GB2221631A - Gravity classifier for loose material - Google Patents

Abstract

A housing (1) has a downwardly inclined bottom (4) with perforations (7) for the passage of an ascending stream of gas, and separating elements (14) accommodated in the housing 1 one above another. A charging chute (2) overlies the uppermost edge (3) of the perforated bottom (4), and a pipe (5) discharges the coarser product of separation from the lower most edge (6). Pipes (11) withdraw the gas and finer product of separation from the upper part of the housing 1 which is partitioned (12) into vertical channels. Barriers (8) are provided on the perforated bottom (4) across the direction of the advance of the material being separated. The upper edge of each barrier (8) has the shape of alternating peaks (9) and valleys (10) and functions to even out the distribution of sizes of the mixture to be separated over the length of the bottom (4) and to allow larger coarse particles to move downwards thus reducing clogging thereby. <IMAGE>

Description

GRAVITY CLASSIFIER FOR LOOSE MATERIAL The present invention relates to classification of various loose materials, e.g. in an ascending flow of air, and, more particularly, it relates to a gravity separator or classifier.

The invention can be successfully implemented in the metallurgical, foundry, mining, chemical and other industries involving operations of handling loose materials requiring separation or classification by their particle size, particularly within a range from 0.5 to 5 mm, into either two or several products of different fineness/coarseness.

The problem of providing highly efficient devices for separation or classification of loose materials is quite topical nowadays, as the advent or ever newer and improved technologies in powder metallurgy, in preparation of moulding materials in foundries, and in other fields presents ever stricter requirements to the particle size of the initial materials, semi-finished and final products. Pneumatic classification is one of the most promising present-day classification techniques.

In implementing the principle of pneumatic classification of loose materials, there has been successfully employed the general design of a cascade gravity separator or classifier, comprising a spatial housing divided by successive vertical partitions into a series of shafts overlying an inclined perforated bottom. Classification of the material in the shafts is effected either by separating elements mounted on the inner walls of the shafts (SU-A-787113) or owing to the zig-zag shape of the walls of the shafts themselves (GB-B-2623038).This design of the classifiers has allowed one to attain an efficiency expressed by the Eder-Mayer indicator "be = d75/d25 .100% as high as 70 to 758 (where d75 and d25 are the sizes of particles recovered into the finer product, respectively, by 75 and 25%) with a flow concentration of the initial loose material ij = 2 - 2.5 kg/m3 and a throughput of 60 - 100 t/hour. The power input generally does not exceed 2.5 kW/t (initial material).

In classifiers of the above-described type the process of separation of the initial loose material is based on creating a fluidized bed in a gaseous stream ascending through the perforated bottom, the fluidization supporting the progress of the material down the incline of the perforated bottom, and separating the material into finer and coarser classes of particles under gravity forces assisted by aerodynamic resistance. The finer particles are carried in the ascending air stream into the separation shafts, where their additional separation and release of incidentally entrained coarser and heavier particles takes place. Thus, the above-described designs provide for repeated separation of both coarser and finer classes of particles, which yields the above-mentioned satisfactory efficiency rating.

However, the fact should not be overlooked that the above-described types of classifiers or separators have proved to be inadequately efficient in cases where requirements concerning the particle size of the separated loose products are particularly high, e.g. in the production of moulding sands, grinding and abrasive powders, initial materials for powder metallurgy and the like. This is explained by the fact that concentration of the treated material lengthwise of the perforated bottom and in the shafts tends to be different, declining with the progress of the material, as the greater part of the finer product is recovered in the upstream shafts, whereas the downstream ones remain "underloaded".It is known, however, that reduction of the concentration of the material being separated leads to more coarser particles being improperly carried away and to a growth of the "separation barrier", so that different shafts separate the initial material to different barriers", which means that the finer and coarser products of separation become more burdened with coarser and finer particles. respectively. Here and in the disclosure to follow, the term "separation barrier is meant to describe the mean diameter of particles which are recovered in real-life separators or classifiers with equal probability in the finer and coarser products of separation.

