Figure 1. Processes for making space-age metals may b e worked out with this small multistage extractor. Each stage of the 17-stage extractor consists o f a mixer-settler tube. Here the cradle i s rotated so that all mixer-settler tubes lie in a horizontal position
A Countercurrent Liquid-Liquid Extractor A small, workable extractor for simulating production scale extraction of hafnium, zirconium, columbium, tantalum, and yttrium
A
N u h i B E R of multiple-contact extractors have been designed in the Ames Laboratory for liquid-liquid separation of inorganic substances. Some of them have been described briefly in reports on the preparation of high purity zirconium (4-6) and hafnium (2. 6 ) and the separation of niobium from tantalum (2, 6 ) and of yttrium from rare earths ( 2 ) . This report describes one of these extractors and presents data obtained with it in preparing a batch of more than 100 pounds of hafnium essentially free of zirconium. The two liquid phases in these extractors are moved countercurrentwise and are in contact in all stages of the scrub and extract sections. The extractors can be operated under steady-state conditions to give continued output. They have some features in common with the extractor developed by Craig ( 7 ) . I n his design, however, only one liquid phase is moved Lvhile the other phase remains essentially stationary. The extractor described here simulates a continuous countercurrent flow vertical extraction column and. therefore, differs in design and function from the Craig apparatus.
Design and O p e r a t i o n of Extractor
For the 17-stage extractor described. Present address, Research Department, U. S. Industrial Chemicals Co., Cincinnati, Ohio.
HARLEY A. WILHELM and RAYMOND A. FOOS' Institute for Atomic Research and Ames Laboratory, U. S. Atomic Energy Commission, Iowa State College, Ames, Iowa
each stage consists mainly of a mixersettler tube, made from a section of glass tubing about 24 inches long and 2 inches in diameter. These stages are mounted by lugs in an orderly fashion in an angle iron cradle supported by two end shafts that allow rotation on a horizontal axis (Figure 1). h'ine mixer-settler tubes are side by side in one bank, with eight tubes similarly mounted in the bank on the opposite side of the cradle and its axis of rotation. This assembly is mounted in an angle-iron framework about 3 feet wide, 3 feet deep, and 5 feet high. The larger sprocket wheel, > 'W is secured to the end shaft, 0, of the cradle. Turning the hand crank, T , on the small sprocket wheel of the bicycle-chain drive mechanism rotates the extractor stage assembly on its cradle axis, which is supported by bearings mounted on the framework. The input liquids are delivered by gravity from the three reservoirs, R: at the top to the three feeders, E, mounted over the two end shafts. By electrically controlled valves, the feeders are filled and caused to deliver measured amounts of liquids to the extractor assembly through three funnels, F , that come to positions under the feeders a t the proper point in the rotation cycle. These funnels are mounted on the end shafts of the cradle and are out of the receiving position (out of view) in Figure 1 . The liquids pass countercurrently through the extractor and the product solutions from
the extractor are delivered by gravity flow through the tubes shoMn extending from the hollow end shafts and to the two receivers, b-. Because the feeders are operated by magnetic valves actuated at certain positions of the larger sprocket lvheel. deliveries of liquids to and from the extractor are essentially automatic. All parts of the extractor from the reservoirs to the receivers that come in contact with liquids are constructed of glass with some plastic tube connections. T h e mixing, settling, separation, and flow of the two phases in the extractor of Figure 1 can be demonstrated with the aid of the tcvo-stage model (Figure 2). Assume that the mixer-settler stage,
.M, is in a horizontal position and below the axis of rotation. 0, and contains sufficient immiscible phases to fill about half of its total volume. The phases are mixed by causing them to flow7 from end to end of stage M bv an oscillatory motion of the assembly through a total This stage is held arc of roughly 40'. at a nearly horizontal position until separation of the phases is essentially complete. The unit is then rotated slowly through 90", dockwise about the axis. The position of the open end has been preof tube L inside stage adjusted so that it stands just above the interface of the light and heavy liquid phases as the mixer-settler tube of stage M comes to the vertical position. At this point in a 360' rotation cycle, the light-liquid phase has moved out of stage M through tube L to stage M ' . VOL. 51, NO. 5
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The next 90" of clockwise rotation moves the heavy-liquid phase from M through tube H to M ' , as M' approaches a horizontal position below the axis of rotation. The phases are then mixed and allowed to settle in M' as they did in M . The light and heavy liquids can then be caused to flow, in a similar manner, back to M through tubes L' and H', respectively, as the unit is rotated clockwise to complete the 360" cycle and two stages of extraction. In a multistage assembly, countercurrent flow can be provided by connecting tubes such as L and H' to the next mixer-settlers upstream and downstream from M and M ' , respectively. In the multistage extractor shown in Figure 1, the mixing, settling, separation, and flow seauence for a number of stages take place concurrently on each successive cycle during