Patent Application
20160367997
The present invention is directed to a vertical bowl mill for producing coarse ground particles, and more specifically to such a vertical bowl mill having a coarse particle transport area located radially outward from the grinding area, the coarse particle transport area being configured to allow the coarse particles to freely exit the grinding area and to mitigate and/or prevent the coarse particles from being circulated back into the grinding area and to reduce the production of fine particles.
Various types of grinding mills are typically employed to grind solid materials such as minerals, limestone, gypsum, phosphate rock, salt, coke, biomass and coal into small particles for use in a wide range of processes such as for combustion in furnaces and for chemical reactions in reactor systems. There are many types and configurations of grinding mills including ball mills, roller mills and bowl type vertical grinding mills. The ball mills typically include a horizontal rotating cylinder containing a charge of tumbling or cascading balls. The roller mills are sometimes referred to as pendulum mills which include a support shaft rotationally supported by a bearing housing. One end of the shaft is coupled to a drive unit for rotating the shaft. An opposing end of the shaft has a hub mounted thereto. A plurality of arms extend from the hub. Each of the arms pivotally supports a roller journal which has a roller rotatingly coupled to an end thereof. The rollers rollingly engage the grinding ring. During operation of the roller mill, centrifugal forces drive the crushing members against the grinding ring. The crushing members pulverize the solid material against the grinding ring as a result of contact with the grinding ring.
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The prior art bowl mills 100 are generally suitable for producing fine particles, but cannot produce coarse particles coarser than 40-50 percent passing 200 mesh. As used herein, the percent of material passing a sieve screen refers to the amount of material smaller than that particular sieve opening. However, there is a need to produce a larger amount coarser particles in certain applications. For example, certain types of sorbents (e.g., limestone for a circulating fluidized bed (CFB) boiler) and biomass fuels are required to be ground or pulverized to coarser particle sizes such as for example, particles having a fineness of 99 percent passing a 1 or 2 mm sieve opening. In addition, such sorbents and biomass fuels have limitations on the amount of fine particles that are included in the pulverized product. For example, fine particles can be limited to 25 percent passing 200 or 325 mesh. For the case of sorbent applications, the fine particles can be blown out of the CFB boiler before completely reacting with sulfur. For biomass applications, the fine material less than 200 mesh (75 μm) is an explosive dust and should be limited. Thus, there is a rather narrow range of acceptable particle sizes for certain sorbents and biomass fuels. The prior art bowl mills 100 are generally not capable of producing a large percentage of coarse particles and tend to overgrind the particles. Such overgrinding causes an undesired increase in power consumption, a decrease in throughput of the bowl mill 100, and thus increasing the amount of sorbent usage for the case of sorbent application.
Based on the foregoing, there is a need for an improved bowl mill that is configured to produce such coarse particles in the acceptable range while limiting the amount of fine particles.
In one aspect, the present invention resides in a bowl mill for producing coarse ground particles. The bowl mill includes a substantially closed body, a bowl assembly, a plurality of grinding rolls, and a coarse particle transport enabling area. The substantially closed body has an interior area. The bowl assembly includes a rotatable grinding table mounted for rotation in a direction of rotation in the interior area. The grinding table has a grinding surface thereon. The plurality of grinding rolls are positioned proximate the grinding surface. The grinding rolls and the grinding surface define a grinding area therebetween. The coarse particle transport enabling area is located radially outward from the grinding area. The coarse particle transport enabling area is configured to allow the coarse particles to freely exit the grinding area and to mitigate and/or prevent the coarse particles from being circulated back into the grinding area.
In one embodiment, the coarse particle transport area is defined by an upwardly facing exposed surface on the bowl assembly. In one embodiment, the upwardly facing exposed surface is on the grinding table.
In one embodiment, the bowl mill further includes a wear insert secured to the grinding table by a clamp ring that circumferentially surrounds the wear insert. The coarse particle transport enabling area is defined by an upwardly facing exposed surface on the clamp ring.
In one embodiment, the bowl mill further includes a hollow cone shaped structure secured to an upper portion of the body. The hollow cone shaped structure defines a free space therein. In one embodiment, the hollow cone shaped structure tapers radially inward in a direction of flow therethrough. In one embodiment, the hollow cone shaped structure tapers radially outward in a direction of flow therethrough.
