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Cyanuric Acid Technology John A Cyanuric Acid Technology John A. Wojtowicz Chemcon Cyanuric acid was identified as a chemical Cyanuric Acid >99% substance over two centuries ago. However, it was not until the late 1950’s that it attained industrial Ammelide and Ammeline ≤0.5% significance with the introduction of chlorinated Water ~0.1%* isocyanurates by Monsanto and FMC. Although the pH of saturated solution at 25°C 4.5-5.0 majority of cyanuric acid production is used in the manufacture of chlorinated isocyanurates, some of it *Water content will vary with manufacturer; some is also used as a swimming pool available chlorine products can contain up to 4% water. stabilizer. Cyanuric acid is also used in the manu- facture of specialty intermediates used in the pro- Table 1 – Typical Analysis of duction of plastics and coatings. The properties, chemistry, uses, etc. of cyanuric acid and chloroiso- Commercial Cyanuric Acid cyanurates have been comprehensively reviewed (Wojtowicz 1993a and 1993b). This paper discusses cally referred to as isocyanuric acid whereas the the structure, properties, analysis, chemistry, manu- enol form is simply cyanuric acid. Both forms are facture, and uses of cyanuric acid. collectively referred to as cyanuric acid. Properties Physical Typical Analysis Cyanuric acid is a white crystalline solid. Some of its physical properties are summarized in Typical analysis of commercial cyanuric acid Table 2. The aqueous solubility data for CA in Table (CA) is shown in Table 1. The product also typically 2 refers to distilled water, consequently these solu- contains small amounts of ammonium sulfate or tions have a pH around 4.8. A plot of the data is ammonium nitrate (<0.2%). shown in Figure 2. The extrapolated solubility of CA in distilled water at 0ºC is 525 ppm. The solubility Structure OH O H H Cyanuric acid can exist in either of two N N structures as shown in Figure 1. The keto form N N predominates in the solid whereas the enol form predominates in solution. The keto form is techni- HO N OH O N O H Originally appeared in the enol keto Journal of the Swimming Pool and Spa Industry Cyanuric Acid Isocyanuric Acid Volume 4, Number 2, pages 9–16 Copyright © 2001 by JSPSI Figure 1 – Cyanuric Acid All rights of reproduction in any form reserved. Structures 80 The Chemistry and Treatment of Swimming Pool and Spa Water at pool pH would be much higher because of ioniza- Ionization in Aqueous Solution tion to cyanurate, which amounts to 73 to 91 % over Cyanuric acid (H3Cy, where Cy represents the the normal pool range of 7.2 – 7.8. triisocyanurate anion) is a weak acid, even weaker than carbonic acid. The pH of a saturated solution Sublimation is only 4.8. Ionization forms hydrogen and cyanu- Solid cyanuric acid does not melt, instead it rate ions as shown below: undergoes sublimation, i.e., it volatilizes directly + – from the solid to the gaseous state. Significant H3Cy + H2O H + H2Cy sublimation commences at about 350°C as shown by the reaction below, where (HNCO)3 is a linear Effect on Swimming Pool Water representation of the tricyclic formula of CA. Alkalinity Addition of cyanuric acid will lower the pH of (HNCO) (solid) (HNCO) (gas) 3 3 pool water. When the pH is restored to the normal range, total alkalinity will be increased, due to Dissociation in the Gaseous State formation of cyanurate ions. Cyanuric acid is 73– Heating gaseous cyanuric acid above 400°C 91% neutralized at pool pH. Total alkalinity is causes dissociation into isocyanic acid as shown corrected for cyanurate alkalinity by the following below. equation to yield carbonate alkalinity (Wojtowicz 1995): (HNCO)3 (gas) 3HNCO (gas) AlkCORR = AlkT – 1/3•CA Solid Forms Granular, powder Toxicity Aq. Solubility @ 25°C 0.15% a Cyanuric acid is essentially non–toxic to hu- mans, animals, wild–life, and aquatic life. The toxi- “ “ “ 50°C 0.70% a cology of cyanuric acid and chlorinated isocyanurates “ “ “ 90°C 2.6% a have been reviewed (Hammond et al 1986). The b “ “ “ 150°C 10.0% acute toxicity (oral LD50) is >10 g/Kg for rabbits. Density (anhydrous) 1.75 g/mL “ (dihydrate) 1.66 g/mL Biodegradability Melting Point Does not melt up Cyanuric acid undergoes biodegradation by some soil bacteria (Bagnall et al 1984). to 350°C a) Nelson 1967 Analysis of Cyanuric Acid b) Zagranichnyi and Polyakova 1963 Solid Cyanuric Acid Table 2 – Some Physical Solid cyanuric acid dissolved in water can be Properties of Cyanuric Acid titrated with sodium hydroxide solution using a Figure 2 – Cyanuric Acid Solubility John A. Wojtowicz – Chapter 5.1 81 recording pH meter; the equivalence point is at (i.e., heating) urea (see Figure 3). A molten or a about pH 8.8–9.0. Ammelide and ammeline are also concentrated aqueous solution of urea is sprayed titrated (Wojtowicz 1974). Ammelide and ammeline onto a moving bed of crude cyanuric acid granules in do not interfere if the titration is performed in a a directly heated rotary kiln. The crude cyanuric dimethyl sulfoxide (DMSO)–benzene mixed solvent acid is maintained at about 250°C for about one using tetrabutyl ammonium hydroxide instead of hour. About 90% of the crude cyanuric acid exiting sodium hydroxide (Morales 1968). Ammelide and the kiln is recycled to the front of the kiln. Urea ammeline can be determined by gas chromatogra- dissociates into isocyanic acid and ammonia in the phy after dissolving cyanuric acid in kiln. The isocyanic acid trimerizes to cyanuric acid. trimethylsilylacetamide (Wojtowicz 1974). 