Prediction of power requirement in mixing of clay based pastes

Abstract

The successful design of mixers is known to depend primarily on the mixed fluids properties as well as the geometry of both impeller and vessel. A classical approach commonly used to predict the power requirements in the mixing process is to make use of the relation between the Power number (N_p) and the Reynolds number (Re). Here, Re = ρ.n.D² /μ, where ρ is the density of the liquid mixture, μ its viscosity, n the rotational speed of the impeller (rps), and D its diameter. On the other hand, N_p = P / ρ.n³.D⁵, where P is the power required for mixing. This approach has been conventionally applied to Newtonian fluids and suspensions but not to pastes. In the present paper, relationships were developed for mixing of ceramic pastes used in the manufacture of sewer pipes. A standard mix was chosen and the effect of the following variables on the power number - Reynolds number relation was investigated: water content, grog content and particle size of clay. All paste mixes investigated followed a relation of the type: N_p = a / Re^b, where a and b are constants that depend essentially on the fraction of grog used and to lesser extent on the water fraction and clay particle size. The values of the constant b were close to unity in all cases, emphasizing the fact that the laminar regime mixing relation N_p = a / Re, applicable for Newtonian fluids is also valid for pastes.