Table of Contents
The conventional processing sequence for treating sulfide flotation concentrates containing Cu, Ni, Fe, and platinum-group metals (PGM) includes matte smelting followed by leaching of the metal values. In mid-1990, Stillwater Mining Company began operation of a production smelter in Columbus, MT, to treat the flotation concentrate produced at the Stillwater Mine in Nye, MT. The smelter operation, which consists of a submerged arc electric furnace followed by a top blown rotary converter, produces 1.5 ton-per-day of a high sulfide, Cu-Ni matte containing approximately 2 pct combined PGM and Au. The matte is shipped to Belgium for toll refining due to the excessive penalties charged by domestic refiners for high base metal and sulfide content.
The first stage of Baglin’s process sequence utilized an atmospheric pressure sulfuric acid leach to selectively remove Ni and Fe without solubilizing the Cu or PGM, according to the reaction:
MS + 2H+ → M²+ + H2S (g) M=Fe, Ni……………………………………………………(1)
The second-stage oxidizing leach was conducted to remove Cu from the first-stage residue and leave a high-grade precious metals product. Ferric chloride in a hydrochloric acid medium or ferric sulfate in a sulfuric acid medium were found to be excellent second- stage lixiviants for oxidizing and removing Cu from the matte residues, without co-solubilization of PGM.
Cu2S + 4Fe³+ → 2Cu²+ + 4Fe²+ + S°………………………………………………………….(2)
Cu2S + 2CuCl2 → 4CuCl + S°…………………………………………………………………(3)
Materials and Methods
A sample of converter matte was obtained from Stillwater Mining Company. This matte was granulated by passing molten matte through a stream of water. This material was then screened at the smelter to produce a minus 10- mesh (2.0 mm) product.
Chemical Leaching Tests
Chemical leaching was carried out in 1-L jacketed reactors connected to circulating water baths for temperature control. Each reactor was fitted with a sealed shaft stirrer, a thermometer, a condenser to minimize evaporative losses, and a sample addition port.
Description of Bacteria and Culturing Techniques
Strains of the bacterium Thiobacillus ferrooxidans were grown in sterile American Type Culture Collection (ATCC) nutrient medium 64 at 30° C and pH 2.0.
A mixed culture of T.ferrooxldans was adapted to the matte over a period of several months by adding small amounts (approximately 0.1 g) of the matte to culture tubes and transferring biweekly until consistent growth in the presence of the matte was achieved.
Biological Leaching Tests
Two stirred bioreactor leach tests were conducted in jacketed 3-L reactors containing minus 10 mesh matte in nutrient medium 64 with a 10 wt pct inoculum of adapted T. ferrooxidans culture. The pulp density was 5 wt pct in the first reactor and 10 wt pct in the second reactor. The slurry pH was adjusted to 2.0 using concentrated sulfuric acid. Reactors were sparged with water-saturated air to provide sufficient dissolved oxygen for biological activity.
Chemical and biological leach solutions were analyzed for base metal, PGM, and total sulfur concentrations by inductively coupled plasma (ICP)/atomic emission spectroscopy (AES). Sulfate content was determined by ion chromatography (IC).
Results and Discussion
Chemical Leaching Tests
Chemical leaching was performed to remove Cu, Ni and Fe from the matte and to produce a high grade precious metals residue that can be refined domestically to recover the PGM and Au. A two-stage approach, which included an atmospheric sulfuric acid leach to selectively remove the Ni and Fe, followed by either a ferric salt or cupric chloride leach to extract the Cu, was used.
First-Stage Sulfuric Acid Leaching: Initial test conditions for the first-stage sulfuric acid leach were obtained from Baglin’s work with the synthetic and green mattes. Screening tests were conducted to determine which variables, including particle size, leach time, sulfuric acid concentration, acid to matte ratio, and temperature were key factors affecting the extent of Ni and Fe extraction from the converter matte. In addition, the benefit of retreating first- stage residue with fresh sulfuric acid was assessed.
The detection limits were 0.1 to 1 ppm, which indicates less than 1 pet of the PGM was solubilized under any of the first-stage leaching conditions tested. First-stage leaching reduced the matte weight by about 50 pct, thereby increasing the PGM content to approximately 4 pct.
Second-Stage Chemical Leaching: Ferric salts were not effective under the conditions tested for extracting Cu from either the granulated or pulverized converter mattes. Combined Cu and Ni content of the residues exceeded 50 pct after the two-stage leach. Differences in mineralogy between the converter matte and the synthetic and green mattes tested by Baglin may be responsible for the much lower base metal extractions from the converter matte.
The weight of the first-stage residue was decreased by over 70 pet during the second- stage cupric chloride leach, thereby concentrating the PGM in the residue. The PGM content of the converter matte was upgraded from about 2 pct to 13 pct after the two-stage chemical leach process.
Biological Leaching Tests
Biological leaching oxidized over 90 pct of the sulfide and co-solubilized 99 pct of the Cu and Ni in the matte in 21 weeks for the first reactor and 17 weeks for the second reactor. The Cu and Ni concentrations in the leach solution ran as high as 6 to 8 g/L, while sulfate concentrations of up to 28 g/L were achieved. The weight of the matte was reduced by about 60 pct in both reactors during the biooxidation process.
Previous research on the Stillwater flotation concentrate showed that operation in a continuous mode greatly enhanced reaction kinetics. Overall leach time was decreased by about 80 pet when operation was switched from batch to continuous flow. It is expected that overall leach time could also be decreased significantly by using a continuous stirred tank bioreactor system to treat the converter matte.
Although no experiments were performed on the matte leach solutions produced during this study, solutions from chemical and biological leaching can be treated by conventional methods to remove the Fe, and to recover the Cu and Ni. Possible methods include cementation, solvent extraction, and electrowinning. One method for treatment of acidic sulfate leach solutions containing Cu, Ni, and Fe was suggested by Lake and Snelgrove (1978). Sodium ion is added to precipitate the Fe in the form of natrojarosite.