CA1082647A - Methods and apparatus for continuous centrifugal classifying of a continuous flow of particulate material - Google Patents

Abstract

COANDA EFFECT CLASSIFIER
FOR PARTICULATES

ABSTRACT OF THE DISCLOSURE

Method and apparatus is provided for continuous centrifugal classifying in a deflected flow within a wide cut-size range at high and low throughputs using the Coanda effect wherein the classifying flow is a deflected curved flow which is bounded internally by a deflecting wall and has a free jet external boundary adjacent an outer flow. The centrifugal forces occuring in the deflected wall jet separate the particles of material while the free outer boundary ensures that coarse material is removed from the classifying flow and cannot return to it.

Description

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This inventio~ rela-tes to c method, and apparatus or perfor~ing the ~.ethod, for continuous centrifugal classifying o~ a continous flow of par-ticulate material into at least one fraction of coarse material and at least 5. one fraction of fine material in a deflected flow, either in a gaseous ~luid at cut-sizes between approx. 1 ~m and 100 ~m, i~ the mass flow ratio of the supplied material to the flo~ of classifying gas is up to approx. 10, or in a liquid at cut-sizes between approx. 10 ~m and 1 ~m.
lO.By a deflected flow is meant a flow which is in the process of deflection, i.e. one which is proceding along a generally curved path rather than along a straight line. The in~ention is particularly applicable to deflected flows in which the Reynolds number related to the radial transverse extension 15. (i.e. the dimension o~ the flow cross-section in a direction passing through the local radius of cur~ature of the deflected flow) of the classifying flow being bet~e~n approximately 2,000 and a million or over. The Reynolds number is defined as:
20. Re = v . d/~
in which v is the speed of the fluid, is the kinematic viscosity of the ~luid~ and d is the radial transverse extension of the deflected flow.
25. In some known classifying methods and devices, separation occurs in a flow deflected by walls. The most well-known and widely-used application, which also applies to the separation of material uniformly distributed in a flow of fluid, is deflection classification in a deflection 30. or "slat" classifier. Slat classifiers are used e.g. in oval fluid-energy mills. Another embodiment of a slat classifier is described in US Patent Specification 3 006 470.

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, ~ ~ 2 ~ ~7 In slat classifiers, the fluid uniformly charged with the material for sifting flows in a channel which is usually straight. A~ter leaving the channel, some of the fluid is sharply deflected by a lateral slat system comprising a 5. relatively large number of parallel slats forming parallel outflow channels between them, and is thus discharged. The deflected fluid entrains the fine material, whereas the coarse material remains in the fluid which flows in straight lines. The front edges of the slats are relatively sharp.
1~. Consequently, the flow is deflected around relatively sharp edges having a radius of curvature which is ~ery small compared with thè diMensions of the straight channel and -the entire length of the slats in the ~low direction. The material for sifting is relatively uniformly distributed in 15. the in~low channel. Ot~ing to these characteristics of deflection classifying in a slat classifier9 the selectivity is relatively low for classifying below 100 ~m and relatively high loads of material. The relatively sharp deflection also necessitates a high pressure drop, i.e. a high energ~r 20. requireme~t. The deflected flow in a slat classifier is a curv~d non-parallel flow which separates at the sharp deflection edges. In the flow, similar particles of material move along different trajectories, depending on the centrifugal force exerted on them by the deflectionO
25. It is also kno~n for classification to occur in flows which are not curved by deflecting walls or guided along walls. Such classifica~ion is performed e.g. in "spiral air classifiers" having a housing which is annular in cross-sect.ion .
and in which an axially symmetrical flow is maintained in an 30. inward spiral. Such a method, therefore, is not comparable with deflection classification. In the case of spiral air classification, the fine material ili the curved spiral flow ~ - 3 -~ .

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moves in~ards whereas the coarse material ~lows outwards, relative to the curved flow, towards the outer ~lall of the classifier housing, and is then removed. Spiral flow is suitable for fine classifying and is widely used for that 5. purpose, but has a serious disadvantage in that particles of material which are at or near the cut-size accumulate in the clas~ifying chamber as a result of the e~uilibrium between centrifugal force outwards and entraining force inwards and, as a result o~ the concentration gradient, are diffused and 10. discharged partly with the coarse material and partly with the fine material, thus reducing the selectivity. Since the classi~ier flow charged with fine material emerges ~xially from the classifier chamber, there are limi-tations to the axial width of the chamber and the throughput.
15. A disadvantage common to siat classifiers and spiral air classifiers is that the material can be separated into only two fractions.
Deflection classifying must also be distinguished from cross-current classifying as disclosed in British Patent 20. 1 088 599 and the corresponding US Patent 3 311 234, and British Patent 1 194 213 and the corresponding US Patent

3 520 407 and the Canadian Patent 834 558 of the present Applicant, in which the material is introduced at a given initial speed into a flow extending at an angle or almost 25. in the opposite directiont through which the coarse material travels. On the other hand, the particles of fine material are decelerated and deflected in the flow, the deceleration distance and the acceleration distance in the flow direction both depending on the particle size. These classifiers are `
30. unsuitable for very fine separation. This is clear from the fact that the deceleration distance of a particle of material having a diameter of 10 ~m and a density of 1 g/cm3 .' is only 5 mm in stagnant air at an initial speed of 30 m/sec.
Counter-current and cross-current classifiers of this ~ind are not centrifugal classifiers in which the particles of material suspended at the centre of the flow are subjected to centrifugal force owing to the curvature of the flow.
Instead, they are deflected in the flow to an extent depending on their size, but only because their entry speed differs from that of the flow.
The present invention provides method and apparatus for continuous centrifugal classifying in a deflected flow, so as to obtain very selective classifying within a wide cut-size range, more particularly at very fine cut-sizes below approximately 10 um in gaseous fluids and below approximately l00 ~m in liquid fluids and at comparatively high throughputs, and also at low throughputs~ Classification is achieved in which the fine material contains substantially no particles of coarse material above a given size, and the coarse material contains substantially no fine material below a certain particle size.
In the art, the requirements on the selectivity of :
classifying vary considerably. For example, when classifying is combined with grinding, the coarse material must usually be substantially free from fine material. However, the absence of coarse particles in the fine material is usually less important, e.g. when cement is ground and simultaneously classified. On the other hand~ increasing importance attaches to applications to very fine classiEying, e.g., of fillers . ':' i ` . .. ..

and clay, in ~hich the fine material ~as to be sub-stantially free from particles of coarse material and has to have very low particle sizes, e.g. 10 ~m or less, in which case the cu-t-sizes must be con-5. siderabl~ lower. In the case of known classifiers,these requirements are impossible to meet or are possible only in conjunction with small throughputs, i.e. amounts of the order of 100 kg/h or less.
No classifying is absolutely selective. I~ the 10. particle size which is divided by the classifying process in the proportion 50;50 between coarse material and fine material is called d50 (cut size) and the particle size out of which 10%, 25yo, 75%, 90~ etc., goes into the coarse material is denoted 15. by dlo, d25, d75, dgo etc., very selective classi~
cation is denoted by the selectivity coe~icient K = ~ _ ~.7. Many industrial classifying processes, ~ 5 in contrast to the process for analyzing the particle size distribution, have a selectivity coefficient K
20. o~ less than 0.5. As explained, however, the co- `~
ef~icient K is not adequate to characterize the classifying quality. If the fine material has -to be .~ree! ~rom relatively coarse particles, the critical ~ ;
particle sizes are dg~, dgg 9~ dloo. In practice 25. they can be measured only in a given sample quantity, e.g., by wet mechanical analysis or micro-analysis o~ a 10 g sample. The following Table gives charac teristic average values o~ the ratio between the particl~ sizes dgo:d50 ~or highly selective classifying 30. (K = ~ and ~or moderately selective classifying ' ~ - ' . .......
~ 6 -... .

