Abstract
M. Tech.
The separation of minerals such as coal, diamonds and others have involved the use of a heavy
medium. The heavy medium separation technique, used in the diamond industry as a primary
concentrator, is a physical process that separates minerals of different specific densities based on
their relative movement and resistance to motion in a viscous fluid. It is made up of a suspension
of finely ground solid particles constituting a stable suspension. There is a wide range of
materials that are used to prepare suspension media in Heavy Medium Separation (HMS), but
Ferrosilicon has found a wider usage. Major demand for dense medium Ferrosilicon powder is
from the diamond industry which accounts for 77 percent of milled sales. In HMS, either
dynamic or static separators can be used, even though; dynamic separators are widely used
because they give higher separation efficiencies.
A number of accounts have been reported on the effects of stability and viscosity to the medium.
It is, however, observed that the effect of ore mineralogy, which is a major contributing factor
during ore and medium interaction, during separation, has not been properly taken into account,
apart from the fact that ore mineralogy form a basis for HMS efficiency.
The literature, however, does not cover a thorough investigation of the relationship between the
recovery of diamond bearing ores and stability of Ferrosilicon grades media, as a function of
physicochemical properties of ferrosilicon.
Eight samples of Ferrosilicon, four milled and four atomized were used to prepare media to
recover value from kimberlitic ore, alluvial ore and synthetic ore in a laboratory set-up HMS.
Both Ferrosilicon and ore samples were characterized with the following techniques before use:
XRD, XRF, SEM-EDAX and Screening, to investigate physico-chemical properties of
ferrosilicon and ore mineralogy. The used Ferrosilicon was also characterized to investigate any
v
changes as Ferrosilicon medium interacted with the ores. Settling tests were performed on each
Ferrosilicon grade to investigate media stability before and after recovery exercise. The stability
of the media was then related to the recovery of each ore. In-circuit sample of ferrosilicon was
collected from Letšeng diamonds to compare any changes with Ferrosilicon used in the
laboratory, and also used to study the effects of contamination on the degradated medium. All
recovery results were done in a static HMS, but in practice dense medium cyclones are widely
used.
The screening characterization technique revealed that there was a loss of finer particles size
fraction, predominant in sieves 75 microns and 45 microns and that loss was highest with
atomized grades than milled ferrosilicon grades. Minimal changes in density and chemical
compositions were observed for each ferrosilicon grade. The loss of the finer fraction was found
to effect changes in the settling rate of each ferrosilicon grade differently, hence changes in the
stability of their media. The efficiency (Ep) of separation was found to vary with each grade of
Ferrosilicon used, accompanied by a shift in cut point density indicating the influence of grade
on the separation efficiency. The effects of medium stability on recovery for both ores showed
that although both ores percentage recoveries differ, the trend of medium stability to recovery,
with each Ferrosilicon grade, is the same. However, the recovery was found to be more
dependent on the density of the medium, as the effects of loss of finer particle size fraction did
not show any significant contribution to recovery, but rather on the medium loss. The mineralogy
of the ores also had been observed to give a minimal or no contribution to the separation
efficiency, if related to recovery.
Milled Ferrosilicon is further recommended over atomized Ferrosilicon, based on cost and
stability. The highest loss of atomized Ferrosilicon could escalate the operating costs and affect
the stability of the medium. It should be remembered that the purchasing costs of atomized
grades is higher than that of milled grades.