Mesoporous silica recovery from phlogopite by aqua regia leaching

Abstract

Phlogopite is a mineral found in the Phalaborwa complex, which is mined in abundance and has little to no industrial use. Phlogopite in its raw form consists of between 40 50% silicon. Previous studies have yielded various forms of silica through acid leaching of phlogopite. The aim is to leach phlogopite with aqua regia to obtain mesoporous silica with the maximum specific surface area possible. Interest in mesoporous silica utilization has greatly increased in areas such as catalysis, polymer reinforcement, adsorption and medical uses. Phlogopite is leached in a temperature controlled batch reactor with aqua regia, nitric acid and hydrochloric acid to recover the remaining silicon product. The leaching conditions are varied in order to obtain the silicon oxide with the largest specific surface area. The leaching conditions such as acid concentration, nitrate ratio, leaching time, temperature, solid to liquid ratio, stirring speed and particle size are varied to determine the effects on the surface area. With aqua regia as leaching agent the maximum specific surface area of 99.41% silica achieved was 515 m²/g at acid concentration of 8.40 M, nitrate ratio of 0.64, leaching time of 12 hrs and leaching temperature at 50 ?C. This experiment provided mesoporous silica with average pore width of 2.62 nm and average pore volume of 0.36 cm³/g. Similar specific surface area values were achieved with 6.33 M nitric acid, leaching time of 6 hrs and leaching temperature at 60 ?C. A regression (R²= 0.90) was developed which can accurately estimate the specific surface area of the recovered silica with the known acid concentration, nitrate ratio, leaching time and temperature.