In one embodiment, the bowl mill further includes a mill plow secured to the body. The mill plow has a leading edge, a trailing edge, and an angle of incline. The mill plow is positioned on a downstream side of one of the plurality of grinding rolls. The leading edge of the mill plow faces into the direction of rotation of the rotatable grinding table. The mill plow is configured to loosen material that is caked on the grinding surface.
In another aspect, the present invention resides in a bowl mill for producing coarse ground particles. The bowl mill includes a substantially closed body, a bowl assembly, a plurality of grinding rolls, and a hollow cone shaped structure. The substantially closed body has an interior area. The bowl assembly includes a rotatable grinding table mounted for rotation in a direction of rotation in the interior area. The grinding table has a grinding surface thereon. The plurality of grinding rolls is positioned proximate the grinding surface. The grinding rolls and the grinding surface define a grinding area therebetween.
In one embodiment, the hollow cone shaped structure tapers radially inward in a direction of flow therethrough. In one embodiment, the hollow cone shaped structure tapers radially outward in a direction of flow therethrough.
In another aspect, the present invention resides in a method for controlling particle size in a bowl mill. The method includes providing a bowl mill that includes a substantially closed body, a bowl assembly, and a plurality of grinding rolls. The substantially closed body has an interior area. The bowl assembly includes a rotatable grinding table mounted for rotation in a direction of rotation in the interior area. The grinding table has a grinding surface thereon. The plurality of grinding rolls is positioned proximate the grinding surface. The grinding rolls and grinding surface define a grinding area therebetween. The method further includes transporting coarse particles from the grinding area in a coarse particle transport enabling area. The coarse particle transport enabling area is configured to allow the coarse particles to freely exit the grinding area and to mitigate and/or prevent the coarse particles from being circulated back into the grinding area.
In one embodiment, the method further includes controlling fineness of the particles by adjusting air flow through the interior area.
In one embodiment, the method further includes controlling fineness of the particles by adjusting a speed of the grinding table.
In one embodiment, the method further includes controlling particle size by adjusting a pressure applied to the plurality of grinding rolls.
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In one embodiment, the angle θ is from 0 to 20 degrees.
In one embodiment, the cone 230 has a cylindrical transition portion 230T mounted proximate an outlet 231 of the cone 230.
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In one embodiment, the angle δ is from 0 to 15 degrees.
In one embodiment, the cone 330 has a cylindrical transition portion 330T mounted proximate an outlet 331 of the cone 330.
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The grinding table 212T, 312T defines a coarse particle transport enabling area 233, 333 that enables the coarse particles exit the grinding areas 229 and 329 unimpeded. The coarse particle transport enabling area 233, 333 is located radially outward from the grinding area 229, 329. In one embodiment, the coarse particle transport enabling area 233, 333 is defined by an upwardly facing (i.e., in the direction of the arrow V) exposed surface 215E, 315E on the bowl assembly 212, 312, for example, formed on the respective clamp ring 215, 315. The exposed surfaces 215E and 315E are configured to allow the coarse particles to freely exit the grinding area 229, 329 and to mitigate and/or prevent the coarse particles from being circulated back into the grinding area 229, 329 by an impeding structure, such as the prior art dam ring 114, as shown in
While the coarse particle transport enabling area 233, 333, is shown and described in one embodiment as being defined by the upwardly facing exposed surface 215E, 315E formed on the respective clamp ring 215, 315, the present invention is not limited in this regard as other configurations of the particle transport enabling area may be employed including but not limited to a grinding table having an upwardly facing exposed surface configured to allow the coarse particles to freely exit the grinding area 229, 329 without being circulated back into the grinding area 229, 329; a castellated or segmented dam ring secured to the clamp ring and having circumferential openings configured to allow the coarse particles to freely exit the grinding area 229, 329 without being circulated back into the grinding area 229, 329 and/or a location of and edge of the grinding rolls 218, 318 proximate a radially outermost portion of the grinding table 212T, 312T.
