Local solids concentration measurement in a slurry mixing tank

Abstract

The local solids concentration in a mixing tank was measured using both sample withdrawal and a new conductivity probe. The conductivity probe was used to assess the errors associated with various sample withdrawal techniques and to measure solids concentration profiles in the mixing tank. The effects of sampling tube design (tip shape, face angle and inside diameter), sampling position, bulk solids concentration and particle size on the sampling errors were examined in detail. Solids concentration profiles were also measured as a function of particle size, bulk solids concentration and mixer rotational speed. The experimental results indicated that on the impeller plane, the sample withdrawal techniques (tapered sample tube) gave a lower solids concentration than that in the tank at a sampling velocity ratio of unity. These results suggest that the flow at the impeller plane was three-dimensional. The errors associated with sampling techniques depended on the sample tube shape and location in tank, and were significant for the coarse sand particles of 1000 μm. When sampling at right angle to the flow, sampling errors increased with particle size, especially when a small diameter sample tube was used. The data provided in the present study can be employed to correct for the errors associated with the sampling withdrawal techniques over a wide range of parameters. Solids concentration profiles in the mixing tank were found to be a function of the particle size, bulk solids concentration and mixer rotational speed. Solids concentration varied with the radial position, except when fine sand particles of 82 μm were used. A strong variation in solids concentration with the axial position was observed at the impeller plane for the sand particles examined in the present study. This variation increased with the particle mean size and the mixer rotational speed.