US3434593A - Methods and apparatus for air classifying and screening of finely divided material - Google Patents

Description

March 25, 1969 D suL v 3,434,593

METHODS AND AFPARATUS FOR AIR CLASSI FYING AND SCREENING OF FINELY DIVIDED MATERIAL Filed Jul ye, 1966 Sheet of s 1 i l l P L M E He.a .L; I... 6 f2 an a 3 & A46

M I... 6a a I ea March 25, 1969 J, SULUVAN 3,434,593

METHODS AND APPARATUS FOR AIR CLASSIFYING AND SCREENING OF FINELY DIVIDED MATERIAL Filed July 6. 1966 Sheet 2 of'3 E a 6 64 L 64 1e I6 I,

March 25, 1969 [3.1 SULLIVAN 3,434,593

METHODS AND APPARATUS FOR AIR CLASSIFYING AND SCREENING 0F FINELY DIVIDED MATERIAL Filed July 1966 Sheet 5 of a United States Patent 0 f 3,434,593 METHODS AND APPARATUS FOR AIR CLASSIFY- ING AND SCREENING OF FINELY DIVIDED MATERIAL Daniel J. Sullivan, Milton, Mass., assignor to Sturtevant Mill Company, Boston, Mass., a corporation of Massachusetts Filed July 6, 1966, Ser. No. 565,648 Int. Cl. B07b 9/02 US. Cl. 209-31 14 Claims ABSTRACT OF THE DISCLOSURE An air classifier for classifying finely divided materials comprises inner and outer casing members and a fan member for inducing a flow of air upwardly through cascading particles introduced at the top of the casing members. Adjustable screen means of the invention are located adjacent to the fan member and function to selectively provide for a separation of coarse particles not removed by conventional air classifying apparatus.

This invention relates to improved methods and ap paratus for processing finely divided materials and, more particularly, the invention is concerned with machines for air classifying and screening finely divided substances such as minerals, vegetable substances, chemicals and other similar products. V

In Patent No. 2,774,476, owned by the assignee of the present invention, a typical form of air classifying ap paratus of the general type referred to is disclosed wherein a cascading mass of finely divided material is subjected to a centrifuging action and the centrifuged material is displaced radially outwardly into a fan-induced current of air flowing upwardly through the centrifuged mass to obtain a separation of fine particles from relatively coarser particles.

Machines of the class referred to are very effective in removing controlled percentages of relatively fine particles and separating them from relatively coarse particles. This is accomplished by the fine particles being lifted upwardly by a fan-induced flow of air and these fine particles are led into an outer casing which is separated from an inner tailings casing for coarse particles.

It will be appreciated that there are limitations in the capability of such machines to lift and separate some ranges of relatively larger sized particles. More recently it has been found that industrys need for specialized classifications of finely divided materials have created requirements for extending the range of classification on the coarse side. One such instance is exemplified in requirements for processing a product such as limestone, and particularly in the preparation of an agricultural limestone where definite percentages of slightly coarser particles, together with controlled percentages of fines, are now being specified by users.

It is a chief object of the present invention, therefore, to improve air classifying machines of the class referred to and, in particular, to devise an improved method and means for carrying out air classifying operations and simultaneously extending the range of particle sizes produced on the coarse side for meeting requirements of users of agricultural limestone and similar products.

3,434,593 Patented Mar. 25, 1969 Another object of the invention is to provide a method of combining the steps of air classifying with novel mechanical screening in such a manner that there may be realized a controlled extension of the range of coarse particle sizes recovered while also obtaining a desired recovery of relatively fine particles in a suitable product mixture.

Another object of the invention is to provide an improved apparatus for processing finely divided material such as limestone and the like whereby standard forms of air classifying machines of the type disclosed in the above-noted patent may, by relatively simple modification, be adapted to a novel type of screening operation.

Still another object is a method of combined classifying and screening of finely divided materials to produce a product mix occurring in a range of predetermined relatively coarse sizes from which a specified range of fines may be substantially removed.

