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What is The Reseparation potential of middings for scarce coking coal?

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Coking coal is an important chemical raw material, and scarce coking coal refers to main coking coal such as fat coal, coking coal and lean coal. With the continuous development of coal industry, the high quality coking coal with shallow burial has been vigorously developed, and the coking coal resources have gradually turned to deep mining. With the large-scale development of coking coal, the raw coal quality of coking coal deteriorates. About 20% of coking coal is high-sulfur coal which is difficult to be directly used, and the available low-sulfur coal also has poor selectivity. The existing coking coal separation process will produce a large amount of medium coal, which is closely formed from inorganic mineral and organic coal rock components. If the one stage separation is adopted, it will lead to a large number of middings products with a combustible content of about 65%, resulting in a large number of high-quality scarce coking coal resources waste seriously.
The middlings can be dissociated and reprocessed to recover a large amount of qualified scarce coking coal, which can produce good social benefit and economic value. Many experts and scholars have studied the middlings reseparation technology. For example, TBS, dense medium cyclone and flotation methods are used to carry out the application research on the reseparation technology of middlings. Many middlings dissociation and reseparation technologies are also proposed, but few systematic analysis on the reseparation potential of coking middlings. One of the technical difficulties of middlings reseparation is the coordination between the degree of fragmentation and the effect of flotation. The disseminated grain size of gangue affects the upper limit of dissociation particle size. When the disseminated grain size is small, the upper limit of fragmentation is also small. Due to the over crushing, the content of fine particle size increases sharply after crushing. If the selective crushing ability of coal is poor, it is likely to lead to the production of a large amount of high ash slime. The flotation selectivity of high ash slime is poor. However in the process of separation, equipment parameters adjustment can improve the flotation quality. Also the interaction between particles and bubbles can be regulated by changing the ionic environment of the pulp to reduce the phenomenon of fine mud cover and entrainment. But, it is still very difficult to efficiently recover high quality and scarce coking clean coal under the premise of low crushing upper limit.
In order to study the feasibility of middlings reseparation in Qianjiaying coal preparation plant and promote the development and perfection of the middlings reseparation process. Through analyzing the characteristics of lump middlings, fine middlings and floatation middlings produced from Qiangjiaying coal preparation plant, the experimental research of crushing, dissociation and reseparation was carried out.
Conclusion
(1)The dissociation test of block middlings shows that, the theoretical yield of full-grain clean coal after dissociation from hammer crusher is 47.08%. When the ash content of cleaned coal product is required to be 12.5%, the yield of clean coal is 70.04% and the ash content of clean coal is 25.03% after the > 0.25mm grain size part is sorted by the spiral separator. The experiments of particle size material < 0.22mm were carried out, such as reagent optimization, flotation of high foam layer, rough and select, etc. Only rough and select could produce qualified cleaned coal products, but the yield was only 24.14%, which was low.
(2)The dissociation test of the final middlings shows that under the upper limit of crushing particle size of 1mm, the theoretical yield is very low when the particle size of 1 ~ 0.22mm is 12.5% of ash content, and the ability to directly recover qualified cleaned coal through separation is very low. The actual yield released by particle size distribution < 0.22mm was less than 1/3 of the theoretical yield. After the optimization of flotation conditions, the ash content of flotation cleaned coal is still difficult to reach below 12.5%, and it is difficult to re-separation.
(3)The flotation middlings after spiral chute , the ash content of raw coal decreases from 49.30% to 26.50%, and the yield is 46.28%. The ash reduction effect is obvious. When the ash content of cleaned coal is required to be 12.5%, the theoretical yield of cleaned coal is about 61%. The cumulative curve of cleaned coal released by steps in the flotation after dissociation shows that the actual separation yield reaches the highest when the grinding time is 3.5min and the yield is 50.05%. The flotation of coal has a good value of re - separation.
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