~C~8Z6~7 (K = 0 5~
d75/d50 dgo/d50 dg9/d50 K - 0.7 1.2 1.4 1.8 K = 0.5 1.4 2.0 3.3 The requirement that a given dloo must be reached~
is considerably harder than e.g. dgg g or dg9 99, since it is found by experience that in each flow-classifying process, it is often extremely difficult to ensure that no "oversize"
particles above a certain size enter the fine material.
Consequently, even in selective very fine classifying, dloo/d50 is often above 4.
In many industrial classifying processes, particular-ly at high loads, the values d25, dlo, d5 are never reached at all, since, for example, more than 25% of all particle sizes below d50 reach the coàrse material.
According to one aspect, the present invention provides a method of continuous centrifugal classifying of a continuous stream of particulate material into at least one fraction of`coarse material and at least one ~-fraction of fine material in a deflected flow, the material to be classified being classified in a gaseous fluid at cut-off sizes between approximately 1 ,um and 100 ~m and a mass flow ratio up to 10, between the supplied stream of material and a classifying gas flow, and being classified in a liquid fluid at cut-off sizes between approximately 10 ~m and 1 ,um, comprising: (a) providing a curved inner deflection wall curved from a beginning over an inner deflection angle greater than 45;
(b) establishing a classifying fluid flow which is deflected in a classifying region by the curved inner deflection wall and has, as an inner boundary, the curved inner ' deflection wa~1 and has a curved outer boundary which is . ~ ;
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~0~ 7 not covered by a wall over an outer angle smaller than r ' the inner deflection angle, the classifying fl~w being substantially parallel to the inner deflection wall and abutting the inner deflection wall at least over the inner deflection angle; (c) establishing an outer flow for carrying away the fraction of coarse material, the outer flow establishing the outer ~oundary of the classifying flow over the outer angle, the ratio of radii between the outer boundary and inner deflection wall of the classifying flow being less than approximately 5:1; (d) introducing a stream of material to be separated into the classifying flow in a thin layer in the vicinity of the beginning of curvature of the inner deflection wall in a direction such that the vector component of its velocity in the -i;
direction of the classifying flow is at least half the value of the velocity of the classifying flow and in a ':
direction which does not deviate more than 45 from the direction of the classifying flow, whereby fine material, after being fanned out by centrifugal force is discharged primarily with the out-flowing classifying flow, and the coarse material passes through the outer boundary of the classifying flow which is not covered and is `
discharged primarily with the outer flow.
According to another aspect, the invention provides an apparatus for continuous centrifu~al :
classifying of a continuoUs stream of particulate material into at least one fraction of coarss material and at least one fraction of fine material in a deflected ~low, the material to be classified being classified in a gaseous -fluid at cut-off sizes between approximately 1,um and 100 ~m and a mass flow ratio up to 10, between the supplied - 8 - ;
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stream of material and a classifying gas flow, and being classified in a liquid fluid at cut-of sizes between approximately lO ~m and l ~m, comprising: (a) a curved inner deflection wall curved from a beginning over an inner deflection angle greater than 45; (b) a flow channel fox conveying a classifying fluid flow ha~ing a classifying zone in which the fluid flow is deflected by the curved inner deflection wall and which has a curved outer boundary not covered by a wall and defining a coarse material discharge aperture over an outer angle smaller than the inner deflection angle; (c) means establishing an outer flow for carrying away the fraction of coarse material and for establishing the outer boundary of the classifying flow over the discharge aperture, the ratio of radii between the outer boundary and inner deflection wall being less than approximately 5:1; (d) means for introducing a stream of material to be separated into the classifying flow in a thin layer in the vicinity of the beginning of curvature of the inner deflection wall in a direction such that the vector component of its velocity in the direction of the classifyin~ flow is at least half the value of the velocity of the classifying flow and in a direction which does not deviate more than 45 from the direction of the classifying flow.
The inner deflection angle should be at least 60 for medium-fine cut sizes and at least 90 for very fine cut sizes. Usually it is between 90 and 180. Preferably, particularly in the case of a gaseous fluid, the velocity component of the stream of material in the direction of the classifying flow is approximately equal to the speed ~! _ 9 .. . . ~ . . . .. `
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~8Z~i~L7 of the classifying flow at the point of introduction.
The ratio between the radii of the outer and inner curvature of the flow (technically called the "deflected wall jet") deflected by the deflecting wall in the classifying region is preferably approximately 3:1 to 2:1. The radius of curvature of the inner deflecting wall of the flow channel should be at least 1 cm.
Advantageously, it may decrease in the flow direction.
The speed of the classifying flow, in the case of a gaseous fluid, is preferably between 10 m/sec and 300 m/sec, the precise figure depending on cut size. The stream of material can be supplied to the classifying 10w either mechanically or, preferably, in a carrier flow in which the particles are suspended. In the neighbourhood of the flow deflection, the inner deflecting wall is used for classifying. Upstream of ~'' ` ` ' ` ` ` ' .

32~i4~7 and downstream of the deflection wall or the coarse-material outlet aperture, the ~low can be radially subdividea into a number of inflow and outflow channels at an angle to the average direction of flow.
The invention is described further, by way of illus-tration, with reference to the accompanying drawings, in which:
Figure 1 shows the parts of a 1at or planar classifying device in the neighbourhood of the classifying zone, comprising two coarse-material outElow channels;
Figure 2 shows a flat classifying device in which the coarse-material discharge device comprises a collecting vessel for coarse material;
Figure 3 shows a flat classifying device in the neighbourhood of the classifying zone, comprising a number of inflow and outflow channels in order to illustrate their position and extent;
Figure 4 shows a flat classifying device in the neighbourhood of the classifying zone, for the purpose of illustrating the place of origin of possible secondary flows in an outflow channel;
Figure 5 shows a flat classifying device in which the 10w channel is subdivided in the neighbourhood of the in10w to and the outflow from the classifying zone;
Figure 6 shows a flat classifying aevice in the neighbourhood of the classlfying zone, the uth of a feed device being displaced into the inflow;
Figure 7 shows a flat classifying device having an . .
inner deflection wall constructed as a rotating cylinder;
Figure 8 is a diagram of a complete air classifying apparatus comprising a flat classifier;

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~D8~647 Figure 9 is a cross-section through an annular or axially symmetrical classifying device showing onl~ those parts in the neighbourhood of the classifying zone, this zone being supplied with material by a centrifugal platei Figure 10 is a diagrammatic vertical section throuqh an axially s~mmetrical classifying device in which material is pnevmatically supplied to the classifying zone rom above and ~elawi Figure 11 is a diagrammatic vertical section through an axially symmetrical classifying device in which material is pneumatically supplied to the classifying zone at a distance from the inner boundary wall of the inflow channel;
and Figure 12 is a diagrammatic vertical section through an axially symmetrical classifying device in which material is pneumatically or hydraulically supplied to the classifying zone vertically upwards.
It will be understood that the invention makes use of the "Coanda" effect for the classifying flow. This effect occurs at a curved deflection wall or a curved wall jet. Consequently, the sifting or classifying flow is a deflected, curved flow which is bounded internally by a deflecting wall but is not externally guided by a wall but has a free jet boundary, which is adjacent an outer flow.
The centrifugal forces occurring in the deflected wall jet are used for separating the particles of material, whereas the free outer flow boundary ensures that the coarse material is removed from the classifying flow and cannot return thereto.
~; The systematic use of a deflected wall jet, more particularly for classifying, particularly in the case of relatively high loads (a large amount o~ material per average amount of fluid) and high requirements on the selectivity and ' ~;`. ,~.
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.. , , . ,.. .. , -. . , . -~8264~7 fineness of the cut size, necessitates a number of geometrical and flow features and features relating to the motion of the material, which clearly demarca-te the invention from the prior art. Selective very fine classifying at cut sizes between 1 and 20 ~m are extremely difficult because the fine particles of material follow turbulent fluctuations in the flow and each disturbance to the flow. Frequently the disturbances are caused or . , : :
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accen-tuated by the material itself~ Consequently, tlle ~low conditions for material-free flows ~r for coarse classifying cannot be applied -to very fine classifying.
5. It is believed that the method according to the invention can be used in the following situa-tions: for ~ine classifying do~n to cut sizes of the order of l ~m, where there are extremely high requirements on the absence o coarse particles in lO. the fine materials and ~rhere the throughput is relatively high, ~or selecti~e separation with coarse material .~ree from fine material and ~rhere there is an even greater ~hroughput requirement and there are some-wha~ lower requirements relating to the absence of coarse material in the fine material. In many appli-cations it has a very valuable advantage in that a number of fractions can be sharply separated in a single passage.
The invention can be applied to gaseous and 20. liquid fluids. It is believed -to be applicable to classifying in a de~lected flow at Reynolds numbers o~ 2000 to about l,000,000, related to the radial transverse extension of the classifying flow, i.e., outside the laminar region. The main application is to 25. dry classifying, i.e. classifying in gaseous fluids, more particularly air, and ~Jet classi~ying, e.g. in the treatment of ore. The invention may also be applied to continuous classifying of small quantities for on-line measurement and adjustment of grinding 30. installations.

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~(~8Z6~7 According to the invention, in ~hich for the first time a classifying flow is deli~erately deflected for classifying purposes at a not too sharply bent deflection ~all using the Coanda effect, 5. the flow is adjacent the wall o~!ing to the resulting negative pressure, and is therefore deflected~ A
jet~can be deflected in known manner, e.g. by inserting~ a finger into the side of a water jet.
Hot~ever, it is not easy to deflect it through a given 10. angle, since the negative pressure at the wall boundary layer returns to normal and the flow comes away from the ~all. If the flow is charged with particles of material~ the material moves outwards owing to the centrifugal force in the flow and exerts an additional 15. radial, outwardly di~ected force on the flow, thus further increasing the tendency to separate. In the case of fine cut si~es, it is also necessary to produce deflection around a maximum deflection angle in which flow occurs parallel to the curved deflection 20. wall. It is for this reason that,according to the invention, the outer deflection angle (aa, see Figures l`and 3) along which the classifying flo~r does not touch the wall but is adjacent the oute~ flow used ~or removing the coarse material, is less than the 25. inner deflection angle (ai, see Figures 1 and 3) at ~he cur~ed inner deflection wall. Usually the outer deflection angle is within the limits of the inner deflection angle. As far as possible, the classifying flow is positively guided upstream of and downstream 30. of its free outer flow boundary, through which the ' ' ~