The nature of the invention and its objects will be more fully understood and appreciated from the following description of a preferred embodiment of the invention selected for purposes of illustration and shown in the accompanying drawings, in which:

FIGURE 1 is a plan view of an air classifying apparatus of the invention in one preferred form;

FIGURE 2 is a side elevational view with portions broken away at one side thereof;

FIGURE 3 is a vertical cross section of the machine shown in FIGURES 1 and 2;

FIGURE 4 is another fragmentary cross sectional view of the machine with portions thereof broken away to indicate diagrammatically the path of flow of fine particles and coarse particles induced by rotative movement of a fan member; and

FIGURE 5 is a detail fragmentary exploded view of a portion of the inner casing of the machine and screening means of the invention.

With the foregoing objectives in mind, I have discovered that in utilizing a machine of the general type shown in the drawings for air classification of fines particles, it is possible to carry out, Without interference with normal air classification, a unique and important mechanical screening step. This mechanical screening step can be very precisely regulated and controlled to not only isolate a specific range of coarse particles, but also to introduce these particles into the discharge flow of air classified fines while allowing relatively coarser particles to pass downwardly through a tailings conduit in the usual manner.

I have found that I may accomplish this screening operation by displacing particles of a specific desired size radially outwardly through protectively arranged screen means having openings or perforations arranged to avoid interference with normal air classifying and whose size, arrangement, and accessibility in the machine may be adjusted in accordance with a particular coarse particle size requirement.

In one preferred embodiment of the invention, I provide a plurality of screen elements detachably secured in the walls of an inner casing of the air classifier and provided with adjustable screen covers by means of which the screen apertures may be opened or closed as desired to regulate passage of particles outwardly therethrough.

The screen elements are disposed immediately below radially projecting casing top edges and protective flange portions so that the screen elements occur in a relatively sheltered relationship and fine particles passing downwardly over the outer edges of the inner casing top are thus prevented from entering between the screens and the screen covers. In this preferred embodiment I further locate the screen elements in the inner casing of the machine in positions so chosen with respect to the air classifier fan as to subject particles to a maximum exposure of the radially displaced stream of air occurring in the immediate vicinity of the fan tips.

Considering first the conventional portions of the air classifying structure in the drawings, numeral 2 denotes a closed cylindrical casing which is formed with a conically shaped lower section 4 and a top section 2a. The section 2a has vertically supported therethrough a bearing B in Which is received a shaft S driven by a belt L and a motor M. The bearing B and the motor M are preferably supported on transverse beam members 3 in turn secured at the upper side of the top section 2a. A pulley P is driven by the belt L as suggested in FIGURE 3. Located in spaced relation to the outer casing 2 is an inner cylindrical casing 30 carried on supporting brackets 32 projecting inwardly from the inner periphery of casing 2. The casing 30 is provided with a top rim portion 31 which projects radially outwardly as shown in FIGURES 3 and 4 for an appreciable distance and also provided immediately under the rim 31 are reinforced channel shaped members 33 providing overhanging lower edges 35.

At the lower end of shaft S is provided a rotary a'pparatus including the distributor plate 10 which is supported on suitable bearing means 10a. Located in spaced relation above the distributor plate 10 and rotatable with it is a plate member 11 to which is secured vanes as .13. The spacing of the vanes 13 may be varied to provide a means of rejecting certain ranges of coarse particles as is disclosed in Patent No. 2,774,476.

Finely divided material is delivered into the machine through an inlet chute 6 formed at the upper side of the machine as shown in FIGURE 3. Also supported for rotation with the distributor plate 10 and the member 11 is a fan member F which is adapted to rotate and induce an upwardly directed pulling force on material centrifugally displaced from the distributor plate 10.

