TECHNOLOGY
Metals recovery modification promising Solvent extraction coupled with centrifugal contacting may lead to economic recovery of several metals Metallurgical engineers are watching with interest two commercial-scale operations in which solvent extraction has been wedded with centrifugal contacting—two old chemical processing techniques. One is a vanadium plant that Union Carbide's mining and metals division brought into production last summer at Wilson Springs, Ark. The other is a rare earths recovery unit that Denison Mines is now operating at Elliot Lake, Ont., in conjunction with its uranium production facilities there. At Wilson Springs and Elliot Lake, Pod centrifugal contactors replace the usual battery of mixers and settlers that are part of hydrometallurgical liquid ion exchange processes. (Contactors that metals people are most interested in are: Podbielniak Pods, made by Baker Perkins, Inc., headquartered in Saginaw, Mich., and Quadronics, made by Chicago-based Liquid Dynamics.) Because a single contactor achieves the same results as a series of mixer-settler tanks, valuable plant space is conserved, proponents of contactors note.
They also point to other savings. For example, contactors don't need anything like the large volume of the organic liquid phase that mixer-settlers do, says Baker Perkins* Cameron Hopper, Denver-based manager of Podbielniak Pod sales to metallurgical industries. A Pod that processes 500 gallons per minute of aqueous feedstock may require 3000 gallons of the organic phase. On the other hand, as much as 75,000 gallons may be involved when mixer-settlers are used for the same system, Mr. Hopper notes. "A company that uses contactors obviously doesn't have a sizable amount of cash tied up in solvent inventory," he adds. Moreover, less organic phase is lost through entrainment and evaporation, he says. Short time. The great speed at which the organic and aqueous phases zip through a contactor cuts the time that they are in intimate contact with one another to a matter of seconds from the usual several minutes for mixer-settlers. Very short intimate contact time favors the extraction
Solvent extraction includes preferential solubility Solvent extraction is an imprecise term used to describe two different means of selectively separating chemicals. One, liquid ion exchange, is the basis of production sequences in the hydrometallurgy industry. It involves ion exchangers that take part in the transfer of ions from one liquid to another. The other, ion pair extraction, involves preferential solubility. The two processes share an important common factor, however; both involve the mixing of immiscible liquids that separate readily into discrete phases. The principle underlying liquid ion exchange is identical to that involving solid ion exchange resins, the only difference being that the ion exchanger is a liquid. Like their solid resin counterparts, liquid ion exchangers are either cation exchangers or anion exchangers, depending on their chemical makeup. The compounds are usually dissolved in an inert hydrocarbon diluent, or a carrier, such as kerosine. There are two steps involved in solvent extraction. In the first, the extraction step, the chemical to be recovered migrates to the organic phase from an aqueous solution that's usually impure and very dilute. During the stripping step that follows, the procedure is reversed and the chemical passes from the "loaded" organic phase back to a new aqueous phase. The net result of this selective separation process is conversion of a dilute solution of the chemical into a highly concentrated one that's essentially free from unwanted impurities. The organic phase, meanwhile, recycles throughout the system. Adequate mixing of the aqueous and nonaqueous phases with one another is an important requirement for the success of the solvent extraction process. Mixing and settling tanks or countercurrent gravity-flow towers are the most common means of mixing the two phases.
