BENEFICIATION
about two billion bricks a year; however,
their bulkiness generally excludes them
from being an export product – though
Corobrik Midrand does export a small
proportion of its production, due to the
advantages of the rand’s weak exchange rate,
says Von Wielligh.
The crushing activity reduces the clay
to the ideal size of between one and two
millimetres. “With that, we add water
and extrude the clay in order to make
a brick. We have a weekly maintenance
programme of all equipment, which
considerably reduces downtime. The mix
for the bricks is highly precise. “We don’t
use a single clay for a brick, but mix several
clays in proportion. Any mix would have
a minimum of four components, and our
challenge is to ensure the proportions in the
mix are exactly the same on a daily basis for
colour and consistency to SABS-approved
brick sizes. We have a 2.5mm dimensional
tolerance in any direction.
“It’s all fully-automated with no hands
touching bricks, and what is amazing
about this plant is it’s 30 years old and the
technology is still efficacious.” The plant
manufactures on average 150 000 bricks a day.
Colours
Corobrik Midrand has possibly the widest
range of colours in Africa, claims Von
Wielligh – ranging from white to black and
every form of red and grey in between. To
achieve this variety it imports up to 85%
of its clay from other quarries, but also
broadens the range by adding pigments.
The fired colour of clay bricks is
influenced by the chemical and mineral
content of the raw materials, the firing
temperature, and the atmosphere in the
kiln. “Getting the range is therefore all
about consistency,” he says. For example,
pink bricks are the result of a high iron
content, white or yellow bricks have a
higher lime content. Most bricks burn to
various red hues; as the temperature is
increased the colour moves through dark
red, purple, and then to brown or grey at
higher temperatures.
An impervious and ornamental surface
may be laid on brick by the use of a slip,
which is a glaze material into which the
bricks are dipped. Subsequent firing in
the kiln fuses the slip into a glazed surface
integral with the brick base.
The seven-day manufacturing
process
In brief: the raw material enters a bin
feeding system, then goes through a rolls
crusher, and from there though a set of
screens and on to two wheels rotating in
opposite directions, dropping to a grid
Clay blending and feeding into brick factory.
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QUARRY SA | JULY/AUGUST 2019_33