A sedimentation column device is described and tested for determining the effect of surfactants on the static stability of coal-oil mixtures (COM). The sedimentation ratio (bottom-sampled wt % coal in treated slurry/untreated slurry after 24 h settling) was found to be an effective screening parameter for the effectiveness of commercial surfactants on COM stability. From the settling behavior of 25 wt % coal slurries, determined with a sedimentation balance, a compressive settling (subsidence) model was found to be appropriate. This led to an interpretation of COM stability in terms of network stability rather than classical isolated colloidal particle stability. The effects of water and surfactant structure were assessed and led to identification of the most effective classes of surfactants and interpretation of COM stability at the macroscopic, microscopic, and molecular levels.

The concept of colloid stability is explored in a practical study of the effectiveness of additives as stabilizers for mixtures of powdered coal (80% -200 mesh) and no. 6 fuel oil. Further insight is added by the results of a parallel study of the stability of colloidal suspensions of carbon black at high concentratln In mineral oil. Sedimentation, subsidence, consistency, and viscosity are measured and interpreted in terms of practical stability arid colloid stability. It is shown that flocculation and network formation are important in powders as well as in colloids. The dependence of the effectiveness of stabilizers on molecular weight of the additive suggests a mechanism of bridging flocculation and network formation. Certain anionic surfactants stabilize carbon black in mineral oil as shown by the slow sedimentation of dilute suspensions and the low viscosity of pastes, but the same surfactants destabilize coal-oil mixtures (COM) since the particles readily settle to a closely packed bed. Surfactants that confer practical stability on COM by promoting rapid flocculation and network formation Increase the viscosity of carbon black pastes.

A microelectrophoretic investigation of different carbonblacks suspended in n-butano1 was undertaken to observe the relationship between particle size and zeta potential of each black. Brownian motion was used to size an individual particle which was subsequently subjected to an electric field to obtain the zeta potential on the same particle. For three blacks studied, the zeta potentials were independent of size within the range measured.

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