Evaluating and Selecting Chemical Equipment
W I L L I A M
T. MARSTON
F. V. S C H N E I D E R G. E. C O F I E L D
DESIGN F O R WET S E P A R A T I O N Wet class$ication is economical and dependable, especially f o r finer separationsproblems in material transport and Jow control are simpler evaluating a classification problem, the first step Often, characteristics of the material automatically eliminate one of these choices, or operations preceding or following the classifyins step may be controlling. If the material is damp or wet, dry methods may he uneconomical. Size of operation can he important. A general rule is availaiile : For separations coarser than 20 mesh, use dry screening; and for finer separations, use wet classifications. Normally, wet processing involves simpler probleins in material transport and flow control.
I is choosing between a wet or dry process. II
General Considerations
Assuming that a wet separation process is selected, the mesh of separation must be considered. Generally, rake- or screw-type mechanical classifiers are used for separation in the 28- to 200-mesh range, and hydroseparators or bowl rake types for the 65-mesh to 25micron range. Other factors to be considered are dilution of feed and coarseness of particles in the feed. .4n excess of coarse particles in a pulp may eliminate hydroseparators, even though the desired point of separation may he finer than 200 mesh. In some cases, a combination of a classifier and hydroseparator is the solution. Here, the classifier receives the primary feed and removes a coarse fraction that would he difficult to rake in a hydroseparator. T h e classifier overflow is then fed to a hydroseparator where the originally desired particle size is removed. For highly diluted pulps, however, a reverse procedure can be used. A hydroseparator o r scalping tank can receive the primary feed, separate particle sizes finer than those desired, and a t the same time remove most of the dilution water. Underflow then provides a feed to the classifier, and dilution can he adjusted to that required to separate the originally required coarser size. However, when removing fines in the hydroseparator, fast settling pulp for the classifier can result, which makes the desired separation difficult. Both of these approaches were not uncommon a few years ago. Now, however, the hydrocyclone provides a simpler solution by r e p l x i n g the hydroseparator. 44
INDUSTRIAL A N D ENGINEERING CHEMISTRY
I n selecting such equipment, settling tests usually provide adequate data. Wet classification processes can be classified : -Desliming. Removing a discrete slime fraction --Sizing. Separating the particles in pulps into two or more sizes -Closed-circuit grinding. Controlling product size from a grinding mill -Washing and leaching. Using a solvent to remove a constituent of the substance being treated -Concentration. Segregating particles on the basis of specific gravity -Draining. Removing solids from a liquid-solids mixture
Desliming. A typical example of desliming is a procedure used in a western glass sand plant. T h e material being treated contains silica sand, essentially plus 150 mesh, and a clay, -325 mesh. T h e material is hydraulically mined, and at a dilution of 20% solids, it is pumped to a hydroseparator where must of the clay is removed as the overflow product. This is pumped to a neighboring plant and used for firebrick. The sand which is the underflow product is processed further. T h e hydroseparator in this instance is used for desliming, hut cyclones and bowl-type classifiers can serve just as well. T h e wide difference in particle size between the sand and clay makes this a simple separation. Often desliming incorporates a mild scrubbing effect, particularly for screw classifiers where the rolling action rubs particles together, loosens surface coating, and breaks u p lightly consolidated undesirable materials. Sizing. Perhaps the most common use of classifiers is in preparing sands for aggregate use. Natural sandk in some areas contain a n excess of 100-mesh fractions, and excess fines are usually eliminated in screw- or AUTHORS William T . Marston is General Sales Manager, Dorr-Oliver Co., Stam,ford, Conn. F. V. Schneider is Manager and G. E. CoJeld is Assistant Manager, Process Equipment Defiartment, Wemco Division, Western Machinery Co., San Francisco, Calif.
