Technology
Processes recover fluoride from phosphate New systems for recovering fluorides, problem compounds in phosphate rock processing, provide marketable products, lessen environmental worries
1979 Washington, D.C Phosphate rock, the raw material for making phosphoric acid and other phosphates of commerce, typically contains 3 to 4% fluorine, chemically combined as fluorides. Those fluorides present technical, economic, and environmental problems for phosphate processors. At a symposium on fluorine chemistry in the fertilizer industry, organized by the Division of Fertilizer Chemistry, several possible solutions to the "fluoride problem" were presented. The toxic fluorides naturally must be removed from phosphate products intended for use in foods and feeds. In current practice, phosphate fertilizers are not defluorinated; however, there's growing concern that fluoride buildup in soils eventually could adversely affect soil fertility. In addition, volatile fluorine compounds released during processing must be contained to prevent air pollution. Nevertheless, many fluorine compounds are useful substances. Large amounts of the mineral fluorspar, for example, are used as a fluxing agent in steelmaking. Significant quantities of fluorides are used in the aluminum industry and in making fluoropolymers. At present, the U.S. imports most of its fluorides, mostly in the form of fluorspar from Mexico and South Africa. At the same time, most of the fluorine contained in phosphate rocks now goes to waste. One approach to the fluoride problem, described by Erhart K. Drechsel of Pennzoil Chemical, is called SAFE II. SAFE is an acronym for sequential acidulation fluoride elimination. SAFE II is an outgrowth
of an earlier process designed to pro- retical 10% saving in sulfuric acid, duce potassium phosphates. It rep- compared to the conventional proresents a considerable departure from cess. In practice, Drechsel says, the the traditional method of making acid savings likely will be even greater phosphoric acid, in which phosphate as acid producers turn to lower grades rock is acidulated with sulfuric acid. of phosphate rock. With conventional In the SAFE II process, the rock is technology, he explains, lower-quality first solubilized with recycled phos- rock requires higher sulfate levels to phoric acid to produce monocalcium produce filterable gypsum; this isn't phosphate (in solution) and a sludge the case with SAFE II. The monocalcium phosphate containing iron and aluminum oxides (R2O3) and insoluble phosphates crystallization step serves several (P2O5). The R2O3/P2O5 sludge is useful purposes. With it, the wetthickened and filtered. Clean gyp- process acid manufacturer is in a posum, produced at a later stage of the sition to produce any quality of process, may be added as a filter aid phosphates he chooses. "Although he may not require and to adjust the phosphate content to yield a 0-24-0 superphosphate food-grade phosphoric acid to proproduct. The filtrate is recycled to the duce industrial or animal feed phosphates," Drechsel points out, "he now primary reactor. The clarified monocalcium phos- has that prerogative." Also, the phate/phosphoric acid overflow so- quality of raw material is less imporlution also contains the bulk of the tant; processing of matrix might even rock's fluorine, in the form of calcium be feasible. fluosilicate. Monopotassium phosThe new process also could faciliphate is added to precipitate the flu- tate the recovery of uranium, another oride as potassium fluosilicate . The constituent of phosphate rock, mixture is thickened and decanted Drechsel says. With conventional and the underflow component is practices, organic matter in the treated with steam. The potassium phosphate rock reacts with strong fluosilicate is hydrolyzed, in the sulfuric acid to produce detergentlike presence of monocalcium phosphate, compounds that interfere with the to produce synthetic fluorspar phase separation of uranium ion. (CaFVSi02) and regenerate the po- Phosphoric acid doesn't attack these tassium monophosphate. The hy- organics; they are removed with the drolysis products are centrifuged to R2O3/P2O5 sludge. recover the synthetic fluorspar; the The economics of the SAFE II liquid portion is recycled to the fluo- process still "require clarification," ride precipitator. Drechsel says that Drechsel says. He notes that an incritics might debate the utility of the dependent consulting firm has been synthetic fluorspar; however, there engaged to make a technical and appear to be no real problems in economic appraisal and that "preupgrading it to any required liminary indications are favorable." quality. Although the new acidulation The clarified monocalcium phos- process appears to have its merits, phate/phosphoric acid overflow from there won't be any rush to convert the fluoride precipitator is cooled to existing phosphoric acid plants. But yield relatively pure crystals of mo- according