Final Report: A Novel Approach To Prevention of Acid Rock Drainage (ARD)

EPA Contract Number: 68D00276
Title: A Novel Approach To Prevention of Acid Rock Drainage (ARD)
Investigators: Olson, Gregory J.
Small Business: Little Bear Laboratories Inc.
EPA Contact: Richards, April
Phase: II
Project Period: September 1, 2000 through September 1, 2002
Project Amount: $224,941
RFA: Small Business Innovation Research (SBIR) - Phase II (2000) Recipients Lists
Research Category: Watersheds , SBIR - Water and Wastewater , Small Business Innovation Research (SBIR)

Description:

The goal of this research project was to determine the effectiveness of thiocyanate as an agent to control and/or prevent acid rock drainage (ARD), a serious environmental problem in the United States and around the world. ARD occurs from the uncontrolled oxidation of sulfide minerals exposed to air and water by mining. This oxidation, accelerated by the activities of iron- and sulfur-oxidizing acidophilic microorganisms, produces acidic drainage containing heavy metals. Little Bear Laboratories, Inc., chose thiocyanate because it is highly and selectively inhibitory toward acidophilic microorganisms at low concentrations, relatively inexpensive, relatively stable in acidic environments, and readily and completely biodegraded in "normal" neutral pH environments.

The dose and efficiency of thiocyanate for stopping biocatalyzed ARD was evaluated in laboratory-accelerated weathering tests with several types of sulfidic mine tailings and waste rock from base and precious metal mining operations. Sulfidic materials were loaded into humidity cells, columns, or trays at a scale of 1 to 50 kg. Thiocyanate was either blended with the rock initially or added by trickle irrigation at the start of or during the tests. The efficacy of thiocyanate in reducing ARD was evaluated by analysis of leach solutions for ARD components (iron, acidity, sulfate, heavy metals). Results with thiocyanate-treated material were compared to untreated controls. Laboratory investigations of the fate of applied thiocyanate also were performed, as were tests of the potential for acidophilic microorganisms to adapt to thiocyanate.

Four major mining companies participated in this project. Teck Cominco, Ltd.; Phelps Dodge Corporation; Barrick Gold Company; and Homestake Mining Company (which merged with Barrick during the course of this project) played a key role in supplying samples of tailings and waste rock for testing. These four companies also provided matching funds in supporting the "option" proposal. In the "option" project, Barrick and Teck Cominco provided facilities, personnel, and analytical results in larger scale tests (13.6 to 100 metric tons) of thiocyanate performance in large columns and test pits at their mine sites.

Practical aspects of commercializing thiocyanate for ARD control were addressed in a series of reports produced by project consultant Dr. Terry Mudder. These reports covered: (1) chemistry and commercialization, (2) design and economics of a full-scale thiocyanate application process, (3) evaluation of encapsulation of thiocyanate for slow release, (4) thiocyanate ecotoxicity and environmental regulations, and (5) the degradation of thiocyanate.

Summary/Accomplishments (Outputs/Outcomes):

Thiocyanate significantly reduced ARD from samples of tailings and waste rock in accelerated weathering tests. The extent of reduction depended on the specific ARD parameter measured and the test system. Examples of reduction of sulfate and acidity from laboratory and field tests of several types of sulfidic materials are shown in Table 1. The percent ARD reduction based on sulfate leaching was less in most tests than the percent ARD reduction based on iron leaching. This was most likely due to iron precipitation in the rock in columns and humidity cells.

The mechanism of action of thiocyanate in reducing ARD (metal sulfide oxidation) is by inhibiting microbial iron oxidation-leach solutions in test systems treated with thiocyanate had lower redox potentials than controls. It is likely that at higher doses of thiocyanate, microbial metal sulfide oxidation was almost completely inhibited. Remaining ARD production in these situations probably resulted from abiotic metal sulfide oxidation by oxygen. When thiocyanate concentrations in leachates declined to less than 10 mg/L, redox potentials increased and biocatalysis of ARD resumed.

In some cases, thiocyanate was applied only at the beginning of a test at an initial dose of 20 to 200 mg/kg. Rainfall was simulated by periodic application of deionized water over a period of several months. In other cases, thiocyanate was applied throughout a test by adding water-soluble KSCN to leach water. Small column adsorption tests indicated test samples bound from 8 to 48 mg SCN/kg.

Copper thiocyanate was shown to be an effective slow- or controlled-release form of thiocyanate. It is not water soluble, but it is soluble in acidic, oxidizing solutions characteristic of ARD. In large (approximately 25 kg) humidity cell tests, CuSCN performed as well as KSCN, but was not as rapidly leached from sulfidic rock. The disadvantage of CuSCN is that copper is released with thiocyanate. In situations where dissolved copper can be tolerated, CuSCN is promising as a slow-release form of thiocyanate. Additionally, it may be less susceptible to biodegradation at neutral pH than soluble forms of thiocyanate.

Microbial adaptation to thiocyanate was not significant, nor was there evidence for biodegradation of thiocyanate at low pH. However, thiocyanate was slowly hydrolyzed at low pH abiotically, producing ammonia. Hydrolysis was faster at higher Fe3+ concentrations. These observations are consistent with published reports of the abiotic autoreduction of Fe3+-thiocyanate complexes.

Field tests at the 13.6 to 100 metric ton scale at mine sites showed thiocyanate (as KSCN) applied at the start at a dose of 25 to 57 mg/kg could reduce ARD by 50 percent or more from waste rock and sulfidic ore over a test period of 4 to 5 months. Similarly, these results indicated that microorganisms, as opposed to chemical oxidation by atmospheric oxygen, were responsible for more than 50 percent of the ARD formation.

