United States Patent (10) Patent N0.: US 6,761,872 B2 Roensch Et Al

US006761872B2 (12) United States Patent (10) Patent N0.: US 6,761,872 B2 Roensch et al. (45) Date of Patent: *Jul. 13, 2004

(54) METHOD FOR GENERATING CHLORINE 4,986,990 A 1/1991 Davidson et al. DIOXIDE 5,031,700 A 7/1991 McDougall et al. (75) Inventors: L. Fred Roensch, Glen Allen, VA (US); (Llst Connnued on next page‘) Richard H. Tribble, Richmond, VA FOREIGN PATENT DOCUMENTS (US); Dick Hilliard, Glen Allen, VA (Us) CA 1 264 659 1/1990 EP 0 448 659 B1 1/1997 (73) Assignee: ChemTreat, Inc., Glen Allen, VA (US) OTHER PUBLICATIONS ( * ) Notice: subleet_to any disclaimerg the term of this Aieta et al., “Determination of Chlorine Dioxide, Chlorine, Patent 15 extended or adlusted under 35 Chlorite, and Chlorate in Water,” Research and Technology, U50 154(b) by 0 days- Jan. 1984, pp. 64—70. Aieta et al., “Chlorine Dioxide Chemistry: Generation and This patent is subject to a terminal dis- Residual Analysis,” Symposium on Chemistry and Chemi claimer. cal Analysis of Water Intended for Reuse, 179th National Meeting of the Amer. Chem. Soc., Houston, TX, Mar. 1980, (21) Appl. N0.: 10/179,201 pp- 1—22 _ _ Masschelein et al., “Chlorine Dioxide—Chemistry and (22) Flled' Jun‘ 26’ 2002 Environmental Impact of Oxchlorine Compounds,” Ann (65) Prior Publication Data Arbor Science Publishers, Inc., 1979, pp. 1—191. US 2003/0003015 A1 Jan. 2, 2003 Primary Examiner—Robert J. Warden, Sr. _ _ Assistant Examiner—Sean E. Conley Related U-S- Apphcatlon Data (74) Attorney, Agent, or Firm—Harness, Dickey & Pierce, P.L.C. (62) Division of application No. 09/814,927, ?led on Mar. 23, 2001, now Pat. No. 6,436,345. (57) ABSTRACT (51) Int. Cl.7 ...... C01B 11/02 The present invention provides a new method for the in Sim (52) US Cl- ~~~~~~~~~~~~~ ~~ 423/477; 422/28; 422/37 generation of chlorine dioxide from a solution of sodium (58) Field of Search ...... 423/477; 422/28, chlorite and carbon dioxide. The present invention produces 422/37 effective levels of chlorine dioxide Without having to resort to the use of hydrochloric acid, sulfuric acid, hydrogen (56) References Cited peroxide, chlorine, sulfur dioxide, or methanol. By elimi nating the use of toxic and/or hazardous compounds, the U.S. PATENT DOCUMENTS present invention provides a safer means for generating and 3,591,515 A 7/1971 Lovely using chlorine dioxide in a Wider variety of applications than 4,247,531 A 1/ 1981 Hicks previously possible. For instance, the present invention can 4,414,193 A 11/1983 Fredette et al. easily be adapted for the treatment of combustion exhaust 4,547,381 A 10/1985 Mason et al. gases, ?ue gases, cooling toWers, chilled Water systems, 4,585,482 A 4/ 1986 Tice et al. contaminated groundWater, and agricultural produce or 4,590,057 A 5/1986 Hicks products. 4,683,039 A 7/1987 Twardowski et al. 4,689,169 A 8/1987 Mason et al. 4,861,514 A 8/1989 Hutchings 17 Claims, 3 Drawing Sheets

Solution Collection CollectionFlask B Reaction Flask A " ' ‘ Flask Carbon Dioxide US 6,761,872 B2 Page 2

US. PATENT DOCUMENTS 5,674,466 A 10/1997 Dahl et a1. _ 5,676,920 A 10/1997 Lipsztajn 5,091,107 A 2/1992 Hutchlngs 5,719,100 A * 2/1998 Zahradnik 61 a1...... 502/417 5,110,580 A * 5/1992 Rosenblatt et a1...... 423/472 5,770,171 A 6/1998 Sundblad et a1_ 5,126,070 A 6/1992 Lelfheltet 91- RE36,064 E * 1/1999 Davidson 618.1...... 424/665 5,158,658 A 10/1992 cawl?eld er 91- 5,922,776 A 7/1999 Wellinghoff 61 211. 5,234,678 A 8/1993 Rosenblatt et 211. 5,980,826 A 11/1999 Barenberg et a1_ 5,378,447 A 1/1995 Jackson 9191- 6,436,345 B1 * 8/2002 Roensch 61 a1...... 422/37 5,567,405 A 10/1996 Klatte et 211. 5,651,996 A 7/1997 RooZdar * cited by examiner

US 6,761,872 B2 1 2 METHOD FOR GENERATING CHLORINE chlorine dioxide -liberating compound and a hydrolyZable DIOXIDE organic acid-generating polymer (such as a methylvinylether/maleic anhydride copolymer) for loWering This invention relates to a method and apparatus for the pH of the composition and sloWly releasing the chlorine generating chlorine dioxide using carbon dioxide and dioxide. sodium chlorite and is a divisional of application Ser. No. 09/814,927 ?led Mar. 23, 2001 now US. Pat. No. 6,436,345 Chlorine dioxide is generally formed in one of tWo Ways, B1, Which issued Aug. 20, 2002, the entire contents of Which either by reducing a chlorate ion (Q05) in an acidic are incorporated by reference. medium according to reaction [1]: 10 BACKGROUND OF THE INVENTION 1. Field of the Invention or by oxidiZing a chlorite ion (ClO2_) according to reaction Chlorine dioxide is a strong oxidiZer and is Widely used [2]. as a bleaching and/or disinfectant agent, With hundreds of 15 tons being generated and used each day in the paper and Water treatment industries. Chlorine