Screening process

I. Basic principle of sieving The process of dividing a group of broken materials with different particle sizes into a single layer or a plurality of layers of sieve holes uniformly into a plurality of different levels is called sieving. In theory, the particles larger than the mesh are left on the sieve surface, which is called the sieve on the sieve surface, and the particles smaller than the sieve (the smallest two-dimensional smaller than the sieve size) pass through the sieve hole, and the sieve called the sieve surface Under the object.
The sieving process of the broken material can be regarded as consisting of two stages: one is that the fine particles smaller than the mesh size reach the sieve surface through the material layer composed of the coarse particles; the other is that the fine particles pass through the sieve holes. In order to complete the above two processes, the most basic condition must be met, that is, there should be relative motion between the material and the screen surface. To this end, the screen box should have appropriate motion characteristics, on the one hand, the material layer on the screen surface is loose; on the other hand, the coarse particles blocked on the screen hole are flashed off, and the fine particle permeable screen is kept clear.
The actual screening process is: a large number of different particle sizes, after the coarse and fine mixed materials enter the sieve surface, only a part of the particles are in contact with the sieve surface, and in the part of the material contacting the sieve surface, not all the fine particles smaller than the sieve pores Most of the particles smaller than the size of the mesh are distributed throughout the entire layer. Due to the movement of the screen box, the layer on the screen surface is loosened, so that the larger gap existing in the large particles is further enlarged, and the small particles take the machine through the gap and transfer to the lower layer. Since the small particle gap is small, large particles cannot pass through, and therefore, the large particles are constantly in position during exercise. Therefore, the original disordered particle group was separated, that is, layered according to the particle size, and the arrangement rule of small particles underneath and coarse particles was formed. The fine particles reaching the sieve surface are sieved through the sieve, and finally the coarse and fine particles are separated to complete the screening process. However, sufficient separation is not available, and during sieving, a portion of the undersize is typically left on the screen.
When the fine particles are sieved, although the particles are smaller than the mesh holes, the ease of their screening is different. It is known from experience that the smaller the particles, the easier the sieve is, and the particles with similar mesh sizes are transparent. Screening is more difficult, and it is more difficult to pass through the large particle gaps in the lower layer of the screen.
Second, the evaluation of the screening effect <br>Assess the effectiveness of the screening equipment in the operation process, often using two indicators: processing capacity and screening efficiency. The processing capacity is a measure of the quantity of the product, and the screening efficiency reflects the completeness of the screening process and is an indicator reflecting the quality of the product. Only when the quality is guaranteed, the quantity is meaningful; therefore, the processing capacity and screening efficiency are the main indicators reflecting the performance of the screening machine.
There are many ways to assess the effectiveness of screening, and each country has its own assessment criteria. To sum up, there are two main types: one is to use screening efficiency, and the other is to adopt possible deviation. The former is relatively simple to apply, so the application is extremely wide, and the latter is more complicated to apply, and is usually applied in the approximate screening.
The purpose of sieving is to use the size of the mesh to perform the fractional separation. Ideally, the particles smaller than the mesh size will all enter the undersize, and the larger the mesh size, the more on the sieve surface. The matter is discharged. However, in the actual production process, due to various factors, there are always some fine particles left on the sieve surface and cannot enter the sieve. However, due to different screening methods, such as the use of large sieve pores according to the small size separation of the approximate screening method, or the production of mesh damage, etc., there are also particles larger than the separation particle size into the sieve; therefore, the calculation of the sieve Efficiency should take into account the impact of these factors.
There are two calculation methods used in China and currently being applied: the volume efficiency formula and the total efficiency formula.
(1) Quantity efficiency formula
In the quantitative efficiency formula, the screening efficiency refers to the ratio of the mass of the product actually entering the sieve to the mass of the sieve actually contained in the pellet (below), expressed as a percentage.

Screening efficiency calculation chart

Where η _amount - screening efficiency (%);
C—the number of products under the screen (t);
Q—screen feeder feed amount (t);
a — content in the granules smaller than the sieve fraction (%). [next]
In actual production, because the operation is continuous, it is difficult to accurately measure the quality of the incoming and under-sand products. However, it is easier to separately determine the percentage of the feed and sieve contents which is smaller than the sieve size by the sieving test method. Therefore, the C/Q term in equation (1) can be obtained from the quantity balance relationship, namely:

From the formula (2): D = QC, substituted into the formula (3), after finishing:

Substitute formula (1)

Where θ is the fine fraction (%) of the sieve size smaller than the size of the sieve;
D — the number of objects on the sieve (t).
a, θ can be measured by taking a representative sample from the feed and the sieve to perform a screening test.
It can be seen from the formula (1) that the amount of the sieve material is regarded as 100% less than the mesh material when the mass efficiency formula is calculated. In actual production, it is not entirely true. With the development of screening technology, probabilistic sieves, equal-thickness sieves, relaxation sieves, etc., the sieve size is 1.1 to 2.0 times larger than the required classification granularity, and even larger, sometimes a sieve In the case of the surface, the sieve mesh is used. Therefore, the “super-grain” material is inevitably present in the sieve. In this case, the mass efficiency formula is not suitable.
(2) Total efficiency formula
The total efficiency formula was proposed by American scholar R.T. Hancock in 1918. Later, according to Ken and Kim's Lu made the same formula so call it that Hancock $ Lu according to Ken efficiency formula, the physical meaning of its formula is:
Total sieving efficiency η _S = recovery rate of target η ₁ - miscellaneous rate of non-target η ₂ ( 5 )
Target recovery rate

