Claims:We claim:
1. A crushing and grinding unit (100) for coal preparation plant comprising:
? a primary crusher (10) adapted to receive raw coal and perform primary crushing operation to form primary crushed coal,
? a secondary crusher (20) adapted to receive the primary crushed coal and perform secondary crushing operation to form secondary crushed coal,
? a silo (30) adapted to store the secondary crushed coal,
? a tertiary crusher (40) adapted to receive the secondary crushed coal from the silo (30) and perform tertiary crushing operation to form coal of a predetermined size suitable for washing unit,
wherein a screening means (50) is located intermediate of said silo (30) and tertiary crusher (40).
2. The crushing and grinding unit (100) for coal preparation plant as claimed in claim 1, wherein the screening means (50) are adapted to screen the secondary crushed coal of the predetermined size coal suited for washing unit.
3. The crushing and grinding unit (100) for coal preparation plant as claimed in claim 1, wherein the screening means (50) comprises means to transport the coal of predetermined size to washing unit without performing tertiary crushing operation.
4. The crushing and grinding unit (100) for coal preparation plant as claimed in claim 1, 2 and 3, wherein the predetermined size is -13mm.
5. The crushing and grinding unit (100) for coal preparation plant as claimed in claim 1, wherein the screening means (50) comprises of a sieve.
6. The crushing and grinding unit (100) for coal preparation plant as claimed in claim 1, wherein the primary crusher (10) comprise of a pair of rollers spaced 200 mm apart.
7. The crushing and grinding unit (100) for coal preparation plant as claimed in claim 1, wherein the secondary crusher (20) comprise of a pair of rollers spaced 75 mm apart.
8. A method for crushing and grinding (200) in a coal preparation plant comprising the steps of:

? performing a primary crushing operation (201) on raw coal to form primary crushed coal,
? performing a secondary crushing operation (202) on the primary crushed coal to form secondary crushed coal,
? storing (203) the secondary crushed coal in a silo,
? performing screening (204) of the secondary crushed coal received from the silo to screen coal of a predetermined size and transporting the coal of predetermined size to washing unit,
? performing a tertiary crushing (205) operation on the secondary crushed coal to form coal of predetermined size and transporting to the washing unit.

9. The method of crushing and grinding as claimed in claim 8, wherein the predetermined size is 13mm.
10. The method of crushing and grinding as claimed in claim 8, wherein the primary crushing operation is performed by primary crushers (10).
11. The method of crushing and grinding as claimed in claim 8, wherein the secondary crushing operation is performed by secondary crushing (20).
12. The method of crushing and grinding as claimed in claim 8, wherein the screening means comprises a sieve.
13. The method of crushing and grinding as claimed in claim 8, wherein the tertiary crushing is performed by a tertiary crusher (40).
, Description:FIELD OF THE INVENTION
[001] The present invention generally relates to coal preparation plants, and more particularly to a crushing and grinding unit of a coal preparation plant for coal screening and a method thereof.
BACKGROUND OF THE INVENTION
[002] Coal is one of the very important raw materials for iron and steel making, where the quality plays a major role in its utilization. Ash content is the key parameter and several unit operation steps are carried out prior to its end use. Separation of intrinsically distributed ash bearing minerals is very difficult; hence crushing and grinding is carried out to improve the minerals. Different stages of crushing and grinding designed to give the required particle size, at which, maximum separation is feasible.
[004] Coal is a mixture of carbonaceous matter, minerals and pore network; and is extracted by different mining operations. The carbonaceous matter and minerals are combined with the Vanderwaal’s forces of attraction. First step of mining is the blasting, where coal is blasted to a size of less than 1200 mm by crack generation from the point of blast. The immediate unit operation after the mining process is crushing and grinding to reduce the coal to a size at which, the coal washery operates. Three different levels of crushing and grinding generates the product of less than 13 mm size, where, first level of crushing (also known as primary crushing) is done through roller mills having a gap of 200 mm between the rolls. Crushed material is carried to the secondary roller crusher with a product size of 75 mm (equivalent to the roller gap) through silo and a conveyor belt. Output of secondary crusher is taken to the tertiary crushing unit through conveyor belt, where the -45 mm (less than 45 mm) coal is grounded to less than 13mm (coarse coal) by two stages of grinding and carried by conveyor belt C for storage in hopper. The crushed material is then de-slimed to separate -0.5 mm fine material (fine coal) and then fed to the different physical separation units. In this process, the grinded material with a size of less than 13 mm is over grinded and it also generates fine coal, which are very difficult to handle. Thereby, reducing the efficiency and increasing the energy consumption of the coal preparation machine. The coarse coal (-13 mm) is pre-liberated and it has very good washability characteristics, which are lost by conventional grinding methods.
