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Reverse flotation as a method of coal cleaning for preparation of coal-water slurries Pawlik, Marek

Abstract

Due to strongly attractive hydrophobic forces that operate between hydrophobic particles in water, aqueous suspensions of bituminous coals are characterized by high yield stresses and behave as non-Newtonian systems. Since coal must be cleaned before it can be used to prepare coal-water slurries (CWS), flotation is commonly applied to clean fine coal. However, coal flotation has never been optimized with regard to CWS and the use of flotation reagents in coal flotation renders the final clean coal product even more hydrophobic making CWS preparation even more difficult. The objectives of this thesis include the following: (i) to study reverse coal flotation as a cleaning method for the preparation of coal-water slurries, (ii) to study the applicability of amines in reverse coal flotation, and (iii) to elucidate the mode of action of various polymers used as either coal flotation depressants or CWS dispersants. In order to study the effect of coal surface properties on coal reverse flotation, a very hydrophobic bituminous coal, oxidized bituminous coal and sub-bituminous coal were utilized. As flotation and rheological experiments showed, anionic (polystyrene sulfonate, carboxymethyl cellulose, humic acids) and nonionic (dextrin, hydroxyethyl cellulose) polymers of low molecular weight were capable of acting as both flotation depressants and CWS dispersants for bituminous coals. Using humic acids as a model additive, this dual action of polymers was attributed primarily to increased coal hydrophilicity and to a higher coal surface charge in the presence of these additives. Contrary to literature reports, dodecyltrimethyl ammonium bromide (DTAB) was shown not to be able to depress the flotation of high rank coals below the critical micelle concentration, as determined through contact angle and flotation studies. It was concluded from adsorption and zeta potential measurements that DTAB molecules assume a flat orientation on a hydrophobic coal surface. DTAB behaved as a weak flotation collector for low rank/oxidized coals. It was shown that the quaternary amine did not increase water contact angles on these coals despite a very high amine adsorption density. This behavior was related to a highly chaotic orientation of the adsorbed amine molecules at the "gel-like" coal/water interface. The adsorption mechanism involved electrostatic interactions between the cationic surfactant and the negatively charged coal surface. Reverse flotation experiments on artificial coal/silica mixtures revealed that while humic acids were necessary in the reverse flotation of a hydrophobic bituminous coal, the reverse flotation of a sub-bituminous coal did not require any depressant. The separation of silica from coal was a strongly kinetic process taking place in a very narrow range of relatively high DTAB dosages. Contact angle and adsorption studies revealed that, despite a low adsorption density, DTAB could strongly increase the hydrophobicity of silica. This collecting action of DTAB in the flotation of silica was associated with the deposition of DTAB molecules at the air/silica interface upon collisions with air bubbles. The observed beneficial effect of a short conditioning time with DTAB on the selectivity of coal reverse flotation supports such a mechanism.

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