dioxide is also used in The choice of reducing agent for chlorine dioxide gen considerably smaller quantities in treating agricultural pro eration from chlorate has a great bearing on optimum reaction conditions, byproducts, and economics. Production duce and certain medical applications. Chlorine dioxide is from chlorite ion (ClO2_) is rather uneconomical. Indeed Well knoWn as an algaecide, fungicide, germicide, reaction [2] is reversible and chlorite is commonly synthe deodorant, bleach, and general antiseptic. Currently, the siZed from chlorine dioxide. Reducing agents typically used equipment required for generating chlorine dioxide usually for producing chlorine dioxide from chlorate are sulfur requires gaseous chlorine, sulfuric acid, or a combination of dioxide (S02), methanol (CH3OH), chloride ion (Cl'), and sodium hypochlorite (bleach) and acid With either sodium hydrogen peroxide (H202). The associated half reactions are chlorite or sodium chlorate. Because one or more haZardous represented by reactions [3]—[6]. materials are typically required to generate the chlorine dioxide or produced as a byproduct, the use of chlorine dioxide has been someWhat limited. The cost of chlorine dioxide generating units and the need for trained personnel 30 to operate and maintain the generating units has also ham pered the Wider utiliZation of chlorine dioxide. Accordingly, [6] there exists a need for a method that is capable of readily and Combining reaction [1] With reactions [3]—[6] produces safely producing chlorine dioxide Without requiring the use reactions [7]—[10]. of more haZardous chemicals or generating them as reaction byproducts. 2. Background Art [8] Chlorine dioxide is a haZardous material. Pure chlorine dioxide is an oily, dark amber liquid and is extremely 40 [9] unstable at temperatures above —40° C. Chlorine dioxide is [10] also explosively unstable as a gas in concentrations greater than 10% by volume in air or at partial pressures above 76 The byproducts formed are sulfate ion ($042“), formic mm Hg (1.46 psig). Above these levels chlorine dioxide may acid (HCOOH), chlorine (C12), and oxygen (O2). The acid detonate if it contacts ?ammable organic solvents or other equivalents required per mole of chlorine dioxide produced oxidiZable materials. Chlorine dioxide is also pressure sen differ and are Zero for sulfur dioxide, one for methanol, tWo for chloride, and one for hydrogen peroxide. Acid consump sitive and may decompose violently if compressed, making tion is also in?uenced by the process conditions used in it impractical to store or ship. Pure solutions of chlorine particular commercial designs. dioxide may also detonate if exposed to bright light or an In all systems, a side reaction may occur, reduction of ignition source such as heat, a spark, or an open ?ame. The chlorate to chloride according to reaction [11]. upper boundary for safe chlorine dioxide concentrations in aqueous solutions is about 8 g/L at 30° C., With most generating systems being designed to operate Well beloW 55 Steps must be taken to minimiZe this reaction by careful that limit, typically producing solutions having 1—3 g/L choice and control of reaction conditions. The basic mecha chlorine dioxide. nism of chlorine dioxide formation has been extensively It is Well knoWn that chlorine dioxide is formed by studied and has previously been described in detail else reaction of sodium chlorite and an acid. For instance, US. Where by Haller and Northgraves in ZAPPI (a publication of Pat. No. 3,591,515 to Lovely teaches the use of various the Technical Association of the Pulp and Paper Industry) acidifying agents to generate chlorine dioxide at pH levels (April 1955) and LenZi and Rapson in the Pulp Paper Magazine of Canada (1962). In each of the disclosed beloW 6 and form a dry poWder fungicide. US. Pat. No. mechanisms, chloride plays a crucial role as evidenced by its 4,330,531 to Alliger discloses a tWin-compartment container presence in all chlorate-based reaction media and by the for generating chlorine dioxide by reacting solutions of 65 trace amounts of chlorine in the chlorine dioxide formed. No lactic acid and sodium chlorite. US. Pat. No. 4,585,482 to chlorine dioxide is formed if chloride ion is not present in Tice, et al. discloses a biocidal composition that uses a the reaction medium. Chloride ion is introduced into the US 6,761,872 B2 3 4 system by reduction of chlorate to chloride (reaction [11] had led to the increasing use of Rapson’s R2 process. The above) or by addition of chloride in the feed. In a 1956 overall reaction in the R2 process, based on reaction [9] can ZAPPI article, Rapson proposed the following mechanism be represented by reaction [18]. Where chloride ion is the reducing agent.

