Influence of alumina on the zinc slag fuming processes : an experimental study on physical properties and leaching behaviour of the generated fayalite-type slag

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 of the operation. Alumina is one of the most common and significant impurities that enter the furnaces in different ways, including recycling of waste electrical and electronic equipment (WEEE). In order to control and optimize the outcome of the process, a deep understanding of influences of alumina on slag properties is an essence. In the current work, a combination of different experimental techniques and thermodynamic calculations was carried out to investigate the effects of alumina on some of the most important properties of an industrial iron-silicate or fayalite-type slag, from a Swedish base and precious metals producer. For this purpose, a reference slag sample, devoid of alumina addition, and three different mixtures of slag with 5, 10, and 15 wt-% alumina additions were prepared and used for evaluation of density, surface tension, thermal diffusivity, viscosity, melting behaviour, mineralogy and leaching characteristics. Results show that in this type of slag, aluminium can be tetrahedrally coordinated and behave as a network former cation. As a result, increasing the alumina concentration leads to polymerization of the melt, and consequently, an increase in viscosity and thermal diffusivity. It is observed that alumina addition to the slag leads to formation of units with relatively higher molar volumes, which reduces the measured density of the material. Slag samples with higher alumina contents exhibit higher surface tensions, primarily due to the relative lower concentration of the surface-active components. By 5 wt-% alumina addition to the system, the liquidus temperature decreases slightly. Further increase in alumina content (10 and 15 wt-% alumina additions) causes a change in primary crystallization phase field of the material from fayalite (Fe2SiO4) to hercynite (FeAl2O4). This change is associated with a rise in liquidus temperature; however, the solidus temperature of the slag remains almost constant. The most important industrial characteristics of the slag (copper loss to the slag phase and foaming behaviour of the material) are definitely affected by such changes in slag composition and physical properties. Influence of alumina on these two phenomena were not investigated experimentally in the current work, but discussed, theoretically, based on the available literature. Mineralogical studies of the semi-rapidly solidified samples reveal that alumina addition to this type of slag changes the composition and concentration of major phases that are present in the material, which directly controls the leachability of different elements from the slag. It was found that in the current system, spinel and anorthite are the most leaching resistant phases, which are followed by olivine, clinopyroxene and orthopyroxene, respectively. Therefore, among the investigated samples in this study, the mixture of slag with 10 wt-% alumina addition shows the lowest leachability of most of the elements. As expected, by changing the cooling condition of the samples (i.e. water granulation), a totally different dissolution rate of the elements was observed. It is shown that leaching of the generated amorphous structures is a complex phenomenon that is controlled by other mechanisms than that of the semi-rapidly cooled samples.