Hypochlorite brine

High yields of NaOCl are obtained electrolyticaHy by oxidation of CT at dimensionally stable anodes (219). Sodium hypochlorite is prepared using small diaphragmless or membrane cells, with a capacity of 1—150 kg/d of equivalent CI2, which produce a dilute hypochlorite solution of 1—3 and 5—6 g/L from seawater and brine, respectively (see Chemicals from brine). They are employed in sewage and wastewater treatment and in commercial laundries, large swimming pools, and aboard ships. 

Almost 40 years later the Lummus Co. patented an integrated process involving the addition of chlorine along with the sodium chloride and sodium hydroxide from the cathode side of an electrolytic cell to a tertiary alcohol such as tertiary butanol to produce the tertiary alkyl hypochlorite. The hypochlorite phase separates, and the aqueous brine solution is returned to the electrolytic cells. The alkyl hypochlorite reacts with an olefin in the presence of water to produce a chlorohydrin and the tertiary alcohol, which is returned to the chlorinator. With propylene, a selectivity to the chlorohydrin of better than 96% is reported (52). A series of other patents covering this technology appeared during the 1980s (53—56). 

The major chemical problem met in ion-exchange practice is the fouling or poisoning of the anion resins by organic matter. The various counter measures deployed include pre-flocculation, oxidation of the organic material, the use of specially developed resins, and treatment of the fouled resins by brine and/or hypochlorite. 

Hypohalite ions, XO, are formed when a halogen is added to the aqueous solution of a base. Sodium hypochlorite, NaClO, is produced from the electrolysis of brine when the electrolyte is rapidly stirred, and the chlorine gas produced at the... 

The reactor is designed to provide sufficient residence time (for recirculating liquid) for the reaction producing chlorate (started in the electrolyzers) to be completed. This involves further reaction of intermediates formed by the complex reactions in the electrolyzer, such as hypochlorite and hypochlorous acid, to produce chlorate. The reactor receives weak chlorate liquor from a crystallizer (not shown), fresh brine feed (also not... 

The chlorine value can be converted into hypochlorite bleach by absorption into lime or caustic soda. It can also be used to produce hydrogen chloride by combining with hydrogen in a burner. The latter technique is particularly useful when the acid can be consumed on site in brine acidification or ion-exchange resin regeneration. Another approach that has found some favour in the past is the absorption of the chlorine into a solvent from which it could then be stripped and returned to the liquefaction plant. 

Recycle and cathodic reduction. The most elegant solution for the Diaphragm Electrolysis Plant (DEP) appears to be recycling of the hypochlorite solution and reduction of the chlorate and bromate on the cathode of the electrolysis cell - the hypochlorite solution is added to the feed brine of the cells and the chlorate and bromate are converted to chloride and bromide at the cathode. 

There are several crucial steps in the process of recycling hypochlorite solution. First, hypochlorite is formed in the chlorine destruction where chlorine reacts with the sodium hydroxide solution. This solution is added to the brine-degassing unit. Partial conversion to chlorate and bromate takes place, which continues in the anolyte... 

The feed brine of the DEP contains a large quantity of carbonates. Therefore, at pH5 carbon dioxide is degassed. When hypochlorite is added, chlorate and bromate are formed in the feed of the electrolysis cells. These reactions have a slow velocity. The result of this is that conversion is only partial ... 

After extensive research and several tests, the option selected was recycling hypochlorite to the feed brine of the electrolysis cells. For this purpose, hypochlorite feed pipes were manufactured and the hydrochloric acid feed capacity to the brine degassing tanks was enlarged. 

There are no longer any chlorate and bromate emissions from the chlorine and hypochlorite destruction units. All hypochlorite is recycled into the feed brine. This process has been operating efficiently since July 1999. 

An additional advantage of the hypochlorite recycling process is the chlorination of the feed brine in the brine-degassing unit. Organic and nitrogen-containing components are oxidised. The reaction products are removed via the vent-gas to the chlorine destruction unit. Less NCI3 is formed in the electrolysis cells because part of the... 

