| The high pressure roller mill is the new technology since the late 1980's. The advantages of these new grinding machines are less energy consumption,less wear ,less noise emission and less space requirement than with ball mills. Today, these mills are commonly used for grinding cement clinker,coal,limestone, diamond ore,silicon carbide and other brittle materials. The high pressure roller mill brings still high pressure to bear on the particle beds and makes them destroy inside to numerous tiny cracks. At last, the particle beds get lest granularity by extrusion. The process is as followed: The particles that meet the demand of the high pressure roller mill brings are fed from the hopper into the acceleration zone. In the acceleration zone, the relative bulk density of the particle is always . The next is compression zone which is from the end of acceleration zone to the gap between the two rollers and form the angle . The particle beds start to be compressed by the rolls. The last is expansion zone with the angle , where there is no pressure on the rolls and the compacted flake expands in the horizontal direction. The compression-expansion properties of particle beds are the basis for studying the mechanism of the high pressure roller mill. According to the experimental data on particle bed compression, the most suitable equation to describe the compression, the horizontal pressure in the middle of the roll is: (1) where--specific pressure of the material of the particle beds,MPa；--initial relative bulk density,=； n -- compression curve factor；δr-- relative bulk density at the end of the expansion phase ；-- expansion ratio；--relative bulk density at the end of the compression phase；k –expansion curve factor ,k≥1 ；m—an exponent,； ； ,Z—the distance from a point to the middle of the roll. Determine the operation parameter The grinding force is equal to the pressure acted on the surface of rolls. The pressure in the compression zone and expansion zone is much bigger than in the acceleration zone. So the grinding force can be calculated by the integral in the compression zone and expansion zone. (2)The driving torque T of one roll is also composed of two components in the compression zone and expansion zone. (3)The power P of the mill is then: (4)where:u is the circumferential speed of the rolls.The throughput and the energy consumption per unit throughput are the two major values for evaluating the performance of high pressure roller mills. If the material does not slip at the compression zone, the throughput Q is : (5)The energy consumption per unit throughput is therefore: (6)When s is kept constant, the maximum pressure rises with the increase of u. There is a speed at which the maximum pressure on the surface of the roll reaches a minimum. The throughput is not linear with the speed of the roll, the higher the speed of the roll, the lower is the increase in rate of throughput. The driving torque rises with the speed. When the speed is higher than a certain limit, the required driving torque will decrease. 二,Deaeration of particle beds in high pressure roller millsThe upstream gas with a certain velocity causes an upward pressure on the particle beds. This pressure affects the vertical force balance in the gap. The vertical force balance is the perquisite for a stable operation of the high pressure roller mill. In the compression zone, the gas flow rate though plane with angle can be calculated by the integral:= (7)The area of the voids in plane is (8)The relative velocity of the gas flow through the plane in the compression zone is = (9)Since the relative bulk density of the particle beds is constant in the acceleration zone, no gas is added to the gas flow. There... |
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