The R2 process, like the Mathieson process, is also subject to an unWanted side reaction, based on reactions [6] and [11], resulting in the production of chlorine according to reaction [19]. The formation of byproducts, other than those identi?ed 10 in reactions [7]—[10], is governed by the chlorate salt and As can be seen in reactions [18] and [19], the R2 process acid selected. In all commercial processes, the chlorate salt generates chlorine, typically in a 0.6:1 Weight ratio With the used is sodium chlorate (NaClO3) and, to date, the most desired ClO2. Aportion of the chlorine, approximately 1 g/L commonly used acids have been sulfuric (H2504) and 15 of chlorine, usually remains in the chlorine dioxide solution hydrochloric (HCl). Consequently, the most common With the balance being separated and used to produce byproducts are sodium sulfate (Na2SO4) and sodium chlo sodium hypochlorite. Because the R2 reaction is much faster ride (NaCl). Depending upon process conditions, sulfate is than the Mathieson or Solvay reaction, the process can be recovered as neutral crystalline sodium sulfate, sodium carried out in a single vessel. HoWever, because the R2 sesquisulfate (Na3H(SO4)2), or is dissolved in an acidic reaction does not form the in situ acid of the Mathieson and effluent. If hydrochloric acid is used, sodium chloride is Solvay processes, and because additional Water is needed as recovered in a crystalline form or in an internally recycled the result of the addition of sodium chloride, the addition of solution. approximately 4.5 ton of acid per ton of C102 is required for Commercial chlorine dioxide generation systems can be the R2 process, Which limits its practicality for many indus broadly divided into atmospheric and sub-atmospheric pro trial locations, particularly in areas Where Water consump cesses. Atmospheric processes include the Mathieson, tion is an issue. Solvay, and Rapson R2 processes Which use sulfur dioxide, In response to the need to decrease the amount of Waste methanol, and sodium chloride, respectively, as the primary acid produced, efforts Were made to ?nd methods of crys reducing agents. Each of the processes use air to strip and talliZing or recycling the sodium sulfate from the Waste acid. dilute the chlorine dioxide and have an over?oW of spent One solution Was the application of an evaporator sulfuric acid. In the 1950s, the Mathieson process Was crystalliZer that could function as a chlorine dioxide gen dominant, folloWed by the Solvay process. The Mathieson erator With the steam and vacuum serving to control chlorine process Was developed in 1950 by Olin-Mathieson Chemical dioxide partial pressure. This led to the development of the Corporation and generated chlorine dioxide by reducing R3/SVP process in Which the basic R2 chemistry Was sodium chlorate With sulfur dioxide in the presence of 35 performed under vacuum, at higher temperatures, and in the sulfuric acid. presence of proprietary catalysts, to achieve suitable pro The Mathieson process chemistry generally folloWs reac duction rates and yields With a reaction solution at acidities tion [7] above, i.e. beloW 4.5 N H2504, the point beloW Which neutral anhy drous sodium sulfate can be crystalliZed and ?ltered. The 40 R3/SVP process also produced byproduct chlorine, typically HoWever under loW acidity conditions an unWanted side in about the same 0.6:1 ratio as the R2 process, With C102 reaction, re?ected in reactions [3] and [11] above, can and about 2 g/L of Cl2 remaining in the chlorine dioxide considerably reduce the yield. solution and With the balance again being separated and used to produce chlorine Water or hypochlorite. 45 As pulp mills decreased sodium and sulfur losses and also In order to suppress this side reaction, an excess of sulfuric increased their use of chlorine dioxide, the amount of acid is typically fed to a Mathieson-type generator to create sodium/sulfur byproducts formed exceeded the pulp mills’ a 450—500 g/L acid concentration (9—10 N H2504). The acid needs. Further, the use of byproduct chlorine to produce (typically 2—2.5 tons of acid per ton of ClO2 produced) hypochlorite also became less attractive as mills Worked over?oWs from the generator and must be recovered and toWard obtaining both higher brightness pulps and suppress used elseWhere or, less preferably, neutraliZed and dis ing or eliminating chloroform formation. These conditions charged. Some unreacted sodium chlorate also leaves the and demands led to processes using hydrochloric acid as a generator With the acid. The loss of the sodium chlorate and replacement for part or all of the sulfuric acid in an R3 or the contribution of reaction [16] typically limits the yield of SVP process (see reaction [20] beloW). The hydrochloric a Mathieson-type generator to less than 90%. 55 acid could either be purchased or made by burning byprod The Solvay process uses methanol as the reducing agent uct chlorine With hydrogen in a hydrochloric acid burner, or and, like the Mathieson process, typically utiliZes a 450—500 alternatively, the chlorine could be reacted With sulfur g/L sulfuric acid solution. The primary reaction, based on dioxide and Water to make a mixture of hydrochloric and reaction [8], can be Written as shoWn in reaction [17]. sulfuric acids (see reaction [21] beloW). These changes, along With the partial replacement of sulfuric acid, signi? cantly reduced the byproduct sodium sulfate and virtually eliminated the chlorine Water and hypochlorite byproducts. Both the Mathieson and Solvay processes are capable of 2NaClO3+2HCl+H2SO4—>2ClO2+Na2SO4+Cl2 [20] producing chlorine dioxide solutions having loW chlorine concentrations. 