GB2078553A - Pneumatic classification of finely-divided solids - Google Patents

Abstract

The classification is performed by circulating solid particles within the main body of the classifier housing 1 (Fig. 1). To improve the classification, the circulation is increased by cutting a portion of the coarse product stream separating in the lower section of the housing to pass upwards and around and along the inner periphery of the housing by means of the gas medium in circulation within the housing. This is accomplished by means of curved stationary flanges (41) attached to the inside front and back walls of the lower part or collecting hopper (4) of the classifier.

Description

SPECIFICATION Method Of and Apparatus for the Pneumatic Classification of Finely-divided Solids This invention relates to a method of and apparatus for the pneumatic classification of finely-divided solids. More particularly, the invention provides an improvement in the method of pneumatic classification proposed in British Patent Specification No. 1 391 892 and in the pneumatic classification apparatus by which the classification is carried into effect.

Classification methods and apparatus at present in industrial use have so far been based on the principles specified in relation to the first preferred embodiment as shown in Fig. 1 in the British Patent Specification No. 1 391 892. The existing embodiment of the classifier, as developed and modified for industrial application, is illustrated in Fig. 1 of the accompanying drawings, which figure is a schematic vertical section of the apparatus as presently in use.

One would expect that in conventional classification methods, the upper internal periphery of the classifier housing would be the part of the apparatus particularly subject to wear.

Accordingly, the corresponding surfaces would be ground clean and well polished. No possibility should exist for accumulation of any kind of coatings of the material under treatment.

In industrial practice, however, harmful exceptions have emerged. For example, in certain closed-circuit cement grinding operations using this type of classifier, coatings of substantial thickness and of varying roughness arise, especially on the internal surface of the upper inner periphery of the classifier housing. It is obvious that the existance of such a coating prevents smooth circulating sliding motion of the solids within the housing, slows down the speed of travel of the entire suspension in circulation and distorts the otherwise undisturbed flow pattern within the classifier. As a result, alien oversize particles find their way into the fine product, the overall sharpness of size separation becomes poorer and the capacity of the apparatus to perform separation deteriorates.

In the case of cement, further investigations have shown that the accumulated coatings consist principally of gypsum which is normally introduced with the clinker into the grinding unit.

In the ground mill product which is the feed to the classification apparatus, the gypsum concentrates into the very finest size fractions. The tendency to form coatings increases with increasing fineness of the classifier feed and with increasing gypsum content in the feed.

.'An object of the present invention is to improve the existing method of pneumatic classification and the corresponding classification apparatus to avoid the formation of the internal surface coatings and eliminate the resulting harmful effects.

A further object of the invention is to optimise the internal circulation of solids within the classifier in an effort to achieve the best possible overall sharpness of size separation.

To prevent the formation of the said coatings a simple, but significant and inventive improvement in the known classification method and in the apparatus is herein proposed. Pursuant hereto, the present invention provides a method for the pneumatic classification of finely divided solids in which internal circulation of solid particles within the main body of the classifier housing is increased by cutting a portion of the coarse product stream separation in the lower section of the housing so as to proceed upwards around and along the inner periphery of the housing by means of the gas medium in circulation within the said housing. The additional fraction which is forced into circulating motion by the gas medium in the upper part of the housing provides the polishing sand needed for removal of any coating and for keeping the surfaces clean and well polished.

In a preferred method the additional fraction is cut from both sides of the separating coarse product stream.

Insofar as concerns the apparatus the invention provides a pneumatic classifier comprising a housing provided with curved stationary flanges attached to the inside front and back walls of the lower part of classifier housing and disposed to cut a portion of the coarse product stream separating in the lower section of the housing so as to proceed upwards, around and long the inner periphery of the housing by means of gas medium in circulation within the said housing.

The invention will be described further, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a schematic sectional elevation illustrating the known pneumatic classifier construction; Fig. 2 is a similar view, but showing a preferred embodiment of the classifier of the present improvement; Fig. 3 is an enlarged cross-section of the coarse-product collecting hopper of the classifier of Fig. 2, but showing a modification; Fig. 4 is a view similar to Fig. 3 but showing another modification; and Fig. 5 is a cross-section showing an arrangement similar to Fig. 4, but having special means added for effective washing of the coarse separation product (sands) with gas medium.

In the following the numbering used in the British Patent Specification No. 1 391 892 has been followed as far as possible.

