Glen Douglas
The treatment process is essentially one of crushing, washing and screening to produce the various grades
(sizes) of dolomite. The plant presently produces 17 different products, and has considerable flexibility to
switch production between grades and product sizes to meet market requirements. Clearly, only high or low
silica rock should be processed at any one time. Before a switch is made, live tonnage on the plant stockpile
is run down to avoid cross contamination. Obviously this is more important when switching from high to low
silica rock to avoid contamination of metallurgical grade products. This is not an issue on the product side,
since the various products are stored in separate stockpiles, which provide supply for one market when
products for the other are being produced.
The existing plant configuration and capacity has been unchanged for a considerable period, and no major
modifications are planned. Ore from the pit (either metallurgical or aggregate grade) is fed to the primary
gyratory crusher, set at 150mm. The crushed ore is conveyed to a 3,000t live stockpile. Two minus 150mm
products are produced through the primary crusher: minus 150mm dump rock (aggregate stone)
(product 959) and “Sub Base” (product 602).
Ore is withdrawn from the stockpile by three vibratory feeders. Maximum feed rate to the main plant is about
600tph. At present production rates, target feed rate is 460 – 480tph. The ore is fed to a double-deck washing
screen fitted with a 80mm opening top deck and a 10mm bottom deck. The +40mm rock is conveyed to one of two secondary gyratory crushers. The crushed product is returned to a second, similar, washing screen in
closed circuit with the crusher. The –40+10mm fractions from both screens constitutes product: 40mm
metallurgical dolomite, or 37.5mm stone, depending on whether <2.5% or >2.5% SiO2 rock is being
processed.
These products may also be fed to a 19mm screen, which produce further products and feed for further
processing. Minus 40+19mm rock is conveyed to separate sections of G-heap (<2.5% or >2.5% SiO2) for
further processing. The live capacity of this stockpile is 12,000 – 15,000t. A –53+19mm aggregate product,
53mm ballast, may be produced. Also, a 53mm metallurgical dolomite may be produced. These require
alterations to secondary crusher setting and screen sizes.
The ‘lumpy’ products (–40+10mm and –53+10mm) are stacked separately and reclaimed for dispatch by
means of a stacker-reclaimer installation. The 19mm screen undersize is an aggregate product designated
19mm stone.
The –10mm slurry from the double deck screens is dewatered in a spiral classifier. The fines and water is
pumped to a clay-lined earth settling dam. These fines, which constitute about 6% of the new feed, are
subsequently reclaimed for agricultural lime production. The spiral underflow (coarse –10mm fraction), is
screened on a 5mm vibrating screen; the +5mm fraction constitutes the –13.2mm flat stone while the –5mm
fraction is designated washed crusher sand. These are both aggregates.
It is possible to send the entire +5-40mm fraction to the G-heap, for example when metallurgical grade rock
is being processed. The water used in the plant, largely on the washing screens, is ground water pumped out
of the pit, or from the nearby Bass Lake (previously the A-pit).
The sinter plant produces sinter sand and BHK limestone from metallurgical grade rock from the G-heap. The
–40mm rock is fed to four Hazemag impact crushers at a rate of 155tph. The crushed material is screened
on a double-deck vibrating screen fitted with a 5.5mm top deck and a 1.5mm bottom deck. Oversize is
returned to the crusher, the intermediate product, about 160tph, is the sinter sand, and the –1.5mm fraction,
>15tph is the BHK limestone.
Minus 40mm rock from the aggregate grade section of the G-heap is fed to two vertical impact crushers. The
crusher discharge constitutes Crusher Run product. Crusher feed rate is around 150tph.
When super sand is produced, feed rate is reduced to 50tph and crusher discharge is fed to 5-deck
Morgenzon sizers. Oversize from the screens is returned to the impactors. A fine (–4.75mm screen) fraction
is the super sand product.
An intermediate –13mm+4.75mm fraction is fed to the survival plant. Here the material is fed to a double deck
screen with a 10mm top deck and a 5mm bottom deck; the plus 10mm fraction is designated 13.2mm round
stone and the intermediate product is 9.5mm round stone. The –5mm fraction is returned to the G-heap.
Two agricultural lime products are produced. The first is BHK, produced in the sinter plant. The second,
designated silver. Lime, is a mixture of BHK and main plant classifier fines reclaimed from the settling ponds.
The two fractions are mechanically mixed by front end loader and fed to a horizontal impact crusher acting as
a de-clogger. Crusher discharge is screened on a 7.5mm screen. Oversize is returned to the de-clogger or
discarded. The screen undersize fraction is conveyed to the lime store.
Plant monitoring and control is by way of PLC controllers in the main and super sand/sinter plants. Emergency
stops are provided at machines. This control system is about six years’ old.
Two wet scrubbers and a dry, Donaldson filter, are employed to extract dust. Chemically dosed water sprays
are applied at conveyor discharge points to allay dust.
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