Stability of CWM in long-distance pipelines

The change in apparent viscosity of CWM occurring during long-period operations was measured using a loop test facility in which a pipe of 200 mm inside diameter was used. The apparent viscosity of CWM showed a tendency to increase while CWM was circulated in the loop. It was hypothesized that the agglomerated coal particles were dispersed, attributed to the high shear rates created by the CWM fluid flowing through the pipe and the pump. The coal particles in the CWM, being pulverized, adsorbed greater amount of dispersant. It was, thus, considered that the amount of dispersant in the liquid phase was reduced to an extent too little to maintain the viscosity of the CWM at low levels. For the purpose to investigate this phenomenon, the rheological properties were examined experimentally in detail at high and low shear rates. High shear rates were produced by allowing CWM to flow into a small pipe at various velocities. Low shear rates were created by a rotary viscometer. As a result, the apparent viscosity of CWM increased far more quickly at higher shear rates. Based on these findings, a model for rheological change of CWM was proposed. Increase in viscosity of CWM was predicted using this model, which exhibits the relation between shear rate and apparent viscosity. The apparent viscosities of CWM calculated by this model were in good agreement with the measured values. It became clear that an increase in apparent viscosity occurred in the flow of pipeline at low shear rates rather than during pumping operation at high shear rates. Based on this model, the required initial amount of dispersant can . be determined in order to maintain the apparent viscosity of CWM almost constant.