65 C12+SO2+2H2O—>2HC1+H2SO4 [21] By the 1960s, the groWing recognition of the crucial role Hydrochloric acid processes can be operated indepen of the chloride ion in chlorine dioxide synthesis processes dently or integrated With an onsite chlorate plant. The key US 6,761,872 B2 5 6 reactions are the production of chlorine dioxide according to 2NaClO3+H2O2+H2SO4—>2ClO2+O2+2H2O+Na2SO4 [29] reaction [22] With sodium chlorate being produced by elec trolysis of the byproduct salt according to reaction [23]. The use of hydrogen peroxide as the reducing agent has the advantage of producing no byproduct chlorine and directly producing neutral sodium sulfate, but the relatively high cost of hydrogen peroxide has limited its Widespread use. Efforts have also been made to develop various electro The byproduct chlorine from the generator and supplemental chemical processes Which can split the salt cake byproduct chlorine are then reacted With the byproduct hydrogen from and/or the sodium chlorate feed into their respective acids the chlorate reaction to produce HCl according to reaction and bases according to reactions [30] and [31]. [24], producing an overall stoichiometry for the integrated process, i.e., the combination of reactions [22]—[24], as re?ected in reaction [25]. NaClO3+H2O—>NaOH+HClO3 [31]

15 The electrochemical processes, hoWever, are not yet cost competitive With the more traditional chemical processes so This integrated process produces no sodium sulfate and their adoption tends to be driven more strongly by environ requires chlorine input in a Weight ratio of approximately mental pressures or restrictions relating to plant discharges 0.7:1 to that of the product ClO2 (reaction [24]) to make the and disposals. necessary quantity of hydrochloric acid and help balance the Although not discussed here in detail, those of skill in the NaOH/Cl2 needs. The integrated process does, hoWever, art Will be familiar With a Wide variety of alternative increase the space and capital requirements compared With processes for generating chlorine dioxide including: other alternative chlorine dioxide plants. The Modi?ed Mathieson process in Which small amounts The interest in eliminating byproduct chlorine, decreasing of sodium chloride (NaCl) Were added to the reactants in the byproduct sodium sulfate, and improving the overall ef? 25 primary generator to improve the reduction ef?ciency of the ciency and production rate in turn led to the development of chlorate and increase generator capacity. an alternative methanol-based process. The overall reaction The Hoist process Which is a batch process similar to the for this R8/SVP Methanol (MeOH) process can be repre Mathieson process With the signi?cant difference being sented by reaction [26]. found in the solution concentrations and the batch-Wise operation. The Kesting Day-Fenn, or Day-Kesting, process utiliZes hydrochloric acid (HCl) for reducing NaClO3 and is suitable The reaction represented in reaction [26] does not take into for integration With an electrolytic plant for the production account the smaller amounts of methanol Which typically of chlorate. leave the generator in the gas phase and/or dissolved in the 35 The R1 process, the ?rst of the “R” processes developed chlorine dioxide solution. Some of the formic acid by Dr. HoWard Rapson, relied on reacting sodium chlorate (HCOOH) reacts further according to reaction [27]. and sodium chlorite in a strong acid to form chlorine dioxide. The R2 process used a mixture of NaClO3, H2504, and Because formic acid has a similar vapor pressure to that of 40 NaCl for reducing the chlorate, thereby eliminating the need Water, most of it is stripped from the generator. Atypical 10 for S02. The R2 process, hoWever, increased the formation g/L ClO2 solution Will also contain 0.2—0.9 g/L of CH30H, of chlorine (C12) Which Was then absorbed (usually to form 1.7 g/L of CHOOH, 0.4 g/L of CO2, and 0.1 g/L of C12. sodium hypochlorite (NaOCl)) after the C102 absorption Because the R8/SVP MeOH processes typically operate at toWer. acidities above 5 N, sodium and sulfate are recovered as 45 The R2H process replaced NaCl and half of the HZSO4 sodium sesquisulfate (Na3H(SO4)2). This process has the used in the R2 process With hydrochloric acid (HCl). advantages of virtually eliminating the chlorine byproduct The R3 process Was another modi?ed R2 process in (0.1 g/L of Cl2 in a 10 g/L solution of ClO2), producing less Which the reaction temperatures are salt cake than the R3/SVP processes, increasing chlorine increased to boiling and concentrations increased strength dioxide yield to over 95%, and improving production capac ened to permit crystalliZation of sodium sulfate (Na2SO4) in ity. The residual chlorine is a product of the loW concentra the reaction vessel. This process, commercialiZed as the R3 tion of chloride ions in the generator. If this loW concentra process by Erco Ltd. (noW a part of Sterling Pulp Chemicals, tion of chloride ions is eliminated, the production of chlorine Ltd.) and as the SVP process by the Hooker Chemical dioxide ceases, and the reactor enters a condition knoWn as Company (noW Eka-Nobel), is also sometimes referred to as “White-out” in Which the reactor generates a toxic White gas 55 the “effluent-free” process because since the byproduct (comprising primarily chlorine and Water vapor) and a removed is crystalline Na2SO4. grayish-White generator liquor. Maintaining loWer acidities The R3H process, like the R2H process, replaced the (approximately 5—7 N) helps maintain a sufficient concen NaCl and half of the HZSO4 of the R3 process With HCl. tration of chloride ions but results in