As a long term means of preventing the onset of acidic drainage, potentially acid generating and acid consuming waste rock can theoretically be blended or layered to produce a geochemically benign composite. This report presents the general theory behind blending and layering for prevention and control of acid rock drainage (ARD), and presents selected case studies on the use of this technique at mines in Canada, United States and other parts of the world.

The focus of this report is the blending, mixing and layering of waste rock, and, to some extent, the addition of limestone and other alkaline materials. The addition of alkaline materials was initially considered beyond the scope of this MEND sponsored project (Project2.37.1). However, the scope was expanded as very few case studies were found which exclusively examined blended waste rock, especially in the coal fields. Both coal and metal mines are covered in the review.

Coal Mines

Most of the reviewed coal case studies were located in the Appalachian region of the eastern United States with pyrite sulphur values in the range of 0.1 to 1 %. At these sites, the effect of waste rock blending was often confused or masked by additional management measures such as lime and flyash addition, and/or the presence of acidic drainage from adjacent historic sites. Comprehensive reports, providing compilations and assessments of acid-base accounting (ABA) data and kinetic tests, predictions of long-term effluent quality, pre and post mining monitoring of rock characteristics, and pre and post mining monitoring of drainage quality, were not found in the course of this project. However, numerous papers in the published literature reviewed specific and general management practices at individual sites.

Researchers in Appalachia have compiled estimates of volume-weighted ABA data derived from drill core analyses, and compared these values to effluent quality at associated seepages. These compilations (diPretoro and Rauch, 1988; Ericksen and Hedin, 1988; Brady et al, 1994) have resulted in the development of general criteria for overall volume-weighted waste rock blends considered likely to produce net alkaline drainage but not necessarily prevent metal leaching. The commonly cited criteria in the Appalachia (P. Ziemkiewicz, pers. comm., 1997) are:

Net Neutralization Potential (NNP) > 10 kg CaCO3 equivalent/tonne;

Neutralization Potential (NP) > 15 kg CaCO3 equivalent/tonne; and,

Ratio of Neutralization Potential to Acid Potential (NP/AP) ratio > 2.

The reviewed studies on coal wastes tended to examine the addition of alkaline materials rather than the deliberate blending of acid generating and acid consuming waste rock types. These studies indicated that:

a) Fine alkaline material distributed uniformly throughout a waste pile were effective in delaying the onset of acidic effluent;

b) Layering of limestone within test piles was effective in reducing the amount of net acidity produced, and in delaying the onset of acidic effluent, but did not effectively prevent acid conditions from developing within the potentially acid generating (PAG) portions of the piles;

c) Lime kiln dust was more effective in reducing the amount of net acidity produced and in preventing the onset of acidic effluent than limestone, possibly due to the higher surface area and the greater reactivity of CaO;

d) The order in which overburden was placed in relation to acid drainage generating strata had a significant effect on leachate quality; and,

e) Placement of overburden as soon as possible after excavation was beneficial in reducing ARD.

The critical ratio of alkaline addition to acid potential which would permanently prevent the onset of acidic drainage at specific sites was generally not predicted.

Metal Mines

Few metal mine sites were identified that had deliberately examined or applied blending and/or layering as a prevention method. Typically, older sites that provided long term water quality data to support the success or failure of this technique Jacked sufficient documentation on materials in the waste piles and the level of blending employed to allow conclusions to be drawn. Newer sites that characterized their materials and operational procedures have had limited time in which to demonstrate success or failure.

Site data and laboratory test data for metal mines tended to indicate that:

a) The majority of cases evaluated blends with NP/AP ratios less than 2;

b) Blending did not reduce sulphide oxidation rates in the potentially acid generating material unless highly reactive neutralizing material (limestone) was applied and the blending was near ideal. Perfect mixing was generally only possible in column or humidity cell tests. Such ideal blending was not considered feasible at field scale;

c) Layering thicknesses down to 10 cm did not reduce sulphide oxidation rates in potentially acid generating material;

d) Blending and layering were effective in delaying the onset of acidic effluent, and reducing the metal and acidity loadings exiting from the combined material. The presence of alkaline drainage from neutralizing materials appeared to temper the locally produced acidic leachate, resulting in less acidic drainage and lower metals levels due to decreased solubility of hydroxide and carbonate phases at the elevated pH.

e) Prevention of acidic effluent from the blended or layered materials did not necessarily prevent dissolved metal levels from being problematic.

The reviewed metal mine case studies did not identify safe waste rock blends which would prevent ARD and metal leaching at those sites.

From the literature and case studies reviewed, it appears that blending and layering of acid generating and acid consuming waste rock may be a legitimate method of neutralizing acidic drainage with a consequent reduction in metal leaching through precipitation of metals within the pile. However, there remains considerable uncertainty, and further investigation into the application and practice of this method is required. Specifically, more information on large scale, controlled field studies is needed. The identification and detailed documentation of additional sites should also be encouraged, in association with predictive laboratory tests. At issue is what portion of alkalinity is available to neutralize the acidity produced by sulphide oxidation, and the extent that this is influenced by the thoroughness with which alkaline and acid producing waste rock are blended at a given site.