rake-type machines. Size split is controlled principally by dilution in the classifier pool. For applications requiring multiple size fractions, multiple sizers are frequently employed. Although several types are used, they have in common a controllable, uprising current. T h e more recent designs provide avtomatic control discharge and are a true hindered settling classifier. Hydraulic types are used for sizing minerals prior to other forms of concen:ration, preparing specification sands, and foundry sands, sizing abrasive sands, and more recently, in preparing fracturing sands for oil-well application. T h e separations are not sharp and there is some overlapping of sizes. Closed-Circuit Grinding. A typical circuit consists of a ball mill paired with a classifier usually a rake- or screw-type. T h e mill discharges into the classifier pool where dilution is maintained to oi.erflow pxticles of the desired fineness. T h e coarser sands become the rake product and are returned to the n d l feed for further reduction. Thus, thc classifier controls product size of the circuit and prevents overgrinding. .Also, the classifier should be long enough so that raked sands are elevated sufficiently to flow by gravity into the ball mill feed or scoop box and, a t the same time, the mill discharge also should he able to flow by gravity to the classifier feed entry. This eliminates the need for pumps or conveying devices. T h e circulating load, the coarse fraction returned to the mill for further reducing, can amount to several times the original feed. Often several stages of grinding are used. Cylcones are replacing classifiers increasing-ly for closed-circuit grinding, both alone and in combination with classifiers. When used in combination, the classifier is a conveying device which returns oversize material to the mill and overflows a product of coarser size. This constitutes the feed which is pumped to the cyclone where it is separated a t the desired size. T h e cyclone overflow is the finished product of the circuit and the underflow joins the classifier-rake product a t the mill feed box. Thus, the coarse portion of the mill discharge bypasses from the pump and cyclone; this reduces considerably on wear and maintenance costs. Washing and Leaching. Classifiers can be used for ivashing and leaching where particle size and contact times permit. I n producing a n activated alumina, it was desired to remove excess caustic from a 10- by 30-
100
80 0
$ 2e 6o s 9
5
3
40
d
a W 20
I
I
TYLER MESH 12'aturai sands in some areas contain excess - 700 mesh jraclions. ciassa~erpooisprincipal controi in size spiit is dilution
In
inesh sized fraction and, a t the same time, pre\rent later generation of undesired fines by chipping sharp edges from tlie particles. .A screw classifier equipped with specially designed scrul,hirig flights \vas selected. As the alumina particles pass through the classifier pool, excess caustic is clissol\-ed in the water hath and overflowed. At the same time, the mild scrubbing action Iireaks off or aliracles the angular edges of the particles, and the fines thus procluccd arc carried out in the classifier pool overfloi\.. Optimum caustic removal is ensured Iiy using rvashing spra);s directed on thc rake product as it emerges from the pool arid moves u p the drainage deck of the classifier. Rake- or screw-tj-pe classifiers can be used i n counter~ Tu.0 or more classificrs current decantation s ? -terns. i n series are arranged so [hat the sands of the first niachine are fed inlo the second rriachine and so 011 through the multiple units. Qverflort-s from the classi-
In Addition to Simple Sizing, Hydraulic Classifiers Can Also Produce Multiple Size F ~ a s l i o n s
Feed
Indiiiidual _ _ _ _7 _0 _Retained ____~~_-_____,SpiXot Producfs. Cumuiatiiv
Sieve
...
in.
...
4
0.4 37.6
97.3
8 14 30 50 100
100.0
Pan
0.1 15.8 36.1 57.4 84.9
80.3 96.9
100.0
30.6 75.2 95.1
99.6 100.0
21.2 61 .2 94. 0 100.0
5.7
27.5 81 . h 100.0
0.9 45.0 99.5
100.0
VOL. 5 4
7.7 88.4 99.4 100.0 NO. 1 1
53.4 94.0
100.0
53.6 93.8 100.0
NOVEMBER 1 9 6 %
45
fiers have a counterflow from the last machine to the first. Advantages are compactness a n d comparative high solution recovery in each washing stage. T h e Dorr multideck or washing classifier is especially designed for this application. I n this manner, screw classifiers are used by a number of plants in uranium extraction. Concentration. Hydraulic sizers are used to separate a mixture of particles having different specific gravities into groups of uniform specific gravities. T h e material is first screened into closely sized fractions which are then
fed to a single sizer and separated into groups according to specific gravity. However, excessive water requirements a n d recent developments in equipment for fine screening have limited this application. Draining. Draining is not a true classification operation, but classifiers are often used for this purpose. Rake- or screw-types are particularly suitable. T h e raked product has a comparatively low liquid content, and because the material is conveyed u p an inclined ramp, a desirable gain in elevation results which is frequently an advantage in plant design.
