In solving the abrasion device, media recovery, problems such as the simplification of the process, the medium re-selected fraction of its high separation efficiency of coal, coal adaptability of strong, low-density separation can be achieved, easy to operate and easy to achieve automatic control, etc. It is highly valued by the world's major coal producing countries. In recent years, China's heavy medium coal preparation technology has made breakthroughs in simplifying the process system, large-scale equipment, increasing the upper limit of the feed of the heavy medium cyclone, reducing the effective separation limit and automatic control of the production process. But so far, there are three major disputes in the selection of coal in the heavy coal preparation process.

Heavy medium coal removal mud removal or not

Raw coal has been preliminarily delimed and not delimed before entering the heavy medium cyclone. It is also one of the main issues debated by different coal preparation workers.

The main argument for supporting pre-desilting is that pre-dewatering is beneficial to improve the efficiency of sorting of heavy media systems. Therefore, most of the coal preparation plant design or general contracting projects undertaken by these companies adopt the method of deliming. It is believed that although the de-sifting screen is added, the average particle size of the cyclone feed is increased, the sorting precision is improved, the material entering the cyclone is reduced, the cyclone model is reduced, and the power consumption is reduced; Decrease, the desorption efficiency is improved; the secondary coal slime amount is reduced, which is beneficial to the slime water treatment; the small amount of coal sludge entering the heavy medium system can greatly reduce the partial flow rate, thereby reducing the medium consumption. In the past 10 years, such coal preparation plants have been innumerable, and it is also a reflection of the recognition and rapid promotion of foreign coal preparation concepts in China.

The idea of ​​not removing mud is that the lower limit of the sorting of the heavy medium cyclone can be as low as 0.2 mm, and the deliming is usually carried out in several ways of 1.5 mm, 1 mm, and 0.75 mm. Taking 1.5mm de-sludge as an example, the 1.5-0.2mm part can also be effectively sorted in the cyclone, so there is no need to remove the mud. Pre-desilting increases unnecessary processes, and the non-de-sludge process simplifies the process and reduces investment costs. This view has had a major impact on China's coal preparation technology. The coal preparation plant using the raw coal non-dewatering heavy medium cyclone technology exists in large quantities, and its typical representative is the three-product heavy medium cyclone system.

First of all, these two viewpoints are considered that the heavy medium cyclone is the highest sorting precision in the current common coal preparation technology, but there is debate about whether or not to remove mud. From the sorting mechanism of the heavy medium cyclone, the essence is the process of separating the two according to the density of coal and vermiculite. In general, the pursuit is to maximize the yield of clean coal under the premise of meeting the quality of clean coal, that is, to improve the efficiency of sorting, regardless of which process, which equipment, the impact of particle size on density sorting is objective Existence, minimizing the impact of particle size plays a positive role in improving sorting accuracy. Therefore, the ideal sorting method is to classify the raw coal into a number of very narrow particle sizes and separate them separately. The effect of particle size on the density sorting is neglected, and the equal lambda sorting can be realized, which is also the “grading selection”. The origin of the. From this point of view, pre-selection de-sludge can reduce the particle size difference of the feed, and under the same sorting conditions, it can really reduce the influence of particle size on the sorting, and thus improve the sorting precision.

The author believes that even if the lower limit of the sorting of the cyclone can reach 0.2mm, in order to ensure that the particles near 0.2mm can be effectively sorted, it is necessary to increase the centrifugal force by increasing the feed pressure of the cyclone. Will greatly increase power consumption and equipment wear. Secondly, because this part of the clean coal is too fine, it is difficult to separate from the less than 0.2mm material that has not been effectively sorted. It is also difficult to dissociate. There is always the problem of fine mud pollution. In order to ensure the total clean coal ash, other grades are needed. "Back ash", which in turn reduces the total clean coal yield. Third, the fine-grained classification accuracy near 0.2 mm is lower than that of coarse particles, and the Ep value is about 0.1 kg/L, which is close to other coarse slime sorting equipment, such as spiral sorting machine, interference bed sorting machine, etc. And the special coarse coal slurry sorting equipment has the advantage of low operating cost. Fourth, portions of less than 0.2 mm that cannot be effectively sorted by the cyclone also enter the cyclone, increasing the load on the cyclone and the amount of media circulation.

