Method and apparatus for the classification of fine material from a stream of material in a circulating air classifier

Method and apparatus for classifying articles of varying size from a stream of particulate material in which the stream is subjected to centrifugal forces to provide an annular dispersion, and a stream of air is directed substantially perpendicular through the dispersion to aid in separating the fines from the coarser particles.

BACKGROUND OF THE INVENTION 
Field of the Invention 
This invention is in the field of circulating air classifiers of the type 
having an inner and outer housing concentric with each other, and 
employing centrifugal spreading means to propel the stream of material 
into the inner or outer housing, depending upon the mass of the particles. 
DESCRIPTION OF THE PRIOR ART 
Classifiers which employ concentric inner and outer housings, together with 
a circulating air system for separating fine particles from coarser 
particles are well known in the art. In such classifiers, the material is 
supplied to a rotating spreader plate and distributed by the spreader 
plate perpendicular to the axis of the housings. Above the spreader plate, 
there is provided a ventilator which produces an air current passing 
through the fine material as well as the grading chamber of the 
classifier. The current of air produced by the ventilator is directed 
through the grading chamber parallel to the axis of the classifier and 
flows through the material centrifugally propeled perpendicular to the 
axis of the classifier by the spreader plate, pulling the fine material 
upwardly and delivering the fine material suspension into a fine material 
chamber from where particles are dischared through discharge members. In 
the case of such classifiers wherein the fine material is lifted upwardly 
against the force of gravity and must be guided through the wall of the 
chamber into the fine material chamber, there are substantial difficulties 
in transporting portions of fine material with larger granule diameters, 
for example, up to 3 mm. out of the grading chamber to the collection 
zone, as the air speeds must rise substantially in order to accommodate 
higher granule diameters. For example, with a classifier which separates 
material to a 1 mm. granule size, the air speeds must be tripled in order 
to sift out particles from material with a diameter up to 3 mm. This means 
increased rates of rotation for the fan or blower, coupled with larger 
drive units, and larger blade diameters for the fan or blower, whereby the 
classifier as a whole is made larger, more expensive and uneconomical. 
SUMMARY OF THE INVENTION 
The present invention provides a method for the classification of fine 
material from larger diameter material which makes it possible to effect a 
separation even with low speeds and small fan or blower capacity. In the 
present invention, the stream of material is directed in a cylindrical or 
conical suspension in the direction of the axis of the classifier, and is 
subjected to a current of air which is substantially perpendicular to the 
dispersion axis to thereby improve the separation of fine material from 
coarse material. With this system, particles of material with granule 
diameters smaller than 3 mm. are deflected by the auxiliary stream of air 
from the vertical direction which extends parallel to the axis of the 
classifier. A small deflection is sufficient to cause these fine particles 
to be directed into the fine material chamber which is located between the 
inner and outer housings of the classifier. The particles of material in 
the present process are not carried by an air current contrary to the 
effects of gravity in an updraft, for which purpose high air speeds are 
necessary. Instead, the method of the present invention may use relatively 
small fans or blowers, and small classifier dimensions to separate fine 
materials having granule diameters up to 3 mm. from a stream of larger 
materials containing such fine particles. 
The annular dispersion according to the present invention is formed to be 
substantially coextensive with the inner wall of the inner housing, and is 
formed above the fan or blower means. By this means, the material 
suspension drops down as close as possible to the inner wall of the inner 
housing, so that a small stream of circulating air is sufficient to 
deflect the particles of fine material from the vertical direction and 
permit them to drop into the fine material chamber surrounding the inner 
housing. In addition, it is possible with the apparatus of the present 
invention to adjust for different degrees of separation by control of the 
rate of rotation of the drive and the speed of the air flow. 
The apparatus of the present invention provides a circulating air 
classifier which has an inner housing and an outer housing concentric with 
each other. A fan or blower is provided coaxially with the housings. A 
centrifugal spreading means is provided above the fan or blower to propel 
the stream of material radially outwardly into a cylindrically shaped 
dispersion which descends through gravity into the effective area of the 
fan or blower. 
In the preferred form of the present invention, the scattering or spreading 
device is surrounded coaxially with and spaced from a cylindrically shaped 
baffle. This strucure is particularly advantageous when using scattering 
or spreading devices which are rotary mixing tables or turntables and are 
provided with peripheral vanes. The material centrifuged off the rotary 
mixing table or turntable is deflected by the baffle wall and drops 
independently of the material particle size from the inner surface of the 
baffle wall as a uniform annular dispersion. Consequently, there is a 
continuous dispersion of material through which the circulating air which 
is directed perpendicularly to the dispersion may flow, and a uniform 
deflection of the finer particles of material is attained. 
The baffle wall is preferably constructed as a hood and is open toward the 
bottom, so that larger particles of material are prevented from being 
thrown into the fine material chamber. The outer wall of the hood extends 
approximately coextensively with the inner wall of the inner housing so 
that the inner diameter of the hood is only slightly smaller than the 
inner diameter of the housing, and the annularly shaped material 
suspension decreases from the lower edge of the baffle hood near the inner 
wall of the inner housing. Thus, a small stream of air is sufficient to 
deflect the finer particles of material from their vertical drop and to 
carry them into the fine material chamber of the outer housing. 
In a particularly preferred form of the invention, the inner housing is 
provided with an inlet in the vicinity of the fan or blower consisting of 
a wall in the form of a truncated cone. Accordingly, the inner housing may 
be relatively small in diameter with the wall under the fan or blower in 
the area of the lower edge of the baffle having a larger diameter, so that 
the coarse particles of material which are not deflected from the stream 
of air or are only slightly deflected, drop into the larger particle 
chamber of the air housing. Preferably, the conical sleeve is constructed 
to be slidable longitudinally. In this way, it is possible with a constant 
rate of rotation of the fan or blower to alter the effective diameter 
through a simple shifting of the conical sleeve. Thus, for example, a 
coarse adjustment of the separating section may take place through a 
shifting of the conical sleeve while the fine adjustment is placed through 
a change in the rate of rotation of the fan or blower.

