SODIUM FLUOSILICATEA Neglected CLIFFORD
A.
IIAMPEL,
Chemical
Armour Research Foundation of Illinois Institute of Teclinology, Chicago, 111.
Easily available, s o d i u m fluosilicate has m a n y properties which s h o u l d m a k e its a p p l i c a t i o n attractive i n a h o s t o f p r o c e s s e s a n d p r o d u c t s . . . T o d a t e o n l y m i n o r a p p r e c i a t i o n o f its p o t e n t i a l i ties h a s h e e n e v i d e n c e d b y b o t h r e s e a r c h a n d i n d u s t r i a l w o r k e r s JL HK great forward strides taken by the chemical industry have had one unfavorable result: at times, the rapid progress in the development of new chemicals and novel processes lias caused the older, or more common, agents to be overlooked and neglected. A pertinent example is the indifference exhibited toward sodium fluosilicate, a well-known, cheap, and easily available product. This chemical has many properties which should make its application attractive in a host of processes and products, but only minor appreciation of its potentialities has been evidenced by both research and industrial workers. Mellor, for instance, devotes about two thirds of a page to the literature concerning it, as of 1930 {11). In this paper a description of the sources, properties, and present and potential uses of sodium fluosilicate will be presented, with the object of calling attention to the many possible applications for this compound. The data presented were obtained during a yearlong survey sponsored by Blockson Chemical Co., Joliet, 111., to develop new uses for sodium fluosilicate. Phosphate Rock Chief Source Almost all of the sodium fluosilicate produced today is a by-product of the phosphate industry and is derived from the calcium fluoride and fluorapatite in phosphate rock. The fluorine is converted in part to silicon tetrafluoride during the acid processing of the rock. Gaseous silicon tetrafluoride reacts readily with water to form hydrofluosilicic acid, H2SiFe, which can be changed to the sodium salt by reaction with sodium hydroxide, sodium carbonate, or sodium chloride. A variable portion of the fluorine content of the phosphate rock evolves as silicon tetrafluoride when the rock is acidified, depending upon the processing techniques, and only a minor part of the silicon tetrafluoride formed is recovered and transformed to hydrofluosilicic acid, or its salts. Domestic rock carries from 3 to 4% fluorine; between 11 and 42% of the fluorine is volatilized in the phosphate plants in this country. 2420
Thus, from 10 to 30 pounds of fluorine are evolved per ton of rock treated. At most plants, the silicon tetrafluoride is allowed to escape to the atmosphere or is absorbed in water and the solution run to waste (4)· A minor commercial source of hydrofluosilicic acid and its salts is the production of hydrofluoric acid, in which the silica content of the fluorspar, CaF«, used as a raw material is converted to hydrofluosilicic acid by the reaction: 3CaF 2 + Si0 2 + 3II 2 S0 4 ^ 3CaS0 4 + H 2 SiF 6 + 2H 2 0 (1) Additionally, whenever hydrofluoric acid is allowed to react with materials containing silica, hydrofluosilic acid or a salt is formed. For example, the silica in the alumina treated with hydrofluoric acid for the preparation of synthetic cryolite is changed to a fluosilicate. Potential Production Greater Than Consumption Of the three sources mentioned above, the phosphate rock treating plants offer the greatest potential production. Currently, about 4 million tons of rock are treated annually to produce phosphate fertilizers, and another million tons enter into the production of phosphate chemicals, phosphoric acid, and phosphorus. If 20 pounds of fluorine per ton of rock treated were converted to sodium fluosilicate, some 82,000 tons of this salt would be produced annually. Domestic production of sodium fluosilicate is much less than this figure, and the apparent current consumption is about 10,000 tons per year. These data reveal that the demand for this salt is only a fraction of the potential production and also shows why the phosphate fertilizer plants have to dispose of silicon tetrafluoride and hydrofluosilicic acid as wastes. At the present time, it is uneconomical to convert these chemicals to hydrofluoric acid or some other fluorine compound. However, as the high grade fluorspar becomes scarcer, with resultant increase in cost, phosphate rock as a source of fluorine products will become more attractive. CHEMICAL