[005] Thus, there is a need for a screen in between different crushing equipment of crushing and grinding unit for coal preparation plant that enhances the efficiency and reduces energy consumption and method thereof.
OBJECTIVES OF THE INVENTION
[006] The primary object of the invention is to provide a screen in between different crushing equipment of crushing and grinding unit for coal preparation machine that enhances efficiency of the coal preparation machine by better utilizing the coarse coal.
[007] Another object of the invention is to provide pre-screening crushing and grinding unit for coal preparation machine that reduces the energy consumption by reducing the grinding operation.
[008] Yet another object of the invention is to provide pre-screening crushing and grinding unit that reduces production of coal fines in a coal preparation machine pre washing stage.
[009] Yet another object of the invention is to provide a method to enhance the efficiency of a coal preparation machine by better utilizing the coarse coal.
[010] Still another object of the invention is to provide a method that reduces the energy consumption by reducing the grinding operation.
[011] Still another object of the invention is to provide a method that reduces production of coal fines in a coal preparation machine at pre washing stage.
SUMMARY OF THE INVENTION
[012] The present invention provides pre-screening of coal in a crushing and grinding unit of a coal preparation plant and a method thereof.
[013] In accordance with an embodiment of the invention, a crushing and grinding unit for coal preparation plant is provided. The unit comprises of a primary crusher adapted to receive raw coal and perform primary crushing operation to form primary crushed coal, a secondary crusher adapted to receive the primary crushed coal and perform secondary crushing operation to form secondary crushed coal, a silo adapted to store the secondary crushed coal, a tertiary crusher adapted to receive the secondary crushed coal from the silo and perform tertiary crushing operation to form coal of a predetermined size suitable for washing unit. The unit further comprises of a screening means is located intermediate of said silo and tertiary crusher.
[014] In accordance with an embodiment, the screening means are adapted to screen the secondary crushed coal of the predetermined size coal suited for washing unit. The screening means comprises means to transport (for example conveyer belt) the coal of predetermined size to washing unit without performing tertiary crushing operation and a sieve. The predetermined size (coarse coal) is not limited to any particular size, according to the preferred embodiment it is 13 mm.
[015] In accordance with said embodiment, the primary crushers comprise of a pair of rollers spaced 200 mm apart. The secondary crushers comprise of a pair of rollers spaced 45 mm apart.
[016] In accordance with another embodiment of the invention, a method for crushing and grinding in a coal preparation plant is provided. The method comprises the steps of performing a primary crushing operation on raw coal to form primary crushed coal, performing a secondary crushing operation on the primary crushed coal to form secondary crushed coal, storing the secondary crushed coal in a silo, performing screening of the secondary crushed coal received from the silo to screen coal of a predetermined size and transporting the coal of predetermined size to washing unit, performing a tertiary crushing operation on the secondary crushed coal to form coal of predetermined size and transporting to the washing unit.
BREIF DESCRIPTION OF THE DRAWINGS
[017] The following drawings are illustrative of preferred embodiment for enabling the present invention, are descriptive of some methods, and are not intended to limit the scope of the invention. The drawings are not to scale (unless so stated) and are intended for use in conjunction with the explanations in the following detailed description.
Figure 1 A: illustrates the crushing and grinding unit, in accordance with the present invention.
Figure 1B: illustrates the crushing and grinding unit of the prior art.
Figure 2A & 2B: illustrates size distribution at different crushing stages (A) Differential size analysis (B) Cumulative size analysis
Figure 3A & 3B: illustrates washability data for -13+0.5 mm fraction at primary crusher output (A) -13+0.5 mm coal directly by screening (B) +13 mm crushed down to -13 mm.
Figure 4A and 4B: depicts washability data for -13+0.5 mm fraction at secondary crusher output (A) -13+0.5 mm coal directly by screening (B) +13 mm crushed down to -13 mm.
Figure 5A and 5B: depicts washability data for -13+0.5 mm fraction at tertiary crusher output (A) -13+0.5 mm coal directly by screening (B) +13 mm crushed down to -13 mm.

Figure 6: refers to a flow chart depicting the method of crushing and grinding in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION
[018] It should be appreciated by those skilled in the art that any block diagrams herein represent conceptual views of illustrative systems embodying the principles of the present subject matter. Similarly, it will be appreciated that any flow charts, flow diagrams, state transition diagrams, pseudo code, and the like represent various processes which may be substantially represented in computer readable medium and executed by a computer or processor, whether or not such computer or processor is explicitly shown.
[019] While the present disclosure is susceptible to various modifications and alternative forms, specific embodiment thereof has been shown by way of example in the drawings and will be described in detail below. It should be understood, however that it is not intended to limit the present disclosure to the particular forms disclosed, but on the contrary, the present disclosure is to cover all modifications, equivalents, and alternative falling within the spirit and the scope of the present disclosure.
