The Application of Diagnostic Leaching to Copper and Gold Ores

Transcription

1 The Application of Diagnostic Leaching to Copper and Gold Ores by Dr. Corby G. Anderson Director of The Center for Advanced Mineral & Metallurgical Processing Montana Tech Room 221 ELC Building Butte, Montana Tel/Fax CAnderson@mtech.edu Web: Abstract In the processing of copper and gold ores, it is important to understand the nature of the metal bearing minerals from a metallurgical standpoint. While traditional mineralogy can provide some qualitative insight into this, diagnostic leaching allows the ore to be evaluated quantitatively by mineral type and can potentially lead to a metallurgical process. The application of diagnostic leaching is important for both initial exploration and when the ore is already being treated in the process plant. This paper outlines the basic methodology of diagnostic leaching applied to copper and gold ores. Also, examples of applications to various ore types are provided.

2 Introduction In the processing of copper and gold ores, it is important to understand the nature of the metal bearing minerals from a metallurgical standpoint. While traditional mineralogy can provide some qualitative insight into this, diagnostic leaching allows the ore to be evaluated quantitatively by mineral type and can potentially lead to a metallurgical process. The application of diagnostic leaching is important for both initial exploration and when the ore is already being treated in the process plant. Diagnostic Leaching Methodology for Copper Ores The basis of diagnostic leaching for copper ores is the mineral s distinct thermodynamic and kinetic response in a given lixiviant. As the technique and application is relatively new, there are several ways to carry out a diagnostic leach campaign. A typical leach sequence and resultant mineral content calculation methodology for copper ores are shown in Table 1. Here four 10 gram samples are leached for one hour with 500 ml. of the chosen lixiviant to speciate chrysocolla, the total hydrous copper carbonates (malachite plus azurite) and chalcocite, chalcopyrite and copper metal. First the sample of copper ore is pulverized to 100% passing 200 mesh. Then the total copper is determined using pyrosulfate fusion followed by ICP/AAS analysis. The content of malachite and azurite in the sample is determined by leaching with a 5% solution of hot acetic acid. The content of chrysocolla (+/- other copper silicate minerals) is determined using an 85 o C solution of 3.0% sulfuric acid and 0.1% sodium sulfite solution on the sample. The content of chalcocite (plus covellite and digenite), malachite and azurite in the sample is determined using a 10% solution of room temperature cyanide solution adjusted to a constant ph of 12 with NaOH. Finally, the metallic copper content is determined by the displacement of silver from room temperature 15 g/l silver nitrate solution. Table 1. Example Diagnostic Leach Analytical Flowsheet for Copper Ores Leaching and Analytical Sequence A- Total Copper Analysis by ICP/AAS B- Copper Carbonates by Acetic Acid Leach ICP/AAS Analysis C- Copper Oxides and Carbonates by Sulfuric Acid- Sodium Sulfite Leach ICP/AAS D- Reprecipitated Copper Sulfides and Carbonates by Cyanide Leach ICP/AAS E- Native Copper by Silver Nitrate ICP/AAS Analysis Calculations Copper Oxides and Silicates = C-B Reprecipitated Copper Sulfides = D-B Primary Copper Minerals = A-E ((C-B) + D) Native Copper = E

3 The Application of Diagnostic Leaching to Copper Ores Utilizing the diagnostic flowsheet presented in Table 1. several copper ores were analyzed. The results and suggested metallurgical process route are summarized in Table 2. Table 2. Applications of Copper Ore Diagnostic Leaching Ore Total Cu Oxides and Silicates Reprecipitated Sulfides Metal Primary Minerals Possible Process % 80% 20% 0% 0% heap/vat leach 2 0.3% 0% 30% 0% 70% flotation 3 0.4% 5% 90% 5% 0% oxidizing leach Diagnostic Leaching Methodology for Gold Ores The basis of diagnostic leaching for gold ores is the mineral s specific thermodynamic and kinetic response in a given lixiviant. Thus, a given mineral containing gold will be selectively destroyed allowing the encapsulated gold to be directly recovered by cyanidation. As the technique and application is new, there are several alternative ways to carry out a diagnostic leach campaign. A simple leach sequence and resultant mineral content calculation methodology are shown in Table 3. First, the sample is pulverized to 100% passing 200 mesh. Then it is analyzed by fire assay for total gold content. The first diagnostic leach test is direct cyanidation of the pulverized ore sample. This is done in a 1% NaCN solution at 50% solids for 24 hours. The ph is held at 10.5 using NaOH. Fire assay analysis is performed on the leached solids and the solutions and resultant gold recovery is calculated. This test delineates the free gold in the sample. The second diagnostic leach test involves leaching of the pulverized sample with a 3 molar solution of HCl at 70 o C for six hours at a slurry density of 40% solids. Then the solids are subjected to direct cyanidation. This is done in a 1% NaCN solution at 50% solids for 24 hours. The ph is held at 10.5 using NaOH. Fire assay analysis is performed on the leached solids and the solutions and resultant gold recovery is calculated. This test delineates the gold in oxide, carbonate and pyrrhothite minerals. The third diagnostic leach test involves leaching of the pulverized sample with a 50% solution of HNO 3 at 70 o C for six hours at a slurry density of 40% solids. Then the solids are subjected to direct cyanidation. This is done in a 1% NaCN solution at 50% solids for 24 hours. The ph is held at 10.5 using NaOH. Fire assay analysis is performed on the leached solids and the solutions and resultant gold recovery is calculated. This test delineates the gold in sulfide minerals like marcasite, pyrite and arsenopyrite. The fourth diagnostic test involves roasting of the pulverized sample in a furnace for six hours at 600 o C in an oxidizing atmosphere. Then the solids are subjected to direct