44 C&EN MARCH 11, 1968
of some metals, such as vanadium. This metal is recovered from ore leach liquors either as a vanadyl cation or as a vanadate anion. To recover vanadyl ions from solution, a liquid cation exchanger such as an organic phosphate is used. (Vanadate anions call for liquid anion exchangers, the most common being substituted amines.) In either case, short contact time is an advantage because this increases the selectivity of vanadium extraction by reducing chances that competing ions, present as impurities, may be pulled out of solution with the vanadium. Some metals people, though, argue that the vanadium extraction operation is hindered if contact time is too short. They point out, for example, that there are different species of vanadate anions, some of which load onto an ion exchanger at a slower rate than others. Economics dictate that these slower acting species be recovered from solution. But there's less chance of this happening if residence time is too short. This reasoning applies, too, to copper extraction with General Mills' LIX-64, a cation exchanger that's highly selective in taking copper out of acid solution (C&EN, April 17, 1967, page 6 2 ) . LIX-64 works quite well in mixer-settler units but so far hasn't proved too satisfactory in centrifugal contactors. The reason may be that the kinetics of ion exchange are too slow for the quick mixing rate encountered in contactors, muses Clifford J. Lewis, director of chemical research at the Colorado School of Mines Research Foundation in Golden, where he directs an active research and development program into the application of Podbielniak Pods to hydrometallurgy. Few details. Much of the foundation's findings are proprietary and details aren't available. Neither Carbide nor Denison Mines says much about its operations. In Arkansas, Carbide admits only to using Podbielniak Pods for recovering vanadium from ore that the company mines locally. Denison, working with a very dilute rare-earth solution, a by-product of uranium processing, uses Pods to convert the solution to a more concentrated one. The rare earths are then precipitated, and
Centrifugal contacting includes density differential Two U.S. companies make centrifugal contactors. One, Baker Perkins, Inc., Saginaw, Mich., tradenames its units Pods, a word derived from the name of Dr. Walter P. Podbielniak, who was the first to make the units commercially some 25 years ago. The other maker, Chicago-based Liquid Dynamics, calls its line of equipment Quadronics. These contactors, or to give them their proper names, liquid-liquid counter current extractors, consist of a drum with a rigid shaft running horizontally through its center. The drum rotates at rates that can go as high as 10,000 r.p.m., depending on the size of the unit. Fixed inside the drum so that they rotate with it are a number of concentric, perforated, cylindrical plates. The diameter of these plates decreases from the largest, which is slightly less than the diameter of the drum's periphery, to the smallest, which is just greater than the diameter of the central shaft. The plates extend the full width of the drum, dividing it into a number of annular spaces. The two liquids that are to be mixed together enter the contactor separately through piping within the central shaft. The liquid with the greater density discharges into an annular space between the plates at a point close to the axis. The less dense phase enters an annular space nearer the periphery. When the drum spins, the heavy liquid moves outward toward the drum's periphery, while its less dense counterpart travels inward toward the axis. Intimate mixing of the two phases takes place as they are forced through the orifices in the plates in opposite directions to one another. These zones of rapid mixing are followed by the relatively quiet annular spaces between the plates in which the two phases have a chance to settle out again. Finally, the two liquids leave the contactor by way of separate ducting within the central shaft. During their countercurrent passage across the radius of the drum, the two liquids undergo repeated mixing and settling. A single contactor takes the place of towers or mixer-settler tanks used for countercurrent solvent extraction. A process cycle involves at least two contactors. In the first, the organic liquid extracts the material to be recovered
the mixture is sold for processing into individual rare-earth compounds. Why is it that hydrometallurgists are only now becoming interested in centrifugal contactors? For one thing, the huge demand for metals within the past decade or so has put a strain on available reserves. Thus, metallurgists are being forced to find economical ways of processing feedstocks that were once considered too low in metal content to be worth troubling with. For another, application of liquid ion exchange to hydrometallurgy is a fairly recent development. Companies like Archer Daniels Midland, General Mills, Rohm and Haas, and others are working with metallurgical engineers toward developing new liquid ion exchange systems or improving ones already available. Some engineers voice concern over leakage that may occur through the mechanical seals of centrifugal contactors. "There have been cases of
from the aqueous solution. The resulting "loaded" organic phase passes on to a second, smaller unit where the dissolved material is stripped from it by a second aqueous phase. The recovered liquid organic is recycled back to the extractor. Pods and Quadronics are basically similar although the details of their internal design are different. In the case of Baker Perkins* units, for example, the spacing is the same between each of the concentric plates, while in Liquid Dynamics' units, the annular spacing increases as the radius of the plates increases. The contactors come in different sizes. The biggest Pods have a drum diameter of 60 inches and contain 60 plates; they can cope with a total liquid throughput of more than 600 gallons per minute. Baker Perkins also makes a small bench-top unit that processes a fraction of a gallon per minute. Prices are from a high of about $80,000 for the large Pods to about $7500 for the bench-scale model. Baker Perkins indicates that it's entirely feasible to make contactors with capacities of 5000 or 10,000 gallons per minute where specific applications warrant the design effort. Liquid Dynamics says it has a unit that can process 2000 gallons per minute of the combined streams. Although the company hasn't sold any Quadronics of this size as yet, they carry a price tag of about $150,000. Two companies make centrifugal contactors that are built around a vertical shaft. These are the Lurgi-Westfalia units made in West Germany, and the DeLavals made in Sweden. Hydrometallurgical engineers haven't evaluated the vertical shaft machines to any great extent because the maximum throughput capability of the models now available isn't great enough to cope adequately with very large volumes of liquids. Centrifugal contactors came into their own during the mid-1940's when penicillin makers found that they could reduce the time involved in the preparative extraction steps and thereby markedly improve the yield of the labile antibiotic. The contactors are now used fairly extensively in a wide variety of industrial operations, including petroleum and oil processing and chemicals manufacturing.
