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Fluidized bed roasting of zinc sulfide concentrate : factors affecting the particle size distribution Constantineau, Jean Pierre

Abstract

An experimental programme was established to study the effect of process parameters on the agglomeration behaviour of particles within a fluidized bed zinc concentrate roaster. A 102 mm internal diameter pilot scale roaster was designed, constructed and commissioned. Tests were then conducted with two industrial zinc concentrates at different temperatures (875-975C), superficial gas velocities (0.25-0.5 m/s), oxygen enrichments (inlet oxygen concentration: 21-30 vol%), stoichiometric excess oxygen (0-80%), initial inert bed materials (silica and alumina) and average bed particle sizes (81-223μm). With the exception of one experiment, all experiments were carried out with a zinc concentrate containing 53-54 wt% Zn, 30.5-31 wt% S, 4.5 wt% Fe, 3.5 wt% Pb, 0.34 wt% Cd, 0.145 wt% Cu, and 1.5 wt% SO4/S, with 80% of particles smaller than 24 pm. One experiment tested a different zinc concentrate, containing 51 wt% Zn, 30 wt% S, 8.1 wt% Fe, 4.7 wt% Pb, 0.14 wt% Cd, 0.05 wt% Cu, 1.5 wt% SO4/S, with 80% of particles smaller than 33μm. The experimental results indicated that the temperature and excess oxygen had the greatest effect, followed by bed material, its size distribution and the superficial gas velocity. Agglomeration increased when excess oxygen approached 0%. Lead, present in small proportion in the concentrate (3.5 wt%Pb), was observed to segregate to the larger bed particles. Lead sulfide volatilized from the zinc concentrate and deposited as a lead oxide/sulfate melt onto inert bed particles which caused fine particles to stick to the surface of larger particles. The lead concentration in the agglomerated particles suggested that the agglomeration mechanism, similar to the coating mechanism, relies on the transport of lead species from reacting particles to inert particles. The generalized slugging-bubbling fluidized bed reactor model (GSBM) handled seamlessly the transition from bubbling to slugging fluidization, using probabilistic averaging. The ratio of the bubble diameter to the column diameter was employed to correlate the probability of each of these fluidization flow regimes. The generalized fluidized bed reactor model was coupled to a solids reaction model and used to evaluate the effect of roasting parameters on the oxygen concentration in contact with the particles. Modelling of the fluidized bed under industrial (bubbling) and laboratory (slugging) conditions indicated that the effect of various parameters on the particle-averaged oxygen concentration depended greatly on the reactor in question (industrial vs laboratory). For the laboratory roaster, the effect of particle size was negligible, while the effect of excess oxygen was significant. For the industrial roaster, the effect of excess oxygen depended on the average particle size. For a relatively large average bed particle size (150 μm), the effect of excess oxygen was limited. For a small average bed particle size (65 μm), the effect of excess oxygen was large, comparable to that in the laboratory roaster.

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