The influence of crystallization on the flow of coal ash-slags

Numerous technical applications in the energy and metallurgical industries demand a fundamental knowledge of the flow of slags. In particular, the operation of an entrained flow gasifier is challenging, as the oxide slag has to be reliably discharged. Crystallization in the slag influences strongly the flow behavior of the slag because precipitations occur. In this study, the process of crystallization during flow of two coal ash slags was investigated. Therefore, isothermal viscosity measurements were conducted in order to examine the rheological evolution over time caused by the crystallization. It has been demonstrated that the evolution of viscosity of a sub-liquidus melt depends strongly on time, as well as on temperature and composition. Using a rotational high-temperature viscometer to investigate coal slags, it was found that the crystallization during flow could be separated into three time regimes: a lag-time, in which the undercooled melt behaved as an Arrhenius-liquid; the kinetic-driven crystallization; and, finally, the rheological equilibrium that is represented by a constant viscosity. Furthermore, an increase of relative viscosity caused by crystallization was accompanied by a shift from Newtonian to non-Newtonian flow; here, pseudoplastic flow indicated the existence of precipitations. The results demonstrate that the flow behavior has to be divided into dilute, semi-concentrated and concentrated particle bearing fluids. A view into the morphology of the partly crystallized slag was taken by scanning electron microscope. Differential thermal analysis of the slags was conducted, to underline the results of the isothermal viscosity measurements. The degree of supercooling promotes the kinetics of crystallization. Our results demonstrate that time-dependency has to be considered for an accurate description of flow during crystallization, as well as the influence of degree of supercooling.