Title:
METHOD AND APPARATUS FOR PULVERIZING MATERIALS
United States Patent 3614001


Abstract:
Material to be pulverized is passed into a countercurrent heat-exchanging relation with cold vapors produced by evaporation of a low-temperature liquefied gas. The material is then immersed into the liquefied gas and is pulverized in a mill.



Inventors:
BEIKE HANS
Application Number:
04/786179
Publication Date:
10/19/1971
Filing Date:
12/23/1968
Assignee:
HANS BEIKE
Primary Class:
Other Classes:
241/65, 241/DIG.37
International Classes:
B02C19/18; B02C21/00; (IPC1-7): B02B1/08; B02C11/08
Field of Search:
241/23,65
View Patent Images:



Primary Examiner:
Mcgehee, Travis S.
Claims:
What is claimed is

1. A method of pulverizing material in a crushing mill comprising the first step of passing the material to be pulverized into a countercurrent heat-exchanging relation with cold vapors produced by evaporation of a low-temperature liquefied gas, the second step of immersing the material in the liquefied gas from which the vapors have evaporated, the third step of conveying the material from the liquified gas to the mill, and the fourth step of pulverizing the material in the mill, the fourth step taking place in the mill predominantly operating with crushing elements, the grain size of the pulverized material being determined in accordance with dimensions of the crushing elements of the mill, and the material being conveyed through the mill solely by mechanical force.

2. A method of pulverizing materials comprising the first step of passing the material to be pulverized into a countercurrent heat exchanging relation with cold vapors produced by evaporation of a low-temperature liquefied gas, the second step of immersing the material in the liquefied gas from which the vapors have evaporated, and the third step of conveying the material from the liquefied gas to a mill and pulverizing the material in the mill, the third step taking place in a mill predominantly operating with crushing elements, the grain size of the pulverized material being determined in accordance with dimensions of the crushing elements of the mill, additionally the fourth step of passing the cold pulverized material in a countercurrent heat exchanging relation with a vapor, the vapor being cooled by the fourth step and then being employed to assist precooling of the material before pulverization.

3. A method as claimed in claim 2, in which the vapor cooled by the fourth step comprises the vapor heated by the first step, which vapors after being cooled in the fourth step are recirculated in the first step.

4. A method as claimed in claim 2, in which the vapor cooled by the fourth step is dry air.

5. A method as claimed in claim 2, in which the liquefied gas is liquid nitrogen.

6. In an apparatus for pulverizing materials comprising a hopper for the material to be crushed, a first countercurrent heat exchanger communicating with the hopper whereby the material may be fed from the hopper, inlet means in the first heat exchanger for controllably allowing a low-temperature liquefied gas to form an immersion region of a controlled depth at the bottom of the heat exchanger, a mill communicating with the heat exchanger for pulverizing the material after immersion in the liquefied gas, including a second countercurrent heat exchanger for cooling a vapor, and means for leading the resultant cooled vapor back to the first heat exchanger to assist in precooling of the material before its immersion in the liquefied gas.

7. Apparatus as claimed in claim 6, in which the hopper is connected to the first heat exchanger by a charging valve and is surrounded by a cooling jacket, a nonreturn valve being situated between the top of the first heat exchanger and the jacket to allow passage of the vapor from the evaporating liquefied gas to pass to the jacket on an excess pressure being attained at the top of the first heat exchanger, and the wall separating the jacket from the material contained in the hopper being pervious to the vapor.

8. Apparatus as claimed in claim 7, including a first vapor outlet adjacent the top of the first heat exchanger, a blower connecting the first vapor outlet to a first vapor inlet of the second heat exchanger, a second vapor outlet in the second heat exchanger connected to a second vapor inlet at the bottom of the first heat exchanger above the immersion region.

9. Apparatus as claimed in claim 7, including a source of dry gas, a vapor inlet in the second heat exchanger, a blower and a vapor outlet connecting the inlet to the source, and the vapor outlet being connected to the said jacket.

10. Apparatus as claimed in claim 6, wherein the mill is a crushing mill.

Description:
BACKGROUND OF INVENTION

This invention relates to methods of, and apparatus for pulverizing materials. The materials are preferably substances rendered brittle by immersion in low-temperature liquefied gas and the materials are pulverized by mechanical methods.

In the plastics-processing industry there is a growing demand for pulverized synthetic materials with a preferably uniform grain size. The demand for these powders differs as regards type (polyethylenes, P.V.C., polyamides, etc.,) and also as regards grain sizes for different applications. For example, fine-grain material (approximately 10 to 75 microns) is needed in the preparation of suspensions and for the electrostatic spraying and fusing of plastics when making protective coatings. A grain size of 75 to 200 microns is suitable for the so-called fluidized bed sintering process. The coating of carpets requires a grain size of preferably 200 to 500 microns, whereas the centrifugal casting of components requires a grain size of 400 to 800 microns.

Various methods are known for the manufacture of pulverulent materials, none of which is entirely satisfactory. There is, for example, the solution/precipitation method. However, the grain size of the particles cannot be selected arbitrarily by this method. Moreover, this process is economic only for large plants (for example, for manufacturing 1000 tons or more per year of a single type of synthetic material having a certain grain size).

