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Submitted on: 05 January 2000.
Revised on: 18 April 2000.
Accepted on: 27 May 2000.
___________________Journal of Mining Institute---SAINT-PETERSBURG MINING UNIV
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Submitted on: 07 June 1998.
Revised on: 28 July 1998.
Accepted on: 22 August 1998.
___________________Journal of Mining Institute---SAINT-PETERSBURG MINING UNIV