4 cyanidation. This is done in a 1% NaCN solution at 50% solids for 24 hours. The ph is held at 10.5 using NaOH. Fire assay analysis is performed on the leached solids and the solutions and resultant gold recovery is calculated. This test delineates the gold in carbonaceous material. The remaining gold in the tails from the cyanidation of this fourth diagnostic test indicate what fraction is encapsulated by quartz and silicates. Table 3. Example Diagnostic Leach Analytical Flowsheet for Gold Ores Leaching and Analytical Sequence A -Total gold analysis by fire assay analysis B - Direct cyanidation of gold ore and fire assay analysis C - Leaching of ore with hydrochloric acid followed by direct cyanidation and fire assay analysis. D - Leaching of ore with nitric acid followed by direct cyanidation and fire assay analysis. E - Roasting of ore followed by direct cyanidation and fire assay analysis F - Fire assay analysis of cyanidation tails from E. Calculations % Au Recovered in B = % of free gold. % Au Recovered in C-B = % of gold in oxides, carbonates and pyrrhotite. % Au Recovered in D-C = % of gold in sulfides. % Au Recovered in E-D = % of gold adsorbed/associated with carbonaceous material. % Au Recovered in F-E = % of gold in quartz and silicates. The Application of Diagnostic Leaching to Gold Ores Utilizing the diagnostic flowsheet presented in Table 3. several gold ores were analyzed. The results and suggested metallurgical process route are summarized in Table 4.

5 Table 4. Applications of Gold Ore Diagnostic Leaching Au in pyrrhotite oxide & Au in Au in Au in Ore Total Au Free Au carbonate sulfides carbonaceous silicates Possible Treatment Process oz/t 90% 5% 0% 0% 5% Gravity/direct cyanidation oz/t 5% 0% 95% 0% 0% Flotation and/or pre-oxidation oz/t 0% 0% 5% 95% 0% Roasting, chlorination oz/t 5% 0% 5% 0% 90% Fine grinding Summary In the processing of copper and gold ores, it is important understand the nature of the metal bearing minerals from a metallurgical standpoint. While traditional mineralogy can provide some qualitative insight into this, diagnostic leaching allows the ore to be evaluated quantitatively by mineral type and potentially classified by a metallurgical process. This is important for both initial exploration and when the ore is already being treated in the process plant. This paper outlined the basic methodology of diagnostic leaching of copper and gold ores. Also, examples of applications to various ore types were provided. References Malholtra, D. and Armstrong, S., 1993, Characterization of Refractory Gold Ores Through Diagnostic Leaching Procedure, Trans. SME., 1993 SME Annual Meeting, Denver, Colorado. Henley, K.J., 1989, A Combined Mineralogical/Metallurgical Approach to Determining the Nature and Location of Gold in Ores and Mill Products, Minerals Engineering, Vol. 2, No. 4, pp Sibrell, P.L., Wan, R. Y., and Miller, J. D., 1990, Spectroscopy Analysis of Passivation Reactions for Carbonaceous Matter from Carlin Trend Ores, Trans. SME., 1990 Annual Meeting, Gold 90 Symposium, Salt Lake City, Utah. Hiskey, J. B., 1997, Diagnostic Leaching of Copper Bearing Materials, SME Pre-print 97-83, 1997 SME Annual Meeting, Denver, Colorado.

6 Parkinson, G. A. and Bhappu, R. B., 1995, The Sequential Copper Analysis Method Geological, Mineralogical and Metallurgical Implications, SME Pre-print 95-90, 1995 SME Annual Meeting, Denver Colorado. Tumilty, J. A., Sweeney, A.G., and Lorenzen, L., 1987, Diagnostic Leaching in the Development of Flowsheets for New Ore Deposits, Proceedings of the International Symposium on Gold Metallurgy, Canadian Institute of Mining and Metallurgy, Winnipeg, Manitoba.