US3826015A - Device for continuous cooling of hot powder - Google Patents

Abstract

In cooling a hot powder containing coarse grains or lumps by a fluidizing method, the course grains or lumps spontaneously accumulate at the bottom portion of the fluidizing device in use and impede the fluidization of the powder, making it necessary to remove the accumulated coarse grains or lumps from the device. This invention provides an improved device wherein the bottom is inclined and a groove incorporating a rotary screw and having at one end thereof a coarse grain discharge means is formed at the base of the inclined bottom. This device provides continuous cooling of the hot powder and at the same time permits discharge of coarse grains or lumps.

Description

United States Patent Kuroyama et a1.

[ 1 July 30, 1974 DEVICE FOINQNTINUOUS COOLING 0F HOT POWDEI Shigefumi Kuroyama; Masaki Kaneda; Hiroshi Tamura, all of Yokohama, Japan Showa Denko Kabushiki Kaisha, Tokyo, Japan Filed: May 24, 1973 Appl. No.: 363,586

[75] Inventors:

[73] Assignee:

[30] Foreign Application Priority Data June 13, 1972 Japan 58196-47 References Cited UNITED STATES PATENTS 4/1960 Walker 34/57 A X 2/1968 Newman ct al.. 209/474 X 9/1968 Larwill 34/57 A 12/1970 Horner et a1. 34/57 A ltahashi 34/57 A Robertson 34/10 [5 7] ABSTRACT In cooling a hot powder containing coarse grains or lumps by a fluidizing method, the course grains or lumps spontaneously accumulate at the bottom portion of the fluidizing device in use and impede the fluidization of the powder, making it necessary to remove the accumulated coarse grains or lumps from the device. This invention provides an improved device wherein the bottom is inclined and a groove incorporating a rotary screw and having at one end thereof a coarse grain discharge means is formed at the base of the inclined bottom. This device provides continuous cooling of the hot powder and at the same time permits discharge of coarse grains or lumps.

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DEVICE FOR CONTINUOUS COOLING OF HOT POWDER BACKGROUND OF THE INVENTION This invention relates to a device for cooling a hot powder.

Devices for cooling hot powders are used extensively in various industries.

In the Bayer Process for the production of alumina from bauxite, for example, aluminum hydroxide is continuously calcined in a rotary kiln at 1,000C to l,400 C to produce alumina. The alumina is discharged in the form of powder from the kiln at outlet temperatures of from 400 C to 600 C. In many cases, this hot alumina powder contains grains and lumps, because local superheating due to the irregular flow of charge within the kiln in the course of calcination accelerates the growth of coarse grains and the attendant thermal shock tends to chip the lining bricks of the kiln wall. This alumina powder which thus contains coarse grains and lumps must be cooled to temperatures below 100 C for the convenience of subsequent processing.

For this cooling, there is used a cooling device which uses the phenomenon of fluidization. This device has the shape of a slender box provided at one end with an inlet for the hot alumina powder and at the opposite end with an outlet for the cooled powder. This box is provided on the inside with a multiplicity of water cooling pipes. A perforated plate designed to permit injection of compressed air into the box interiorforms substantially the entire area of the bottom of this box. The flow of the compressed air fluidizes the alumina powder inside the box, so that the fluidized alumina particles are cooled upon contact with the water cooling pipes and the cooled alumina powder is discharged via the outlet. For the perforated plate, there is used a wire netting, a punched metal sheet or a sheet of reinforced asbestos or rockwool. For protection against heat, this box may be lined with a fireproofing material or a heatinsulating material. A fluidizing device designed mainly for use in the production of portland cement has been suggested by US. Pat. No. 3,362,083. Also in this device, there are used means for fluidizing a powdered matter, water cooling pipes for cooling the powder, etc.

Similarly to the alumina powder described above, a given powder often contains crude grains or lumps having diameters greater than the tolerable limit of the grain sizes of that powder. In all of the fluidizing devices of the-type described above, such coarse grains or lumps fall and accumulate on the aforesaid perforated plate to impede smooth fluidization of the powder and degrade the cooling capacity of the device. The accumulation of coarse grains or lumps, if allowed to continue, will eventually bring the fluidizing operationto a complete stop. If the fluidization is stopped, then the operation of the cooling device must be interrupted to permit removal of the accumulated coarse grains or lumps. Interruption of the operation has inevitably been made at intervals of l and days in conventional commercial devices used for the cooling of alumina powder. for example.

