Changes in an ore dump during a 40-year period of commercial scale and spontaneous natural bioleaching

A dump located in the proximity of the Vlaikov Vrah mine, Bulgaria, and containing about 120,000 tons of run-of-mine mining wastes (low-grade ores and rock mass), since 1968 was subjected initially to spontaneous natural bioleaching based on the rains as irrigating leach solutions, then from 1972 to 2003 to commercial-scale bioleaching by means of irrigation with acidified leach solutions, and since 2004 again to spontaneous natural bioleaching. The monitoring of these processes during this 40-year period revealed considerable changes in the composition, geotechnical and acid–base properties of the dump material, as well as in the diversity, number and activity of its microflora. The data obtained during this study were used to stimulate the bioleaching process in the huge commercial-scale copper dump leaching operation at Vlaikov Vrah mine in which about forty million tons of such mining wastes were treated.