leakage early in the development of the contactors," Cameron Hopper admits, "but improved techniques of sealing the machines have overcome this defect." Liquid Dynamics' Dr. Collin Doyle adds, "leakage at the seals isn't a problem when they are properly designed." Other critics cite cases where the machines become plugged with particulate matter. A filtration step isn't always an answer to this problem because there are some systems in which formation of solids is an inherent part of the operational sequence. When this happens, the units must be cleaned out periodically. Strong position. "There's little doubt in my mind that centrifugal contactors will someday find a strong position in liquid ion exchange processes used in hydrometallurgy," ventures Mr. Lewis, whose group is looking into the application of the machines in recovery of a number of
metals. "As is the case with all development work, the units must be studied carefully for each specific application." The availability of benchscale models that operate with throughput of a few gallons per minute should result in more application of the contactors into research and development programs, he believes. Hydrometallurgy isn't the only field that stands to gain from the wedding of solvent extraction with centrifugal contacting. For example, the union may lead to an economically attractive route for water desalination. Typical of the approach that several research groups throughout the country are taking along this line is that of Dr. Richard R. Davison at Texas A&M University's chemical engineering department. H e plans to hook up a Quadronic contactor to an experimental system that makes potable water from brackish water which contains solids of 5000 to 10,000 p.p.m. Key MARCH 11, 1968 C&EN 45
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to the process is the selective solubility of water in various amines such as triethylamine and diisopropylamine. "Water is extracted from the salts rather than vice versa," Dr. Davison points out. He recovers fresh water simply by heating the extract, be cause a temperature increase of 30° F. greatly reduces the solubility of water in the solvent. A final steam-stripping step rids the water of any entrained solvent. One advantage of using centrifugal contactors, Dr. Davison notes, is that the solubility equilibrium between the two phases is reached just minutes af ter the units start up. "Moreover, sol vent loss due to evaporation will be cut down, as will the size of solvent inventory required for the mixer-set tler units we use now," he adds.
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46 C&EN MARCH 11, 1968
For $45,000, an analyst can now have a computer data processing system for mass spectrometers. The low-priced system is being offered by Picker Corp., White Plains, N.Y., for use with ΑΕΙ (Associated Electrical In dustries of England) spectrometers it sells or with any others. Two of the systems are now in op eration—one at Argonne National Lab oratory and the other at ΑΕΙ. A third is due to go in shortly at Wyeth Lab oratories. Basically, the system consists of an interface and the computer, a PDP-8 manufactured by Digital Equipment Corp., Maynard, Mass. A complete software program library is provided. It was developed and written specifi cally for Picker by Applied Data Re search, Princeton, N.J. Picker says that it settled on the small, dedicated computer approach only after it had completely investi gated alternative routes to computer
data handling. It ruled out timeshare operation for now because the high data rate from high-resolution mass spectrometers makes its feasibil ity questionable. Digitizing of data for later use on a large computer turned out to be more expensive than the small computer approach. A big advantage of using the com puter is the speed with which the machine can handle spectrum analysis. The mass spectrometer can produce a spectrum in 10 to 40 seconds. But it takes an inordinate amount of time to measure masses manually to obtain the data so that elemental composi tion of ions can be determined using tables. With the computer, an ana lyst can evaluate the data immedi ately. The computer program operates in several phases. The first—data col lection—takes sample data from the spectrometer and prints run statistics, such as number of peaks, base peak, intensity, and the like. It also pro vides a digital spectrum listing so ref erence peaks can be identified and data collection quality can be imme diately verified. In the next phase, the operator is required to identify, from the digital spectrum listing, three consecutive peaks of the reference compound and enter their respective nominal mass numbers through a teletype terminal. Interpolation then proceeds automati cally. The final phase prints out measured mass, logarithmic peak intensity, and peak width (number of samples taken). A composition is determined for each measured mass within limits specified by the operator. The limits include an error tolerance and a basic element content of C, 13 C, Η, Ν, and O. In addition, up to three heteroatoms can be specified and included as part of the composition determina tion. No limit is placed on the num ber of atoms of each element or iso tope selected.
IMMEDIATE DATA. Mass spec trometer (center and left) "weighs" ions to an accuracy of 10 p.p.m. Picker Corp. systems' in terface equip ment feeds this information into computer (right) where it is trans lated into atomic compositions and printed out within seconds
