R.
P. Kite was born in Wilmington, Del., in 7897. H e received his B.S. in chemical engineering from the University o f Delaware foollowed by graduate work at the Massachusetts Institute of Technology. He became associated with The Dorr Company in 7921 and is now assistant manager of the Development Department in New Yo&, specializing in chemical end industrial projects. Kite is a member of the American Institute of Chemical Engineers and is the vice chairman of the Committee on Admissions of the American Institute.
HYDRAULIC CLASSIFICATION R. P. Kite and A. 1. Fischer WO compensating factors are involved in preparing a rcview of recent developments in almost any field at this time. The helpful factor is the stimulus to new equipment development for war industries. The hindering factor results from development programs being deferred during the war. Such projects will shortly bring new ideas into the market, but thcy involve products unproved at this time. Some liberties have been taken with the scope of the operations designated as sedimentation and hydraulic classification. Where the definition has been stretched, it was for the purpose of including proved devices which were interesting and at least bordered on the operat,ions being discussed. In the field of sedimentation the most significant’ advances during the past fern years have been in mechanical “conditioning” or flocculating the feed prior to actual sedimentation, and to improvements in settling tank inlet and outlet to increase settling efficiency. Other developments sufficiently new to warrant description are combination type units where flocculation and sedimentation are carried out in the same container and/or where sedimentation is carried out in two or more compartments. The various types of blanket-filtration units such as the Precipitator, .4ccelator, and Hydrotreator may be regarded as falling into the former category. Mechanical flocculation of solids-in-liquid suspensions involves a slow mixing whereby the solid particles are caused t o agglomerate so that their settling rate is increased. It is obvious that once a rather fragile aggregate is formed, the suspension must be transferred into the sedimentation tank at relatively low velocity so as not to destroy the delicate floc structure. Tests have shown that the critical velocity ranges between 0.6 and 1.2 feet per second for a wide variety of materials. A relatively short fall over an inlet weir is even more destructive to some floes than is an excessive velocity in the flocculating chamber. The Clariflocculator was developed for the purpose of avoiding floc break-up as the feed is transferred from the flocculating to the settling zone. Here the fiocculat,orand settling compartments arc concentric and circular in plan. Feed is introduced into the inner flocculator compartment at the surface through a siphon feed pipe or overhead trough. It then passes down into the settling zone through a central opening in the floor of the flocculator tank. Clarified liquor overflows into a peripheral laundcr.
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Mechanisms are provided in both t,he tlocculator and t,he settling tanks, both mechanisms and flocculator tank being support,ed on beams or a center pier. The flocculator mechanism consists of a rotating paddle made up of a series of V-shaped vertical blades extending up from two horizontal arms supported from B vertical drum. Intermeshing with these vertical blades are four sets of do~7nmard-extendingstationary V-blades supported from beams above the liquid surface. The purpose of this design is to afford maximum particle contact without causing excessive local velocity currents. This unit was originally developed for use in the sewage Irentment field. A number of units are now in use for water treatment, brine purification, and t.rade waste treatment. Flocculation periods generally employed are 30 to 40 minutes; settling tank overflow rates range from 600-1200 gallons per square foot per 24 hours (3.3 to 6.6 feet per hour), depending on the material being handled and the results desired. The Multifeed clarifier was developed for the cane sugar indust,ry. This unit is a multitray settling unit with a special flocculating compartment at the top. The design of the flocculator mechanism is similar to that used in the Clarifloeculator. The feed is transferred into the sedimentation compartments from the flocculation zone at relatively low velocity. The over-all benefits derived from this unit as well as the Clariflocculator are increased capacity per unit of floor space and lower overflow turbidities. The various sludge-blanket type of clarification units were originally developed for water softening by the lime or lime-soda process. The common features of all units are the introduction 0’ the feed into the mixing or coagulating zone where chemicals ari added in the presence of large amounts of sludge, and upwar1 “filtration” of the treated liquor through a sludge blanket. The differ in the arrangement of flocculating and settling zones and i mechanism design. In addition to water treatment, sludg blanket type units are now in operation for clarification of whi water from paper mills, production of magnesia from sea watc and brine purification. In water softening they are common operated at overflow rates up to 2700 gallons per square foot 24 hours (15 feet per hour), and thus effect a considerable s a e in ground area and installation cost. Their operation, honcx is more deliciitc than the older conventional low-rate units.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 38, Nc
