MechChem Africa November-December 2024

⎪ Minerals processing and materials handling ⎪

While the solids on the screen are being de watered, 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 centri fuge 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 mo tor 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 pro cess 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 inter locked 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 abnor mal 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 oth ers 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 con trol 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 pro cess 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 economi cally sustainable,” concludes Gerrit du Plessis. www.multotec.com 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.

The Tema Process fluidised bed dryer.

November-December 2024 • MechChem Africa ¦ 19