1. School of Process, Environmental and Materials Engineering, University of Leeds, Leeds, LS6 2EQ, UK. Phone: 0113 3432800; Fax: 0113 2467310; E-mail: d.w.dixon-hardy@leeds.ac.uk
2. Department of Mining Engineering, Eskişehir Osmangazi University, Meselik Campus, 26480 Eskişehir, Turkey.
3. Department of Mining Engineering, Dumlupinar University, 43100 Kütahya, Turkey.