Study On Recovering Iron Concentrate From Pyrite Slag

A considerable amount of waste slag is produced from the pyrite processing procedure every year and accumulated as wastes. The main components of the slag are iron oxides, recovery of which is of high commercial value. In this study, based on its composition, structure and morphology, the waste slag was divided into two categories, namely, black slag and red slag, for separation and recovery, with the iron concentrate in the final products meeting the general requirement for the blast furnace iron-making. Meanwhile, the side products from the treatment were investigated for further utilization.For the recovery of black slag which has high content of Fe3O4, a two-step method of combining physical ore-dressing with chemical ore-dressing was developed, including magnetic separation and alkali dissolution. Namely, magnetic separation was used firstly to enrich iron oxide preliminarily; and then, by using the sodium hydroxide to dissolve a part of silica, the iron content of slag was further promoted. The side product sodium silicate from the second procedure was recycled for preparing silica. Experimental results showed that, the iron content of the raw black slag (40%) after washing and ball milling for8times (with each5min) could reach to55%. This new slag was further treated with10%sodium hydroxide solution at the solid-liquid ratio of1:2and the temperature of80℃for8hours. Eventually, the iron content arrived at more than60%with the impurity content of S below0.5%, which is complied with the general requirements of blast furnace iron-making. The resulting sodium silicate solution was used to prepare silica gel by using a high pressure carbonation process. Supercritical CO2drying was employed to obtain the final silica product with a surface area of686m2/g and a pore volume of3.27cm3/g. This material can be used as the coating matting agent, catalyst carrier, insulation material, and so on. For the recovery of red slag which has a high content of Fe2O3, a three-step method of chemical ore-dressing was developed, including sulfur dissolution, transformation of the precipitate, and dissolution of the precipitate. Namely, at normal temperature, the slag was treated with sodium sulfide solution (5%) for40min to remove the elemental sulfur; the slag was further treated with sodium carbonate solution (5%) to remove the calcium sulfate, and finally hydrochloric acid was used to dissolve the resulted calcium carbonate. The iron content of the final sample was improved from55%to62%, and the sulfur content was reduced from2.8%to0.46%, which is complied with the general requirements of blast furnace iron-making.The economy of the two recovery processes were preliminary analyzed with only considering the raw material costs. For the recovery of black slag, the main profit lies in the high value-added silica products.0.099ton of silica can be recycled while one ton of iron concentrate is produced; the profit can reach several thousand yuan/ton. For the recovery of red slag, due to its simple operation, less operating costs are needed, and the iron concentrate and the by-product polysulfide both make profit. When the content of the elemental sulfur in the raw material is1%, the profit reaches485yuan/ton.In this paper, through the research for the two types of pyrite slag, we established comprehensive processes for recovery of iron, silicon and sulfur to hopefully solve those problems such as process complexity, low product quality, low recovery rate and high cost in the traditional pyrite slag recycling processes.