Zinc recovery from steelmaking wastes

Abstract

During the production of steel, a dust waste stream is created that contains a significant amount of zinc which makes it difficult to treat and to recycle. Various furnaces have been developed that can deal with those dusts like rotary hearth furnaces and the DK process but none are fully optimal yet. HIsarna, a furnace in development by Tata steel, is able to treat those zinc bearing dust by separating zinc from iron whilst being an ironmaking furnace. Basic oxygen furnace (BOF) dust, blast furnace (BF) dust and historical oxy chalk sludge (HOKS) have been the dusts under investigation for recycling in HIsarna. BF dust is a source of zinc and carbon whilst both BOF dust and HOKS contain a large share of iron. Microgranulates have been made from BOF dust and HOKS with different additions of BF dust which provides carbon that can be used for reduction reactions. With the help of thermo gravimetric analysis (TGA) and a horizontal furnace for isothermal experiments at 1000°C and 1300°C, the best two microgranulates (one HOKS/BF dust, one BOF dust/BF dust) were chosen that could completely reduce all zinc and iron oxides in a nitrogen atmosphere. Because HIsarna has a reducing atmosphere, this thesis focuses on answering the question how the self reduction behaviour of zinc in the microgranulates compares to the reduction behaviour in an atmosphere of 50% CO and 50% CO2. It appears that during isothermal experiments at 1000°C, the zinc separates more efficiently in nitrogen than in the 50/50 atmosphere. However this was not seen at 1300°C. At this temperature, the zinc concentration in the BOF dust microgranulates decreased in similar ways in both atmospheres while the zinc in the HOKS microgranulates reduced much faster in this reducing atmosphere. Besides isothermal experiments, a vertical TGA furnace was used for a non-isothermal experiment up to 1500°C at a large enough scale to analyse the samples after treatment. However, not much difference in zinc reduction ability was found between the different atmospheres. The dust from those experiments was collected and showed to contain zinc and lead present in elemental form, oxides and carbonates as well as potassium chloride. The results show to be promising for the recycling of dusts in HIsarna.