to Agrico Chemical's P. S. nocalcium phosphate (0-60-0), which O'Neill, there still are opportunities are separated by filtration. Only then for increasing fluoride recovery in is the remaining dissolved monocal- conventional plants. Agrico, for excium phosphate treated with sulfuric ample, is working on a method of reacid to form more phosphoric acid covering calcium fluoride from cool(which is recycled to the primary re- ing pond water. O'Neill notes that about 48% of the actor) and gypsum. According to Drechsel, the SAFE II fluorine in phosphate rock is retained sequence is almost ideally suited for in product acid and by-product gypphosphoric acid manufacture. The sum and is not recoverable in current system is essentially isothermal. It practice. The other 52% is vaporized yields relatively clean products and as silicon tetrafluoride and hydrogen coproducts. Also, it affords a theo- fluoride and recovered, as fluosilicic Sept. 24, 1979 C&EN
37
SAFE II process facilitates fluoride recovery Phosphate rock
Iron and aluminum oxides (R 2 0 3 ) sludge removal loop
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acid, in scrubber water, which also is used for cooling. The contaminated water is re-used after cooling in a storage pond. Its fluorine content remains at a steady level of about 1%, mainly as a result of precipitation. Many phosphoric acid plants employ fluorine recovery units, which, using recirculated fluosilicic acid as scrubbing liquid, recover about 15% of the rock's fluorine in the form of an almost pure 20 to 25% solution of fluosilicic acid. That material has commercial value, O'Neill points out; it finds use in aluminum smelting, in water fluoridation, and in making fluoride salts. But the remaining fluorine is trapped in the recirculating pond water. It has no value and it isn't recovered. 38
C&EN Sept. 24, 1979
Underflow
Separation (filter)
Gypsum
Nevertheless, O'Neill adds, calcium fluoride is attractive to a phosphoric acid manufacturer as a by-product— if its production doesn't interfere with plant operation, and if it's economically attractive, independent of environmental or other benefits. In the Agrico process, pond water is fed, along with ground calcium carbonate, to a "main reactor." The fluorine contained in the pond water is precipitated as calcium fluoride. About 80% of the fluorine can be removed before substantial phosphate precipitation occurs, O'Neill says. If the pond water contains more than 0.4% sulfate, it must be pretreated with calcium carbonate, in a separate mixing tank, to remove the excess sulfate as gypsum.
Next comes an acid wash, in which the calcium fluoride-rich sludge is resuspended with fresh pond water containing a small quantity of fluosilicic acid. The resulting slurry is thickened and the solids-rich fraction is sent to a sulfate wash unit. There the sludge is treated with a recycled alkaline solution. The sulfate dissolves, giving a final solid product having at most 0.3% sulfur (dry basis). Details of this wash step are proprietary. Small quantities of an alkali such as caustic soda or ammonia are consumed. However, O'Neill notes, regeneration of the spent washings with hydrated lime keeps that cost to a minimum. The Agrico project is in the smallpilot-plant stage; some 500 lb of product has been made. According to O'Neill, a 1000 ton-per-day P 2 0 5 plant could yield 30,000 to 40,000 tons per year of 75% calcium fluoride. Product testing is under way. The synthetic fluorspar contains more sulfur and phosphorus than the natural product used in steelmaking. But in pilot tests in a basic oxygen furnace, O'Neill says, the product had good fluxing properties and there was no significant increase in the phosphorus and sulfur levels of the steel. The Agrico process is quite sensitive to the size of the calcium carbonate particles used, O'Neill notes. With larger particles, sulfate in the solution coats them, and as much as 10% of the dried solid phase is unreacted carbonate. The difficulty can be overcome by using finely ground calcium carbonate. In contrast, another fluoride recovery process, developed by geologist William C. Warneke and associates at Borden Chemical's SmithDouglass division, uses relatively large particles of calcium carbonate or magnesium hydroxide, which react with aqueous hydrofluoric acid or ammonium fluoride from the phosphoric acid plant's scrubber system. The reaction takes place in a countercurrent reactor. Because of a phenomenon known as pseudomorphic replacement, the carbonate or hydroxyl anions are replaced by fluoride anions, but the particles retain their original sizes and shapes. The phenomenon, although not well understood, is well known; examples found in nature include petrified wood and silicified oyster shells and coral. The advantage of the process, Warneke notes, is that it produces a high-grade granular calcium fluoride product that can be pumped, handled, and dried easily, with essentially no dust or drying problems. In addition, it removes from solution fluorides that might otherwise cause disposal problems. D