A primary object of this invention is to provide a fluidization type cooling device which is capable of uninterrupted cooling of a hot powder containing coarse grains or lumps and at the same time permits continu- SUMMARY OF THE INVENTION This invention relates to a fluidization type cooling device for a hot powder. The device of this invention is provided with an inlet for the hot powder and an outlet for discharging the cooled powder. The bottom of this device is inclined and provided at the lower end of its inclination with a groove which incorporates a screw. The bottom of the device and the base of the groove are both perforated. A means such as a blower which is designed to blow air through the perforations into the device interior to fluidize the powder is also incorporated in conjunction with a pressure adjusting means which serves to regulate the pressure of the air. The device of this invention is further provided on the inside with cooling pipes for passing a coolant for maintaining the surface of the pipes at low temperatures, so that the powder is cooled by being brought into contact with the cooled surface of the pipes.

The powder is fluidized by the flow of air introduced via the bottom and at the same time cooled by the cooling pipes. Gravitation causes the coarse grains or lumps contained in the powder to fall to the bottom, slide down the inclined surface of the bottom and enter the groove. The screw rotating inside the groove advances the coarse grains and lumps and collects them at one end of the groove. A coarse grain discharge means at the end of the groove is actuated each time the volume of coarse grains collected at the end of the groove reaches a predetermined level, to discharge the collected coarse grains from the device. A unit which has an open-close mechanism disposed at the point of accumulation and a unit which has an air injection nozzle disposed below the point of accumulation and a discharge pipe placed thereabove so as to force the coarse grains out of the device by means of air are used advantageously as means for discharge of collected coarse grains and lumps.

Other objects and characteristics of this invention will become apparent from the description given in fur ther detail herein below with reference to the accompanying drawing.

BRIEF EXPLANATION OF THE DRAWING:

FIGS. 1, 2, 3, 4 and 5 are model diagrams illustrating the phenomenon of fluidization of powder.

FIG. 1 is a diagram illustrating the ideal state of fluidization of a powder.

FIG. 2 is a diagram illustrating the initial stage of the fluidization of powder having coarse grains mixed therein. I

FIG. 3 is a diagram illustrating the fluidization of powder in which incorporation of coarse grains has increased to a higher level than in FIG. 2,

FIG. 4 is a diagram showing the fluidization of powder inwhich incorporation of coarse grains has increased to a higher level than in FIG. 3.

FIG. 5 is a diagram illustrating a system wherein the fluidization of powder is slight because of the advanced accumulation of coarse grains.

FIG. 6 shows one preferred embodiment of the device according-to the present invention, wherein the center part is a side elevation, and either side is a partial longitudinal cross section showing the inner parts of the device.

FIG. 7 shows one preferred embodiment of the device according to the present invention, wherein the center part is a plan view, and left and right sides are partial cross sections taken in the directions of lI' and II-II' of FIG. 6 respectively.

FIG. 8 is a longitudinal cross section taken in the di- DETAILED DESCRIPTION OF THE INVENTION In a fluidizing device for cooling a hot powder containing coarse grains or lumps (hereinafter referred to collectively as coarse grains), the coarse grains accumulate and degrade the cooling ability of the device. The mechanism of this phenomenon will be clarified herein below with reference to FIG. 1 through FIG. 5.

In the diagrams of FIG. 1 through FIG. 5, 1 denotes a fluidizing device proper, 2 a perforated plate at the bottom, 3 a pipe for introducing air used for causing fluidization and 4 coarse grains mixed in the powder. The arrows show the path of the air for causing fluidization at the indicated point. with the length of the arrow representing the relative velocity of the flow of air.