continued operation. The flow tubes in such an assembly are so connected that there is progressive movement of the heavy-liquid and light-liquid phases to opposite ends of the extractor axis as the cvcles are continued. Once in each complete cycle, portions of feed, scrub, and extract liquids are added at the proper stages and the product solutions are delivered from the end stages. The result of continued operations is a continued countercurrent movement of the two liquid phases through the extractor. All stages of the multistage assembly have essentially the form shown for the stages of Figure 2. The positions of the light-liquid take-off tubes, such as L and L' in their corresponding mixer-settler stages, are adjustable through the plastic sleeve connections to the stages. The breather tubes ( B and B ' ) are open to the outside and are curved at their open ends for returning to the stages any liquid that might get into the breather tubes. Spouts corresponding to S and S' on the mixer-settler tubes serve for special additions or removals of liquids. Figure 3 shows a feeder system supported from the extraction framework. Liquid flows by gravity from a large reservoir on top of the extractor to the metering chamber, E, through tube R and the glass valve within coil -4. When the glass bulb, float D, in the metering chamber reaches a predetermined height, the snap-action switch, C, which is in series with coil A , breaks the circuit and shuts off the flow of liquid to the feeder. Funnel F is secured to an end shaft of the cradle of the extractor by means of a
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Figure 2. A twostage model aids in demonstrating mixing, settling, separation, and flow of liquid phases in the multistage assembly
clamp, G, and the funnel spout is connected by tubing to the proper stage. When the stage assembly rotates to the position for feeding, the glass valve within coil B is activated and the metered liquid flows from the feeder to the funnel and thence to the proper stage. Later in the cycle after the valve at B closes, more liquid for delivery on the next cycle is metered into the feeder chamber. The two immiscible solvents used in the equipment presented here flow from the feeders through the funnels and to the end stages of the extractor. The materials to be separated are dissolved in a portion of one of these immiscible liquids to give the feed which flows likewise to an intermediate stage through spout P (Figure 2) or through a tee connection in one of the regular stage connecting lines. For simplicity in extractor design and operation, all input liquids enter stages in the same stage bank. Operation of the electromagnetic valves of the feeder system is timed by side segments attached to the sprocket wheel on the drive shaft. Figure 4 is a close-up view of this arrangement. Switch A is operated normally closed and B normally open for the valve circuits, and these switch connections are in series with coils A and B, respectively, of the glass valves of Figure 3. When side segment K of Figure 4 moves into position to close switch B and allow the feed to flow into the extractor, segment J holds switch A open and prevents the flow of more liquid into the feeder chamber. The feeder chamber can fill only later in the cycle after segment J releases switch A. For the system with three feeders, all corresponding valves are activated simultaneously through switch A or B. Switches A and B of Figure 4 are double-pole double-throw ; those corresponding to switch C of Figure 3 are single-pole double-throw. Some poles of these snap-action switches, not used directly in the feeding operations, are wired to lights that warn the operator of any irregularities, such as depletion of the supply of one of the solutions. At N (Figure 4) is a revolution counter which registers once on each 360' cycle as a pin on the sprocket wheel engages the arm of the counter. This pin also slides by a ratchet mechanism mounted on the frame near the sprocket wheel, thus preventing a backup and more than one feeding period per cycle.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Separation of Zirconium from Hafnium
More than 100 pounds of high-purity zirconium-free hafnium metal were needed in the research program at the Ames Laboratory. A liquid-liquid extraction purification of a hafnium salt was proposed, to supply a purified compound for reduction to the desired hafnium metal. Hafnium containing 2 to 3% zirconium, with smaller amounts of other impurities, was available as the tetrachloride in relatively large quantities. This impure hafnium compound was a by-product resulting from the thiocyanate extraction process ( 3 ) employed in the production of low-hafnium zirconium for thr atomic energy program. It was decided to investigate further the nitric acid-tributyl phosphate extraction process ( 8 ) as a means for removing the remainder of the zirconium from this hafnium. In this latter process the zirconium is preferentially extracted into the organic phase, while in the thiocyanate process the zirconium follows the aqueous phase. The work on the purification of hafnium is described here to demonstrate some possible uses of extractors of the design presented
B
F Figure 3. The feeder system delivers a measured volume of liquid to the extractor Valves within coils A and B a r e electromagnetically operated
