Chlorates electrolytic process

Figure 11.8 shows the Kvaemer Chemetics electrolytic process for the production of sodium chlorate (in a concentrated solution of strong liquor ) and hydrogen from sodium chloride solution ( brine ). The overall reaction is NaCl + 3H20 - NaC103 + H,(g). The part of the system shown consists primarily of brine electrolyzers, a degasi-fier, a chlorate reactor, and an electrolyte cooler. 

Sodium chlorate is produced by the electrolysis of a sodium chloride solution in an electrolyzer without a diaphragm, having an iron cathode and a magnetite or graphite anode. For the manufacture of potassium chlorate either the sodium chloride solution is used, or a mixed solution of both sodium and potassium chloride. The chlorate solution obtained will finally be converted with potassium chloride into potassium chlorate, which is separated by crystallisation while the mother liquor is returned to the electrolytic process. The potassium chloride solution will not be electrolyzed directly as potassium chlorate is sparingly soluble and the potassium chloride entrained in the chlorate crystals is not easy to remove. 

However, the process is more complex than this, and the usual molar ratio of chlorine dioxide to chlorine produced is about 1 1. The process is integrated with an electrolytic process for making the sodium chlorate, such that the liquor from the reduction step is recirculated to the electrolytic step. The product gas, a mixture of chlorine with chlorine dioxide, is washed with water, which preferentially dissolves the chlorine dioxide. The resulting solution is used for pulp bleaching. 

Potassium chlorate, KC103.—The chlorate is obtained by methods similar to those employed for the corresponding sodium salt. The electrolysis of the chloride 4 affords a means of manufacture, and it is also obtained by the interaction of potassium chloride and calcium chlorate. Large quantities are made by the electrolytic oxidation of sodium chloride to chlorate, and conversion of this salt into potassium chlorate by treatment with potassium chloride. Sodium chlorate is much more soluble than potassium chlorate, so that the electrolytic process is not impeded by crystallization of the salt. 

Attempts to develop an activated cathode for chlorate cells have not yet been successful, and a material for the application faces many constraints. Some important properties for a chlorate cathode are (a) low overpotential for hydrogen evolution, (b) high stability during hydrogen evolution (resistant to the mechanical stress from gas bubbles and no detrimental hydride formation), (c) resistant during shut downs (low corrosion rate at open circuit in chlorate electrolyte), (d) low activity for hypochlorite decomposition, (e) low activity for reduction of hypochlorite and chlorate in the presence and in the absence of the chromium hydroxide film (the latter a step in the search for a chromate-free process), (f) relatively resistant to impurities in the electrolyte, (g) easy to manufacture, (h) easy to install in existing cell concepts, and (i) cost-effective. 

Fig. 5.10 An industrial plant for the production of sodium chlorate (the Erco process). Production is typically up to 42000 tons per year of sodium chlorate. The process operates at a current density of 0.21-0.28 Acm and a temperature of 80-90° C. The electrolyte, which has pH 6.4-7, contains 1.5-5 gdm Na2Cr207.2H2O. Typical raw-material consumption per tonne of product are NaCl, 550 kg HCl, 15 kg NaOH, 10 kg Na2Cr207.2H2O, 2.5 kg. (Courtesy Albright and Wilson Americas.)...

Tlie anhyd salt is obtained when samples are recrystd from w above 53° below this temp a monohydrate is obtained (see below). The pure salt is best obtained on a lab scale by dissolving pure Na carbonate in a slight excess of dil aq perchloric ac, the soln partly evapd, cooled to 50°, the solid centrifuged off, and dried in a current of air at 250°. Similar results were obtained starting with pure Na chloride (Ref 2). On a coml scale it is prepd by the electrolysis of Na chlorate (see Vol 2, C197-R). Processing details and economics of the prepn are given in Refs 5 11. Coned solns are used, and modern plants use continuous electrolytic cells. In 1960 prodn was estimated to be ca 10000 tons/year at a cost of 17.56 /lb (Ref 11, p 87)... 

In chlorate production the EMOS system has also been used to determine the formation of deposits on the electrodes, either the anode or cathode and combined with the information on process and electrolyte composition the system determines the need for cell cleaning or acid rinsing. The close monitoring of individual cell voltages has allowed plant engineers to establish the most appropriate current density for production lines dependent upon the state of the anode coatings. This allows for the same overall production capacity while permitting the operation of two different cell lines in the cell room at different current densities based upon the state of the anodes and cathodes in the cell. 

In 1851, C. Watts1 patented a process for preparing chlorine, soda, hypochlorites, and chlorates by the electrolysis of soln. of alkali chlorides but little progress was made for many years. In 1882, A. P. Lidoff and W. TichomirofE described the preparation of hypochlorites by this process, and in 1883, E. Hermite patented a process, for the preparation of electrolytic bleaching liquor, which has been used in several countries, but is now regarded as an obsolete process. 

Electrolytic production of white lead is based on this principle during this process the lead anode is immersed in a solution of alkali chlorate or acetate with a small quantity of alkali carbonate. In the course of electrolysis the lead is dissolved and forms at first a soluble chlorate or acetate the ions Pb++ diffuse and migrate into the bulk of the solution where the necessary amount of OH- and CO3 ions is found with which they react and precipitate as insoluble, basic lead carbonate. The solution is thus deprived of the carbonate ions which are then supplemented by a continuous absorption of carbon dioxide in the electrolyte. By using chromate, instead of carbonate chromium yellow is formed. Analogously also zinc white could be produced. 

Chlorate concentration increases during electrolysis, but exerts no influence upon the process, as its decomposition voltage is much higher than that of chloride. Chlorate ions are oxidized into perchlorate ions not sooner than tho chloride concentration in the electrolyte drops below 5 % of the initial value. Therefore, chlorate can accumulate in the solution up to the saturation. 

Electrolysis in this case only differs slightly from the sodium chlorate process. A sodium chloride solution or a mixed solution of sodium and potassium chloride is used as the electrolyte. The sodium chlorate in the electrolytically treated brine is converted to potassium salt by double decomposition with potassium... 

It will be seen from this equation that the dissolving component of the electrolyte, i. e. sodium acetate or chlorate, is continually regenerated in the course of the process. The sodium carbonate and hydroxide consumed in the reaction are supplemented by the migration of C(K and OH- ions from the catholyte. At the cathode made of lead or of iron hydrogen is liberated whereby the concentration of hydroxyl ions in the catholyte increases. In order to maintain their concentration within suitable limits and to replace the carbonate ions consumed in the production of white lead the eleotrolyte is continuously saturated by carbon dioxide thus converting the hydroxide to carbonate. 

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