HYDROFLUORIDE PROCESSING OF TAILINGS FROM WET MAGNETIC SEPARATION OF TITANOMAGNETITE TO OBTAIN AMORPHOUS SILICON DIOXIDE

Abstract

A method for the opening of silicon-containing technogenic feedstock using an aqueous solution of ammonium bifluoride (NH4HF2) with subsequent production of amorphous silica has been developed. In this work we examined wet magnetic separation tailings (WMST) – iron titanium-magnetite ore enrichment rejects that are formed in huge quantities and stored in sludge depositories at concentrating plants. The main rock-forming minerals of WMST are resistant silicates – pyroxene, amphibole, plagioclase and olivine with the total SiO2 content of about 50%. It was established that fluorination of silicon-containing minerals begins during mechanical dispersion of a dry reaction mixture at room temperature. The thermodynamic calculations confirm active interaction of components in the composition of mineral complexes already at 25 ℃, some of the processes being exothermic with entropy reserve. When the temperature increases to 90 ℃ and the duration of leaching to 5 h, the extraction of silicon in the form of soluble ammonium fluorosilicate grows. Insoluble calcium and magnesium fluorides and sparingly soluble under leaching conditions ammonium fluorometallates, as well as a part of unreacted rock are formed in the insoluble residue. Under the most effective desilication conditions (20% NH4HF2 solution, L:S=4:1, 2 h), the residual content of SiO2 decreases to 1–2%. The synthesis of finely dispersed amorphous silica (SiO2) was performed by hydrolytic deposition when the value of рН of clarified ammonium fluorosilicate solution was reduced by adding ammonia. It was shown that gradual introduction of hydrous ammonia (5 ml/h) into diluted fluorosilicate solution with a concentration of less than 5 g/l Si while constant stirring leads to the formation of spherical silica nanoparticles. The degree of silica extraction from tailings in the process of hydrofluorination of 20% NH4HF2 solution at 90 ℃ for 2 h was 98%, and the content of impurities in amorphous silica did not exceed 1–2%. Separation of finely dispersed suspension of insoluble fluorides and hydroxide impurities formed during stepwise hydrolysis of fluorosilicate solution to рН value less than 5 is a necessary condition for producing pure silica SiO2. The final product, similar to “white soot” in properties, is characterized by predominant particle size in the interval of 2-20 nm, a large surface area 230-360 m2/g and a low pycnometric density 1.62 g/cm3. For citation: Medyankina I.S., Pasechnik L.A. Hydrofluoride processing of tailings from wet magnetic separation of titanomagnetite to obtain amorphous silicon dioxide. ChemChemTech [Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol.]. 2023. V. 66. N 2. P. 70-77. DOI: 10.6060/ivkkt.20236602.6706.