The management of waste rock produced from mining of sulphidic ores poses a challenge to the mining industry. Acid generation occurs when sulphide minerals (principally pyrite and pyrrhotite) contained in the rock are exposed to air and water. In the absence of sufficient alkaline or buffering minerals, the resulting leach water becomes acidic, and is characterized by high sulphate, iron and, metal concentrations. This water, sometimes called acid rock drainage (ARD), can contaminate surface water and ground water courses, damaging the health of plants, wild life, fish and, possibly, humans.
A study was initiated by Noranda Technology Centre (NTC) and the Centre de recherches minérales (CRW to evaluate the relative effectiveness of various techniques for controlling ARD in waste mine rock. This study was undertaken at NTC as part of the MEND (Mine Environment Neutral Drainage) Program. The techniques investigated were water cover, soil cover, wood bark cover, and addition of limestone and phosphate rock (apatite).
Potentially acid-generating waste rock samples used in the investigation were obtained from the Stratmat site, located on the Heath Steele Mine property, near Newcastle, New Brunswick, and from Les Mines Selbaie, located near Joutel, Québec. Both types of rock samples were crushed to particle sizes between 25 and 50 mm. The investigation involved outdoor lysimeter tests and indoor or laboratory column experiments. Cover techniques investigated were a 1 m water cover, a soil cover consisting of a 150 mm thick water-saturated clay layer sandwiched between two 75 mm thick sand layers, and a 150 mm thick wood bark layer. Limestone and phosphate were added at 1 and 3% dosages. Control experiments, using waste rock without cover and additive, were also installed for comparison. The outdoor tests were subjected to natural weather conditions (rain, freeze-thaw and evaporation). The laboratory or indoor tests were run at an average temperature of 20°C and subjected to a cycle of 8 weeks of dry conditions and 8 weeks of wet conditions (water addition). Water was added to simulate the average annual precipitation for a nearby municipality, Dorval, Québec. All tests were conducted in triplicate.
Monitoring of the effluent water quality to three years (154 weeks) indicated the control waste rock started producing acid very early in the tests (about the 5th week). The rate of acid production was quantified (mg of CaCO3 per day per kilogram of rock) and found to be higher in the laboratory than outside. Higher laboratory temperatures are most probably responsible for the higher rate. The Stratmat rock generated acid at a higher rate than the Selbaie rock, although the latter has a higher pyrite content. A detailed post-testing investigation of the two rock types was conducted at The University of Western Ontario, using mercury intrusion porosimetry, surface analytical techniques and X-ray fluorescence and diffraction methods. The results indicate the fresh, unoxidized Stratmat and Selbaie rocks have a similar pore structure, but different gangue mineralogy. The Stratmat rock consists of pyrite and minor amounts of metal sulphides held in a matrix of silicate minerals including illite and feldspar. The Selbaie rock, on the other hand, contains mainly pyrite and quartz. Trace amounts of metal sulphides appear to be in solid solution with the pyrite. Results of accelerated leaching tests clearly showed that the gangue composition of the Stratmat rock has a major influence on its acid generation ability and would explain the difference in acid production between the Stratmat and Selbaie rocks.
Water cover was found to be the most effective control technique during the three years of indoor testing, followed by 3% and 1% limestone, soil cover and, finally, 3% and 1% phosphate. The effectiveness of the various techniques observed in the outdoor tests were as follows: water cover, 99%; 1% limestone, 93%; soil cover, 70%; and 1% phosphate, 9%. 10-15% increase in effectiveness was observed (from 83 to 98%) when the amount of limestone added to the rock was increased from 1 to 3%. A similar increase in the amount of phosphate yielded higher effectiveness, from 10 to 70%. All the techniques, with the exception of the water cover, were found to be slightly more effective in the laboratory than outside. The water cover maintained the same effectiveness (> 99%) in both laboratory and outside tests. The soil cover was more effective in the laboratory (98%) than outside (70%). The difference may be explained by the effects of adverse natural climatic conditions (for example, freezing and thawing) which were not resent in the laboratory. It is also believed that oxygen and water enter the soil covered waste rock mostly by the side walls of the lysimeters. The phosphate was found to contain some carbonate mineral (calcite) which probably delayed acid production (at the 3% dosage) for some time. An increase in acidity and a decrease in pH were observed in both the Stratmat and Selbaie rocks when all the calcite was presumably consumed. It should be noted that the relative effectiveness of the different techniques is likely to change with time, due to depletion of alkalinity or phosphate materials.
The wood bark accelerated acid production by about 60% in the laboratory and 500% outside. The role of the iron oxidizing bacteria (Thiobacillus ferrooxidans) was invoked to explain this acceleration. This was confirmed when a bactericide (0.02% thymol solution), added to the wood bark cover, reduced acid generation considerably. The iron oxidizing bacteria are mostly autotrophs (that is, they require inorganic carbon for their metabolism), and would become more active by using CO2 produced from fungal decomposition of the wood bark. Other heterotrophic iron oxidizing bacteria would use organic carbon from the wood bark for metabolism. Thus, a wood bark cover is not considered a good technique for reducing acid generation in sulphide-bearing waste mine rock. The water covered waste rock began to release low concentrations of metals (zinc, iron and lead) after two and a half years of operation. The delay in metal release may be attributed to the presence of trace amounts of alkaline minerals which were probably depleted after the initial two and a half years. The results of the study thus far indicate that, although. a water cover may not completely prevent oxidation, it will reduce acid generation considerably. In fact, when considering both feasibility and efficiency, it is the most promising ARD control technology known to the industry. The rate of oxidation is decreased in two important ways: first, the oxidation will begin much later if fresh rock is covered (two and a half years in this case), and second, the oxidation will continue at a considerably reduced rate, due to the oxygen diffusion barrier the water presents. The delay before oxidation begins is probably proportional to the neutralization potential of the rock. If oxidized waste rock is covered with a layer of water, it is likely that the alkaline materials will be depleted and that the oxidation will begin immediately.
The effectiveness of the water cover may be enhanced by increasing the depth of the water or applying an organic layer on top of the waste. With an organic layer, the oxygen may be consumed by biodegradation before it can reach the sulphides. The practical implementation of a water cover scheme presents some other questions (for example, maintaining the required depth of water and long-term stability of holding structures) which still have to be addressed through hydrological and engineering studies. Laboratory studies such as this one are also necessary to address initial uncertainties prior to implementation.