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~3Z647 `

coars~ material leaves it. Except when the jet deflection is small, the inner and outer def]ection angles cannot be equal without seriously a~fecting the separation. Even ~hen the de~leGtion is small, 5. however, the outer deflecticn angle should be made smaller.
The out~r flow, which occurs outside -the free outer flow boundary of the classifying flow5 is an important feature of the invention. It is used for 10. removing the coarse material. The result is that substantially no coarse material flows back through the jet boundary into the classi~ying ~low. This condition, which is extremely important ~or selective classifying,becomes increasingly dif~icult in propor-15. tion to the length of the free outer flow boundarsr.This is another reason for selecting a small outer deflection angle. `
Selective classifying is achieved by substantial parallelism of the classifying flow. It is essential -20. to a~oid disturbances to parallelism in which the streamlines locally approach or overlap. Disturbances can be avoided if the direction of the inflow into the classifying zone and the direction of the outflow o~ ~luid from the classi~ying æone are not parallel 25. ~o the i~ner de~lection wall; another method is to produce flow separation during inflow and a backwash or reflux and flow separation from the boundary walls and edges o~ the flow channel or channels durin~ the outflow. A feedback` effect on the classifying flow 3Q. is produced by dammin~ or flow separation in the outflow ,' ~, - _, . l S

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channel, or in the outîlow ch~nnels if a nu!l~ber of discharg~ channels for the classifying flow or the outer flow are provided for discharging the coarse material in order to divide the fine material or 5. coarse material each into a num.ber of fractions.
The feedback ef~ect is intensified by the material in the flow. A material-free flow is much easier to make parallel, although it should be noted that, as a result of the Coanda effect, the deflected jet 10. has a tendency to become constricted, ~hich is disadvantageous in the case of very fine separation.
~ing to the interaction between the flow and the material in it, simple experiments are the most rapid way of finding the optimum adjustment in dependence 15~ on the material load. The ~low can be observed if the lateral walls of the flow channel are made transparent.
In order to deflect the flow a certain pressure drop is required. It depends on the ratio between the 20. radii (ra:ri, Figure 1) of the outer and the inner curvature of the classifying flow. This also has an important effect on the parallelism of the classi~ying ~lo~r. Accordingly, therefore, the ratio between the radii should be less than 5:1 and preferably between 25. 3:1 and ~
The curved deflected classifying flow ~ay also be influenced and stabilized by the outer flow used for discharging the coarse material. In most cases the outer flow is selected to be slower than the 30. classifying flow, thus resulting in a flow or jet ~ .

~)826~7 boundary w~lh turbu]ent mixing. Howe~er, this mixing can be avoided by bringing the two flow speeds close tcgether or making them equal. The latter method is specially advantageous when the speed of the classi-5. ~ying flow is relati~ely low, i.e. for relativelycoarse separation, or when the coarse material in the~outer flow has to be classified into a number of fractions.
The stream of material or particles is supplied 10. at the material introduction point in a thin layer, i.e. a layer which is thin compared with the radial dimension of the classifying flow, at a speed which is at least half and preferably the same as and in the same general direction as the classifying ~low, 15. the deviation being not greater than 45. In this manner, the material is fanned out in a particularly ef~icient mar~er by centri~ugal ~orce in the classify-ing flow, the coarse particles moving further outwards than the ~ine particles. In order to make use of the 20. ~anning-out, the supply point must be near the inner deflection wall at or upstream o~ the beginning of the curve.
If the material .introduction point is actually at the curved inner deflection wall, very fine particles 25. may sticls there. It has ~een obs~rved that in many cases the amount of stuck particles does not increase, ; so that there is no disturbance. In many other cases, however, it may be advantageous to introduce the stream of material at a radial distance from the curved inner 30. de~lection wall, the distance from -the inner deflection .

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~08Z~7 `
wall being less than the radial distance from the outer classifying current boundary. In such cases, the classifying flow directly adjacent the de~lecting wall remains substantially free from material, so 5. that material cannot stick. This also reduces the dange~ of ~low separation in the material-free region.
On the other hand, there is an increase in the amount o~ fluid required and in the pressure drop, if the radial extension of the entire classi~ying zone is 10. relatively large.
Out of the material fanned out in the classifying region, the fine material is usually discharged only by the outflowing classi~ying flow and the coarse material only by the outer flow, ~rom which it can 15. be subse~uently separated by conventional means. It may, however, be advantageous to discharge a small part o~ the outer ~low tGgether with the classif~ing ~low, or a small part of the classifying flow together with the outer flow. If a number of fractions of 20. ~ine and/Qr coarse material have to be obtained, a number of outflow channels may be provided at increas-ing radial distances on the coarse-material or fin~-mat~rial side, as is known in the case o~ ~ `
transverse flow classifying (see Bri-tish Pa-tent Speci-25. ~ication 1,088,5g9 and the corresponding US Patent Specification 3,311,234).
The flow should be exactly parallel to the deflec-tion wall; this is particularly important at very low cut sizes, e.g. between 1 and 10 ~m in the case of a 30. gaseous ~luid. kThen the separation is coarser, the ~ ~ 2 6 47 flo~ parallelism is not so critical. It may even be advantageous to accelerate all or part of ~he cl~ssify-ing flo~. in the classifying zone by reducing the inlet apertures of the outflow channels. In this 5. flow system, an inwardly directed flow component occurs in the acceleration region at the place where tlle~curved ~low runs parallel to the inner deflection wall. This reduces the fanning-out in the region of ~iner particle sizes, whereas the fanning-out is 10. increased in the region of coarser particle sizes.
This is advantageous for coarser separation. The reason is that if the curved classifying flow is ad~usted to very fine separation, particles above a certain si~e suffer only slight deflection, and are 15. only slightly fanned out. If the speed is adjusted, the maximum ~anning-out can occur in -the desired cut-size region. The speed in the classi~ying region can be varied within very wide limits.
Advantageously, in the case of fine separation 20. with maximum parallelism of the stream in the classify-ing ~egion, the speed there is kept constant. This is the easiest method of avoiding flow disturbances caused, for example, by differences in the in~low speed of the flow layers. On the other hand, in 25. order to reduce the amount of fluid required, it may be advantageous for the fluid to move at maximum speed in the inner flow layer and at lower speeds further out. However, the decrease in speed outwards is limited by the ~low stability conditions.
30. Advantageously, in the case of very ~ine classifyinO

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wllere there are high requirements on selectivity and absence of oversized particles in the fine material, the material is supplied in the flow direction at the same speed as the fluid. In that case, fa~ning-out 5. is produced by centriIugal force alone. As a ~irst approximation, the radial travel of the particles is proportional to the speed at ~hich they si~k, their pelipheral speed and the deflection angle. The cut size between the fractions is determined by the 10. position o~ the leadin~ or front edge of the walls bounding the out~low channels. If, ~or example, limestone has to be classified in air at a cut size at l ~m, and i~ the deflection angle is 180 and the speed of the classifying M ow is 200 m/sec7 it is 15. calculated that the radial travel o~ the particles tabout 1 ~m in size) and consequently the radial distance bètween the front edge of the inmost boundary wall and the de~lection wall or the radial position o~ the material introduction point, is almos-t 6 mm.
20. I~ the cut size is 2 ~m, the aforementioned radial minimum distance is 19 mm. These calculated values are substantially ~alid ~or practical classiiying~
if the parallelism of the ~low of ~luid is properly adjusted.
25. I~ there are no extreme requirements on the selectivit~ o~ Yery fine classifying, there is some-~hat greater freedom in the choice of the direction and speed of the flow of material at the in-troduction point. In that case, neither the direction nor the 30. value of the in~low speed need be exactly equal to that ~: .
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~8Z6~7 of the flow speed. It may be advantageous, ~hen the strcam of particles is introduced, ~or the veloci-ty to be gi~ren a certain radial component, though this should not be greater than the component in the 5. direction of the classifying ~low. As a result of this component, the coarse particles move further outward than the fine particles. This may be advan-tageous in the case of ~anning-out in an average range o~ particle siæes.
10. Preferably, the ma~erial is introduced into the classifying flow in a carrier flow, i.e. by pneumatic means when a gaseous fluid is used for classifying.
In this case, the classifying flow may be disturbed if the material and its carrier flows in at a different 15. direction from the classifying flow; in this case, therefore, care should be taken that the classifying flow and the ~low of incoming material are in the same direction.
The trajectori~s o~ the coarsest particles pro-20. ~idè a limit up to which the classifying flow (or each partial ~lo~ if the classifying flow is sub-di~ided) can be guided in the flow direction during the inflow, be~ore coarse particles strike the chalmel walls. The latter should be avoided at any cost.
25. On the other hand, to ensure maximum guidance, the outer boundary wall, or the boundary wall of the outer channel, should end not far in front of the trajectory o~ the coarsest particles. The boundary walls of the inflow channels should be streamlined 0. to prevent turbulence from being produced therein as ' . :. :- ~ . . . .