decreased efficiency of The R5 process is basically the same as the R3 process methanol use. Alternatively, operation at higher acidities With the exception that the NaCl and all of the HZSO4 are (approximately 8—10 N) leads to more ef?cient methanol replaced With HCl, leaving HCl and NaClO3 as the only feed use, but additional steps, including perhaps the addition of streams into the generator With the byproduct being crys sodium chloride, are typically added to guard against a talline NaCl suitable for reuse in chlorate manufacture. White-out condition. Further, the R5 process is distinguished from the R2 process When hydrogen peroxide is used as the reducing agent the 65 by the higher operating temperatures and concentrations that overall reaction based on reaction [9] can be represented permit the recovery of the crystalline byproduct and incor reaction [29]. porates technology developed by DoW Chemical Canada. US 6,761,872 B2 7 8 The R6 process Was an “integrated” process used in conjunction With an electrolytic plant producing NaClO3. Although With simple mixing the initial chlorite is even Variations of this basic “integrated” process are also knoWn tually consumed, the yield of ClO2 tends to be loWer than as the Lurgi integrated process or the Chemetics integrated expected, typically 60—80% depending on the starting pro process. portions of the chlorite and acid reactants. Indeed, a con The R7 process Was another modi?cation of the R3 current reaction appears to be re?ected by reaction [36]. process in Which chlorine gas from the exit stream Was reacted With SO2 to form a mixed acid of HZSO4 and HCl that is then returned to the generator. The only substantial byproduct of the R7 process is anhydrous Na2SO4. 10 As a result of reaction [36] and other competing side The R8 process utiliZes methanol as the reducing agent reactions, the C102 produced tends to be impure and contain and produces an acid salt, sodium sesquisulfate [Na3H(SO4) both chlorine and chlorate. The most common industrial 2], as a byproduct. The sodium sesquisulfate must then be applications rely on hydrochloric acid, sulfuric acid, and neutraliZed With caustic soda before recovery in the liquor acetic acid, With hydrochloric acid being the most Widely system. 15 used. Catalysts for this family of reaction include sodium The R9 process is an extension of the R8 process in Which peroxide, potassium perborate, and cobalt sulfate. Typically the sodium sesquisulfate is diluted With Water and separated yields are improved by running the synthesis With excess into caustic soda and sulfuric acid in a membrane cell. acid, typically about 300%, to increase the synthesis yields The R10 process is another extension of the R8 process in to levels approaching 100% (i.e., 4 moles of ClO2 produced Which the sodium sesquisulfate is diluted With both Water 20 from 5 moles of NaClO2). A much less frequently used and methanol folloWing removal of sodium sulfate in a synthesis described in SWiss patent 481,839 (1970) relies on second ?ltration stage With the ?ltrate being returned to the a combination of poWdered NaClO2 and poWdered citric generator. acid that are dissolved in 1.5 liters of Water to produce a The R11 process, uses hydrogen peroxide as the reducing solution having about 5 g ClO2 in a solution of citric acid. agent for C102 generation. This process has seen limited use 25 Chlorite can also be reacted With chlorine to produce due to the higher operating costs associated With hydrogen ClO2 according to reaction [37]. peroxide. The R12 process electrochemically converts sodium chlo rate to a mixed feed of sodium chlorate and chloric acid HoWever, implementing this synthesis using solid chlorite (HClO3~7H2O) Which is fed to the C102 generator, Which 30 and gaseous chlorine introduces contact problems such as proportionally reduces sodium sulfate formation, While pro the chlorite becoming coated With salt and local heating that ducing sodium hydroxide as a by-product. increases the danger of an explosion. As a result, it is The R13 process, uses chloric acid from the R12 process common to implement this reaction in solution under sub to produce ClO2 Without the byproduct Na2SO4. atmospheric conditions to reduce the danger of explosion. The SVP-Lite process is another methanol based process 35 BRIEF SUMMARY OF THE INVENTION similar in some respects to the R8 process. The main difference betWeen the SVP-Lite and R8 processes is the It is an object of the present invention to provide an acid strength in the generator. improved method for the generation of aqueous chlorine The SVP-HP process is similar to SVP-Lite except that it dioxide solutions using as reactants only carbon dioxide gas utiliZes hydrogen peroxide rather than methanol for reducing 40 and sodium chlorite that can implemented With less complex the chlorate, is operated at subatmospheric pressures, and apparatus and With greater safety. produces as its byproduct neutral sodium sulfate (Na2SO4). It is a further object of the present invention to produce The SVP-HPA process is an atmospheric process that is chlorine dioxide in an aqueous system Without adding similar to the SVP-HP process in that it also utiliZes hydro corrosive ions such as sulfate and chloride. gen peroxide and produces NaZSO4 as a byproduct. In 45 It is a further object of the present invention to produce addition to the Na2SO4, the reactor also produces a spent chlorine dioxide in an aqueous system Without increasing acid stream. the dissolved solids. The Lurgi process is an integrated process similar to the It is a further object of the present invention to produce R6 and Chemetics processes in Which an onsite electrolytic chlorine dioxide in an aqueous system While reducing the process is used to produce ClO2 from chlorine and Water. 