Therefore, non-uniformity of the concentration lengthwise of the perforated bottom and among the separation shafts adversely effects the quality of classification. Furthermore, classifiers of the above-described general designs display a tendency towards non-uniform distribution of the concentration of the material across the width of the perforated bottom, be it on account of minor faults in either manufacture or mounting of the perforated bottom, or else, which is more likely, of the charging of the initial material being non-uniform across the width of the bottom.In either case, the initial material moves in a denser stream along one side of the bottom, closer to the side wall, whereas its concentration on the bottom of the classifier at the opposite side wall is substantially lower. Correspondingly. finer particles carried in the denser stream of the material- are recovered with the coarser classes to a greater degree, contaminating the coarser product and thus adversely affecting the classification quality. In other words, non-uniformity of the concentration of the material being separated, either lengthwise or crosswise of the bottom and among the separation shafts, impairs the quality of classification.

It is also known that the efficiency of classification generally grows with a longer time of residence of the loose material in a classifier, owing to its greater susceptibility to the separating action of the air stream, which brings down the probability of contamination of the coarser product with finer particles. In its turn, the time of residence of particles in classifiers or separators of the type being described is greatly dependent on the inclination angle of the perforated bottom, i.e. the less steep the bottom, the greater is the time of resistance of particles in the classifier.However, in a majority of practical cases the angle of inclination of the bottom with respect to a horizontal plane cannot be made smaller than 9". This is explained by the fact that most of practically encountered loose materials have a broad range of particle sizes, and the biggest ones of the particles in operation of the classifier would not submit themselves to fluidization, displaying a tendency to accumulate on the bottom, clogging its perforations, as their progress along the bottom towards the means for withdrawing the coarser product is due only to their rolling down the inclined bottom. Thus, both the inclination angle and the residence time it governs are limited in separators or classifiers of the type being described, which further impairs the quality of classification.

To summarize, in the above-described designs of classifiers the quality of classification is adversely affected by non-uniformity of the concentration of the material being separated lengthwise and crosswise of the perforated bottom and by the limited time of residence of the loose material in the separator.

An attempt to prolong the time of residence of a loose material in a classifier has been made in another known gravity classifier (SU-A-486814). This classifier has a housing divided into shafts by vertical partitions.

Each vertical shaft is further divided by additional partitions into channels whose walls carry separating members overlying one another. The housing has an inclined perforated bottom of a stepped shape, the inclination of the bottom being from the charging means for feeding the initial material, mounted on the housing of the classifier, towards the means for withdrawing the coarser product of separation, likewise mounted on the housing. The charging means is situated above the uppermost ledge of the bottom, whereas the means for withdrawing the coarser product is situated adjacent to the lowermost edge of the bottom. Pipes for withdrawal of air and the finer product of separation are mounted at the upper part of the housing. A gap is left between the lowermost edges of the vertical partitions and the perforated bottom, sufficient for the passage of the loose material.Each partition of the shafts is provided with a gate pivoted to the lowermost edge of the partition and facing the respective step of the bottom. Each gate is intended to reduce the flow of air with finer particles of the material from one shaft into another. Furthermore, it assists in lowering the velocity of the progress of the material along the bottom, thus increasing its time of residence in the classifier and somewhat improving the classification quality.

However, the last-described design of classifier has not improved to any significant extent the quality of classification of loose material, as it does not cope sufficiently with either non-uniformity of the concentration of the material lengthwise and crosswise of the perforated bottom or the limited time of residence of the loose material in the classifier.

In operation of the last-described classifier, the loose material is fed by the charging means into the housing of the classifier, onto the upper ledge of the perforated bottom. A fluidized bed of the material is formed on the perforated bottom owing to the air stream ascending through its perforations. The fluidization is accompanied by the progress of the material lengthwise of the inclined bottom towards the means for withdrawing the coarser product. In the course of this progress, the loose material is separated into coarser and finer fractions, the latter being carried away in the stream of air into the separation shafts where they are subjected to repeated separating action of the separating elements.This flow sheet of the process of separation in the classifier results in the concentration of the material both lengthwise and crosswise of the perforated bottom and among the separationshafts being non-uniform, varying several times over. Thus, in classification of potassium chloride in a fertilizer production technology, the greater part of the material is recovered in the upstream shafts; e.g. with the total yield of the finer product being yf = 35.7S, its distribution among the shafts is: Ylf = 24.34S, 2f = 7.08%, y3f = 4.28%. It can be seen from the above data that the yield of the first shaft is nearly 6 times as great as that of the third shaft. which means that the flow concentration of the material lengthwise of the bottom may also vary as much as 6 times.

