GB2073048A - Grinding methods and apparatus - Google Patents

Abstract

The size reduction of plastics, rubber, or other materials is effected with reduced consumption of a cryogenic fluid such as liquid nitrogen by introducing the materials to be ground into a hopper 16 and supplying a liquid cryogen from a conduit 37 to the throat 17 of the hopper 16 which is disposed in communication with a standpipe 18. A screw conveyor 20 is provided for metering chilled material, equilibrated in temperature, from the exit of the standpipe into a grinding mill 22 where such material is reduced in size. Temperature control means are preferably provided so as to sense the temperature existing in the hopper throat to enable the flow of liquid cryogen thereto to be controlled. By passing chilled materials through the standpipe 18, sufficient time will elapse to enable a temperature equilibration of the materials and, thus, enable grinding thereof with less energy and hence less heat generated by the grinding device which in turn requires less, if any, additional flow of liquid cryogen to the grinding mill 22 and hence minimises the consumption of such cryogen. <IMAGE>

Description

SPECIFICATION Grinding methods and apparatus The present invention relates to methods and apparatus for reducing the size of materials and more particularly, to the grinding of discrete materials at subambient temperatures.

One technique for reducing the size of materials such as rubber crumbs or plastic bits is to subject such materials to extremely low temperatures thereby embrittling the same so that upon subsequent impact by hammers or the like, such materials shatter into minute pieces. These size reduction processes are typically referred to as cryogrinding methods. Although these techniques may be effective to achieve desired size reductions, the use of cryogenic fluids such as liquid nitrogen (LN2) is required and thus, cryogrinding techniques may be costly unless efficient use is made of the particular cryogen employed. In order to improve the economics of cryogrinding systems, it has been proposed to prechill the materials to be ground by contacting such materials with liquid nitrogen as is illustrated in US patent No. 3 771 729.It has been found, however, that utilization of relatively long screw type prechiller units as illustrated in this reference significantly adds to the capital costs of cryogrinding systems and thus degrades the economic desirability thereof.

It has also been proposed to effect a degree of prechilling materials to be ground by passing materials through a hopper into which a liquid cryogen is introduced as is illustrated in US patent No. 4 073 443. The vaporised liquid cryogen is to a large extent passed downwardly through the grinding device or mill from which it is permitted to escape and is returned to the upper reaches of the hopper in an attempt to prechill material introduced therein. In operation of such systems, considerable quantities of the liquid cryogen are passed directly into the grinding device which tends to utilise the refrigeration of such cryogens inefficiently and permits the downward escape of substantial quantities of vaporised liquid cryogen thus not fully exploiting the refrigeration available in this cryogen.It is also known to use liquid cryogens to "shock" prechill materials supplied to a hopper in a deburring system as is shown in US patent No. 4 030 247. The shock prechilling of such materials, although suitable for deburring, generally fails to render such materials sufficiently chilled to enable the grinding thereof in a hammer mill or other device without generating excessive quantities of heat and thus requiring unacceptably large quantities of heat and thus requiring unacceptably large quantities of the liquid cryogen in order to achieve desired size reductions.

Accordingly, a clear need exists for a cryogrinding method and apparatus which economises on the quantity of liquid cryogen in effectively grinding material to required sizes without the use of unnecessarily complex and costly structure or equipment, and it is the aim of this invention to go at least some way in meeting such need.

According to the present invention there is provided a method of reducing the size of material comprising the steps of passing said material through a restricted area, supplying a liquid cryogen to said area to contact said material thereby chilling the same, permitting the temperature substantially to equilibrate throughout the material at a temperature below a predetermined value, metering the temperatureequilibrated materials into a grinding device and reducing the size of said temperature-equilibrated material in said grinding device.

The invention also provides apparatus for reducing the size of material comprising a hopper for receiving said material having a throat through which said materials are discharged from the hopper, means for introducing a flow of liquid cryogen into the vicinity of said throat to chill said material passing therethrough, a standpipe adapted to receive said chilled material discharged from said hopper with the volume of said standpipe being sufficient to retain said chilled material for a time required for the temperature of said material substantially to equilibrate, grinding means, and means for metering said chilled temperature-equilibrated materials from said standpipe to said grinding means.

By assuring that each of the materials to be ground is equilibrated to a temperature at or below a predetermined embrittlement temperature, less power or energy must be supplied to the grinding device in order to achieve a desired size reduction of the materials. As a consequence of providing less power or energy, less heat is generated during operation of the grinding device and the tendency to increase the material temperature to a level above the predetermined or embrittlement temperature is thereby abated.Consequently, the chilled materials will be ground without requiring excessive additions of the liquid cryogen to the grinding device (or mill) in order to retain temperature of the material below a predetermined or embrittlement level and consequently, the grinding process according to the invention will enable efficient cryogrinding of materials with less liquid cryogen per pound of material ground than has heretofore been utilised.

