THEORETICAL AND EXPERIMENTAL-STUDY OF THE SURFACE-CHARGE DENSITY AND THE SURFACE-POTENTIAL OF COAL-WATER SUSPENSIONS IN DISSYMMETRICAL ELECTROLYTES

To estimate the physicochemical interactions between dispersed particl es and the stability of a suspension it is necessary to know the surfa ce charge and surface potential of the particles. In practice, the sur face potential can reach very high values (+/-100 to +/-400 mV). In th at case, the solution of the Debye-Huckel equation becomes inaccurate. The same holds if the particle radius a becomes very small (500-10 an gstrom) and if kappaa < 100 where kappa is the Debye-Huckel reciprocal length. In the latter case, the Poisson-Boltzmann equation (implying parity between a spherical particle and a plane surface) becomes unsui table. A more suitable and precise solution of the non-linear Poisson- Boltzmann equation for charged spherical particles in the presence of dissymmetrical electrolytes was worked out. The relationship between t he electrostatic potential PSI(x) and the surface charge sigma0 was es tablished. It was found that electrolytes with trivalent anions (1-3 a nd 2-3 electrolytes) give higher surface densities. These theoretical results were confirmed by studying the surface charge and potential of coal-in-water suspensions in the presence of various dissymmetrical e lectrolytes such as Na2SO4, Na5P3O10, CaCl2, Na3PO4 and NaCl. Using ou r theoretical model and experimental results we were also able to obta in the changes in electrostatic potential as a function of the distanc e x (from the particle surface) in various electrolytes and for differ ent pH values. Our experimental results were in good agreement with th e theoretical calculations. It was demonstrated (in order of decreasin g surface charge density) that sigma0(NaTPP) > sigma0(Na3PO4) > sigma0 (Na2SO4) > sigma0(NaCl) > sigma0(CaCl2) or sigma0(1-5) > sigma0(1-3) > sigma0(1-2) > sigma0(1-1) > sigma0(2-1)