It has been already mentioned that reduction of the concentation of the material at successive shafts lifts the "separation barrier" from shaft to shaft. All this impairs the efficiency of the performance of the classifier and adversely affects the quality of classified products. Furthermore, as it has been already discussed, faults in either manufacturing the bottom or mounting it, and, in the first place, non-uniform charging of the material across the width of the perforated bottom, may cause the progress of the material in a denser layer at one side wall of the housing and in a significantly less dense layer at the opposite side wall. This results in finer particles being entrained in the denser layer and delivered with the coarser product, which contaminates the latter and impairs the'classification quality.

The experience of operating the last-described classifier has shown that the angle of inclination of the perforated bottom with respect to a horizontal plane cannot be made smaller than 90 for most loose materials having a broad range of the sizes of their component particles1 lest the perforations of the bottom become clogged with the coarsest of the particles, as they are not engaged in the fluidization, but progress solely by rolling down the inclined surface of the perforated bottom. With the angle of inclination of the perforated bottom being thus limited on the smaller side, the time of residence of the material being separated in the classifiers is likewise limited, which adversely affects the quality or classifications.

It would be desirable to improve the quality of classification or a loose material by levelling out its concentration over the entire area of the perforated bottom and among the separation shafts, and also owing to prolonged time of residence of the material in a classifier.

It would also be desirable to be able to enhance the throughout of the classifier with the same classification efficiency, by providing for stepping up the flow concentration of the loose material.

The present invention provides a gravity classifier for loose material, comprising a housing with an inclined bottom having perforations made therethrough for the passage of gas and at least one threshold provided on the bottom across the direction of the material flow; charging means for feeding the initial material into the classifier, situated above the uppermost edge of the bottom: means for withdrawing the coarser product of separation, situated at the lowermost edge of the bottom; and means for withdrawal of the gas and finer product of separation, mounted at the upper part of the housing.

This design of classifier or separator, with at least one threshold being provided on the perforated bottom, results in the material being arrested upstream of this threshold, so that the concentration of the material in the area upstream of the threshold is stepped up, both in the fluidized bed on the perforated bottom and in the space of the housing (or a separation shaft) overlying the threshold. The degree of stepping up this concentration is governed by selecting the height of the threshold. Thus, by installing the threshold or thresholds, it is possible to influence the concentration of the material lengthwise of the perforated bottom (and within the space of each separation shaft, if provided) towards the levelling out of this concentration, to enhance the quality of separation or classification of loose material.

Furthermore, the provision of the threshold or thresholds slows down the progress of the material along the inclined perforated bottom towards the means for withdrawing the coarser product, thus prolonging the time of residence of the material in the classifier and further enhancing the quality of separation or classification.

It is expedient for the threshold to comprise a plate having its upper edge shaped as alternating peaks and valleys1 the bottom level of each valley being substantially at the level of the perforated bottom.

Owing to this structure of the threshold, the coarsest particles of the material, not engaged in the fluidization, can unobstructedly progress by rolling down the perforated bottom through the valleys of the threshold, towards the means for withdrawing the coarser product, preventing the clogging of the perforated bottom.

It is further expedient that, with two or more such thresholds being provided on the perforated bottom, each successive threshold should be arranged for its peaks to face the valleys of the preceding threshold.

With the thresholds being so relatively arranged, their peaks would break up the dense layer of the loose material, brought about by either improper-manufacture of the perforated bottom, or its faulty mounting, or else non-uniform charging of the classifier across the width of the perforated bottom. This provides for levelling out the concentration of the loose material crosswise of the bottom, opposes the carrying away of finer particles in the dense layer of the material into the coarser product being discharged, and thus enhances the quality of classification.