~ 8Z64'7 a result o~ separation of the outflo~, and being transferred -to the classif~ing flo~.
For siLQilar reasons, the b~undary walls of the out~lo~ cha~nels should also be streamlined and smooth.
5. Preferably, they are slightly rounaed at the front edge, thus preventing flow separation in the outflow ducts. Slight rounding is also advantageous so as to reduce wear. The position o~ these front edges in the direction of de~lection or curvatu~e is sho~n in 10. Figure 3 (to which detailed re~erence will be made below) by a deflection angle ~1 to ~4, measured from a flxed point, e.g. the beginning o~ the curvature of ~he inner deflection wall. The innermost angle B
coincides with the .inner deflection angle 1 of the 15, classifying flow. In Figure 3 it is approximately 180. At smaller deflections, as used for coarser separation, *he front edges of all -the boundary walls o~ the outflow channels can have tlle same angle of deflection. The edge (21) of the flow chan~el separat-20. in~ the coarse material from fine material and ~orming the end of the coarse-material outlet aperture need ~ `
not lie on the same radius as the front edge of the outer boundary ~all of the inflow chann~l or of the outèr inflow cha~lel of the classifying ~low. It can 25. be somet~hat further inward or somewhat ~urtlier outward.
I.~ the outer flow ~or separating the coarse material is used in at least two fractions, a part of the ~low (as sho~n in ~igure 3) may also flow away through -the -outer outflow chan~el for the classifying flo~r.
30. It has been found tha~ disturbance-free ~low ' . ' '`~

3Z~47 with a ma~imu~.n de f lection angle OI 1~0 Ior the ~inest fraction, i.e. selec-tive very fine classifyingg can be achieved only if the outflo~ chalmel of the flo~
chanllel is radially subdivided at an angle to the 5. classifying ~low by parallel guide vanes or the like, or if a number (at least two) of out~low channels are proyided for the classifying flow and the front edges of the guide vanes and/or outer boundary walls are disposed at deflection angles which decrease outwardly 10. (~ 2~ ~3 ~ ~4)- This ensures that the ~low has the desired strict parallèlism with maximum inward deflection of the flow. If the radial dimension of the classifying flow is less, fewer boundary walls are needed, e.g. only two outflo~ channels.
15. On the other hand, there should not be excessive distances between the outer ~ront edges of the outflow channels in the deflection direction, since slig~lt secondary flows are produced at the inner boundary wall of a curved flow and are intensified by friction 20. betwe~n the material and the wall. These secondary ~lows (indicated by arrows 26 in Figure ~) are propa-gated obliquel~r in~ards in the flow direction and entrain relatively coarse material inwards. They may penetrate across the ~ront edge o~ the adjacent inward 25. boundary wall into the next inward outflo~ channel if the distance o~ the last-mentioned fron-t edge in the de~lection direction is excessive. The permlssible distance in the deflection direction depends on the radial distance and on the load and the particle size.
30. For the same reasonS difficulties arise regardin~
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~8~69~7 the selecti~ity ~nd absence of ovcrsized ma-terial in the ~ine m~terial if the outer flow for removing the coarse material is ex~ernally guided in a channel whose bo~mdary wall is substantially parallel to the 5. classifying flow. This is possible only when the material being tre~ted is very fine and does not ~ -contain any relatively coarse, rebounding particles and if there is a sufficiently large distance between the outer wall (33 in Figure ~) o:E the coarse-material 10. discharge device and the outer classifying-~low boundary and a su~ficiently small distance between a) the front edge o~ the outermost outflow channel `
~orming the classi~ying-flow boundary or bounding the coarse-material outlet aperture in the flow 15. direction and b) the edge of the outermost in~low channel of the classifying flow, i.e. if the ~ree flow boundary and the jet boundary are short. In ``
such cases, ~low disturbances do not penetrate in~ards into the classi~ying region from the outer wall of :~
20. the coarse-material discharge device.
The curvature of the inner deflection wall ad~acent the classifying flow can be circular, i.e~ `
the radius of curvature can be constant. This however is not a necessary condition for the success of the 25. method according to the invention. On the contrary, it has been found that under certain special condi~ ;
tions the op~imum flow shapes may dif~er from circular curvature; more particularly the curvature may increase in the flow direction, i.e. the radius of curvature 30. may decrease (see Figure 6). Xn this case the inner .
, :-`'":

~ 6 ~7 wall ~f the flow channel,the outer wall of the flowchannel and tlle ~lo~ channel itse]f do not have a uninlle centre of curvature but a locus of the centres of curvature as the radius o~ curvature varies. In 5. this case the various centres of curvature re~erred to should each be understood as meaning the centre o~ gravity o~ the appropriate locus.
The turbulence of the classifying flow may be a dis~urbing factor, particularly in the case o~ very 10. fine classi~ying in gaseous fluids. ~ccordingly, the turbulent mixing paths of the particles at an an~le to the trajectories thereof must be small com-pared with the lèngth of the traJectories. This limits the flow length. Either the de~lection ang]e can be 15. increased (in the case of small radii of curvature) or the deflection angle can be reduced (i~ the radîi ` of cùrvature are larger). Selective separ~tion in ; gaseous fluids can be obtained i~ the average radius of curvature of the inner wall is between 0.5 and 20 20. cm, more particularly between 1 and 10 cm. The radius o~ curvature can be made even larger when the deflection angles are small.
The cut size between coarse material and ~ine matèrial is determined by the position of the front 25. edge of the outer boundary wall o~ the outermost channel ~or the classifying flow, the a~orementioned ;~
~ront edge ~orming the ooarse-material outlet aperture.
The coarse mater.ial penetrates into the outer ~low, which must be so guided that no pa~ticles above the 30. coarse-cut size return into the classifyine flow.
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. . ~

1082~4'7 The outer flo~ should be free from material or at least free from relatively coarse particles ~hen it is supplied parallel to -the in~er deflection ~!all near the outer classifying flow boundary. In such 5. cases, coarse particles are not returned to the classifyin~ flow as a result of turbulent mixing bett~een the outer flow and the classifying flow at the classifying-flow boundary.
The return of coarse particles may also be due 10. to uncontrolled particle motion, e.g. resulting from collisions with the walls. This may be largely preven-ted i~, as is preferred9 the outer flow is supplied substantially material-free and substantially parallel -to the classifying flow and is discharged 15. outwards together with the coarse material approxi-mately in the average direction of travel o~ the coarse material, i.e. in the direction o~ the coarse-material trajectories (13, Figures 1 and 10-12).
Another advantageous method of preventing coarse 20. ma~erial from being returned to the classi~ing flo~
is as ~ollows: the outer flow, which is supplied free from material, flot~ls along the classifying-~low boundary and then reaches a wide coarse-material collecting chamber (13a, Figures 2, 8). It is there ~ ;
25. co~veyed substantially through a half-circle and discharged together t~lith the coarse material through an outlet aperture in the outer wall. Some of the coarse material can also be removed from the coarse-material chamber by gravity, e.g. through the bottom 30. funnel of the coarse-material collecting chamber~

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using a bucket-~heel lock. The outer ~low conveyed in a semicircle produces an inner vortex or whirlpool flow in the coarse-material chamber (18, Fi~ures 2,~).
Advantag~ously, the flow is guidedS by discharging 5. and supplying the outer flow and by subse~uen-t de~lection at the ~Jall (19, Figures 2, 8) so that the particles of material therein are driven only in the direction towards the outer wall. This is achieved by a complete, like~ise approximately semicircular, 10. dcflection back to the direction of the incoming flow.
~dvantageously, the distance between the outer wall of the coarse-material discllarge device or coarse-material collecting chamber and the classifying-flow boundary is at least as great as the path tra~elled 15, by the coarsest rebounding particles.
In the case of very fine classifying, the cut size for cut material is often below 50 ~m, e.g. 15-25 ~m. It is then possible to separate the coarse material additionally at coarser cut sizes, by using the 20. outer flow to classify the coarse material ~nto two or more fractions. In this case, the outer flow can be supplied through one or more inflow channels. The flow must be discharged in two or more partial flows.
The outermost partial flow carries the coarsest fraction.
25. Advantageously, the preriously-mentioned precautions are taken to avoid the return of coarse material. ~he classification by the ou-ter fl~ow may be~ a combined transverse-flow and deflectlon classifying or may be a pure transverse-flow classifying. In transverse-flow 30. classifyin~, the material to be classified is as a rule~

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~08Z647 introduced constantly at an angle into a flow. In the present case there is the additional advantage that the coarse material entering the outer flo~
has already been fanned out in a manner very advan-5. tageous ~or transverse-flow classi~ying. Advantage-ously, tlle outer flow is adjusted so that it produces the maximum intensification in the ~anning-out of the coarse material over the desired range o~ cut si~es.
It is kno~m to increase the selectivity of 10. classifying, more particularly in a gaseous fluid, by connecting two c]assifiers in series and recycling the middle fraction, i.e. adding it to fresh material to be cl~ssi~ied. An advantage of the classifying me~hod according to the invention is that classifying 15. into more ~han two fractions can occur simultaneously.
There thus may be no need of further classifying in order to recycle a middle fraction and thus increase the selectivity between the two neighbouring fractions.
In any case, this is necessary only when the selecti-- 20. vity has to be extremely high.
Classifying according to the invention can be carried out in a "~lat" system, i.e. in a single plane or in two dimensions only, in a flat classi~ying flow, a typical flat Glassifying device comprising a 25. ~lo~ channel having a rectangular cross-section or in three dimensions, ~or example in an annular or ~-axially symmetrical device, an axially symmetrical classifying device comprising a flow channel having an ~nnula~ cross-section. Examples of a flat c1assi~y-ing device are shown in Figures 1 to 8 and examples of , .~ . , .

