Modernisation of the leaching circuit at Pasminco metals—EZ

Abstract

The calcine leaching circuit at the Pasminco Metals —EZ plant has recently been extensively upgraded, with the pachuca leaching and iron purification stages being replaced. The modernised leaching circuit is a conventional counter-current plant, composed of two low-acid calcine leaching stages, (neutral leach and weak-acid leach), an iron precipitation stage (jarosite precipitation) and three high-acid leaching stages (preneutralisation, hot-acid leach and strong-acid leach Integration of the new leaching circuit and closure of the old pachuca leaching section commenced in March 1990. On completion in September 1991, the leaching capacity increased to produce 220 000 t zinc per annum. The thickening and filtration sections, in the high-acid leaching stages of the modernised leach circuit, did not perform satisfactorily from the outset, because of the presence of very fine particulate material containing high concentrations of silica. The silica-bearing particles formed a recirculating load within the high-acid leaching stages, increasing the silica content of thickener feeds. The resultant low density thickener underflows, containing less than 200 g solids/1, increased recycle flows through the plant and severely limited the capacity of the filters used for filtering the secondary leach residue from the high-acid leaching stages. Zinc production was only maintained by reducing the lead and silica content of zinc concentrates and utilising all available filter capacity for secondary leach residue. Silica in the preneutralisation stage liquors is partially precipitated in the jarosite stage. The residual silica in the jarosite precipitation stage liquors was found to be a significant contributor to the particulate silica in the high-acid leaching stages. Various options were explored to increase silica precipitation in the jarosite precipitation stage, with the addition of a coagulant to this stage being the most cost effective. Thickeners previously full of low-density pulps are now devoid of pulp, with underflows containing up to 400 g solids/1. The secondary leach residue filters now have surplus capacity, and the lead content of the residue has increased from 26 to 36%. The restriction on lead and silica in zinc concentrates has been removed.