Agglomeration of particles during roasting of zinc sulfide concentrates
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I,
INTRODUCTION
R O A S T I N G of zinc concentrates in a fluid bed reactor is always associated with some degree of agglomeration of the reaction products. A certain amount of agglomeration in the bed is desirable. This results in the formation of granules measuring 1 to 2 mm in diameter which help to stabilize the bed. Under certain operating conditions, however, excessive agglomeration occurs, causing large particles to settle and prevent further fluidization, eventually blocking the air tuyeres. In extreme cases the entire bed can "freeze" and the operation has to be halted in order to remove the solidified bed by mechanical means. The objective of this study is to identify the causes of agglomeration, particularly of the catastrophic kind. This excessive agglomeration of particles is normally associated with the facility of forming stable liquid phases at the reaction temperatures which promote sticking. Zinc sulfide concentrates are composed mostly of ZnS (75 pct) and FeS (25 pct) with small amounts of FeS_~, PbS, and traces of gangue-forming compounds such as CaO and SiO2. It should be noted that FeS in the naturally occurring mineral also forms a range of solid solutions with ZnS normally designated as (Zn, Fe)S. A review of the literature was carried out on the phase equilibria of the Fe-Zn-S-O system at temperatures between 1173 and 1373 K in order to identify the composition regions of possible reaction products that can form liquid phases. Additional elements in the concentrate can be present either as a solid solution or as a separate phase, and each can play an important role in contributing to agglomeration during the roasting process. For example, the Fe-Pb-S system presented by Brett and Kullerud ~ shows a liquid phase as low as 990 K, and this liquid region increases rapidly with increasing temperature. Studies by Dutrizac 2 on the PbS-ZnS binary also show a eutectic at 13 tool pct ZnS at 1323 K. The addition of FeS to the PbS-ZnS M. BENLYAMANI, Graduate Student, and E AJERSCH. Professor, are with Dep. de g~nie m6tallurgique, Ecole Polytechnique, C.P. 6079, Succ. A. Montr6al, PQ. Canada. H3C 3A7. Manuscript submitted March 10, 1986. METALLURGICALTRANSACTIONS B
system produces a ternary eutectic which melts at a temperature as low as 990 K. During the latter stages of roasting, Mountvala and Ravitz 3 have also shown that a reaction between FezO3 and PbO can also form a eutectic which melts at 1003 K, and Bouleke and McDowell 4 have shown that the PbO-ZnO system can form a liquid phase at 1134 K for a composition of 11 mol pct ZnO. The experimental work in this study will focus only on FeS, ZnS, and one specific (Zn, Fe)S solid solution which is representative of the industrial zinc concentrates normally used. Since these are the major constituents, the identification of agglomeration phenomena in these cases is sufficient in order to explain its occurrence in industrial cases.
II.
EXPERIMENTAL
A. Materials
Iron sulfide was prepared synthetically by slowly heating a homogeneous mixture of iro
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