The idea of ​​not removing mud is that the presence of slime can stabilize heavy media. Studies have shown that with the increase of medium density, the highest allowable content of slime in the suspension is decreasing under the same solid volume concentration. A set of data is given in the literature: when the solid volume concentration is 30% (the actual production does not generally exceed this value), the suspension density is 1.45g/cm3, the slime content can be up to 48%, and the suspension density If the increase is 0.01g/cm3, that is, 1.46 gg/cm3, the slime content can be up to 36%, which is drastically reduced by 12%. When the suspension density requirement is higher, the allowable coal slime content is lower. In actual production, it is a common phenomenon to adjust the separation density by 0.1 g/cm3. In this case, in order to ensure the content of slime in the suspension, it is necessary to increase the partial flow. When the split is not guaranteed, as the slime accumulates and the viscosity of the suspension increases, the sorting effect is inevitably affected. Increasing the shunt increases the load on the magnetic separator and increases the dielectric loss. Therefore, if you want to use coal slime to stabilize the medium without removing mud, you can't achieve the expected effect. On the contrary, it will have a negative impact because the coal slurry is too much higher than the maximum allowable slime in the medium. In addition, when the de-sludge process is adopted, due to the de-sludge efficiency and the presence of secondary slime, there will be sufficient amount of coal in the medium to ensure the stability of the medium. Another point of view that supports no mud removal is that it does not remove mud and reduces the de-sludge process, which simplifies the process and reduces investment. The author believes that although the mud removal screen is omitted without the mud removal, the number of models of the heavy medium cyclone and the de-screening and magnetic separator are larger than that of the mud removal process; the mud removal sorting equipment is saved without removing the mud, but A 0.2 mm classification device has also been added. Therefore, the non-desilting process does not necessarily reduce investment.

Any process is based on the optional equipment. The de-sludge process is based on the subsequent separation of coarse coal slurry. The current coarse coal slurry separation equipment is mature and the sorting cost is very low. There is no need to use heavy medium cyclones to sort coarse coal slime, and there are many benefits mentioned above after de-sludge, which naturally becomes a reasonable choice. The non-de-sludge process occurs when the coarse coal slurry sorting equipment is not mature. In order to exert the function of the heavy medium cyclone as much as possible, it is also a big measure not to remove the mud and wash it. If it can solve the clean coal 0.2mm graded off The problem of mud, the current non-de-sludge process will be more perfect. If there is a high-efficiency raw coal 0.2 mm classification equipment, it is believed that 0.2mm de-sludge, greater than 0.2mm heavy medium cyclone sorting, less than 0.2mm flotation or non-sorting process will be highly praised.

Through the above comparative analysis, it is concluded that the pre-selection de-sludge can improve the sorting precision and reduce the medium consumption under the current technical conditions, and is theoretically better. If the non-de-sludge process can solve the problem of removing high-ash fine mud from 0.2mm clean coal, it will be more perfect.

Two-stage two-product cyclone or three-product cyclone process

When sorting coking coal, fat coal and other coal types, when it is necessary to produce medium coal, there are also different options for using two-stage two-product cyclone or three-product cyclone.

Supporting the use of two-stage two-product process: using two-stage two-product cyclone, since the sorting density of each section can be automatically adjusted, both sections can ensure the best sorting effect; two of the three product cyclones The segment density can not be adjusted online, which affects the sorting accuracy; the discharge capacity of the three-product cyclone is limited, especially when it is not suitable for the meteorite content; the three-product cyclone is large in order to ensure the normal sorting of the second section. High energy consumption.

Supporting the use of the three-product cyclone process, the three-product cyclone requires only one set of density control system, and three kinds of products can be produced with one medium. The system is simple; two-stage two-product cyclone is considered to require two rotations. Flow device, 2 feeding pumps, 2 sets of density control system, one more de-screening, complicated system and inconvenient management; three product cyclones have low requirements on medium fineness, and one section of three-product cyclone is round The cylindrical shape has a weak effect on the concentration of the medium, and the cone section of the two product cyclone has a very strong concentration effect on the medium. Under the condition of requiring the same medium density, the medium particle size required by the three-product cyclone can be coarser.

It can be seen from the main arguments of the above two parties that the three-product cyclone process mainly emphasizes that the process can be simplified and the system is simple. The two-stage two-product process emphasizes the coal quality adaptability and sorting precision. As for media adaptation, general quality standards magnetic iron ore can be adapted, not meaningful comparison.

