Final Report: Sulfide Mineral Coating Process To Control Acid Rock Drainage

EPA Contract Number: 68D03028
Title: Sulfide Mineral Coating Process To Control Acid Rock Drainage
Investigators: Olson, Gregory J.
Small Business: Little Bear Laboratories Inc.
EPA Contact: Richards, April
Phase: I
Project Period: April 1, 2003 through September 1, 2003
Project Amount: $70,000
RFA: Small Business Innovation Research (SBIR) - Phase I (2003) RFA Text |  Recipients Lists
Research Category: Watersheds , SBIR - Water and Wastewater , Small Business Innovation Research (SBIR)

Description:

Acid rock drainage (ARD) is a highly significant environmental problem, resulting from oxidation of sulfide minerals in mine tailings and waste rock. Chemical and biological processes cause ARD, which is characterized by acidic water containing heavy metals. Existing technology for combating ARD consists of treating the acidic effluents or isolating problematic tailings or waste rock; however, these methods do not attack the problem at the source.

Recently, Little Bear Laboratories, Inc., has made significant progress in developing an approach to stop the biological component of ARD with thiocyanate. Thiocyanate is an inexpensive, widely available chemical, also found in some mineral processing solutions, that is selectively toxic toward acidophilic bacteria that accelerate formation of ARD. However, abiotic, chemical oxidation remains problematic. ARD cannot be eliminated unless both chemical and biological components of sulfide oxidation are arrested.

This research project tested the feasibility of stopping ARD by applying a novel process that coats or armors sulfide mineral particle surfaces to stop further chemical oxidation and neutralize acidity. The armoring process was used in combination with thiocyanate treatment to stop the biological oxidation of sulfide minerals. The goal of this research project was to determine the effectiveness of the armoring process in: (1) preventing ARD from developing (ARD prevention), and (2) mitigating an existing ARD problem (ARD control). Solid and soluble forms of the armoring agent at different concentrations were applied to waste rock or tailings in accelerated weathering, laboratory-scale (1 kg) humidity cell tests. The test system was biologically active (i.e., sulfide-oxidizing bacteria were added to the waste rock). In this manner, Little Bear Laboratories, Inc., determined if sulfide mineral coating alone is effective in stopping ARD, or if coating also required treatment to stop the activity of microorganisms.

Test results provided a preliminary estimate of treatment costs and process efficiency in stopping chemically and biologically catalyzed ARD. If successful on a large scale, the process would be a major breakthrough in the control of ARD, a comprehensive solution stopping both chemical and biological oxidation of sulfides. The process could be applied to source material, to existing piles of mine waste producing ARD, and for closure of spent heap leach pads. The process would significantly lower the costs and increase the effectiveness of treating ARD. These costs amount to billions of dollars worldwide. The process would help to protect the thousands of miles of streams adversely affected by ARD. Not only does ARD threaten the environment, it also threatens economic viability of the mining industry worldwide.

Summary/Accomplishments (Outputs/Outcomes):

Treatment of oxidizing sulfide ore with sodium triphosphate solution, until the solution was near neutral in pH, resulted in significant reduction in ARD in subsequent accelerated weathering tests. Phosphate treatment reduced the abiotic rate of sulfide oxidation by nearly 50 percent. Thiocyanate co-treatment reduced the amount of phosphate required, increased the effectiveness of phosphate treatment, and may provide for longer term ARD control than phosphate treatment alone.

ARD prevention was most effective with a combination of dicalcium phosphate and thiocyanate, which reduced ARD (sulfate formation) from sulfidic ore by 95 percent compared to the untreated control. Phosphate rock and thiocyanate also were highly effective in preventing ARD, as was dicalcium phosphate alone. Abbreviated weathering tests showed that combined dicalcium phosphate and thiocyanate also reduced leaching of sulfate and zinc by 81 percent and 95 percent, respectively, from waste rock from a zinc mine. Copper leaching from copper tailing material also was reduced by more than 90 percent.

A preliminary comparison of ARD treatment costs was made using solutions produced from accelerated weathering tests. The amount of caustic required to bring leachates to pH 9.5, precipitating most metals as hydroxides, was reduced 95 percent from control values with leachate produced from the combined dicalcium phosphate-thiocyanate treatment. This would represent a large reduction in lime costs and in sludge volume. Furthermore, savings in capital costs also could be realized if a single-stage, high-density sludge neutralization process was sufficient for water treatment, as opposed to a two-stage water treatment facility to treat severe ARD.

Conclusions:

Control of an existing ARD problem may be achieved by treating oxidizing sulfide ore with sodium triphosphate solution until effluent solution is near neutral in pH. This treatment significantly reduced subsequent ARD production from sulfide ore in accelerated weathering tests. Furthermore, combined phosphate-thiocyanate treatment appears somewhat more effective than phosphate treatment alone and may provide longer term ARD control.

For ARD prevention, certain combinations of solid phosphates and thiocyanate may produce conditions that prevent ARD from becoming established. Dicalcium phosphate alone at 10 g/kg was nearly as effective as the combined treatment in preventing ARD. The dicalcium phosphate system may produce conditions similar to those achieved with oxidizing ore treated with sodium triphosphate solutions.

Improved water quality of effluents draining waste rock, tailings, or ore, resulting from treatment with phosphate and thiocyanate, would result in significantly lower treatment costs, including lower capital costs for simpler processes and lower operating costs for reagents such as lime. In addition, there would be far less sludge production. Effluents could be sufficiently low in metals to allow for options other than treatment.

The results noted in this research have demonstrated that phosphate rock or its derivatives, alone or in combination with thiocyanate, applied to sulfide-containing materials capable of producing ARD, represents a potentially cost-effective commercial approach to controlling ARD at the source. Controlling ARD initially at the source would dramatically reduce not only its long- term environmental impacts, but also the financial and bonding concerns arising from the enormous costs of mine closure and long-term water treatment.

Journal Articles:

No journal articles submitted with this report: View all 1 publications for this project

Supplemental Keywords:

acid rock drainage, ARD, biocides, mining wastes, phosphates, phosphate rock, thiocyanate, acidic effluents, tailings, waste rock, acidophilic bacteria, triphosphate, sulfidic ore, dicalcium phosphate, small business, SBIR., Scientific Discipline, Water, POLLUTANTS/TOXICS, Chemical Engineering, Environmental Chemistry, Chemicals, Chemistry, Analytical Chemistry, Engineering, Chemistry, & Physics, acid mine drainage, biological oxidation, environmental contaminants, acid rock drainage (ARD), chemical oxidation, sulfer oxide, mine drainage

SBIR Phase II:

Sulfide Mineral Coating Process To Control Acid Rock Drainage