dicating little need to condense the feed urea phosphate further, Mr. Sher idan says. Some biuret forms during the con densation and subsequently may pre cipitate, limiting the grade of liquid fertilizer that can be made. The amount of biuret varies with pyrolysis .conditions and reaches a maximum concentration with a polyphosphate chain length of 2.7. Adding monoam monium phosphate (MAP) to the urea phosphate before pyrolysis reduces by product biuret, but the rate of pyroly sis decreases. Adding ammonia to the reactor during pyrolysis also reduces biuret formation. With MAP or ammo nia, the biuret can be limited enough to allow making solutions equivalent to 11-37-0 (as N, P 2 0 5 , and K 2 0). Although polyphosphates have spe cial properties such as increasing the solubility of ammonium phosphates and sequestering metallic impurities, which help to make high-analysis sus pension fertilizers, they are relatively expensive, Thomas N. Jones of TVA points out. With polyphosphates mak ing up at least half of the phosphate content of a fertilizer, a base suspen sion grade of 12-40-0 will have satisfac tory flow characteristics. If the phos phate is all phosphoric acid, the grade limit runs 10-30-0 to 11-33-0, thereby increasing cost of handling the fertiliz er because of the extra water present. A way around this problem is a pro cess now being tested in a 20 ton-perhour demonstration plant that TVA is operating. In this process, which will give a grade of 11-39-0—nearly as high a grade as obtained with superphosphoric acid containing polyphosphates —the phosphoric acid is ammoniated in two stages followed by a clay mixing step. About 75% of the ammonia is added in the first stage, Mr. Jones says. No cooling is provided for this stage—tem perature is about 225° F., allowing the contents to boil, an inexpensive way to dispose of the heat of reaction. The relatively high temperature aids crystallization of the metallic impuri ties as precipitates of the metal, am monia, phosphate, and fluorine. Other
critical variables for large crystals, in contrast to amorphous hydrated gel like materials that give high viscosi ties, are a pH of 4 in the first amraoniating stage, a product concentration of 39% P 2 0 5 , a ratio of Ν to P 2 0 5 of 0.28, and an atomic ratio of fluorine to the sum of aluminum, iron, and mag nesium of 1.2. Temperature in the second stage of ammoniation ranges between 120° and 160° F., with cooling provided by evap oration from air sparging. Retention time in the first stage is 30 to 60 min utes; in the second, 15 to 30 minutes. Storage testing of 11-39-0 suspensions shows little change in viscosities with time, according to Mr. Jones and his associate, John O. Getsinger. The vis cosity ranges between 500 and 800 centipoises. Suspension fertilizers in vari ous grades such as 21-7-7, 14-14-14, or 5-15-30 have all been stored for two weeks while retaining satisfactory (under 1000 centipoises) viscosities. No large crystals of more than +20 mesh developed and no crystals settled and packed on the bottoms of containers.
New ways to separate organic mixtures 168THrta MhTionhL meeTiNG Separation of organic mixtures by se lective permeation through membranes usually has foundered because of the dissolution of the membrane by the permeant. Precisely because the mem branes are usually organic, they are seldom compatible with the solvents. But this dilemma may be yielding to research being conducted at Gulf South Research Institute, New Or leans. That research and a related de velopment involving hollow fiber ion exchangers were described in a sympo sium on recent developments in mem brane processes. A number of polymeric "alloys" have been developed that can effect separa-
Two-stage ammoniation process gives relatively low-viscosity, high-analysis suspension fertilizers
Ammonia
Wet-process orthophosphoric acid (36.5% P205)
pH4 225° F. (boiling)
Air
tions of organic mixtures including a number of binary azeotropes, the insti tute's Dr. I. Cabasso told the Division of Industrial and Engineering Chemis try. Polymer "alloys" are mixtures of polymers that are compatible on the molecular level. One example of a polymer alloy is a mixture of cellulose acetate and a polyphosphonate (PPN). When formed into a sheet membrane, the alloy can be used to separate mixtures of benzene and cyclohexane with separation fac tors high enough to be of interest in in dustry. Even though benzene normally would dissolve PPN, it does not de stroy the alloy membrane. Nor does cy clohexane affect it. Two other binary azeotropes that were separated with alloy membranes were styrene/ethylbenzene and ethanol/heptane. The criteria for selecting polymers to form alloy membranes are based on Hildebrand-Hansen solubility parame^· ters. In general, a given alloy might be effective in separating mixtures if one alloy component is soluble in one mix ture component but not in the other mixture component. A related development, also from Gulf South Research Institute, involves hollow fiber ion exchangers that may be useful in removing such undesirable effluent contaminants as chromate ions from plating wastes. The institute's Dr. Elias Klein says that hollow fiber res ins were used because of the competing requirements that effective membranes must be thin and strong. No other ge ometry was found to be suitable. In one series of experiments, a crosslinkable anionic exchange monomer was made by adding 2 moles of N,Ndimethylamino ethyl methylacrylate to either 1 mole of l,4-bis(chloroethyl)benzene or 1 mole of l,4-dichloro-2butene. The monomer was permeated through the walls of a hollow polysulfone fiber and polymerized in situ. A 500-fold reduction of chromate concen tration was achieved with this mem brane. Another potentially useful ap plication of this fiber is the removal of divalent cations from boiler feedwaters or other chemical process water. Re ductions in calcium ion concentration of 10 times and in magnesium ion con centration of 7.3 times were achieved in a single pass through the fiber bundle.
Hollow fibers offer enzyme containment
Ammonia
pH6 120°-160° F.
Clay
120°-140° F.
Suspension 11-39-0 (1.5% clay)
168THrfG IWIOIW rhŒÎING A new method of immobilizing entire enzyme-coenzyme systems has been developed by a research group at Virginia Polytechnic Institute and State University under the direction of Dr. Continued on page 24 Sept. 16, 1974 C&EN
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