Acidic effluent generation from sulphidic tailings, waste rock piles and mine workings can continue long after cessation of mining activities, posing a long-term liability to Canadian mineral operations. Lime neutralization is a common treatment practice to neutralize acid and precipitate heavy metals. This methodology is simple and economically advantageous. However, it produces large volumes of potentially hazardous sludge, has high maintenance requirements and may fail to meet new, more demanding environmental standards. Hence, the improvement or replacement of lime neutralization technology with new, advanced techniques is becoming a necessity. A data bank on the characteristics of acid mine drainage (AMD) streams in Canada will serve as an important input in helping to define strategies for improving AMD treatment techniques.
Fourteen companies participating in the MEND program were contacted by phone and in writing with respect to their currently acid generating sites. Data on the raw AMD streams at these sites were solicited using a written questionnaire. Detailed information on 72 untreated AMD streams from across Canada has been compiled and is presented in this report. The data were also input into a Lotus 1-2-3 spreadsheet designed to allow further analysis of the stream parameters based on selected criteria. The data on stream sources, flowrates, heavy metal concentrations, acidity, oxidizing conditions and suspended solids content have been extracted from the spreadsheet and are presented in a concise tabular format to allow seasonal comparisons. Graphical concentration distributions are provided for several of these parameters.
Seasonal trends were observed for some of the stream characteristics. As expected, flowrates are almost always higher in the spring and fall compared to summer and winter. When only those streams with measurements for all four seasons are considered, five of nine heavy metals reveal seasonal trends. Copper and zinc concentrations tend to be highest for individual streams in the spring while lead, iron and cobalt concentrations tend to be highest in the winter. Zinc concentrations tend to be lowest for individual streams in the summer and fall, while copper and cobalt concentrations tend to be lowest in the fall alone. Sulphate concentrations and acidity tend to exhibit matching trends: they are highest in winter and lowest in spring. It would appear that confounding factors such as adsorption phenomena, interactions along the AMD migration pathway or other site specific features which are not seasonal have an overriding impact on many AMD stream characteristics.
A small number of the streams have concentrations of copper or zinc which are high enough for metal recovery to be potentially feasible as a treatment approach. However, many of the streams are diluted by other sources of water (e.g., runoff, mill effluent, etc.) reducing the base metal concentrations. Hence, the metal recovery option may require the interception of AMD streams closer to the source of metals dissolution yielding lower flowrates and higher concentrations.
Very little data were provided on oxidizing conditions in AMD streams (e.g., either as Fe2+ concentrations to allow calculation of Fe2+/Fe3+ ratios, or as redox potentials), probably because measurements of these parameters are difficult and the results are often suspect. However, this information is necessary for the consideration of many treatment options. Therefore, mining companies may need to be encouraged to collect these data.
Information on problems and successes in dealing with AMD at the various sites was extracted from the questionnaire responses and is summarized in terms of generic issues. The two most common concerns/accomplishments involve the collection of all contaminated flows at a site and the quality of the final effluent.
This report, along with the associated computer spreadsheet, provides a good data bank of Canadian AMD stream characteristics which may be used to assist in developing new or improved AMD treatment techniques. As well, it will allow the formulation of synthetic AMD solutions, which are relevant to the Canadian context, for use in general AMD research.
