At the request of the Mine Environment Neutral Drainage (MEND) Program, Natural Resources Canada conducted a survey of mine drainage treatment and sludge management practices. A detailed survey was prepared which requested information such as site background and history, mine drainage characteristics, type of treatment and reagents used, treatment issues, sludge composition, sludge management practices and issues. The response from these questionnaires accounted for about 52% of the database. The remaining 48% of the data were extracted from technical papers, company press releases, website information, and public information. Data on treatment practices and sludge management were collected on over 100 sites. Most sites in the database are located in Canada, but other sites that populate the database are in the USA, UK, Australia, Mexico, Peru, China, South Africa, Germany, Brazil, New Zealand and Hungary. The majority of the mines surveyed were base metal mines (46%) followed by precious metal mines (23%), coal (7%), uranium (5%), and other (19%). Other types included molybdenum, antimony, diamond, tin and non-mining operations with acidic drainage issues.
The majority of sites surveyed reported that they expect to treat in perpetuity and as such, their choice of treatment is critical not only for economic but also for environmental reasons. Roughly thirty percent of the sites control their influent flow through water management practices. Active treatment processes were the most prevalent with chemical treatment more common than physical (membrane) and biological processes combined. Roughly the same number of basic (simple) treatment processes as high-density sludge (HDS) processes were recorded. Of the sites applying chemical treatment, lime was the most prominent reagent used. Flocculant was used in approximately 42% of the treatment operations and a range of flocculant types were recorded. Magnafloc 10 was the most common flocculant used.
Treatment issues recorded included the following:
•gypsum scaling,
•the control of total suspended solids TSS)in the final effluent,
•managing high flows,
•algal blooms in collection ponds,
•poor settling,
•lime handling and mixing,
•polymer mixing during winter,
•difficulty in maintaining high density sludge,
•manganese and sulphate concentrations in the final effluent,
•inefficient mixing and acidity in water due to residual thiosulphate derived from mill processing.
Capital costs ranged considerably from $0.02M to $42M with the average of approximately $7.5 M. The average cost to treat one cubic metre of mine drainage was $1.54.
The majority of the sites surveyed utilized sludge ponds for dewatering and permanent sludge disposal. On average, sites produced about 9,500 tonnes of dry sludge per year with production ranging from 20 dry tonnes to 135,000 dry tonnes per year. Depending on the percent solids of the sludge, the volume factor ranged from approximately 2 to 70 times the sludge mass. Generally, sites reported having sufficient sludge storage capacity for an average of 25 years.