Although the price of sodium fluosilicate has fluctuated to some degree during the past few years, it is now fairly stable at about 4 cents per pound. The large potential supply from the fertilizer plants effectively prevents an appreciable price rise and at the same time offers a reservoir capable of meeting almost any future demand which may arise. Physical Properties Sodium fluosilicate is a crystalline, nonhygroscopic salt characterized by its rather low solubility in water. Although it is odorless, it has an acid taste. No hydrates of it are known. Its molecular weight is 188.05 and its density is 2.679. The salt crystallizes in the pseudohexagonal (orthorhombic) system as prismatic crystals of which the indexes of refraction are: Ng, 1.3125 and Np, 1.3089. Like other fluosilicates, sodium fluosilicate decomposes at elevated temperatures and, therefore, has no true melting point. The decomposition begins at about 500°C. according to the reaction: Na*SiF6 >· 2NaF + SiF 4 (2) The solubility of sodium fluosilicate in water is the lowest of those of the common sodium compounds, ranging from 0.76 grams per 100 grams of water at 25°C. to 2.45 at 100°C. Solubility data are given in Table I. The density of saturated solutions is low. Worthington and Haring (16) report a value of 1.0054 at 20°C. In the presence of ammonium sulfate the solubility of sodium fluosilicate is increased greatly. Previously unpubTable I·
Solubility of Sodium Fluosilicate in Water
Temperature ° C. 0 10 16 17 17.5 20 21 25
Grams NasSiFe Grams Na2SiFe per 100 cc. per 100 grams solution 1 H2O2 0.435 0.542 0.637 0.62 0.65 0.733 0.69 0.762
35 0.940 40 1.05 45 1.120 50 1.293 55 1.328 60 1.49 7S 1.822 80 1.95 100 2.45 1 Data from Carter (S). 2 Data from Cheplevetski and Bol'ts (S), Nikolaev et al. (12), Rees and Hudleston (14), Stolba (see S), and Worthington and Waring (16).
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Table II. Solubility of Na 2 SiF e (NHOoSO* Solutions at 30° C. (NH02SCU °7o by weight 0
in
Table III. Sodium Fluosilicate Screen Analyses, Apparent Density, and Par ticle Size Range Regular —40 mesh 100% —60 mesh 91% —100 mesh 85% Apparent density—72 lb. per cubic foot Particle size range— 3 to 150μ
Na 2 SiFe % by weight 0.85
Fluffy —200 mesh 99.5 4 H + -4- 6 F " + S1O2
(3)
Λ two-step mechanism is offered by Rees and Hudleston (14), a slow re action : SiF e > SiF< + 2 F (4) followed by the relatively rapid reaction : SiF4 + 3H2G > 4HF + H 2 Si0 3 (5) A different silica product is mentioned by Jacobson (a), who favors the follow ing neutralization reaction: Na2SiFe 4- 4NaOH
> 6NaF + H4S1O4 (6) Studies of the hydrolysis show that the equilibrium constant for reaction (3) is about 1 χ 10"28 at room temperature. The hydrolysis allows of the direct analytical determination of fluosilicate by titration with a standard alkaline solution. Physiological Properties Sodium fluosilicate is a toxic material although it has not been regarded as being a dangerous poison in the past. VOLUME
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NO.
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» » AUGUST
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than any other factor related to this compound. As far as killing insects is concerned, sodium fluosilicate is one of the most generally effective agents known. Unfortunately, its acid reaction has been found to harm plants acting as insect hosts, and as a result the treat ment of insects on plants with it has been practiced but little. It has found application as a grasshopper poison, in the form of a mixture with bran which is spread on fields exposed to grasshopper attack, and as a mothproofing agent in a popular trademarked product, where its property of adhering tenaciously to wool is utilized. The chemical division of the Bureau of Foreign and Domestic Commerce reports that some 3 million pounds of sodium fluosilicate were used in 1947 for insecticidal purposes. For several years the Bureau of Entomology and Plant Quarantine has distributed be tween 700 and 900 tons per year for grasshopper control. Some sodium fluosilicate is used as a constituent in metal fluxes to protect metals like magnesium and aluminum during secondary melting. Effect on Microorganisms The high toxicity of sodium fluosilicate toward the lower organisms, such as bacteria and fungi, as well as insects, coupled with its relatively low toxicity to warm-blooded animals, suggests that it should find a host of applications as a preservative for a wide variety of products. Although the preservative properties are frequently mentioned in the literature, only limited data have been published about its effect upon specific microorganisms in comparison with that of other agents. Studies were made at Armour Re search Foundation by L. J. Vinson and S. W. Schwartzman to obtain such in formation. Using a modified penicillin antibiotic assay technique it was found that sodium fluosilicate was much more effective than sodium fluoride and mono basic sodium arsenate in inhibiting the growth of such typical microorganisms as Actinomyces gnseus, Bacillus subtilis, and Staphylococcus aureus, all three agents being compared at the same molarity. Its action upon the "dry rot" fungi, Porta incrassata and Poria monticola, which attack cellulose, was determined by inoculating samples of cellulose in sulating board with pure cultures of these fungi, the boards having been im pregnated with various concentrations of sodium fluosilicate. P. incrassata, which destroys millions of dollars worth of coniferous timber in buildings in this country each year, chiefly in the Gulf Coast and Pacific Northwest regions, was controlled by 0.1% sodium fluosilicate even though the board contained 40% water, a condition extremely favorable for fungus growth. The more resistant 2421