FIG. 1 shows the powder free from coarse grains and it is consequently fluidized in an ideal way. FIG. 2 shows the state of fluidization which occurs when coarse grains 4 begin to mingle into the powder. Here, coarse grains 4 descend and reach the perforated plate 2 when the pull of gravity thereon overcomes the force with which the injected flow of air tends to lift them. Around the coarse grains 4, resistance to the passage of air is reduced and the volume of air passage is proportionally increased, with the result that the flow ofair will be deprived of uniformity. FIG. 3 illustrates a powder system in which admixture of coarse grains 4 has advanced. In this case, additional coarse grains are shown to have settled around the formerly accumulated coarse grains, with the fluidization deprived of uniformity to a greater extent. Consequently, there are formed a low-density fluidized layer indicated by 5 and a high-density fluidized layer indicated by 6. FIG. 4 shows the powder system with a further increase in the number of coarse grains. When the uniformity of fluidization is degraded to the extent illustrated in FIG. 4, the distribution of air paths becomes so uneven as to stop the fluidization completely. Thus, a portion 7 composed of accumulated particles of the powder is formed. The volume which is occupied by the fluidized layer 8, consequently, is reduced proportionally. This accumulated portion 7 gradually grows in size so that the upper segment of the portion collapses and spreads out in all directions. When this occurs, the fluidized layer is formed only in a limited zone in the upper section as illustrated in FIG. 5. Those cooling pipes which fall in the zone in which the fluidization is consequently stopped can no longer serve the purpose of cooling the powder. Thus, the overall cooling ability of the device is degraded. Nevertheless, large motive force is required to maintain what little fluidization there is in the device.

It is plain that quick removal of entrained coarse grains from the interior of the device will suffice to stop the development of such phenomenon and maintain the fluidization of powder in the idealcondition.

The inventors have discovered that continuous cooling of a hot powder containing coarse grains can successfully be effected by a device which is capable of separating the coarse grains from the powder in the course of fluidization and removing the separated coarse grains without producing any effect on the phenomenon of fluidization. The present invention has been accomplished on the basis of this discovery. This invention, therefore, relates to a device for continuous cooling of a hot powder characterized by being provided at one end thereof with an inlet for the hot powder and at the opposite end thereof with an outlet for the cooled powder; on the inside with a coiled cooling device for passing a coolant within; at the bottom with a groove which contains a screw fitted with a rotating mechanism and possesses a perforated base; below the bottom with a device for injecting a gas for fluidizing the powder being introduced into the device; and at one end of the groove with a mechanism adapted to discharge from the device those coarse grains which have been collected and advanced through the groove. Now, the present invention will be explained herein below with reference to one preferred embodiment of the invention illustrated in FIG. 6 through FIG. 9.

In these diagrams, 11 denotes a box constituting the device proper. This box is divided into a plurality of blocks (usually into two to six blocks) as occasion demands. The numerals 12 and 13 denote an inlet for the hot powder and an outlet for the cooled powder respectively. Denoted by 14 is a cooling pipe for passing a coolant within. The coolant is delivered via feed pipes 15 and 16 and passed within each block generally in a direction opposite the direction of the flow of the powder. Generally water is used as the coolant. A perforated plate 17 constitutes the bottom which is downwardly inclined toward the center from each lateral wall of the device. The air which has been forwarded by a blower 29 and adjusted in pressure by pressure adjusting means P and P is delivered through an air inlet 18 and injected via the perforated plate 17 to produce an upward flow of air within the device. This flow of air is used for fluidizing the powder. The spent air is discharged through an air outlet 19 disposed at the top of the device. The powder is fluidized and at the same time moved within the device and, during the movement, cooled by being brought into contact with the water cooling pipe 14. In the meantime, coarse grains contained in the powder fall onto the perforated plate 17, slide or roll down .the inclined surfaces of the bottom and reach a groove at the center. The groove 20 incorporates a screw 21 which is driven by a motor 22. The coarse grains collected within the groove are advanced through the groove because of the rotation of this screw and collected at one end of the groove. A coarse grain discharge means 23 disposed at the said end of the groove is actuated in a manner to be described in detail hereinafter, to discharge the coarse grains out of the device.

