John Besida

Production of Ultra Clean Coal, Part I – Dissolution behaviour of mineral matter in black coal toward hydrochloric and hydrofluoric acids

The mineral matter in an Australian black coal has been isolated using a low-temperature ashing (LTA) procedure. This LTA procedure is a modification of the Australian Standard for LTA at 370 degrees C, and alleviates adverse effects to thr: minerals caused by the heat of combustion. The leaching behaviour of the mineral matter towards aqueous HCl and hydrofluoric acid (HF) is presented. HCl can dissolve simple compounds such as phosphates and carbonates, yet it cannot completely dissolve the clays. HF resets with almost every mineral in the mineral matter, except pyrite, and most of the reaction products are water soluble. However, at HF concentrations greater than that required to dissolve the aluminosilicate compounds in the mineral matter, insoluble compounds form. These compounds include CaF2, MgF2 and a compound containing Na, which is believed to be NaAlF4. It is proposed that HF reacts preferentially with the aluminosilicates in the mineral matter to form largely AlF2+, AlF3 and SiF4, and that the concentrations of free fluoride (F-) and AlF4- are not high enough to complex cations such as Ca2+, Mg2+ and Na+. When the mineral matter is treated with HF concentrations greater than that required to dissolve all of the aluminosilicates, AlF3, AlF4- and SiF62- form, the concentration of F- is high enough to complex Ca2+ and Mg2+ and form insoluble CaF2 and MgF2, and the concentration of AlF4- is high enough to complex Na+ and form insoluble NaAlF4. This work has application toward the development of a process for producing Ultra Clean Coal with less than 0.1% by weight mineral matter

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

A model fur determination of the concentration of fluoride complexed aluminium and silicon species, free fluoride (F-), II+ 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 CaF2, MgF2 and NaAIF(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.

Spent Potlining – A Hazardous Waste Made Safe

The electrolytic production of aluminium is undertaken in large smelters containing carbon-lined pots in which carbon acts as the cathode, the pots having a life typically of 2 to 6 years. Eventually the potlining is incapable of performing electrically and a toxic waste product of enormous tonnage is created. The potlining is heavily contaminated with metals, fluorides, cyanide and other compounds. The carbon content is reduced to less than 50%. Spent potlining (SPL) has been dumped in rivers, at sea and on land, creating major environmental problems. Today most spent potlining is stored at great expense awaiting a safe treatment process. The paper describes a low temperature chemical extraction process which has been demonstrated to recover valuable by-products from the SPL, leaving an environmentally acceptable landfill solid product. A demonstration plant has been built and plant trials have been performed. Cyanide is adequately removed in the water wash stage and fluoride is recovered as CaF 2 . The acid washes remove the rest of the fluoride and a significant quantity of this is recovered as AlF 3 .

Production of ultra clean coal: Part III. Effect of coal's carbonaceous matrix on the dissolution of mineral matter using hydrofluoric acid

An Australian bituminous coal was treated with increasing concentrations of hydrofluoric acid (HF), and the extraction levels of Al, Si, Fe, Ti, K, Na, Ca and Mg were determined. These extraction levels were compared to those obtained when the mineral matter alone, produced by ashing the coal at a low temperature, was treated with HF, in order to quantify the extent that the carbonaceous matrix inhibits extraction. The carbonaceous matrix inhibits the dissolution of Ti to a large extent. Si and Fe are the next most inhibited elements. It is proposed that the Ti is present as extremely small particles, of possibly less than 1 mum in length, which are finely disseminated throughout the coal.

Arsenic pentafluoride equilibria in anhydrous hydrogen fluoride

Abstract Earlier work has indicated that arsenic pentafluoride when dissolved in anhydrous HF is present largely as the anion As 2 F 11 - , particularly temperatures much below ambient. Raman spectra and conductance measurements are used here to show that, at and near room temperature, there are significant concentrations of molecular AsF 5 , AsF 6 - and As 2 F 11 - in equilibrium and that on reduction of temperature, As 2 F 11 - is formed at the expense of AsF 5 and AsF 6 - . The implications of the Lewis acid and oxidant strengths of AsF 5 are discussed as affecting synthetic procedures in anhydrous HF.

Recent chemistry of chromium and uranium in superacidic media

Abstract An extensive Raman spectroscopic investigation of Lewis acid/Lewis base behaviour of CrO2F2 in HF of varying acidity and basicity has shown that it exhibits no significant Lewis acidity in basic HF and that considerable enhancement of the acidity of HF is required to cause fluoride transfer the solute to the medium. Amphoteric behaviour of the fluoride, fluorosulpate and triflate of chromium (III) has been studied for each of these solutes in the HF, HSO3F and CF3SO3H solvent systems. Following the stabilization of solvated U3+ in acidic HF and the subsequent demonstration of its disproportionation as the acidity of the medium is reduced [1], lower oxidation state of uranium have been studied in HSO3F and CF3SO3H to which appropriate Lewis acids have been added.