Bioleaching shows promise for Adex Mining

New metallurgical processes and higher prices for its commodities may improve the economics of the Mt. Pleasant mine in southwestern New Brunswick.

The mine produced tungsten, molybdenum and several byproduct metals from 1983 to 1985, and its new owner believes it has the potential to become an economic tin mine and a major producer of indium metal.

Adex Mining (TSE) acquired the property in June by taking over Piskahegan Resources and now plans to complete feasibility studies that would lead to a 1996 startup. The deposit has proven and probable reserves in its North and Fire Tower zones.

The North zone contains 7.1 million tonnes and has an average grade of 0.62% tin and 0.65% zinc, plus 91 grams indium per tonne, with quantities of bismuth, copper, molybdenum and tungsten. Most of these reserves are accessible from the existing underground workings.

The Fire Tower zone, a tungsten-molybdenum deposit, has 9 million tonnes, all proven, with 0.03% tin, 0.08% zinc, 0.2% molybdenum sulphide and 0.4% tungsten oxide, plus 31 grams indium. Also, 29 million tonnes are classified as possible reserves in the Halo, Bismuth and North zones and in the recently discovered Saddle zone.

The biggest development in the feasibility picture at Mt. Pleasant has been the design of a biological leaching process using oxidizing bacteria that already exist in surface waters at Mt. Pleasant. The bacteria oxidize sulphide minerals, releasing the base metals bonded with the sulphides into an acidic water solution. The metals are then back-extracted from the solution.

Early tests on Mt. Pleasant samples show that conventional flotation recovers 86% of the tin, and the sulphide concentrate from flotation can be bioleached with recoveries on most metals ranging from 73% for bismuth to 79% for indium. Copper recovery is lower, at 59%.

Previous troubles with metal recovery at Mt. Pleasant were a major factor in its 1985 shutdown. However, its current owners believe that increased capacity and better recovery should improve the deposit’s economics considerably.

The changing face of tin on the metal markets may also signal better economics for Mt. Pleasant. The price of the metal, which suffered with the collapse of the tin cartel in the 1980s, has edged up slowly since 1993, and the market may yet experience some tightness of supply through 1996 and 1997. The London Metal Exchange spot tin price now hovers just below US$3 per kg. Prices for indium, a metal used in electronics and specialty alloying, have also risen and demand appears to be increasing. Spot market prices now are around US$540 per kg.