A perforated plate 24 is disposed below the screw 21 to form the base of the groove 20. The air which has been adjusted in pressure is introduced through an air inlet 25 and spurted upwardly through the perforated plate 24 so as to maintain the fluidized layer of the powder in the central part and, at the same time, cause the part of the powder which has fallen in conjunction with coarse grains into the groove to be fluidized and rejoin the main flow of powder. Since no part of the fluidized powder is conveyed even by the screw 21 within the groove 20, coarse grains can be selectively withdrawn out of the device. FIG. 6 through FIG. 8 illustrate an embodiment in which such groove is situated at the center and the bottom is inclined in the shape of V from opposite sides, whereas FIG. 9 shows an embodiment in which the groove is situated along one edge of the bottom which is inclined only in one direction. FIG. 10 is a detailed drawing of one preferred embodiment of the construction of the bottom. As illustrated, a perforated plate 17 is used to form the upper cover of a box 30 and an air inlet 18 is incorporated in the box 30. The box is attached to the device proper 11 by means of stud bolts 31. Since the powder temperature considerably differs at the inlet and at the outlet, it is necessary to adjust the amount of fluidizing gas being passed through the perforated plate 17 depending on the relative position of gas passage in order for the fluidization to be uniformized throughout the entire device interior. This necessity and the conveniences of fabrication, repairs and maintenance should be taken into account in determining the dimensions of such box. If a plurality of such boxes is required, the number of boxes must also be taken into consideration in conjunction with other conditions.

Since the powder being handled is hot, the perforated plate I7 must be resistant to heat. Besides, it must have a smooth surface in order that coarse grains falling onto the plate may readily slide or roll down the surface. The inventors experience has indicated that a perforated plate of sintered metal powder or of ceramic material advantageously serves the purpose. Particularly, a sintered plate of finely divided stainless steel particles (pore diameter about 3 to 10 t) proves to be most suitable for the purpose. lrrespectively of the kind of perforated plate to be used, it is essential that use of a fastening frame or other means apt to stop the motion of the coarse grains should be avoided in fixing the perforated plate 17 in position.

The maintenance of the fluidized layer of powder is found to be difficult where the perforated plate 17 is inclined to an excessive degree. The suitable degree of this inclination, therefore, should be determined by balancing the ease with which the fluidized layer can be maintained against the ease with which coarse grains slide down the inclined surface. In the operation dealing with alumina powder, for example, an experiment conducted by the inventors shows that the inclination with respect to the horizontal plane is required in many cases to exceed 30 but that any inclination exceeding 40 proves to be disadvantageous. When the inclination is greater than 40, it becomes difficult to maintain the fluidization uniform throughout the entire device interior. Once the fluidization is deprived of its uniformity, part of the powder begins to accumulate because of the mechanism explained previously with reference to FIG. 1 through FIG. 5. Essentially the same thing can be said of other operations involving powders other than alumina powder. In any case, the inclination of the bottom may well be calculated empirically on the basis of such factors as density, grain size distribution and friction angle of the powder which is to be cooled. In the case of a device of large capacity, it becomes necessary to divide the bottom into two or more sections and introduce air of different pressure into each section in order that the fluidization may be carried out smoothly.

Inside the device of this invention, one or more baffle boards may suitably be so disposed that the flow from the inlet to the outlet of the powder which is fluidized and at the same time moved forward may be somewhat elevated and subsequently lowered. Thus, the baffle boards serve the purpose of increasing the distance of powder movement and, consequently, lengthening the retention time of the powder within the device. Incorporation of baffle boards is essential particularly where the amount of heat to be taken from the powder by cooling is large.

As is evident from the preceding description, the device of this invention, when operated to cool a hot powder, permits continuous separation of extraneous matter such as coarse grains or lumps contained in the powder and never allows the extraneous matter to accumulate within the device. Thus, efficient fluidized cooling can be performed without interruption for a long time.