.

~ 0 8 ~ ~ ~ 7 an a~iall~ s~r~e-tric~l classifying device are sho~n in Figures 9 to 12.
In the ~'flat" system the classifying flot~ occurs in planes parallel to the plane of the drawing 5. bet~lee~ a front and rear wall bounding the flow channel. The width o~ the classifying region at right an~les to the pl~ne of flo~ or of the drawing can be given any required value.
In order to describe the efficiency, it is 10. advantageous to give the mass flow o~ material and fluid in the ~orm o~ specific mass ~lows related to the width of the classifying region. In many ~pplications, the specific mass flow of supplied material can be kept at a value of the order of 100 15. kg/h . cm width of classifying region. I~ there are extremely high requirements regarding the fi~eness and the absence of oversize particles in the fine material, the specific mass ~low for air classifying is made lower, e,g. between 20 and 50 kg/h . cm. Very 20~ high requirements can be satisfied with regard to the cut size and selectivity and absence of oversize particles from the ~ine ma-terial. It is believed that this is the only method of ensuring that, ~ a cut si2e of 2 ~m, substantially no coarse material 25, occurs on a screen having a mesh width of 6 ~m ~hen 10 g of ~ine material is classi~ied. (d50 - 2 ~m9 dloo = 6 ~m). When the width of the classifying region is 50 cm, the amo~mts o~ material treated may be from 1 to 2.5 t/h~ I~ 0.1% of residue above 10 ~m 30. is permitted, a specific throughput of 150 kg/h . cm .
, 2 6~

may be obtai ned in ver~ ~ine ai~ classiI'ication o ( 50 ~ ,um; dloo = 6 llm). This corresponds to a throughput of 7.~ t/h when the width of the classifying region is 50 cm.
5. These outputs are thought to be several orders of magnitude higher than tl~e throughputs of e~isting air classifiers for similarly high finenêss require-ments Xn addition, the existing ~ery fine classiiers have rotating parts and are much more expensive to 10. produce. It is thought that classifiers in accordance with the invention can be built ~or specific through-puts up to several hundred kg/h . cm with selective classifying and cut sizes about 10 ~m. In the axially sy~metrical devices (Figures 9 to 12) the ~idth of the 15. classifying chamber in the flat classifier corresponds to the circumference of the circle with the average 'diamèter (sho~m in the drawing) of the inner deflection ' '' wall (D in Figure 8). Thus, at a diameter of 1 m, there is obtained an equivalent classifying-chamber width of ~0. approximately 3 m, and a possible throughput of 60 t/h at 200 kg/h cm.
In ~he "axially symmetrical" system9 the flow is axially symmetrical with respect to a central axis and is e~ual i~ all radial planes extending therethrough 25. The axia].ly symmetrical system, compared with the flat system, has an additional possibility in that a rotating flow component around the central axis or axis of symmetry of the classifier can be imparted to the supplied material and the classifying flow.
30. Separation in the classiying zone is little .
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~ 8 2 ~ 4~7 influenced by gravity. Consequently,--the classifying zone can be oriented in any mar~er required in space, i.e. material can be supplied horizontally (Figures 1 to 7), obliquely (Figure g), vertically do~rnwards 5. (Figures 10, 11) or vertically upwards (Figure 12).
If the flow and the supply o~ material to the classi~ying region is vertical and if it is deflected outwards relative to the central axis of the classifier, the de~lection o~ the classifying flow around the 10. inner de~lection wall can be increased by its rotational component around the central axis, thus intensi~ying the Coanda ef~ect.
Advantageously, therefore, in classi~ying accord ing to the in~rention~ a rotating ~low component around 15. the central axis o~ the system is imparted to the inner ~low layer, which is pre~erably supplied free ~rom material between the material inlet and the inner deflection wall. A possible embodiment is shown in Figure 11.
20. To ensure sharp separation it is also pre~erable that all particles of material having the same size should enter at approximately the same speed and in approximately the same direction. In addition5 all the particles of material, irrespective of their size, 25. can be given the same speed on entry, with various accuracy depending on the manner in which the material is supplied. This is possible when material is supplied on a conveyor beltj more particularly on a belt covered by another belt moving at the same speed, 30. the two belts entraining the feed between them. In '.

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the axially symmetrical system, the conveyor belt may be r~placed by a centrifugal plate, more particularly a plate in which the wall in contact with -the feed i~
in the form o~ a concave conical or concave curved 5. surface of rotation, at least in the outer region, and is covered to a short distance from its edge by a cover extending to the feed point.
It is very advantageous, in both the ~lat and the axially symmetrical systems, for the material to 10. be supplied in a fluid carrier, e.g. by pneumatic means~
Very advanta~eously also, in the case o~ the axially sy~netrical system, the material is supplied either ~rom above or below. The classi~ying ~low can be de~lected either outwardly or inwardly relative -to 15. the central axis. In the latter case, depending on the distribution of particle sizes in the material, it may be impossible to prevent the smaller particles having a higher average speed than the coarser particles.
Pre~erably, therefore,the components o~ the inflow 20. speed o~ the coarsest particles in the direction of ~low o~ the classifying fluid at the point o~ entry is approximately e~ual to the speed o~ flow of the classi~ying ~luid, ~hereas the smaller particles, entering in the same direction, have an entry speed 25. which increases continuously or stepwise with increas-ing particle size. Consequently the coarser particles are subjected to approximately the entire centrifugal ~orce from the beginnin~ and perhaps also to an additional radial component o~ motion, as a resul-t of ~0. a radial component at entry.

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~8;~647 The advantage of pneumatic or hydraulic material supply is that the flow of material can easily be kept constant by maintaining a constant pressure drop along all or part of the conveying distance by regulating the flow of material.
The invention may be carried into practice in various ways, but a number of forms of classifying apparatus and their method of use in accordance with the invention will now be described in detail with reference to the accompanying diagrammatic drawings.
In all the classifying devices shown in the drawings, classifying occurs in the classifying region 7 of a flow channel, which is continuously bent to a varying extent .. . . .

'~ 8 Z 6 in the neighbou:rhood of the cia~sifying region. The fluid, e.g. air, flows to the region 7 e.g. from a fan or blower, through a part of the flow channel shown as the inflow channel 2 (see Figure 1) and 5. flows out therefrom through a part of the flow channel sho~m as the outflo~:r chal~nel 3 (see Figures 1 and 12).
The.inflow cha~lel can be subdivided into a number o~ inflo~ channels by boundary ~alls. For example, ~igures 3, 9 and 11 each sho~ two inflow channels 2 10. and Figures 5 and 8 show three inflow channels 2 or 2a, 2b and 2c. Instead of a single outflow channel 3 (see Figure 12) there can be two outflow channels 3, 4 (see Figures 1, 2, 4, 6, 7, 9, 10, 11)~ three outf1ow cllannels 3, 4, 5 (see Figure 5) or four out-15. flow channel~ ~, 4, 5 and 6 (see ~igures 3 and 8), depending on the extent to which the classifying ~low has to be de*lected or the number o~ fractions r into ~Jhich the fine material has to be divided ~Yhen withdra~m from the classifying region 7 into 20. the out~lo~J channel or channels. In the region where :
-the ~low channel curves, the inflow channel is con ``
nected to the radially inward outflow channel 3 by a convexly curved deflection wall 1, which can extend aroùnd an angle of 45 to 180 or more. The angle 25. around which the inner deflection wall 1 extends is the inner deflection angle ~i~ which is abou~t 130 ` `~
in the embodiment in Figure 1. It extends from the beginning of the cur~ature of the inner de~lection wall 1 to the front edge~of the outer boundary wall ;`
30. of the inner outflow cha~Inel 3 (see Figure 1). The . .:

. ~ .
~ 34 `', 3Z6~7 inner deflection ~all 1 is continuously curved aroundthe inner deflection angle ~i. On the side of the inner de~lection wall or the inner side of the flow channel, ~ material supply device terminates in the 5. neighbourhood of the beginning of the inner channel curvature, near the inner deflection wall 1. The supply device is used for supplying a stream of material to be classi~ied in a thin layer and in a direction deviating by less than 45 from the classif~ing 10. flow inside the ~low channel. In the embodiments shot~n in Fi~ure 1 to 8 and 10 to 12, the material supply device comprises a channel 10 ~or supplying a ~low o~ carrier substance, e.g. air or water? charged with a flow of material to be classified9 the chamlel 15. terminating at the flow-channel wall (see Figures 1-5, 7, 8, 10, 12) or at a slight distance therefrom inside the flow channel (see Figures 6, 11). The width of the opening, radially and transverse to the classifying-flow direction, is small compared 20. with the radial dimension of the flow channel.
On the side opposite the point at which material is introduced into the flow channel, the channel ~all is ~ormed with a coarse-material outlet aperture 8 ~rom which coarse material escapes from the 25. classifyin~ flow. Its end ~orms an edge 21 oblique to the tra~ec+ories of the material, the edge being on the outer boundary wall of the outflow channel 3 (see Figure 12) or of the outer outflow channel 4 ~ .
(see Figures 1, 2, 49 6, 7, 9~11) or of the outer 30. outflow channel 5 (see ~igure 5) or of the outer .

