Production of Ultra Clean Coal: Part II—Ionic equilibria in solution when mineral matter from black coal is treated with aqueous hydrofluoric acid

Abstract

A model for determination of the concentration of fluoride complexed aluminium and silicon species, free fluoride (F −), H + ions and molecular HF in solution when aluminosilicate compounds are treated with aqueous HF is presented. The model elucidates chemical mechanisms governing both the dissolution behaviour of the mineral matter in coal towards aqueous HF, and the unwanted precipitation of various fluoride compounds, such as CaF 2, MgF 2 and NaAlF 4. The controlling parameter for the precipitation of fluoride compounds is the free F − concentration in solution. The model has application toward the development of chemical strategies for dissolving virtually all of the mineral matter from coal and avoiding the unwanted precipitation of fluoride compounds. The model also has application toward the development of a strategy for recovering fluoride from spent leaching solutions. Ultimately, this work will assist in the development of a process for the production of Ultra Clean Coal (UCC) containing less than 0.1% by weight mineral matter.