A. J.
Fischer of The Dorr Company, N e w York, was born in Philadelphia In 1902 and graduated from the University of Pennsylvania with a B.S. in chemical engineering in 1 9 2 4 . H e then became a junior chemist a t the N e w Jersey Sewage Investigations Laboratory. In 1 9 2 5 he resumed studies a t Rutgers University on a Dorr Sewage Research Fellowship. H e received his M.S. in 1 9 2 6 and Ph.D. in 1 9 2 8 . Since 1928 he has been in charge of sanitary engineering development for The Dorr Company except for June-September, 1945, when he was on a Technical Industrial Intelligence Committee mission in Germany for the Government. Fischer is an associate member of the American Society of Civil Engineers. H e has been granted twenty-eight United States patents involving flocculation, sedimentation, digestion, and filtration. H e is author of thirty technical articles on sewage treatment.
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Rarely has it been necessary to design continuous sedimentation devices for operation under pressur industry it is conventional t o use pressure separators, but these. devices are almost exclusively used for liquid-liquid separations. However, a problem was presented early in the aviation gasoline program which involved the removal of micron sizes of catalyst from a continuous flow of oil at 400-500" F. This was accomplished by modifying the conventional multicompartment thickener and auxiliaries t o make it suitable for oxygen-free pressure operation. Feed stock from the fractionating tower at the rate of 900-2500 barrels per day and containing 15-80 pounds catalyst per barrel was introduced through a closed, compartmented, feed box which distributed the flow equally t o all thickener compartments. Clarified oil, usually containing from 0.2-1.0 pound catalyst per barrel was collected in a closed overflow box for transfer t o the succeeding operation. Recovered catalyst in slurry form, containing from 125-400 pounds catalyst per barrel, was removed continuously and also transferred to the succeeding step. Pressures were equalized by suitable vents, interconnected with a related operation. The original work was done by the Standard Oil Development Company, and the idea was used widely in Fluid catalytic cracking plants. A novel design of multicompartment sedimentation unit is the Trebler clarifier which has been used commercially in connection with milk waste treatment by means of trickling filters. The unit consists of a standard clarifier separated into two compartments by a vertical partition that extends from above liquid level t o the mechanism rake arms. One compartment is used for primary settling and the other for secondary settling. Thus, one tank and mechanism-for example, 40 feet in diameter -will serve the same purpose as two '28-foot-diameter units, resulting in a lowering of plant installation costs. This type of unit is applicable only where a liquid is settled in two stages and where the initial settling step is followed by coagulation or oxidation ahead of final clarification. Short-circuiting of raw feed under the partial vertical partition may be prevented by hydraulically causing a backflow from the secondary t o the primary compartment. Many studies have been made in recent years on rqethods of improving feed distribution and overflow take-off in order t o minimize short-circuiting. I n most laboratory studies of feed and overflow arrangements, average detention time as measured and computed from dye and salt tests has been taken as the criterion of settling efficiency. Large scale tests have shown, however, that such is not the case, and that elaborate feed distribution and baffling means have little or no effect on actual solids removal. The most exhaustive studies of the behavior of circular centerfeed settling tanks were carried out on 126-foot-diameter units a t the Sanitary District of Chicago. These tests showed that the only worth-while improvement was when the overflow launder was moved 12 feet in from the tank periphery. Similar tests run on cross-flow tanks, either rectangular or circular in plan, January, 1946
