Title of Invention

AN IMPROVED ROLL CRUSHER

Abstract

An improved roll crusher with improved crushing index and reduced micro fines in the crushed output. The improvement is achieved by providing a simple modification to conventional roll crushers viz. adapting the rolls of the roll crusher to rotate at differential speed. Substantial increase in the crushing index and reduction in the % micro fines is achieved thereby. Roll crushers of the present invention are useful in various applications especially where the size distribution of the output plays an important role in the end use of the crushed particles for example in crushing of coke.

Full Text

Field of invention The present invention relates to an improved roll crusher. The present invention particularly relates to a roll crusher with improved crushing index and reduced micro fines in the crushed output. The present invention further relates to a method of improved crushing. Roll crushers of the present invention are useful in various applications especially where the size distribution of the output plays an important role in the end use of the crushed particles for example in crushing of coke. Background of the invention Crushing index is described as the percentage of weight of particles which pass through 3mm screen after crushing in roll crusher. Commonly used roll crushers are double roll crushers. One of the several applications of roll crushers is the crushing of nut coke/coke breeze which is used as fuel in the sinter plant. Over size (+3mm) particles in the crushed sinter mix cause overheating of the charge mix whereas micro fines (-0.5mm or below) are not able to generate sufficient heat for sintering. Hence it is essential that -3mm fraction is maximum and at the same time micro fines (-0.5mm) are minimum to the extent possible. Similarly in other applications it is desired that the particles in the crushed output are within a predetermined size range. Conventionally roll crushers are used for crushing of fuel in raw material handling plant or sinter plant. In such roll crushers either two rolls or four rolls are used for crushing the coke. The output of the first two rolls is used as input of the other two rolls. Alternatively there are two crushers, first in the circuit is known as primary crusher and the other crusher which receives the output of primary crusher as input is known as secondary roll crusher. Both of the rolls are driven through same speed motors with same reduction ratio, so that their roll speed is same. Several attempts have been made in the past to improve the crushing index. Certain standard practices include maintaining of optimum gap between two rolls, maintaining size of the input material which is conductive for desired output size and regular replacement of rolls after their wear for monitoring the roll gap. The above practices although improve crushing index, but also lead to increase of micro fines. Various methods are employed to reduce fines (0.5mm) such as mechanical screening, air classifier etc. However, these methods cannot be introduced in industry due to the need of fundamental changes and reconstruction of crushing departments. Such changes require substantial investments and in some cases are difficult to implement due to space constraints. Objects of the invention Therefore the primary objective of the present invention is to provide an improved roll crusher which exhibits better crushing index. Another object of the present invention is to provide an improved roll crusher which results in decreased amount of micro fines in the output. A further object of the present invention is to provide an improved roll crusher which is achieved by simple and easy modification of known roll crushers. Yet further object of the present invention is to provide a process of improved crushing with better crushing index and lower amounts of micro fines. Summary of the invention Accordingly, one aspect of the present invention provides an improved roll crusher with improved crushing index and reduced micro fines of crushed output, the roll crusher comprising two rolls which create a nip through which the materials pass and are subjected to crushing, characterized in that the two rolls of said roll crusher are adapted to rotate at a differential speed. According to another aspect of the present invention there is provided a method of crushing comprising: feeding the material required to be crushed into a roll crusher wherein the rolls of the said roll crusher are adapted to rotate at a differential speed; and receiving crushed material from said roll crusher. Detailed description of the invention Improved crushing index is achieved along with reduction of micro fines by the roll crusher of the present invention. Whenever crushing is carried out in conventional roll crushers of, say for example, of coke either in four roll crusher or two roll crusher with target of higher crushing index, over pulverization of coke takes place. It is due to the fact that the smooth rolls of the crusher work as a pair on the principle of crushing and partial abrasion. A piece of coke pulled into gap between the rolls, which rotate at the same speed is acted upon by pressure forces of equal magnitude and opposite sign. Crushing of brittle material is accompanied by cracking resulting in the formation of