Non-target mixture rate

Similarly, using the product balance relationship:

Substituting the above formula:

Where η _S - total screening efficiency (%);
a - fines content (%) less than the specified particle size in the feed;
Β—the content of fine particles (%) less than the specified particle size in the sieve;
θ — the fine particle content (%) of the sieve having a smaller than the specified particle size.
From equation (5), the significance of total efficiency is to deduct the loss rate of the coarse material that should not be recovered from the efficiency value that should be recovered in the product. It not only reflects the fine particles that should be sifted through the sieve, but also the loss of the coarse-grained sieve.
The mass efficiency formula is different from the total efficiency formula, but there is no substantial difference. When the amount of superfine particles in the sieve is small, β≈100%, the mass efficiency formula is a special case of the total efficiency formula.
The total efficiency η _S is a comprehensive indicator reflecting the screening process. It only shows the overall effect of the screening operation. In order to specifically analyze the quality of each product, the screening operation can be evaluated according to the requirements for the screening product. In 1979 the Ministry of Coal Industry made a ministerial standards for coal screening. The total efficiency formula is used as a comprehensive index to evaluate the screening effect, and the limit rate and the upper limit rate are used as auxiliary indicators to comprehensively evaluate the process effect of the screening equipment.
Limit rate: The mass percentage of the product on the screen that is less than the specified particle size. The limit rate is determined according to the Ministry of Coal Industry Standard MT _{1 -} 82 "Measurement Method of Commodity Coal Containment Rate and Limit Rate".
Limit rate: The mass percentage of the product under the sieve that is larger than the specified particle size. Can be calculated as follows:

Where G is the mass (kg) of the coal sample taken under the sieve;
B—The mass (kg) of the coal sample that is larger than the specified particle size. [next]

(III) Calculation of double-layer sieve screening efficiency
When calculating the screening efficiency of a single-layer sieve surface, it is only required to feed the sieve machine, and the sieve material and the sieve material are respectively taken as coal samples for screening test to find the fine particle content a, β and less than the specified particle size. θ, substituted into formula (6) can be used to obtain the total screening efficiency. However, for a double-layer screen, sampling can only take the first layer of the screen surface, the first and second layer of the screen and the second layer of the screen. It is impossible to obtain the input sample of the second sieve surface (ie, the sieve under the first sieve surface). Therefore, the screening efficiency cannot be calculated directly by the formula (6); however, the second layer can be calculated by the least square method. The number of each grade of the sieve feed.
Specific steps are as follows:
(1) Separately adopting double-layer sieves for various coal samples: the feed material, the sieve on the first sieve, the sieve on the second sieve and the sieve, and carry out the screening test. Fill out the screening test report form, as shown in Table 1.
(2) Determine the classification particle size of the two-layer sieve according to the application conditions. In this example, the classification particle size of the first sieve surface: d _f1 = 25 mm, and the classification particle size of the second sieve surface is: d _f2 = 6 mm.

Table 1 Double-layer grading sieve inspection screening data

(3) According to the particle size composition of each product in Table 1, the yield of each product was determined by the least squares method (also known as the Grignard method). Calculated as follows:
The sieve yield yield formula γ _{1 of the} second sieve surface:

The sieve yield yield formula γ _{2 of the} second sieve surface:

Yield γ _{3 of the} second sieve sieve:
γ ₃ =100%-γ ₁ -γ ₂
The calculation process is listed in Table 2, and then the yield is calculated using equation (8):

(4) Calculate the particle size composition of the second layer of sieve input: the yield calculated by the formula (8) is the total sample of the upper sieve surface. Therefore, the feed of the second sieve is the sum of the on-screen yield (γ ₂ ) of the lower sieve and the yield under the sieve (γ ₃ ), so γ ₂ and γ ₃ are respectively used as the fourth in Table 3 By multiplying the five columns, the particle size composition of each product of the second layer of the screen surface as a total input material (the total sample of the upper layer sieve) can be obtained. The calculation results are listed in Tables (3). Columns 6 and 7 are added to the third column, which is the input of the lower sieve.

(5) Calculate the screening efficiency of the double-layer sieve: according to the columns 2, 3, 4, 5 and 9 of Table 3:
The fine particle content of the first sieve input is a ₁ = 84.39%;
The first layer of sieve mesh has a fine particle content θ ₁ = 14.96%
The first layer of sieve mesh under the fine particle content β ₁ = 98.18%;
The fine particle content in the second sieve input is a ₂ = 71.16%;
The second layer of sieve mesh has a fine particle content θ ₂ = 13.05%;
The second layer of sieve mesh has a fine particle content of β ₂ = 90.60%.
The first sieve screening efficiency η _sl :

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