Another method is the grinding of a charge introduced at room temperature with the aid of various mechanical grinding devices. The grinding of polyethylene with rebound crushers has gained some popularity. Here, too, the grain size of the produced powder can only be predetermined within certain limits and, up to now, many types of synthetic material could not be satisfactorily ground with this device. The output of rebound crushers is comparatively low in relation to the consumed energy (50 to 100 kg. of powder/hour, depending on grain size, with an energy consumption of 50 to 70 kw.-hr.).

Another known method of producing powder from granular materials is first to cool them in liquid nitrogen and then to use a fast-running pin mill (see German Pat. specification No. 1,004,460 ). All types of synthetic material can be ground in such a manner at a temperature of approximately -196° C. (the temperature of liquid nitrogen), the hourly output being 400 to 600 kg. and more, with an energy consumption of approximately 25 to 35 kw.-hr. However, the grain size of most of the powder produced cannot be varied at will to any great extent in a pin mill, small predetermined variations only being produced for example by varying its speed, the number of pins and the throughput per unit of time. The powder produced by this method is characterized by a wide spectrum of grain sizes so that it cannot usually be used directly as it still has to be sifted into more or less uniform fractions, for example by sifting or like operations. This requires at least one additional operation. Another disadvantage in known manners of carrying out this method is the excessive consumption of a comparatively expensive, liquefied low-temperature gas such as liquid nitrogen. Owing to the high-energy consumption of the fast-running pin mill (e.g. 25 to 35 kw.-hr. for a production of 500 kg. of powder per hour) undesirable heat is induced in the system. Furthermore, the powder often leaves the mill at an undesirably low temperature (approximately -50° C. to -80° C.), involving again a considerable waste of energy.

Hitherto known methods require approximately 1 kg. of liquid nitrogen for every kilogram of powder produced, irrespective of the wide spectrum of grain sizes. However, if only 75 percent or even 60 percent of the produced total is usable -- this being normal for most applications -- it therefore becomes necessary to separate the excessively fine or coarse grains for regrinding and the nitrogen requirement per kilogram of usable powder may amount to 2 kg. or more. Besides, the removal of the unusable powder component by grading or air sifting introduces a further operational step which is expensive and complicated.

SUMMARY OF INVENTION

According to the present invention a method of pulverizing materials comprises the first step of passing the material to be pulverized into a countercurrent heat-exchanging relation with cold vapors produced by evaporation of a liquefied gas; the second step of immersing the material in the liquefied gas from which the vapors have evaporated; and the third step of pulverizing the material in a mill. Preferably, the method comprises also the fourth step of passing the cold pulverized material in a countercurrent heat exchanging relation with the gas or vapor, the gas or vapor being cooled by the fourth step and then being employed to assist precooling of the material before pulverization.

Preferably, the vapor cooled by the fourth step comprises the vapor heated by the first step, which vapors, after being cooled in the fourth step, are recirculated in the first step.

By operating with such a closed gas circulation system, not only is the amount of cold produced by the evaporation of the liquid gas (e.g. liquid nitrogen) efficiently used, but also about 90 percent of the cooling capacity of the liquefied gas converted into the vapor state are exploited.

The method of the invention is applied with advantage to materials rendered brittle by means of liquefied low-temperature gases, such as thermoplastic or resilient synthetic materials, natural materials having viscous, sticky or elastic properties, i.e. material which either exhibit such properties under normal conditions or assume such properties as a result of the heat caused by the energy produced during comminution in conventional devices. The method can also be applied in the case of other solids which cannot be crushed in some other way without impairment to their specific properties, for example aromatic substances or the like. The type of pulverizing device used according to the invention is preferably a mill with crushing elements, this requiring a comparatively low amount of energy for comminution and, if necessary, also enables the grain size of the powder to be selected by varying the distance between the grinding elements and/or using grinding elements having different serrations in their surface, the powder obtained having a largely uniform grain spectrum.

The undesirable heat increase, produced in the charged during the crushing operation, is therefore low, so that the ground material remains brittle. Depending upon the type of material, the heat increase caused by grinding is such that, provided that preliminary cooling in e.g. liquid nitrogen has taken place, a temperature is produced at which nearly all substances which are viscous under normal conditions, are still very brittle.

Further according to the invention an apparatus for carrying out the above method includes a hopper for the material to be crushed, a first countercurrent heat exchanger to which the material may be fed from the hopper, an inlet in the first heat exchanger for controllably allowing a low-temperature liquefied gas to form an immersion region of a controlled depth at the bottom of the heat exchanger, a mill for pulverizing the material after immersion in the liquefied gas, a second countercurrent heat exchanger containing the communited material for cooling a gas or vapor and means for leading the resultant cooled gas or vapor back to assist in precooling of the material before its immersion in the liquefied gas.

The crushing device suitably used in accordance with the invention is a mill operating generally with shearing and frictional effects, but predominantly with a compression effect, for example a mill with grinding or crushing disks, cones, wheels or cylinders the surface of which has been serrated suitably and the distance of which can be altered; in which the charge substantially does not follow long, accelerated random paths of movement as is the case, for example, with pin, hammer, rebound crusher mills, etc., which operate either predominantly or exclusively on the impact or rebound principle.

