Chlorate chromium removal

The sodium chlorate manufacturing process can be divided into six steps (/) brine treatment 2 electrolysis (J) crystallisation and salt recovery (4) chromium removal (5) hydrogen purification and collection and (6) electrical distribution. These steps are outlined in Figure 3. 

Chromium Removal System. Chlorate manufacturers must remove chromium from the chlorate solution as a result of environmental regulations. During crystallization of sodium chlorate, essentially all of the sodium dichromate is recycled back to the electrolyzer. Alternatively, hexavalent chromium, Cr, can be reduced and coprecipitated in an agitated reactor using a choice of reducing agents, eg, sodium sulfide, sulfite, thiosulfate, hydrosulfite, hydrazine, etc. The product is chromium(III) oxide [1333-82-0] (98—106). Ion exchange and solvent extraction techniques have also... 

The SRS process is versatile in chlorate plant production. The design and operating conditions can be adapted to meet the need of the different plant requirements, e.g. production of R8 solution with reduced chromium, recovery of chlorate from the purge liquor sent to effluent, or removal of sulphate from the liquor loop. 

Chromate, dichromate, permanganate, chlorate and hypochlorite and other oxidants are readily reduced hy hydrazine for example, removal of chromate from wastewater may he achieved fuUy hy converting water-soluhle chromate to insoluble precipitate of chromium hydroxide, Cr(OH)3 ... 

In membrane-cell plants fed with well brine, evaporation of depleted brine can be used to remove the excess water and maintain a water balance. This approach does not require a crystallizing evaporator. The solids-recovery section of a salt evaporator also is not required, and evaporator design is simpler. However, the problem of sulfates in the brine remains, and the materials of construction mentioned above may not be suitable. Free chlorine should be scrupulously removed from the depleted brine before it enters an evaporator, and even the chlorate that forms in the cells can be a problem. The working materials may have to be upgraded to types with higher nickel contents. Thus, the evaporator bodies and liquor-handling components may be upgraded from Monel to one of the Incoloys, which contain about 20% chromium and up to 45% nickel. 

The roasted clinker is then leached out with hot water and the resulting liquor filtered to remove insoluble silica, along with aluminum and iron oxides present as impurities in the original chromite ore. Pure sodium dichromate is then crystallized and dried. The anhydrous sodium chromate can then be reduced to chromium trioxide by simple carbothermic reduction. Note that the sodium chromate produced can be converted into the dichromate by dissolving it in sulfuric acid and is the basis for the manufacture of all industrially important chromium chemicals. Once produced, chromium trioxide is mixed with aluminum powder and lime and placed inside a refractory-lined steel vessel. The exothermic reaction is started by igniting a pyrotechnic mixture made of barium peroxide and aluminum powder or potassium chlorate aluminum powder ... 

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