BENEFICIATION
to the glass). For instance, in glass used for
solar heating where maximum sunlight is
wanted, negligible iron is added. Anthracite
and sodium sulphate (saltcake) are added
to remove defects from the glass, especially
bubbles,” Lotter explains.
Cullet is recycled glass, mainly off-cuts
from PFG’s own operations, but also
recycled glass acquired externally. Cullet
is easier to melt than silica.
The raw materials are transferred from
their respective silos to a mixer where all
the additives are batched in appropriate
proportions along with a small amount
of water to control dust. The sand particle
size cannot be too big or it will not melt
in the furnace. Other materials also have
a tight particle size specification as too
much fines would lead to particulates in
the waste gas and too much oversized
particles may lead to unmelted stones
especially if it is contaminated with
chromite, silica and corundum.
It is transferred by a conveyor belt to a
hopper at the furnace (called ‘the bunker’)
where it can be stored for hours. This
ensures that the furnace is continuously and
uniformly fed at all times and never runs
out. Lotter explains that this assists both
efficiency and quality. If there are drops in
the quantity fed, it causes defects. It is fed
to the furnace and melted at a temperature
of up to 1 600°C and all irregularities are
melted out of the glass at this stage.
“Approximately 18% of a batch is gas,
because the raw material comes in as
carbonates, and this has to be removed or it
forms bubbles in the glass. As it is melted in
the furnace, most of this gas is dispersed in
the waste gas. Some gets dissolved into the
glass and is refined out after the melting.
One way of doing this is to introduce large
bubbles into the molten glass, into which
smaller bubbles merge and the entire
bubble is released.”
Glass forming and annealing
From the furnace, the material enters
the ‘working end’ which controls the
temperature and readies the material
for the float process where the glass is
actually formed: this is a continuous ribbon
of molten glass which floats along the
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John Lotter, PFG process technology manager at the Springs facility.
surface of molten tin – it floats due to the
difference in densities of tin and glass. The
temperature is controlled during the float
process which forms the glass into shape,
thickness and width.
In the ‘annealing lehr’ the glass is
gradually cooled to relieve stresses in the
glass and prevent splitting and breaking
in the cutting phase. This occurs over
a period of between 15 minutes and an
hour, depending on the glass width and
thickness, so that the internal stress of
the glass is acceptable for the end use and
cutting process. The ribbon moves to the
‘cold end’ of the line where it is washed
and automatically cut, as it travels along
the rollers.
Lotter explains that it is a simple task
to interchange the settings for different
sizes and widths, as most glass comes in
standard sizes. The thickness ranges from
1.6mm to 15mm. Thereafter, the glass
sheets are offloaded and stacked. The
glass is then warehoused for distribution
throughout South Africa and exported
into regional and overseas markets.
QUARRY SA | MARCH/APRIL 2019_31