The restricted area through which the material initially passes may comprise the throat of a hopper which is coupled to the top of a standpipe or the like. The material to be ground is chilled upon contact with liquid cryogen introduced into the vicinity of such throat and by virtue of having to pass through a standpipe, a time delay of passage of such material to the grinding device or mill is achieved. The purpose of establishing such a time delay is to permit equilibration of temperature substantially throughout the material being chilled.The contact between the liquid cryogen and the material being chilled is also effective to reduce the average temperature thereof below a predetermined or embrittlement value such that not only are temperature equilibrated materials supplied through a feeding means from the standpipe to the grinding device, but such materials actually yield refrigeration to the mill without causing the temperature of such materials to exceed the predetermined or embrittlement value upon grinding.By so refrigerating the mill, the heat which is inevitably generated upon dissipation of energy, i.e. power consumption, in the grinding device is compensated and does not result in raising temperatures of the materials above the predetermined or embrittlement point and thus enables such materials to be ground without excessive additions of liquid cryogen to the mill itself which typically is an ineffective device for cooling or refrigerating such materials.

Consequently, crvogrinding of materials is effected with reduced consumption of the liquid cryogen.

The invention will be more clearly understood by reference to the following description of exemplary embodiments thereof in conjunction with the following drawing which is a diagrammatic view of cryogrinding apparatus utilising a hopper prechiller and standpipe in according with the invention.

Before describing the method and apparatus according to the invention in detail, it is believed helpful to define certain terms utilised herein. The term "material" refers to discrete bits or crumbs of solid substances such as rubber derived from automobile tyres or the like or plastic substances which have been cut or reduced in size to dimensions on the order of about 0.25 in (0.6 cm).

Alternatively, the material to be ground may comprise food substances such as spices or the like and typically, materials ground in accordance with the invention will be reduced in size to dimensions of approximately 0.1 in (0.25 cm) as will be subsequently described. The term "liquid cryogen" is used herein to mean the liquid phase of an inert gas such as liquid nitrogen or liquid argon etc. The former is generally less costly than the latter and consequently, the term "liquid nitrogen" will be used interchangeably with the term liquid cryogen. A further characteristic of liquid cryogen is that such substances are typically supplied in a very dry condition in that the dew points thereof are usually in the order of-500F (-450C) or lower.As those skilled in the art will appreciate, liquid nitrogen and liquid argon are commercially available from suppliers which derive the same from the separation of ambient air. Finally, the term "embrittlement" refers to a condition of the crystal structure of materials which upon impact tends to shatter into many smaller pieces. In some instances it may simply be desirable to grind materials under relatively cold conditions, e.g. conditions which do not require embrittlement but which are effective to avoid polymerisation of certain plastics, etc.

Accordingly, the term "embrittlement" will be utilised interchangeably with chilling of materials to predetermined, low temperatures.

Referring now to the drawing illustrated therein is an exemplary embodiment of cryogrinding apparatus 10 essentially comprising a hopper 16, standpipe 18, metering means 20 and grinding device 22 with associated valves and conduits for supplying a liquid cryogen thereto. More particularly, a conveyor 14, or other suitable means is provided to supply material 12 to the inlet or upper end of hopper 1 6 which is disposed with a throat area 1 7 adjacent to the upper end of a standpipe 1 8 such that material 12 passes through hopper 16 and through standpipe 18 preferably under the influence of gravity.A metering means 20 which may comprise a screw conveyor or auger or the like is disposed to receive material 12 from standpipe 1 8 and pass the same at a predetermined rate into grinding device 22 which may take the form of a mill having a screen 25 and a plurality of hammers 24 mounted for rotation therein. Grinding device 22 is typically provided with an outlet 26 for discharging material 12 which has been considerably reduced in size with respect to material 12 supplied to the inlet of hopper 16.

A flow of liquid cryogen such as LN2 (liquid nitrogen) is supplied through conduit 30 and may be divided into conduits 32 and 34 having valves 33 and 35, respectively, disposed therein.

Conduits 32 and 34 are preferably joined to form conduit 37 through which the liquid cryogen flows and is discharged from the lower end of nozzle thereof at a point in the vicinity of throat 1 7 of hopper 16. A temperature sensing means 36. is provided in the vicinity of the discharge end of conduit 37 and preferably within 6 inches of the point at which LN2 is introduced into hopper 16.