[020] The terms “comprises”, “comprising”, or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a setup, device or method that comprises a list of components or steps does not include only those components or steps but may include other components or steps not expressly listed or inherent to such setup or device or method. In other words, one or more elements in a system or apparatus proceeded by “comprises… a” does not, without more constraints, preclude the existence of other elements or additional elements in the system or apparatus.
[021] In the following detailed description of the embodiments of the disclosure, reference is made to the accompanying drawings that form a part hereof, and in which are shown by way of illustration specific embodiments in which the disclosure may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the disclosure, and it is to be understood that other embodiments may be utilized and that changes may be made without departing from the scope of the present disclosure. The following description is, therefore, not to be taken in a limiting sense.
[022] Hereinafter, a crushing and grinding unit (100) for a coal preparation plant and a method for crushing and grinding in a coal preparation plant will be discussed in detail.
[023] Coal is a carbonaceous mineral, consisting of organic macerals and inorganic minerals. Organic macerals are considered as the combustible materials and the inorganic minerals remains as ash after combustion. For metallurgical application, coal is the basic raw material, which accounts almost 60% of the total cost of production. Quality of the coal, mainly ash content, plays a major role in determining coke strength, which directly impacts blast furnace operation for producing molten iron. But, Indian run of mine coals are of drift origin with distributed mineral patterns and high ash, due to which, these coals are not suitable for direct use. However, Indian coals are used after segregation of maceral rich part through physical beneficiation route prior to its use in boiler or coke ovens depending on the end use. The process of physical cleaning mainly depends on liberation of ash bearing particles, for which, different grinding equipment were used. As the grinding process involves high energy consumption; selection of the method of comminution is of high importance for achieving the required particle size with low power consumption and high liberation. Selection of the comminution mainly depends on the input material characteristics and ease of liberation. Overgrinding of the material benefit recovery of the required material, but it also generates fine material. Generation of coal fines is energy intensive and it creates fine slurry handling challenges and finally generates high moisture coal cake, which reduce the process efficiency of steel making process.
[024] Coal miming process carried out by blasting, which generates the cracks in such a way that, the Vander waal’s forces between minerals and maceral will break, which intern liberate the material. The immediate unit operation after the mining process is crushing and grinding as depicted in figure 1B. The crushing and grinding unit (100) adopts three levels of crushing and grinding as depicted in figure 1B. First level of crushing is also known as primary crushing operation, which is done through the primary crusher (10) as depicted. A gap of 200 mm between the rolls generates the feed material with a size of 0-1500 mm size to -200 mm size. This material is carried to the next level of crushing through the conveyor belt.
[025] The conveyor belt carries the material to the second level of crushing unit, i.e., secondary crusher (20) to perform secondary crushing. Again roller mill is used for secondary crushing purpose and it has a gap of -75 mm between the rollers. The secondary crushed material is carried through the conveyor belt and the material is stored in silo (30).
[026] Output from the silo (30) is taken to the tertiary crusher (40), where the -45 mm coal is grounded to -13mm (coarse coal) by two stages of grinding. The crushed material is then de-slimed to separate -0.5 mm fine material (fine coal) and then fed to the different physical separation units.
[027] During the process of crushing and grinding, high impact forces and compression forces are applied, which grinds the coal and mineral simultaneously and it will results in homogeneous distribution of minerals in the coal. However, during the process of blasting, crack generates at the point of blast and it propagates through the mineral-coal barrier by breaking the Vander waal’s force of attraction between them, and hence, the coarse fraction is assumed to have good liberation property, which is very much essential for washability of coal. But the above property is degraded by homogenous grinding. Hence, a clear washability study conducted to check the feasibility of screening the coarse fraction at different crushing stages without loosing the overall plant yield.
[028] Coarse fraction is taken for size analysis and the overall size analysis is depicted in Figure 2. The differential and cumulative size analysis is depicted in Figure 2A and Figure 2B respectively. Differential analysis shows that, the maximum amount of coarse coal lied in 10-1 mm for primary crusher output, 6-1 mm for secondary crusher output and 1-0.5 mm for tertiary crusher output respectively. This indicates that, maximum amount of 1-0.5 mm coal generated at tertiary crusher.
[029] Cumulative study indicates that, 23.22% coal from secondary crusher is -13 mm and 29.74% coal from tertiary crusher input belt is -13 mm. This shows the scope of separation of coarse coal at tertiary crusher input reduces the tertiary crusher energy consumption of 29.74% and productivity improvement.
[030] Washability chart of coarse coal fraction and grinded lump coal fraction for belt no. A (primary crusher output) are shown in Figure 3A and Figure 3B respectively. 58% yield observed for -13 mm at an ash level of 18%, whereas-13+0.5 mm generated from +13 mm by grinding gives 43% yield at same ash conditions. This clearly indicates the best liberation due to natural breakage phenomenon. A difference in yield of 15% is being observed.
