Case Studies

Dewatering Process-Control of A Preparation Plant’s Slack System

In order to reduce the impact of coarse coal slime moisture on commercial coal quality and coal preparation plant profit, we analyze the following factors which have influenced the coarse slime dewatering effect from the managerial point of view. The factors include secondary hydro-cyclone underflow’s diameter, sieve bend’s dip angle, sieve bend’s seam, centrifuge screen basket’s seam, centrifuge’s working condition etc. The results show that the underflow’s diameter of secondary hydro-cyclone decreases from 90 mm to 75 mm in the slack system of A coal preparation plant, then the coarse slime moisture will decrease by 2. 2% simultaneously. If the sieve deck seam increases from 0. 35 mm to 0. 5 mm, the moisture of coarse coal mud decreases by 1. 45%. If the seam of coarse coal centrifuge increases from 0. 35 mm to 0. 5 mm, and the coarse slime moisture decreases by 0. 91% .

Project Introduction:

Located in Shenmu County Shanxi province, A coal preparation plant is an oversized modern underground plant, which undertook the sorting, processing, loading and transport function of 2 mines and outsourcing coal. The design scale is 34Mt/a.

Slack system is all started and fully operated, slack cleaned coal, unselected raw coal, slack coarse slime and slack fine slime are mixed into blended coal. According to the calculation 2017 whole year’s B mine’s blended coal regression formula of calorific value, 1% moisture reduced the coarse slime’s calorific value 303.88KJ and the entire blended coal’s 41.87KJ. According to blended coal sales price, if the calorific value lifts to 1kJ, the sales price will increase US$0.0111/ton, if the 1% moisture reduction will increase US$0.111/ton of B mine’s blended coal. In 2007, B mine’s blend coal output is 8.72Mt, so it could increase profit for US$968,500.00 annually.

The existing problem and rectification measures of Coarse slime dewatering.The coarse slime size is almost close to slurry, usually more than 0.3mm, which is unsuitable for flotation. Coarse slime recycling of slack coal system of A coal preparation plant adopted spiral to discharge reject and hydraulic cyclone classifier, sieve bend, centrifuge to combine dewatering, which finished separation of water, slurry and recycling. The slack system cleaning process of A coal preparation plant: coal in slurry tank is sized via cyclone classifier, overflowed to thickening tank, underflow is classified by spiral and entered into product tank. Then sized by second hydraulic cyclone classifier, overflowed to thickening tank, the underflow is thickened and dewatered by sieve bend and enter into coarse coal centrifuge to produce the final product, Coarse Slurry.

During the whole process flow, the main influence factors are the thickening and sizing procedure of second hydro-cyclone classifier，dewatering procedure of sieve bend, dewatering procedure of centrifuge and management except procedure etc.

1. Secondary hydro-cyclone classifier

The working principle of secondary hydro-cyclone classifier is that the ore pulp is feed into cyclone through tangential feeder which formed circle current under certain pressure. Under the combination of gravity and centrifugal force, the coarse size moves outward/downward and is discharged via underflow port to form high concentration underflow mainly based on coarse size, while fines move inward/upward and is discharged via overflow port, to from low concentration underflow mainly based on fine size.

During the whole process flow, the secondary hydro-cyclone classifier is the preparation for dewatering. The main purpose of using the secondary hydro-cyclone classifier is as following. One is to control material size composition entered into sieve bend. According to the theory of specific surface area, the smaller the coal size is, the more water will be absorbed and the harder the coal will be dewatered. Through the sizing function of hydro-cyclone classifier, the proportion of fine-grained material in coarse slurry could be controlled. The other is to control material concentration entered into sieve bend. Through the thickening function of secondary cyclone classifier, the material concentration and processing pressure entered into sieve bend could be reduced.

The working influence factor of hydro-cyclone classifier theoretically includes feeding diameter, overflow diameter, underflow diameter, cyclone cone angle, feeding pressure, feeding properties etc. The most intuitive method to regulate secondary cyclone classifier operating condition is adjust second cyclone classifier underflow diameter. Through site experiment, when the underflow port is 75mm, 80mm, 85mm, 90mm, the coarse slurry moisture is 23.6%, 24.1%, 24.3%, 25.8% respectively. The smaller the underflow port diameter, the higher the concentration cyclone underflow, the final products moisture is reduced. During experiment, if narrowed continually, the underflow port diameter would be blocked because of coarse sized accumulation.

