Classifying – separating – and assorting solids – Fluid suspension – Gaseous
Reexamination Certificate
1999-03-26
2001-08-21
Walsh, Donald P. (Department: 3653)
Classifying, separating, and assorting solids
Fluid suspension
Gaseous
C209S714000, C209S713000, C209S710000, C209S143000, C209S150000
Reexamination Certificate
active
06276534
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention pertains to an apparatus for classifying powders. In general terms, classification of powders refers to the separation of a feed powder containing particles having a variety of particle sizes into a coarse fraction and a fines fraction in accordance with a selected “cut” size. One known method to evaluate the air classifier's cut size and sharpness is to construct a grade efficiency curve that plots size selectivity (&eegr;
D
) versus particle size (D). The relationship can be calculated by analyzing the particle size distributions of the feed and final product to determine what percentage of a particle size in the feed goes into the coarse fraction. Size selectivity is defined as:
η
D
=
Quantity
of
size
D
entering
course
fraction
Quantity
of
size
D
in
feed
The cut size (x
50
) is the particle size corresponding to &eegr;
D
=0.5 on the grade efficiency curve. Cut sharpness of the classification can be determined by intersecting the curve with the &eegr;
D
=0.25 and &eegr;
D
=0.75 lines and placing the particle sizes in the line intersections in relationship to each other. Cut sharpness (x
25
/x
75
) is often used to quantify air classifier performance. x
50
is the equiprobable cut size, i.e., the particle sizecorresponding to the 0.5 size selectivity value. x
25
is the particle size corresponding to the 0.25 size selectivity value. x
75
is the particle size corresponding to the 0.75 size selectivity value. Cut sharpness values range from 0.0 (almost no classification) to 1.0 (ideal but not achievable classification). In a production operation, an air classifier's cut sharpness typically ranges between 0.3 and 0.7. For a laboratory scale classifier, the cut sharpness can reach about 0.9. A good classifier has a wide adjustable cut size range and can achieve a very fine cut size and high cut sharpness.
There are a number of prior art references directed towards powder classifying apparatuses and methods. In general terms, most prior art powder classifiers comprise a means for dispersing the feed powder and a means for separating the dispersed powder at a specified cut size in order to obtain a coarse and a fines fraction. The prior art takes a variety of approaches in order to achieve the desired classification.
For example, a number of references disclose classifiers which employ the same basic design concept wherein a dispersion disk(s) is used to initially break up the feed powder and subsequently a classifying means such as a rotor is employed to impart a centrifugal force to the particles. The classification is typically achieved by applying a current of air to the dispersed powder stream, whereby the fine particles are removed from the particle stream by the air current and directed to a fines discharge outlet and the coarse particles travel through the air current and into a coarse particle discharge outlet. Among the references which describe variations of this basic design concept include U.S. Pat. Nos. 2,188,634; 2,542,095; 2,796,173; 3,720,313; 4,066,535; 4,100,061; 4,066,535; 4,388,183; 4,560,471; 4,604,192; 4,759,943; 4,869,786; and 5,024,754.
The references cited above provide a variety of designs in an attempt to optimize the same basic design concept. For example, some of the above references disclose designs wherein the current of air directs the fines inwardly towards the center of the classification chamber. see e.g. U.S. Pat. Nos. 4,560,471; 4,759,943; 2,796,173; 4,869,786 Others of the references disclose designs wherein the current of air directs the fines to an outer portion of the classifying chamber. see e.g. U.S. Pat. No. 4,066,535; 4,388,183. Many of the prior classifiers disclosed in the above references exploit the effects of gravity in that upon classification of the powder, the fines fraction and the coarse fraction are directed to separate discharge ports located in the bottom portion of the classifier housing. see e.g. U.S. Pat. Nos. 4,066,535; 4,388,183; 4,560,471; 4,759,943, 5,024,754. However, there are some prior art classifiers wherein the fine material is lifted upwardly against the force of gravity and is discharged from the upper portion of the classifier. see e.g. U.S. Pat. No. 4,661,244. A number of the references mentioned above disclose classifier systems wherein the dispersion means and the classifying means are separately driveable in order to achieve optimum particle dispersion and classification. see e.g. U.S. Pat. Nos. 5,024,754; 4,869,786; 4,661,244; 4,388,183; 4,100,061; 2,188,634.
However, there remains a need for an improved powder classifier which allows for control of a number of variables in order to obtain a more precise cut of the coarse fraction and fines fraction while also maintaining a high throughput of the feed powder. The present invention provides a novel design for such a classifier, the features of which are not disclosed or suggested by any of the prior art classifiers, either alone or in combination.
SUMMARY OF THE INVENTION
The present invention provides an improved powder classifier which provides a precise classification of a feed powder stream into a coarse fraction and a fines fraction, while also allowing a high throughput of the feed powder. The improved powder classifier of the present invention employs a powder dispersion, preclassification, secondary classification and primary classification in order to obtain the precise classification of the feed material.
In particular, the present invention is directed to a powder classifier which comprises a classifier rotor fixedly secured to a rotatable shaft and having (i) an interior portion defined by an upper plate and a lower plate, and (ii) an impeller wheel having upper and lower surfaces and a plurality of vanes therethrough. The vanes form a plurality of channels through the impeller wheel, the upper plate has a rounded outer edge along its outer circumference, and the interior portion is in communication with a fine particle discharge outlet. The powder classifier also includes a first annular ring having an inner circumference, an outer circumference, an upper surface and a lower surface, the first annular ring being disposed about the outer circumference of the classifier rotor, and a first gap formed between the inner circumference of the first annular ring and the outer circumference of the classifier rotor, with the first annular ring being positioned so that a preclassification of a feed powder stream occurs at the first gap such that a fraction of fine particles is separated from the feed stream and flows through the first gap and into the interior portion of the classifier rotor for primary classification.
In this device, it is preferred to provide a transition portion beneath the first annular ring with an inwardly tapered configuration in order to enhance particle separation therein. With this design, the first annular ring may be a solid ring having upper and lower surfaces where the lower surface includes the inwardly tapered configuration.
The powder classifier may also include a second annular ring having an inner circumference, an outer circumference, an upper surface and a lower surface, wherein the second annular ring is disposed about the outer circumference of the first annular ring. Preferably, the second annular ring has a plurality of air guide vanes located between the upper surface and lower surface thereof, with the air guide vanes forming a plurality of channels through the second annular ring. Advantageously, these air guide vanes are evenly spaced from each other and positioned such that a radial vector projecting from the center of the classifier rotor intersects a vector projecting along the centerline of an air guide vane to form angle &bgr; which is between about 60 to 90°.
When the first annular ring includes a hollow central opening, it is advantageous for a se
Huang Ching-Chung
Pavlosky Joseph M.
Hosokawa Micron Powder Systems
Schlak Daniel K
Walsh Donald P.
Winston & Strawn
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