Cyanide alkaline chlorination

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) ... 

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). 

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. 

C41 Cyanide Oxidation -- Alkaline Chlorination R09 Reuse as Fuel - Other... 

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. 

Ozonation treatment can be used to oxidize cyanide, thereby reducing the concentration of cyanide in wastewater. Ozone, with an electrode potential of +1.24 V in alkaline solutions, is one of the most powerful oxidizing agents known. Cyanide oxidation with ozone is a two-step reaction similar to alkaline chlorination.22 Cyanide is oxidized to cyanate, with ozone reduced to oxygen as per the following equation ... 

FIGURE 9.5 Cyanide reduction via alkaline chlorination. (Adapted from U.S. EPA, Meeting Hazardous Waste Requirements for Metal Finishers, Report EPA/625/4-87/018, U.S. Environmental Protection Agency, Cincinnati, OH, 1987.)... 

The term A, as in Cyanide A, shall mean amenable to alkaline chlorination. 

A typical reaction condition for the alkaline chlorination of 1 kg (2.2 lb) of cyanide to cyanate requires 6 kg (13.2 lb) each of sodium hydroxide and chlorine. The reaction is carried out at pH 10, and at least 15 min contact time is required to drive the reaction to completion. If metal cyanide complexes are present, extended chlorination for longer periods may be necessary. Complete destruction of cyanate requires a second oxidation stage with approx 45 min retention at a pH below 8.5. The theoretical reagent requirement for this second stage is 4.1 kg (9.0 lb) of chlorine and 1.1 kg (2.4 lb) of caustic per kg (2.2 lb) of cyanide. 

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. 

Cyanide may be detoxified to cyanate by oxidation with chlorine or hypochlorite (Chap. 15). Or the oxidation may be conducted by air in the presence of sulfur dioxide and copper ion at pH 9 or 10 [44]. Einally BOD reduction to 80-90% of the original values will be achieved via either biological waste treatment or alkaline chlorination methods before discharge. 

Cyanide hydrolyzing enzymes (i. e. reaction schemes 12.1-4 and 12.1-5) have, with the exception mentioned above, not been reported to hydrolyze organic nitriles and thus appear to be highly specific for inorganic cyanide. Therefore, they are mainly of interest in waste water treatment as a biological alternative to conventional chemical detoxification of cyanide by alkaline chlorination and will not be treated further. 

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... 

A typical cyanide neutralization is shown below. For alkaline chlorination ... 

Molten sodium cyanide reacts with strong oxidizing agents such as nitrates and chlorates with explosive violence. In aqueous solution, sodium cyanide is oxidized to sodium cyanate [917-61 -3] by oxidizing agents such as potassium permanganate or hypochlorous acid. The reaction with chlorine in alkaline solution is the basis for the treatment of industrial cyanide waste Hquors (45) ... 

On the large scale, cyanuric chloride is produced by the trimerization of cyanogen chloride. The cyanogen chloride is produced by chlorination of hydrogen cyanide and is trimerized by passing it over charcoal impregnated with an alkaline-earth metal chloride at a high temperature (250—480°C). 

Benzofuroxan may be obtained by oxidation of o-quinone dioxime. The first benzofuroxan derivative, 1,2-naphthofuroxan, was obtained by this method. Suitable oxidizing agents include alkaline ferri-cyanide, bromine water, chlorine, and nitric acid. The method is of practical value only when the o-quinone or its monooxime (o-nitrosophenol) is readily available, and since this is not generally the case, other routes, e.g., the oxidation of o-nitroanilines and the thermal decomposition of o-nitrophenyl azides/ are more commonly used. 

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