Study Of Low Temperature Heating Pretreatment Followed By Dry Magnetic Separation Of Low Rank Coals

This thesis mainly deals with a new process in removing ash and sulfur contents from low rank coals with high sulfur content. In this process, low rank coal is upgraded by heat treatment and then by magnetic separation. The influence of heating temperature and heating time on concentrate yield, sulfur content and ash content were studied. The effect of magnetic separation on ash and sulfur removal was also studied. Furthermore, the possibility to remove organic sulfur of low rank coals by magnetic separation using iron as a catalyst was discussed.Heat treatment experiments were conducted with Low rank coal samples with high sulfur content collected from Qianjin and Hongxin coal preparation plants. According to the results, with the increase of heat treatment temperature, moisture and volatile of coal escaped as gas. Also, pyrite and organic matter of coal was decomposed at high temperature. Therefore, an increase of concentrate ash and a decrease of clean coal ash was seen. While the sulfur content shows an upward trend at first, but a decrease tendency later. At a certain heating temperature, with the increase of heating time, the concentrate yield drops down while ash rises up as heating time goes by.Magnetic separation experiments of upgraded coals were carried out. Results showed that at the heating temperature between 0 to 300 oC, yield of magnetic material is low. At the temperature of 400 oC, yield of magnetic material reaches the highest point. According to the XRD analysis, pyrrhotite was found in the coal sample in the temperature of 400 oC. And a reasonable de-ash and desulfurization effect by magnetic separation was obtained at a heating temperature between 400-550℃. In detail, the sulfur content of the concentrate could be as low as 1 %, which is much lower than that in the raw coal with a sulfur content of 7 %. The lowest concentrate ash of Qianjin and Hongxin coal were both obtained at 400 oC, the ash is 11.01% for the former and 13.74% for the latter, which is 2.04% and 3.34% lower than the raw coal respectively.In order to remove organic sulfur from low rank coals, the study of heat treatment followed by magnetic separation with iron as a catalyst was carried out. ShengLi lignite with a high content of organic sulfur but little non-organic sulfur was used here. At the magnetic separation tests without iron, only a small proportion(5%) of magnetic material was recovered. The value increases greatly to 30% once the iron was added. According to XPS analysis, iron sulfide was found after heat treatment when adding iron as catalyst. The sulfur content of concentrate is 0.5%, which is much lower than that in the raw coal(1.8%). This shows that the addition of iron catalyst is favorable for organic sulfur removal.