A Polyethylene Box-Type Mixer-Settler Extractor
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LESTER KNAPP,’ R O M A N SCHOENHERR, J O H N BARGHUSEN, and MORTON SMUTZ Institute for Atomic Research and Department of Chemical Engineering, Iowa State College, Ames, Iowa
This mixer-settler is capable of handling liquids of varying degrees of viscosity and density difference. It is especially useful in studying multistage extraction in that, with it, it is possible to perform a stagewise evaluation of the system
A having individual
extractor interface control in each stage was developed a t Ames Laboratory. The actual design was based on extractors originally made by Whatley ( 2 ) and Bochinski ( 7). The extractor is a horizontal contactor constructed in the form of a rectangular box, partitioned into discrete stages which, in turn, are divided into two chambers-an antechamber and a settling chamber. A mixing chamber is suspended in the antechamber from an overhead clamp. Each mixing chamber is provided with an impeller which enters through the top of the chamber. Each impeller is connected by a belt and pulleys to a shaft which drives all the stages simultaneously. The drive shaft is driven by a belt from an electric motor. Liquid flow through the extractor is countercurrent. T h e heavy phase from 1 and the light phase from stage n stage n - 1 flow through underflow and overflow ports into the antechamber of stage n. I n the antechamber the heavy phase rises to the level of the inlet weir to the mixing chamber. The light phase accumulates as a film on the surface BOX-TYPE
MIXER-SETTLER
of the heavy phase. The two phases flow simultaneously through the inlet weir into the mixing chamber of stage n. I n the mixing chamber, by the action of the impeller, the two phases are mixed and then pumped through an overflow arm into the settling chamber of stage n. Here the phases separate by gravity and then flow through the appropriate ports into the adjoining stages. The interface between the two liquid phases in the settling chamber of stage n 1 is maintained at any desired height by adjusting the position of the inlet weir to the mixing chamber of stage n. The position of the interface depends not only upon the densities of the individual phases but also upon the difference in the densities. The more nearly identical the densities of the two phases, the more critical the position on the inlet weir becomes. A system with a
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density difference of 0.03 gram per ml. could be handled only if the position of the inlet weir could be set accurately to inch. A deviation of l1/32 inch would produce a 1-inch change in the level of the interface. The mechanism for holding the mixing chamber was designed so that, by a simple adjustment, the height of the inlet weir could be positioned accurately in the antechamber. Consequently, every stage has its own interface control. If only the densities of the two phases and the depth of the settling chamber are known, the height of the interface in each stage can be preset by adjusting the position of the mixing chamber in the adjacent stage. Construction of Mixer-Settler
Several separation processes recently developed at the Ames Laboratory in-
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n-@ REFLRENOE
PLANE
Present address, Aluminum Co. of America, New Kensington, Pa.
/STAGE n + i / STAGE n
LEGEND
The extractor i s a horizontal contactor in the form of a rectangular box
I. MIXER m m E , I. x 1.5. 2 MIXER TUBE, 55’ DEEP, 1.2’ D. I ORGANIC OVERFLOW FROM PRECEDING SETTLER 4. INNER RESERVOIR 5 ORGANIC LAYER 6 INTERFACE BETWEEN PHASES 2 SETTLING CHAMBER, 9” M E P , 3’ 0 8. AOUEOUS LAYER S EXIT TUBE FOR AOUEOUS LAYER IO. OVERFLOW FROU SETTLER TO FOLLOWING STAGE II. INTERFACE CONTROL WEIR 12. OVERFLOW DELIVERY ARM TO ADJOINING SETTLER
Flow through the box-type mixersettler extractor is countercurrent
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The position of the interface depends on the densities of the individual phases and the difference in densities h. Total height of liquid in settling chamber
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of stage n 1 hi. Height of interface in stoge n 1 h,. Height of inlet weir to mixing chamber of stage n pn. Density of heavy phase p ~ Density . of light phase h i p , = h,pn (h - hi) PL hi - hwpn-hPL Pn - PL 1 . Mixing chamber 2. Stirrer 3. Inlet weir to mixing chamber
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VOL. 51, NO. 5
M A Y 1959
639
The mixer assembly includes a polyethylene drinking cup, a saran plate for mounting, a stainless steel shaft, a cover, a polyethylene impeller, and a pulley (not shown)