P. monticula, the "dry rot" fungus most frequently encountered in the cooler regions, was inhibited with 0.8% sodium fluosilicate when the board contained 40% water. At a 5% water content, 0.1% sodium fluosilicate was completely effective for both fungi. The addition of sodium fluosilicate t o cellulosic insulating boards to furnish protection against fungal and insect attack is eminently feasible, the chief limitation being the water solubility of the salt. For products not subject t o external exposure or weathering this odorless and colorless compound should be very suitable. Rodent Repellency An unexpected property of sodium fluosilicate discovered recently is rodent repellency. Wlien incorporated in food, in amounts as low as 0.5 to 1.0%, i t caused rats to refuse the food over a four-day test period. For that matter, both wild and domesticated animals and fowl find it very distasteful when present in food. This observation immediately suggests that the presence of sodium fluosilicate in paperboard cartons might create sufficient repellency to prevent attack and penetration of shipping cartons by rats. Critical studies are now being conducted to evaluate this possibility with the aid of James B. De\Vitt of the Fish and Wildlife Service at the P a tuxent Research Refuge in Maryland. The demand for such an agent is indicated by the annual loss of several hundred million dollars (some estimates place it at over $1 billion) caused b y the depredations of rats upon packaged food and other products. Warehousers assume that they will suffer an average loss of 15% of the value of stored articles due to rodent attack. For this application, the material might be coated on the outer surface of the carton in the form of a slurry in any of a number of vehicles, thereby obtaining a high local concentration and, at the same time, a water resistant coating for the sodium fluosilicate. It also might be applied in the glue lines or incorporated in the paperboard itself a t the time of manufacture of the chip layer and liner sheets. Defoliation of Cotton Plants T h e advent of the mechanical cotton picker requires the use of chemical defoliants to kill the leaves prior to t h e harvesting of the cotton bolls in order t o prevent bolls' and leaves' being stripped together. Calcium and sodium cyanamides are the chief agents now used, but Marcovitch found that the use of a mixture of sodium fluosilicate with calcium cyanamide gave enhanced efficiency (9). Preliminary trials on test plots by W. H . Tharp at the U . S . Department of Agriculture station a t
2422
Fayetteville, Ark., indicated a synergistic effect due to sodium fluosilicate, and further field tests are projected for this summer. For example, both 0.75 and 1.5% of a monosodium cyanamide gave 66% defoliation in 12 days, and 1.0% Na2SiF e gave 13%, but a combination of the two agents at the same concentrations increased the defoliation to 87%. As a weed killer, sodium fluosilicate has been given scant consideration in this country. One trial application against Bermuda grass gave indifferent results; according to L. W. Kephart of the Bureau of Plant Industry, U. S. Department of Agriculture, this is the only test made in the U. S. for weed killing purposes. However, it was used in Germany before the last war along railroad right-of-ways with considerable success in controlling weed growth. Soil Sterilization Several of the annual specialty crops, such as tobacco, require that the soil be sterilized, prior t o seed planting, to destroy microorganisms, insects, and weed seeds which subsequently might hinder the young plants' development. Heat pasteurization is commonly used, but in recent years chemicals have been applied for the purpose. Sodium fluosilicate promises to be suitable as a soil sterilizing agent. I t is known to kill termites, cutworms, and other insects, and its lethal effect upon soil microorganisms has been established. Whether or not it kills seeds has not been determined, but tests are underway to measure this effect. When mixed with recognized seed-killing chemicals, such as calcium cyanamide, it should enhance the use of these chemicals. Soil sterilization chemicals are commonly added to the soil in the fall, allowed to do their work during the winter, and are removed by the spring thaws and rains before planting time. tnsecticidai Applications