Marek Pawlik

Effect of Humic Acids and Coal Surface Properties on Rheology of Coal-Water Slurries

In the tests described in this paper, humic acids were used to modify coal surface properties and to study the effect of such modification on the rheology of coal-water slurries (CWS). Adsorption of humic acids onto a hydrophobic bituminous coal surface renders the surface hydrophilic; also the electric charge on such a surface becomes more negative. Such a modified surface of a bituminous coal resembles very closely the surface of an oxidized and/or low rank coal as indicated by the results of our contact angle and electrokinetic measurements. The presence of humic acids results in improved dispersion of bituminous coal and increases the coal loading of CWS. Slurries prepared from hydrophobic coals in distilled water exhibit high yield stress values and high apparent viscosities. In the presence of humic acids the yield stress values decrease almost to zero and the apparent viscosity is lowered as well. Since oxidized and/or lower rank coals contain large quantities of humic acids, the addition of humic ...

Coal Reverse Flotation. Part II. Batch Flotation Tests

Coal reverse flotation, a process in which coal mineral matter is floated instead of the organic matter, was studied through a series of batch flotation tests on artificial coal/silica mixtures using two coals varying in rank. Dodecyltrimethyl ammonium bromide (DTAB) was utilized as a mineral matter collector, while humic acids (sodium salt) were added as a coal depressant. It was shown that the separation of silica from coal by reverse flotation is a kinetic process in which silica floats first followed by clean coal. The selective recovery of silica was possible only in a very narrow range of DTAB dosages and the amount of DTAB needed for best selectivity was a function of coal rank. A lower rank coal required much higher amine doses than a bituminous coal (depressed with humic acids), which was attributed to the higher amine adsorption on the more hydrophilic coal.

Effect of coal surface wettability on aggregation of fine coal particles

Fine coal aggregation in concentrated coal-water suspensions was studied through steady-state rheological measurements. The wettability of coal particles was modified by dry oxidation of a bituminous coal. The degree of hydrophobicity was monitored using film flotation. The rheological measurements revealed that hydrophobic coal particles aggregate in aqueous suspensions over a wide pH range, and produce slurries with high yield stresses. As the degree of coal oxidation increases, the coal particles become more hydrophilic and such suspensions exhibit lower yield stress values. The yield stress values for suspensions prepared from oxidized/hydrophilic coal become a strong function of pH and correlate well with the surface charge as determined through electroacoustic measurements. For the suspensions prepared from the samples with intermediate degree of oxidation the effect of coal surface heterogeneity becomes very important. These results indicate that the coal particle aggregation, and thus the yield stress of aqueous suspensions of coal, is strongly affected by wettability of coal particles.

Coal Reverse Flotation. Part I. Adsorption of Dodecyltrimethyl Ammonium Bromide and Humic Acids onto Coal and Silica

Adsorption of dodecyltrimethyl ammonium bromide (DTAB) and humic acids (HA) on bituminous, oxidized bituminous, and subbituminous coals, as well as on silica, was studied through direct adsorption, electrokinetic, and contact angle measurements. It was concluded that the adsorption of DTAB and HA on a hydrophobic coal surface takes place through hydrophobic interactions between the hydrocarbon chains of these compounds and the hydrophobic coal surface. The adsorbed DTAB molecules cause a gradual decrease of coal hydrophobicity due to their tail-to-surface orientation. The adsorption density of DTAB on a hydrophilic/oxidized coal is much higher than on a bituminous coal. The results indicate that the adsorption mechanism involves some strong interactions between the cationic head group of the surfactant and the negatively charged oxygen groups on the coal surface. Despite the apparent head-to-surface orientation, the adsorbed DTAB molecules do not render the surfaces hydrophobic; this probably results from the fairly chaotic orientation of DTAB molecules on the surface of oxidized coal. The well-ordered conformation of the DTAB ions adsorbed onto the negatively charged silica surface, combined with significant adsorption of this surfactant at the solution/air interface makes the silica surface very hydrophobic.

Influence of pH and temperature on the rheology of aqueous quartz-bitumen suspensions

The rheology of slurries prepared from artificial mixtures of bitumen with fine quartz of varying bitumen content, as well as slurries of actual oil sand ores, was investigated under different physicochemical conditions (pH and temperature). All rheological measurements were conducted using a Haake Rotovisco VT550 rotational viscometer, connected to a fixture specifically designed to measure the rheological properties of settling suspensions. It was found that when the bitumen content in slurries prepared from quartz-bitumen mixtures was low (0.7 vol. %), the pH of the slurries was the most important parameter that controlled the rheology of these slurries. At higher bitumen contents (8 vol. %), it was a combination of high temperature and high pH that gave the lowest apparent viscosities and yield stresses of the model mixtures. These model observations correlated well with the rheology data obtained for slurries of actual oil sand ores. The overall results were discussed in terms of aggregation-dispersi...