MechChem Africa November-December 2024

Scroll centrifuge and bed dryer for salt recovery MechChem Africa talks to Multotec’s solid-liquid separation specialist, Gerrit du Plessis, about the advanced Siebtechnik Tema screen scroll centrifuge and Tema Process shaking fluidised bed dryer systems which, in combination, offer continuous and highly effective dewatering and drying solution for recovering salts such as sodium sulphates.

These salts are then dissolved in ammonium hydroxide to form ammonia metavanadate (NH 4 VO 3 ). From the final saleable product, va nadium pentoxide (V 2 0 5 ) is formed, which can be added directly into the mix during vanadium steel making. “The sodium sulphate used is recovered by precipitating it out of the ‘waste’ liquor stream, which also contains valuable ammonia and other chemicals that are separately processed. This improves operating costs, reduces waste and contributes towards processing circularity,” he adds. Turning attention to the Siebtechnik Tema centrifuge on display, Du Plessis says the so dium sulphate feed comes into the machine at between 105 and 110°C. “The idea is to dewater this ‘slurry’ to a moisture level of 5 to 7% before passing it on to the thermal dryer if lower product moistures are required. Not shown here is a hydrocyclone stage, which we use to pre-thicken and reduce the volume of the

feed slurry before it enters the centrifuge – via a flexible hose, because we need to accommodate for vibration,” he tells MCA. The rotating dewatering drum of the centri fuge is conical, and it has a wedge wire screen with an aperture of about 210 µm that is cus tomised to suit the feed density and particle size of the sodium sulphate feed. The centrifuge is rotating at about 2 100 rpm and can produce centrifugal forces of up to 1 000 to 1 100 g at the largest diameters of the drum basket. On the feed inlet on the front of the machine Du Plessis points out a spiral pathway to pre accelerate the feed uniformly before reaching the dewatering screen. “We are really trying to get the product to sit on the screen rather than scrape against it, which would cause accelerated wear,” he explains. So, by the time the product reaches the drum basket, it is rotating at the same speed as the drum, so the liquid starts to drain through the wedge wire mesh into the out side casing and channelled out of the machine.

S ourced from sister companies, Siebtechnik Tema and Tema Process, Multotec's centrifuges and dryers offer efficient and cost-effective solid/liquid separation for the worldwide minerals process ing industry. “The units we have on display here achieve precise separation while requiring minimal energy and space,” Gerrit du Plessis tells MCA at Multotec’s Electra Mining stand. “One of these two units are destined for a client using sodium sulphate salts for roasting vanadium,” Du Plessis explains. In this process, vanadium-bearing ores or slags are roasted at elevated temperature in the presence of a crys tallised salt, such as sodium sulphate (Na 2 SO 4 ) or sodium carbonate (Na 2 CO 3 ). This is followed by leaching, which forms complex vanadium salts.

Multotec’s Siebtechnik Tema centrifuge on display at Electra Mining Africa 2024.

18 ¦ MechChem Africa • November-December 2024