The circulating air classifier shown in the drawings has a vertical axis 
and includes an outer housing 1, a rotary mixing table or turntable 3 
carried by a shaft 2, the lower end of the shaft carrying an impeller 
means such as a fan or blower 4. Above the rotary mixing table 3 there is 
provided a material supply hopper 6 which extends through a cover 5 of the 
outer housing 1, and is spaced from the shaft 2. Below the fan 4 there is 
an inner housing 7 which has funnel-shaped collecting housing 8 for the 
coarser particles. The area between the inner housing 7 and an outer 
housing 8 is provided with air conducting blades 15 so that the inner 
housing 7 is in open fluid communication with the outer housing 1. The 
collecting housing 8 is provided with a discharge pipe 14 and other 
withdrawal members (not shown). The outer housing 1 has a discharge means 
13, together with suitable mechanical devices for transporting the 
collected dust. 
Surrounding the rotary mixing table 3 and coaxial therewith is a 
cylindrical baffle hood 9 which is stationarily connected with the inlet 
6. However, the hood may also rotate with the mixing table 3. The baffle 
hood 9 has an open lower end which terminates shortly above the upper edge 
of the fan 4. The diameter of the baffle wall 10 of the hood 9 is less 
than the largest diameter of the inner housing 7. The inner housing 7 has 
an inlet end in the area of the fan 4 which is in the form of a wall 
widening into a truncated cone which exends beyond the inner diameter of 
the baffle wall 10 of the baffle hood 9. The conical sleeve or cone-shaped 
shell may be shifted longitudinally, as by means of a telescoping 
relationship, so that the diameter of the inner housing 7 in this vicinity 
may be altered. 
The operation of the illustrated device is as follows. A continuous stream 
of material is delivered to the inlet 6 as, for example, from a 
comminuting device. The stream of material is delivered through the inlet 
6 onto the rotary mixing table 3 and is centrifuged therefrom by means of 
the peripheral vanes 11 on the rotary table 3. The particles strike 
against the inner wall 10 of the baffle hood 9. From the lower edge 12 of 
the baffle hood 9, the material drops in a uniformly distributed 
cylindrically shaped suspension by the effect of gravity, and arrives at 
the effective area of the fan or blower 4 which is coupled to the shaft 2. 
The current of air produced by the fan or blower 4 is directed 
perpendicularly to the axis of the classifier through the material 
suspension and carries the fine particles of material beyond the inner 
housing 7 and into the space between the inner housing 7 and the outer 
housing 1. The fractions of fine material are separated off within the 
outer housing 1 and withdrawn through a discharge pipe 13. 
The coarse material parts are not influenced greatly by the transverse flow 
of air and are therefore delivered to the inner housing 7 and thence into 
the collector housing 8 from which they are discharged through a discharge 
pipe 14. The air flowing into the outer housing 1 is drawn through the 
air-conducting blades 15 of the fan or blower 4 and flows through the 
inner housing 7 in an upwardly directed current to thereby circulate to 
the fan or blower 4. 
If the separation desired between the coarse and fine materials is to be 
shifted to larger granule diameters, then the rate of rotation of the fan 
or blower is increased by adjusting the speed of the blower motor 16 so 
that a stronger current of air deflects coarser particles of material from 
the suspension into the outer housing 1. If the separating action is to be 
shifted in the other direction, namely, to accommodate only smaller 
granule diameters, then the rate of rotation of the fan or blower is 
reduced so that some of the finer particles of material drop into the 
inner housing 7. The shifting of the dividing line between the coarse and 
finer particles may also be accomplished with a constant rate of rotation 
by manipulation of the conical sleeve of the inner housing 7 to catch more 
of the coarser particles. It is, of course, possible in the course of the 
present invention to adjust both the disposition of the conical inlet to 
the inner housing 7 and the speed of rotation of the fan or blower 4, 
providing a coarse adjustment by changing the length of the conical sleeve 
inlet to the inner housing 7 and achieving a fine regulation by means of 
changing the speed of the blower motor 16. 
It should be evident that various modifications can be made to the 
described embodiments without departing from the scope of the present 
invention.