Abstract:
An apparatus that pulverizes feedstock material with crushers injects a liquefied inert gas into the feedstock material, evaporation of the gas causing the material to become very cold, brittle, facilitating further processing. Very fine finished particle sizes, passing −200 micron mesh screens, result from the use of the apparatus. The apparatus generally requires little maintenance to achieve such small particle size as compared to mechanical crushers. Because of the effect of the inert gas upon the feedstock material, pulverization generally occurs by impact only. A method for pulverizing is also described.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application No. 60/251,677 filed Dec. 6, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to pulverizing and, more particularly, the use of an inert gas, such as carbon dioxide or nitrogen, to cool a feedstock of material, which is then pulverized. 
     2. Description of the Prior Art 
     Equipment which processes material by reducing its size and increasing its surface area is widely known in the art. Generally, various types of mechanical reduction methods are applied to materials to reduce the size of the material and increase its surface area as a function of weight or unit volume. There are four basic ways to reduce material: by impact, attrition, shear or compression. Apparatus known in the art employ one or a combination of these four methods to process material for subsequent use. Materials can be processed by impacting in one of two ways: by gravity impact or by dynamic impact. In gravity impact, the material is dropped onto a surface harder than the material being processed. This method is most often used to separate two materials which have different friability. The more friable material is broken while the balance remains intact. The friable material can then be separated from the remainder by screening. Gravity impact is frequently employed in the processing of coal by dropping it onto a hardened steel plate. 
     Dynamic impact typically involves a rotary hammer that accelerates the feedstock material towards breaker blocks or other hammers. Dynamic impact is used when a well-graded cubicle particle is the desired result of processing, to break ores along cleavage lines removing inclusions, and for materials too hard or abrasive for hammermills. 
     The attrition process reduces materials by subjecting them to forces between two hard surfaces. Typically, the material is first processed by the impact method using hammers and then further reduced by attrition of the material against screen bars. The process is appropriate for materials that are abrasive, such as limestone and coal. 
     In comparison, shear processing involves a trimming or cleaving action, as opposed to the rubbing action that occurs during attrition. Shear material processing is typically used during primary crushing when the desired reduction ratio is in the range of 6:1. 
     Compression processing is achieved by crushing material between two surfaces, one or both surfaces acting on the material being processed. Compression processing is used on extremely hard, tough, abrasive rock that breaks cubically. Compression material processing can be combined with attrition processing. 
     One or more of the crushing methods described above is used to process raw feedstock material into sizes more suitable for end use. Often, the methods are employed in series, each step reducing the size and increasing the surface area of the material being processed. For example, precious and semi-precious metal ores, like gold, silver and copper, are reduced in size and the surface area increased to facilitate the further refining of the ore into a finished product. Coal may be processed in a similar manner for use as a fuel source particularly in fluidized bed reactors or boilers. Generally, the combustion efficiency of a fluidized bed boiler using processed coal increases in proportion to the surface area of the coal fuel source. A third example of a finely processed material would be Portland cement, wherein finer particle sizes result in a stronger, more dense finished concrete composite product. 
     Processing material into finer and finer particulate sizes requires increasing energy consumption. Such processing also accelerates the wear on the equipment being used, particularly when the finished product is “pulverized” for passing a −200 micron screen. The preprocessing of material feedstocks to facilitate a pulverized finished product, apart from using mechanical means, is unknown in the art. 
     SUMMARY OF THE INVENTION 
     An apparatus is provided for using a source of inert gas to significantly reduce the temperature of a feedstock material, increase its brittleness, and render the feedstock more susceptible to further processing, preferably pulverization. A method of using the apparatus is also described. The apparatus has a feedstock hopper which, by its dimension, controls the rate of material entering the apparatus. The feedstock is then exposed to a source of liquefied inert gas. As the feedstock material travels through the inlet, the liquefied inert gas expands and absorbs a tremendous amount of heat energy. The heat necessary to convert the liquefied gas to a gaseous state is absorbed from the feedstock material, cooling it significantly. In this cooled state, the feedstock material is extremely brittle and very little mechanical energy is required to pulverize it. The feedstock material then travels from the inlet into a crusher, such as a flywheel turbine. The blades of the flywheel turbine dynamically impact the feedstock material, reducing the material into powder. The housing of the flywheel turbine is adapted such that processed material is forced through an outlet once it has reached the proper particle size. A screen may be placed over the outlet to ensure uniform gradation of the finished product. 
