GB2283484A

GB2283484A - Bromide removal

Info

Publication number: GB2283484A
Application number: GB9421964A
Authority: GB
Inventors: David Macdonald Bonnick
Original assignee: Wallace and Tiernan Ltd
Current assignee: Wallace and Tiernan Ltd
Priority date: 1993-11-03
Filing date: 1994-10-28
Publication date: 1995-05-10
Anticipated expiration: 2014-10-28
Also published as: CA2220112C; GB9421964D0; GB2283484B; WO1996034999A1; CA2220112A1

Abstract

A chlorination system in which brine is converted to chlorine or sodium hypochlorite by an electrolyser. Brine fed to the electrolyser is passed through a filter which is capable of adsorbing bromine or hypobromous acid. Some of the chlorine or sodium hypochlorite produced in the electrolyser is fed back to a point in the brine feed upstream of the filter such that any bromide in the brine is oxidised to bromine or hypobromous acid and therefore adsorbed by the filter.

Description

BROMIDE REMOVAL The present invention relates to a method and apparatus for removing bromide from an electrolytic chlorination system.

It is well known to produce sodium hypochlorite from sodium chloride brine by converting the brine to sodium hypochlorite in an electrolyser. Sodium hypochlorite is used to treat potable water.

Unfortunately, if the brine includes traces of bromide, the electrolyser causes the conversion of the bromide by oxidation to bromate. It is desirable to produce bromate-free sodium hypochlorite for the treatment of potable water as experiments have indicated that bromate may be carcinogenic.

It is an object of the present invention to provide a method and apparatus for removing bromide from an electrolytic chlorination system so as to obviate or mitigate the problem outlined above.

According to the present invention, there is provided a method for removing bromide from an electrolytic chlorination system in which brine is converted to chlorine or sodium hypochlorite in an electrolyser, wherein the brine is fed to the electrolyser through a filter containing a medium capable of adsorbing :romine or hypobromous acid, and chlorine or sodium hypochlorite S TCc back from the electrolyser and mixed with the brine feed upstrean of the filter to oxidise any bromide in the brine to bromine or hypobromous acid.

The present invention also provides an electrolyser or inverting brine to chlorine or sodium hypochlorite, means or feeding brine to the electrolyser, a filter through which the brine is fed to the electrolyser, the filter being capable of adsorbing bromine or hypobromous acid, and means for mixing chlorine or sodium hypochlorite from the electrolyser with the brine upstre am ot tne filter such that bromide in the brine is oxidised to bromine or hypobromous acid.

Preferably hydrochloric acid is mixed with the brine ups@@ @m of the filter to maintain a low pH and thereby ensure effectiv oxidation of the bromide.

An embodiment of the preser,t invention will rcw .e Cs by way of example, with reference to the acornoa'-' - -- - - illustrates a bromide removal system incorporated into a conventional on-site electrolytic chlorination system.

The illustrated electrolytic chlorination system comprises a salt saturator i in which brine is prepared. The saturator 1 may have a capacity of, for example, 15 cubic metres. Brine flows from the saturator 1 through a filter 2 to an electrolyser 3, the brine flow being maintained by a brine transfer pump 4. A flow rate through the pump 4 of 51.5 litres per hour may be established. The electrolyser 3 is of conventional form and is effective to oxidize the brine to sodium hypochlorite. The contents of the electrolyser are mixed with dilution water supplied through line 5 at a rate of 540 litres per hour, the resultant sodium hypochlorite being transferred to a product tank 6 with a capacity of 13 cubic metres.

With the exception of the filter 2, the components shown in the drawing and described above are entirely conventional. With the operation of such a conventional system, however, traces of bromide in the brine flow are converted to bromate and reach the product tank 6. This is undesirable as the content of the product tank 6 is delivered via line 7 to a treatment plant (not shown) in which the contents of the products tank is mixed with a supply of potable water.

In accordance with the present invention sodium hypochlorite from the product tank 6 is fed back through line 8 to a point upstream of the filter 2. The flow of hydrochlorite 5 maintained by a hypochlorite transfer pump 9. vor ochloric acid is also delivered to-the brine flow upstream of the filter 2 via line 10, the flow of hydrochloric acid being maintained by a pump 11. In the illustrated system the flow of sodium hypochlorite was l litre per hour and the flow of hydrochloric acid was 1 litre per hour. The supply of hydrochloric acid is controlled to maintain a low pH in the brine flow upstream of the filter 2. Typically y the acidity will be controlled to approximately 4 pH. At such a low pH, bromide in the brine flow is rapidly converted into bromine and hypobromous acid which is adsorbed by the filter 2. . appropriate selection of the stoichiometry and reaction times the further oxidisation of the hypobromous acid to bromate can be minimised or substantially eliminated. (It is believed that bromate production will not be a problem providing the pH is maintaine, at bei @@. Accordingly, providing the filter is capable of adsorbing bromine or bromine released by the reduction of hypobromous acid, substantially no bromine compounds reach the electrolyser and accordingly substantially no bromate reaches the product tank 8.

Appropriate materials for the filter 2 are activated carbon and zeolites.

It can be advantageous to produce chlorine and sodium hydroxide rather than sodium hypochlorite. This can be achieved by placing a suitable membrane between the electrodes of the electrolyser such that chlorine evolved at one electrode is prevented from mixing with sodium hydroxide evolved at the other electrode. Sodium hypochlorite could then be produced by mixing the chlorine and sodium hydroxide in a separate stage of the process. However, if chlorine rather than sodium hypochlorite is fed back from the electrolyser to be mixed with the feed upstream of the filter, this would enable the removal of bromide and in addition make it unnecessary to add hydrochloric acid to the brine to maintain a low pH.

Claims

1. A method for removing bromide from an electrolytic chlorination system in which brine is converted to chlorine or sodium hypochlorite in an electrolyser, wherein the brine is fed to the electrolyser through a filter containing a medium capable of adsorbing bromine or hypobromous acid, and chlorine or sodium hypochlorite is fed back from the electrolyser and mixed with the brine feed upstream of the filter to oxidise any bromide in the brine to bromine or hypobromous acid.

2. A method according to claim m, wherein hydrochloric acid is mixed with the brine feed upstream of the filter to maintain a low pH and thereby ensure effective oxidation of the bromide.

3. An electrolytic chlorination system comprising an electrolyser for converting brine to chlorine or sodium hydrochloride, means for feeding brine to the electrolyser, a filter through which the brine is fed to the electrolyser, the filter being capable of adsorbing bromine or hypobromous acid, and means for mixing chlorine or sodium hypochiorite from the electrolyser with rhe brine upstream of the filter such that bromide in the brine is csidised to bromine or hypobromous acid.

4. An apparatus according to claim 3, comprising means for mixing the brine upstream of the filter with hydrochloric acid to maintain a low pH and thereby ensure effective oxidation of the bromide.

5. A method for removing bromide from an electrolytic chlorination system substantially as hereinbefore described with reference to the accompanying drawing.

6. An electrolytic chlorination system substantially as hereinbefore described with reference to the accompanying drawing.