BACKGROUND OF THE INVENTION Sodium hypochlorite has been widely used in a variety of industrial and recreational water systems to control biofouling. However, unstabilized sodium hypochlorite degrades very quickly in field use conditions. There are several methods known in the art for stabilizing hypochlorite (See, e. g. , U. S. Patent Nos. 3,328, 294 and 3,767, 586).

Bromine is preferred over chlorine for use in water treatment because of its lower volatility and better performance at high pH and amine environments. However, like sodium hypochlorite, sodium hypobromite is unstable in typical storage conditions, so it is also preferable to provide sodium hypobromite in a stabilized form. U. S. Patent Nos. 5,683, 654 and 5,795, 487, as well as the references disclosed therein, teach various methods for stabilizing sodium hypobromite. The'654 and'487 patents disclose batch methods which utilize sodium hypochlorite and sodium bromide as starting materials, followed by the addition of a stabilizer. U. S. Patent No. 6,015, 782 similarly discloses a process which includes a hypobromite formation step followed by a bromine stabilization step. However, a disadvantage associated with this technique is that unstabilized hypobromite is formed in a separate step at a high concentration and pH. It

is known that unstabilized hypobromite degrades quickly under such conditions to form bromate, a non-biocidal compound that is very toxic and a suspected carcinogen.

Also, U. S. Patent No. 6,037, 318 discloses a process that first forms stabilized chlorine compounds and then converts them to stabilized bromine compounds.

However, this type of process is limiting because only hypochlorite-releasing compounds can be used as the oxidizing source.

In addition, U. S. Patent No. 6,270, 722 discloses a method of adding hypochlorite to a mixture of bromide and sulfamate and then adjusting the pH of the mixture. Alternatively, a method is disclosed for adding hypochlorite and sulfamate simultaneously to bromide and then adjusting the pH to form a stabilized bromine product.

Because the demand for stabilized bromine solutions is expected to increase in the future due to its advantages over chlorine, there is a need for other cost-effective methods of manufacturing stabilized bromine which are flexible and can generate a high throughput. One such method may include preparing a stabilized bromine and chlorine mixture wherein the stabilized chlorine gradually oxidizes the residual bromide in the mixture into stabilized bromine.

Accordingly, it would be desirable to provide a method of making a stabilized bromine and chlorine mixture which can be carried out as a batch or continuous process either at the manufacturing facility or right at the site of the commercial application. It would also be desirable to develop a method of making a stabilized bromine and chlorine mixture which generates a high yield, is flexible and does not require precise pH control.

SUMMARY OF THE INVENTION The stabilized bromine and chlorine mixtures of the present invention are prepared by simultaneously combining a bromine source, an oxidizer and a stabilizer.

An alkaline source may optionally be added to the stabilized bromine and chlorine mixture to adjust the pH of the mixture to at least 13.

The inventive method is economically appealing because it can be carried out as a batch or continuous process and because it can be done at a very high throughput. The method of preparation is also flexible because it can be performed at the manufacturing facility or right at the site of the commercial application and the method does not require precise pH control.

DETAILED DESCRIPTION OF THE INVENTION The present invention is directed to a method of making stabilized bromine and chlorine mixtures. As used herein, "stabilized bromine and chlorine mixtures"means that both the bromine and the chlorine are stabilized in the mixture. In accordance with this invention, a bromine source, an oxidizer and a stabilizer are combined simultaneously. An alkaline source may be added to the stabilized bromine and chlorine mixture to adjust the pH of the mixture to at least 13.

The bromine sources which may be used in the practice of the present invention include hydrobromic acid, elemental bromine and alkali or alkaline earth metal bromides, such as sodium bromide, potassium bromide and lithium bromide. The preferred bromine source is an alkali or alkaline earth metal bromide.

The oxidizers which may be used include chlorine gas, hypochlorous acid, hypochlorite salt, chlorite, chlorate, persulfate, permanganate and peracetic acid.

Chlorine gas, hypochlorous acid and hypochlorite salt are the most preferred oxidizers.

It is believed that other peroxy compounds can also be used in accordance with this invention.

The stabilizers which may be employed in this invention have the chemical formula R-NH-R', wherein R and R'are selected from the group consisting of R2 CO, R2 SO2, R2 CF2, R2 CHF, H, OH and PO (OH) 2, and R2 is an alkyl group or an aromatic group.