     A wide range of materials can be processed using this invention. The pulverizer can be adapted to produce a wide variety of finished products by simply varying the dimensions of the inlet hopper, the inclination of the inlet tube, the rate of flow and type of inert gas used, and the dimensions and configuration of the crusher. Qualities of the finished product can be varied by regulating the flow rate of the inert gas, controlling cooling by using different inert gases, each having its own latent heat of evaporation, regulating the time to which the feedstock is exposed to the inert gas, and by varying the energy imparted by the crusher upon the material following cooling. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an elevation view of an apparatus according to a preferred embodiment of the present invention; 
     FIG. 2 is a cross-sectional view along line A-A′ of the apparatus shown in FIG. 1; and 
     FIG. 3 is a cross-sectional view of the inlet tube and jet shown along line B-B′ of the apparatus, of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     For the purposes of the description hereinafter, the terms “upper”, “lower”, “right”, “left”, “vertical”, “horizontal”, “top”, “bottom”, and derivatives thereof, shall relate to the invention as it is oriented in the drawing figures. However, it is to be understood that the invention may assume alternative variations and step sequences except where it is expressly specified to the contrary. It is also to be understood that the specific devices in processes illustrated in the attached drawings, and described in the following specification, are simply exemplary embodiments of the invention and specific dimensions and other physical characteristics related to the embodiments disclosed herein are not to be considered as limiting. 
     FIG. 1 shows an inert gas-aided pulverizer  10 . The pulverizer  10  consists of three sections: a material inlet section  44 , a pulverizer device  46  and a drive device  48 . The material inlet section  44  consists of an inlet hopper  12 , an inlet tube  14  and inert gas nozzles  16 . The material to be processed is placed into the inlet hopper  12 . The physical dimensions of inlet hopper  12  and the initial size of a feedstock material  50  to be processed control the rate of input of feedstock material into the inert gas-aided pulverizer  10 . The feedstock material  50  travels from inlet hopper  12  into inlet tube  14 . Inlet tube  14  is inclined from the horizontal approximately 30° to control the rate of travel and time that the feedstock material  50  is exposed to the inert gas. The inert gas enters the inlet tube  14  through inert gas nozzles  16 . The source of the inert gas is not shown, but may include pressurized tanks of liquefied nitrogen, or a suitable equivalent. The invention will now be further described using liquefied nitrogen as an exemplary source of inert gas. 
     FIG. 3 shows a cross section of the inlet tube  14  of the inert gas-aided pulverizer  10 , shown in FIG. 1, along line B-B′. Two inert gas nozzles  16  are shown penetrating inlet tube  14 . Each inert gas nozzle  16  is approximately 15° from vertical, as viewed in FIG.  3 . Inert gas nozzles  16  allow for the flow of liquefied nitrogen from a source, not shown, into inlet tube  14 . As the liquefied nitrogen flows from inert gas nozzle  16 , it evaporates into a gaseous state, absorbing heat from feedstock material  50  in so doing. The exact degree of cooling of feedstock material  50  is a function of the rate of feedstock material  50  that passes through inlet hopper  12 , the degree of inclination of inlet tube  14 , the rate of inert gas that flows through inert gas nozzles  16 , and the shape of the outlet pattern of that gas as it emerges from inert gas nozzles  16 . The temperature of the feedstock material  50  entering housing inlet  18  (as shown in FIG. 1) can be controlled by varying the dimensions of inlet hopper  12 , the size of the feedstock material  50 , the inclination of inlet tube  14 , and the rate of gas flow through inert gas nozzles  16 . 
     Generally, feedstock material  50  is cooled to between approximately −100° C. to −150° C. depending on the rate of gas flow through inert gas nozzles  16  and the initial size of feedstock material  50 . For example, copper ore may be pre-processed such that it has an average dimension of ½″, whereas coal, a more friable material, may be as large as 1¼″ in size before pulverization. The rate of flow of nitrogen, or other inert gas, the adaptation of inert gas nozzles  16 , including additional nozzles in more complex arrangements, the manner in which the inert gas nozzles  16  are manifolded to the source of nitrogen, the inclination of inlet tube  14  and the dimensions of inlet hopper  12  can be varied relative to each other during operation to achieve the desired finished product based on empirical results without departing from the spirit and scope of this invention. 
     Once the feedstock material  50  is cooled by the exposure to the inert gas, it becomes brittle, loses mechanical strength and is rendered susceptible to pulverization by mechanical means. The feedstock material  50  is reduced in size and its surface area increased simply by traveling down inlet tube  41  to housing inlet  18  in its cooled state. 
     Returning to FIG. 1, the feedstock material  50  enters the pulverizing device  46  through housing inlet  18 . During operation, a motor  30  turns a drive shaft  28  by turning a motor pulley  34  connected to a drive shaft pulley  36  via two belts  38  and  40 . The drive shaft  28  is mechanically attached to a flywheel turbine  20 . The drive shaft  28  turns the flywheel turbine  20  causing turbine blades  24  to rotate in a circular pattern within a housing  32  of pulverizing device  46 . As the feedstock material  50  enters the pulverizing device  46 , the rotating turbine blades  24  of flywheel turbine  20  impact the feedstock material  50 . Because of its extremely cool and brittle state, the impact of the turbine blades  24  on feedstock material  50  pulverizes the feedstock material  50  into a very fine powder capable of passing a −200 micron screen. 
