Quality management of chromium containing steel slags from melt phase to cooling

Abstract: Outokumpu Stainless Oy Tornio works produces in close future around 1.7 million tonnes steel slabs annually. As all in the metallurgical processes the production of stainless steel is producing by-products as slags. The different kinds of processes produce about 300 000 tonnes slags annually, almost the whole amount of this has been deposited after the metal separation up to recent days. The project to utilize the steel slag started during 2001 with the aim to increase the amount of recycled by-products and to develop new slag-products.By-products are produced from the different process part as Electric Arc Furnaces (EAF1, EAF2), Argon Oxygen Decarburization converters (AOD1, AOD2), ladle stations and Chromium Converter (CRC). The chemical composition of slags from a process has a slight variation, but between the different processes the variations in analyses are greater. The chemical composition of the slags is dependent to the characters of the slag, as volume stability by basicity of the material. The aim of this work was to investigate the quality control of chromium containing steel slags during cooling into solid phase. The product should be an aggregate with sufficient hardness and low leaching behaviors. The investigation started when the slag was poured into a slag pot at melting shop, any addition of stabilizers or other chemical components has not been done. The aim was to investigate the dependence of different cooling methods to spinel forming. The physical properties of the product of the tests were tested by Nordic ball mill test ( prEN 1097-9). The chemical analyses were: total analyses (XRF), and leaching test (Shaking test prEN 12457-3). A powder X-ray diffraction meter analyses (XRD) was used to estimate the mineralogy of the samples. The solid aggregate samples were studied with Scanning Electron Microscope (SEM). Simulations with respective chemical analysis of the slags were done by the data program FactSage. The slags, included in this work, were mainly EAF2 and CRC slags because of less variation in chemical analysis of these slags. These slags contain chromium, a minor part of chromium is not bound in spinels, which is a reason for leaching. During the first tests, which were carried out by water-cooling, no significant decreasing of leaching of chromium was observed. When pouring out the slag as a thin bed the leaching of chromium was lower compared to the massive bed of the same slag part. Lower leaching of chromium from the massive material from the thin bed can be caused by the smaller active surface of material. Reference test with semi-quenched slag were made and they showed low leaching values compared with the slags from normally pouring practices. Semi-quenched slag is porous and cannot be analyzed in SEM and was hence milled as powder and analyzed in XRD. The mineralogy of semi-quenched slag is varying in some degree from the mineralogy of normally poured slag. The semi-quenched slag, pumice, is partly amorphous which can be the reason for lower leaching values of chromium. Granulation tests have been made earlier with all of the three types of slag, but all of these showed higher leaching values than those of pumice. During granulation material may come in contact with air and become oxidized and chromium oxides leach more than other compounds of chromium. The lowest leaching values of slag can be attained by semi-quenching of material but the product is not an aggregate and cannot be used in all of the civil engineering applications. Aggregates can be produced by cooling on the slabs, but the slag bed should be maximum 10 cm thick, so that the gases can flow out before solidification of material.