INCORPORATION OF ARSENIC IN SILICATE SLAGS AS A DISPOSAL OPTION

Arsenical residues are often produced as by-products in the non-ferrou s metallurgical industry. They are regarded as toxic wastes and must b e treated to immobilize the arsenic. Pyrometallurgical operations prod uce arsenical residues in the form of As2O3-containing flue dust and s peisses. If the flue dust is of high purity, it can be sold for use as an active agent in wood preservatives, although even this is coming u nder challenge from the use of safer substitutes. Often the dust quali ty is not amenable to upgrading and no uses can be considered. Traditi onal practice is to dissolve the dusts in acid prior to precipitation as ferric arsenate. Recent work has highlighted concerns over the suit ability of this process, and such a solution may be seen as inappropri ate by an established pyrometallurgical plant. Hence, alternative meth ods of disposal have to be considered. Tests have shown that up to 10 wt% As can be incorporated into 'glassy' silicate slag with very low l evels of arsenic leaching. The slags that were considered in the prese nt investigation are based on the system SiO2-Fe(x)O(y)-CaO-Al2O3, whi ch is considered to be relevant to non-ferrous metallurgical operation s. Calcium arsenate was added to a range of synthetic slag mixtures an d melted under p(O2) conditions of around 10(-7) atm. The leachability of arsenic from glassy slags that had been quenched in water was very low relative to that from slowly cooled, crystalline slag samples. Th e effects of CaO/SiO2 ratio, p(O2) in the atmosphere above the melt an d iron oxide content on As immobilization were studied. It would appea r that water-quenched silicate slags represent an environmentally acce ptable method for the disposal of many arsenical residues.