Chlorination cyanide alkaline process

Cyanide destmction by alkaline chlorination is a widely used process. With alkaline chlorination, cyanide is first converted to cyanate with hypochlorite [7681-52-9] at a pH greater than 10. A high pH is required to prevent the formation of cyanogen chloride [506-77-4] which is toxic and may evolve in gaseous form at a lower pH. With additional hypochlorite, cyanate is then oxidized to bicarbonate, nitrogen gas, and chloride. The pH for this second stage is 7—9.5 (6). 

Chemical Oxidation Design Considerations 2.4.1. Cyanide Alkaline Chlorination Process Design... 

Introduce the cyanide alkaline chlorination, excess chlorination (superchlorination), and acid hydrolysis processes, which treat industrial effluents containing cyanide. 

The cyanide alkaline chlorination process uses chlorine and caustic to oxidize cyanides to cyanates and ultimately to carbon dioxide and nitrogen. The oxidation reaction between chlorine and cyanide is believed to proceed in two steps as follows, according to Eqs. (11) and (12) ... 

Industrial cyanide-bearing waste is to be treated in a batch process using alkaline chlorination. In this process, cyanide is reacted with chlorine under alkaline conditions to produce carbon dioxide and nitrogen as end products. 

The pretreatment miit operations for various types of treatment facilities are shown in Table 24. The pretreatment processes generally involve separate treatment of cyanide wastes and other acid wastes containing metal ions. The cyanide wastes can be treated with ferrous sulfate and lime to convert highly toxic cyanides to less toxic cyanates or cyanide complexes, or can be oxidized to COj and Nj with chlorine under alkaline conditions. The acid waste streams are treated first to reduce hexavalent chromium to trivalent chromium, using ferrous sulfate, scrap iron, or sulfur dioxide, and then precipitating the metal ions (Cr +) as metal hydroxides. 

Ozone can be used to completely oxidize low concentrations of organics in aqueous streams or partially degrade compounds that are refractory or difficult to treat by other methods. Compounds that can be treated with ozone include alkanes, alcohols, ketones, aldehydes, phenols, benzene and its derivatives, and cyanide. Ozone readHy oxidizes cyanide to cyanate, however, further oxidation of the cyanate by ozone proceeds rather slowly and may require other oxidation treatment like alkaline chlorination to complete the degradation process. 

Corrective Action Application At a hazardous waste treatment storage and disposal facility in Washington State, a cyanide-bearing waste required treatment. The influent waste stream contained 15 percent cyanide. Electrolytic oxidation was used to reduce the cyanide concentration to less than 5 percent. Alkaline chlorination was used to further reduce the cyanide concentration to 50 mg/1 (the cleanup objective). The electrolytic process was used as a first stage treatment because the heat of reaction, using alkaline chlorination to treat the concentrated cyanide waste, would be so great that it would melt the reactor tank. 

Hydrogen peroxide is an efficient oxidizing agent (Section 16-3), particularly in alkaline solution. The excess peroxide is usually decomposed by boiling the alkaline solution the process is hastened by a number of catalysts, including nickel salts, iodide, and platinum black. Schulek and Szakacs removed the excess with chlorine water and then added potassium cyanide to destroy the excess chlorine. Examples of oxidation reactions are the oxidation of Cr(III) to chromate in 2Af sodium hydroxide, Co(II) to Co(III) in bicarbonate solution, Mn(II) to Mn(IV) in the presence of tellurate, and Fe(II) to Fe(III) followed by titration with ascorbic acid. Sodium peroxide, an even more vigorous oxidant, is applied in alkaline fusions. The fusion of chromite ore to form chromate has been critically studied, ... 

A classical example of the application of chlorine in industrial waste treatment is the alkaline destruction of cyanide wastes. The process is maintained at pH greater than 10 and cyanide is destroyed in a two step process using separate... 

In the metal plating industry, cyanides are frequently eliminated from waste discharges by treatment with chlorine at high pH (alkaline chlorination process). Two steps are involved (1) the oxidation of cyanide, CN , to cyanate, CNO , and (2) oxidation of CNO to Nj and CO2. 

Alkaline chlorination of wastewaters is one of the more widely used methods of treating cyanide wastes. In this process, cyanogen chloride (CNCl) is formed, which is hydrolyzed to the cyanate (CNO ) at alkaline pH. If free chlorine is present, CNO can be further oxidized. The use of sulfur dioxide in a high dissolved oxygen environment with a copper catalyst reportedly reduces total cyanide... 

Redox electrodes are mainly used as sensors in processes involving redox couples. An important application is in the metal plating industries where cyanide has to be removed from the plant effluents. Cyanide is efiminated in alkaline solutions by oxidation with chlorine gas in the first step to cyanate ions and then continuing the oxidation process to produce nitrogen and carbon dioxide as the final products ... 

Wastewater can be treated by a variety of chemical processes, including acid/base neutralization, precipitation, and oxidation/reduction. Sometimes these steps must precede biological treatment for example, acidic or alkaline wastewater must be neutralized for microorganisms to thrive in it. Cyanide in the wastewater can be oxidized with chlorine and organics with ozone, hydrogen peroxide promoted with ultraviolet radiation, or dissolved oxygen at high temperatures and pressures. Heavy metals can be precipitated with base, carbonate, or sulfide. 

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