individual smaller pieces. These pieces are abraded by each other as they are without rotation into the convergent roll gap. This also helps to produce a large number of small dust sized fractions of coke in the crushed product. According to the present invention the rolls are rotated at a differential speed. As a result, when the top of the rolls rotate at different speeds, one side of the lump travels faster than the other side and the lump is reoriented so that it falls and pulled through the rolls of its narrow sides. This helps to reduce the applied forces and amount of over-pulverized material (micro fines). Also since one side of lump travels faster, there is some change in the action of the breaking forces. Shear forces appear along with the crushing (compressive) forces. The shear forces crush the thin wall of the cavities in the lump before the compressive forces have a chance to act, thus effectively diminishing the latter. Since the shear action takes place over the weakest parts of the cavities, this type of fracture requires less force than the fracture by crushing. It will, therefore precede the latter. The newly formed small lumps rotate between and pass through rolls without experiencing additional compressive forces or wearing by each other. Also less force will be needed to further crush largest of these lumps, which also serves to reduce over pulverization of the fuel. In usual and existing practice roll crusher rolls are driven by electric motors and the required reduction in speed is achieved through pulleys. One smaller pulley is mounted over the shaft of motor and other bigger pulley is mounted over the roll shaft. Power transmission from motor to roll takes place through a set of belts. Both of the rolls are rotated at same speed with set value of speed reduction. For achieving the advantages of present invention, a very simple modification is required in conventional roll crushers. In the present system size of one of the pulley over motor shaft is changed marginally, so that one roll rotates at differential speed of 5-15 rpm. This change in speed changes the nature of forces and reduces the micro fines as discussed above. Conventional roll crushers have two rolls. The diameter of the crusher rolls is usually around 1000mm. The two rolls are supported over bearings and incased in close space. Gap between the rolls is kept as per requirement of the output size, for example 1.5 to 2.0 mm. One pulley is mounted over the shaft of each roll. Each roll is driven by a motor and pulley. The pulley which is mounted over the motor shaft is usually smaller in size. Power is transmitted to crusher rolls from drive through belts. The pulleys over the crusher rolls are three to four times bigger in diameter of the pulleys on motor shaft, so that a substantial reduction in speed of 3 to 4 times depending on the motor speed and the desired speed of the rolls is achieved. The entire system is mounted over a rigid frame. In case of differential speed crushing of the present invention, speed of one of roll is varied, which can be either increased or decreased for getting a differential speed. This can be achieved by a variation in the size of the either of the pulleys mounted on the roll crusher or the one mounted on the motor with respect to one of the rolls. However, procurement of smaller pulley with minor change in diameter is easier and economical and marginal increase in speed of one of the crusher roll is achieved by increasing the diameter of one of the pulley on motor shaft. Same can be achieved by also decreasing the diameter. Other dimensions of the pulley on motor shaft are unchanged. Therefore, a simple change in the diameter of a pulley is required. Value of the differential speed is a very important parameter in the present context. If the difference is too small, the system is not effective. If the difference is too large, the system has adverse effect on the life of the crusher rolls. The present invention also provides a method of crushing where the material for crushing is fed into a roll crusher as described above. The material is crushed and the output is received. The micro fine content in the output is found to be substantially less and the crushing index is also high. - Description of preferred embodiment The invention will now be illustrated with reference to the accompanying drawings wherein: Figure 1 shows a schematic arrangement of roll crusher of the present invention. Figure 2 shows a sectional view of the drive pulley. Figure 3a, 3b and 3c show the forces acting on the particle being crushed in the roll crushers. Schematic arrangement of roll crusher is shown in Figure 1. The roll crusher has two rolls (1). The diameter of the crusher rolls is 1000mm. The two rolls are supported over bearings and encased in close space. Gap between the rolls is kept as per requirement of the output size. In the preferred embodiment the gap is 2.0mm. One pulley (2) is mounted over the shaft of each roll. One of the pulleys is slightly bigger than the other to impart differential speed of the rolls. Each roll is driven by a motor and pulley (3 and 4). One pulley, usually the smaller one is mounted over the motor shaft. Power is transmitted to crusher rolls from drive through belts (5). The pulleys over the crusher rolls are three to four times bigger