It is generally known that out of two classifiers having different efficiency, with the same requirements of the purity of the classified products, the classifier with the higher efficiency is operable with higher flow concentrations of the material being separated. There is further known that the throughput Q1 (kg/s) of a classifier is defined by an expression Q1 = 'L.Q21 where ij is the flow concentration of the initial loose material, kg/m3; and Q2 is the flow rate of the air through the classifier, m3 /s.

It can be seen from the above expression that the throughput of the classifier rises in direct proportion to the flow concentration of the loose material. Thus, in a number of technologies, e.g. in production of construction materials, fertilizers, etc., not requiring the utmost purity of the classified products, a classifier constructed in accordance with the present invention is operable reliably with a higher flow concentration of the initial material, stepping up its throughput.

A prototype of a gravity classifier constructed in accordance with the present invention for classification of moulding sands has shown the following characteristics; the separation efficiency by the above-cited Eder-Mayer indicator iN = about 80%, with the initial material flow concentration IL = 3.5 3 kg/m3 and the throughput Q1 = 20 t/h, the overall dimensions being 1600 mm (height) x 400 mm (width) x 1900 mm (length). The energy input including the purification of the air from dust is within 1.5 kW/t (loose material).

The invention will be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic general perspective view of a gravity classifier for separating loose material; Figure 2 is a schematic sectional view lengthwise of the progress of the material of the classifier, taken on line II-II of Fig. 1; and Figure 3 shows on a larger scale a sectional view taken on line III-III of Fig. 2.

The gravity classifier illustrated, e.g. intended for classification of moulding sands according to their particle size1 comprises a housing 1, charging means for feeding the initial moulding sand, in the form of a chute 2 overlying the uppermost edge 3 (Fig. 2) of a perforated inclined bottom 4, and means for discharging the coarser product of classification, including a downwardly directed delivery pipe 5 mounted at the lowermost edge 6 of the perforated bottom 4. The perforated bottom 4 has holes 7 (Figs. 2 and 3) for the passage of an ascending stream of air, and it is inclined with respect ot a horizontal plane in the direction of the progress of the material to be classified, i.e. from the charging means towards the discharging means.The perforated bottom 4 has successive thresholds 8 (Fig. 2) mounted on its surface, in the form of transverse plates of which the upper edge is shaped as alternating peaks 9 (Fig. 3) and valleys 10. Pipes 11 (Fig. 1) are mounted at the top of the housing 1 for withdrawal of the air carrying the finer product of classification.

The housing 1 of the classifier is divided by vertical partitions 12 (Fig. 2) into a succession of separation shafts 13 accommodating therein separating elements in the form of inclined ledges 14 overlying one another within each shaft 13. The ledges 14 are mounted on the vertical partitions 12 and on the side walls 15 (Fig. 3) and 16 of the housing 1 (Fig. 2), one above another.

To create a stream of air ascending through the perforated bottom 4 the classifier includes a centrifugal fan 17 (Fig. 1) whose suction pipe 18 is connected via an air duct 19 to a manifold 20. Each pipe 11 for withdrawal of air with the finer product leads to a cyclone separator 21 of any suitable known structure, intended for separating the finer product of classification from the carrier air. The outlet pipe 22 of each cyclone 21 is connected with the manifold 20, and each bin 23 of the respective cyclone 21 overlies a belt conveyer 24 of any suitable known structure for carrying away the separated finer product. Another belt conveyer 25 is provided for feeding the initial sand into the chute 2. Still another belt conveyer 26 for carrying away the coarser product of classification underlies a feeder 27 of any suitable known structure, communicating with the delivery pipe 5.

The gravity classifier is operated as follows.

The initial moulding sand is fed in by the belt conveyer 25 (Fig. I)in the direction of arrow a through the chute 2 onto the uppermost edge 3 (Fig. 2) of the perforated bottom 4 of the housing 1. Owing to the suction created in the housing 1 by the centrifugal fan 17 (Fig. 1), the stream of air ascending through the holes 7 (Fig. 2) of the perforated bottom 4 lifts sand particles into the successive shafts 13, with a fluidized bed of the sand being produced on the surface of the inclined perforated bottom 4, and the gravity forces and aerodynamic resistance of the particles initiating the separation of the sand into coarser and finer fractions.