Because of the plant injury caused by sodium fluosilicate, the use of the material as an insecticide is limited t o insects not living on plants, or to methods of application of such a nature as not to damage foliage. For example, it can be incorporated in baits, or it can be sprayed or dusted onto surfaces over which insects pass. I t has been observed that some injects, notably cockroaches, avoid contact with sodium fluosilicate unless it is disguised in a bait. This leads to t h e belief that the compound may have insect repellency, a property extremely useful in several fields. Articles attractive to insects could be treated by impregnation or coating with sodium fluosilicate t o discourage insect attack or passage. In 1928 Marcovitch (8) showed that sodium, fluosilicate in solution at a con-
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centration of 1 in 80,000 is fatal to Culex mosquito larvae in 24 hours. This concentration is not harmful t o humans or other warm blooded animals. It appears that a safe method of treating mosquito breeding pools has been overlooked for several years. T h e well-known effect upon the common clothes moth could be more widely utilized to achieve mothproofing in a variety of commercial woolen products, such as yard goods, clothing, drapes, furs, rugs, rug pads, and upholstery. T h e desirable concentration range is 0.5 to 1.0% and can be reached by immersing or spraying the materials with an aqueous solution of sodium fluosilicate. Weevils of many sorts, such as the vegetable weevil and the strawberry crown borer, crickets, and cutworms can be killed by the bait technique. Several household pests are susceptible to sodium fluosilicate. I t can be applied as a dust t o control cockroaches, bedbugs, silverfish, and others in much the same manner as that in which the more expensive sodium fluoride is now used, or it can be used in a bait. T h e house fly can be controlled to a great extent on farms b y treating the manure in which the flies breed with a dilute sodium fluosilicate solution. T h e residues left in the underlying soil prevent pupation of fly larvae and will not injure subsequent plant growth (10). Industrial Acidifications T h e rather high acid pH of 3.5 to 4.0 developed in solutions of the sparingly soluble sodium fluosilicate should be applicable to many industrial processes requiring an acidification step. As mentioned previously, commercial laundries now use sodium fluosilicate for this purpose. T h e textile finishing mills, which convert the rough, heavily-sized cotton yard goods into finished material, remove the starch sizing by an alkaline kiering operation, wash the cloth, and neutralize residual alkali by dipping or running the cloth into a bath containing an acid, commonly sulfuric. Extreme care must be exercised to prevent damage to the cloth b y the use of too much acid or too low a final pH. Sodium fluosilicate could easily be applied to this operation with a minimum of control and no danger of harm to the cloth. The incorporation of a small amount of sodium acid fluoride, N a H F 2 , in the sodium fluosilicate will assist in removal of iron stains picked up in the kiers. T h e compound also might be used rather simply to maintain an acid p H in the manner of a buffer by keeping an excess of undissolved sodium fluosilicate in the system; as the portion in solution is consumed, more dissolves to replace it and hold the p H at a more or less constant value. Several other potential uses for sodium (Continued on page 2456)
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Cited
Anosov, V., and Chirkov, S., J. Applied Chem. ( U S S R ) , 6, 224-7 (1933). Carter, R. H., Ind. Entj. Chcm., 22. 886-7 (1930). Chepelevetski, M., and Bol'ts, T., J. Applied Chem. ( U S S R ) , 10, 1183-93 (1937). Jacob, K. D., Marshall, H.L·.,Reynolds, D. S., and Tremarne, T. H., Ind. Eng. Chem., 34, 722-8 (1942). Jacobson, C. Α., J. Phys. Chem., 28, 506-9 (1924). Kubelka, P., and Pristoupil, V., Z. anorg. allgem. Chem., 197, 391-4 (1931). Kofoid, C. Α., et al., "Termites and Termite Control," Berkeley, Uni versity of California Press, 1934. Marcovitch, S., Tenn. Agr. Exp. Sta. Bidl. 139 (1928). Marcovitch, S., J. Econ. Entom., 38, No. 31, 395-6 (1945). Marcovitch, S., and Stanley, W. W., Tenn. Agr. Exp. Sta. Bull. 182 (1942). Mellor, J . W., "A Comprehensive Treatise on Inorganic and Theo retical Chemistry," \ r ol. V I , page 947, London, Longmans Green & Co., Ltd., 1930. Nikolaev, N., et al., J. Applied Chem. ( U S S R ) , 9, 1183-90 (1936). Parker, J . R., and Schweis, G. G., J. Econ. Entom., 37, 309-10 (1944). Rees, A. G., and Hudleston, J . J., J. Chem. Soc, 1936, 1334-8. Ryss, I. G., and Slutskaya, M. M., J. Phys. Chem. (USSR), 14, 701-7 (1940). Worthington, Κ. Κ., and Haring, M. M., Ind. Eng. Chem., Anal. Ed., 3, 7 (1931).
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