Closed hammer mill circuits
United States Patent 2400382

This invention relates to improvements in closed hammer mill circuits. An important object of the invention is to substantially eliminate loss of finely ground material pneumatically conveyed from a hammer mill. It is conventional practice to use a hammer mill or the like for grinding material...

Arnold, Gerald D.
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Arnold, Gerald D.
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Other Classes:
62/57, 62/320, 241/48, 241/65, 241/79.2, 241/185.5
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This invention relates to improvements in closed hammer mill circuits.

An important object of the invention is to substantially eliminate loss of finely ground material pneumatically conveyed from a hammer mill. It is conventional practice to use a hammer mill or the like for grinding material to a fine powder and to employ pneumatic convection currents for delivering the material from the hammer mill and to separate the ground material centrifugally from the convection current. The separators have Invariably discharged the convection gases because of considerations which have precluded the recirculation thereof. It is the object of the present invention to provide means whereby such gases may be recirculated, thus substantially eliminating loss.

Another important object of the invention is to cool the product in process of grinding instead of heating it. The invention includes the method, 2( as well as the apparatus.

Other objects of the invention will be apparent to those skilled in the art upon analysis of the following disclosure thereof. 2 In the drawing: Figure 1 is a diagrammatic view in side elevation partially broken away to expose the interior mechanism and illustrating a hammer mill having a closed circuit pneumatic convection system 3 embodying the invention.

Figure 2 is a view of the hammer mill partiallY in section and partially in elevation as viewed in a plane at right angles to the showing of the hammer mill in Figure 1.

Like parts are designated by the same reference characters throughout the several views.

The present application is a companion to an. application entitled Improvements in apparatus and methods for spray-drying and cooling and filed July 29, 1943, Serial No. 496,574.

The hammer mill 15 (Figs. 1 and 2) comprises a casing which may contain a screen at Is6 and a rotor 17. The material to be ground in the mill is admitted through the pipe I8 and the rotary charging valve 19 which excludes all atmospheric air other than such as is carried in the interstices between particles of material.

The hammer mill casing preferably comprises part of a closed circuit in which convection gas circulates. Ordinarily the gas may comprise ordinary atmospheric air. The pipe 20 leading to the hammer mill casing may have one branch 21 opening into the casing above the rotor 17 and another branch 22 opening into the base below screen 16. From the hammer mill,'base an outlet pipe 23 for the circulating gas and the material entrained therewith leads into the tangential inlet 27 of a centrifugal separator 30 which is preferably made in accordance with the disclosure of my co-pending application Serial Number 466,576, filed November 23, 1942, and entitled Centrifugal separators.

The separator has an undercut screw-like shoulder at 31 for urging all solids toward the rotary discharge valve at 32. Gases escape centrally through the hood 33 to the central eye or inlet of a centrifugal blower 25 having a tangential discharge.

The location of the blower is preferred to be as shown but since the circulatory gas circuit is closed, the blower may be associated with the hammer mill in accordance with conventional practise or may be located elsewhere in the circuit whenever the special advantage of the indicated position is unnecessary to a particular, device.

For reasons explained in the companion application above identified, the inlet of the centrifugal blower casing is preferably axially aligned with the discharge boot 33 of the separator casing 30 and the two casings are arranged for opposite rotation of the vortices respectively formed therein. Since the tangential inlet 21 to the separator casing 30 establishes a vortex rotating counterclockwise as viewed in plan, the centrifugal blower is arranged for clockwise rotation of its runner to establish a clockwise vortex in the blower casing. This clockwise vortex tends to propagate itself downwardly through the axially aligned boot 43 of the separator casing and to 35 interact within the separator casing with the counterclockwise vortex therein to establish a zone in which the gases will not be in rotation at all, but will be moving axially in a vertical direction. The elimination of the rotative veloc40 ity of the gases enables the entire cross section of this neutral zone to be filled with gas in uniform and relatively low velocity movement, thus facilitating the discharge of almost all pneumatically borne solids by gravity therefrom. 45 The cooling coils 36 in the refrigerating chamber 35 are only shown on a small scale where the door 31 of the cooling chamber 35 is broken away in Figure 1. However, it will be understood that they are as smooth surfaced as pos50 sible and although they may be made as suggested in the companion application above identiffed, they preferably comprise pipes arranged in spaced banks with fins common to all coils in a given bank and spaced in positions paralleling 55 air flow to be. secured by circulating gases for affording extended heat absorption surfaces. An: conventional refrigerating mechanism may bi used with the coils 36. I have shown a motor a 38 driving a compressor 39 to deliver compresse( refrigerant through a condenser 40 to the coils If, notwithstanding all precautions, dust or mois. ture accumulates on the coils, the coils may read. ily be cleaned by opening the door 37 whic] affords access to the spaces between banks oJ pipes and enables the pipes and fins to be reached for cleaning.

