David G. Wood

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 .

Effects of drying methods on the low temperature reactivity of Victorian brown coal to oxygen

The effects of air drying and thermal dewatering on the low temperature oxygen reactivity of Victorian brown coal have been investigated in the temperature range 35–55 °C and at 100 kPa oxygen pressure using coal samples ground to < 100 mesh. An attempt has also been made to relate the low temperature oxygen reactivity of the coal to its free radical concentration as measured prior to oxidation. Two rate models, the Schmidt and Winmill models, have been adapted to include the initial free radical concentration of the coal samples as the drying method sensitivity parameter in lieu of the concentration of oxygen-reactive sites in the coal material. The experimental results show that air drying, which reduces the free radical concentration of the coal, causes a decline in its oxygen reactivity whereas thermal dewatering, which causes an increase in the free radical concentration of the coal, enhances its oxygen reactivity. Air drying does not affect the distribution of the consumed oxygen in the oxidation products. A difference is observed in the case of the thermally dewatered coal samples. The correlation of the two rate models adopted is considered equally satisfactory. However, only the values obtained for the two activation energies in the Winmill model reflect the changes caused by thermal dewatering in the oxidation pattern of the coal. The activation energy values obtained from the two models are within the range of those quoted in the literature for the abstraction of hydrogen from various arene structures by free radicals.

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.

Investigation of catalytic combustion within a fin boundary layer

A mathematical model of a catalytic fin, a flat plate coated with a catalyst, operating under steady-state conditions where air carrying a fuel flows parallel to the surface, is developed. The model equations are derived from the basic equations of change, and model predictions of fin and boundary layer temperature are compared with experimental data for the combustion of propane and carbon monoxide (CO) over the flat plate coated with platinum(Pt)/alumina catalyst. Good qualitative agreement is found between the results of the experiments and the model predictions, although the model generally predicts higher fin temperatures and ignition of reaction to occur at lower temperatures.

Combustion of a gas flowing over a catalytic flat surface: : Experimental investigation of heat and mass transfer

Abstract An experimental investigation was conducted on the heat and mass transfer effects of combustion of a gaseous fuel at a catalyst surface. Air containing either propane or carbon monoxide was passed over a flat fin coated with an alumina/platinum catalyst. The bulk flow of gas was parallel to the surface resulting in boundary layer formation at the surface. Combustion of the fuel caused a distinct increase of fin temperature. The temperature profile across the boundary layer was measured, as was the mean fuel concentration close to the fin surface. The results demonstrate the effects, under steady state conditions, of the bulk gas properties and radiation on the fin temperature and boundary layer profiles.

Non-adiabatic catalytic combustion at a fin surface involving heat extraction from the fin edge

The steady state catalytic combustion of fuel borne air, flowing parallel to a catalytic fin surface, has been studied. Experimental studies determined the temperature profiles of a catalytic fin when heat is extracted from the fin by a heat sink at the fin edge. A mathematical model of the catalytic fin was developed from the basic equations of change and the model predictions of fin temperature are compared with experimental results with combustion of propane or carbon monoxide fuel/air mixtures over a platinum/alumina catalyst. The heat fluxes from the fin to the heat sink were calculated and the effect of process variables on this are reported.