Computer model for chemical process design
1979 Washington, D.C The era of the computer has permitted long-awaited attempts to organize better the often bewildering routes through the maze of organic syntheses. With more or less success, numerous researchers engaged in organic syntheses have produced highly computerized systems that combine organic chemistry, information retrieval, data manipulation, processing conditions, and the like. However, there have been very few nonproprietary attempts to pursue the virtues of the computer beyond the realm of the research chemist into the purely industrial area of the chemical engineer. Such an attempt is now under way. It was described by R. B. Agnihotri of the department of chemical engineering at Washington University, St. Louis, at a Division of Industrial & Engineering Chemistry symposium on computer applications to chemical engineering process design and simulation. Of the systems developed for computer-aided organic synthesis, nearly all are either direct-associative or logic-centered. There also are combinations of the two approaches. In the direct-associative approach, the chemist relies on experience to manipulate compounds in reaction sequences that have proved effective in the past. By its nature this is empirical and, even if it has been quite fruitful in the past, it also is somewhat haphazard. Logic-centered approaches depend on generation of sets of chemical intermediates that correspond to the branches of a decision tree leading to the target molecule. Each branch of the tree corresponds to a potential alternative route in the synthesis of the target molecule. According to Agnihotri, the computer-aided systems that have been devised usually are keyed to familiar, discrete laboratory procedures, which may be effective for pure research purposes but also can be unrealistic when dealing with reacting systems of industrial importance. The new system devised at Washington University is still in a formative stage but it aims to favor industrial considerations. The core of the system under development is a set of
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programs collectively labeled Chem ical Engineering Investigation of Reaction Paths, or CHIRP. In devising CHIRP, great use was made of a number of mathematical models of constitutional chemistry proposed by several investigators over the years. One hallmark of the models is the concept of isomerism of mo lecular ensembles. The right and left sides of the conventional reaction equation, for example, represent two different ensemble isomers from the same family of ensembles. From this point of view, a chemical reaction is a transformation of ensembles. The ensembles are represented in CHIRP symbology by bond-electron matrices. If there are η atoms in an ensemble, the corresponding matrix would be an η Χ η matrix that depicts the chemical constitution of the en semble. Off-diagonal entries in the matrix represent covalent bonds be tween atoms. For example, the entry by represents the covalent bond(s) between atoms Aj and Aj. The diago nal entry represents the number of free, unshared valence electrons of Ai. In the CHIRP system, any reaction can be regarded as the difference be tween two matrices. The difference is, identically, the reaction matrix R. Entries of the R-matrix indicate which bonds are made and/or broken
as well as the changes in distribution of the free electrons. An R-matrix represents a class of reactions that is independent of the individual react ing system. The general approach in using the CHIRP system is to operate (manip ulate and rearrange bonds) between atoms in a reactant set under the di rection of a preselected R-matrix. The resulting product matrix is then sorted to yield product molecules that are subsequently checked against a compound catalog. Enough flexibility is included to accommodate new product molecules that are not in the catalog. A limited number of R-matrices can generate a great number of products. There are further restric tions built into the system to avoid the product mixture's getting out of hand. At present, the reaction types included in the system include hy drogénation, cracking, oxygenation, alkylation, hydration, halogenation, dehydrogenation, dehydration, and dehydrohalogenation. New types can be added when desired. A series of reactions that may constitute a total synthesis with the production of intermediates is handled by the successive application of the appropriate R matrices. Thermodynamic feasibility, as measured by Gibbs free energy, is the criterion
CHIRP uses electron-bond matrices to study reaction paths Decomposition of α-hydroxy acetonitrile into formaldehyde and hydrogen cyanide can be represented by the conventional notation... C2H3ON = CH 2 0 + HCN . . . or by the bond notation...
