Introducing micro-pelletized zinc concentrates into the Zincor fluidized solid roasters

Abstract

Zincor, a refinery in South Africa, uses the conventional roast-leachelectrowinning process to produce zinc metal. The roasting process of ZnS concentrate makes use of four Lurgi fluidized bed roasters to produce calcine (contains ZnO and ZnFe2O4 as zinc products) and SO2 gas. The roasting plant consists of two 18 m2 and two 35 m2 cross sectional area roasters. Prior to 1996, Zincor utilized air as the only oxidant and fluidizing medium in its roasters. The maximum dry feed rates that the roasters could process were 6.5 t/d.m2. In an attempt to increase production, oxygen enrichment was first trialled and then introduced into the fluidizing air. The ability of oxygen enrichment to increase the rate of the ZnS oxidation reaction allowed higher feed rates to the roasters. This was successful and oxygen enrichment was permanently implemented. That enabled dry feed rates to be maintained at 7.0 t/d.m2 and 7.3 t/d.m2 for the small and big roasters respectively. Oxygen enrichment up to 26% in the fluidizing air is utilized. Due to the highly competitive nature of the zinc industry, innovative processing techniques are necessary to be competitive. The aim of this study is to determine whether oxygen enriched air can be reduced by introducing micro-pelletized concentrate into the roaster feed blend, whilst maintaining current roaster feed rates and calcine quality. This study was executed in four parts. Firstly, the role entrainment played in influencing average particle residence time. Secondly, a study of production methods for stable micro-pellets. Thirdly, a study of the influence of oxygen enrichment and particle size on the roasting of micropellets. The fourth part of the study was introduction of micro-pellets into the Zincor roasters to determine whether oxygen enrichment could be reduced. The particle size distribution of a blend of feed concentrate to the roasters is 50% passing approximately 48 μm. This leads to entrainment values between 87% and 91%. The micro-pelletization process reduces the –500 μm fraction from 87% to 10%, which degrades to 30% during roasting. This requires that approximately 48% of the concentrate needs to be micro-pelletized to restore the 70% designed entrainment target. It was determined that entrained particles spend on average 0.46 hr to 2.44 hr in the bed compared to particles in the overflow that have residence times between 3.93 and 4.00 hr. The calculated times for entrainable particles are somewhat higher and for the bed overflow lower compared to the values measured by Spira, 1970. The required reaction time for micropellets was found to be far below their residence time inside a Zincor roaster. With a load of 20% micro-pellets introduced into the feed concentrate, the oxygen enrichment could be reduced by 60%. The quality of the calcine produced was maintained above the target of 98.8% ZnS to ZnO conversion. The results of this study have shown that the use of micro-pelletization of concentrate at Zincor reduces entrainment of particles successfully. Manipulation of entrainment through micro-pelletization can be used successfully to reduce oxygen enrichment, whilst improving production and maintaining quality at Zincor. Copyright