Steady-state modeling of zinc-ferrite hot-acid leaching
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I.
INTRODUCTION
THE extraction of zinc by hydrometallurgical means has a relatively long history. More than a century ago, in 1881, Le´on Le´trange from France proposed and patented the production of zinc by dissolving zinc compounds into acid solutions and recovering the metal by electrolysis.[1] Today, the electrolytic or ‘‘roast-leach-electrowin’’ (RLE) process accounts for the biggest share of the world zinc production, even if some more modern hydrometallurgical methods are slowly gaining ground in this area.[2,3] One of the major problems in the classical RLE process and its variants is zinc ferrite, a complex zinc-iron oxide that forms upon oxidative roasting of zinc sulfide concentrates. This compound, which accounts for about 15 pct of the total zinc produced in a typical RLE plant, remains practically inert under mild acidic conditions that are used for zinc oxide leaching. It dissolves only in relatively concentrated sulfuric acid solutions at temperatures close to 373 K (100 7C), i.e., by hot-acid leaching (HAL).[4] Still, it has been reported that even under these more harsh conditions, the ‘‘leachability’’ of zinc ferrite may not be stable, but may vary significantly,[5] causing a fluctuation in the required HAL circuit residence time between 4[6,7,8] and 12 hours.[9] Zinc plant operators have observed that zinc-ferrite residues with different origin do not dissolve at the same rate under the same leaching conditions.[10] This is what forces them to intervene and alter the HAL operating conditions. If the zinc recovery from ferritic residues is too low, they increase the acid concentration in the leaching tanks and the residence time.[11] Apparently, for the same reason, some RLE plants have also introduced additional stages for zinc-ferrite treatment such as ‘‘strong-acid leaching,’’ where the concentration of sulfuric acid exceeds 100 g DIMITRIOS FILIPPOU, formerly Doctorate Student, is Research Associate, and GEORGE P. DEMOPOULOS, is Professor and Head, Department of Mining and Metallurgical Engineering, McGill University, Montre´al, QC, Canada H3A 2A7. Manuscript submitted March 19, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS B
L21.[12] All the above point out the need for process optimization via modeling and control. In the present publication, the development of a mathematical model of the HAL process is described, which is based on the dissolution kinetics of zinc ferrite. The work that led to the determination of the dissolution kinetics is described elsewhere.[13,14] The interested reader may also find full details of the HAL kinetics and modeling work in Reference 15. According to these kinetic investigations, industrial zinc-ferrite particles were found to be porous with an approximate composition Zn0.9Fe2.1O4 (i.e., (Zn0.9, 31 Fe21 0.1 )Fe2 O4) and dissolve according to the reaction Zn0.9 Fe2.1 O4 (s) 1 4H2SO4 (aq) →
[1]
→ 0.9ZnSO4 (aq) 1 2Fe2 (SO4)3 (aq) 1 0.1FeSO4 (aq) 1 4H2O
The rate (r) of this reaction was found[13,14,15] to depend on the value of hydrogen-ion activity (aH1)
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