3H2NCONH2 ® 3HNCO + 3NH3 In Swimming Pool Water 3HNCO ® (HNCO) By Test Kit – The swimming pool test kit 3 analysis is based on the turbidity caused by precipi- The ammonia offgas from the kiln is scrubbed in the tation of cyanuric acid as melamine cyanurate. urea feed solution to remove small amounts of Since melamine cyanurate has a small solubility volatilized urea and cyanuric acid and can either be (5–10 ppm), the method is not suitable for very low flared (i.e., burned) or recovered as ammonium cyanuric acid concentrations (<10 ppm). The accu- nitrate by reaction with nitric acid for use as fertil- racy of the test kit method is ±10%. The reaction is izer. shown below. Byproduct Formation (HNCO) + (H NCN) ® (HNCO) •(H NCN) 3 2 3 3 2 3 Pyrolysis of urea also produces undesirable cyanuric acid + melamine melamine cyanurate aminotriazine byproducts which can form by reac- tion of ammonia with isocyanic acid to yield cyana- Other Methods – Dilute aqueous solutions of mide. cyanuric acid can also be analyzed by liquid chro- matography (Downes et al 1984), ion chromatogra- HNCO + NH3 ® H2NCN + H2O phy (Wojtowicz 1985), uv spectrophotometry (Downes et al 1984), and differential pulse polarog- Cyanamide can react with isocyanic acid or with raphy (Struys and Wolfs 1987). These methods are itself to produce ammelide (Ad), ammeline (An), or more accurate than swimming pool test kits. melamine (Mm). The formulas for Ad, An, and Mm shown below are linear representations for the structural formulas shown in Figure 4. Crude cya- Manufacture of Cyanuric Acid nuric acid usually contains 80% cyanuric acid, 18% ammelide, 2% ammeline, and <0.1% melamine. Pyrolysis Of Urea Cyanuric acid is manufactured by pyrolyzing 2HNCO + H2NCN ® (HNCO)2(H2NCN) Ad NH H O Acid H O 3 Vent Gases 2 2 Amino Urea Pyrolysis CA Urea Triazine Scrubber Kiln Filtration Hydrolysis Wet CA Purge to Dryer Acid Figure 3 – Process Flow Diagram for Manufacture of Cyanuric Acid 82 The Chemistry and Treatment of Swimming Pool and Spa Water O NH2 NH2 H H H N N N N N N H2N N O H2N N O H2N N NH2 Ammelide Ammeline Melamine Figure 4 – Structures of Aminotriazines HNCO + 2H2NCN ® (HNCO)(H2NCN)2 An Neutralization (Salt Formation) On reaction with sodium hydroxide, cyanuric 3H NCN ® (H NCN) Mm acid forms a series of sodium salts, i.e., mono–, di–, 2 2 3 and trisodium cyanurate as shown by the reactions below, where Cy represents the triisocyanurate 3– Hydrolysis of Aminotriazines anion, i.e., (CNO)3 . The crude cyanuric from the kiln is pulverized to a powder, slurried in dilute sulfuric (or nitric) H Cy + NaOH ® H NaCy + H O acid, and heated at the boiling point for about one 3 2 2 hour. This hydrolyzes the aminotriazines to cyanu- ric acid as shown below. H2NaCy + NaOH ® HNa2Cy + H2O + + HNa Cy + NaOH ® Na Cy + H O Ammelide + H2O + H ® CA + NH4 2 3 2 + + Ammeline + 2H2O + 2H ® CA + 2NH4 The solubilities of these salts are greater than that of CA, e.g., the solubilities of mono–, di–, and triso- dium cyanurates are: 0.9, 5.7, and 14.1 g/100 mL of Melamine + 3H O + 3H+ ® CA + 3NH + 2 4 solution at 25°C. Recovery of Purified Cyanuric Acid Chlorination The purified cyanuric acid crystals from the hydrolysis step are filtered, washed with water to Preparation of Dichlor – Chlorination of an remove the digestion acid, and flash dried. The dry aqueous slurry of disodium cyanurate produces powder is compacted and granulated. dichloroisocyanuric acid. HNa Cy + 2Cl ® HCl Cy + 2NaCl Reactions of Cyanuric Acid 2 2 2 Hydrate Formation The dichloroisocyanuric acid is recovered by filtra- tion and is washed with water to remove the Cyanuric acid forms a dihydrate on reaction byproduct sodium chloride. It is reslurried in water with water. and reacted with sodium hydroxide to produce so- dium dichloroisocyanurate (Dichlor). (HNCO)3 + 2H2O ® (HNCO)3•2H2O HCl2Cy + NaOH ® NaCl2Cy + H2O The dihydrate effloresces, i.e., it loses water on exposure to air of low to moderate humidity. When The sodium dichloroisocyanurate crystals are heated to 57°C, the dihydrate undergoes a transi- isolated by filtration.
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