Chlorate formation

The brine feed to the electroly2ers of all the processes is usually acidified with hydrochloric acid to reduce oxygen and chlorate formation in the anolyte. Table 14 gives the specifications of the feed brines requited for the membrane and diaphragm cell process to reali2e optimal performance. 

Anhydrous hypochlorites ate oxidi2ed to chlorates by CI2O (176). The rate of chlorate formation decreases in the order Na > Ba > Si > Li > Ca. In the presence of gaseous chlorine, dry hypochlorites decompose in two ways ... 

Strontium, Li, and Ca hypochlorites react primarily by the first path and NaOCl mainly by the second. In the presence of moisture, chlorate formation is the predominate reaction in all cases. 

Calcium hypochlorite can also be prepared by reaction of soHd lime (212), quicklime (213), or CaCl2 (214) with CI2O chlorate formation is a competing side reaction. 

A competing side reaction is chlorate formation, 3 CIO — 2 Cl + CIO3, which decreases the yields of dibasic magnesium hypochlorite [11073-21-5] and of evolved chlorine. The difficult to filter slurries can be dried with Httle loss to a white, powdery soHd with av CI2 in the range of 52—59%. The yields of isolated product and recovered chlorine are typically about 40% each, while product solubiUty loss and chlorate formation amount to about 10% each. 

The conditions for chlorate formation are high pH, low reactant concentrations, and the presence of excess chlorine or hypochlorous acid. Thus, the addition of free chlorine or hypochlorite to chlorine dioxide treated water, which contains chlorite as a by-product of the chlorine dioxide treatment, predominandy forms chlorate in the pH 5—8 range typically used in water treatment (140). 

This is the reaction by which sodium chlorate is manufactured commercially. In the present context, however, the formation of chlorates is generally undesirable since they tend to be explosive and toxic. The reaction given by Equation 26.2 is strongly affected by pH and temperature as well as the hypochlorite ion concentration. As the alkali becomes exhausted, and the system is thus over-chlorinated, the rate of chlorate formation is greatly accelerated as the reaction becomes auto-catalysed by hypo-chlorous acid. The acid is formed by... 

It might be noted that, upon burial in an argillaceous matrix, the process of both glauconite and 7 8 chlorate formation is largely stopped. 

The reduction of the hypochlorite, at the cathode, and the chlorate formation at the anode,become more difficultbyincreasing the current densities at these electrodes,... 

Oxidants often tend to disproportionate by interaction of the acidic or undissociated oxidant with the anion of the same oxidant 1 this is a nucleophilic displacement, by the OX- anion, of X- from the oxygen atom of the electrophilic HOX [Eq. (11)]. Two points should be emphasized here. First, the maximum decomposition occurs when HOX and OX- are in equal concentrations, that is, when the pH of the reaction is equal to the pKa of the oxidant [Eq. (12)]. Second, since this is a displacement, the reaction goes more readily when the group X- is easily displaced thus formation of iodate from hypoiodite is more extensive than chlorate formation from hypochlorite. 

Sodium chlorate, formation of, in preparation of chlorine (I) oxide in carbon tetrachloride solution, 5 159n. 

In Eq. (18.1), sodium hypochlorite is produced by feeding chlorine into a 30% aqueous caustic solution in a circulating reactor/cooler system. To avoid sodium chlorate formation, the reaction temperature is kept below 30°C and NaOH concentration is kept below 1 g/liter. Typical reaction temperature is 5 °C132. 

The Raschig/Olin process (see Figure 18.2) is used to make anhydrous hydrazine. In this process the NaOCl production occurs at a low temperature to prevent decomposition and chlorate formation. The excess NaOH is kept to a low level132. 

From tho above relation it is evident that the chlorate formation rate is proportional to the concentration of the hypochlorite ions CcIO-, but rises with the square of the hypochlorous acid concentration cHOci- Tn order to achieve a sufficiently high reaction rate, it is necessary to introduce surplus chlorine into the hydroxide, so as to ensure a high concentration of hypochlorous acid in the solution. 