LIQUID-LIQUID E X T R A C T O R above for experiment and for production. Samples of the hafnium tetrachloride were dissolved in water and filtered and the solution was concentrated by evaporation. Hafnium oxychloride was later crystallized from this concentrate, which was about 6M in hydrochloric acid. These crystals, which after washing with acetone were free from impurities other than zirconium, Were used in making up the aqueous feed solutions for the experimental work on the removal of the zirconium by liquid-liquid extraction. T o set up adequate operating conditions for a larger scale extraction, a number of preliminary single-stage tests !%eremade in separatory funnels. Chloride, nitric acid, and hafnium concentrations, as \%,ell as relative amounts of phases, were studied with respect to variations in distribution and separation behaviors of zirconium and hafnium in nitric acid-tributyl phosphate system. O n the basis of the single-stage data, a few countercurrent extractions were made employing 15 stages of a small extractor similar in principle of operation to the larger one described here but having one tenth the volume capacity per stage. Stagewise and other data obtained on the smaller extractor were used in setting up operating conditions for larger scale tests with the extractor of Figure 1. Fifteen stages of this larger extractor were used in the test. The aqueous feed was 1.5M in hafnium oxychloride (containing the zirconium) and 6.2M in nitric acid, the aqueous scrub was 5.8M in nitric acid, and the organic extract (solvent) consisted of 60 volume yo tributyl phosphate and 40 volume yo di-n-butyl ether. O n each cycle of operation, 200 ml. of the aqueous feed, 50 ml. of aqueous scrub, and 200 ml. of the organic extract entered the extractor at stages 7, 1, and 15, respectively. About 30 cycles of operation were required to reach essentially steady state. After 65 complete cycles, the operation was discontinued and the liquids were removed from the odd-number stages. The organic and aqueous volumes from each stage were measured and the various portions analyzed for total zirconium and hafnium (as oxide) concentration, per cent zirconium oxide in the oxide, and total acid and chloride concentrations; the nitrate concentration was determined indirectly by difference. Some of the stagewise data obtained by this experiment are shown graphically in Figures 5 and 6. I n Figure 5 the product hafnium (as oxide) that was ready for discharge in the aqueous phase at stage 15 contained only about 20 p.p.m. of zirconium oxide; the hafnium (as oxide) in the organic phase that was ready for delivery from the extractor at
Figure 4. Timing is by side segments attached t o the sprocket wheel on d riveshaft Snap-action switches A and 6 are manipulated b y roller contacts on segments J and K, respectively
stage 1 contained 34% zirconium oxide. Stagewise data on the chloride and combined hafnium and zirconium concentrations are shown graphically in Figure 6. The chloride in the organic phase had a maximum of only 0.3M, which accounts for the very low chloride concentrations in the aqueous phase of the scrub section. The above purity and concentration data for hafnium and zirconium with corresponding phasevolume data showed that at least 90% of the hafnium was recoverable from stage 15 with a purity of 99.9980/, with respect to hafnium and zirconium. The nitrate concentrations, not shown, generally varied between 5.5 and 6.5'44 for the aqueous phase and between 3 and 4M for the organic phase. Because the purity and yield of hafnium in this larger scale test extraction were considered satisfactory, work was directed toward the purification of over 100 pounds of the hafnium. The solutions were prepared and introduced into the extractor of Figure 1 essentially
as described above. except that all 17 stages \$.ere emp1o)ed. The t i t o additional stages were used in the organic extract section. More than 250 liters of aqueous feed solution were processed and approximatelv 90% of the hafnium \vas recovered xvith a zirconium content of much less than 30 p.p.m
Discussion Starting an extraction with any apparatus generally requires a systematic procedure. The above description of this equipment and its operation should enable one to set up a satisfactory program for charging the equipment, bringing the system to equilibrium, and completing an extraction. However, one principal feature of this extractor, the adjustable take-off tubes ( L and L', Figure 2), must be closely attended, especially during the first charging of the stages and while approaching equilibrium operating conditions. The position of a take-off tube for the less dense phase in a stage is determined
Figure 5. Hafnium ready for discharge in the aqueous phase contained 20 p.p.m. of zirconium oxide
VOL. 51, NO. 5
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by the position of the liquid-liquid interface in that stage. The amount of the heavier liquid in the mixer-settler tube, therefore, determines the position of adjustment of the take-off tube. If some of this heavier liquid is delivered with the less dense liquid to the next stage, the position of the interface there may be raised sufficiently to throw that stage out of adjustment. Therefore, the open end of the take-off tube for the less dense phase should always stand above the liquid-liquid interface as the tube approaches a vertical position during the phase separation part of a cycle. Consequently, a small amount of the less dense phase is intentionally delivered with the heavier phase to the next stage. In starting an extraction, the take-off tubes are generally adjusted when in a vertical position so that the open ends are well above the liquid-liquid interfaces. As the system approaches equilibrium on subsequent cycling, however, the tubes are readjusted to approach ideal phase separations more nearly. A procedure for starting and