Chlorine dioxide, ClO2, is one of the most effective bleaching agents for use in industrial and domestic process and services, and for commercial and consumer products. The strong oxidative potential of the molecule makes it ideal for a wide variety of uses that include disinfecting, sterilizing, and bleaching. Concentrations of chlorine dioxide in an aqueous solution as low as 1 part per million (ppm) or less, are known to kill a wide variety of microorganisms, including bacteria, viruses, molds, fungi, and spores. Higher concentrations of chlorine dioxide, up to several hundred ppms, provide even higher disinfection, bleaching and oxidation of numerous compounds for a variety of applications, including the paper and pulp industry, waste water treatment, industrial water treatment (e.g. cooling water), fruit-vegetable disinfection, oil industry treatment of sulfites, textile industry, and medical waste treatment.
Chlorine dioxide offers advantages over other commonly used bleaching materials, such as hypochlorite and chlorine. Chlorine dioxide can react with and break down phenolic compounds, and thereby removing phenolic-based tastes and odors from water. Chlorine dioxide is also used in treating drinking water and wastewater to eliminate cyanides, sulfides, aldehydes and mercaptans. The oxidation capacity of ClO2, in terms of available chlorine, is 2.5 times that of chlorine. Also, unlike chlorine/hypochlorite, the bactericidal efficiency of chlorine dioxide remains generally effective at pH levels of 6 to 10. Additionally, chlorine dioxide can inactivate C. parvum oocysts in water while chlorine/hypochlorite cannot. Hypochlorite and chlorine both react with the bleached target by inserting the chlorine molecule into the structure of the target. Though this mode of reaction can be effective, it can result in the formation of one or more chlorinated products, or by-products, which can be undesirable both from a economic sense (to eliminate hydrocarbons from the reaction media) and a safety and environmental standpoint. In addition, the step of bleaching by hypochlorite and chlorine results in the destruction of the bleach species itself, such that subsequent bleaching requires a fresh supply of the chlorine bleach. Another disadvantage is that certain microorganisms that are intended to be killed by these two commonly-used bleach materials can develop a resistance over time, specifically at lower concentrations of the chlorine or hypochlorite.
Chloride dioxide is generally used in an aqueous solution at levels up to about 35%. It is a troublesome material to transport and handle at high aqueous concentrations, due to its low stability and high corrosivity. This has required end users to generate chlorine dioxide on demand, usually employing a precursor such as sodium chlorite (NaClO2) or sodium chlorate (NaClO3).
A typical process for generating chlorine dioxide from sodium chlorate salt is the acid-catalyzed reaction:NaClO3+2HCl→NaCl+½Cl2+ClO2+H2O
Sodium chlorite is easier to convert to chlorine dioxide. A typical process for generating chlorine dioxide from sodium chlorite salt is the acid-catalyzed reaction:5NaClO2+4HCl→4ClO2+5NaCl+2H2O
Further details on the acid-catalyzed reactions of chlorites and chlorates to produce chlorine dioxide can be found in “Chlorine Dioxide Generation Chemistry” (A. R. Pitochelli, Rio Linda Chemical Company), Third International Symposium: Chlorine Dioxide Drinking Water, Process Water and Wastewater Issues, Sep. 14, 15, 1995, La Meridian Hotel, New Orleans, La., incorporated herein by reference.
A common method of making chlorine dioxide uses a multi-chamber electrolysis cell that converts the chlorite salt into chlorine dioxide. This method uses separately an anode compartment and a cathode compartment that are separated by an ion permeable membrane. The separate compartments operate with significantly different reactants, and contain solutions with different pH values. One example of a multi-compartment electrolysis cell is disclosed in U.S. Pat. No. 4,456,510, issued to Murakami et al. on Jun. 26, 1984, which teaches a process for forming chlorine dioxide by electrolyzing a solution of sodium chlorite in an electrolysis cell that contains an anode compartment and a cathode compartment separated by a diaphragm, preferably a cation exchange membrane. Another example of a two-chamber electrolysis cell is disclosed in U.S. Pat. No. 5,158,658, issued to Cawlfield, et al. on Oct. 27, 1992 which describes a continuous electrochemical process and an electrolytic cell having an anode chamber having a porous flow-through anode, a cathode chamber, and a membrane there between.
While separate-compartment, membrane-containing electrolysis cells have been used to make chlorine dioxide on a commercial scale, they have not been completely satisfactory. Even though they may have convenience advantages over the conventional acid catalysis production of chlorine dioxide, the electrochemical approach has proven to be more expensive to produce large volumes of chlorine dioxide. The electrolysis cells in commercial use, and disclosed in the prior art that utilize ion permeable membranes or diaphragms, require that the anolyte solution be substantially free of divalent cations, such as magnesium and calcium, to avoid the formation of precipitated calcium or magnesium salts that would quickly block and cover the membrane, and significantly reduce or stop the electrolysis reaction.
Consequently, there remains a need for a simple, safe method and apparatus for manufacturing chlorine dioxide to meet a wide variety of commercial and domestic uses, under a wide variety of situations. The present invention describes a method and an apparatus for making chlorine dioxide inexpensively, easily and effectively.