New Centrifuge Technology For Dewatering Sodium Sulphate (September 2017)
Minerals specialist Multotec Process Equipment has improved a customer’s sodium sulphate dewatering system by introducing its innovative Conturbex screen worm centrifuge technology to achieve better results when dealing with this crystalline salt product.
The successful result – which included better plant availability and nearly double the production – led to a further order for two more of these machines. Sodium sulphate is one of the chemical reagents used in roasting processing plants as part of the steel making process.
According to Gerrit du Plessis, product specialist in solid liquid separation at Multotec Process Equipment, the company worked closely with the customer to establish the operational and process parameters of the plant, so that the Conturbex centrifuge machine could be custom designed for the application.
“As it is relatively new technology, we also arranged for the customer to visit existing installations where these units were operating in similar applications,” says Du Plessis. This allowed the customer to gain the necessary confidence in the technology by talking to companies where it was already employed.
“To be effective in the downstream process, the levels of moisture in the crystalline sodium sulphate must be as low as possible,” he explains. “The centrifuge therefore has a strategic role in the main stream of a salt roasting plant, and it was vital that we should improve both plant availability and production capacity.”
He says that Multotec recommended the screen worm centrifuge as it features a number of benefits in this application. With the centrifuge rotating at a high speed of over 2,000 rpm to ensure effective dewatering the product is moved forward across the screen by a scroll mechanism, also spinning but with a speed differential of just 20 rpm.
“This distributes the product evenly, which minimises the possibility of vibration due to uneven loads around the screen,” he says. “Vibrations can become an issue with centrifugal machines if material gathers unevenly, causing stresses and eventual mechanical failure which of course could lead to unplanned and expensive downtime.”
Du Plessis highlights the short residence time – only about two seconds – that the product spends in the machine, which has important implications for the continuity of the whole process.
“This factor equips this Conturbex unit well to deal with the kind of process fluctuations that are common in a plant, and allows the machine to absorb variations in feed rates,” he says. “This makes the unit much more ‘forgiving’ of these variations, easily absorbing lower solids concentration streams. As a result, it also consumes less energy and is subject to fewer line blockages, which in turn improves the overall plant availability.”
Should there be a sudden plant stoppage or power outage, the machine’s design allows it to clean itself as it runs down without mains power; it can then be easily re-started without the time consuming cleaning process usually necessary in many types of machines.
Another design advantage is that the feed outlet of the solid liquid mixture inside the rotating components occurs at the smallest diameter of the drum, where the majority of the liquid is separated by the screen.
“Since the mixture is not subjected to high G-forces in this area for dewatering, the power load required to spin the drum is much lower,” he says. “This allows a smaller motor to be utilised, which in turn means less power consumption and lower operating cost.”
He highlights that the feed into the smaller diameter also transmits lower mechanical action onto the solids, reducing particle damage and breakage.
“There is a further important advantage in the design of the screen worm centrifuge, and that is the formation of thin layers of the solids on the screen,” he says. “As it travels from the small diameter to the large diameter, the layer of solid matter remains loose and is constantly becoming thinner due to the increasing surface area of the screen. This allows further dewatering without requiring high G-forces that would need greater energy inputs.”
The demanding conditions in the crystalline streams of these process plants – where there are high temperatures along with extremely corrosive and abrasive conditions – requires that Multotec utilise specialised, abrasion resistant materials to ensure long operational life and durability.
“Centrifuge technology is based on high speed rotating equipment and fine tolerances,” says Du Plessis. “For instance, the worm flight tips have a clearance distance from the screen surface of just 0,5 mm to 4 mm. This is where Multotec’s high quality design and precision engineering are vital to achieving maximum availability and performance.”
The grinding action of the hard crystals on the flight surface is particularly abrasive, requiring the application of specialised material that is high on the MOH scale of mineral hardness.