MEND Report 2.35.2a
October 1993

Summary

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 Center (NTC) and the Centre de recherches minérales (CRM) 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, Quebec. 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, Quebec. All tests were conducted in triplicate.

Monitoring of the effluent water quality to about 60 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 (the dominant sulphide present) content. The reason for the difference is not exactly known but could be attributed to a difference in the pore structure of the two rocks.

The most effective control technique over the 60-week period was water cover, 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%. Only a slight increase in effectiveness was observed (from 93 to 96%) 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 9 to 71%. 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 present 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. These 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 sulphidebearing waste mine rock.

Aqueous geochemical modeling of effluents, collected during the first 4 weeks of testing, indicated the concentration of iron could be regulated by amorphous ferric hydroxide, that of lead by anglesite, and sulphate concentration by gypsum. In the effluents from the limestone-amended waste rock, calcium, magnesium, zinc and manganese concentrations were found to be controlled by carbonate minerals: calcite, dolomite, smithsonite and rhodochrosite, respectively. Very few minerals were observed to be saturated in the effluents from the wood bark covered waste rock because of the low pH resulting from accelerated acid generation. Sulphate concentrations were up to 29 g/L and modeling results indicated saturation with respect to gypsum and sodium jarosite.

Monitoring will be continued for a further two-year period as part of the ongoing work to establish the effectiveness and longevity of the limestone in controlling acid production. Future work will also include a detailed interpretation of the geochemistry of the various metals in the drainage water.

A draft of this report was peer-reviewed. This final copy has incorporated most of the comments raised by the reviewers. Other comments were, however, found to be more related to the proposed future work and will be addressed in the next and final report on the project.

Last Modified: 2003-11-26		Important Notices

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