It is pointed out that the inner casing 30 extends upwardly to a point just below the fan member F as shown in FIGURE 3 and also includes a lower conically shaped section 12 around which is supported a plurality of adjustable vanes. Supported immediately below the vanes is a second conically shaped section 15, at the lower extremity of which is attached a tubular tailings conduits 18. The latter member projects downwardly through one side of the conical section 4 as best shown in FIGURE 3. As will be observed in FIGURE 3 the tailings conduit 18 occurs in spaced relation to inner peripheral portions of the conical section 4 so as to define a fines outlet 24.

In the conventional operation of the parts now described, finely divided material is directed into the feed inlet 6 and passes downwardly around the bearing and rotating shaft S to fall onto the rotating plate 10. As the material drops down onto the distributor plate it is centrifugally displaced towards the inner casing 30, and some of the relatively coarse particles begin to fall downwardly around the inner peripheral surfaces of the conical section12.

As particles start to fall the upwardly induced draft forces of the fan F, acting upwardly through the path of movement of the particles, lifts some of the falling fines and travels these fines upwardly and inwardly through the rotating vanes 13 and the fan F and then outwardly and downwardly through the passageway 17 between the outer casing 2 and the inner casing section 30.

From passageway 17 the fines drop downwardly through the conical section 4 to be discharged from the fines conduit outlet 24. The remaining coarse particles of the finely divided material pass downwardly through the conical 4 section 12 and then out through the conical tailings conduit 15 and discharge outlet 18.

In accordance with the method of the present invention, relatively fine particles are classified as described above and, in addition, I provide for discharging a selected range of relatively coarse particles radially outwardly through the inner casing section 30 and into the path of downward fiow of the fines particles passing down through passageway 17.

As one suitable means of accomplishing this I construct the inner casing member 30 with a plurality of spaced apart openings 40, one of which is shown in FIGURES 2 and 5. The openings 40 may, if desired, be fitted with enclosing frames of some suitable nature detachably secured therein. In these framed openings 40, I further provide a plurality of screen elements as 42 (FIGURE 2). The screen elements may, for example, consist of rectangularly shaped plates of perforated metal or other suitable screen material and each of the screen elements are further curved into an arcuate shape to coincide with the curvature of the cylindrical casing 30.

In the arrangement of screen elements 42, shown in the drawings, it will be observed that the upper edges of the screen elements are located close to and directly under the overhanging portions of edges 35 of the reinforcing members 33, and it is also pointed out that the outer edges of the casing rim 31 extend somewhat further beyond the reinforcing members 33 so that the screen elements are thereby disposed in a somewhat recessed and sheltered position. Therefore, fine particles moving downwardly over the rim 31 and through the passageway 17, are prevented from being drawn inwardly towards the screen elements.

Cooperating with the screen elements and arranged to move against and close the openings in the screen elements are complementary curved cover elements 44 which as shown in FIGURES 2, 3 and 4 are supported on rods 46 slidably received in bars 48 in the outer casing 2. By means of this arrangement the screen elements may be opened or closed as desired by a manual adjustment of any one or all of the rods from outside the machine. It will be understood that any desired number of screens and screen covers may be employed spaced around the casing members as suggested in FIGURE 1.

An important feature of the invention is the location of the screen elements and screen covers in the recessed position described relative to the casing rim 31 to provide a sheltering action. It will be appreciated that when the fan F is revolving at relatively high operating speeds, it exerts a considerable upward suction force in the direction indicated by the arrows A and there conceivably could arise a tendency for this suction force to draw fine particles from the passageway 17 inwardly through the screen elements unless this is prevented. This would result in an undesirable recycling of fine particles.

I find that by maintaining the screen elements in a recessed position under the casing rim 31 and the flanged portions 35, particles falling downwardly through the passageway 17 are effectively prevented from coming into contact with the screen elements. Also, by varying the spaced relation of the screen covers 44 to respective screen elements, I find that a partial vacuum effect may be induced in the spaces between the screen elements and the screen covers so that no fines are drawn in and little resistance is presented for centrifuging particles to be directed against the screen element peripheries and out through these openings. Therefore, a selected range of sizes may be discharged through the screen element openings.