-
SEDIMENTATION
Pool-Type Classifiers. Depending on geography and usage, the oldest pool-type classifier is called a \7box, classifying launder, Spitzkasten, scalping tank, and various other terms, depending on geography and usage. This machine has survived many years, and is still used today, particularly in the sand and aggregate field. M a n y methods of discharging such launder-type classifiers have been developed. Spigot discharge is impossible to control manually, and therefore automatic valving is usually needed, based on the immediate area of the individual spigots. Often the lack of sensitivity will cause intermittent discharge ; consequently, the sharpness of classification is not as good as it should be. T h e oversize discharge will be relatively dilute because free flow through the spigot is necessary and adjustments for size separation or sharpness of separation are lacking. Actually, no flexibility a t all is available, but practical feed fluctuations are accommodated by adjusting the portions of the spigots which are accepted or rejected.
Cone Classifier. This is a logical outgrowth of the launder-type machine. Essentially, it is a cone with a center feed which distributes itself radially and a t decreasing velocity toward the overflow launder. Normally oversize product builds up in a manner similar to that shown in the illustration. I t is then withdrawn intermittently, which usually means as continuously as possible, by a variety of spigot control methods similar to those used in a launder classifier. This machine has the same limitations as the launder classifier. I t is nearly as old, has almost as many variations, and is still used in sand plants and some of the old bleaching plants. 46
INDUSTRIAL A N D ENGINEERING CHEMISTRY
iiss
COARSE
SOLIDS STILL LESS
COARSE SOLIDS FINEST SOLIDS
+
FINE SOLIDS T h e pool classijier is a low cost rough unit which fills a definite process need, particularly for sand and gravel
FEED
u I n the cone classzfer, oversize material buzlds u@ i n the manner shown, and undernze particles ouerjow
OVERSIZE
Unit Classifier. For particle size sedimentation, the unit classifier numerically outranks all other machines. Developed in the early part of the century by John Van Sostrand in the Black Hills of South Dakota, it was the first machine designed for particle size separation by sedimentation. Basically, it is a launder classifier with an oversize collecting mechanism instead of spigots, which moves sand u p the slope and out of the pool. Coarse particks are dewatered as they travel. Many modifications are available but all have a pool into which the feed is distributed as uniformly as possible, a tank with a sloping I,ottom, a mechanism to scrape u p and out the oversize material as it settles, and an overflow weir which forms the other boundary of tile classifier pool. T h e feed stream enters across the kvidth of the pool, turns down toward the oLVerflow iveir, and then levels out a t a depth where its densit)- is equal to that of thc pool material. Thus: most of the rnaterial in zone C is feed pulp which moves toward the orrerflow weir. T h e oversize material settles through zone B and almost immediately into zone A. Zone B is the densitystabilization zone created by the physical boundaries of the pool, together with agitation provided by the sandcollecting mechanism. T o p of the zone is delineated by the overflow stream which sweeps out that material which is just finer than the mesh of separation. O n the other hand, the definitely coarser material sinks easily through zone B and is collected by the sanrlcollecting mechanism. T h e critical sizes, those close to the size of separation, have a relatively loiig residence time in zone B in which to make u p their mind whether to !)e undersize or oversize. T h e machine is flexible and modifications to meet changing process conclitions can be made easily-e.g.,
Hydroseparator. Essentially tlie cone classifier opened u p to a cylinder, the hydroseparator is provided Lvitli a mechanism to move oversize material to a coneshaped discharge outlet a t the center of the tank hottom. This results in the same type of irnprovcnient over tlie launder clasrificr as \vas obtaincd l)y the rake-type. HoLvever, the hydroseparator has an advantage--. n-hen the quantity of \vater which must go to the overflow is disproportionately large compared to the quantity of o\.ersize solids, the hydroseparator provides a large pool area more economically than can \)e ul)tainecl from unit classifiers. I t is a relatively inflexible machine---vast changes in feed dilution are the only adjustment availalile. Agitation of the deiisit)stal)ilizatio!i zone tiy tlie rake arms is mild, and therefore changes in speed of tlie rakes do not affect separation. T h e oversize solicls are relatively dilute and thus arc dirtier than Lvith the unit classifier. However, wash Lvater can lie put i n the cone to improve cleanliness some\\hat.
