Pittsburgh, PA: U.S. Department of the Interior, Bureau of Mines, TN 426, 1993 Nov; :1-2
Objective: To recover metals as sulfide concentrates from contaminated waste streams using hydrogen sulfide (H2S) generated by the bacterial digestion of waste organic materials. Approach: The goal of this research is to develop a treatment method for metal mine effluents that will rival the cost and convenience of conventional lime treatment, and that will provide better effluent water quality, result in less expensive sludge disposal, and allow for the selective recovery of metals. How it works: In this method, indigenous sulfate-reducing bacteria are used to generate H2S gas in an anaerobic bioreactor containing sulfate-rich mine water and inexpensive, degradable organic matter such as food processing wastes or primary sewage sludge. As H2S is formed, it is sparged from the bioreactor by an inert carrier gas to create a gas stream containing about 0.3 pct H2S. When the gas comes in contact with a metal-contaminated mine effluent, the heavy metals precipitate as relatively insoluble sulfides. Elemental sulfur (S(0)) also may be formed. By adjusting the pH, the composition of the precipitated concentrates of metal sulfide can be manipulated. For example, the pH of mine water containing Cu, Zn, Fe, AI, and Mn (pH <3) can be controlled so that a copper sulfide (CuS) and S(0) concentrate forms in Reactor-Clarifier 1, a zinc sulfide (ZnS) and S(0) concentrate forms in Reactor-Clarifier 2, and a sludge containing Fe, AI, and Mn is produced in Reactor-Clarifier 3. After precipitation of the metal sulfide is complete, the "biogas" depleted of H2S and the metal-depleted, sulfate-rich water are recycled to the bioreactor. Results: The treatment process has been used at the bench scale to treat water from the Rio Tinto Mine, an inactive copper mine in Nevada. The untreated Rio Tinto Mine effluent had a pH of 2.5 and contained 600 parts per million (ppm) Fe, 150 ppm AI, 100 ppm Cu, 84 ppm Mn, 75 ppm Zn, 5 ppm Co, and 2 ppm Ni. The treated water bad a neutral pH and all metal concentrations except Mn were below detection limits (100 parts per billion). Final Mn concentrations ranged from 2 to 4 ppm. The Cu content of the CuS and S(0) concentrate ranged from 30 to 50 pct Cu, with Zn and Fe contaminants comprising 0.2 and 0.02 pct, respectively. The ZnS and S(0) concentrate contained 28 pct Zn, 0.74 pct Fe, 0.36 pct Ni, 0.34 pct Cd, and 0.31 pct Co. Beet molasses, an inexpensive byproduct of sugar refining, was used in this trial of the treatment system as the source of organic carbon for the sulfate-reducing bacteria. The beet molasses was pumped directly into the bioreactor at a rate of 0.8 gram per liter of water treated. The sulfate required for H2S production came entirely from the mine water. The estimated cost of the beet molasses, trace nutrients, and alkaline agents required'for the biogenic H2S treatment of Rio Tinto Mine water is estimated to have cost $0.20 per 1,000 liters of water. However, this cost would be reduced significantly by using a low-cost waste material such as primary sewage. Experiments now in progress indicate that such an approach is both feasible and practical.
Pittsburgh, PA: U.S. Department of the Interior, Bureau of Mines, TN 426