# Review of Coal-Fired Electrification and Magnetic Separation Desulfurization Technology
## 5 High Gradient Magnetic Separation Technology
### The Development of High Gradient Magnetic Separators
With the development of material science, rare earth permanent magnetic materials have been developed rapidly, which sparked the design of a new permanent magnet high gradient magnetic separator. In 2004, the Changsha Research Institute of Mining and Metallurgy cooperated with the Moscow University of Mining to develop a CRIMM double-box reciprocating permanent magnet high gradient magnetic separator. The machine is equipped with multi-dimensional magnetic separation medium stack in the double sorting box, when entering the magnet, the magnetic induction intensity on the surface of the medium is more than 1.3 T. In 2006, it was applied in the production of iron removal from feldspar, nepheline and kaolinite, and good technical and economic benefits were obtained [61]. In December 2005, Baotou rare Earth Research Institute successfully developed a new type of high gradient, a strong magnetic rare earth permanent magnetic separator. The magnetic separator can raise the grade of iron concentrate by one percentage point compared with the existing equipment, while ensuring that the iron ore recovery rate will not be reduced, and, on the basis of no reduction of concentrate grade, the mineral recovery increased by two percentage points [62]. Superconducting HGMS has become one of the main research directions of HGMS due to its high efficiency and low consumption. Outokumpu and Eriez, two of the top foreign manufacturers, have made many improvements to their original Cryofilter, Powerflux series. The famous manufacturers in recent years are: ACCEL instruments (Alton, CA, USA), GmbH (Frankfurt am Main, Germany), Aguuafine Company (Valencia, CA, USA), Cryomagnetics Company (Oak Ridge, TN, USA), Kyushu Electric Power Company (Fukuoka, Japan) and so on [63]. In China, the research on superconducting high gradient magnetic separation has been carried out by the Beijing Nonferrous Research Institute, but no great progress has been made; only a superconducting HGMS developed by the Institute of Electrical Engineering of the Chinese Academy of Sciences, the diameter of the sorting tank is 0.5 m, the processing capacity is 3 t/h, and the field strength is 3.96 T [64]. In recent years, a new type of high gradient magnetic separator developed by Henan University of Technology has designed a rectangular magnet-tapered magnetic medium system. The optimized taper angle is 60 degrees and the polar distance is 0.005 m, the rotating disc structure is adopted, and the assembling technology of rectangular magnet with the same pole extrusion and different pole interval is adopted in the assembling of magnetic system, which avoids the damage of magnets. Setting an acidic system to strengthen pulverized coal magnetism in order to improve desulfurization rate, the research shows that, when the particle size of Chongqing coal, Gansu coal and Luoyang coal is 0.074 mm, the desulfurization rate of pulverized coal before and after magnetic strengthening increases by 22.77%, 16.93% and 19.99%, respectively [65]. There are some problems in these kinds of high gradient magnetic separators: electromagnetic high gradient magnetic separator uses electromagnets to produce magnetic field, which requires a lot of energy consumption, and also increases the investment and maintenance work of the equipment. Compared with a electromagnetic high gradient magnetic separator, the permanent magnet high gradient magnetic separator has the advantages of low operating cost, high processing capacity, long service life, simple and convenient maintenance; however, it still has the disadvantages that the magnetic structure is unreasonable, the \"long beard\" phenomenon of the magnetic system internal drainage port and the magnetic pole working surface is not easy to remove, the mechanical inclusions in the sorting process and the blocking of the magnetic medium are obvious. Superconducting high gradient magnetic separators also have fatal disadvantages, such as high investment cost and complex equipment structure.