have indicated that feed stilling wells and directional vanes are of some advantage, especially in wide tanks. As in the case of circular tanks, moving the overflow launders back from the discharge end of the tank has also been advantageous. A departure from circular-tank conventional feed and overflow arrangements is used in the Spiraflow clarifier. I n this unit the feed is introduced peripherally near the tank bottom and overflowed a t the surface into a circular launder near the tank center. Equal feed distribution is obtained by introducing the influent tangentially a t a relatively high velocity into the circumferential feed well. A radical departure from standard settling practice has been recently introduced in the sanitary and chemical fields where solid particles having a specific gravity close to 1.0 are caused to float at reduced pressure after pre-aeration. The flotation of the solids is caused by the attachment of minute air bubbles to the solids particles during aeration and their tendency to rise as the pressure is lowered. The floated solids are removed by a skimming device, and the clarified liquor is drawn o f f from a point below the liquid surface. The Vacuator as used in sewage and trade waste treatmept, and the Pedersen and Adka Savealls as used in paper mill white-water recovery are specific examples of units based on this principle. The use of the Vacuator has recently been extended to the separation of high- from low-gravity solids in the sewage treatment field. In this application the settled sludge consisting of grit and organic matter is delivered t o a classifier where the lighter organics are separated out and recycled t o the aerator preceding the Vacuator. I n this way a three-product separation giving relatively clean grit, scum, and clarified effluent is obtained. I n the' metallurgical field it is difficult t o define sharply those developments of the past year or so which belong strictly t o concentration and those which belong to hydraulic classification. There is considerable overlapping. Development and commercial work on a large scale have taken place on float-and-sink methods, using heavy-density slurry mediums. However, such developments are not considered as falling within the scope of this 'review. A new device known as the selective-media concentrator was first placed under field development in 1944 on the Iron Range in Minnesota, and it may prove useful in other fields. Briefly, it consists of a rotating, ribbed construction cone surrounded by a stationary conical shell with the annular space between the cones available for holding a relatively high-density slurry of the material being treated. The high-density slurry is formed and held in place by the cone speed and a restricted lower opening a t a point near the apex of the cone, The base of both cones is at the top, and feed in dilute slurry form is introduced into the annular space. The finer and lower-density particles which cannot pass through the selective-media bed are rejected a t the top about 270 from the point of feeding. The coarser high-density particles (Continued on page 81) are discharged through the restricted O
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(25) Reavell, J. A,, Ind. Chemist, io, 119-30 (1944). (26) Rubush, J. P., and Seavoy, G. E., Paper Trade J.,116, No. 24, 25-30 (1943). (27) Shvavskf, V., Chem. Zentr., 1941, 11, 123. (28) Schambra, W. P., Trans. A m . Zmt. Chem. Engrs., 41, 35-51 (1945). (29) Schiebl, K., Centr. Zuckerznd., 50, 63-4 (1942). (30) Seavoy, G. P., Pulp Paper Mag. Can., 45, 455-7 (1944). (31) Semple, D. M., Trans. Znst. Chem. Engrs. (London), Chem. En&. Group, Glasgow Sect., advance copy, March 26, 1943. (32) Staub, S., Intern. Sugar J., 45, 40-3 (1943). (33) Waeser, B., Chem. Tech., 16, 252-4 (1943). (34) Webre, A. L., Intern. Sugar J., 47, 65-8 (1945). (35) Weiss, J. M . , Heat Engineering, 19, 110-15 (1944). (36) Wulfinghoff, M. F. A., Southern Power and Znd., 62, No. 2, 107-12, NO. 4, 100-4 (1944). (37) Zieder, J. G., Sugar, 38, No. 7, 22-4, 27, No. 8, 22-4 (1943).