THE DRAWINGS

The single FIGURE shows an apparatus constructed in accordance with the invention.

DETAILED DESCRIPTION

The hopper 1 is charged with the granular or lumpy material to be crushed. When a motor-powered cell-type charging valve 2 is switched on the container of a countercurrent heat exchanger 3 is charged. A sensing device 4 disconnects the motor of the charging valve 2 when the upper charging limit 5 has been reached and switches it on again as soon as the lower charging limit 6 has been reached during the operation of the crushing plant. When the heat exchanger 3 has been fully charged a valve 7 in a pipe 8 conducting the liquefied low-temperature cooling gas slowly opens. The valve 7 may be motor driven or may be a solenoid valve and controlled by a temperature-sensing device 9. The evaporated gas rises through the charge to be pulverized and, during its passage, cools the charge, emerging at the surface 5, or 6, nearly at the temperature of the noncooled charge. The volume of the countercurrent heat exchanger 3 is dimensioned such that this result is guaranteed. When a slight excess pressure has been attained in the space 10, the nonreturn valve 11 opens and the gas can escape through the connecting duct 12 into the space 13 of the hopper. The wall 14 separating the space 13 from the hopper 1 may consist of perforated sheet metal covered by a fine-mesh tissue. The gas passes through the wall 14, rises through the charge up to the surface 15 and escapes to the atmosphere. While it rises the gas surrenders any residual low temperature to the charge.

As soon as the desired low temperature has been attained in the vicinity of the sensing device 9 (approximately the temperature of the liquid gas), the motor 16 of a worm conveyor 17 is activated. The speed of the worm conveyor 17 or the inlet opening to said worm conveyor is variable so as to adapt the amounts conveyed by the worm to the output of a crushing device 18 in relation to the desired grain size. Above the highest level 19 of the liquefied gas 24 in the container the worm conveyor housing 17 has a pressure compensation orifice 20 preventing the liquid from being urged upwards in the worm conveyor housing, due to overpressure. The cooled brittle charge is conveyed through the connection duct 21 to the crushing device 18. The drive motor of the latter is switched on simultaneously with the worm conveyor motor, and so is the motor driving the cell-type charging valve 22 which takes the pulverized material to the countercurrent heat exchanger 23 which operates, for example, according to the fluidized bed principle, as illustrated in the drawing. The grain size of the pulverized material leaving the crushing device can be controlled, if desired. If the countercurrent heat exchanger 23 operates according to the fluidized bed principle it consists of a gas chamber 25, a porous plate 26 and a container 27 for the pulverized material. When the charge has reached a suitable level in the container 27 the blower 28 is switched on, this sucking in the warm vapors from the space 10 in the first heat exchanger through the pipe 29 and forcing it into the gas chamber 25. The warm vapor penetrates the porous plate 26 and rises finely distributed through the cold pulverized material in the container 27. The valve 30 is set such that the pulverized material assumes the state of a fluidized bed with a clearly defined surface, and without a lot of dust developing. The gas, after cooling during its passage through the cold, pulverized material, is forced through a pipe 31 towards a distributor which, if necessary, is provided in the shape of a hood 32 from which it issues into the charge to be crushed. The gas is led to the distributor 32 in similar manner also where a heat exchanger of different type is employed. Incorporated in the connecting pipe 31 is a nonreturn valve 33 which prevents the gas from flowing in the reverse direction while the plant is started up. The cooled gas rises through the charge to be comminuted into the space 10 and surrenders its low temperature to the charge.

As a result of the gas being circulated, the pulverized material assumes something like the temperature of the induced warm gas directly above the porous plate 26, whereas the material is kept cold at the surface of the fluidized bed owing to the crushed cold material continuously introduced from the crushing device 18. By means of the motor-driven cell-type discharge valve 34 the heated pulverized material is continuously transferred to the storage container 35 or filled in bags in amounts per unit of time substantially corresponding to the amounts fed in from the crushing device.

All the cold parts of the plant are preferably externally insulated against heat (this insulation being represented by shaded portions in the drawing).

The gas may be circulated in the described manner several times, if necessary. An amount of gas corresponding to the amount continuously evaporating from the liquefied gas escapes through the nonreturn valve 11 into the space 13, permeates the charge in the hopper 1 and then issues into the atmosphere or is used for other purposes after having surrendered any residual low temperature to the charge. To replace the amount of hot gas leaving the closed system, an equivalent amount of liquefied low-temperature gas is introduced therein.

To effect the exchange of heat between the cold pulverized material and the warm gas in the heat exchanger 23, a gas other than the gaseous medium represented by the evaporated liquefied gas may be employed, for example dry air, which may be drawn by the blower 28 and fed through the countercurrent heat exchanger 23 and through the connecting pipe 31 within the circuit to the space 10 in the described manner or, through a separate pipe, to the space 13. In the latter case it would be an advantage to insulate also the container 1 against heat losses.

As used in the claims the term vapor is intended to means a gas or a vapor.