:
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H-C-CSN:
• H^-C^O: + H-C=N:
© I ® 0 ® H
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. . . in which the circled numbers identify the atoms and are pot stoichiometric subscripts, or by CHIRP matrix notation, in which the axis numbers refer to the individual atoms as above 1
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40
C&EN Sept. 24, 1979
for selection of product sets that are preferred in multistep syntheses. In the future, as CHIRP develops, Agnihotri expects that manipulations of the matrices will reflect the presence or absence of catalysts, deliberate and intrinsic. This possibly may provide insights into the nature of catalysts, particularly in-situ, homogeneous catalysts that might be the controlling step in a sequence. D
Ultrafiltration poised for new uses
mk 1979 ^ S i ^ Washington, D.C. Like most new technologies, ultrafiltration has not achieved all the expectations of its founders 15 years ago. Even so, the promise remains great, and the performance of the past decade and a half has been better than some suggest. One of the more sober assessments of ultrafiltration's current status was provided for a Division of Colloid & Surface Chemistry symposium on ultrafiltration membranes and applications by one of ultrafiltration's pioneers, chemical engineering professor Alan S. Michaels of Stanford University. The beginnings of practical ultrafiltration are traced by Michaels to a collaboration between Dorr-Oliver Co. and Amicon Corp. in the early 1960's. The objective then was development of an economical process for large-scale removal of colloidal and macromolecular impurities from secondary sewage effluent. That objective is still elusive, but ultrafiltration did succeed elsewhere. The first big success for the technique was connected with the electrocoat process for electrophoretic deposition of colloidal, resinous particles in aqueous dispersions on metal surfaces under the influence of an impressed dc potential. This process produces uniform, coherent, and defect-free coatings very rapidly. However, the economic benefits depended on the concurrent development of a reliable way of recovering paint colloids economically from rinsings of electrocoated parts and removing soluble impurities and corrosion products from the plating baths. This application has only the most primitive requirements, which were easily met by ultrafiltration. Another important industrial application is purification and concen-
tration of biological macromolecules such as vaccines, peptide hormones, and plasma proteins. In pharmaceu tical manufacture the value of such products is so high on a unit basis that the capital and operating costs of separation and purification are minor compared with the ultimate yield of the products. Ultrafiltration mini mizes losses of labile biologicals due to denaturation or decomposition and is the preferred means of operation for biologicals production. An early potential application that is re-emerging after suffering from some materials problems is the re covery and concentration of protein values from cheese whey. Interest originally stemmed from the desire to eliminate a pollution problem and to capitalize on the sale of milk proteins as food supplements and processed food components. One of the prob lems has been that whey proteins are notorious membrane foulers. Another has been that required sanitation procedures are destructive to the membranes. Michaels believes that much of the renewed optimism for ultrafiltration in this application comes from the development of "superrefractory" membranes that can survive contact with active chlorine and steam, and with new membrane configurations such as hollow-fiber bundles, which minimize fouling. In the case of whey ultrafiltration, the principal interests still are pollu tion abatement and resale value of recovered proteins. But there also is interest in production of fuel-grade ethanol by direct fermentation of lactose. The return from whey pro cessing has been barely enough to justify the investment but this may change with the rising cost of energy for thermal processing and with in creasing demand for ethanol. Another industrially important new application is production of ul trapure water for use in electronics and pharmaceuticals manufacture, as well as in the preparation of sterile fluids for medical use. The sole re quirement here is for removal of trace concentrations of colloidal or macromolecular debris. The absolute con centrations are so low that virtually full water throughput can be realized at low operating pressures and can be sustained for prolonged periods. Finally there is the use of ultrafil tration membranes in artificial kid neys, which Michaels believes are on the verge of commercialization. Some of the membranes appear to be better at removing toxic metabolic wastes than are conventional hemodialyzers without the membranes, and the membranes are more effective in controlling chronic uremia. Original estimates for the ultrafil-
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Sept. 24, 1979 C&EN
41
