Magnetic separation is a key stage of any screening, crushing and sorting plant. MK Vega-Po LLC (Radex) of St Petersburg in Russia purchased six Electro Overband magnets from Master Magnets, based in Redditch in the UK, for their coastal coal processing complex in Nakhodka on the Trudny Peninsula, approximately 85 km east of Vladivostok in Russia. Radex started work on the Nakhodka project in early 2013 and their task was focused on integrating new equipment into an existing coal processing plant specifically for the screening, crushing and removal of magnetic tramp metal prior to feeding the loading vessels.

The end specification of the coal dictated that all the output product needed to be below 50 mm with a maximum ferromagnetic content of 0.5 kg per 10,000 t (0.000005%), which also could be no more than 50 mm in size in any one dimension. Magnetic separation, to remove the tramp metal and protect the screens, crushers and conveyors and guarantee the high output specification of the coal, formed two important stages within each of the three process lines.

The finished plant receives around 9,000 t/h of coal, which is then split into three lines each handling 3,000 t/h. At the first stage of the process, material can either be fed directly onto the output conveyor feeding the loading vessel or into the screening and crushing plant. The material that is fed through the screening and crushing plant immediately passes under the first Master Magnets Electro Overband Magnet. This is installed to remove large tramp iron such as digger teeth, blasting caps and tramp metal to protect the roller screen. After the screening stage, material under 50 mm (approximately 95% of the main infeed) in size is fed straight to the main output conveyor feeding the loading vessel, whilst between 150 and 250 t/h of 50-300 mm coal (between 5% and 8% of the main infeed) is conveyed to an impact crusher. After crushing, the material is returned to the original screening stage, but not before passing under a second Master Magnets Electro Overband Magnet which removes any metal that has been liberated or introduced during the crushing process.

The Electro Overband Magnet is the most powerful overband separator from the Mastermag range and is designed to operate and separate at increased suspension heights where the operating gap (the distance between the magnet and the conveyed material) will also be higher. The higher strength magnetic power of the Electro Overband Magnet enables the extraction of ferrous metals even from material conveyed along deep troughed conveyors. An Electro Overband Magnet consists of a coil, wound of aluminium wire, centrally located inside a mild steel fabricated box and mounted around a solid steel core. When powered, the coil produces a magnetic field that is focused and projected downwards by the steel core and a thick steel back plate. All the Electro Overband Magnets supplied by Master Magnets for the Nakhodka project are oil-cooled, where the coil is efficiently maintained at an optimum working temperature by recirculating oil inside the stainless steel box. Oil-cooled Electro Overband Magnets produce higher magnetic fields that other designs such as those where the coil is cooled using air. Master Magnets use a computer model to assess applications where there are set separation objectives and this enables the size, type and design of the Electro Overband Magnet to be considered in detail. Other parameters, such as operating at potentially low temperatures around minus 20°C, had to be considered when finalising the design. The high strength magnetic field meant that ferrous metal would be attracted and propelled up onto the magnet at speed with a high impact force and so the whole base plate of all the Electro Overband Magnets was made out of wear resistant manganese steel.

Radex ordered the Electro Overband Magnets in two stages, with an initial order for two placed in May 2013 and a second order for placed in February 2014. All the equipment was designed and built by Master Magnets at their UK manufacturing and test facility in Redditch, near Birmingham, UK. With nearly a 40 year history, Master Magnets is one of the world’s leading suppliers of magnetic separation equipment to the mining and mineral processing sectors, with representatives and partners located globally. The initial order in May 2013, for the first line processing 3,000 t/h of coal, was for one model 200 OCW 45 in and one model 105 OCW 45 in Master Magnets Electro Overband Magnet and supplied in July 2013. Both were supplied with transformers. The model 200 OCW 55 in was positioned over the 2 m wide main infeed conveyor running at 3 m/sec prior to the roller screen, with the model 105 OCW 45 in situated on the 1.2 m wide return conveyor post crushing of the oversize fraction. Both Electro Overband Magnets are positioned inline with the conveyor over the head pulley, with separated ferro-magnet material being lifted and then discharged away from and out of the natural trajectory of the coal. Electro Overband Magnets used in the inline orientation generally achieve a better separation performance than when positioned across the conveyor belt as the conveyor belt flattens as it passes over the head pulley, breaking the surface of and opening the conveyed material and enabling the more effective removal of tramp metal, especially when located at the bottom of the burden.

The Model 200 OCW 45 in is currently the largest Electro Overband Magnet ever manufactured by Master Magnets, weighing approximately 10 tonnes and drawing up to 15.6 kW from the 17.5kW transformer rectifier set. The overall dimensions of the Overband Magnet, including the running gear, are 4.3 m long by 2.67 m wide and 2.07 m high, which presented several manufacturing challenges. The Master Magnets’ manufacturing facility in Redditch is equipped with heavy cranes to handle and manufacture equipment of this size and greater and there was a heavy reliance on the skills and experience of the engineering and production teams. Subtle changes to the standard design were required as the size and weight of the Electro Overband Magnet increased and this knowledge is already being used for future projects where larger Electro Overband Magnets are required. As a standard size Electro Overband Magnet, the smaller Model 105 OCW 45 in posed no such manufacturing challenges, weighing approximately 5.6 t with overall dimensions of 3.275 m long by 2 m wide and 1.49 m high and drawing up to 8.7 kW from a 9 kW transformer rectifier.
Following the successful installation of the first two Electro Overband Magnets, Radex placed a second order in February 2014 and were collected for shipment to Russia on June 2014 for the second and third screening and crushing lines, replicating the equipment purchased for the first line. The plant is now fully operational, processing over 9,000 t/h of coal and maintaining the high quality specification of the final coal product.