Sodium Chlorate

Introduction
Sodium Chlorate is produced by the electrolysis of a brine solution. More than 95% of the sodium chlorate produced today is used in the pulp and paper industry where it is a primary raw material for the production of chlorine dioxide. Chlorine dioxide is a strong and selective oxidizer and is used in the pulp bleaching process.

Chemistry
Sodium Chlorate is formed through electrolysis of brine according to the following equation:

NaCl + 3H2O + 6 e-  → NaClO3 + 3H2

This reaction is a multistage reaction where the electrode reaction products, chlorine and hydroxide, form hypochlorite, which then chemically is converted to sodium chlorate. The conversion rate and efficiency is very dependent on pH and temperature. The main part of this reaction takes place in the electrolyte cell but a small part reacts in the reaction tank in the electrolysis system. Due to pH influenced reaction inefficiencies, the hydrogen gas will contain small amounts of oxygen and chlorine. If the pH in the electrolyte is lower, more chlorine is formed, if it is higher, more oxygen is formed. It is thus very important to keep constant optimum pH.

Process Description

Brine System
Process water used in the plant is purified by filtration and by passing through ion exchange filters. This water is used mainly for dissolving salt, NaCI, and for make-up purposes. Depending on the quality of the salt, different methods of brine purification will be applied. If a pure vacuum salt is used, no pretreatment is necessary and the brine is passed directly via polishing filtration to the ion exchange unit from the salt dissolvers. If a less pure salt is used the brine first has to be treated with precipitation chemicals. In a brine settler the precipitated impurities are separated and the clear overflowing solution is filtered before being fed to the ion exchange unit.

When producing crystallized chlorate in a closed plant concept, the brine then could be further purified in a salt evaporator unit where water is evaporated and the salt crystallized. The slurry from the evaporator is dewatered on a filter or with a centrifuge and the solid salt is then added to the electrolyte system. By evaporating the water in the brine and crystallizing the salt, the water balance in the plant will be positive and dissolved impurities in the salt, e.g. sulfate, will be accumulated in the evaporator mother liquor and can thus easily be removed from the salt.

Electrolyte System
The incoming electrolyte is fed to the lower part of the cellbox. Hydrogen and discharged electrolyte leave the box through the top nozzle and are separated in the gas ducting above the cellbox. The DC-current enters the cellbox via busbars from the rectifier. When the current passes through the electrolyte in the cell box, the electrode reactions forming chlorine and hydroxide, results in the formation of sodium chlorate.

As a result of the electrolysis, heat is formed in the cellboxes, which means that the temperature of the electrolyte will increase. In order to maintain a constant electrolyte temperature, some heat has to be removed. This will take place outside the cellboxes in an external-cooling loop. The product chlorate solution is taken out from the cooling loop and fed to the second reactor tank.

Electrolyte purification and crystallizing
After the reaction tank the electrolyte is alkalized with hydrogen scrubber liquid and caustic soda. A high pH is necessary to avoid chlorine stripping off in the crystallizer.
The crystallizer is operated under vacuum by means of a vacuum pump or a steam booster/ejector system. Heat is added, if necessary, to the crystallizer via a plate heat exchanger, which is installed in the crystallizer circulation loop. The water vapor from the crystallizer is condensed in an indirect or direct condenser.

Chlorate slurry is taken out from the bottom of the crystallizer and fed to a screw thickener. The thickened slurry is then transferred to a table filter for dewatering. The remaining mother liquor is removed and the crystals are washed with water.

Drying, storage, and loading
Moist chlorate crystals are dried in a fluidized bed type dryer. Drying air is heated with either electrical power or steam. The dryer and the transportation equipment are connected to a scrubber for chlorate dust removal. Dry chlorate is normally stored in silos before being loaded into trucks, rail cars or big bags.

Hydrogen system
The hydrogen gas leaving the cellboxes contains some oxygen and chlorine. The gas mixture is thus washed with caustic soda solution in absorption towers for removal of chlorine before it can be used for burning or released to the atmosphere. From a safety point of view, the oxygen content in the hydrogen is continuously monitored in order to operate the plant below any dangerous concentrations. When the hydrogen will be used as a chemical raw material for other purposes, it might be necessary also to eliminate the oxygen content. This can easily be accomplished by passing the gas through a catalytic bed where any remaining oxygen reacts with hydrogen to form water.

Tail gas and miscellaneous systems
To prevent the chlorine containing air in some of the tanks to be released into the atmosphere, those tanks are vented through a tail gas system. The tail gas system consists of a fan(s) and scrubber tower where the chlorine is removed.

For safety reasons the hydrogen system has to be put under nitrogen atmosphere after a shutdown and before a start-up. A special nitrogen system is installed that flushes nitrogen into the cellboxes and the different parts of the hydrogen system at any shut-down or start-up,

Finally a hot water system is installed to transfer heat from the electrolyte to the chlorate electrolyzer and salt evaporator. No externally generated heat is required for these purposes.

Information
Eka Chemicals is the world's largest producer of sodium chlorate. Eka Chemicals Cellchem maintain a highly skilled staff involved in research and development with both chlorate and chlorine dioxide technologies. Through lab analysis, pilot plants and full-scale mill trials, the knowledge of sodium chlorate / chlorine dioxide is continuously increasing.
For further information and technical details, contact a Eka Engineering Sales Representative.