MechChem Africa visits the South African manufacturing facilities of Multotec’s Gravity Separation division and talks to the unit’s general manager, Wynand Erasmus, about spiral technology and the company’s latest product enhancements.
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Gravity Concentration forms part of Multotec’s mining and minerals processing equipment offerings, alongside Samplers; Magnetic Separators; Cyclones; and Industrial Slurry Pumps. “I manage the Gravity division, which specialises mostly in spiral separation and concentration equipment,” says Multotec’s Wynand Erasmus.
“As a South African processing original equipment manufacturer (OEM), we are the leading supplier of spirals to the coal and chromium industries. Spiral concentration technology is a cost effective way of separating materials according to density, particle size and shape. As no moving parts are involved, the equipment used is robust and very reliable,” he tells MechChem Africa.
Breaking down how spirals work, he says material, generally sized at -1 mm, is mixed in a solids to water ratio of about 35%. The slurry is fed via distribution systems at a controlled rate into the top of a bank of spirals that usually contains between two and 12 spiral separators.
The slurry flows under gravity down the spiral. The rotating motion of the flow creates centrifugal forces on the slurry causing the suspended light and low-density particles to be displaced away from the spiral’s centre column and up the side slope.
Heavier and more dense particles tend to sink to the working surface of the spiral, where drag and friction forces slow them down. The slower speeds mean they are subjected to lower centrifugal forces. As a result, heavier particles tend to follow a downward path closer to the centre of the spiral. “More dense particles take the inside path, while lighter less dense and more easily suspended particles get spread across the outside path,” Erasmus explains.
“At Multotec, we design and manufacture spiral-based gravity concentration technology solutions for all major minerals as well as for specialised niche applications. In addition, we manufacture slurry feed distributors, launder systems and spiral accessories. We also produce test units to optimise and improve concentration efficiency for existing and new processing plants,” he says.
“We feed individual units at feed rates from as little as 1.5 tph through to 7.5 tph on wider diameter units.
Multiple spiral stages are coupled together to raise the grade of the concentrate and to recover as much of the mineral product as possible. Chromium, copper, cobalt, tin and mineral sands such as ilmenite, rutile and zircon are all dense/heavy ores that will flow down the inside path. A spiral has three dart splitters at its base which cut the product or concentrate (heavies), near density material (middlings) and the lighter tailings on the outside. Multotec can add a fourth super concentrate off-take, depending on the application. “We can shave off a super concentrate from the innermost channel, for example, which can be directed away for final processing,” says Erasmus.
Although conceptually simple, spiral designs differ from ore to ore and efficient separation of the different ore grades depends on careful optimisation of spiral diameter, the turns, pitch and profile. “Our biggest spirals have a 1.0 m diameter, while our smallest is 680 mm. We manufacture spirals with three to 12 turns, which offer much better retention times and, therefore, better separation efficiencies,” he adds.
Coal and the new SX10
Describing some of Multotec’s recent developments in spiral separation, Erasmus first lifts out the new 10-turn coal spiral, the SX10. “We are getting a really low density cut of below 1.5 with this spiral, which is exceptionally good. This makes an ideal material for coking applications in the steel smelting industry,” he tells MechChem Africa.
The South African coal industry currently uses Multotec’s SX4, SX7 and MX7 spirals, with tailings, middlings and concentrate splitter darts, “but the new SX10 spiral with tailings, middlings, concentrate and super concentrate darts can deliver a product with lower silica, which reduces the ash content on combustion.”
Chrome fines
“We are also very excited about the development of our new ultra-fine spiral, which uses a customised pitch and profile to allow more effective separation of finer particles,” he continues.
“Our testing has delivered excellent results on the recovery of ultra-fine chromite from UG2 platinum tailings,” he says. “We are hoping to move towards commercialising this technology very soon.”
UG2 circuits involve the reprocessing of the ultra-fine tailings from the platinum recovery process, beneficiating the discards. In today’s constrained mining environment, new markets for minerals recovery are becoming of more and more interest.
Mineral sands solutions
Multotec has an increasing range of products designed to suit heavy minerals such as ilmenite, rutile and zircon. “For these mineral sands we developed a 12-turn spiral, the NHM12, which offers significantly longer retention times. In principle, using two six turns in series would have a similar effect, but the reduction in distributors and auxiliaries makes 12-turns a more cost effective solution,” he tells MechChem Africa. “While it’s tricky to put a 4.5 m spiral into a plant, a bigger spiral can provide significant savings in terms of overall production efficiency,” Erasmus adds.
“Testing using Multotec’s NHM12 spiral is going very well at sands plants in Australia, where banks of our new spiral technology are being used alongside magnetic and electrostatic separators for better product yields,” he says.
Multotec spirals are manufactured in the company’s Spartan factory in Gauteng, South Africa, along with all feed distributors and product boxes. “We have gone through the SABS local content verification process and, with verified local content greater than 60%, we exceed the requirements of the new Mining Charter.
Our products are exported all over the world and used on all six continents across a full spectrum of minerals. Our process engineers understand process flow sheets better than anyone in the market, so our purpose-built spiral concentrators can quickly and seamlessly be integrated into mineral processing circuits,” Erasmus concludes.