tration membrane market put the value at $30 million to $50 million by the mid-1970's. These estimates have not been met. The reasons offered by Michaels are persistent and inherent limitations in membrane and module design. A serious limitation is depression of the permeation flux by solute polar ization at the upstream membrane surface. This reduces hydraulic per meability, relative to the pure solvent rate, a factor of 10, and it requires a corresponding increase in membrane area to compensate. Even more serious is the problem of membrane fouling, which is inde pendent of solute concentration in the feed and independent of the hydro dynamics of the system. Fouling has been ascribed to such things as pro gressive pore plugging and the grad ual development of a gelatinous sol ute layer on upstream surfaces. Such fouling is highly unpredictable and often irreversible. Growing recogni tion of the importance of colloidal and surface phenomena in membrane fouling has directed development toward membrane modification as a way to overcome the phenomenon. Some emerging new ultrafiltration technologies are in the areas of immunochemistry, where the volumes may be low but the values are high, and removal of particulates in aque ous suspensions, where the volumes are high and values sometimes low. One particular area where high vol umes of materials must be handled is hydrometallurgy, where selective separation and concentration of trace metals is desired. The desired metal ion-containing solution is treated with a water-solu ble polymer, which chelates or com plexes with the desired ion, thereby forming a metal-rich complex. This complex can be removed by ultrafil tration to yield a metal-rich concen trate, which is then treated to de couple the metal from the polymer. The resulting solution is subjected to a second ultrafiltration, yielding a metal-free polymer concentrate that can be recycled and a metal-rich so lution that can be treated conven tionally to remove metal values. Some people are skeptical about some of the suggested new applica tions for ultrafiltration, but Michaels retains his optimism. One application in particular that gives rise to his op timism is the proposed use of asym metric, hollow-fiber bundles for the continuous culture of mammalian cells. By inoculating viable mamma lian cells on the shell side of the bun dle and passing appropriately con stituted nutrient solutions on the tube side it is possible to nourish and maintain the cells properly. If the 42
C&EN Sept. 24, 1979
cells synthesize and secrete a biolog ical product, then it becomes possible to produce the product contin uously. An application similar to that de scribed by Michaels is under investi gation by Clark K. Colton of Massa chusetts Institute of Technology's chemical engineering department, in collaboration with Barry Solomon of Amicon Corp., Pierre Galletti of Brown University, and William Chick of Harvard. The aim is to produce an implantable, artificial pancreas con sisting of beta-cell cultures on the outside surface of tubular membranes that are permeable to glucose and insulin but retain antibodies and lymphocytes. The device would be implanted in the cardiovascular system as an ar teriovenous shunt so that the cells have access to oxygenated blood. This
eliminates need for the immunosup pression that is required in ordinary transplants. The approach also offers the possibility of using animal beta cells in place of human tissue, and it relies on natural biological feedback for insulin release and control of blood concentration. The first results, reported by Col ton at the symposium, indicate that the membrane does constitute an ef fective means for restoring carbohy drate tolerance in experimental dia betes in rats and dogs. There are some compatibility problems, in particular with respect to blood clotting. Clot ting appears, however, to be reduced by larger membrane tubes. At the same time, larger tubes mean thicker walls with correspondingly higher diffusional resistances. All these in fluences must be optimized in any successful system. D
N 2 0 enhances electron capture detection
1979 Washington, D.C.
reacts with electrons (emitted by the detector's radioactive foil elements) as follows: e" + N 2 0 -> Ο" + N 2 0 " + N 2 0 — N O " + NO
A little nitrous oxide greatly increases the utility of electron capture detec tors in gas chromatography, chemis try professor Robert E. Sievers of the University of Colorado, Boulder, told members of the Division of Analytical Chemistry. Sievers notes that the electron capture detectors are quite sensitive to substances that readily capture electrons, such as halogen- and nitro-containing organic compounds. But they are quite insensitive to nonelectrophilic compounds, in cluding hydrogen, carbon dioxide, and hydrocarbons. That situation can be altered by adding a few parts per million of ni trous oxide to the nitrogen used as a carrier gas, Sievers says. He dubs the technique selective electron-capture sensitization (SECS). With SECS, Sievers says, it's pos sible to detect picogram levels of hy drocarbons and to increase the sen sitivity to certain compounds by several orders of magnitude. With carbon dioxide, for example, sensi tivity is enhanced by a factor of about 5000; that's useful, because flame ionization detectors also are insensi tive to carbon dioxide. For methane, peaks are about 10 times larger with nitrous oxide added than without. For hydrogen, the increase is about 40fold at about 350° C. The sensitization occurs, Sievers explains, because the nitrous oxide
N O " + N 2 — NO + N 2 + e" In the absence of other reactants, there is no net loss of electrons in that sequence; there would be no appre ciable loss of electron density in the detector unless something else was also going on. However, Sievers points out, O" and N O " are quite reactive. For example, O" reacts readily with methane, ethane, propane, butane, carbon dioxide, hydrogen, and many others. Apparently, O" is the species that is destroyed, interrupting the regeneration of electrons, decreasing free electron density, and producing the detector signal. As might be expected, adding ni trous oxide doesn't improve sensi tivity for electron capture detection of compounds that have high electron affinities. In fact, the response is sometimes diminished. And because of a competing "associative detach ment" reaction, hydrogen actually gives a negative response at lower temperatures, and carbon monoxide responds negatively at all detector temperatures. Much of the SECS research was done by graduate student Michael P. Phillips, Sievers notes. Others partic ipating in the work include Fred C. Febsenfeld, Paul D. Golden, and William C. Kuster, all of the National Oceanic & Atmospheric Administra tion's Environmental Research Lab oratories at Boulder. D
