Reduction of lead minerals by CO/CO 2 gas mixtures: Application of the grain model
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I.
INTRODUCTION
INVESTIGATIONS of the chemical conditions and the associated energy effects within the lead blast furnace of Brunswick Mining and Smelting Ltd., New Brunswick, Canada, may be summarized as follows: (1) Chao e t al. 1 interpreted measurements of gas composition, temperature, and pressure within the furnace in terms of an oxygen potential diagram. This diagram indicated that the thermodynamic driving force for the transfer of oxygen from PbO to the gas phase was large (1150[ to [ 2001 kJ tool -~) throughout the bed. It was concluded that this large driving force was perforce maintained in order to reduce Fem in the sinter feed to Fe ~ for incorporation of iron in the slag. (2) Despite the large thermodynamic driving forces noted by Chao et a l . , Morris e t al. 2 found that the chemical reactions were essentially confined to the region of about 2.5 m depth from the tuyeres. It was interpreted that kinetic factors were retarding the reactions in the upper regions. The principal lead bearing phase in the commercial sinter feed to the blast furnace was found to be Pb3Ca2Si3Ott. (3) As a consequence, the thermodynamic efficiency of the lead blast furnace is substantially lower ( - 1 0 pct) than that of an iron blast furnace, 3 in which the thermodynamic driving force for the transfer of oxygen from FeO to the gas phase is much smaller than that noted above in the lead blast furnace for PbO reduction. In order to elucidate the kinetic factors involved in the reduction reactions of the lead blast furnace sinter, a laboratory investigation has been conducted. This investigation involved reduction experiments using CO/CO2 gas mixtures with PbO, Pb3Ca2Si3Oll, a synthetic sinter, and a commercial sinter. A summary of the experimental results was recently published; 4 in this paper the results are presented in more detail and some predictions are made regarding the influence of structural parameters and temperature on the conversion of lead calcium silicate, with the aid of the grain model.
MANSOOR M. HUSSAIN, Graduate Student, and DAVID R. MORRIS, Professor, are with the Department of Chemical Engineering, University of New Brunswick, Fredericton, NB, Canada, E3B 5A3. Manuscript submitted October 17, 1985.
METALLURGICALTRANSACTIONS B
II.
LITERATURE REVIEW AND THEORY
There are few reports of kinetic studies relating to lead bearing compounds in the literature and none relating to Pb3CaESi3Ou, the principal lead bearing compound in the Brunswick sinter. 2 Studies by Tsvetkov e t al., 5'6'70ates and Todd, 8 and Lambiev and Kurchatov 9 on the reduction of lead compounds implicitly assumed that in the absence of external diffusional resistance to mass transfer, the overall rate was controlled by chemical reaction. Activation energies for reduction of PbO by CO were reported as 112 kJ mol-~ 5,6,7 and 85 kJ mol -~,9 and for reduction of PbzSiO4 the activation energy was reported as 200 kJ mol-l. 5 The initiation temperature for reduction of PbO by CO was reported to be 258 ~ 8 More recently, Rao and Lin 1~stud
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