SEDIMENTATION AND HYDRAULIC CLASSIFICATION CONTINUED FROM P A m
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opening against a controlled flow of hydraulic water into a dewatering device. Operating data are not yet available for publication. Also in the field of classification, the Bird continuous centrifugal has been further proved during the past few years. The advantages of centrifugal classification are best realized when particle size cuts are made in the micron range, particularly finer than 20 microns. For example, a considerable tonnage of titanium dioxide and lithopone is classified in continuous centrifugals. In the case of titanium pigment, the slurry is fed a t ball mill consistency or with limited dilution, and the fine fraction which is 100% -5 microns is discharged at about 20% solids. The coarse fraction is returned t o the ball mill in the conventional manner. An operation of this type permits direct filtration of the slurry comprising the fine fraction. Other materials such as clay, calcium carbonate, and other coating or filling materials are similarly classified. The continuous centrifugal has been used in the same manner in the cement industry for closed-circuit grinding; i t produces a thick slurry suitable for direct blending and calcination. I n this instance the slurry is ground t o 85% -200 mesh. I n the cement industry it has also been used t o classify cement slurry into two fractions, the cuts being made from 10 to 30 microns, and the adjustment in performance is controlled by variable-speed motor drive. Approximately one million tons of solids have been processed in two units, 54 inches in diameter and 70 inches long during the past two years. A still newer use for centrifugal classification is the desliming (rejection of 2-3 micron particles) of the slurry feed t o froth flotation cells. Another application is in the separation of magnesium from water softening sludge where the sludge is burned for reuse in the system. During the 1944 season on the Iron Range there was an interesting development in hydraulic classification. The mining companies were pressing to ship the maximum tonnage of ore to the blast furnaces. Washing (concentrating) plant tailings were examined for the purpose of determining how t o recover additional tonnage of blast-furnace grade quickly and economically. In one typical case the tailings from a washing plant at -35 mesh contained about 22-23% iron and 63-65% silicon and represented about one third of the crude ore treated. ‘ Normal concentrate from this plant contained about 61% iron and 9% silicon. January, 1946
A tailings retredtment plant was developed and proved; it involved a three-step operation: (1) dewatering and desliming the tailings, (2) hindered sizing of the deslimed tailings, and (3) dewatering the concentrates produced by selected pockets of the Dorrco sizer. Briefly, this sizing device consists of a number of rectangular pockets, each fitted with a perforated bottom through which is introduced a controlled volume of hydraulic water for sizing. Feed is introduced into pocket No. I, and all particles which will settle through the controlled water velocity are automatically discharged at the bottom. Particles carried over by the hydraulic water pass t o succeeding pockets with diminishing upward velocities. The operation is controlled by pressureregulating instruments set in accordance with the particle sizing desired. I n hydraulic sizing of iron ore and silica gangue, it has been established that a silica particle of a certain size and an iron ore particle 2-2.8 meshes finer will overflow a t substantially the same rate. The commercial significance of this fact is shown by the following figures which represent plant performance over a 24hour period: Feed
No. 1 No. 2 No. 3 No. 4 No. 5 No. 6 No. 7 No. 8
% Fe 41.83 60.02 62.82 63.15 61.45 62.18 60.56 59.11 32.78 %Si 36.42 7.63 6.90 6.53 8 64 8 12 10.28 12.54 48.76
Products from pockets 1 to 7, inclusive, were combined to produce a shipping grade of concentrate containing 61.45% iron and 8.92% silicon. The weight recovery for the washing plant aa a whole was increased from 66.6 t o 71%, representing ore which otherwise would not have been delivered t o the blast furnace during the war. Hydraulic classification was also used in a war industry for grading abrasives in the micron size range. Beginning in 1942 and continuing through the war, some grades of abrasives for lens and crystal grinding became so scarce t h a t it was necessary t o recover the abrasive grains by the same method used on iron ore tailings. There may be other developments along the lines covered in this review, but any omission is due t o the fact that they have not been brought t o our attention or have not been available for publication.
HIGH TEMPERATURE DISTILLATION CONTINUED FROM P A Q E 9
are charged into the retort. The open end is then sealed, and a vacuum of 0.05 t o 0.15 mm. mercury is applied. Eight hours later the vacuum is broken, the lid opened, and a “crown” of magnesium is removed in the condenser sleeve. A new sleeve is inserted and the retort is ready for recharging. From 200 pounds of charge, about 37 pounds of magnesium may be recovered. Some magnesium oxide does distill over with the magnesium metal, and this is removed by fluxing during molding. An interesting side light on this operation is the technique of extending the life of the retorts by “reblowing”. At temperatures in this range the steel in the retorts flows under the differential pressure in the furnace proper and inside the retort. It has been found possible, when a retort collapsed, t o apply pressure t o the inside of the retort and thus cause i t to resume a shape suitable for continued operation. I n this manner the useful life of retorts has been extended from a few days t o well over a year,
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