Recovery of iron oxide concentrate from high-sulfur and low-grade pyrite cinder using an innovative beneficiating process

A high quality iron oxide concentrate, suitable as a feed for blast and electric reduction furnaces was recovered from high-sulfur and low-grade pyrite cinder. Pyrite cinder was treated with a hot strong alkali solution which dissolved most of the silica and alumina, leaving a solid residue and forming a precipitate which was an acid-soluble salt of aluminosilicate hydrate. The residue and precipitate were then treated with a diluted aqua regia to dissolve the precipitate and other impurities. Impurities such as the soluble silicates, aluminates and heavy metals can be extracted and produced high value-added products with several simple methods in later research. After analyzing the chemical composition and crystalline phase of raw material, the effects of different parameters on recovery efficiency of iron were carried out. The optimum reaction parameters were proposed as the following: melting at 200 °C for 1.0 h with 3.0 M NaOH in a liquid–solid ratio of 1.0:1.2 at atmospheric pressure, followed by leaching with aqua regia solution (3.0 vf.%) in a ratio of liquid-to-solid of 1.2:1.0 at room temperature for 1.5 h. In optimum parameters mentioned above, the raw residue only containing 38% Fe could be purified to about 65% and the recovery yield of Fe was 86%. Furthermore, the sulfur content of product could be reduced to 0.090% from 8.0%. To reveal the mechanism of this process, the phase transformation and microstructure of the process were studied by XRD and SEM with EDS.

Research highlights
► One of the most important problems is how to dispose high-sulfur and low-grade pyrite cinder in the sulfuric acid industry in China. This work is to state an economical and environmentally acceptable way to recover high quality iron oxide concentrate from high-sulfur and low-grade pyrite cinder by combining sodium hydrate melting and acid leaching process.
► This process investigated presents some distinct advantages over the physical processes. Firstly, an as-received only containing 38% Fe could be purified to about 65% and the recovery rate of Fe was 86%. Furthermore, the sulfur content of product could be reduced to 0.090% from 8.0%. Secondly, through the analysis of chemical composition, the iron oxide concentrate was considerably less toxic than the original pyrite cinder in that most of the impurities such as silicates, aluminates and heavy metals were leached into the solution. The soluble silicates and aluminates can be extracted and produced high value-added products, such as 4A zeolite. Meanwhile, these heavy metals were enriched into the solution, which provided the possibility for extracting them with several simple methods in the following research. Therefore, we can basically realize comprehensive utilization and zero emission of pyrite cinder waste.