UNDERSIZE
For separation by sedimentation, the unit da,siJier is the olifeit and most frequently used device
the pool area can be raised or lowered hy raising or lowering the overflow weir. This increases or decreases the pool area and consequently either or i,oth capacity arid mesh of separation. Oversize particles are \vel1 dewatered, nornially t o the point where they can be carried in a conveyor I d t . This results in less void-filling material a s well as a n oversize product which is cleaner than \vith the laundertype classifier. Also, independent of pool arca adjustment, feed dilution and rake speed can lie changed to alter size of separation or to adjust for changing feed conditions. However, like all pool classifiers, this machine does not produce a clcan oversize product. A wash on thr oversize deck may or may not improirc this aspect.
FEED
OVERSIZE
UNDERSIZE
Essentially, the hydroseparator zs the cone cliiJ.ii/irr o , b m d up t o n cjiinder. I t has a mechanijm l o moue ouersizt nicitoiiil tn t h f out/i’/ at the bottom
(Continued on next p a g e ) VOL. 5 4
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NOVEMBER 1 9 6 2
47
Bowl Classifier-Bowl Desiltor. T h e hydroseparator a n d unit classifier were combined into the bowl classifier which has some of the advantages a n d disadvantages of both. I t is used where the hydroseparator does not provide a n oversize product sufficiently clean or dewatered, and where a large pool area is needed because of dilute feed. FEED
/
+
v
UNDERSIZE
OVERSIZE
The bowl classifier i s a combination of the hydroseparator and unit classijcr
Fluid Bed Classifiers. These machines have the same theoretical requirements as pool classifiers plus a barrier in the form of a fluidized bed, which separates the oversize product from the undersize product. Oversize material must force its way down through the fluidized bed to escape into the oversize stream. Thus, the critical and undersize material is eliminated more efficiently. Little or n o void filling occurs and therefore the oversize stream is not contaminated. T h e major disadvantage is the large quantity of water needed, and nearly all developinental efforts have been devoted to correcting this deficiency. F A H R E N W A L D SIZER. I n the basic single pocket fluidized bed classifier, feed enters a rectangular crosssection pocket. Pool-type classification occurs in zone B with resulting contamination of the fluidized bed A by critical sized and undersized particles. However, FEED
m
OVERSIZE Theoretical equirements f o r the jfutdized bed or Fahrenwald classq?er are the same a s f o r the Fool classiJier, but i t has a barrier which separates oversize f r o m undersize particles 48
I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
Operation is the same as for the hydroseparator o r unit classifier. T h e pool has a density-stabilization zone with rotary collecting mechanism which gathers the oversize material and deposits it into the bottom dewatering a n d washing compartment. T h e water, concentrated out of the underflow from the bowl compartment along with any wash water used in the rake compartment, returns countercurrently to the bowl through the same openings. Also, a type of siphon or airlift is usually used to pull crirical size material from the density-stabilization zone : this also goes u p into the oversize collecting compartment to be returned to the howl compartment. T h e bowl classifier is more flexible than the hydroseparator. I t produces a cleaner and well dewatered oversize product more economically than the unit classifier. Nevertheless, concerning changes in feed conditions, it is still relatively inflexible.