Temperature sensing means 39 is effective, in known manner, to generate an electrical signal which is supplied through line 40 and is utilised to control the setting of valve 33 which may take the form of a conventional solenoid valve. Valve 35 is preferably a need valve which may be manually set or adjusted. A further flow of liquid cryogen is supplied under the control of valve 42 through line or conduit 41 to grinding device 22 for the purpose of maintaining desired, low temperatures therein during the grinding of chilled materials 12.

Hopper 16 which has a half angle of 300 or less is provided as a convenient and effective device for collecting material 12 to be ground and, more importantly, is utilised to prechill such material as the same is passed downwardly therethrough to standpipe 18. As the liquid cryogen is discharged from conduit 37 into throat 17, the liquid is vaporised and the resulting cryogen vapour is caused to flow upwardly through material 12 in hopper 16. By utilising LN2 which generally exhibits low dew points, "bridging" in throat 17 is avoided due to lack of sufficient moisture. By selecting a hopper having a sufficient cross sectional area at the upper edge thereof, and assuring that a predetermined level of material exists therein, the velocity of such gas will be low enough to avoid fluidising material 12 and will thus not impede the passage thereof through hopper 1 6. Also, by disposing the discharge end of conduit 37 in the vicinity of throat area 17, contact between the liquid cryogen and material 1 2 passing through the hopper is maximised due to the concentration of such material in this restricted area.Consequently, an effective heat transfer occurs between the liquid cryogen and material 1 2. The resulting cryogen vapour will ascend upwardly throughout the greater length of hopper 1 6 and yet be in contact with material 12 thereby prechilling the same. Hopper 1 6 is filled to a level with material 12 such that the cryogen vapour leaves hopper 1 6 at virtually ambient (i.e.

room) temperature. Thus, substantially all of the refrigeration available in this vapour is utilised to prechill material 12. Consequently, efficient use is made of both phases of the cryogen in the course of chilling material 12 and by sensing the temperature in hopper 1 6 immediately adjacent (i.e. a distance of up to 6 in from) the discharge end of conduit 37, the flow of cryogen may be controlled by controlling the operation of solenoid valve 33. Preferably, a gas flow of liquid cryogen is established through valve 35 and conduit 34 and by effecting an "on-off" operation of valve 33, the base flow may be pulsed with additions of liquid cryogen supplied through conduit 32 in response to the sensing of temperature in the throat of hopper 16 by sensing means 39.It will be appreciated that by sensing the temperature in the throat area 1 7 of hopper 1 6 as mentioned above, the actual minimum internal temperature (e.g.

-2000F or so) to which materials will be chilled can be carefully sensed and controlled thereby enabling a more efficient utilisation of the liquid cryogen.

Standpipe 1 8 is in communication with the throat area of hopper 1 6 and metering means 20 is provided for a specific purpose. That is, the volume of standpipe 1 8 is selected so that a sufficient time delay occurs between the passage of materials 12 through throat 1 7 of hopper 16 and entry into metering means 20 and hence grinding device 22. This time delay is important to enable the temperature of the material 12 to equilibrate thereby assuring such material is chilled throughout to at least a predetermined or embrittlement temperature. Consequently, upon grinding of such materials in device 22, power consumption by device 22 will be minimised.It will be appreciated that in the event that discrete particles of material 12 are not fully equilibrated as is common with utilisation of prior art cryogrinding systems, only the exterior or skin portions of such materials are easily shattered during grinding. Thus, further power, i.e. energy, is required to grind such discrete particles to desired sizes which in turn generates further heat in the grinding device and additional, substantial flows of liquid cryogen to the grinding device are required to maintain required temperatures (e.g. -1 500F (-1 000C)) in the grinding device. Such additional flows of liquid cryogen, however, further degrade the operating efficiency of typical prior art systems.Hence, by assuring a temperature equilibration of materials passed through standpipe 18, a more efficient (in terms of liquid cryogen consumption) cryogrinding of such materials is enabled.

It will be understood that the time delay of material 12 in standpipe 1 8 will be determined by the volume thereof. For a given cross sectional area, the height of standpipe 1 8 will determine this time delay although the diameter or width or standpipe 1 8 may be greater than similar dimensions of the exit of hopper 1 6.

In accordance with a further aspect of the invention, it is preferable during cryogrinding to chill materials 12 to temperatures below the embrittlement temperature thereof. By "subcooling" materials 12 upon contact between the liquid cryogen and such materials in throat area 17 (e.g. to about 200a F), as the chilled and equilibrated materials 12 are fed into grinding device 22 by metering means 20, the materials themselves will effect a cooling or refrigeration of the grinding device 22. This cooling is effective ta compensate for heat generated in device 22 as hammers or the like impact material 12 and grind the same.Consequently, providing refrigeration to device 22 integrally with the passage of materials 12 therein results in less additional liquid cryogen being supplied through conduit 41 to maintain device 22 at a temperature below the embrittlement temperature of materials 12.