It will be evident that I may vary the size of the openings in the screen elements to control the range of particle sizes which are screened. It will also be apparent that by closing some of the screen elements and leaving others open, greater or smaller quantities of any desired range of screened products may be realized. It is still further pointed out that these operations can be carried out without interference with the normal air classifying of fines in the usual manner. The result is a more carefully regulated product mix of desired fines and a controlled percentages of coarse particles within predetermined limits.

In the preferred arrangement referred to, the screen elements 32 are disposed so as to form a multiplicity of openings all the way around the inner peripheral surface of the casing 30. Moreover, the tops of these screen elements are located immediately below the tips of the fan F. I find that in this region of close proximity to the tips of fan F, centrifuged material is continuously swept around and against the screen openings by the air stream of the fan and a maximum screening action is produced as regulated by the size of the openings employed in the screen and the speed of the fan induced.

In operation material is fed into the member 6 and passes downwardly onto plate 10. The combined effect of the fan F, the rotating vanes 13, and distributor plate 10, acting on the continuously cascading mass of particles, is to provide a spiralling stream of air and particles of varying sizes which move outwardly in response to centrifugal forces.

As indicated diagrammatically in FIGURE 3, some of the relatively finer particles in response to the pulling forces of the fan F move outwardly only a short distance and then are lifted and travelled inwardly and upwardly as indicated by the arrows A. Other relatively coarse particles are centrifuged outwardly a further distance and start to move in a downwardly spiralling stream along the inner coned surface of the casing 30 as suggested diagrammatically by the arrows D.

In accordance with the invention still other relatively coarse particles of a selected range of sizes are displaced radially outwardly sufficiently to move against and pass through apertures in the screen elements 42 and then into the passageway 17 as denoted by the arrows T. These selected coarser particles are thus combined with the air classified fine particles passing down through passageway 17 and a product mixture of air classified fines and a predetermined percentage of screened coarse particles is thus obtained.

As illustrative of one typical application of the method of the invention, there may be cited processing of limestone aggregates for producing agricultural limestone with a required percentage of both fines and coarse particles regulated to meet specific customer requirements.

It should be understood that in conventional processing of limestone aggregates to produce agricultural limestone, standard requirements have in the past specified that particles be of a size such that a 1000 pound sample of material be sutficiently finely divided to pass through a 16 mesh screen and 95% of the sample should pass through a .30 mesh screen. However, more recent requirements in the trade now specify that a 1000 pound sample of agricultural limestone include a fraction of at least 20% of particles of a size sufliciently coarse to be retained on a mesh screen with 80% of the material passing through a 10 mesh screen.

Since a 10 mesh screen has openings of approximately .062 inch and a 100 mesh screen has openings of approximately .0058 inch, the requirement is quite selective and involves retaining coarse particles which cannot be satisfactorily classified by conventional air classifying machines. That is so since the large size particles are not effectively lifted by a fan-induced air stream. By using the method and apparatus of the invention however, as described above, the requirements can be met satisfactorily and I have indicated below two examples which compare conventional air classifying operations with combined air classifying and screening of this invention.

Example I A conventional agricultural limestone product was produced from 1000 pounds of limestone aggregates having the following analysis:

650 pounds of particles passed through. a 14 mesh screen and were retained on a 10 mesh screen,

150 pounds of particles passed through a 10 mesh screen and were retained on a mesh screen,

200 pounds of particles passed through a 100 mesh screen.

This example, after being subjected to an air classifying operation of conventional nature, provided 20 pounds of particles which were held on a 100 mesh screen, and 1160 pounds of particles which passed through a 100 mesh screen.

From this analysis of the sample processed, it will be observed that the conventional air classifier failed to provide the required percentage of coarse particles and the processed fines fraction, together with the coarses removed, held only 180 pounds of processed agricultural limestone.

Example II There was provided a similar 1000 pound sample of limestone aggregates with the same relative percentages of coarses and fines specified in the sample of Example I, i.e.