in diameter of the pulleys on motor shaft, so that a substantial reduction in speed for example 3 to 4 times depending on the motor speed and the desired speed of the rolls. The entire system is mounted over a rigid frame (6). One of the pulleys (3) on the motors is slightly smaller to achieve a differential speed of 5 rpm. A sectional view of the pulley on the motor shaft is shown in Figure 2. The forces acting on a particle while being crushed is shown in Figure 3. Figures 3a and 3b shows rolls of conventional crushers where the rolls rotate at the same speed. Whenever a particle (1) is crushed between rolls (2), the smooth rolls of the crusher work as a pair on the principle of crushing and partial abrasion. The particle is pulled into gap between the rolls, which rotate at the same speed is acted upon by pressure forces of equal magnitude and opposite sign. Thus only compressive forces act on the particle. Figure 3c shows rolls of the crusher in accordance to the present invention, which rotate at differential speed of AN. As a result, one side of the lump travels faster than the other side and the lump is reoriented so that it falls and pulled through the rolls of its narrow sides. This reduces the applied forces and amount of over pulverized material (micro fines). Also since one side of lump travels faster, there is some change in the action of the breaking forces. Shear forces appear along with the crushing (compressive) forces which lead to the advantages of the present invention. Example: Demonstration of improved crushing index and reduction of micro fines A comparative study of the performance of a conventional crusher with the crusher of the present invention in terms of the Crushing Index and percent micro fines produced was carried out. Crushing operations were carried out in both the crushers with different settings of crusher gap. The roll speed in the conventional crusher was 326 rpm for both rolls. In the crusher in accordance with the present invention, the speed of one of the rolls was increased to 331 rpm. The result of the study is shown in table 1. Table 1 As seen from the above table, the roll crusher of the present invention exhibits increased crushing index and decreased % micro fines in the output. We claim: 1. An improved roll crusher with improved crushing index and reduced micro fines of crushed output, the roll crusher comprising two rolls which create a nip through which the materials pass and are subjected to crushing, characterized in that the two rolls of said roll crusher are adapted to rotate at a differential speed. 2. An improved roll crusher as claimed in claim 1, wherein said rolls are rotated by a set of pulleys and power transmission belts. 3. An improved roll crusher as claimed in claim 2, wherein said set of pulleys comprise one pulley mounted on the roll and one pulley mounted on the motor shaft and connected by a belt. 4. An improved roll crusher as claimed in any of the preceding claims, wherein said differential speed of said rolls is achieved by slightly varying the diameter of at least one of the roll pulley and motor pulley of one the said rolls. 5. An improved roll crusher as claimed in claim 4, wherein said differential speed is achieved by varying the diameter of the motor pulley of one of the said rolls. 6. An improved roll crusher as claimed in any of the preceding claims, wherein said differential speed is selected between 5 to 15 rpm. 7. An improved roll crusher as claimed in any of the preceding claims, wherein the gap between said rolls is selected as claimed in the desired size of the particles in the crushed output. 8. An improved roll crusher as claimed in claim 7, wherein the gap is selected between 1.5 to 2mm. 9. An improved roll crusher as claimed in any of the preceding claims, wherein said roll pulley is 3 to 4 times bigger than the corresponding motor pulley. 10. A method of crushing comprising: feeding the material required to be crushed into a roll crusher wherein the rolls of the said roll crusher are adapted to rotate at a differential speed; and receiving crushed material from said roll crusher. 11. A method as claimed in claim 10, wherein said rolls are adapted to rotate at a differential speed of between 5 to 15 rpm. An improved roll crusher with improved crushing index and reduced micro fines in the crushed output. The improvement is achieved by providing a simple modification to conventional roll crushers viz. adapting the rolls of the roll crusher to rotate at differential speed. Substantial increase in the crushing index and reduction in the % micro fines is achieved thereby. Roll crushers of the present invention are useful in various applications especially where the size distribution of the output plays an important role in the end use of the crushed particles for example in crushing of coke.

Documents:

558-kol-2003-granted-abstract.pdf

558-kol-2003-granted-claims.pdf

558-kol-2003-granted-correspondence.pdf

558-kol-2003-granted-description (complete).pdf

558-kol-2003-granted-drawings.pdf

558-kol-2003-granted-examination report.pdf

558-kol-2003-granted-form 1.pdf

558-kol-2003-granted-form 18.pdf

558-kol-2003-granted-form 2.pdf

558-kol-2003-granted-form 3.pdf

558-kol-2003-granted-pa.pdf

558-kol-2003-granted-reply to examination report.pdf

558-kol-2003-granted-specification.pdf