Surfactants - coal extraction and processing

In Russia, a so-called Interdisciplinary Group was established which was in charge to develop HWCS suspensions and their hydro-transportation at distances over 200 km [270, 271]. The investigations developed in two main directions 1) stabilisation mechanism of the suspensions and control of their flow properties 2) the choice of additives - surfactants, polymers, inorganic substances which control the suspension properties. 

The main laws of the flow properties of disperse HWCS systems are well described in [272, 273]. The main conclusions of this study consist in that the process of degradation of HWCS structures is described by an exponential equation which is determined by the depth and the rate constant of the degradation. Then, it is proposed to consider, along with the steady-mode suspension flow curve, an initial-moment flow curve, which provides additional information on the flow behaviour of the suspension in transition regimes. 

Bedenko et al [275 - 276] have, to a certain extent, developed the ideas by Rehbinder and Urjev, of the effect of disperse phase particle size on the strength of the structure and viscosity T using HWCS as an example. In mixtures of fine (e.g. df =21.8 pm) and coarse coal fraction (e.g. dc = 180 pm), the finely divided fraction as a continuous medium can completely fill out the entire void volume between the particles of the coarse fraction. Experimental data have shown that at dc/df 4, a synergistic effect of the packing density of particles in the mixture is 

The plasticising activity of ethoxylated alkylphenols and EO/PO block-copolymers depends on the number (m) of the EO units in a surfactant molecule. Ethoxylated alkylphenols with m 40 have a substantial effect on the viscosity reduction of HWCS. It may be connected with the fact that they can form rather thick hydrated layers when adsorbing on the particles, which play the role of a structure-mechanical barrier. The addition of ethoxylated alkylphenols with m 40 produce a thickening effect on the suspensions which may be connected with the features of coal particle wetting by aqueous surfactant solutions. 

Also the effect of block-copolymer surfactants (Synperonic NPE) on the viscosity and stability of suspensions as a function of the PEO chain length is studied [278], The critical volume fraction of coal in suspension and maximum stability were observed for a surfactant with 48 PEO units. At 27 PEO units, flocculation appeared. In the interval from 79 to 174 PEO, desorption resulted leding to an increase in suspension flocculation. Thus, the choice of an optimum PEO content is very important to obtain liquid and stable suspensions, considering the features of the coal nature. 

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