Hypochlorite destruction

HYDECAT [Hypochlorite destruction catalyst] A continuous process for destroying unwanted hypochlorite streams. A heterogeneous catalyst containing nickel converts the hypochlorite ion to chloride ion and oxygen gas ... 

Owing to a change in government regulations from 6 February 1999, regular discharges from the hypochlorite destruction unit were prohibited. Thus, it is necessary to devise an alternative method for the decomposition of the hypochlorite. 

Start-up of the hypochlorite recycling process was successful in July 1999 and has been working very well since. The procedure has proved that hypochlorite recycling is indeed an excellent way to eliminate completely chlorate and bromate emissions from the hypochlorite destruction unit. 

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. 

The abatement of chlorine vents and the subsequent destruction of the resulting sodium hypochlorite has been the subject of many studies. There are a variety of approaches to the waste hypochlorite destruction including chemical dosing, homogeneous and slurry catalysis as well as fixed-bed catalysis. For the most part these processes treat the hypochlorite at its natural strength the stoichiometric equivalent strength of the caustic soda fed to the scrubber. 

Generally, although not exclusively, a scrubber with a recycle loop of the caustic scrubbing liquor is used cases of once-through scrubbing liquor operation do exist. The scrubber may be operated in batch, semi-batch or continuous mode with respect to the liquid. Process hazards exist in batch and continuous mode, the most significant of which is over-chlorination. Batch-wise operations leads to periodic high loads on the hypochlorite destruction unit. In order to even out these loads, and improve the process safety, a study of alternative treatment options has been undertaken. 

The destruction of the resulting sodium hypochlorite has been the subject of many studies. There are a variety of approaches to waste hypochlorite destruction. 

Fig. 26.3 Cost of hypochlorite destruction as a function of inlet concentration. 

As noted above, batch and semi-batch-based operations result in periodic high loads and subsequent over-design and increased capital cost. By destroying the hypochlorite in situ, within the scrubber recycle loop, the end of cycle concentration can be reduced and the load on the end-of-pipe hypochlorite destruction system lowered allowing an overall cost reduction. The reduced free chlorine concentration also leads to improved process safety, although increased heat removal is required. 

Dynamic simulations of a batch-operated scrubber with pump-around will be presented showing the effect of process configuration on destruction demands, where the method of hypochlorite destruction is a fixed-bed catalytic reactor. 

E.H. Stitt, F.E. Hancock, and K. Kelly, New Process Options for Hypochlorite Destruction. In J. Moorhouse (ed.). Modem Chlor-Alkali Technology, vol. 8, Blackwell Science, Oxford (2001), p. 315. 

The combination CD/S is a measure of the ease of reducing hypochlorite. For best results, it must be kept small. A low concentration of hypochlorite and a thick diffusion layer are preferred. Hence, Brockmann s no stirring and the use of low flow rates through the cell are normal practices. High current densities improve the current efficiency because the rate of the desired reaction increases in proportion, while the rate of hypochlorite destruction increases only as the diffusion layer becomes thinner. The hydrogen bubbles generated at the cathode agitate the solution and reduce the thickness of the diffusion layer. Krstajic et al. [63] showed that... 

The entire electroplating process generates complex wastewater streams. For the purpose of treatment these can be considered to occur in four segments — acid, alkali, chromium and cyanide streams [29]. The acid and alkali wastes are combined and the pH adjusted. The chromium is reduced from Cr to Cr " with sulphur dioxide at pH 2-3. The cyanides are destroyed with chlorine or hypochlorite. Destruction of cyanides is essential for effective precipitation of the heavy metals, as they form strong complexes with the metals and thereby increase... 

It was not their reactivity but their chemical inertness that was the true surprise when diazirines were discovered in 1960. Thus they are in marked contrast to the known linear diazo compounds which are characterized by the multiplicity of their reactions. For example, cycloadditions were never observed with the diazirines. Especially surprising is the inertness of diazirines towards electrophiles. Strong oxidants used in their synthesis like dichromate, bromine, chlorine or hypochlorite are without action on diazirines. Diazirine formation may even proceed by oxidative dealkylation of a diaziridine nitrogen in (186) without destruction of the diazirine ring (75ZOR2221). The diazirine ring is inert towards ozone simple diazirines are decomposed only by more than 80% sulfuric acid (B-67MI50800). 

As with the 5-amino-4-phenyl-l,3-dioxane auxiliary47 53, the rert-leucine ester group has to be removed by oxidative degradation, in this case by a regioselective decarboxylation using fe/7-butyl hypochlorite. The expense of this auxiliary, coupled with its destruction, limits the practical value of this interesting procedure. 

