Pyrometallurgical reduction of zinc leach residue by using pretreated non-fossil reductants
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School of Chemical Engineering |
Master's thesis
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en
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108
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Abstract
Hydrometallurgical zinc production creates between 0.5-1 ton of residue for every ton of zinc produced. The formed residue is stabilized and landfilled. That can result in extensive land use and potential leaching of metals (As, Pb), in addition to the loss of Zn and valuable metals (Ag, Au, Sb) due to their entrapment in the residue. Thermal processing of jarosite has been investigated as a possible solution to these problems and to produce a clean slag for construction use. The research includes the use of biochar as an alternative reducing agent in the pyrometallurgical treatment of jarosite to reduce global warming and the environmental impact by the industry. In this work, two non-fossil biochars were investigated as reducing agents to replace traditional metallurgical coke. The biochars were tested for their ability to form a clean slag that can be reused for construction purposes, and for the collection of valuables in a metallic phase, with the removal of volatiles in the gas phase. Experiments were carried out to test the applicability of the biochars, and they consisted of three steps – pre-treatment, oxidation and reduction – with the biochars being used in the reduction step. In addition, the aim was to explore LCA as an analytical tool to determine its suitability for comparing the two biochars. The experiments were carried out in a vertical tube furnace at 1300 °C. Pre-treatments were carried out under air atmosphere for 60 min and the oxidations were carried out under oxygen atmosphere (65 ml/min) fed into the furnace by a gas lance for 60 min with a Fe/SiO2 ratio of 1.86 after fluxing with SiO2 and an addition of 10 wt-% MgO. The reductions were carried out under Ar atmosphere (300 ml/min) with test durations of 30, 60 and 240 minutes. The reductions were done using two biochars, LIGBR600 derived from hydrolyzed lignin and BP600 derived from softwood bark – both previously pyrolyzed at 600 °C – with different mixing techniques during the course of the experiments and with different stoichiometric carbon ratios. A literature review was conducted to investigate the usefulness of LCA in assessing the impact of different reducing agents. The objectives of reducing the concentration of S below 1 wt-% were achieved after oxidation experiments, and those of Pb to below 0.03 wt-%, and Zn below 1 wt-% after reduction, although the jarosite used in this work was high in S and Zn. Phase separation into slag and metal phases was successful to some extent, with slag and magnesiowüstite phases forming in addition to smaller patches of calcium magnesium silicate and forsterite. The metallic phases included base metal and Fe speisses based on As and Sb, and other metallic phases such as Pb-rich droplets. Mixing ground oxidized jarosite with biochar gave more homogeneous results, with a stoichiometric carbon ratio of 1.0 giving the best results. In addition, slightly better results were obtained with the LIGBR600 biochar in terms of Zn removal, but BP600 also gave satisfactory results. It was concluded that LIGBR600 has a lower ash content and therefore less reaction retardation, in addition to having a higher surface area than BP600. A longer reaction time was found to be beneficial for slag cleaning and metal phase formation, but 60 min of reduction was sufficient if a sufficiently high carbon ratio was used. LCA was found to be a useful tool for comparing reducing agents when the functional unit (FU), system boundaries, and data are appropriately selected.Description
Supervisor
Lindberg, DanielThesis advisor
Klemettinen, LassiSalminen, Justin