50 potential for calcium carbonate scaling. The hydrogen produced is reacted With Cl2 to form HCl, Which is, in turn, used in the process. Additional Cl2 is It is a further object of the present invention to produce required to provide suf?cient HCl for the process. chlorine dioxide in an aqueous system to reduce organic The Chemetics process is another integrated process stack emissions. similar to the R6 and Lurgi processes. 55 It is a further object of the present invention to produce An alternative to the chlorate-based ClO2 synthesis is the chlorine dioxide in an aqueous system for treatment of chlorite-based synthesis. In the chlorite-based synthesis, a ground Water contamination including, but not limited to, chlorite solution, typically sodium chlorite, is mixed With an phenols, bacteria, manganese, and iron. acid to form an unstable chlorous acid, Which in turn, It is a further object of the present invention to produce disproportionates into chlorine dioxide according to reac 60 chlorine dioxide in an aqueous system for conversion of tions such as [32]—[34]. hydrogen sul?de into hydrogen sulfate. It is a further object of the present invention to provide a method for generating an aqueous solution of chlorine dioxide or a gaseous mixture including chlorine dioxide 65 suitable for the treatment of agricultural products including leaf products such as tobacco, root crops such as potatoes, The resulting overall reaction corresponds to reaction [35]. and other fruits and vegetables. US 6,761,872 B2 9 10 BRIEF DESCRIPTION OF THE DRAWINGS Using the experimental apparatus illustrated in FIG. 1, optimal collection of chlorine dioxide Was achieved by FIG. 1 is a representation of the experimental apparatus initially injecting carbon dioxide into the sodium chlorite used to con?rm the utility of the present invention and solution for ten minutes folloWed With a ten-minute incu includes a carbon dioxide source, a ?oW meter, a reaction bation period (during Which no further carbon dioxide Was ?ask holding a sodium chlorite solution and a series of tWo added). Thereafter, the carbon dioxide injection Was collection ?asks initially ?lled With deioniZed Water (“DI resumed for periods of tWo minutes, each injection period Water”). folloWed by another ten-minute incubation period. The ?rst FIG. 2 is a representation of the application of the present one-liter collection ?ask Was then removed and the chlorine invention for treatment of a contaminated Well and sur 10 dioxide levels measured after each gas injection period and rounding groundwater. changed to a fresh ?ask. Total carbon dioxide injection FIG. 3 is a representation of the application of the present periods have ranged from 30 minutes to almost seven hours invention for treatment of the exhaust stream generated by using a nominal carbon dioxide injection rate of 5 SCFH. the combustion of organic fuels in Which the carbon dioxide Based on the applicants’ experimental Work, it does not from the exhaust stream is utiliZed in the production of 15 appear that constant gas ?oW is necessary. Indeed, it Was chlorine dioxide. found that having a period of carbon dioxide injection folloWed by an incubation period (during Which no carbon DETAILED DESCRIPTION OF THE dioxide Was injected) helps prevent loss of chlorine dioxide INVENTION into the atmosphere. With incubation times included, experi 20 ments have been run up to almost seven hours Without As re?ected in the discussion above, there are many exhausting the initial charge of sodium chlorite solution. methods for generating chlorine dioxide. Each of the prior art methods, hoWever, either requires the use of a strong acid EXAMPLE 1 or other oxidiZer, produces unWanted byproducts, or is prohibitively expensive. The present invention resolves the The results for Example 1 provided in Table 1 beloW Were de?ciencies of the prior art methods by utiliZing as the 25 generated using one initial 10-minute gas exposure and primary reactants only a solution of sodium chlorite and seven 2-minute periods of carbon dioxide injection, fol carbon dioxide gas to produce chlorine dioxide. loWed by seven 10-minute incubation periods. To maximiZe Using the apparatus illustrated in FIG. 1, gaseous carbon chlorine dioxide collection, both ?rst and second collection ?asks Were used in series. dioxide Was injected into a solution of sodium chlorite to 30 produce chlorine dioxide that Was, in turn, captured in the ?rst and second collection ?asks. The volume of sodium TABLE 1 chlorite used Was 100 ml of a 10% sodium chlorite solution having an initial pH of 9—10. It Was found that the chlorine C102 C102’ C103’ dioxide yield from a conventional sodium chlorite solution, 35 Reaction Flask: 606 mg/L 23.02 g/L 1.73 g/L typically stabiliZed With carbonate at a pH of about Collection Flask 1: 19 mg/L 26 mg/L 2.1 mg/L 11.8—12.4, could be improved by at least partially removing Collection Flask 2: 19 mg/L 19 mg/L 2.7 mg/L the stabiliZing carbonate. It Was found that suf?cient car bonate removal Was achieved by adding 500 g of calcium EXAMPLE 2 chloride to one liter of 10% sodium chlorite to remove a 40 portion of the carbonate and loWer the solution pH to Using the same conditions as in Example 1, a second approximately 10.2. experiment Was run to produce the results provided in Table It appears that during the carbon dioxide injection step, 2. the formation of carbonic acid begins to loWer the pH of the sodium chlorite solution. The formation of chlorine dioxide 45 TABLE 2 begins When the pH of