Xiao Bian believes that the feed of the second section of the three-product cyclone comes from the underflow of a section, and the feed pressure and density are closely related to one section, which is difficult to obtain effective control, resulting in the sorting density and pressure of the second-stage cyclone. With the change of the density of the first stage, even if the method of artificial control is adopted, the stability of the two-stage sorting of the cyclone cannot be ensured, thus affecting the sorting precision; the second-stage density adjustment cannot be completed automatically, once the coal quality is actually produced. There is a change in the amount of feed, the amount of vermiculite, etc., and production adjustment cannot be carried out in time; after the second stage adjustment, the change of the second stage feed is caused, which in turn affects the sorting of one section. In this sense, even if the separation precision of the three-product cyclone is the same as that of the two-product cyclone, the actual sorting accuracy will be affected because of the influence of the two-stage. Due to structural reasons, the three-product heavy medium cyclone has a small cross section at the second feeding section, limited material passing capacity, a small diameter of the second section, and limited meteorite emission capacity, and is generally more suitable for high yield of clean coal. In the case of high coal and vermiculite yields, in order to improve the drainage capacity, it is necessary to select a large type of cyclone, and the processing capacity of one section will be seriously wasted. Since the two sections of the two-stage two-product cyclone are independently fed, the density and pressure can be automatically adjusted, and the two sections do not affect each other, and the sorting stability and applicability are higher.

For the choice of process, only consider the choice of one piece of precision and the highest accuracy of the second stage. Other questions about the size of the equipment and the amount of investment are not technically different. In terms of operating costs, the three-product cyclone process consumes more energy, and the two-stage two-product cyclone has more equipment, and the difference between the two is small.

Regarding the problem of high precision, both are heavy medium cyclones, and the sorting density is on-line adjustment. Theoretically, the separation precision is not much different, but the two sections of the two-stage two-product cyclone do not affect each other. In practice, the sorting accuracy should be higher. If there is a big difference in the precision of sorting during actual production, it indicates that there are problems with the structural parameters and operating parameters of the device itself, not the problem of the process itself.

Regarding the accuracy of the two-stage precision, the two-product cyclone should be separately arranged and adjusted separately due to its two-stage density, and its sorting accuracy and stability should be higher.

In addition, the density of the heavy medium suspension in actual production cannot be configured too high, and generally can only reach about 1.8g/cm3. Therefore, for the case of higher density sorting, the three-product heavy medium cyclone has its unique function. Because of the concentration of one section, the sorting density of the two sections can be much higher than the density of the medium, that is, the medium can be lower. Density achieves high sorting density for high density drainage. The author believes that these two programs are mature and reliable coal preparation process, and there are also more suitable situations.

In summary, the two-stage two-product process has certain advantages in terms of two-stage sorting accuracy and coal quality adaptability. The three-product heavy medium cyclone process has advantages in system simplicity. Have their own adaptability. For difficult coal selection and high content of vermiculite, it is more suitable to adopt two-stage two-product process; for easy coal preparation and high-density separation, three-product cyclone can be used. In the actual process selection, the most suitable process should be selected according to the specific conditions such as coal quality.

Pressure feed and pressureless feed are better or worse

It is also controversial whether the swirler is fed in a pressurized or unpressurized manner.

Supporting the no-pressure feeding material: the material enters the cyclone directly without pressure feeding. Before it is in contact with the medium suspension, it will not cause soaking or muddy, which is more favorable for easy muddy coal quality; When no pumping is required, the power consumption is low; the material is mixed with the suspension and then pumped into the cyclone. The material needs to be immersed in the mixing tank for a period of time, and collides with the impeller and the pipeline during pumping. The secondary coal slime is produced by crushing, and the non-pressurized feed has no such link, and the secondary coal slime is small, which is beneficial to the slime water treatment system.

Supporting the pressure feed: the pressure of coal and medium is not as good as that of the pressure when the material is not pressed; the speed of the coal entering the cyclone is too low, and the rotation of the medium with the animal material will inevitably affect the flow field. Disturbance is not conducive to sorting; low feed point is suitable for narrow sites; medium to coal volume ratio is 4:1 or lower when pressed, and medium without pressure The volume ratio to coal is 5:1 or higher. Although the non-pressed feed coal is not pumped, the non-pressure feed pump delivers more media, and the pump power is generally greater than the pressurized feed.

Xiao Bian believes that the non-pressure feed is focused on reducing mud and reducing secondary slime, and the pressure feed is focused on sorting accuracy. In actual production, if the raw coal is easy to be muddy, the pressureless material is more advantageous. When there is pressure feeding, the coal and the medium swirl at the same speed and direction, and the velocity and angle of the coal and the medium entering the cyclone are inconsistent when the material is not pressed. In theory, the flow field of the non-pressure material is more disordered. Secondary slime is mainly caused by strong swirling during the sorting process. Therefore, there is an advantage when the non-pressurized material is processed to easily mud the raw coal, and the crushing effect on the coal is smaller; the pressed material is more advantageous when the site is limited. In the actual design, the most suitable process should be selected according to the specific situation.

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