In one embodiment, the grinding table 212T and/or 312T has an air flow vane wheel 221, 321 mounted to a radially outward facing circumferential surface thereof for establishing air flow rates and velocities, as indicated by the arrow AF in an annular area 222, 322 between the respective grinding table 212T, 312T and the body 210, 310, respectively. The air flow vane wheel 221, 321 is positioned sufficiently radially outward from the respective grinding table 212T, 312T so as not to impede the coarse particles from exiting the grinding areas 229 and 329 and not to interfere with the coarse particle transport enabling area 233, 333.
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The mill plow 250, 350 is preferably located close to an edge 212E, 312E of the grinding table 212T, 312T, respectively and in the path of the grinding rolls 218, 318 with a leading edge 250E, 350E oriented opposite the direction of rotation (R1 in
The present invention further includes a method for controlling particle size in a bowl mill 200, 300. The method includes providing a bowl mill 200, 300 comprising a substantially closed body 210, 310 having an interior area. The bowl mill 200, 300 includes a bowl assembly 212, 312 that includes a rotatable grinding table 212T, 312T mounted for rotation in a direction of rotation in the interior area. The grinding table 212T, 312T defines a grinding surface 216, 316 thereon. A plurality of grinding rolls 218, 318 are positioned proximate the grinding surface 216, 316. The grinding rolls 218, 318 and the grinding surface 216, 316 defining a grinding area 229, 329 therebetween. The method includes controlling particle size by adjusting a pressure applied to the plurality of grinding rolls 218, 318. In one embodiment, the method includes controlling fineness of the particles by adjusting air flow through the interior area. In one embodiment, the fineness of the particles is controlled by adjusting a speed of the grinding table.
Applicant has conducted testing and experimentation to determine the effect on particle fineness, capacity and power as a result of employing the a coarse particle transport enabling area 233, 333 located radially outward from the grinding area 239, 329 in the bowl mill 200, 300. The results of the three of the tests (i.e., Tests A, B and C) are summarized in Table 1, below. Tests A and B were performed using a 100 psi grinding roll pressure and an air flow rate of 5000 cubic feet per minute and the material that was ground in the mill was limestone from the USA. Test C was performed using a 200 psi grinding roll pressure and an air flow rate of 4000 cubic feet per minute and the material that was ground in the mill was limestone from Mexico.
Test A was the baseline test and employed a prior art bowl mill 100 having a 1.5 inch high dam ring 114. For Test A, the prior art bowl mill 100 employed a bed depth G of 21 mm. For Test A, 59.2 percent of the particles were passing a 75 μm sieve (i.e., 200 mesh) and 52.5 percent of the particles were passing a 45 μm sieve (i.e., 325 mesh); and 100 percent of the particles were passing a 2 mm sieve and 99.9 percent of the particles were passing a 1 mm sieve. In Test A, the prior art bowl mill 100 demonstrated a throughput of 2540 lb/hr; and required 12.9 kW-hr/ton total power to operate.
For Test B, no dam ring was employed in the bowl mill 200. For Test B, the bowl mill 200 employed a bed depth G of 3 mm. For Test B, 30.2 percent of the particles were passing a 75 μm sieve (i.e., 200 mesh) and 24.7 percent of the particles were passing a 45 μm sieve (i.e., 325 mesh); and 99.9 percent of the particles were passing a 2 mm sieve and 99.1 percent of the particles were passing a 1 mm sieve. In Test B, the bowl mill 200 demonstrated a throughput of 5710 lb/hr; and required 5.8 kW-hr/ton total power to operate.
For Test C, no dam ring was employed in the bowl mill 200. For Test C, the bowl mill 200 employed a bed depth G of 21 mm. For Test C, 21.9 percent of the particles were passing a 75 μm sieve (i.e., 200 mesh) and 17.0 percent of the particles were passing a 45 μm sieve (i.e., 325 mesh); and 99.9 percent of the particles were passing a 2 mm sieve and 98.8 percent of the particles were passing a 1 mm sieve. In Test C, the bowl mill 200 demonstrated a throughput of 5700 lb/hr; and required 7.8 kW-hr/ton total power to operate.
Although this invention has been shown and described with respect to the detailed embodiments thereof, it will be understood by those of skill in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed in the above detailed description, but that the invention will include all embodiments falling within the scope of the appended claims.
