COAL UDC 622.765 : 622.33 Flotation of oxidized coals by apolar and heteropolar reagents does not give good results [1,2] because part of the oxidized grains remains in the railings. Markedly oxidized coals are relatively seldom used as flotation feedstocks. Due to increased open-pit mining of coking coals, coals intermediate in character between unoxidized and markedly weathered coals are being sent to flotation plants. No effective method of flotation of such coals has yet been found. Sun [3] suggested the use of l~urylamine for flotation of oxidized coals. However, the conditions of selective and complete flotation of oxidized coal grains by cationic reagents have not been fully investigated. The relation between the surface charge of oxidized coal and its flotability has been inadequately elucidated.When oxygen reacts with coal acid oxygen-containing groups (COOH, OH) are formed on its surface; these bestow upon the coal the properties of a polyfunctional cation exchange resin, which in water acquires a negative surface charge because of dissociation of these groups. The sign and value of the surface charge of oxidized coals, and therefore its isoelectric state, may be assessed by the ~-potential [4]. Methods of determining the ~-potential are fairly complex and the results are unreliable [5], particularly in the case of weakly oxidized (transitional) coals, for which its value is low.Bearing in mind the close relation between the ~-potential and the suspension effect [6], and the simplicity of measuring the latter [7][8][9], to assess the sign and relative value of the surface charge of oxidized coals, we used the Wigner and Pallman soil-concentration effect [10,11]. According to the theory of the suspension effect, developed in [10], on the basis of ideas on the Donnan membrane potential in absence of this effect, colloidal particles must remain at the zero-charge point. If the suspension effect is due to a change in the diffusion potential at the boundary between the salt bridge and the suspension [6], here, too, the suspension effect is zero at the zero-charge point.The state of the electrical neutrality of the surfaces of oxidized coal particles was determined from the value of the suspension effect under the following conditions. The suspension effect was determined for a suspension of coal from a seam, oxidized in situ. The overall content of carboxyl, acid hydroxyl, and partly peroxide groups in the coal found by the method in [12] was 306 mg-eq/100 g of coal; the carboxyl content, found by the method in [13], was 36 mg-eq/100 g of coaL The content of weathered grains, determined by the microscopic method on the basis of weathering signs, was 100%.To reduce the effect of mineral components of the coal and of metal cations (largely Ca), which had replaced the hydrogen of the functional groups, the coal was treated with 1 N HC1, then washed with distilled water until no chloride ions were present. Treatment with HC1 reduced the ash content of the coal from 9.1 to 3.7~A weighed sample of coal (30 g),...
The time required for an air bubble to adhere firmly to a mineral particle surface, the so-called induction time, has been investigated tn [1][2][3]. During coal flotation, for a considerable number of particles adhesion of the air babble is preceded by attachment of a drop of an apotar reagent to the particle. To the best of our knowledge, the induction time for adhesion of reagent drops to the surfaces of minerals and coal has never been investigated.Attachment of a reagent drop to a coal surface is determined by its hydrophilicity [4]. It is known that flotation of hydrophilic coals, particularly oxidized types, by the reagents normally used for unoxidized coals is ineffective [4,5].Our paper deals with the time required for adhesion of drops of flotation reagents to the surfaces of coals wtth specific degrees of oxidation, the object being to find ways of speeding up flotation of oxidized coals.The contact time required for firm adhesion of reagent drops to coal particles was measured in a device similar in principle to the contact devices in [2,3], but differing from them by the fact that the contact time was recorded accurately in each experiment.A cell of optical glass or t~ansparent plastic was placed in a microthermostat, placed on the core of a solenoid. Graded, air-free coal was placed in the glass cell. A drop of the reagent was placed on the holder by means of fine-pointed pipet and was brought to a given height from the layer of coal by a three-coordinate manipulator. The diameter of the drop and the distance from the apex of the drop to the coal layer were measured by means of a horizontal microscope equipped with an ocular micrometer with a scale value of 0.01 ram. The cell was lit by an illuminator. A silver mirror, in contact with a needle, which closed an oscillator-counter circuit when the cell was raised, was attached rigidly to the cell. The distance between the silver mirror and the needle was made equal to that between the apex of the drop of reagent and the coal layer by a two-coordinate manipulator. The current was fed from the rectifier to the solenoid by a programmed relay. The cell temperature was kept constant (:~ 0.2"C) by a thermostat and a water-jet pump. The cell was returned to the initial position by a spring.At a normal current frequency of 10,000 Hz in the measuring circuit, the error of the measurement of the duration of contact of the reagent drops with the coal grains was At: 0,0001 + t(1-10;0o)c, where t is the number of pulses on the scale-of-ten counter, and ~e is the frequency of the oscillator at the moment of measurement.With the stabilized quartz oscillator the measurement error was determined largely by the remlving power of the scaler and was +0.0001 sec.The coal used for the experiments was from the Tomusinskii coalfield (Kuzbass). The coal samples, denoted by A, B, C, D, E, and F, were taken at different depths from the surface in order to obtain coals with a gradual * Deceased.KuzNIIUgleobogashchenie, Prokop'evsk.
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