650 pounds were retained on a 10 mesh screen,

150 pounds passed through a 10 mesh screen but were retained on a 100 mesh screen,

200 pounds passed through a 100 mesh screen.

This sample was subjected to the combined air classifying screening steps of the invention as regulated by the use of screen elements 42. The screen elements for this particular operation were formed with openings of approximately inches in size in order to pass particles which would be held on a 10 mesh screen (.065 inch).

The resulting product from the combined air classifying and screening steps produced the following:

200 pounds of particles held on a 10 mesh screen,

pounds passed through a 10 mesh screen and held on a 100 mesh screen,

200 pounds passed through a 100 mesh screen.

It is pointed out that this product had the required quantity of coarse particles 20% (200 pounds) as specified above and held a total product mix of 540 pounds of acceptable agricultural limestone.

It will be understood that the openings of screens 42 may be varied as desired to meet any given specification. It may also be desired to increase the number of screen elements used by forming openings at other points in the cylindrical casing members and locating screen elements therein at different levels. For example, I may as shown in FIGURES 2 and 3, form openings in the conical lower section 12, and I may mount therein screened elements as 60 which are secured to rods as 62 slidably supported in bearings 64 in the outer casing 2.

These relatively lower screen elements may, I find, be designed to pass certain sizes of coarse particles which are not readily processed at other points in the casing because of their rapid downward path of travel. Similarly, there may be employed at even lower levels still other screen elements as, for example, screen element 66 fixed to rods 68 slidably disposed in bars in the lower conical section 4.

The invention may also be practiced utilizing other forms of screen members such as screen cloth, spaced bar sections, and the like. Also, the invention may be utilized with a wide range of finely divided materials other than agricultural limestone and various other advantages may be realized. For example, in making concrete sand, it may be required to have a specified range of coarse particles and only a limited percentage of fines of some smaller range of sizes, and by utilizing the combined air and screen operation there can be realized a sharper control the quantity of fines present in the coarse particles which are passed through the tailings outlet of the machine and thus a better concrete sand product is realized for some requirements.

'While I have shown and described preferred embodiments of the invention, various other changes and modifications may be resorted to within the scope of the appended claims.

I claim:

1. In a method of air classifying and screening a finely divided material, the steps which include delivering a free falling mass of finely divided solid material downwardly upon a rotating distributor plate for centrifugal dispersion within a closed chamber having a top portion with an outlet port formed therein and screen means in the sidewalls thereof, supplying air through a conduit means opening into said closed chamber below said dispersing mass, evacuating the air supplied to said chamber upwardly through said finely divided dispersed material by a vacuum fan rotating at a predetermined speed and located at the outer side of the closed chamber with its inlet connected with said outlet port for entraining relatively fine particles of the dispersed material, travelling the air and entrained fine particles through a rotating sizing device located between the distributor plate and the outlet port of said chamber, discharging the flow of air and entrained fine particles from said vacuum fan into a second closed chamber for deposit of portions of the dispersed material at the bottom portion thereof, and simultaneously displacing air and relatively coarser particles radially outwardly through the screen means to the said second closed chamber and into the path of flow of the fine particles.

2. A method according to claim 1 and including the further steps of varying the screen apertures to regulate the magnitude of the relatively coarse particles which are passed through the screen member.

3. A method according to claim 1 and including the further steps of selectively opening and closing the screen apertures to control the quantity of relatively coarse particles which are mixed with the fine particles.

4. A method according to claim 1 and including the further steps of inducing a vacuum effect externally of the screen means to oppose suction of relatively fine particles inwardly therethrough.

5. A method according to claim 1 including the further steps of radially displacing coarse particles through screen apertures occurring at progressively lower levels in the closed chamber.

6. A method according to claim 1 in which the finely divided material consists of limestone aggregates occurring as a mixture of coarse and fine particles, said method including the further step of displacing relatively coarse particles through the screen means in a controlled manner to constitute at least twenty percent of the total quantity of air classified particles and screen particles processed to provide agricultural limestone.