The device operates as follows: The air or othei gas in the closed circuit is maintained by fan 25 constantly in motion at a velocity such as tc impel from the hammer mill to the separator all material ground in the hammer mill. A hammer mill usually reduces to fine dust a part of the material upon which it acts. Some of the material ground in hammer mills is very valuable.

While the improved type of separator shown at 30 is much more efficient in removing dust than is the ordinary separator, there would be some loss of material but for the fact that the gas is recirculated, thus preventing the escape of any solids whatever other than such as are delivered from the bottom of the separator.

While a closed circuit arrangement saves dust which would otherwise be lost, it would not be feasible in a circuit including a hammer mill if it were not for the refrigeration of the air. An ordinary hammer mill tends to raise the temperature of the material upon which it acts.

The temperature rise averages between 10 and 20 degrees, depending somewhat upon the nature of the material, the speed of the mill, and the coarseness of the screen. Since the air or other gas pneumatically conveying the material receives from the material at least a part of the heat engendered in the hammer mill, it will be apparent that if the air or gas were recirculated without first being refrigerated, its temperature would speedily be raised to the point of causing combustion of the materfal. The rise in temperature is also deleterious to many of the materials handled. Therefore, by refrigerating the air or gas in the closed circuit, I not only keep the temperature of the air or gas low, but I am able to cool the material and, by enabling the hammer mill to operate at lower temperatures, I am able to avoid the possibility of fire and the possibility of injury to the material or to the parts of the mill.

It would not ordinarily be regarded as good practice to refrigerate convection gas which has acted upon finely divided material, as the gas tends to dehydrate to some degree the material which it conveys, particularly where such material has been comminuted or ground, thus opening its cells. Obviously, if the convection gas were to pick up any appreciable quantity of moisture from the material conveyed, it would deposit such moisture upon the coils in the refrigerating chamber 35 and the moistened surfaces of the coils would cause any dust entrained in the recirculated gas to adhere to the coil surfaces and thereby rapidly foul the refrigerating organization and destroy its efficiency, finally rendering it inoperative, or requiring prohibitively frequent cleanings.

Accordingly, in the preferred operation in acy cordance with this invention, I first dehydrate e the material delivered into the hammer mill (unt less it is already dry) until its moisture content I is below ten per cent and preferably below six . 5 per cent. Then, by using coils with very substantial surface areas, I am able to operate such coils to cool the recirculated gas without reS ducing it to the dew point. I prefer that the 1 gas should not be cooled below 32-38 degrees F.

L 10 and that the gas be circulated in sufficient volume so that the return gas passing from the separator through pipe 34 to the cooler would preferably be about 60 degrees F.

S With the material preliminarily dehydrated, S15 gas within the temperature ranges indicated will not pick up any appreciable moisture from the material, nor will it deposit moisture upon the coils. With coils of properly stream-lined form, as for example, the coils shown in my Patent No. 2,266,292, or those of my companion application, the coils will tend to be self-cleaning and will require dust removal only at very long intervals, if at all.

From the foregoing it will appear that in order to prevent loss of the material ground in the hammer mill, it was necessary to recirculate the convection gas, but in order to recirculate the convection gas it became necessary to refrigerate such gas, and in order to refrigerate the gas it became necessary to take special precautions to preclude accumulations of dust and moisture on the refrigerating means. The apparatus and method as disclosed meet these conditions.

I claim: A method as herein disclosed which comprises circulating a convection current of gas in a substantially closed path, excluding the gas of said current from direct communication with the exterior atmosphere whereby to tend to preclude any change in its moisture content, refrigerating the circulating gas of said current to a temperature at which said gas is at least approximately 32 degrees F. and above its dew point by passing such gas in the course of its circulation over a dry refrigerating surface of such large, area that no moisture condensation will occur upon such surface to cause the adhesion theretb of fines entrained in the gas of said current, admitting to the gas of the circulating convection current moisture containing but pre-dried material which is combustible and which an undue rise in temperature is deleterious, dry grinding isuch material in air suspension to a degree of fineness such as to adapt it to be pneumatically conveyed by the gas of said current, maintaining the flow of said current with sufficient rapidity to entrain the ground material, centrifugally separating ground material from the gas of said current and discharging the separated material from said gas prior to the circulation of the gas to the point of refrigeration of said gas, and passing through such point of refrigeration with the convection current of gas such finely divided material as is not centrifugally separated and discharged, and finally commingling such fine material with further material newly admitted to the gas of said current.