Recycling the hypochlorite to the feed brine has provided an excellent possibility of eliminating completely the chlorate and bromate emissions of the chlorine destruction unit of a diaphragm electrolysis plant. The main advantage of the hypochlorite recycling and cathodic reduction procedure is the reduction of bromate to bromide. 

To maintain the acidity of the brine at pH5, more hydrochloric acid is required during hypochlorite recycling to the feed brine. This extra acid demand is the cause of the largest increase of variable production costs - approximately 100 000 Dutch guilders per year. Alternative solutions showed variable costs up to one million Dutch guilders per year. The investment for this project proved to be the best economical alternative to solve the chlorate and bromate emissions problem. 

A new variation of the chlorohydrin process uses t-butyl hypochlorite as chlorinating agent. The waste brine solution can be converted back to chlorine and caustic by a special electrolytic cell to avoid the waste of chlorine. 

Lithium-base greases, especially the stearate, are efficient over an extremely wide temperature range up to 160°C. Lithium hydroxide (LiOH) is a component of the electrolyte in alkaline storage batteries and is employed in the removal of carbon dioxide in submarines and space capsules. Lithium bromide (LiBr) brine is used for air conditioning and dehumidification. Lithium hypochlorite (LiOCl) is a dry bleach used in commercial and home laundries. Lithium chloride (LiCl) is in demand for low-temperature batteries and for aluminum brazing. Other uses of lithium compounds include catalysts, glass manufacture, and, of course, nuclear energy. 

Another way of excluding the undesirable reactions (XI-13) and (XI-14) consists in maintaining a high chloride concentration in the electrolyte. The solubility of gaseouB chlorine in a saturated brine is much lower than in a diluted solution, therefore, the equilibrium concentrations of hypochlorous acid [see the equation (XI-12) ] and of hypochlorite ions in the proximity of the anode are also much lower. 

When using diaphragms there is no risk of the anolyte being mixed by thermal convection so unlike the bell-jar electrolyzer higher temperatures may be used which lower the specific resistance of the electrolyte. The increased temperature has also a positive effect upon the current efficiencies as both, migration of hydroxyl ions from catholyte to anolyte and solubility of chlorine in brine, are reduced. By this, formation of hypochlorite is limited and caustic and chlorine losses are reduced. 

Purification is carried out by adding to the spent brine a sufficient quantity of alkali hydroxide to convert the free chlorine into hypochlorite (XI-36). The brine (at 60 to 80 °C) is then led to the bottom of the towers which arc filled... 

It should be noted that in spite of the easy discharge of hypochlorite ions at the anode a solution with a fairly high hypochlorite content can be prepared olectrochemically this can bo explained as follows. Chlorine liberated at the cathode and subsequently dissolved in the brine reacts with water to yield hydrochloric and hypochlorous acid according to equation (XI-12), or according to equation ... 

It follows from the above explanation that electrolysis of alkali chlorides in an electrolyzer without a diaphragm must be interrupted before curve h which represents the concentration of hypochlorite oxygen changes into a horizontal line only under this condition is the process economical, as a prolonged electrolysis would result in a waste of current without any further increase in th<) hypochlorite content. Moreover, care should be taken to prevent the hypochlorite ions formed from being electrochemically oxidized, as this would result in lower current efficiency and lower hypochlorite concentration in the liquor produced. This process is influenced by a number of factors, e. g. brine concentration, hydrogen ion concentration, anode material, current density, temperature, and last but not least a suitable design of the electrolyzer. 

The best results are obtained if two thirds of the total hypochlorite content is present in the form of free acid, Avhieh corresponds to a concentration of hydrogen ions in the solution pH — 6 to 7 according to the sodium chloride concentration in the brine and according to temperature. Because, a small amount of chlorine escapes into the ambient atmosphere, hydrochloric acid must be gradually added in small quantities to the electrolyte during the process. 

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