# Review of Coal-Fired Electrification and Magnetic Separation Desulfurization Technology
## 5 High Gradient Magnetic Separation Technology
### The Development of Desulfurization by High Gradient Magnetic Separation  
In 1977, Auburn University and Oak Ridge National Laboratory used air entrainment for the first time to conduct dry high gradient magnetic separation tests on several groups of coal in Pennsylvania, 94% inorganic sulfur and 35% ash were removed when the yield of clean coal was 85%. Since then, researchers in Japan, Sweden and the United Kingdom have also conducted similar tests using different high-gradient magnetic separation devices, the results show that: in dry separation of micron grade coal with high gradient magnetic separator, the removal rate of inorganic sulfur is more than 80%, ash content can be reduced by 50-60%, and the recovery of calorific value is more than 90% [37; 38; 39]. In China, the research on coal magnetic separation desulfurization technology started late, and the research on magnetic separation theory and practice is still relatively weak. [PERSON] and others from Beijing Campus of China University of Mining and Technology have measured the specific magnetic susceptibility of different kinds of coal and pyrite and minerals in coal, and have carried out an experimental study on coal slurry wet continuous high gradient magnetic separation desulphurization [40; 41]. Commissioned by the Jiangsu Provincial Environmental Protection Bureau, Xuzhou Environmental Protection Institute conducted the first experimental study on desulfurization of dry coal powder by magnetic separation in China from 1987 to 1988, the study used periodic high-gradient magnetic separators to conduct small-scale tests in the laboratory. The results show that the magnetic field strength is 0.65-0.75 T, and the desulfurization of 100-120 mesh coal powder can reach up to 70% [42]. In recent years, XCG-II roller magnetic separator has been used in the dry magnetic separation tests of Chazhuang raw coal and Qingdao thermoelectric coal in Qingdao Institute of Architecture and Engineering. For the dry magnetic separation test of Chazhuang coal, the desulphurization rate and ash removal rate are 42.21% and 40.23%, the heat recovery rate is only 57.73%; in the magnetic separation test of Qingdao thermoelectric coal, the desulphurization rate and ash removal rate are 49.10% and 47.88%, respectively, and the yield of clean coal is only 49.88%. This shows that dry magnetic separation of coal powder with XCG-II roller magnetic separator cannot effectively separate organic matter carbon from pyrite and ash forming minerals [43; 44]. In 2015, [PERSON] of China University of Mining and Technology \begin{table}
\begin{tabular}{c c} \hline \hline
**Component** & **Specific Magnetic Susceptibility \(\times 10^{-9}\)/kg\(\cdot m^{-3}\)** \\ \hline Pyrite & 3.4 \\ Pyrhotite & Strong magnetism \\ Limonite & 700 \\ Siderite & 4000 \\ Iron silicate & 1260 \\ Calcite & 12.0 \\ Clay & 250–490 \\ Shale & 490–570 \\ Sandstone & 180–250 \\ Coal & \(-5.0\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Specific magnetic susceptibility of major minerals in coal. and others used microwave energy to selectively heat pyrite in fine coal to enhance its magnetism. Then, the inorganic sulfur in fine coal (\(-0.5\) mm) was removed using a 2 T strong magnetic roller magnetic separator. Using the microwave energy-intensive method, it was found that the magnetic susceptibility of Weinan and Xiaoyi coals reached the optimum value in 4 min, and the maximum magnetic susceptibility increased by one order of magnitude [45]. In 2017, [PERSON] et al. studied the complex dielectric constant and loss tangent of fine coal samples from Weinan at microwave frequencies of 0.5-6.5 GHz. It was found that when microwave treatment time was 240 s, the sulfur content of clean coal was as low as 2.92%, the optimum desulfurization degree was 32.01%, and the optimum desulfurization rate was 91.97% [46]. In addition, they studied a new dry separation technology of pulverized coal. Microwave energy was used to enhance the magnetic properties of pyrite and achieve high gradient magnetic separation.