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.
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Multotec’s Siebtechnik Tema centrifuge on display at Electra Mining Africa 2024.
Sourced 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 processing 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 crystallised salt, such as sodium sulphate (Na2SO4) or sodium carbonate (Na2CO3). This is followed by leaching, which forms complex vanadium salts. These salts are then dissolved in ammonium hydroxide to form ammonia metavanadate (NH4VO3). From the final saleable product, vanadium pentoxide (V205) 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 sodium 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 centrifuge is conical, and it has a wedge wire screen with an aperture of about 210 µm that is customised 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 outside casing and channelled out of the machine.
While the solids on the screen are being dewatered, they are transported towards the wider end of the conical basket at the front solids discharge end of the machine. A scroll scraper connected to the solid centre shaft, which is rotating at about 30 rpm faster at
2 130 rpm, scrapes the now dewatered material off the drum and pushes it from the back of the of machine towards outlet raceways at the front.
The differential speed between the centrifuge drum and the scroll is achieved by a V-belt pulley arrangement at the back of the machine, with a slightly larger pulley on the solid centre shaft driving the scroll. “So, there is a continuous flow of materials through the centrifuge, with the feed being dewatered as it moves down the outside of the dewatering basket, and the dewatered product being pushed off the screen by the faster rotating scroll and onto a conveyor at the front,” Du Plessis explains.
A host of other features are incorporated to ensure the system remains reliable and efficient.
A fixed speed motor and gearbox provides the drive for both the dewatering drum shaft and the scroll shaft, with a small differential speed between the two.
A clutch mechanism is incorporated that disengages the solid centre shaft from the motor should any high torque event be detected. For example, if a bolt or a piece of tramp metal wedges between the scroll and the drum. This protects the gearbox by allowing this clutch to mechanically slip. A speed sensor also detects the clutch mechanism slippage and is interlocked with the fixed speed motor and other plant process equipment upstream.
Speed sensors monitor the scroll input speed at the solid centre shaft and the drum speed at the hollow centre shaft and also detects V-belt slippage, breakage and overspeed and is interlocked with the fixed speed motor and other plant process equipment upstream.
A vibration transmitter is installed on the centrifuge bearing housing and monitors the machine vibration continuously to detect abnormal vibration preset parameters for interlocking purposes.
An oil filtration system with condition sensors is used to continuously circulate oil through the bearings and gearbox internals for lubrication and cooling. This system includes an air-cooled heat exchanger- and a closed loop filtration system with replaceable cartridge filters and clog indicators to keep the oil clean.
“The system is designed to run continuously, typically 24/7 for weeks at a time, and offers reliability, reduced operational costs, reduced transport costs and improved product quality,” notes Gerrit du Plessis.
The fluidised bed dryer
“Some customers can bag and use dewatered salt products with residual moisture, while others need dry crystals. We therefore transfer the product from the centrifuge directly into a Tema shaking fluidised bed dryer to drive off the remaining moisture,” Du Plessis tells MCA.
For this system, biogas, steam or other energy sources are used to heat up air to about 150 °C through a heat exchanger. The required gas temperature depends on the application and can be elevated up to 750 ˚C. This air is blown from underneath the dryer to fluidise the moist sodium sulphate, keeping it suspended above the bed to allow the moisture to evaporate uniformly. At the same time, the whole dryer is mechanically shaken to bounce and propel the product along the bed.
“The dryer is divided into two sections: a hot drying section, and a cooling section. Hot moist air is exhausted from the cooling section, and then when the dryer product passes a divide on the outlet side of the bed, cool ambient air is used to drop the temperature of the salt. This prevents moisture being picked up from the atmosphere, which would happen if the salt was hot on exit. This prevents the formation of hard-to-break clumps when the product is heaped,” Du Plessis explains.
With the help of accurate temperature control throughout the drying process, the sodium sulphate coming out of the fluidised bed dryer has a free running texture like a fine table salt, with less than 0.2% moisture and crystals sized between 100 µm and 1.0 mm, making it ideal for downstream processing. And like the preceding dewatering stage in the cyclone, the drying process is continuous, with a conveyor on the outlet taking the recovered salt back into the process or downstream for further processing.
“This dewatering and drying system is evidence of Multotec’s commitment to helping mines and the minerals processing industry better meet their ESG goals: by reducing the consumption of raw materials, water and energy; recycling as much as possible, and by striving towards more cost effective and efficient operations.
“Furthermore, we have the product range and the expertise to make these goals economically sustainable,” concludes Gerrit du Plessis.