Consequently, less liquid cryogen is utilised in the overall grinding process than in typical prior art techniques because the heat transfer between the liquid cryogen and materials passing through throat 1 7 is more complete than heat transfer to liquid cryogen introduced into the grinding device 22 as the cryogen vapour is discharged through outlet 26 at a lower temperature, i.e. the operating temperature (e.g. -1 500F (-1 000C)) of grinding device 22.Thus, not only does cryogrinding apparatus 10 result in lower liquid cryogen consumption to grind materials 12 to a predetermined extent as a result of equilibrating temperatures of chilled particles but by subcooling such particles and supplying the same to a grinding device, less additional or "make-up" liquid cryogen is required to be supplied directly into the grinding mill and accordingiy, a more efficient cryogrinding of such materials is enabled.

It will be appreciated that hopper 16, standpipe 18, metering means 20 and grinding device 22 may be insulated with conventional heat insulation materials. Also, other means than valves 33 and 35 may be utilised in the supply of liquid cryogen through conduit 37. Although materials 1 2 are generally comprised of one type of material, i.e. plastic or rubber, mixtures of such substances may be ground by utilisation of the present invention.

The foregoing and other various changes in form and details may be made without departing from the spirit and scope of the present invention.

Consequently, it is intended that the appended claims be interpreted as including all such changes and modifications.

Claims (15)

1. A method of reducing the size of material comprising the steps of passing said material through a restricted area, supplying a liquid cryogen to said area to contact said material thereby chilling the same, permitting the temperature substantially to equilibrate throughout the material at a temperature below a predetermined value, metering the temperatureequilibrated materials into a grinding device and reducing the size of said temperature-equilibrated material in said grinding device.

2. A method as claimed in claim 1, in which the material is passed through a standpipe in which the material chilled by the liquid cryogen, the standpipe being of sufficient volume for said temperature equilibration to take place therein.

3. A method as claimed in claim 2, in which the material passes to the standpipe from a hopper.

4. A method as claimed in claim 3, wherein the step of supplying said liquid cryogen comprises introducing said cryogen into said hopper at a location of substantially minimum cross section area.

5. A method as claimed in claim 3 or claim 4, additionally including the steps of sensing the temperature in the hopper at a place In the vicinity of the location at which said liquid cryogen is introduced and controlling the tlow of said liquid cryogen in response to said sensed temperature to maintain a predetermined temperature at said place.

6. A method as claimed in any one of claims 3 to 5, wherein the step of supplying said liquid cryogen comprises providing a substantially constant base flow of liquid cryogen and periodically adding a supplemental flow of liquid cryogen to said base flow in response to said sensed temperature in the hopper.

7. A method as claimed in any one of the preceding claims, wherein said material is chilled to a temperature below the embrittlement temperature thereof.

8. A method as claimed in any one of the preceding claims, in which said liquid cryogen is vaporised upon chilling said materials and substantially all of said vaporised cryogen passes in heat exchange relation with said material being passed through said restricted area in a direction countercurrent to the passage of said material.

9. A method as claimed in any one of the preceding claims, in which the material is metered into the grinding device by a screw conveyor.

10. A method of reducing the size of material substantially as herein described with reference to the accompanying drawing.

11. Apparatus for reducing the size of material comprising a hopper for receiving said material having a throat through which said materials are discharged from the hopper, means for introducing a flow of liquid cryogen into the vicinity of said throat to chill said material passing therethrough, a standpipe adapted to receive said chilled material discharged from said hopper with the volume of said standpipe being sufficient to retain said chilled material for a time required for the temperature of said material substantially to equilibrate, grinding means, and means for metering said chilled temperature-equilibrated materials from said standpipe to said grinding means.

12. Apparatus as claimed in claim 11, wherein said means for introducing said flow of liquid cryogen comprise an elongate lance disposed in said hopper such that said liquid cryogen is introduced into said hopper such that said liquid cryogen is introduced into said hopper at a predetermined location in the vicinity of said throat portion.

13. Apparatus defined in claim 11 or claim 12, additionally including means for sensing the temperature in said vicinity of said throat portion at a distance of up to about 14 cm (6 inches) from said location at which said liquid cryogen is introduced.

14. Apparatus as claimed in claim 12 or 13, additionally including means responsive to said sensed temperature to control the flow of said liquid cryogen such that a predetermined temperature is maintained in said throat portion.

15. Apparatus for reducing the size of material, substantially as herein described with reference to, and as shown in, the accompanying drawing.