Re-generation of hydrofluoric acid and selective separation of Si(IV) in a process for producing ultra-clean coal

Abstract

A technique for selectively separating approximately 65 wt.% of the Si(IV) in coal has been developed. The technique first uses aqueous hydrofluoric acid (HF) to react with aluminosilicates and quartz to form fluoride complexed Al and Si species in solution. Aluminium cations, in the form of Al(NO 3) 3, are then added to the solution to complex fluoride as AlF 2 + and hydrolyse the silicon fluoride species to silicon hydroxide, which precipitates as a gel and is removed by filtration. The solution is then distilled to recover a water stream, a nitric acid stream and a solid residue. The water stream is used to pyrohydrolyse the solid residue at temperatures in excess of 500 °C to liberate HF for recycling. To complete the circuit, the solid remaining after pyrohydrolysis is treated with the nitric acid stream to produce Al(NO 3) 3 for recycling. The technique satisfies the objective of not requiring on-going purchase of chemicals. The application of this work is primarily as part of a process for producing ultra-clean coal. As it is a technique for the selective separation of Al and Si from aluminosilicates, it may have application in other areas of mineral processing.