     Feedstock material  50  is pulverized by pulverizing device  46  almost exclusively by impact. The housing  32  is shaped and sized relative to the flywheel turbine  20  to eliminate the occurrence of attrition and to reduce wear of the parts within the housing  32 . Further, by using T1 or carbide steel to manufacture the entire flywheel turbine  20  and a housing lining  26 , maintenance on the pulverizer  10 , during ordinary operation, is significantly reduced. 
     Turning to FIG. 2, the exterior shape of the pulverizing device  46  is shown. Flywheel turbine  20  rotates between 1500 and 3600 revolutions per minute whereby turbine blades  24  impact the feedstock material  50 , pulverizing it. The rate of rotation of drive shaft  28  can be varied to control the finished size of the material, its gradation and the ratio of surface area to weight. Generally, the grain size of the finished product is a function of the rate at which flywheel turbine  20  rotates and the number of turbine blades  24  attached to the flywheel turbine  20 . Typically, the lower the rate of rotation of flywheel turbine  20 , the larger the size of the finished product. Experimentally, with a 3″ diameter inlet tube  14 , flywheel turbine  20  rotation at 3000, 2750 and 2750 RPMs is appropriate for Portland cement, coal (up to 1¼″ feedstock size), and copper ore (up to 1″ feedstock size), respectively. As flywheel turbine  20  rotates within this range, a pressure differential results across housing  32  causing feedstock material  50  to enter housing inlet  18  under a vacuum, and be impacted by turbine blades  24 . The impact of turbine blades  24  accelerates feedstock material  50  toward housing lining  26 . The pressure differential across housing  32  further causes feedstock material  50  to be exited through an outlet tube  42 . The pressure differential across housing  32  is the result of the relative shape and dimensions of the exterior of the pulverizing device  46 , the flywheel turbine  20 , and the position of the outlet tube  42 . One skilled in the art would recognize that alternative configurations would result in similar flows of processed material through the pulverizing device. 
     Once the material has been processed and exits pulverizing device  46  through the outlet tube  42 , it can be further separated by mechanical means, including screening and filtering, to tightly control the final finished product. Additionally, outlet tube  42  can be adapted to include a mesh screen so that relatively larger particles are recycled within the pulverizing device  46  until processed to the desired finished size. 
     Material having the capacity to pass through a −200 micron mesh screen is valuable in a number of situations. There are a number of applications where finer particle size and greater surface area would provide significant benefits. These applications include refining precious and semi-precious metal ores like gold, silver and copper, processing coal for fluidized bed reactors or boilers, and processing Portland cement for specialty construction applications. Each of these different types of materials is currently processed using exclusively mechanical means to reduce the particle size and increase surface area of the materials. 
     Current processing of precious and semi-precious metal ores uses acid and cyanide baths to prepare the ores for refining. Over the past few years, the environmental and safety regulations of such operations have come under increasing scrutiny. Fine particle sizes can be achieved at efficient rates by utilizing the method and apparatus of the present invention. Metal ores with small particle size and increased surface area can be processed more efficiently because the metal can more easily be extracted from the ores, particularly when using acid and cyanide baths. 
     Similarly, the use of fluidized bed technology in the coal burning industry has become more prevalent. Fluidized bed boilers burn coal more efficiently and more cleanly than conventional burners. Coal processed through the method and apparatus of the present invention would result in a product having smaller particle size and greater surface area. This product would burn more efficiently, more completely, and result in fewer emissions as compared to coal processed by traditional mechanical crushing methods. Typically, facilities using fluidized bed technology for coal fuel spend significant sums to maintain their ball mill grinders used to process coal for that application. Maintenance costs would be significantly reduced if the apparatus and method of the present invention were used to process feedstock material for the fluidized bed reactor. The invention could be combined downstream in series with a traditional ball mill. With such an arrangement, the finished particle size output from the ball mill could be much greater. Larger particle size reduces attrition within the ball mill and correspondingly reducing the maintenance necessary to address that attrition. 
     In addition to a flywheel turbine crusher, shown herein, the invention could also be embodied in combination with a rotary hammermill, or with simple gravity feed of the material by dropping onto a hard surface, both preceded by exposure of the feedstock to evaporating inert gas. 
     There are occasions in the construction industry where dense strong concrete is required. The strength and density of any concrete product is, among others, a function of the gradation of the Portland cement used to make the concrete composite. Portland cement processed with the method and apparatus of the present invention would have a smaller particle size and greater surface area. Accordingly, a denser, stronger, concrete composite material can be made by using material that has been processed in accordance with the present invention. Such a concrete composite would be appropriate for those circumstances where a strong, dense, concrete composite is required. Concrete composites, manufactured from cement subjected to the process of the present invention, result in an approximately 30% increase in the strength and density of the finished composite as this result having been demonstrated by standard testing. 
     While the preferred embodiment of the present invention has been described, it will be understood that the invention is not limited thereto, but may be otherwise embodied and practiced within the scope of the appended claims and equivalents thereto.