operating this equipment is presented elsewhere in more detail (7). The time interval necessary to complete one cycle of operation will depend largely on the rate at which equilibrium is approached on mixing and the rate of separation of the two phases after mixing. In the system described for the removal of zirconium from hafnium, 10 to 12 cycles were completed per hour. In terminating an experiment in which stagewise data are desired, the phases in the stages must be at equilibrium before the samples are taken, and the operation must not be stopped until after thorough mixing on the last deliveries to the stages of concern. Deliveries of most of the liquids between the stages are made at a level below the axis of rotation; deliveries of liquids from the extractor are made through the end s h a h on the axis of rotation. Therefore, intermediary holding vessels are needed to receive the deliveries from the end stages. Two
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Figure 6. Molar concentrations of chloride and combined zirconium and hafnium in aqueous and organic phases of odd-numbered stages at completion of extraction experiment
small glass vessels (not shown in the figures) are attached to stages in the bank on the opposite side of the axis from the end stages. Deliveries to these vessels take place concurrently at the corresponding flow positions for the liquids in all stages of the bank containing the end stages. Deliveries out of the extractor take place later in the cycle, when these holding vessels pass above the axis of rotation. For simplicity in operating the extractor, the small sprocket wheel to which the hand crank is attached is specially made with half as many sprockets as the large wheel. This makes each mixing, settling, and flow operation recur at the same position of the hand crank. The opposing banks of stages are about 8 inches apart and are offset endwise (Figure 2). The glass connecting lines between the stages are shaped from 10-mm. tubing. Two feeder chambers (Figures 1 and 3) are made from 75mm. and the third from 50-mm. glass tubing. The main bodies of these chambers are about 6 inches long. The product delivery tubes that pass through the end shafts are connected by swivel joints to the lines leading into the product receivers. Polyethylene and other plastic tubes join the glass parts of the assembly. A number of smaller extractors similar in principle of operation and with mixersettlers made of glass as small as 20 mm. in diameter have been constructed and operated in some experimental extractions. The practical lower limit in size of tubing used in construction of such an extractor will be determined largely by capillary interferences with proper mixing and flow of the phases. There appears to be no upper limit in size of the mixer-settlers for extractors of this design. A number of conveniences are recognized in using these extractors in experimental work. The flow ratios for the portions of input liquids can be readily adjusted and maintained at
INDUSTRIAL A N D ENGINEERING CHEMISTRY
constant known values. The extraction can be interrupted and then resumed at the operator's convenience without interfering with proper functioning of the extractor. The progress and effectiveness of the selected conditions for making a separation can be followed analytically as the number of cycles increases and steady state is approached. O n the basis of these analytical tests, the conditions of operation can be altered to meet the requirements of the extraction. At the end of an extraction experiment, samples are readily available for determining the compositions of both phases in each of the alternate stages. The stagewise data are useful in interpreting the extraction processes and in setting up conditions for other experiments and larger scale production extractions. The data on the purification of hafnium demonstrate possible uses of extractors of this design. Because manipulation of this extractor differs drastically from that of other countercurrent extractors, special care should be exercised by the novice, especially in charging and bringing the extraction to equilibrium. The large stainless steel pan shown near the bottom of the framework in Figure 1 is to catch spilled liquids. However, most operations of the equipment in purification of the larger batch of hafnium described above were made by a nontechnical helper. With extractors of the design presented here, extractions can be tested and completed on relatively small amounts of materials. This is especially convenient when the supply of material is limited or only a small amount of product is desired. Although the applications have so far involved only separations of inorganic materials, the equipment appears to have broader possibilities of application in liquid-liquid extractions.
literature Cited (1) Craig, L. C., Anal. Chem. 22, 1346 (1950). ( 2 ) Foos, R. A . , Ph.D. thesis, Iowa State College Library, 1954. (3) Shelton, S. M., Dilling, E. D., McClain, J. H., Paper P. 533, International Conference on Peaceful Uses of Atomic Energy, Geneva 1955, vol. 8, pp. 505-49, United Nations Publication. (4) Wilhelm, H. .4., U. S. Atomic Energy Comm., Rept. ISC-155 (Jan. 24, 1951). ( 5 ) Zbid., ISC-203, 21-9 (March 31, 1952). (6) Wilhelm, H. A,, Foos, R. A . , Zbzd., ISC-458 (Sept 3, 1954). (7) Zbid., ISC-989 (March 5, 1958). (8) Wilhelm, H. A., Walsh, K. A , , Kerrigan, J. V., Zbid., ISC-144, 7-14 (May 1 , 1951).
RECEIVED for review March 24, 1958 ACCEPTED January 26, 1959 Division of Industrial and Engineering Chemistry, Chemical Processes Symposium, 133rd Mreting, ACS, San Francisco, Calif., April 1958.