[031] Washability study for secondary crusher output coal is carried out for coarse coal and crushed coarse coal. Results of sink and float study are depicted in Figure 4A and Figure 4B respectively. It was observed that, 77.7% yield observed for -13 mm at an ash level of 18% and -13+0.5 mm generated from +13 mm by grinding gives a maximum of 40% yield. Significant change in yield values are resulted due to the fact that, pre-liberated coal generated by mining operation itself.
[032] In order to cross check with the present washery plant conditions, washability characteristics of coarse coal and grounded lump coal from ‘conveyer belt’ (tertiary crusher output) are depicted in Figure 5A and Figure 5B respectively. 40% yield observed for -13 mm at an ash level of 18% and 13% yield with -13+0.5 mm coal generated from +13 mm by grinding. An improvement of 27% yield is observed.
[033] The overall yield calculated for coals from different grinding stages and tabulated in Table 1. It was observed that, pre-separation of coarse coal at secondary crusher output will improve the yield by 0.51%.
Coal source
Weight % Wt. % of +13 mm coal after grinding, % Washability Yield @ 18% ash Total yield

+13 mm -13 mm -0.5 mm -13 mm -0.5 mm +13 mm crushed to -13+0.5 mm +13 mm crushed to -0.5 mm -13 mm -0.5 mm
Primary crusher output 76.78 18.29 4.92 64.77 12.01 44 44 52 100 48.21
Secondary crusher output 70.26 25.13 4.62 56.48 13.78 40 40 78 64 50.66
Tertiary crusher output 6.27 54.38 39.4 6.27 0.00 13 0 40 70 50.15
Table 1: Overall yield calculations

[034] In accordance with the embodiment the screening is performed using screening means (50) wherein the screening means (50) is located intermediate of said silo (30) and tertiary crusher (40). The screening means (50) are adapted to screen coal of predetermined size (coarse coal) from the secondary crushed coal suited for washing unit. The screening means (50) comprises means to transport (such as conveyer belt) the coal of predetermined size to washing unit without performing tertiary crushing operation.
[035] Methodology: Coals from Indian origin are used for the present study. Samples collected from different seams (Layer of coal pit, where coal properties are assumed to be similar). Samples collected at three different intervals for repeatability study. Coal sample collected at different parts of crushing circuit. Primary crusher output, secondary crusher output and tertiary crusher output are collected from the different conveyor belts. Standard sampling technique used for collecting the samples. 1 meter belt length sample collected.
[036] Collected samples were screened to three different fractions namely lump fraction (+13 mm), coarse fraction (-13+0.5 mm) and fine fraction (-0.5 mm). Overall analysis steps are:
Step-1: Initially representative sample (bulk sample) collected using standard technique from conveyor belt.
Step-2: Sample for analysis (~20 kg) is prepared by coning and quartering methodology.
Step-3: 20 kg sample is screened for +13 mm lump fraction, -13+0.5 mm coarse fraction and -0.5 mm size fine fraction. The weight of all fractions has to be taken for yield calculations. +13 mm lump coal is crushed to -13 mm coarse fraction and washability study carried.
Step-4: -13+0.5 mm fraction separated into 9 different size fractions and size analysis, size wise ash analysis and washability study (sink and float analysis) carried out for all the fractions.
Step-5: floatation study is carried out for all -0.5 mm samples.
[037] Yield values of all different fractions are combined and the overall yield is calculated by weight average method. The overall yield values shows the variation in the yield at different crushing stages and also give a clear idea on the benefits of separation of coarse fraction (-13 mm) and fine fraction (-0.5 mm) at different parts of the crushing circuit. Based on the results, it was proposed to screen the material at tertiary crusher input line (Figure 1A).
[038] In accordance with another embodiment, the invention also refers to a method of crushing and grinding (200). Figure 6 refers a flow chart depicting the method step. The method comprises the steps of performing a primary crushing operation (201) on raw coal to form primary crushed coal. The step is performed in a primary crusher (10). Performing a secondary crushing operation (202) on the primary crushed coal to form secondary crushed coal. The step is performed in a secondary crusher (20). Further, the method comprises the step of storing (203) the secondary crushed coal in a silo (30) and performing screening (204) of the secondary crushed coal at the input of a tertiary crusher (40) of a predetermined size and transporting the coal of predetermined size (coarse coal) to washing unit. The non-screened coal are transported to tertiary crusher and tertiary crushing (205) operation is performed on the secondary crushed coal to form coal of predetermined size and transporting the same to the washing unit.
In accordance with the preferred embodiment the predetermined size is 13mm. The primary crushing operation is performed by primary crushers (10). The secondary crushing operation is performed by secondary crushing (20). wherein the screening means comprises a sieve.