the sodium chlorite solution becomes slightly acidic, e.g., When the pH reached about 6.0—5 .0 In C102 C102’ C103’ the experimental apparatus, the sodium chlorite solution pH Reaction Flask: 523 mg/L 86.90 g/L 4.16 g/L tended to stabiliZe at a value betWeen 4 and 6 after about tWo Collection Flask 1: 5 mg/L 99 mg/L 5.9 mg/L minutes of carbon dioxide injection. If desired, the initial pH 50 Collection Flask 2: 5 mg/L 98 mg/L 5.9 mg/L could also be partially reduced With another acid, such as acetic or citric acid, to hasten the production of chlorine dioxide With the injection of carbon dioxide. In most Although minor amounts of chlorate Were detected in the instances, hoWever, it is believed that the carbon dioxide collection ?asks, the applicants believe that the chlorate detected may have been the result of the disproportion injection alone Will be sufficient to accomplish the aims of 55 the present invention. decomposition reaction betWeen the time the sample as collected and the time When the sample Was analyZed. Several stoichiometric reactions have been proposed for the acid activation of a chlorite solution. The tWo stoichio Additional experiments Were then conducted to explore the impact of treating the initial sodium chlorite solution to 60 remove a portion of the calcium carbonate. EXAMPLE 3 Starting With a commercially available 25% sodium chlo The source of acid in the claimed method is the formation rite solution having up to 5% alkalinity, a test solution of of carbonic acid When carbon dioxide is dissolved in Water: 65 10% sodium chlorite and not more than 2% alkalinity Was prepared. The starting pH of the non-neutraliZed solution Was approximately 11.7 and the temperature Was approxi US 6,761,872 B2 11 12 mately 21° C. Table 3 represents data collected With varying calcium chloride to form a calcium carbonate precipitate and incubation periods, each followed by a 2 minute period of then ?ltered to remove substantially all of the calcium CO2 injection at 5 scf/hour (142 liters/hour), for a total test carbonate. The starting pH of the neutraliZed solution Was time of 95 minutes. The CO2 injection loWered the solution approximately 11.7 and the temperature Was approximately pH from the initial 11.7 to betWeen 5.6 and 5.7 and main 21° C. Table 5 represents data collected With varying incu tained the pH Within this range for the duration of the bation periods, each folloWed by a 2 minute period of CO2 experiment. injection at 5 scf/hour (142 liters/hour), for a total test time of almost 8 hours. The CO2 injection loWered the solution pH from the initial 11.7 to betWeen 4.4 and 4.7 over the TABLE 3 course of the experiment. As re?ected in Table 5, despite the 10 Incubation Collection Flask Reaction increased alkalinity, the injected CO2 Was able to form Time ClO2 concentration Flask suf?cient carbonic acid to maintain the loWer pH level and (minutes) (mg/L) pH to continue to produce ClO2 over the course of the experi 2 0.2 5.6 ment for a total of approximately 284 mg. in Flask 1 and 2 0.8 5 .7 Flask 2. 15 2 1.2 5.6 2 1.5 5.6 TABLE 5 2 1.8 5.6 5 1.7 5.6 10 5.1 5.6 Incubation C102 C102 Temperature Time Flask 1 Flask 2 10 6.2 5.6 20 10.8 5.6 20 (O C.) (minutes) pH (mg/L) (mg/L) 20 11.7 5 .7 21 2 4.7 0.5 0.0 21 2 4.5 0.7 0.1 Total 75 41 21 2 4.5 1.5 0.1 21 5 4.5 1.7 0.1 21 5 4.4 1.7 0.1 Over the 95 minute duration of the experiment, 41 milli 25 21 5 4.5 2.9 0.2 grams of chlorine dioxide Was generated from the 10% 25 5 — 5.6 0.4 sodium chlorite solution. 25 5 — 5.1 0.5 25 10 — 7.6 0.7 EXAMPLE 4 25 10 — 7.8 0.9 25 10 — 7.6 0.9 Starting With the same commercially available 25% 30 25 10 — 7.7 0.9 25 20 — 10.1 1.3 sodium chlorite solution having up to 5% alkalinity, a test 25 20 — 10.6 1.1 solution of 10% sodium chlorite With the alkalinity adjusted 25 20 — 10.4 1.2 to 3% Was prepared. The starting pH of the non-neutraliZed 25 20 — 11.0 1.1 solution Was approximately 11.7 and the temperature Was 25 20 — 10.6 1.2 30-33 20 — 12.4 1.1 approximately 21° C. Table 4 represents data collected With 30-33 20 — 12.9 1.1 varying incubation periods, each folloWed by a 2 minute 30-33 20 — 14.0 1.5 period of CO2 injection at 5 scf/hour (142 liters/hour), for a 30-33 20 — 12.6 1.6 30-33 20 — 13.1 1.7 total test time of 95 minutes. The CO2 injection loWered the 30-33 20 — 14.6 1.8 solution pH from the initial 11.7 to betWeen 5.7 and 5.9 for 40 30-33 20 — 12.9 1.6 the duration of the experiment. As re?ected in Table 4, 30-33 20 — 11.7 1.3 despite the increased alkalinity, the injected CO2 Was able to 30-33 20 — 11.2 1.2 form sufficient carbonic acid both to loWer the pH and to 30-33 20 — 12.7 1.1 30-33 20 — 12.3 1.8 produce slightly decreased amount of ClO2 (i.e., 36.8 mg.). 30-33 20 4.6 12.5 1.9 45 TABLE 4 Total 409 255.5 28.5

Incubation Collection Flask Reaction Time ClO2 concentration Flask As illustrated in FIGS. 2 and 3, the basic chlorine dioxide (minutes) (mg/L) pH generator constructed and operated according to the inven tion can be used to create and/or supply loW-level aqueous 2 0.2 5.8 2 0.4 5.7 solutions of chlorine dioxide in a variety of applications 2 0.9 5.8 including treatment of exhaust streams from organic fuel 2 0.9 5.8 combustion or in situ ground Water treatment. 