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~ ~ 8~ 6 47 outflo~ cha~me_ 6 (s~e ~ ures ~, 8).~ e ed~e 21 ~orming the end of the coarse-material outlet aperture 8 is disposed in continuation of the line of the outer channel ~all ending at an edge 25 at the 5. be~innill~ of the coarse-material outlet aperture, or is som~That radially offset therefrom (outw~rdly o~ffset in Figure 3). The outer boundary o~ the classi*ying flo~Y tthe jet boundary of the classifying flo~), which has the form of a wall jet in the 10. classifying region 7, extends between the outer edge 25 of the inflow channel 2 forming the beginning of the coarse-material outlet aperture and the edge 21 ~orming the end of the coarse-material outlet aperture.
The ~ngular extension of the coarse-material outlet 15. aperture corresponds to the outer deflection angle a of the classifying ~low deflected at the inner deflection wall 1. The outer deflection angle should be less than the inner deflection angle ai. In -the embodiments shown, the rat,io between the radii of 20. the outer and the inner curvature of the flow channel is approximately between 3:1 and 2:1. The outside of the classi~ying flow in the flow channel is conti-guous to an outer flow 9 for withdrawing the fraction of c`oarse material alon~ the coarse-material outlet 25, aperture ~ where the classifying flow is not guided by a wall. To provide the outer flow 9, a supply ;
channel 12 for fluid largely free from material is pro~ided on the side of the coarse-material outlet `; aperture ~ near the flow channel a~d opens into a 30. coarse-material discharge device at the upstream end '~ - ' . .

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~ ~2 6 47 of the coarse-~.aterial ou-tle-t aperture 8. The material-~ree ~luid flows out of channel 12 su~stan-tially parallel to the cJ.assi~ying ~low. The material discharge, which is adjacent the coarse-material 5. outlet aperture outside the flow channel, can be construct.ed as a coarse-material collecting vessel 13a having a funnel-shaped lower part9 from which accumulated coarse material can be removed by means o~ a bucket-~Jheel lock 17 (see Figures 2, 37 8).
10. ~he fluid supplied through the channel 12 for the outer flow 9 has to be discharged from the coarse- `.
material collecting ~Tessel. After the flow has been deflected up~ards in the bottom part of the vessel along the semicircular line 15 sho~ in E`igure 2, it 15. reaches the outer wall together ~ith that part o~ the coarse material which has not been ejected by gravity through the de~lection, and is discharged through an upwardly extending outlet channel 16. The fluid, which forms an inner vortex core 18, is further 20. deflected by an upper, substantially semicircular curved wall 19 o~ the collec~ing vessel 13a extending nearl~ as far as the flow channel and in the direction of the classifying flow substantially parallel to the coarse-ma~erial outlet aperture 8.
25. In the flat classifying device sho~m in Figure 1 and the annular or axially symme~rical device sho-m in Figures 10 and 11, the coarse-material discharge device has t~o outflow channels 13, 14 one beside the other e.djacent aperture ~ for partial f10WS of the outer flow laden ~ith a fraction of coarse material.

:

: - 37 -~q~8Z647 The outflo~ cha~lel 14 is used for remo~ing the ~incr fraction of coarse material, ~hereas -the coarser fraction is removed throu~}l channel 13, w}lich extends outwards substantially in the central direction of 5~ travel of the coarse material. Separators for collect;ing the two fractions can be connec-ted to channels 13, 14~ Alternatively, all the coarse material can be removed, suhstantially in the central direction of travel, through a single channel 13 10. (see Figures 7, 12).
In the axially sy~e-trical embodiment, as sho~rn in Figure 10, the out~low channel 1~ ends in a spiral channel 46 in ~hich the fluid is collected and from whic}l the fluid is withdra~n. In the same embodiment, 15. the outflow channels 3, 4 for the classifying flow each endsin a volute channel 45.
In all the embodiments, the material to be classi~ied is introduced through a narrow aperture 11 leading from a ~upply channel 10 into a deflected 20. classifying flow flowing from the cha~nel 2 into the zone 7 and there~rom is fanned out into the outflow channels 3, 4 etc. In the embodiments in Figures 1 to 8 and 10 to 12, the material is suspended in a carrier ~low and acce~erated to the speed at which 25. it is introduced at the material introduction point.
Ad~tantageously, this speed is equal to the speed of the classi~ying flow. In the axially symmetrical embodiment in Figure ~, the material is accelerated by a coaxially ro~ating centrifugal plate 31 on which 30. the material is supplied do~wards via a central shaf-t .

~ 38 .. . . .. .. ... . ... , .. . ~, . .. . .. . .. . . . .. . ... . . . . , , ., . ~ ., 1 ~ ~ 2 ~ ~7 Advantageously, the centriiugal plate ~1, which is driven by a motor 33, is concave and conical.
material supply channel 10 covered by a rotating wall 32 or a non-rotating cover is disposed above 5. plate 31. ~ ~low o~ gas can also be introduced through channel 10, so that the material, particularly the fine particles, is additionally accelerated by pneumatic means. In view o~ the explanations in the previously-mentioned patent specifications of the 10. present Applicant, the skilled addressee does not require additional information abou-~ the construction of an axially symmetrical classi~ier comprising a rotating centrifugal plate, about the supply of material thereto, its flmction, or the construction o~ the 15. ~low channels.
~ en a centrifugal plate is used ~or supplying material, the i~er deflection wall 1 can be stationary (see Figure 9). Alternatively~ it ca~ be connected to the centrifugal plate 31 and rotate therewith.
20. ~hen the de~lection wall is stationary, the particles o~ ma~erial and the flow introduced therewith move round the central axis 50, and this motio1~ is super-imposed on the motion in the radial plane. ~en the wall rotates as well, a rotating component of the 25. classi~ying-flo~ boundary layer and, if required~ o~
; the ~hol~ classi~ying ~low is additionally superimposed. ~i For high throughputs, it is advantageous to use the axially sy~nmetrical device, the material being introduced by pneumatic or hydraulic means (Figures 30. 10-12). ~
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1~8;~ 647 .
In the cl~s~ifyin~ ~evices in Figu~s ~ and 11, the stream of nl.aterial i s introduced at point 11 at a radial distance 27 from the in~er deflection t~all 1. The di~tance 27 should be less than the radial 5. distance 28 from the point ll to the outer boundary ~all o~ tl~ classifying .flow. In this case, a cl~ssify~
ing.flow 29 direc~ly adJacent the inner de~lection wall 1 remains substantially free of material.
The embodiment of Figure 7 has rneans for prevent~
10. ing material frcm sticking to the inner cu~ved deflection ~Jall. The inner wall I is a slowly rotating circular cylinder driven by a motor (not shown). The classifJring M ow is adjacent the ~ront side facing the classifying zone 7. Any adhering fine material 15. is removed ~rom the rear side by scrapers ~0 or brushes or similar devices and collected in a vessel 60 ~ulder-neath.
When material enters the zone 7 through the channel ; lO, the fine material is st.rongly de~lected by the 20. flow and carrièd thereby through the discharge chan~els for the classifying flow. The coarse material, on the : other hand, moves along ~latter trajectories past the edge 21 of the outer boundary wall of the outflow cha~nel or of the outer out~low channels through the ; 25. coarse~material outlet aperture into the coarse-material discharge device. In the classifier in Figures 1, 10, 11 the coarse material ~lowing through the outer classifying-flow boundary flows into a coarse-material channel 13 disposed substantially in the central - 3Q, direction of flow of the coarse material. The outer ~ .
'` ` ' : ~'`.` ' .
_ 40 _ . :
, ~C~82~4t7 flo~, which is supplied largely free-of material, travels through channel 13 out of the infl o~.r channel 12 and entrains the coarse material. However, part of the outcr M OW is discharged through the out~flo~J
5. cha~nel 14 in th~ embodiments in Figures 1, 2, ~0 and 11. In this case9 the coarse materia] i5 further classified through the outer flow 9 into a ~iner fraction ejected thrcugh channel 1~ and a coarser fraction ejected through channel 13. The subse~uent 10. classification of the coarse material outside the classifying zone 79 produced by the outer flow 9 as shown in Figure 1, is a combined process of deflection and cross-flo~r classification. ~lterna-tively, usè
can be made o~ pllre cross-flow classification, as in 15. the device in Figure 10.
Advantageously, in order to adjust the cut sizes, the boundary walls of the outflow channels are made ad~ustable so that the position of their ~ront edges can be changed both in the direction of flow and in 20. the radial direction. The position in the radial direction can be adjusted using flaps or vanesO Dis-placement means can also be provided for adjustmen-t in the direction of flow. It is important ~or the outflow channels to be completely sealed from one 25, another. Often, therefore, it is preferable to use di~erent exchangeable boundary walls rather than ;
ad~ustable walls.
Figure 8 shows a complete air classifying apparatus. -~
Material is supplied pneumatically into the classify~lg ~-30. zone. In order -to operate the apparatus iII accordance ~, ;",.~;, `

