Zinc vapourisation from sludge wastes under thermal processing conditions

Abstract

HIsarna is a new furnace technology in the steel industry, featuring a large reduction of CO₂-emissions of up to 80%, and offering larger tolerances for gangue materials. Additionally, it has potential for zinc enrichment in the flue dust, as it can quickly vapourise zinc in the feed material and reject it to the flue dust. Increasing the zinc fraction in the flue dust up to 50% would allow for zinc recovery in the zinc smelters. The larger tolerances of the furnace make it possible to include material streams currently mostly land filled. In this thesis, the focus has been on three of these waste streams, goethite, Blast Furnace (BF) dust, and Basic Oxygen Furnace (BOF) dust. The research focussed on acquiring fundamental knowledge on the behaviour of these waste dusts at high temperatures and various retention times. This will aid their future use as an alternative feed stream material for enriching the HIsarna flue dust with zinc, without compromising the longevity of the furnace or the quality of the steel.
In a literature study, several alternative materials were investigated. Of particular interest are the sludge wastes from the steel industry for their large iron and zinc contents, the sludge waste from the zinc smelters for the same reason, and galvanised steel scrap, possibly in combination with EAF dust. There are a few challenges to overcome, such as the copper concentration in the zinc smelter residues, or the variation in composition and quality of steel scrap. However, these materials were identified as high potential for inclusion in HIsarna.  It was found that zinc can effectively be reduced and vapourised from the waste dusts regardless of its mineral form, in case there is carbon. The carbon acts as a reductant for the iron oxides, including zinc ferrite, and for zinc oxide. Goethite does not contain any carbon, however, mixing with BF sludge will allow the vapourisation of zinc. A model is proposed for estimation of the amount of carbon, and by extension BF dust, that is needed to achieve a full reduction of the iron oxides with native carbon sources. This will aid future mixing of these waste streams, allowing more control over the composition of the feed stream.
A competing mechanism to direct reduction of zinc oxide was identified. Zinc oxide can react with S₂ gas which forms during the thermal shock treatment, forming a zinc sulfide fine particle. At high temperatures, this intermediate species decomposes over time to release the zinc vapour, however, the exact mechanism could not be deduced from the experiments. The mechanism and timescale was verified at various temperatures, and holds up until 1300 ºC. The timescale shortens significantly when increasing the temperature. Therefore, from the work conducted, it was concluded that the zinc can be effectively removed at a relevant timescale for HIsarna and collected in the flue dust.