The advantage of chlorate formation by the chemical reaction as compared to the electrochemical reaction is obvious, because the process (XVII-12) proceeds without oxygen losses, which is not. the case with the reaction (XVII-11). It is, therefore, easy to understand that the more of the hypochlorite ions is converted to chlorate by the chemical reaction, the better is the current efficiency. In practice, however, the discharge of CIO- ions cannot be eliminated entirely so that current efficiencies attain 75 to 95 per cent according to the electrodes used. 

Chemical chlorate formation can be neglected by estimating reaction rates with known constants. Electrochemical chlorate formation is discussed in Sect. 7.3.3.1. As considered below, side reactions of active chlorine with disinfection by-products are mainly responsible for lowering the chlorine formation efficiency. It was found... 

In the concentration range of in-line water disinfection, the occurrence of chlorate is usually denied by cell users and producers. Only a few data exist where the presence of chlorate is mentioned (Cho et al. 2001). Our own studies, however, clearly showed, both with synthetic and real waters and both in laboratory and technical cells, a high chlorate formation potential. For example, the chlorate concentration in technical reactors using BDD or MIO anodes and real drinking water (44 ppm Cl-) was in the range of some ppm (BDD) and some hundred ppb (MIO) in single-pass operation mode. 

Fig. 7.8 Chlorate formation in discontinuous experiments using rotating Ir02/Ru02 and boron-doped diamond anode (Ir02 cathode, 50mL, 50ppm chloride, 200 Am-2, 20°C, 300rpm, pH = 7 KH2P04/Na0H buffer)...

It is not yet clear if the C102 formation is based on an electrochemical or chemical mechanism or on both. New IC results (not presented here) show that chlorite in low concentration can be present using MIO anodes. (Chlorite formation from ozone and chlorine was also reported recently - Son et al. 2005.) Thus, a peak in electrode polarisation (related to the 10-min value in Fig. 7.10) would allow HC102 to form (see also mechanism of chlorate formation). During this period the pH is... 

This concentration is usually between 0.05 and 0.2 ppm. The large variety of possible C102 consuming reactions makes the analysis difficult. The consumption of chlorine dioxide could contribute to a certain extent to chlorate formation in drinking water (Fig. 7.11). 

The concentration of virtually chloride-free sodium hydroxide in the cathode chamber is between 20 and 35% by weight, depending upon the type of membrane used. With the newest membrane types the current yield with respect to sodium hydroxide is over 97%. This non-quantitative current yield is due to the passage of hydroxide ions into the anode chamber, which causes chlorate formation. Since the brine is recycled, as with the mercury process, appropriate measures have to be taken to limit its chlorate concentration. This can be achieved by feeding in hydrogen chloride, although the pH must not be reduced too much, otherwise the membrane is damaged. 

Jacobs et al. [11] examined the data available on the decomposition of Ba(C 04)2-The ar-time curves consist of an acceleratory process followed by a discontinuous decrease in rate at a about 0.52. They proposed that the acceleratory stage is associated with chlorate formation ... 

A cell similar in principle to the diaphragm cell described above and operated without a diaphragm gives the same initial products but allows these to react with each other (Eqs. 8.24, 8.12-8.16). This is the basis of the chlorate cell. The initial electrochemical products ultimately form sodium chlorate as the final cell product from the electrolysis of aqueous sodium chloride [11]. Part of the product formation involves the chemical chlorate formation just outlined (Eqs. 8.12-8.16), and part of it forms from electrolytic chlorate formation (Eq. 8.28). 

Y Ni, GJ Knbes, ARP van Heiningen. Mechanism of chlorate formation during bleaching of kraft pnlp with chlorine dioxide. J Pulp Pap Sci 19 J1-J5, 1993. 

Lindgren, B. O. and T. Nilsson. 1975. Chlorate formation during the reaction of chlorine dioxide with lignin model compounds. Svensk Papperstidn. 78 66-68. 

B. Chlorate Formation. The soluble chlorine-based species in the anolyte, leading to the chlorine current inefficiency, are dissolved chlorine, HOCl, OCl, and CIOJ. 

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