Air-cooled overbelt magnets available to the African mining industry

The latest developments in air-cooled overbelt magnets are now available to the African mining industry. “The design of the Longi-Multotec air-cooled magnet is completely different from the traditional design,” Willem Slabbert, process and applications engineer at Multotec, says.

The new technology incorporates a split coil design, whereby heat is dissipated over ten different surfaces compared to the conventional six, making this cooling method far more efficient in terms of the overall system. “We are now able to offer this innovative technology to the African mining industry through our agreement with Longi,” Slabbert says.

“By means of this technology we have completely eliminated the use of fans or forced cooling. This lack of forced cooling, along with the improvement in natural convective heat transfer, adds to the safety and efficiency of the equipment,” Stuart Cullum, process engineer, says. The trend towards larger and faster travelling conveyors in the African mining industry has highlighted the vital role of overbelt magnets.

Another feature of the latest range of air-cooled magnets is the circular shape of the magnetic coils and magnetic field distribution. Slabbert explains that circular magnets offer attraction gradients in all directions, including the main conveyor travelling direction, whereas conventional rectangular magnets only offers attraction gradients in the perpendicular plane.

“The advantage of a gradient in the conveyor travelling direction is that these attractional fields assist in slowing down fast moving tramp metal, thereby increasing the residence time under the magnet and ultimately increasing extraction efficiency. This technological advantage is more predominant in fast moving conveyor systems like overland conveyors that can run up to speeds of six metres per second, ” he says.  

“It is critical to understand the role of an overbelt magnet and the various factors that need to be taken into account during its specification,” Slabbert says. These factors include belt speed and width, the belt troughing angle, the burden depth and the material density and bulk density, which are related to the particle size. A vital parameter is the tramp metal extraction requirement as smaller and spherical pieces of material are more difficult to remove than larger flat pieces.

Another critical parameter that needs to be determined by the Multotec engineers is the recommended suspension height, as the overbelt magnets require a minimum clearance between the top of the conveyor. “The height of the conveyor is critical because as the magnetic force is reduced the further the magnets are from the conveyor belt itself,” Slabbert notes. This also means that the type and size of the tramp metal to be taken off has to be taken into account.

“In many instances, overbelt magnets are not viewed as an essential item and are thus relegated as add-ons in Greenfield projects. At the end of the day, when the overbelt magnet does not deliver as expected, additional costs are incurred in a Brownfield project in order to rectify the situation. This is in addition to costs incurred for damage to the installation and consumables, all as a result of the tramp metal entering the plant,” Slabbert comments.

Multotec is able to offer the standard range of air-cooled overbelt magnets from Longi, which are capable of operating in an ambient temperature of 45°C. This range is suitable for standard applications, but for specific instances where a more customised approach is called for, Multotec can size the correct magnet from its range of forced cooled magnets.

The standard range conforms to the standard conveyor widths used in the African mining industry, with international standard widths also available. There are four models available for each conveyor width. While the conveyor belt dimensions remain the same for all these model variants, the difference lies in the coil height, which can be raised in order to increase the strength of the magnet.

“Depending on the process parameters, we will critically evaluate which one of the four model variants is the correct solution for the application. “This gives Multotec the necessary flexibility to be able to carry out custom engineering for specific applications, which depends on the tramp metal that needs to be removed,” Slabbert explains.

Key to this process is the in-depth knowledge and experience that Multotec is able to bring to the table. “Our critical differentiator in the marketplace is the highly skilled people we have at Multotec. It is important to work with an original equipment manufacturer that understands the system parameters, and that is also able to see past the one-size-fits-all option that has tended to be the general approach in the past.” Slabbert explains that Multotec submits detailed questionnaires and offers the assistance of application engineers to its customers as well as plant audits to help in ascertaining these essential parameters.

“Each individual application is evaluated in accordance with all the relevant criteria. That information is then collated and brought back for further review by the team of engineers at Multotec, which has both the necessary depth of experience and reference base across all commodities, including across Africa. It is important to note that we do not only focus on one commodity, but are able to supply suitable solutions to all mining applications, including metals recovery,” Slabbert says.

Multotec’s involvement can extend to supplying the entire overbelt magnet system, from equipment supply to a turnkey service by means of its strategic partners, including even the gantry work. “It is preferable for the Multotec team of engineers to become involved right from the design stage so that all of the necessary parameters can be taken into account. In many instances overbelt magnets are specified in locations where it is not practically possible to install them, which is a problem that can be obviated right from the design stage,” Slabbert points out. Multotec can also adapt its product offering according to the different maintenance and control strategies of their respective clients.

In terms of instrumentation and control, both sections of the split coil contain thermocouples, in addition to main isolators and circuit breakers for overload or underload conditions. “Attention to detail has to be paid to the electrical control and instrumentation in and around the overbelt magnet, such as belt break or slippage sensors,” Slabbert notes.

Another standard feature is a crowned head pulley that allows for belt tracking and self-alignment. This results in a significant reduction in maintenance, in addition to ease of operation. In terms of energy efficiency, Multotec can even specific high-efficiency motors upon request of the client.