~ 41 ~-.~08Z69~7 ~ith the inve~tion, it is very impor'~ant for the material supplied at point 11 to be introduced at a const~nt speed into the classifyin~ region. In the case of pneumat-c supply, this can be achieved 5. in a part.iculariy advantageous manner if a pressure-measuring section is provided in a pneumatic conveying section of channel 10 in .front of the point of supply 11 in the zone 7, the resulting pressure drop in the pressure-measuring section being used to adjust the 10. flow of material. The out~low from the outlet aperture 35 of a collecting vessel 34 constructed as a mass flow bunlcer is adjusted by moving a slide valve 36 or similar valve device. All obliaue walls 37 of the bunker 34 are aerated to ensure a vniform 15. stream of material. The slide valve 36 is provided in front of the outlet aperture 35. The material flow~ng out of the aperture 35 is introduced by an injector 38 into the material supply channel 10 from ~hich it flows into the classifying zone 7~ where it is 20. separated into four fractions of fine material~ which are discharged through channel.s 3, 4, 5, 6. The boundary walls of these channels are adjustable. The discl~arge ~ractions are separated either in .~ilters 39 or cyclones 40. Separation should be as complete 25. as possible. It is therefore advantageous to use ~.
filt~rs if th~ fractions contain relatively hi~h . :.
proportions having a particle size below 5-10 ~m. In the classifier in Figure 8, a filter is provided for the very fine fraction dischargcd throu~h channel 3, 30, whereas ~he coarser fractions of fine material dischar~ed .. :

.

. .

through c}lanne~s 5, 6 are scparated in cyclones 40.
In order to en~ure speci ally sharp separation bet~leen ~he fractions discharged in channels 3 and 5, the ~raction o~ fine mat~rial discharged in char~nel 4 5. is conv~yed in a closed cycle. T~le air carr~Jing it is advantageously used for accelerating the material in cha~nel 10. The air, after being ~reed -from ~ine material in c~yclones 4Q, ~lows back -to region 7 throu~h channels 2b and 2co The air dischar~ed 10. through chal~nel 3 ~lows to atmosphere throu~h ~ilter 39 a~ter the finest fraction has been separ~ted.
corre~ponding amount o~ air is sucked in through channel ~a. The coarse material travels through aperture 8 from zone 7 into the outer ~low 9 supplied 15, through channel 12. The outer ~low travels in a semi-circle in the bottom part of vessel 13a and comes out at the top through an outlet channel 16. Some `:
of the coarse material is ejected throu~h the bucket-wheel lock 17. The rest is separated by ~ cyclone 41 20. ~rom the outer .~low, ~hich is largeJ.y free o~ material ~rllen it is again supplied through channel 12.
Figure 10 shows an axially s~m~etrical annular classi~ier for hi~h throughputs, in which the material ~lows ~rom an annular ~eed vessel 34 (constructed as 25. be~ore as a mass M ow bu~er) through an adjustable ; outlet aperture 35 into an annular vertical supply channel 10~ The feed vessel 34 has a verti~al wall and an obli~ue aerated bottom ~,Jall 37, The amount M o~ing out can be held at a constant value by measur-30. ing pressure drop b-y devices (not sho~ ) in a .. ~ .
,',:
: ' ,:

4~ ~ ~
~ ',.

8 ~ 6 pressure-measur-ng section in -the acceleration part o~ the channel lOj the s].ide valve 36 being corres-pondingly adjust~d by a regulator. A do~ ard flow oGcurs in ch~nne]. 10 and accelerates the material

5. to the .n~teria',-intro~uc~ion speed at ~hich 't leav~s aperture 11 and enters the classify.ing flow a~ the be~.inning of the curved i~ner deflection wall 1 and in the same direction. Acceleration is produced by mainly pneu~atic means. It is not appreciably 10. increased by gravity, except ~or large particles above 1 mm ~nd when the material is supplied at a slow rate. All other features for operating the classifying apparatus in accordance with the method ; of ~he invention can be seen directly from Figure 10, 15. with re~erence to the notation which has already been explained. The outer flow 9 supplied through channel 12 rèsults in cross- or transverse-flow classifying - o~ the coarse material into a coarser fraction ~dis-charged through channel 13 and a ~iner fraction discharged 20. through channel 14. In order to vary the cut size between the two fractions of coarse matel~ial, a kni~e-edge 49 can be vertically moved along the inner edge between chalmels 13 and 14. The channels ~or supplying and subse~.uently discharging ~he fluid 25. ~or ~he classi~ying flow and the e~ternal flow can be constructed in various ways. In Figure 10, ~or example, . material is supplied through c~ntral channels 42, ~7 ~- ~ and discharged through volute casings 45, 46. It can be seen *hat if material is supplied vertically do~
30. wards by pneumatic means, the classi~ying device can ~ ~

- ~.
. 4 ~8Z~

have a compact~ ~ery advantageo~s cons-tl~uction, since the volute casings 459 46 can ~e relativel~J near the central axis 50, and the supply channel 2 for the classifying ~low a~d the channel 12 for the cuter 5, ~low are lilcewise at or near axis 50, at a slight inclination thereto, the main reason for efficiency being that the coarsest fraction is discharged through channel 13 vert.ically downwards and not out~ards, resulting in larger diameters.
10. Figure 11 sho~s an annular or axially sy~netrical deflection classi~ier ~Ihich is somewhat different from Figure 10 but basically has the same structure.
M~terial is introduced at point 11 spaced by a small radial distance 27 from the inner de~lec-tion wall 1, 15, the distance being less than -the radial distance 28 .~rom the outer aperture 8. Consequen-tly~ there is a m~terial-free flow layer 29 between point 11 and wall '~
1. Vanes 48 between channel 10 and the ~low channel wall adjacent the upstream end o~ the deflection wall 20. 1 impart a rotating flow component around QXi S 50 -to the flow layer 29. This facilitates deflection through a larger angle of the classifying ~low adjacent the inner wall 1. A similar rotating ~low component can also be imparted to the classi~ing ~low entering through ~ `~
25. channel 2 and/or to the flow o~ material supplied through channel ~0. In principle, it is possible to produce classifying devices of the general constructions `~
shown in Figures 10 and 11 ~Ihich are either suitable for use with gases or suitable for use with liquids.
30. Figure 12 sho~rs an axlally symmetrical embodiment .',:
.
` ~ .,.'.
45 _ ,.,~ ,. ~ . .

~C38~:647 in ~.hich ~he f;o~i ol material is up~iards to the classifying regi~n 7 -through an annular ch~nnel 10 extending up~ards to its delivery aperture 11. This cons~ruction is particularl~ suitable for combination 5. with a mill, e.g. a bo~rl mill, disposed immediately under the classifying device. Thus the material is conveyed by an air stream immediately ou~ o~ the mill throu~h channel 10 into the classifying region.
As be~ore, the fanning-out of the material in zone 7 10. can be used ~or withdra~in~ a number of fractions of fine material from zone 7 through a number of outflow channels 3, 4 etc. If only one fraction of fine material is required, a single outflow channel 3 is suf~icient, as sho~Ym in the classifier in Figure 15. 12. The coarse material ~lowing through aperture 8 en~ers the outer flow 9, supplied through channel 12, and is discharged through channel 13. The ~lows charged with fine material and coarse material can be further guided in various ways. In Figure 12, the 20. outer flow, charged with coarse material, is supplied via tangential guide vanes 52 to a central c~Jclone 51.
If re~uired, coarse material separated at the centre is returned to the mill through a line 46 in a stream o~ a~r. Advantageously, a number of channels are used 25. ~or discharging the classifying flow charged with ~ine material, which emerges through channel 3 in annular manner out of the classifying æone. Cyclones can be - directly connected do~stream, or alternativeIy the flow can be divided into channels and discharged up~ards 30. through the bend 45 sho~m in Figure 12. As before, the .' - . ~ .

. ~ . .

- . .. . , - - , . ..

L~8Z~47 construction is particularly simplc and compact, this construction being partlcularly suitable for material already suspended in a carrier flow and in ~hich material is supplied ~Jertically upwards into the 5. classifying zone.
However, axially symmetrical embodiments in which matèrial is supplied by pneu~atic or hydraulic means fromabove (Figure 10) or from below (Figure 12) have a common feature in that the classifying flow can be 10. deflected either outwards or inwards. Figures 10 and ` 11 shown down~rard and outward flow but could be inserted to provide upward and outward ~low, ~hile i Figure 12 shows upward and inward flow but could be inverted to provide do~.~ward and inward f]ow~ In the 15. case both of outward and inward deflection, the deflected classifying flow does not have exact parallelism if the annular inflow and outflow channels of the classi~ying flow have equal cross-sections.
The streamlines converge some~at in tlle case of 20. outward deflection, ~hereas they diverge slightly in the case of inward deflection. The latter condi-tion is more ~a~ourable for classifying. ~hen the diameter D (see Figures 9, 10, 12) is large in com-parison with the radial dimension of the classifying 25. M ow, the deviation from parallelism is unimportantO
' .

", . ..