Table 1. Thiocyanate (KSCN) Performance Against ARD: Reduction of Sulfate and Acidity in Leach Solutions Compared to Untreated Controls from Representative Laboratory and Field Tests.

Type of Material Test ID Scale, kg SCN Applied Dose, mg/kg Duration, Months # Soln Applications Total Liters Applied1 %ARD Red'n (Sulfate) %ARD Red'n (Total Acidity)
Carlin ore BC50-3 24 At start only 199 9 6 6.7 85 94
Carlin ore BC50-6 25 At start only2 194 9 6 6.7 81 95
Carlin ore BC-6 1.5 At start only 200 5 4 0.8 91 97
Carlin ore BC-11,12 1.5 At start only 49 4 3 0.6 714 824
Carlin ore BC-3,4 1.0 At start3 57 5 6 1.15 79 84
Carlin ore Field 13600 At start3 57 5 6 1000 56 726
Cu tailings PD 3-6 1.0 At start only 150 5.5 2 0.5 705 705
Cu tailings Tray 3 37 Biweekly 5-8 6.5 14 32 50 52
Cu tailings Tray 4 37 Biweekly 27-40 6.5 14 32 47 46
Waste rock RD-3 45 Biweekly 7 6 13 29 34 34
Waste rock RD-4 45 Biweekly 33 6 13 29 75 74
Waste rock Field 100000 At start only 25 4 rainfall only unknown 52 56


1 After system initially was brought to saturation.
2 CuSCN.
3 Second application of 17 mg SCN/kg was made after 4 months.
4 Mean of duplicates.
5 Mean of four replicates.
6 Free acidity value (total acidity not measured), dissolved iron was reduced by 68 percent.


A series of technical reports by Dr. Mudder identified sources of thiocyanate, regulations on its use, treatment schemes, costs, and degradation. The cost for using thiocyanate to prevent acid mine drainage in a hypothetical 25,000 ton per day mining operation was estimated at 25 cents per ton of ore. This operation was assumed to produce 5,000 tpd of ore, 10,000 tpd of non-acid producing waste rock, and 10,000 tpd of acid-producing waste rock. A commercial facility was designed for the application of thiocyanate to waste rock in haul trucks taking rock from the mine pit to waste rock dumps. Capital and operating costs for the thiocyanate application facility were developed.

Conclusions:

Thiocyanate is highly effective in stopping the biocatalyzed component of ARD. As long as thiocyanate is maintained in piles of waste rock or tailings, a substantial fraction of ARD can be eliminated. The extent of ARD reduction by thiocyanate reflects the extent that microorganisms contribute to ARD production in a particular environment or test system-thiocyanate does not prevent abiotic sulfide oxidation by oxygen. Consequently, thiocyanate should be effective in ARD reduction in environments where microorganisms are active in sulfide oxidation. Where abiotic oxidation of sulfides is more significant, thiocyanate will be less effective. Results from field trials at a 13.6 to 100 metric ton scale over 4 to 5 months showed ARD reductions (as measured by sulfate in leach solutions) of 50 percent to more than 75 percent. Thus, at a minimum, microorganisms were responsible for at least 50-75 percent of the ARD in these environments over the course of the test. However, it is likely that biooxidation was more significant than shown by these figures, because thiocyanate largely was leached out of the test systems after 2 months. In the first 1-2 months following thiocyanate application, ARD parameters were reduced 79 percent (sulfate) to more than 99 percent (arsenic) in the 13.6 tonne test and 74 percent (sulfate) to 79 percent (acidity) in the 100 tonne test. Laboratory scale results (kg) were reasonably predictive of larger scale (tons) performance.

Thiocyanate is relatively stable at low pH, being hydrolyzed only slowly to produce ammonia. There was no evidence of significant adaptation to or metabolism of thiocyanate by acidophilic microorganisms. However, comprehensive control of ARD will require a process to also control abiotic oxidation of sulfides. In this connection, preliminary results using a combination of phosphate and thiocyanate were encouraging. This approach might be an effective tool for the most efficient reduction of ARD in many environmental settings.

Although readily available commercially, thiocyanate also is a component of certain process solutions, especially at precious metal mines. In these situations, existing solution management and heap closure strategies may merit reexamination. For example, upon closure, steps could be taken to maintain residual thiocyanate in a heap rather than rinsing.

Journal Articles:

No journal articles submitted with this report: View all 2 publications for this project

Supplemental Keywords:

acid rock drainage, ARD, thiocyanate, KSCN, CuSCN, sulfidic mine tailings, mining, waste rock, metal sulfide oxidations, redox potential, biooxidation, SBIR., RFA, Scientific Discipline, Toxics, Waste, Water, Ecosystem Protection/Environmental Exposure & Risk, National Recommended Water Quality, Chemical Engineering, Wastewater, Environmental Chemistry, Ecosystem/Assessment/Indicators, Ecosystem Protection, Chemistry, Fate & Transport, Ecological Effects - Environmental Exposure & Risk, Hazardous Waste, Chemistry and Materials Science, Hazardous, Engineering, Chemistry, & Physics, Environmental Engineering, Ecological Indicators, wastewater treatment, industrial wastewater, acid mine drainage, municipal wastewater, mining, acid rock drainage, sulfide, copper, ARD


SBIR Phase I:

A Novel Approach to Prevention of Acid Rock Drainage (ARD)  | Final Report