As shown in Fig. 1, the existing classifier for industrial applications includes a housing 1, a feed channel 2, a cyclone collector 3, a collecting hopper 4 for final coarse product and and air tube 5 connecting the cyclone collector 3 to a blower 6. A primary dispersing ejector 7 includes an air inlet channel 9, a feed material inlet channel 10 and a mixing channel 11 for the air and feed material. The ejector 7 opens to primary dispersion space 12. Centrifugal classifier 8 proper includes a wide inlet opening 15, a circular discharge opening 16 connected tangentially to the cyclone collector 3, and a relatively-narrow downward-leading discharge slot 1 7 for coarser middling product. Air for secondary ejector 1 8 is delivered via channel 1 9.

Fig. 2 shows the construction of the pneumatic classifier embodying the improvement in accordance with the present invention. Essentially the improvement concerns the construction of the coarse product collecting hopper 4 which is relatively enlarged and is effectively separated from the main body of the classifier by a sealed joint 40. Stationary, upward curved flanges, ledges or projections 41 are attached to the inside surfaces of the front and back walls of the hopper 4 in such a way that the resulting flow contour of the lowermost section of the classifier 8 proper follows a smooth continuous curve. The widths of the flanges 41 may vary depending on the material being treated and on the object to be achieved. The flanges 41 define a distinct boundary surface between the classification space above and the coarse product collecting space below.The resulting discharge opening, for coarse product (sands) between the flanges 41 is a longitudinal slot following the curvature of the flanges 41. When desired an air inlet tube 42 is included to direct wash medium, from the high pressure an inlet channel 9 after the blower 6, to the hopper 4 in the space below the flanges 41.

Fig. 3 shows a cross-section of the collecting hopper 4 with the flanges 41 attached to the front and back walls 43 and 44 of the hopper 4. The top surfaces of the flanges 41 are perpendicular to the hopper walls, and the longitudinal discharge slot between the flanges 41 is indicated by the numeral 45. The flanges 41 are made of wear-resisting material.

Fig. 4 shows a simliar section of the collecting hopper 4 but in this embodiment the flanges 41 slope downward toward the centreline of the coarse product discharge slot 45. The slot 45 is here fitted with a multitude of stationary plates forming a grate 46. The plates are orientated and inclined to guide the coarse sands smoothly into the hopper 4.

Fig. 5 shows a modification having sloping flanges 41 provided with special means for effective washing of the separating sand fraction by gas medium. As shown, the angled flanges 41 are constructed to include downwardly-directed inclined side plates 47 and toothlike extensions 48. Additionally provided are a hollow stationary divider member 49 and two stationary inclined wear plates 50, all three of which are substantially ofthe same length and curvature as the flanges 41. The air inlet tube 42 is now connected to the divider member 49. The bottom part of the divider member 49 is provided with narrow slits 51 which serve as air outlets. The spaces in between the plates 47 and 50 form upwardly-leading flow channels 52. Plates forming a grate 46 may or may not be included.

The operation of the pneumatic classifier shown in Fig. 1 involves: dispersion of the feed material into an air suspension in a primary dispersion step by applying the ejection principle; redispersion of the sand fraction from the' primary dispersion step in a secondary dispersion step at a lower elevation; removal of the cleaned coarse product fraction downward; directing the primary and secondary suspensions upward into a centrifugal field wherefrom the fine suspended fraction is removed by the air medium via an eccentricallyplaced channel into the fine product cyclone, to be separated therein as the final fine product while the coarser middling fraction separating in the centrifugal field is returned back to the primary dispersion step; and returning the air medium from the fine product cyclone in closed circuit via the blower back into the primary and secondary dispersion steps.

The improvement illustrated in Fig. 2 changes the operation in such a way that while the dispersion steps, the fine product separation step and the closed circuit air return step occur substantially as before, the internal circulation of solid particles within the main body of the classifier housing is materially increased. A substantial portion of the coarse product stream, normally passing freelyand without any controlling restrictions into the coarse product hopper, is now cut by the flanges 41 and is forced to proceed upwards and around the inner periphery of the housing by the strong gas medium stream in circulation within the housing.

These additional solid particles move primarily along the inner periphery at a high speed, rubbing and polishing the respective surfaces and removing any coatings that otherwise might tend to attach themselves to the said surfaces.

Regarding the construction of the flanges 41 as shown in Figs. 3 and 4 it is obvious that the quality and quantity of the final coarse product separated will be affected not only by their inclination but also by the relative width of the final coarse product discharge opening 45.

It is well known that the sharpness of size separation in all industrial classifiers is far from perfect. The coarse product always carries away a certain portion of fines which should be included in the fine product. The increased recirculation of a portion of the coarse product including some fines, in accordance with this invention, provides a renewed opportunity for the respective fraction to be reclassified after returning, via the middling discharge slot 17, back to the classification space proper. Reclassification of the returning frac;tion results in improved recovery of fines into the fine product and in a cleaner final coarse produCt, in other words in an improved overall sharpness of size separation.

