Identifying critical parameters in the settling of African kimberlite slimes

Abstract

Kimberlite is the host rock from which diamonds are mined. The mineralogical features for kimberlites vary greatly with country, origin, depth and type of kimberlite. Kimberlites can contain various clay species with some kimberlites containing predominantly clay minerals. The presence of these clay minerals in the ore can cause difficulty in dewatering due to high flocculant demand, poor supernatant clarity and low settling rates. Identifying critical parameters that can predict the settling behaviour of African kimberlite slurries will assist the process engineer to predict the settling behaviour of different kimberlite slurries. Especially identifying the kimberlites that will most likely not settle with normal flocculant dosage rates is useful. From first principles the settling of a particle is described by Stoke’s law which incorporates the density of the particle and size of the particle as the inherent particle variables. In this case density is assumed constant and therefore the size of particles influence the settling rate of particles to a great extent. This study therefore investigated the influence of particle size on settling rate and whether the particle size distribution showed correlation with settling rate when regression modelling was fitted on the data. Other variables that were tested for correlation with settling were pH when the kimberlite is mixed in water as well as various mineralogical features of the ore. Fitting a simple model to any of these properties or combinations of these properties was attempted which would allow for prediction of settling behaviour. The mineralogical features were classified by evaluating the mineral composition, fractional elemental analysis, cation exchange capacity and the exchangeable sodium percentage of the different kimberlites. These variables were tested as well as their settling behaviour with 18 different African kimberlite samples. The settling rate and slurry bed compaction during natural settling as well coagulant and flocculant assisted settling were measured for the kimberlite slurries. The best performing coagulant and flocculant for each kimberlite were combined to evaluate potential improvements in the settling rates and slurry bed compaction compared to current settling practices that only utilise flocculant additions. Especially the use of coagulant for kimberlites that did not show settling with only flocculants was evaluated. For these 18 kimberlites only 2 kimberlites did not settle with the use of flocculants with settling rates varying between 10.7m/h and 25m/h. Both these kimberlites also did not settle with the combination of coagulant and flocculant, but could only settle with only coagulant additions at settling rates of 1.9 m/h and 2.2 m/h. Regression analysis fitted to the settling rate investigated the influence of particle size, pH and mineralogical features on settling. For representation of the particle size two data points from the Particle Size Distribution (PSD) was utilised which represented the fine material and the coarser material. These two data points were taken at percentage passing 7.5 μm and 75 μm. Regression data for kimberlite with flocculant additions showed that particle size and the pH of the slurry were identified as significant parameters in predicting settling. The regression data showed a R2 of 0.78 for the settling rate and an adjusted R2 of 0.79 for the slurry bed depth.