The finer particles are carried by the ascending stream of air into the separation shafts 13, where they are subjected to repeated action of the successive separating ledges 14, which relieve the ascending stream from coarser particles incidentally entrained in the stream of air. The stream of air carries the finer particles via the pipes 11 (Fig. 1) into the cyclones 21, where they are separated from the air and collected in the bins 23, to be subsequently carried away by the belt conveyer 24 in the direction of arrow b. The air purified from the finer particles advances via the outlet pipes 22 of the respective cyclones 21 through the manifold 20 and the air duct 19 into the suction pipe 18 of the centrifugal fan 17.

The coarser particles moving down the inclined perforated bottom 4 (Fig. 2) in the fluidized bed are relieved of the finer particles, discharged via the pipe 5 (Fig. 1) and the feeder 27 from the classifier, and carried away by the belt conveyer 26 in the direction of arrow c.

As the initial moulding sand advances along the inclined bottom 4 (Fig. 2), it is arrested in front of each successive threshold 8, so that in the respective areas of these thresholds 8 there takes place a build-up of the concentration of the initial material, i.e. of the moulding sand, both in the fluidized bed above the perforated bottom 5 and in the space of the housing 1 and of the respective separation shaft 13 overlying the threshold 8. This building up of the concentration, governed by the respective heights of the successive thresholds 8, tends to level out the concentration of the moulding sand lengthwise of the perforated bottom 4 and among the separation shafts 13, which enhances the quality of classification.Furthermore, the thresholds 8 slow down the advance of the sand along the bottom 4 towards the withdrawal pipe 5, which prlongs the time of residence of the sand in the classifier and also adds to the enhanced classification quality. With each successive threshold 8 having valleys 10 (Fig. 3) facing the peaks 9 of the preceding threshold 8 (and the first threshold 8 closest to the feed end having such valleys 10, too), the coarsest lumps of the initial sand which would not be engaged in the fluidization unobstructedly roll down the inclined perforated bottom 4 towards the delivery pipe 5 (Fig. 2), so that any clogging of the perforated bottom 4 is precluded.

If and when a denser layer of the initial sand advances down the inclined perforated bottom 4 closer to either one of the side walls 15 (Fig. 3) or 16 of the housing 1 of the classifier, the peaks 9 of the upper edge of each threshold 8 (Fig. 2) breaks up this denser layer of the sand, thus levelling out the concentration of the loose material crosswise of the bottom 4. With the peak 10 (Fig. 3) of each successive threshold 8 (Fig. 2) facing the respective valley of the preceding threshold 8 (Fig.

2), this levelling out of the concentration of the material being separated takes place throughout the length of the bottom 4, i.e. over its entire area, which opposes the carrying away of finer particles in the dense layer of the material into the coarser product and enhances still more the classification quality.

A classifier or separator constructed as described above can be efficiently operable with stepped up flow concentrations of the initial material, thus offering a higher throughput.

Claims (6)

Claims:

1. A gravity classifier for loose material, comprising a housing with an inclined bottom having perforations for the passage of gas and at least one threshold provided on the bottom across the direction of the material flow; charging means for feeding the initial material into the classifier above the uppermost edge of the inclined bottom; means for discharging the coarser product of separation at the lowermost edge of the bottom; and means for withdrawal of the gas and finer product of separation at the upper part of the housing.

2. A gravity classifier as claimed in claim 1, wherein the or each threshold has an upper edge shaped as alternating peaks and valleys, the lower level of each valley being substantially at the level of the perforated bottom.

3. A gravity classifier as claimed in claim 2, having two or more thresholds, each successive threshold being arranged to have its peaks facing the respective valleys of the preceding threshold.

4. A gravity seprator as claimed in any preceding claim1 including separating means within the housing above the bottom.

5. A gravity separator as claimed in claim 4, in which the separating means comprise separation shafts defined by walls substantially aligned with the thresholds.

6. A gravity classifier for loose material, substantially as described with reference to, and as shown in, the accompanying drawings.