The final cleaning of the separating sand product can ultimately be achieved by means of wash medium introduced into the coarse product hopper 4 below the coarse product discharge slot.

As shown in Fig. 2, it can be cut from the circulating medium stream at the high pressure channel 9. If necessary, its pressure can be further increased by a small auxiliary blower (not shown).

'Another possibility is to use fresh air introduced by means of a similar auxiliary blower. In the latter case, however, the additional volume of air must be removed from circulation elsewhere in the system. From the hopper 4, the medium must escape upwards in a counter-current fashion via discharge opening 45 or through the discharge grate 46 whereby sands descending in the opposite direction will be further cleaned of any residual fines.

The coarse product wash system shown in Fig.

5 operates as follows: The sands are divided into two sub-streams descending on both sides of the divider member 49. On their way down, the sand streams are crossed by medium jets emerging from the slits 51. The medium streams, with removed fines, hit the inclined side plates 47, receive an upwardly-directed flow component and pass via the channels 52 upwards. The extensions 48 cause a change in their flow direction, whereafter they cross the separating sand layer once more before reaching the classification space above. The extensions 48 keep the flow channels 52 free of settling sands.

The cleaned sands settle downward by gravity in the hopper 4 to be discharged therefrom.

Other measures that may be advantageously used in optimization of the classificaiton process include regulation of the volume of air delivered by the blower 6, elimination of sharp corners in the junctions between the face surfaces and the peripheral rim surface of the classifier housing and reshaping triangular member 53. By designing its upper downward sloping surface 54 in concave and trough line form, the very coarsest particle fraction of the coarse product is preferentially directed toward the centreline of the apparatus, i.e. toward the centreline of the discharge opening or slot 45 between the flanges 41 to be discharged at the earliest opportunity into the coarse product hopper 4.

By systematic testing of the variables outlined above the degree of recirculation can be optimised whereby the best possible overall sharpness of size separation can be achieved.

The improvement described in this specification can increase the overall sharpness of size separation to such an extent that a desired efficient classification result will now be obtainable by one-stage separation rather than by a two-step process otherwise needed.

Though the invention has been illustrated and described from the standpoint of certain structural details in which it is embodied, it is not, however, tb be considered as being limited by such showing. Rather, as will be apparent to those s'killed in the art, certain modifications or variations may be made in the structure as shown and described, without departing from the scope of the invention as defined in the following claims.

It is, accordingly, intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Claims (11)

Claims

1. A method for the pneumatic classification of finely divided solids in which internal circulation of solid particles within the main body of the classifier housing is increased by cutting a portion of the coarse product stream separating in the lower section of the housing so as to proceed upwards around and along the inner periphery of the housing by means of the gas medium in circulation within the said housing.

2. A method as claimed in claim 1 comprising cutting the additional solids portion for recirculation from both sides of the separating coarse product stream.

3. A method as claimed in claim 1 or 2 which further includes washing the portion discharged downward as the coarse product by upward flowing gas medium streams.

4. A pneumatic classifier comprising a housing provided with curved stationary flanges attached to the inside front and back walls of the lower part of classifier housing and disposed to cut a portion of the coarse product stream separating in the lower section of the housing so as to proceed upwards, around and along the inner periphery of the housing by means of gas medium in circulation within the said housing.

5. A pneumatic classifier as claimed in claim 4 where the upper surfaces of the flanges are perpendicular to the inside walls of the classifier housing.

6. A pneumatic classifier as claimed in claim 4 where the upper surfaces of the flanges slope downward toward the centreline of a coarse product discharge slot between said flanges.

7. A pneumatic classifier as claimed in claim 4, 5 or 6 including a grate in the coarse product discharge slot between said flanges.

8. A pneumatic classifier as claimed in any of claims 4 to 7 including means to direct gas medium under pressure into the space below the coarse product discharge slot between the stationary flanges.

9. A pneumatic classifier as claimed in any of claims 4 to 8 wherein the stationary flanges are curved, and are provided with inclined side plates and extensions, and further including: (a) a stationary divider member; (b) two stationary inclined wear plates; (c) inlet means for gas medium into the divider member; (d) discharge slits for the medium from the divider member; and (e) upward leading flow channels for passage of the gas medium with removed fines up to the classification space proper.

10. A method for the pneumatic classification of finely-divided solids substantially as hereinbefore described with reference to and as illustrated in Figs. 2 to 5 of the accompanying drawings.

11. A pneumatic classifier substantially as hereinbefore described with reference to and as illustrated in Figs. 2 and 3, in Fig. 4, or in Fig. 5 of the accompanying drawings.