Typical results of the performance of this invention I as applied by the inventors to the production of alumina will be explained herein below. A device having a treating capacity of 250 tons/day was operated continuously for one week, with the inlet alumina temperature fixed at 400 C and the outlet alumina temperature at C or below, to provide cooling to a total of 1,750 tons of product alumina. Throughout this treatment, about one ton of unfluidized matter (coarse grains or lumps) was separated and discharged by the device. If any of the conventional cooling devices is used in this scale of operation, then the extraneous matter accumulates to such a degree that the operation must be interrupted every day for about two hours in order that the accumulated matter may be withdrawn from the device.

The means for discharging the coarse grains collected at one end of the groove by the rotation'of the screw may be of a structure in which an openable closure member is formed at the portion of the groove bottom at which the collection of coarse grains is made. With this means, the coarse grains may be discharged by intermittently opening the said closure member at the" bottom for allowing the grains to fall out. In this case, since the coarse grains are discharged downwardly, it becomes necessary to dig a pit in the floor to house conveyance means or a vessel for temporarily receiving the discharged grains or to have the device itself disposed at an elevated level. For the purpose of withdrawing the cooled powder unimpaired from within the device maintained under a pressurized condition, use of a special means is required.

Now, a preferred coarse grain discharge means will be explained. It has a structure like the one illustrated in FIG. 9. A coarse grain accumulating part containing a perforated plate 24 as its base is disposed in such way as to adjoin the end of the groove 20 in the bottom, with the said base falling in a plane lower than that of the base of the groove. In the base plate or side walls of this coarse grain accumulating part, there are disposed an air inlet 26 for introducing fluidizing air and an air spurt 27 for blowing the powder. To the inlet and to the spurt, the air delivered by the blower 29 and adjusted to a required pressure is supplied. The air through the inlet 26 is supplied continuously and that.

through the spurt 27 is released intermittently.

A vertical lift pipe 28 having a downwardly diverging end is disposed directly above the air spurt 27.

In the coarse grain discharge means 23 of the present invention, the coarse grains which have fallen from the groove 20 are fluidized over the perforated plate 24 by the air which is constantly introduced via the fluidizing air inlet 26, with the result that the part of powder entrained by these coarse grains is separated. The coarse grains within the coarse grain discharge means 23, therefore, hardly entrain any powder. Since coarse grains are incessantly conveyed through the groove, the accumulation of coarse grains gradually increases. The air is released through the air spurt when the amount of coarse grains thus accumulated has reached a predetermined level. The spurt of air forces the accumulated coarse grains through the vertical lift pipe 28 and discharges them through the opening at the top.

In this means, continuous delivery of air is not effective because of such undesirable factors as increased amount of discharged powder, increased loss of power used for air blowing, increased amount of dust to be collected, accelerated wear of the device,increased difficulty of discharge of large, heavy particles and consequent possible clogging of the bottom. All these disadvantages are eliminated by using pulsating air.

Since the coarse grains discharged via the lift pipe contain a small amount of powder, they may be screened by a separate means.

The method for operating the said coarse grain discharge means will be explained by assuming a case in which coarse grains of alumina accumulate at a rate of.

200 kg per day. in this case, the coarse grains and lumps contained in the powder can be discharged through the lift pipe 28 when air is released via the spurt 27 under a pressure of 0.3 5 kg/cm preferably 3 4 kg/cm", at a pulsating rate of 0.5 l sec/pulse, preferably 4 5 sec/pulse. in an experiment, a steel bolt about 2 inches in length and about A inch in diameter and-a brick piece about 50 mm in edge length could easily be discharged. The intervals at which the pulsating air is to be released via the spurt should be determined by taking into account the coarse grain content of the powder. Generally, the release of the pulsating air is required to be made at intervals of to 60 minutes.

The perforated plates 24 which are used on the bottom 17, at the base of the groove and on the bottom of the coarse grain discharge means 23 must be resistant to heat. They are desirably made of somewhat soft and flexible material so that they are not be damaged while coarse grains are conveyed by the screw 21. Perforated plates of asbestos or rockwool reinforced with wire netting or punched metal sheet are suitable for this purpose.

