Mineralogical influence of different cooling conditions on leaching behaviour of steelmaking slags

Abstract: The Swedish steelmaking industry produces large amounts of by-products. In 2006, the total amount of slag produced reached approximately 1 375 000 metric tons, of which 30% was deposited. Due to its strength, durability and chemistry, steel slag is of interest in the field of construction due to it's similarities with ordinary ballast stone. However, some steel slags face an array of quality concerns that might hinder their use. These concerns generally involve the following physical and chemical properties: Volume expansion, Disintegration, Leaching of metals By controlling and modifying process parameters during slag handling in liquid state, the physical properties of steel slags can be adequately modified to obtain a high-quality product for external application. The present work was undertaken as a research project within the Minerals and Metals Recycling Research Centre, MiMeR. The major objectives of this work have been to investigate how different cooling methods and cooling rates influence the properties of slag products. Four types of steel slags, Ladle slag, BOF (Basic Oxygen Furnace) slag and two different EAF (Electric Arc Furnace) slags, were characterized and modified by semi-rapid cooling in crucibles and rapid cooling by water granulation. Experiments were conducted in laboratory scale using an induction furnace. Analysis techniques used in this investigation include: thermodynamic calculations using FactSageTM, X-ray diffraction analyses (XRD), scanning electron microscope (SEM) and a standard leaching test (prEN 12457-2/3). The experimental results show that disintegrating ladle slag is volume stabilized by water granulation resulting in a product consisting of 98% glass. However EAF slag 1, EAF slag 2 and the BOF slag formed only 17%, 1% and 1% glass, respectively. The leaching tests showed that water granulation did not prevent leaching of minor elements from the modified slags. The solubility of chromium, molybdenum and vanadium varied in the different modifications, probably due to their presence in different minerals. Variations in crystal size as well as phase composition and distribution were observed in the different materials as a result of different cooling methods. The magnesium content of the wustite-type solid solution (Fe,Mg,Mn)O in BOF slag increased when rapid cooling was used. The reactivity factor, á, was calculated for the BOF and EAF slag 1. A majority of the elements of interests in the slags became more reactive when cooled rapidly. The reactivity for silica in BOF and EAF slag 1 was increased by ~4700% and ~1200%, respectively, and for chromium by ~5300% and ~1500%.