Characterisation and optimisation of the Zincor iron removal process

Abstract

As one of the most abundant elements on earth, iron is nearly always present in metal concentrates. This is specifically true for zinc sulphide concentrates, which can contain up to 18% iron (marmatite). Today more than half of these concentrates are treated in hydrometallurgical- or combined hydrometallurgical¬pyrometallurgical circuits. In hydrometallurgical circuits, iron is solubilised (either in a roast-Ieach-, a direct leach- or bacterial leach circuit) along with zinc and must be removed from the zinc¬rich solution before the electrowinning- or solvent extraction step. Various iron removal processes were developed to address the iron problem in hydrometallurgical circuits. The better known of these include the jarosite-, goethite- and hematite processes also used in the zinc industry. Zincor (Zinc Corporation of South Africa) patented an iron removal process (Zincor Process), which was generally considered to be very similar to the so-called "para-goethite" iron removal process used only in two other zinc smelters notably Porto Vesme (Italy) and Pasminco Hobart (Tasmania). However, since the Zincor Process was patented in 1976, various changes have been made such as a change from a batch parallel to a continuous series process, a change in precipitation pH-profile and the introduction of a pH controlled acid wash in the second tank. The introduction of a weak acid leach step and vacuum belt filters at Zincor's residue treatment plant in the near future and an iron removal process that is not clearly understood, necessitated this further study of the Zincor iron removal process. The study has been conducted in three parts. The first part of the study has focused on the characterisation of the Zincor iron residue and the Zincor process. The second part of the study has been concerned with the definition of an optimum operating window in terms of the filterability of the residue and the third part investigated the use of neutralisation reagents other than zinc calcine to control the pH during iron precipitation. The distribution of iron in the Zincor iron precipitate, which usually contains between 35% and 40% iron, has been found to be as follows: approximately 45% as schwertmannite, 5% as ferrihydrite, 20% as jarosites, 25% as franklinite, trace amounts of pyrite as well as 5% of an unknown phase. This confirmed that goethite is not present in the Zincor iron residue and that iron is mainly removed in the form of amorphous intermediate iron phases such as schwertmannite and ferrihydrite. Of these two phases, schwertmannite was the least expected as most work up until now were done on samples taken from natural environments. The following description of the conditions that promote iron removal, mainly as schwertmannite, is viewed as an expansion of the available literature data, which was gathered at ambient conditions. In terms of the main operating parameters, optimum filterability was achieved under the following conditions: pH of 3.0, temperature as high as possible (70°C) and at least 25 kg/m3 seeding. A retention time of at least 4 hours at a pH of 3.0 and 60°C was required, which decreased by more than 50% at a temperature of 70°C. As these conditions mainly impact on the soluble zinc loss encountered during iron removal, an effort was made to further reduce the insoluble zinc loss, which is the inherent weakness in the Zincor process, and similar processes where zinc calcine is used for pH control, by investigating the use of alternative neutralisation reagents. This study showed that iron can be successfully removed with Ca(OH) 2, a basic zinc sulphate and zinc oxide mixture as well as chemically precipitated CaC03 produced in the paper and pulp industry. Of these alternatives, CaC03 appeared to be the most promising, with filtration rates an order of magnitude higher than the zinc oxide options (calcine and basic zinc sulphate mixture), due to better overall economics than with the use of Ca(OH) 2. Utilisation CaC03 as an alternative neutralisation agent might increase the overall zinc recovery figure at Zincor by up to 1.5%. Based on the findings, it can be concluded that the Zincor process in its current form has a very distinct character compared to what was historically considered to be the very similar patented para-goethite iron removal process, as practiced at the Porto Vesme and the Pasminco Hobart hydrometallurgical zinc plants.