Extractions of size granding classes in undersize

On the basis of averaging of 544 balances of grain-size screening products distribution of the largeness classes extraction coefficients are set in a subsize product at dry and wet preparatory screening of ordinary coal and shallow machine class deslurring. Findings can be used for determination of indexes of the indicated technological operations at the calculations of quality-quantitative and water-slime coal preparations factories charts.


Introduction
Process parameters of coal preparation plant operation depend to a large extent upon efficiency of preparation of run-of-mine coal machine grades prescribing its distribution through the preparation processes. This distribution shall be taken into account, when the actual balance of coal preparation products and qualitative and quantitative, as well as water-sludge flow charts of the designed or modified coal preparation plants are computed.
Actual data of plant operation with the equivalent raw material and equipment, coal preparation plant design codes [1], as well as different regulatory documents [2,3] and scientific literature [4][5][6] are used for their computation. However, variation of coal quality towards increase of rock content, moisture content and fine grade content has caused obsolescence of previously used rates. Computation of consistency of size grade distribution through products separation in process of preparation of run-of-mine coal machine grades under the existing conditions shall be the actual research and production task and its addressing shall facilitate approximation of design and actual coal preparation parameters.
Machine grade preparation at the coal preparation plants shall generally include tree process operations: dry screen splitting, large-size grain machine grading, fine grain machine grading.
The first process operation is performed with dry screening, the second -with wet screening, the third -with deslurring.

Methods
To compute parameters of the above process operations, it shall be required to evaluate size grades of the feedstock separated into bottom-screen products, as well as top-screen product moisture content. These values are computed with due account for averaged product separation grain-size composition balances shown in [7][8][9][10][11][12][13]. Equipment loads and operation parameters met the certificate data. In the process of dry preliminary screening, the nearmesh separation grain size made 50, 25, 13 and 6 mm, for wet screening 13 mm, for deslurring -0.5; 1.0 and 2.0 mm.

Results and discussion
Total 544 balances of the grain-size composition of products split with screens and aqua screens operated at 102 coal preparation plants were considered.
Size grades separated into bottom-screen products shall be computed by the formula: Moisture content reduction factors for the top-screen product w top.


in the process of dry screening: where top W , feed W -moisture of appropriate top-screen and feedstock products, %. Table 1 shows parameters of size grading into bottom-screen products in the process of preliminary dry screening. Table 1 shows relations of coefficients of near-mesh 50-, 25-, 13-and 6-mm size grading into bottom-screen products, thus, as far as the screen operating surface hole size increases, these coefficients get higher. In addition, as far as the near-mesh size is reduced, the bottom-screen product output is drastically decreased. For such size grading the difference between outputs of bottom-screen adjacent screens makes in average 10 % (absolute). In this and the following tables O f.r. is a feedstock-relative output, O p.r is a product-relative output, ε 1,c is a calculated value of size grades separated into bottom-screen products. Table 2 shows computation of moisture content reduction factor for top-screen product ε top.w by the formula (3). The provided data show that as far as near-mesh size grading is decreased, top-screen product moisture content reduces as compared with the feedstock moisture content due to separation of the wettest size grades.
Performed studies allow for the conclusion that in the process of preliminary dry screening, 2-time increase or decrease of size grading results in absolute 10 % reduction or rise of bottom-screen product output, as appropriate.    Table 3 shows parameters of size grading into the bottom-screen product in the process of preliminary wet screening. Table 4 shows computation of coefficients of size grading into the bottom-screen product in the process fine machine grade deslurring.
Based upon Table 4, one may conclude that as far as deslurring size is increased, the top-screen product moisture content is decreased. Based upon the data of Tables 1 -4, the procedure for computation of qualitative and quantitative parameters of run-of-mine coal screening with screens and coal slurry ranging with aqua screens includes the following.
i-size grade output into bottom-screen product: where ε 1i -coefficient of i-size grading into bottom-screen or overflow product, unit friction, shall be assumed in line with Tables 1-4;  feed.i -i-size grade output in the feedstock product.
i-size grade output into top-screen or over-split product γ 2i : The Bottom-screen product output γ 1 :  The ash content of the bottom-screen product d A 1 :