Formation of Massive Gunningite-Jarosite Scale in an Industrial Zinc Pressure Leach Autoclave: A Characterization Study

Abstract

Recent attempts to increase the throughput in an industrial zinc pressure leach autoclave resulted in massive scale formation not encountered previously. According to a comprehensive characterization study, the results of which are presented in this paper, in addition to jarosite and elemental sulphur that commonly form and report in the residue of this process, the scale samples contained significant amounts of the mineral gunningite (ZnSO4·H2O) plus minor amounts of szomolnokite (FeSO4·H2O). SEM-EDS examination of cross sections made from the reduced scale revealed gunningite which acted as a binder forming a matrix within which the natrojarosite lens-like particles were embedded. The presence of gunningite and the other major phases (natrojarosite, hematite, elemental sulphur and szomolnokite) were established unequivocally by applying vibrational (ATR-FTIR and Raman) spectroscopic analysis in additional to conventional XRD method. Finally the quantitative abundance of each of these phases was determined via the development of a 3-step extraction-dissolution procedure. According to this quantitative method, the composition of the bulk of the scale consisted of gunningite (23-55%), natrojarosite (13-37%), hematite (13-26%), elemental sulphur (3-10%), szomolnokite (3-7%) and sphalerite (0.7-3%).