2. Sieve bend

The sieve bend’s material is given to screen deck with certain speed tangent, because of centrifugal forces, the material layer moves adhering to screen deck. During the material layer flows from one screen strip to another, each sieve strip’s arris has a cutting effect on material layer. Under the function of centrifuge forces, the cut material is discharged via aperture and becomes product under sieve. The uncut material passed the screen strips becomes product above sieve.

The working parameter of sieve bend is influenced by screen deck radian, screen deck curvature radius, screen seam width, screen strip width, feeding pressure, feeding concentration etc. Under existing process of coal preparation plant, the main influence factor of sieve bend working condition are feeding concentration, screen seam width and sieve bend’s installation angle etc. The sieve bend’s feeding concentration is 15% to 40%, the screening efficiency is best when the feeding concentration is 25%. To guarantee the final products moisture, the existing sieve screen feeding concentration is about 40% on average.

Because the material separation on sieve bend surface is formed by screen probability, so the screen seam width is 1.5 to 2 times of actual separation size. Coarse slime moisture reduced obviously after screening by 0.50mm screen deck. The larger the installation angle, the faster the material flows on screen deck, the shorter the effective dewatering time. So it is necessary to regulate the angle of sieve bend to guarantee the material flowing well-distributed and slowly. In order to prolong dewatering time, the retaining strip could be installed.

 3. Centrifuge

The last part of coarse slime dewatering is centrifuge, whose dewatering effect has directly impact on coarse slime moisture. The plant uses the horizontal Scraper discharge centrifuge. The material is fed well-distributed to scraper and screen interior from feeding pipe, under the function of centrifuge gravity, feeding reclaimed water and fine sized material penetrated the screen and are discharged, while solid size is discharged under the action of scraper to form final coarse slurry products. There are the following mainly factors to influence coarse coal centrifuge. One is the feeding amount. When the centrifuge’s feeding amount is larger than design capacity, the moisture could not be fully discharged, which leads to the moisture increased. 

Now the plant is equipped 4 sets HOT HHC Intelligent Centrifuge. The design capacity is 230 m3 / h, the actual process capacity is 190 m3 / h，so there is no excessive feeding condition. Second is feeding size composition. The smaller the size, the larger the specific surface area of material ratio, the greater the capillary force, the harder it is to dewater. The component of slimes under 0.178mm is no more than 15%, as the slimes under 0.178mm increasing, the product moisture rises sharply. So the hydraulic cyclone classifier have to better control slimes component of underflow material. The third is feeding concentration. The best feeding concentration of centrifuge is 40% to 50%. Higher feeding concentration may lead to uneven feeding and blockage of feeding port, and affect the normal production. With the same feeding amount, higher moisture will increase the centrifuge workload, which lead to poor dewatering effect and products moisture increase. The forth is mechanical effect. When the screen basket’s rotation speed is lower than design speed, ( HOT HHC Intelligent Centrifuge design speed is 600r/min), the centrifuging factor will be reduced and lead to lower effect of dewatering. There are the following reasons which cause the screen basket rotation speed dropped. The loose V-belt leads to belt slip, under voltage and mulfunction of rotation motor lead to speed reduction. The scraper abrasion will enlarge the centrifuge irregular vibrating, which could not discharge the materials competely, leads to aperture blockage and influence the dewatering. Under normal condition, the interval between scraper and screen basket is 1-3mm.

Conclusion:

Through analyzing coarse slime higher moisture reason of slack coal system, the coarse slime moisture reduced gradually after improvement. It can be seen from the table 4 of coarse slurry products moisture comparison. Through theoretical analysis and practical test, we got the conclusion that the coarse slurry product moisture can be reduced to minimum if the secondary hydro-cyclone underflow diameter is 75mm, screen deck seam is 0.5mm, and coarse coal centrifuge’s screen basket seam is 0.5mm.

Any more information about coal washing plant or centrifuge, can contact us:

Peter

Email：peter.gao@hot-mining.com

Tel/WA: +86 13436553365

Tari

Email：tari.wang@hot-mining.com

Tel/WA:+86 15927439651