Suitable stabilizers include saccharin, urea, thiourea, creatinine, cyanuric acids, alkyl hydantoins, monoethanolamine, diethanolamine, organic sulfonamides, biuret, sulfamic acid and its alkali or alkaline earth metal salts, organic sulfamates and melamine. Sulfamic acid and its alkali or alkaline earth metal salts are the most preferred stabilizers.

Optionally, other water treatment chemicals, such as tracing compounds, surfactants, corrosion inhibitors and scale inhibitors, can be added to the stabilized bromine and chlorine mixture.

It is preferred that the molar ratio between the bromine source and the stabilizer be in the range of about 0.2 to 5. The molar ratio between the bromine source and the oxidizer should preferably be in the range of about 0.5 to 2.

The method of the present invention is maintained at a temperature of less than 120 °F and can be carried out as either a batch or continuous process.

The stabilized bromine and chlorine mixtures which are prepared in accordance with this invention may be used in a wide variety of commercial applications. These applications include, but are not limited to, the use of the stabilized bromine and chlorine mixture in a method for: (1) controlling biofouling on a hard surface; (2)

controlling biofouling occurring on the surfaces of equipment in contact with an industrial water system; (3) controlling biofouling in an aqueous phase of an industrial water system; and (4) controlling microbial growth in an aqueous stream used for transporting or processing food products and on food surfaces and equipment surfaces that come in contact with the aqueous stream.

The types of industrial water systems in which the stabilized bromine and chlorine mixture may be used to control biofouling include, but are not limited to, cooling water systems, sweetwater systems, gas scrubber systems, air washer systems, evaporative condensers, pasteurizers, produce sanitizer streams, fire protection water systems, pulp and paper systems, heat exchanger tubes and oil field waters.

The stabilized bromine and chlorine mixture of the present invention can be prepared at the site of the commercial application. Alternatively, if there is a need to store and transport the stabilized bromine and chlorine mixture, an alkaline source may be added to the mixture to raise the pH of the stabilized bromine and chlorine mixture to at least 13 to provide thermal stability and shelf life during storage and shipping.

The alkaline source is preferably an alkali or alkaline earth metal hydroxide.

Suitable alkaline sources include sodium hydroxide, lithium hydroxide, potassium hydroxide, magnesium hydroxide and calcium hydroxide.

The present inventor has discovered that a stabilized halogen product can be prepared without requiring any particular order of addition of the ingredients. By simultaneously combining a bromine source, an oxidizer and a stabilizer, a stabilized bromine and chlorine mixture is achieved and can effectively be used to control biofouling. Moreover, the method of making the stabilized bromine and chlorine mixture is economical since it can be conducted as a batch or continuous process and it

generates a high yield. In addition, the inventive method is flexible because it can be performed at the manufacturing facility or right at the site of the commercial application and the method does not require precise pH control.

EXAMPLE The following example is intended to be illustrative of the present invention and to teach one of ordinary skill how to make and use the invention. This example is not intended to limit the invention or its protection in any way.

A laboratory reactor was set up for preparing a stabilized halogen product from three raw materials: sodium bromide, sodium sulfamate and sodium hypochlorite. The sodium sulfamate solution was prepared by mixing an equal molar ratio of sulfamic acid with sodium hydroxide in water. In order to use one peristaltic pump with multiple pump heads to conduct the experiment, a 250 ml solution of sodium hypochlorite (12.0% as available chlorine, pH 12.41), a 250 ml solution of sodium bromide (17.42%, pH 7.25) and a 250 ml solution of sodium sulfamate (20. 41%, pH 2. 31) were prepared separately. All three solutions were then pumped at a pump rate of approximately 10 ml/min using PVC tubing to a 1-liter glass reactor. A motorized HDPE stirrer mixed the solutions in the glass reactor. The temperature of the product was controlled at 68 °F using an ice-water bath. The finished product had a total volume of approximately 750 ml, a pH of 12.40 and a total available chlorine content of 3.79% (product yield is therefore 95% since the theoretical chlorine percentage should be around 4%). Further product characterization using DPD colorimetric technical showed that the product contained 54% bromosulfamate and the remaining 46% as chlorosulfamate on a molar basis.

While the present invention is described above in connection with preferred or illustrative embodiments, these embodiments are not intended to be exhaustive or limiting of the invention. Rather, the invention is intended to cover all alternatives, modifications and equivalents included within its spirit and scope, as defined by the appended claims.