The boxes are made from polyethylene sheets
volved multistage liquid-liquid extraction. Therefore, one 16-stage and one 56stage mixer-settler extractor were constructed to carry cut these operations. Because the processes involved the use of corrosive chemicals, both units were made from polyethylene and stainless steel. The boxes were constructed from polyethylene sheets. The sides and bottoms were cut in one piece which extended the full length of the box. The partitions were cut to size and the underflow and overflow ports cut into them. Slots were milled at all joints to provide a semiforce fit. After the boxes were assembled, the joints were reinforced and made water-tight by welding them xrith molten polyethylene. The mixing chamber was made from a polyethylene drinking cup. The inlet weir and overflow arm were made from polyethylene tubing, welded to the cup. The impeller assembly consisted of a saran plate for mounting it into the tube holder, a stainless steel bearing, a polyethylene-coated stainless steel shaft, a cover, a polyethylene impeller, and a pulley. The shaft was force-fitted into the bearing and coated by shrinking
polyethylene tubing over it. A4fter the cover for the cup was added, the impeller was welded to the polyethylene coating on the shaft. The bearing was then forcefitted into the saran plate. T o assemble the mixer, the mixing chamber was tightly fitted into the hole in the saran tube holder, the impeller assembly was mounted to the tube holder with bolts and spacers, the cup lid was snapped on, and the pulley was attached. The assembled holder was attached to a stainless steel frame which extended the length of the extractor. The holder can be positioned to any desired height by adjusting the bolts which affix the holder to the frame. Therefore, the height of the inlet weir can be positioned and the interface controlled by adjusting these bolts. The drive mechanism for the impellers consisted of a system of belts and pulleys. Each impeller was driven by a belt from a single drive shaft, powered by a V-belt from an electric motor. Because the 56-stage extractor could not be fabricated in the form of one box and still retain some degree of portability, four boxes, each containing 14
Cost for 56-Stage Mixer-Settler Material Labor
Item Mixer-settler box and table Polyethylene Angle iron table Steel frame
Mixing holder assembly Bearings, shafts, and spacers Mixing cups Saran tube holders Saran bearing plates Bolts and nuts Drive assembly Main drive shaft Universal joints Drive pulleys from motor Neoprene O-ring belts V-drive belt Aluminum pulleys Supports Total cost
$ 84.24
31.60 74.62 190.46 214.93 5.50 190.00 21.00 159.41 590.84 16.75 26.10 2.00 21.81 1.27 57.70 36.00 161.63 $942.93
$114.76 92.25 132.75 339.76 174.28
174,28
Total r$
199.00 123.85 207.37 530.22 389.21 5.50 190.00 21.00 159.41 765.12
419.63
16.75 26.10 2.00 21.81 1.27 477.33 36.00 581.26
$933.67
$1,876.60
419.63
A 56-stage mixer-settler extractor i s used to process a mixture of rare earth nitrates
stages, were used. No piping or tubing was used to transport the liquids from one box to the next. Instead, the end stages opposite each other were used only to pump the liquids from one box to the next. One mixer pumped the heavy phase into the settling chamber of the end stage in the next box, while the mixer of that stage pumped the light phase into the settling chamber contiguous to the first mixer.
Cost of Construction -4lthough this cost analysis is rather detailed, extrapolation to other units could not be made with accuracy. Considerable labor cost has been omitted from the table-i.e., for construction of the boxes and final assembling of the mixer-settler. If these were included, the total cost would be approximately $2500.
Conclusions This box-type mixer-settler has the following distinguishing characteristics: The position of the interface in each stage can be adjusted independently and while the extractor is in operation. No auxiliary means are necessary to transport the phases through the extractor. The necessary head is provided by the mixer-impeller. The design is compact; no interstage piping is needed. Feed lines can be introduced at any stage without revising the design or construction. Therefore, any number of stages are immediately available for any process. The unit is simple in design and easily constructed. literature Cited
(1) Bochinski, J. H., Ph.D. thesis, Iowa State College, Ames, Iowa, 1954. ( 2 ) Whatley, M. E., Ph.D. thesis, Iowa State College, Ames, Iowa, 1953.
RECEIVED for review July 14, 1958 ACCEPTEDJanuary 14, 1959
Contribution 71 1. Work performed in Ames Laboratory of the U. S. Atomic Energy Commission.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
