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Dense medium separation plants
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26th June 2009
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The use of dense medium separation (DMS) technology to recover diamond indicator minerals from prospecting samples has been largely the domain of diamond-miner De Beers.

However, projects house DRA Mineral Projects has developed a plant capable of recovering diamonds and diamond indi- cator grains down to 300 µm. This plant is available in two variants, the standard micro and the micro plus.

DRA’s plant is designed around a small- diameter gravity fed cyclone. To prevent contamination and to ensure recovery of mineral grains as fine as 300 µm, it is machined from solid stainless steel and attached to a seamless mixing box. An automatic density controller and demagnetising coil are essential to the plant.

The plant is built into a 6-m shipping container with a reinforced roof to serve as a work platform for sample loading and storage. It can recover heavy minerals and diamonds from prepared loam and stream sediment samples or crushed core in the –6+1 mm or –2+0,3 mm size ranges. Excessive fine material and vegetation in the feed can affect performance, so it is important that they are reduced.

The standard DMS plant consists of a feed hopper and vibrating feeder; a mixing box and cyclone; a combined product screen with woven wire screen cloths; a separate concentrate screen with removable sieves; a circulating medium pump; a wet drum magne- tic separator (magsep); a magsep effluent sump and pump; a medium holding tank with agitator; a demagnetising coil; a density control system; and a paperless chart recorder for data logging.

The washed and screened sample, stored in the feed hopper, is fed into the mixing box by the vibrating feeder. It is mixed with ferro- silicon before gravitating to the cyclone where it is separated under pressure. The float product is drained and then washed to recover adhering ferrosilicon. The screen washings are pumped to the wet drum magnetic separator for medium recovery and densification.

The tailings are discarded while the concentrates are collected in a sieve and are handwashed to remove adhering medium, before being dried and sorted to recover heavy mineral indicator grains and diamonds.

A separate concentrate screen with remov- able sieves is used during heavy mineral recovery, while the combined float and sink screen is used for larger samples. The plant is capable of treating twenty 50-kg samples a day, increasing to about 300 kg/h when continually processing crushed kimberlite core.

Cyclone separation and recovery efficiency can be monitored by adding plastic tracers of known density and plotting a tromp curve based on the numbers recovered.

This plant is identical to the standard one, but has additional equipment mounted in the DMS container: a feed preparation screen with a screen under the pan; a plant undersize pump; a mixing box; a cyclone feed pump and a 150 mm cyclone with associated chute work.

There is also a standalone skid-mounted front end consisting of: a feed bin, a belt feeder with variable speed drive and a feed conveyor; a rotary scrubber with a trommel screen for oversize removal; a cat ladder and platform for access to the scrubber feed chute; a scrubber product sump and pump; a jaw crusher; a DMS tailings conveyor; and an electrical control panel with plug-in leads for easy dismantling.

These items are stored and transported in a second unmodified 6-m shipping container. The additional equipment changes the standard micro DMS plant from an exploration tool into a mini bulk sampling facility which can quickly be converted back to treat samples for heavy mineral recovery.

In terms of process, the scrubber accepts –75 mm material and, depending on the amount of fines/clay, can be fed up to 5 t/h. The trommel screen oversize is crushed to –10mm in the jaw crusher and returned by wheelbarrow to the feed bin for reprocessing. The trommel screen undersize is pumped to the feed preparation screen where undersize material is removed. The undersize is then pumped to an optional degritting circuit for water recovery and slimes disposal.

The –10+1 mm material is mixed with ferro- silicon in the mixing box before being pumped to the 150 mm cyclone. Separation is under pressure and the resulting products are drained and washed to recover the adhering ferrosilicon. The screen washings are pumped to the wet drum magnetic separator for medium recovery and densification.

The large-diameter cyclone increases the effective top size of the plant from 6 mm to 10 mm, from which meaningful diamond size distribution can be obtained. The tailings are discarded and the concentrates collected in a secure container for hand sorting. X-ray equipment can be used for large amounts of material. The micro plus DMS cyclone feed rate is limited to a maximum of 1,5 t/h.


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