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
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.
Dissertation (MEng)--University of Pretoria, 2014.