed due to insufficient engineering data available to the 1

3 process engineers even after piloting. Resultant is that full-scale equipment could not be designed from the pilot plant data. Often this is labelled as the problems of "scale-up". Recognizing this problem, CESL decided to build a Demo Plant, the purpose of which was to provide engineering data of sufficient quality that the technical risk in building a full-scale plant would be minimized. With this in mind, the demo plant is not just a larger pilot plant, but a plant in which the full-scale equipment choice is the primary criterion. The design for the Demo Plant is therefore governed by what is intended for the full-scale plant. For example, for the PO filtration, actual scaled down commercial filter presses were chosen, of the same design and material construction. During operation, attention was paid to such details as long term blinding characteristics of the filter cloths, which can wreck the availability of a full-scale plant, but would not necessarily be noticed in a typical pilot plant campaign of a few days or a couple of weeks. CESL has also recognized the benefits of repeated Demo Plant campaigns. Experience gained during the first operating campaign of the plant allows evaluation of data collected on the equipment design. Changes can be recommended, implemented, and assessed in a second campaign. This directs the process towards improved equipment selection for the full-scale design, and therefore reduces the possibility of equipment failure at the full-scale. PROCESS DESCRIPTION The CESL process has been described in detail extensively elsewhere, so only a summary is given here. The process begins with a leaching operation to put copper into solution. Pressure oxidation (PO) of copper concentrates efficiently converts all copper sulphides to an acid-soluble form. This may be soluble copper sulphate or solid basic copper sulphate, depending on acidity. This step requires only about one hour retention time, and does not require heat, as PO is exothermic. This is followed by filtration, and any remaining basic copper sulphate is leached into solution in a short atmospheric leach, at low temperature. PO uses high-pressure oxygen at about 150 C to oxidize the sulphide minerals in the presence of an acidic copper chloride-sulphate solution. This solution is recovered by the filtration step and is recycled back to PO. During PO, iron in concentrate minerals is primarily converted to hematite (virtually no Fe is leached into solution); sulphide sulphur is converted to the elemental form, although a small proportion is oxidized to sulphate, and must later be removed. Thus the solids produced by PO consist of a mixture of basic copper sulphate, hematite and elemental sulphur, together with any insoluble components originally present in the concentrate, such as silicates, etc. Other base metal sulphides generally oxidize along with the copper minerals, except for pyrite, which is only partially attacked. PO operates at a relatively high ph, typically ph ph 3.5, which results in many metals being rendered insoluble at the operating temperature of 150 C, e.g. iron, arsenic. The leach liquor (PLS) produced is treated by solvent extraction (SX) to remove impurities, particularly chloride, and produce a suitable electrolyte for electrowinning. Copper is recovered from the pregnant electrolyte using a conventional EW circuit, operating in sulphate media at normal current density, Amps/M 2. The product is comparable to the best EW cathodes produced by heap leach/sx/ew operations. 2

4 The raffinate from SX is partly recycled to the autoclave, and partly neutralized with limestone to control sulphate levels in the circuit, by precipitating gypsum. Other base metals which tend to dissolve during the leaching circuit include: Zn, Cd, Ni, Co, Mg and Mn. These elements are removed in a small bleed circuit, as metal hydroxides using a lime precipitation, or by other means. Significant reagents for the process are limited to oxygen, limerock, and sulphuric acid. Minor amounts of lime are used. Chloride requirements are modest (to make up for entrained losses in residue only) and can be supplied by hydrochloric acid or a salt. Overall the process produces two residues, a leach residue consisting mainly of hematite, elemental sulphur and any unreacted minerals such as silicates, and a gypsum by-product that is quite pure and certainly marketable. The original CESL process, now denoted Process 1, was operated for part of the 1997 demonstration plant campaigns. Improvements to the process were developed during the operation of the plant in 1997, which addressed the need for constant addition of sulphate to the Pressure Oxidation (PO) autoclave, due to the loss of sulphate from the PO circuit as basic copper sulphate. This sulphate becomes sulphuric acid through SX, and eventually requires neutralization, which produces gypsum. In Process 1B, instead of neutralizing excess acid in the 1 raffinate, it is recycled back into the PO circuit. Evaporation of some water must be done to maintain a water balance in PO. This evaporation has to be done on a highly corrosive and gypsum saturated liquor. SUMMARY OF FEEDS TO DEMO PLANT The following table shows the composition of the copper concentrates processed to date in the CESL demonstration plant. Concentrate Demo Plant Operations % Cu %Fe %S Au g/t %Cp %Bo %Py Cc HVC I Summer HVC II Summer Gibraltar August Chilean Summer Brazilian Fall 1999 Summer As can be seen, a wide variety of concentrates have been tested at the Demo Plant. 3

5 SUMMARY OF RESULTS FROM DEMO PLANT The following table presents the main results from each of the Demo Plant campaigns. As can be seen, high copper extraction and good copper quality was achieved with each of the four concentrates. HVC Gibraltar Chilean Brazilian Copper Extraction % Copper Quality % Gold Extraction % Silver Extraction % Sulphur Oxidation % Throughput t Runtime hours RECENT CAMPAIGN RESULTS - DETAILS FLOWSHEET DEVELOPED FOR BRAZILIAN CONCENTRATE PRODUCTION 120% 112% 100% 85% 87% 100% 80% 60% 40% 40% 53% 47% 61% 20% 0% September October November December January February March April Figure 1 Copper Plant % Design EXTRACTION Copper/Gold/Silver/Sulphur Overall copper extraction from the Brazilian concentrate was 98.0%. An initial extraction of 97.5% was achieved after the pressure oxidation and atmospheric leaching step, and an increase in extraction to 98.0% was observed after solids processing though the total oxidative leach step. The major source of copper loss was to the sulphur flotation tails. Unleached oxide copper and entrained solution copper in this stream is approximately 95% of the lost copper. 4

6 CATHODE QUALITY Purity and current efficiency OPERATING RESULTS Grinding Size of received concentrate: Size of reground concentrate: 6% - 15% +400 mesh Pressure Oxidation 2900 hours operating Temp/Press/Ox ratio Atmospheric Leach 3100 hours operating SX/EW 3700 hours operating Coalescers/KPT Organic entrainment and total losses Neutralization Various circuits required: 1 acid neutralization; 2 impurity precip Flotation 3000 hours operating Recovery: gold/silver/sulphur Sulphur Leach 2500 hours operating PCE losses Sulphur recovery Total Oxidative Leach 800 hours operating Gravity concentration Additional copper extraction Sulphur oxidation Cyanidation 800 hours operating Reagent consumption 5

7 IMPURITY BEHAVIOR AND CONTROL Aluminum Cobalt Magnesium Manganese Nickel Zinc Concentrate 100% 100% 100% 100% 100% 100% Flotation Tails 85% 21% 60% 72% 32% 47% TOL Residue 4.6% 1.3% 7.3% 0.6% 1.8% 0.7% 4 PPT Residue 0.1% 13.7% 2.8% 5.3% 6.5% 8.7% 2 PPT Residue 11% 70% 14% 20% 55% 45% 2 Neut Gypsum 0.2% 3.0% 5.2% 0.7% 2.5% 6.6% Recovery 101% 109% 89% 99% 97% 108% 6