Influences of alumina on fayalite-type slags properties : an experimental study on leaching and melting behaviours
Abstract: Recycling of secondary raw materials by non-ferrous metal producers is steadily increasing. These materials inevitably introduce various impurities to the production furnaces, which can change the chemical composition, and thereafter, the physical and chemical properties of the products and by-products.Alumina is one of the most common and significant impurities that enters the furnaces in different ways. Finding a balance between the recycling advantages and possible adverse effects of the increased alumina content in the process is of great interest to the metals industry.In the current work, effects of alumina on two of important properties (melting and leaching behaviours) of an actual industrial fayalite slag, from a Swedish copper producer, is investigated.A reference slag sample, devoid of alumina addition, and three different mixtures of slag with 5, 10, and 15 wt-% alumina addition were prepared. After homogenization (re-melting) of the mixtures, in an induction furnace, the materials were characterized using scanning electron microscopy (SEM) and X-ray diffraction (XRD). A standard single stage leaching test was used to study the leaching behaviour of the samples. Differential scanning calorimetry (DSC), Pt-Rh pans, was applied to investigate the melting and solidification properties of the mixtures.Results show that, olivine, spinel, and augite based solid solutions are the three main constituents of the samples. Addition of alumina to the slag increases the amount of spinel solid solution and changes its composition from iron rich to aluminum rich. The slag shows an alumina saturation limit between 5 and 10 wt-% alumina additions. Chemical composition of the major phases does not change above the saturation and excess amount of aluminum is consumed for formation of a new aluminum containing phase, anorthite. Below the saturation limit, alumina addition causes a relatively sharp increase in the melting temperature of the slag, and at the same time, an increase in the leached amounts of the elements is seen. Alumina addition, close to the saturation limit of the slag, lowers the leachability considerably, while the melting characteristics are not changed dramatically. However, above the saturation limit, a more complex transition occurs as the partial melting and leached amounts of most of the elements increase. In order to achieve an appropriate melting and leaching characteristics, the total alumina content of the slag must be close to its saturation limit. The grounds and influential parameters on such behaviours are thoroughly discussed.
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