Non-destructive partial stripping techniques for basic dyes on acrylic fibres are carried out at 100 °C (or higher if possible) using, for example, 1-10% o.w.f. anionic retarder and 1 g/1 acetic acid (60%), or 1-5 g/1 Marseilles (olive oil) soap. Destructive stripping requires acidified (pH 5.5-6.0) sodium hypochlorite, followed by an antichlor treatment in sodium dithionite or sodium bisulphite. In some cases a preliminary boiling treatment in 5 gA monoethanolamine and 5 g/1 sodium chloride is said to improve the effect of the stripping treatment. 

The chlorine liquefaction plant comprises a bromine-removal column, a compression-condensation unit and a Tetra absorption/distillation unit (Fig. 14.2). Waste streams of chlorine are absorbed in diluted cell-liquor in the chlorine destruction area. As a result, the destruction liquid contains sodium chloride and less sodium hydroxide than is usual. Bromine from the bromine-removal column is also added to the chlorine destruction unit. The hypochlorite solution that is formed contains a reasonable amount of bromine and salts. However, it is a hypochlorite of non-marketable quality. 

Hypochlorite and hypobromite formed in the chlorine destruction unit are subse-... 

Catalytic destruction. Catalytic conversion of hypochlorite to chloride and oxygen can be facilitated by nickel. With the Hydecat process, ICI has a commercial solution available. 

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 destruction of chlorine in a sodium hydroxide solution is a fast exothermic reaction. Hypochlorite, hypobromite and bromate are formed ... 

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. 

The hypochlorite produced has an active chlorine concentration of 160-180 g l-1 and a free caustic concentration of 4-8 g l-1. Figure 25.1 illustrates the simplified layout of the two-step chlorine destruction unit. All measurements are carried out under steady-state conditions under different peak loads of chlorine. 

The previously published process design [5] was able to achieve total destruction of the hypochlorite down to levels of less than 100 ppm (or even <1 ppm) from a feed... 

Traditionally, processes have used a single destruction technique, and this has historically been the case also for HYDECAT . Thus, nearly all installed processes treat the waste hypochlorite at the concentration it exits the scrubbing system down to concentrations suitable for discharge (Fig. 26.2). The key aspect in the re-evaluation described herein is to question the practices of firstly single technology and secondly end-of-pipe treatment the destruction of the hypochlorite exclusively in the blowdown stream from the scrubber. That is, it is questioned whether installation of a single treatment technique solely to process the effluent at its natural concentration from the scrubber loop is necessarily the best process option. This chapter will consider the two parts of the question paraphrased above sequentially. 

An economic benefit can occur through the retrofitting of a catalytic reactor, designed to perform partial destruction of the hypochlorite, into a plant already equipped with a chemical treatment system. The savings in chemicals consumption can more than offset the capital investment associated with the catalytic reactor and purchase of the initial catalyst charge. At high feed concentrations, say above a few weight per cent, the payback time can in fact be less than six months. 

Abbott, P.E.J., Carlin, M., Fakley, M.E., Hancock, F.E. King, F. (1991) ICIHYDECAT process for the catalytic destruction of hypochlorite effluent streams. In Modern Chlor-Alkali Technology (ed. T.C. Wellington), Vol. 5, pp. 23-34. Published for SCI by Elsevier Science, Amsterdam. 

The relationship between the degradation of organic matrix and dentin lesion formation has been studied both in vitro and in situ. Several authors employed matrix destruction to assess the role of the matrix in de-and remineralization. For example, Apostolopoulos and Buonocore (1966) reported facilitated demineralization of dentin at pFl<5.5 after treatment with ethylene diamine. Inaba and coworkers (1996) found that removal of matrix from dentin lesions by hypochlorite promotes remineralization, consistent with a larger crystal surface available for mineral deposition after ashing (McCann and Fath, 1958). Flypochlorite-mediat-ed destruction also increases the permeability of mineralized dentin (Barbosa et ah, 1994). 

This is a process for destruction of microorganisms. Chlorine, hypochlorite salts, phenol, phenol derivatives, ozone, salts of heavy metals, chlorine dioxide, and so on are effective disinfectants. It may require pH adjustments. 

This is a process mainly used in power plants for destruction of cyanides using chlorine, hypochlorite salts, or ozone. The process removal efficiency is about 99.6% [12-19]. 

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