zone -4 has a substantial depth of particles maintained in fluidized suspension or teetered by hydraulic water introduced a t the bottom of th- fluidized zone through a constriction plate. This machine encompasses all conditions for theoretical treatment of fluidized bed classification. Oversize discharge is controlled by a spigot also, which is normally operated intermittently in accordance with the hydostatic pressure exerted by zone A and B combined. As zone A increases in apparent density and height, it exceeds a set point which in turn operates a spigot. Thus, apparent density and/or height of zone A is lowered to a set point where the spigot closes. Unlike the launder or cone-type of classifiers, the density stabilization zone, which is the fluidized bed, provides a n adequate barrer between the incoming feed stream and the discharging oversize stream. T h e Fahrenwald sizer is a relatively low capacity machine, but its oversized product is exceptionally clean, usually a maximum of 2 to 4 meshes wide. Several pockets are required to accomplish a full size range of classification. However, the machine requires a large amount of water-a minimum of 2 to 2 l / ’ ~and usually 3 tons of water per ton of spigot product. This massive dilution often prohibits use of this type of classifier. HYDROSCILLATOR T h e bowl classifier is combined with fluidized bed classification into the hydroscillator which is basically a two-product fluidized bed classifier, unlike the multiple product and original fluidized bed machine. About seven or eight of such machines are in actual operation. A pool is formed by an oscillating perforated piate which forms the upper boundary of the hydraulic water chamber. Feed enters this pool a n d promptly meets a fluidized lied formed by the critical size material previously fed together with hydraulic water fed beneath the constriction plate. This feed as it spreads radially
WATER
/
UNDERSIZE
-
About seven or eight hydroscillators are in actual operation
and a t decreasing velocity to the overflow launder is converted into the undersize stream---all coarse material drops to the bottom. Those particles too coarse for the teeter hed to suspend drop to the bottom of the bed and collect on the constriction plate. By oscillatinq motion combined with the hydraulic water feedcr constriction plate, these coarse particles then move to the periphery of the perforated plate. Here, in the same manner as with a howl classifier, they climb a small inverted weir and drop into the oversize dewatering mechanism. T h e result is a two-product machine which produces a n exceptionally clean, but by no means perfect, oversize stream. Mechanical energy and complexity are substituted for the large quantity of hydraulic water used hy the Fahrenwald type. Water requirements, ranging from 1 to I'/2 tons per ton of oversize material, are suhstantial but more tolerable. This classifier has a density stabilization zone also (zone A in the illustration) which is the fluidized tied. Flexibility of the machine is good. Size separation and cleanli.iiess of separation can be varied within good limits by varying the hydraulic water. In general, the fluidized bed is in an almost steady state conditionif the feed is varied or turned off and on, the lied will simply accept or reject within any increase or decrease in oversize or undersize particle quantities. However,
compared to the Fahrenwald or Evans syphonizer, this equipment is mechanically complex and expensive. EVANSSIPHONIZER.First used for beneficiation of phosphate rock in Florida, this machine is also a fluidized lied classifier. Like the Fahrenwald sizer and fiydroscillator, water fed beneath a constriction plate forms a teeter bed above this plate. Here, however, the teeter bed is narrowed or necked and the feed enters near the top of this nrcked portion. This coupled with ferd water, hydraulic water, and fluidized bed conditions results in a tIvo-stage or coarser hydraulic separation. T h e undersize product contains some tramp oversize but the oversize stream, siphoned out from the fluidizcd bed, is appreciably cleaner than that from the hydroscillator. $$rater requirements are less than those for either the hydroscillator or the Evans separator, but the oversize is not as well dewatered. However, the process is more economical and has all other advantases of a fluidized bed classifier.
IZE
7 h E~m u siphonizer, also a fluidized bed clnsJiJPr, z r m jirst u s d for benejczation ojphosphate rock in Florida
Functional Ranges of Pool and Fluidized Bed Classifiers
Hydraulic 7 y j e o/ Cirrssi'fier
,tlcsh
u/
S.paratzon'L
Classifier Eficiency
Feed,
7bnsjHr.
A4aximuni Oversize, In.
__
70
OuerJow
Feed Density
Solids
Water
20- I 0 0 Poor 1-100 "4 5-65 5 V-boxes, launders, Spitzkasten, scalpers Unit classifier 20-1 00 Medium 1-350 1-1' ' , J 5-65 c C o n e classifier 28-325 tow 100 1 ' 5-30 d e tow 1 i4 1-20 Hydroseparator 65-subsieve Bowl classifier 65-subsieve Medium plus 100-300 1/?-3iq 5-25 10-75 Fluidized bed (hydraulic classifier) 2-100 1i8-3: 16 5-20 60-1 50 High Fahrenwald 40-60 5-250 >'2-1 15-30 40-80 High Hydroscillator 20-200 High 5-250 Evans siphonizer 20-200 a 1/ 4-1 I 4 5-20 10-40 Few pounn'sper hour lo 500 or 700. Tyler screen. b Or rakepruducl. Not crilical. Not critical; usually 5 to 20%.
Pool
'
I
Underflowb
Required, Tons
25-30
None
70-80 35-60
None
30-50 70-80
None None
40-60 75-83 40-60
1.5-4 1-1.5 0.5-1
None
.O
4
VOL. 5 4
NO. 1 1
NOVEMBER 1 9 6 2
49
