| Drum-type magnetic separators are generally used for magnetite powder recovery in dense-medium coal preparation plants. Even though the recovery efficiency is good, it is difficult to reach 100%. A certain amount of magnetite has been lost in the magnetic separator tailings. Therefore, how to efficiently recover the magnetite powder in the coal slurry is the most urgent task for all coal preparation plants.On the basis of analysis on the magnetite powder properties lost in the tailings and referring to the magnetic separator studies around the world, two types of laboratory devices were designed. A series of tests on the magnetite recovery were carried out.The comprehensive analysis on the magnetite collected from a coal preparation plant show that the magnetite content in the tailings is 0.38%(i.e. 3.8kg/t).The results show that 30% of the magnetite powder has stronger magnetism, but can be easily lost for their large size; and 70% of them are the coenobium of magnetite and quartz, which can be easily lost for their weak magnetism. Two devices for residual magnetite recovery were designed and made. First, the magnetic field strength of bottom poles on concentrate recovery device are calculated and measured. The magnetic induction of SL-2-20-type solenoid increases with the increase of exciting current. The magnetic field intensity in the center of the solenoid is 0.31T and 0.136T on the port. The magnetic induction of high-gradient device with vertical-wheel permanent magnets (Φ170mm) are calculated and measured too. The average magnetic field intensity of the centre plane in separation chamber is measured to be 0.1T. The magnetic induction intensity on the teeth is 4 ~ 6 times that of the former after adding the high gradient medium, and the difference of the magnetic induction intensity between the adjacent teeth sharp and teeth Valley was big.Recycling experiments were carried out employing the two designed devices. The magnetite content in the tailings were measured assess the effect of recovery. So as to concluded that with the increase of solenoid current, the recovery effect of the magnetic recycling device increased gradually; the recovery effect of magnetic separator can be improved by adding additional poles with in the device; and the recovery of periphery-type pole was better than the cuboid; more magnetic media in the magnetic field can also improve the recovery of magnetic effect. The recovery effect when feed concentration is 30g / l is better; higher magnetite content in the feeding may results in better recovery effect; low flow rate also results in better recovery; the flow ratio between concentrate and tailings influences the recovery effect too, Small overflow rate gives higher recovery and better recycling results. The recycling experiments on the high-gradient device with vertical-wheel permanent magnets (Φ170mm) were carried out for slurry of low magnetite content. Results show that increasing magnetite content and increasing feeding concentration can improve the recovery effect. Increase in feed flow rate and the background magnetic field intensity also can improve the recovery effect. Pre-magnetization on the magnetic separator feed will not necessarily improve the recovery of magnetic separator and needs further analysis.High-gradient device with vertical-wheel permanent magnets (Φ1200mm) was designed by making improvement on the small ones, including the improvement of the vertical-wheel and magnetic-circuit calculation. It is multi-chamber type. The entire device width is calculated to be 5500mm and is made up of 16 separation chambers. The diameter of the separation chamber is calculated to be 1200mm and the width 200mm. More than 1200 pieces of NbFeB permanent magnet (140×100×50mm) are required.
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