2 1.3 5.8 5 1.9 5.8 As shoWn in FIG. 2, a chlorine dioxide generator accord 10 4.2 5.9 55 ing to the present invention can be used to generate and 10 4.8 5.9 supply a gaseous mixture of C102 and CO2 to Well Water for 20 9.9 5.8 the treatment of organic or bacterial contamination. In the 20 12.3 5.8 alternative, the C102 and CO2 stream can be mixed With a Total 75 36.8 Well Water stream to form a slightly acidic chlorine dioxide 60 solution that is then injected into the Well. As shoWn in FIG. 3, a signi?cant component of the EXAMPLE 5 exhaust stream is carbon dioxide. As a result, a portion of the exhaust stream can be passed through a sodium chlorite Starting With the same commercially available 25% solution to generate the desired chlorine dioxide. It is sodium chlorite solution having up to 5% alkalinity, a test 65 preferred that in instances in Which the sodium chlorite solution of 10% sodium chlorite that also included a minor solution also includes sodium carbonate the sodium chlo portion of sodium carbonate Was prepared and treated With rite:sodium carbonate ratio be maintained at a value of at US 6,761,872 B2 13 14 least 5:1. The remaining portion of the exhaust stream can 9. A method for producing chlorine dioxide according to also be combined With a chlorine dioxide solution according claim 6; Wherein the reaction solution includes at least about to knoWn methods doWnstream of the chlorine dioxide 10% sodium chlorite. generator to treat the exhaust further; thereby reducing both 10. Amethod for producing chlorine dioxide according to the hydrocarbon and particulate content of the exhaust 5 claim 1; further comprising: stream. combining the mixed gas stream With a contaminated gas Although particular preferred embodiments of the inven stream to form a treated gas stream. tion have been disclosed in detail for illustrative purposes, it 11. Amethod for producing chlorine dioxide according to Will be recogniZed by those of ordinary skill in the art that claim 10; Wherein the contaminated gas stream is a com certain variations or modi?cations to the disclosed method 10 bustion exhaust stream. lie Within the scope of the invention as de?ned by the folloWing claims. 12. A method for producing chlorine dioxide according to We claim: claim 1; Wherein the step of passing carbon dioxide gas 1. Amethod of producing chlorine dioxide comprising the through the reaction solution and the step of forming chlo steps of: 15 rine dioxide are substantially simultaneous. 13. A method for producing chlorine dioxide according to forming an aqueous reaction solution of sodium chlorite; the reaction solution having an initial pH greater than claim 12; Wherein the step of passing carbon dioxide gas 7; through the reaction solution occurs under a pressure of about one atmosphere. passing carbon dioxide gas through the reaction solution; 14. Amethod for producing chlorine dioxide according to reacting a portion of the carbon dioxide gas to form claim 12; Wherein the step of passing carbon dioxide gas carbonic acid in the reaction solution; includes injecting a substantially constant volume of carbon loWering the pH of the reaction solution to a pH of less dioxide gas through the reaction solution during the step of than about 5.5; forming chlorine dioxide. forming chlorine dioxide; and 25 15. Amethod for producing chlorine dioxide according to removing a mixed gas stream including carbon dioxide claim 12; Wherein the step of passing carbon dioxide gas and chlorine dioxide from the reaction solution. further comprises: 2. A method for producing chlorine dioxide according to injecting carbon dioxide into the reaction solution for a claim 1; Wherein the step of forming the aqueous solution of 30 ?rst period; sodium chlorite further comprises the step of removing a majority of any carbonate present in the reaction solution. incubating the reaction solution for a second period 3. A method for producing chlorine dioxide according to during Which substantially no additional carbon diox claim 2; Wherein the step of removing a majority of the ide is injected into the reaction solution; and carbonate present in the reaction solution further comprises injecting carbon dioxide into the incubated reaction solu adding suf?cient calcium chloride to precipitate calcium 35 tion for a third period. carbonate and removing the calcium carbonate from the 16. Amethod for producing chlorine dioxide according to reaction solution. claim 15; Wherein: 4. A method for producing chlorine dioxide according to the ?rst period is betWeen about 2 minutes and 10 claim 1; Wherein the sodium chlorite solution has a sodium minutes; chlorite:sodium carbonate ratio of not less than 5:1. the second period is betWeen about 2 minutes and about 5. A method for producing chlorine dioxide according to 20 minutes; and claim 1; Wherein the step of passing carbon dioxide gas the third period is about 2 minutes. through the reaction solution includes passing a combustion 17. Amethod for producing chlorine dioxide according to exhaust stream through the reaction solution. claim 15; Wherein the step of passing carbon dioxide gas 6. A method for producing chlorine dioxide according to further comprises the steps of: claim 1; further comprising: a) injecting carbon dioxide into the reaction solution for passing the mixed gas stream through Water to form a a ?rst period; collection solution that includes chlorine dioxide and is substantially free of sodium chlorite. b) incubating the reaction solution for a second period 7. A method for producing chlorine dioxide according to during Which substantially no additional carbon diox claim 6; Wherein the collection solution has a pH of less than ide is injected into the reaction solution; and about 7. c) injecting carbon dioxide into the incubated reaction 8. A method for producing chlorine dioxide according to solution for a third period; and claim 6; Wherein the pH of the reaction solution is loWered 55 repeating steps b) and c) at least once. to a value of betWeen about 4.4 and 6.0 during the step of forming the chlorine dioxide. * * * * *