CONDITIONS OF NITRIC ACID TREATMENT OF PHOSPHORUS SLAG FOR REMs RECOVERY AND PRODUCTION OF PRECIPITATED SILICON DIOXIDE

Abstract

Phosphorus slag is the most heavy-tonnage waste of yellow phosphorus production, which is stored in the dump fields for many years causing ecological problems in the regions. One of the relevant and priority directions is rare-earth metals (REMs) production, presence of which in the phosphorus slags allows considering the latter as an acceptable raw material source. Phosphorus slags contain about 30–40 wt% of silicon dioxide, therefore they can serve as a source of production of precipitated silicon dioxide highly required in different industries. The purpose of this work is studying the conditions for REMs recovery from phosphorus slag and further processing of silicon-containing cake to improve a quality of the obtained products.The work shows results of researches on the phosphorus slags’ chemical and phase compositions identification, processes of leaching of phosphorus slag and the obtained silicon-containing cake by nitric acid. Instrumental and chemical methods of phosphorus slag content analysis give following data. It consists of 90–92 % of pseudowollastonite α-CaSiO3, and also there are gyrolite Ca4(H2O)4[Si6O15](OH)2, small amounts of serpentine Mg6[Si4O10](OH)8, hydrated calcium aluminosilicate impurities CaO∙2Al2O3∙2SiO2∙H2O, quartz α-SiO2, calcite CaCO3, hematite Fe2O3, iron phosphate FePO4 and metallic iron with manganese impurity. As a result of kinetic studies of leaching process of phosphorus slag, the apparent activation energy for ΣREMs, calcium, aluminum and iron was determined which amounted to 4.31, 8.53, 7.43 and 12.31 kJ/mol, respectively. This, in combination with value of the Pilling-Bedward Criterion CP-B = 1.1 for orthosilicate acid H4SiO4, indicates that the process is characterized by an intradiffusion region. With a decrease in temperature of nitric acid treatment from 90 to 70 °C, purification degree of precipitated silicon dioxide from iron and aluminium impurities increases. Results of the experimental data will serve as a basis for development of the technology for complex processing of production waste of phosphorus industry and for improving quality of obtained products as REMs concentrate and precipitated silicon dioxide