7. In an apparatus for air classifying and screening finely divided material, the combination of an outer closed casing having a cone with a discharge opening at the bottom thereof and a drum above said cone, said drum including a top wall portion, an inner closed casing composed of a cone with a lower discharge conduit portion extending through the cone of the outer conical casing, said inner closed casing having a drum above the cone and said drum including a top wall portion with a projecting rim, said inner closed casing being spaced from the outer casing and defining a chamber therebetween, driven shaft means projecting through the top Wall of the outer casing and within the inner casing, a centrifugal distributor plate mounted on the lower end portion of said shaft and spaced from the top wall portion of the drum of said inner casing, a rotary sizing device positioned within the inner casing above the distributor plate, means for connecting the shaft, said rotary sizing device and said distributor plate to provide a uniform continuous rotation of the parts, means for introducing finely divided solid material into said inner casing and upon said distributor plate for centrifuged dispersion, an outlet port formed in the top wall portion of the drum of the inner casing, a vacuum fan mounted between the top wall portion of the inner drum and the top wall portion of the outer drum and rotatable with said driven shaft for travelling centrifuged material upwardly through the fan and then downwardly through said chamber between the inner and outer casing, said inner drum having a plurality of screen elements mounted therearound at points immediately below the tips of the said vacuum fan and in recessed relation to the rim of the top portion of the inner casing, said screen elements being of a size sutficiently large to provide for passage of centrifuged particles therethrough and into the said chamber.

8. A method according to claim 7 in which the screen means includes a plurality of adjustable closure plates for opening and closing the openings in the screen means.

9. In an apparatus for air classifying and simultaneously screening :finely divided material, the combination of an outer casing having a bottom outlet for discharging relatively fine particles and an inner casing located in spaced relation to the outer casing to define an annular chamber which communicates with the said bottom outlet, said inner casing being formed with bottom conduit means located through the outer casing for discharging relatively coarse particles, driven shaft means vertically disposed through the outer casing and extending downwardly into the inner casing, a distributor plate fixed to the shaft, means for introducing finely divided material into the inner casing onto the rotating distributor plate to provide a centrifuged dispersion of particles in the inner casing, suction fan means mounted in the top of the inner casing for lifting centrifuged particles upwardly and travelling the particles into the said annular chamber, screen means located adjacent said suction fan means in the wall of said inner casing, and having openings for selectively passing therethrough relatively coarse centrifuged particles into said annular chamber.

10. A structure according to claim 9 in which the screen means includes perforated screen plates detachably secured in the inner casing and cover elements slidably disposed in the outer casing for opening and closing the openings in the said screen plates.

11. A structure according to claim 10 in which the screen plates are arranged in recessed relation to a top rim of the inner casing and the said cover means when moved into a position immediately below the said top rim cooperates with the screen elements to provide regions of partial vacuum for rejecting fine particles in the said annular chamber.

12. A structure according to claim 11 in which the screen elements are disposed in the inner casing in immediate proximity to the region of the tips of the said fan whereby a maximum eifect of the air stream induced by the fan is exerted on centrifuged particles to sweep them around the inner periphery of the inner casing and discharge them through the apertures in the screen elements into the annular chamber.

13. A structure according to claim 9 in which the screen means includes an upper row of screen elements arranged around the peripheral surface of the inner casing and a second row of screen elements also arranged in spaced relation around the peripheral portions of the inner casing at a lower level.

14. A structure according to claim 9 in which the screen means includes a top row of screen elements for discharging a selected range of relatively coarse particles, a second row of screen elements below the first row for discharging another range of coarse particles and a third row of screen elements below the second row for discharging still another range of coarse particles.

(References on foliowing page) References Cited UNITED STATES PATENTS FOREIGN PATENTS 694,739 8/ 1940 Germany.

Anderson 209--305 X Fraser 209 148 X TIM R. MILES, Przmary Exammer. Sturtevant 20937 X 5 L Sturtevant.

Doyle

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