47 _ ... . ..... . ,, . , ..... ~ , ,

Claims (33)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of continuous centrifugal classifying of a continuous stream of particulate material into at least one fraction of coarse material and at least one fraction of fine material in a deflected flow, the material to be classified being classified in a gaseous fluid at cut-off sizes between approximately 1 µm and 100 µm and a mass flow ratio up to 10, between the supplied stream of material and a classifying gas flow, and being classified in a liquid fluid at cut-off sizes between approximately 10 µm and 1µm, comprising:
(a) providing a curved inner deflection wall curved from a beginning over an inner deflection angle greater than 45°;
(b) establishing a classifying fluid flow which is deflected in a classifying region by said curved inner deflection wall and has, as an inner boundary, said curved inner deflection wall and has a curved outer boundary which is not covered by a wall over an outer angle smaller than said inner deflection angle, the classifying flow being substantially parallel to said inner deflection wall and abutting said inner deflection wall at least over said inner deflection angle;
(c) establishing an outer flow for carrying away the fraction of coarse material, the outer flow establish-ing the outer boundary of said classifying flow over said outer angle, the ratio of radii between said outer boundary and inner deflection wall of said classifying flow being less than approximately 5:1;
(d) introducing a stream of material to be separated into the classifying flow in a thin layer in the 48.

vicinity of the beginning of curvature of the inner deflec-tion wall in a direction such that the vector component of its velocity in the direction of the classifying flow is at least half the value of the velocity of the classifying flow and in a direction which does not deviate more than 45° from the direction of the classifying flow, whereby fine material, after being fanned out by centrifugal force is discharged primarily with the out-flowing classi-fying flow, and the coarse material passes through said outer boundary of the classifying flow which is not covered and is discharged primarily with the outer flow.

2. A method as claimed in claim 1 operating with moderately fine cut-sizes, the flow of material being introduced into the classifying flow adjacent a curved inner deflection wall having an inner deflection angle of at least 60°.

3. A method as claimed in claim 1 operating with very fine cut-sizes, the stream of material being intro-duced into the classifying flow adjacent a curved inner deflection wall having an inner deflection angle of at least 90°.

4. A method as claimed in claim 1 or claim 2 or claim 3 in which the stream of material is introduced into a deflected, substantially parallel classifying flow, the ratio between the radii of the said region and of the inner deflection wall being between 3:1 and 2:1.

5. A method as claimed in claim 1 in which the stream of material is introduced at a velocity whose component in the direction of the classifying flow is substantially equal to the speed of the classifying flow at the point of intro-duction.

6. A method as claimed in claim 1 in which the direction of the flow of material and of the classifying flow at the point of introduction are substantially the same.

7. A method as claimed in claim 1 in which the fluid is a gaseous fluid, the flow speed of the fluid being kept substantially constant within the classifying zone at a value between 10 m/sec and 300 m/sec.

8. A method as claimed in claim 1 in which the outer flow for removing the coarse material is supplied substan-tially material-free and substantially parallel to the classifying flow and is discharged outwards together with the coarse material substantially in the average direction of travel of the coarse material.

9. A method as claimed in claim 1 or 7 in which the outer flow for discharging the coarse material is guided substantially parallel to the boundary of the classifying flow adjacent the upstream portion of the classifying zone and then passes into a coarse-material chamber in which it follows a semicircular path and is then discharged at the outer wall of the coarse-material chamber together with at least a part of the coarse-material, an inner vortex being produced by the outer flow in the coarse-material chamber, the inner vortex being so aligned that the particles of coarse material therein are driven towards the outer wall.

10. A method as claimed in claim 1 or 7 in which the coarse material in the outer flow is classified into two or more fractions by dividing the outer flow at the downstream end of the classifying zone into two or more partial flows, the outermost partial flow being used for removing the coarsest fraction.

11. A method as claimed in claim 1 or 7 in which the classifying flow leaving the classifying zone after deflection is divided into an inner and one or more outer flow layers which are discharged separately with the fractions of fine material contained therein.

12. A method as claimed in claim 1 or 7 in which the flow of material is introduced into the classifying flow at a radial distance from the inner deflection wall which is greater than zero but less than the radial distance from the outer classifying flow boundary.

13. A method as claimed in claim 1 or 7 in which the material to be classified enters the classifying flow through a passage in which it is suspended in a carrier, the pressure drop along all or part of the passage being kept constant by adjusting the rate of supplies of material to the passage.

14. An apparatus for continuous centrifugal classifying of a continuous stream of particulate material into at least one fraction of coarse material and at least one fraction of fine material in a deflected flow, the material to be classified being classified in a gaseous fluid at cut-off sizes between approximately 1 µm and 100 µm and a mass flow ratio up to 10, between the supplied stream of material and a classifying gas flow, and being classified in a liquid fluid at cut-off sizes between approximately 10 µm and 1 µm, comprising:
(a) a curved inner deflection wall curved from a beginning over an inner deflection angle greater than 45°;
(b) a flow channel for conveying a classifying fluid flow having a classifying zone in which the fluid flow is deflected by said curved inner deflection wall and which has a curved outer boundary not covered by a wall and defining a coarse material discharge aperture over an outer angle smaller than said inner deflection angle;

(c) means establishing an outer flow for carrying away the fraction of coarse material and for establishing the outer boundary of the classifying flow over said discharge aperture, the ratio of radii between said outer boundary and inner deflection wall being less than approximately 5:1;
(d) means for introducing a stream of material to be separated into the classifying flow in a thin layer in the vicinity of the beginning of curvature of the inner deflection wall in a direction such that the vector component of its velocity in the direction of the classify-ing flow is at least half the value of the velocity of the classifying flow and in a direction which does not deviate more than 45° from the direction of the classifying flow.

15. Apparatus as claimed in claim 14 in which the inner deflection angle is at least 60°.

16. Apparatus as claimed in claim 15 in which the inner deflection angle is at least 90°.

17. Apparatus as claimed in claim 14 or claim 15 or claim 16 in which the ratio between the radii of the outer and inner boundaries of the flow channel is between 3:1 and 2:1.

18. Apparatus as claimed in claim 14 in which the radius of curvature of the inner deflection wall of the flow channel is at least 1 cm.

19. Apparatus as claimed in claim 14 in which the curvature of the inner deflection wall of the flow channel increases in the flow direction.

20. Apparatus as claimed in claim 14 in which the flow channel downstream of the downstream end of the coarse-material discharge aperture is divided into at least two outflow channels for the classifying flow, the wall bounding the outer sides of the outflow channels terminating in front edges which are displaced from the upstream end of the coarse-material discharge aperture by angles about the centre of curvature of the flow channel which are progressive-ly smaller from the innermost outflow channel to the outermost outflow channel.

21. Apparatus as claimed in claim 20 in which the front edges are rounded.

22. Apparatus as claimed in claim 14 in which the means for introducing a stream of material to be separated includes a supply channel for material to be classified suspended in a carrier fluid, the supply channel terminating in an aperture at the flow-channel wall or at a slight distance therefrom inside the flow channel and the width of the supply-channel aperture in a radial direction relative to the centre of curvature of the flow channel being small compared with the width of the flow channel in the same radial direction.

23. Apparatus as claimed in claim 22 in which the means for introducing a stream of material to be separated includes a mass flow bunker having aerated walls and having an adjustable outlet connected to the material supply channel and means for controlling the outlet automatically in dependence on the pressure drop along at least a part of the material supply channel.

24. Apparatus as claimed in claim 14 in which outside the downstream edge of the coarse-material discharge aperture there is a coarse-material channel for removing fluid charged with coarse material.

25. Apparatus as claimed in claim 14 in which outside the coarse-material discharge aperture there is a coarse-material collecting vessel arranged to deflect coarse material passing through the coarse-material discharge aperture along an approximately semicircular path to an outlet in the outer wall of the collecting vessel.

26. Apparatus as claimed in claim 14 in which outside the downstream edge of the coarse-material discharge aperture there are a plurality of contiguous outflow channels.

27. Apparatus as claimed in claim 14 in which the flow channel has a substantially rectangular cross-section.

28. Apparatus as claimed in claim 27 in which the inner deflecting wall is afforded by a circular cylinder and which includes means for rotating the cylinder around its longitudinal axis and means for cleaning the cylinder located at a region of the cylinder remote from the part affording the inner deflecting wall.

29. Apparatus as claimed in claim 14 in which the flow channel has an annular cross-section, the material supply device having an axially symmetrical opening coaxial of the flow channel and on the inner side of the flow channel, the coarse-material discharge aperture being provided coaxially on the outer side of the flow channel.

30. Apparatus as claimed in claim 29 in which the material supply device is constructed as a coaxial centrifugal plate and a central material-supply shaft.

31. Apparatus as claimed in claim 29 in which the inner deflecting wall curves outwardly away from the central axis and guide vanes are provided for imparting a rotational component about the axis of the annular cross-section to the classifying flow in the flow channel.

32. Apparatus as claimed in claim 29 in which the central axis of the flow channel is substantially vertical and the supply channel extends downwards to its aperture inside the flow channel.

33. Apparatus as claimed in claim 29 in which the central axis of the flow channel is substantially vertical and the supply channel extends upwards to its aperture inside the flow channel.