Chemistry of inorganic compounds – Halogen or compound thereof – Chlorine dioxide
Reexamination Certificate
1999-05-06
2001-05-29
Nguyen, Ngoc-Yen (Department: 1754)
Chemistry of inorganic compounds
Halogen or compound thereof
Chlorine dioxide
C252S187210, C252S187230
Reexamination Certificate
active
06238643
ABSTRACT:
FIELD OF THE INVENTION
The present invention is generally directed to a method of producing an aqueous solution of chlorine dioxide from the reaction of a metal chlorite and an acid forming component which do not react to produce chlorine dioxide in the substantial absence of water. The reactants are separated from liquid water by a membrane which allows the controlled passage of liquid water and/or water vapor into contact with the reactants. The chlorine dioxide thus generated passes out through the membrane into the liquid water to produce the desired aqueous solution. Aqueous solutions containing chlorine dioxide produced in this manner can be conveniently used for commercial and domestic cleaning operations such as in the food industry.
BACKGROUND OF THE INVENTION
Chlorine dioxide in low concentrations (i.e. up to 1,000 ppm) has long bean recognized as useful for the treatment of odors and microbes. Its use is particularly advantageous where microbes and/or organic odorants are sought to be controlled on and around foodstuffs, as chlorine dioxide functions without the formation of undesirable side products such as chloramines or chlorinated organic compounds that can be produced when elemental chlorine is utilized for the same or similar purposes. For example, if a low concentration of chlorine dioxide gas can be maintained in contact with fresh produce for several days during shipping from the farm to the local retailer, the rate of spoilage of the produce can be decreased. In addition, chlorine dioxide gas is also generally considered to be safe for human contact at the low concentrations that are effective for deodorization and most antimicrobial applications.
Chlorine dioxide can be explosive at concentrations above about 0.1 atmosphere. Therefore, chlorine dioxide gas is not manufactured and shipped under pressure like other industrial gases, and conventional methods of on-site manufacture require not only expensive generation equipment but also high levels of operator skill to avoid generating dangerously high concentrations. These problems have substantially limited the use of chlorine dioxide to large commercial applications, such as pulp and paper bleaching, water treatment, and poultry processing, where the consumption of chlorine dioxide is sufficiently large that it can justify the capital and operating costs of expensive equipment and skilled operators for on-site manufacture.
Commercially, chlorine dioxide is produced from a variety of aqueous solutions of certain chlorine-containing salts, as disclosed for example in U.S. Pat. No. 5,009,875.
Attempts have also been made to produce chlorine dioxide using mixtures of solid reagents. Generally, the prior art has focused on three systems for chlorine dioxide production using solid reagents. One system employs a solid mixture of a metal chlorite and an acid in a liquid, aqueous environment. A second system combines a metal chlorite and a solid acid where chlorine dioxide gas is released under dry conditions. A third system employs the combination of a metal chlorite and a solid organic anhydride to generate a high concentrated flow of chlorine dioxide which must be diluted with a constantly flowing stream of inert gas.
Each of these solid reagent systems is disadvantageous for any one or more of the following reasons:
a) upon mixing there is normally a sudden, highly concentrated stream of chlorine dioxide generated;
b) the mixture of reactants produces chlorine dioxide gas under dry conditions thereby reducing the shelf life of the reactants; and
c) an inert gas stream must be used to reduce the concentration of chlorine dioxide gas in the atmosphere.
Aqueous solutions of chlorine dioxide are also known in the art. Two types of synthesis processes are generally used to provide chlorine dioxide solutions for commercial uses, such as poultry chiller water purification, washwater purification, potable water treatment and as a teat dip for the control of non-human mammalian mastitis.
The first type of synthesis process is based on the manual combination of two aqueous solutions; one containing a source of chlorite anions and another being acidic. The solution containing chlorite anions is usually a solution of sodium chlorite having a concentration of between about 100 ppm and about 5% by weight and having a pH of about 13. The acidic solution may contain any acid capable of providing a pH below about 8.5 after the solutions are mixed. Such acids include citric acid, lactic acid, hydrochloric acid sulfuric acid, and dissolved carbon dioxide (i.e., sodium bicarbonate). The antimicrobial performance of the resultant solutions depends upon the degree to which the chlorite anions from the chlorite source solution are converted to free molecular chlorine dioxide (“Chlorine Dioxide”) in the solution, as Chlorine Dioxide is the effective agent for both antimicrobial and deodorization activity.
In one variation on this synthesis process the pH of the sodium chlorite solution is reduced from about 13 to about 8 using the acidic solution. Chlorite anion is thus converted to Chlorine Dioxide via the reaction below.
5ClO
2
−
+5H
+
&rlarr2;4ClO
2
+HCl+2H
2
O
Such solutions having a pH of about 8 are generally referred to in the industry as “stabilized” chlorine dioxide solutions, and usually contain between about 100 ppm and 5% of a mixed solution of Chlorine Dioxide and unconverted chlorite anion. Because the acid concentration is relatively low at a pH of 8, the typical ratio of Chlorine Dioxide to chlorite anion in a stabilized chlorine dioxide solution is less than 0.01. Therefore, for a given initial concentration of chlorite anion, stabilized chlorine dioxide solutions are relatively weak antimicrobial agents due to their low conversion of chlorite anion to Chlorine Dioxide. Also, since they are typically supplied at a concentration of less than about 5% by weight sodium chlorite, they are relatively expensive to ship and store due to the high weight of water that must be transported as part of the solution.
Chlorite anion is generally stable in stabilized solutions (pH 8), so they have an advantageously long shelf life. To improve their effectiveness, however, they are typically activated just prior to use by the addition of a strong acid to lower their pH to below about 3.5 and convert more chlorite anion to Chlorine Dioxide via the reaction shown above. Since the activation process involves the addition of a strong acid to lower the pH, it requires a high level of operator skill to handle, measure and mix the acid with the stabilized chlorine dioxide solution. Also, since the activation process results in a solution having a pH of less than about 3.5, such activated solutions are not well suited to work in combination with, for example, detergents which work best under alkaline or neutral pH conditions. Contact of these solutions with many metals should also be limited because of possible metallic corrosion by the acidic solution.
Such activated solutions typically have a ratio of Chlorine Dioxide to chlorite anion below about 0.05 when the solution is acidified to a pH of about 3. It is possible to achieve a higher ratio of Chlorine Dioxide to chlorite anion in such activated solutions, but doing so is dangerous and requires extreme operator skill. Achieving a ratio of Chlorine Dioxide to chlorite anion above about 0.05 requires further acidification to a much lower pH than 3 (typically less than 2) and often requires that the further acidification be performed at concentrations of chlorite anion above about 5000 ppm. Under such conditions of extremely low pH and high chlorite ion concentration it is possible to generate a sufficient chlorine dioxide concentration in solution such that the vapor pressure of gaseous chlorine dioxide in equilibrium with the solution approaches the explosive range. Therefore, it is not common practice to produce solutions having a high ratio of Chlorine Dioxide to chlorite anion by manual acidification (i.e. without chlorine dioxide generation equipment as disc
Speronello Barry K.
Thangaraj Appadurai
Wildman Timothy D.
Engelhard Corporation
Miller Stephen I.
Nguyen Ngoc-Yen
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