The pressure of the fluidizing air is determined by such factors as the dimensions of the device, grain size distribution, shape and density of the powder, and resistance of the perforated plates to the passage of air. The air delivered to the inclined perforated plate 17 and that delivered to below the perforated plate 24 at the base naturally have different pressures from each other. Therefore, the compressed air delivered from the blower 29 illustrated in FIG. 8 is passed through the separate pressure adjusting means P and P to be adjusted to required pressures respectively.

'C in a rotary kiln. Within the kiln, the irregular flow (otherwise called shooting phenomenon") of charge occurs in the course of calcination to give rise to local heating, which results in the formation of coarse alumina grains or sintered lumps. Besides, the thermal shock can cause chipping of lining bricks of the kiln wall. The coarse grains, lumps and chippings defile the powder being cooled.

The powder alumina which now contains such extraneous matter undergoes heat exchange with the air for combustion and is then discharged from the kiln at an outlet temperature of 400 to 600 C.

The device of the present invention is suitable for the cooling of such alumina. It serves to separate coarse alumina grains and other defiling matter from the powder and, at the same time, cool the powder to temperatures below C. The present device is also usable in the cement industry and various organic and inorganic chemical industries.

We claim:

1. A device for continuous cooling of a hot powder, comprising in combination an inlet for a hot powder disposed at one end thereof, an outlet for a cooled powder at the opposite end thereof, a cooling pipe distributed therein for passage of a coolant, a groove having a perforated base plate and incorporating a screw meshed with a rotation mechanism, a bottom plate perforated and inclined toward the said groove, injection means disposed below the said bottom plate and the said groove for spurting air required for fluidizing the powder and a mechanism disposed to adjoin the end of the groove so as to discharge coarse grains out of the device.

2. A device according to claim 1, wherein there is provided a coarse grain discharge means which comprises a coarse grain accumulating part joined to the end of the groove formed in the bottom plate of the device, possessed of a perforated base plate and provided with an inlet for fluidizing air and a spurt for releasing air used to discharge coarse grains and a vertical lift pipe disposed directly above the said part and provided with an upper opening protruding from the device enclosure and a downwardly diverging lower opening.

3. A device according to claim 1, wherein the bottom surface is formed of at least one member selected from the group consisting of perforated plates of sintered powder metal and ceramic material.

4. A device according to claim 3, wherein the powder metal to be used in producing the sintered metal is finely divided stainless steel powder having a pore diameter of 3 to l0 a.

5. A device according to claim 1, wherein the bottom plate is inclined by an angle in the range of between 30 and 40.

6. A device according to claim 1, wherein the perforated plate is formed of one member selected from the group consisting asbestos and rockwool and reinforced with one member selected from the group consisting of wire netting and punched metal sheet.

Claims (6)

1. A device for continuous cooling of a hot powder, comprising in combination an inlet for a hot powder disposed at one end thereof, an outlet for a cooled powder at the opposite end thereof, a cooling pipe distributed therein for passage of a coolant, a groove having a perforated base plate and incorporating a screw meshed with a rotation mechanism, a bottom plate perforated and inclined toward the said groove, injection means disposed below the said bottom plate and the said groove for spurting air required for fluidizing the powder and a mechanism disposed to adjoin the end of the groove so as to discharge coarse grains out of the device.

2. A device according to claim 1, wherein there is provided a coaRse grain discharge means which comprises a coarse grain accumulating part joined to the end of the groove formed in the bottom plate of the device, possessed of a perforated base plate and provided with an inlet for fluidizing air and a spurt for releasing air used to discharge coarse grains and a vertical lift pipe disposed directly above the said part and provided with an upper opening protruding from the device enclosure and a downwardly diverging lower opening.

3. A device according to claim 1, wherein the bottom surface is formed of at least one member selected from the group consisting of perforated plates of sintered powder metal and ceramic material.

4. A device according to claim 3, wherein the powder metal to be used in producing the sintered metal is finely divided stainless steel powder having a pore diameter of 3 to 10 Mu .

5. A device according to claim 1, wherein the bottom plate is inclined by an angle in the range of between 30* and 40*.

6. A device according to claim 1, wherein the perforated plate is formed of one member selected from the group consisting asbestos and rockwool and reinforced with one member selected from the group consisting of wire netting and punched metal sheet.

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