Abstract:
An improved method for inhibiting sulfite oxidation in scrubber liquor of the type containing sulfite species is disclosed. A topping agent selected from the group consisting of polyphosphate compounds, and organo-phosphonic acid compounds, and mixtures thereof, is added to the scrubbing liquor to supplement and enhance the antioxidant activity of sulfite antioxidant agents such as the linear polyethyleneamines and substituted secondary and tertiary aromatic amines.

Description:
FIELD OF THE INVENTION 
     The present invention pertains to the utilization of certain phosphorus containing &#34;topping&#34; agents which are admitted into an aqueous gas scrubber medium to enhance the efficiency of sulfite antioxidant compounds which are also admitted to the aqueous scrubber medium. The aqueous scrubber medium is adapted to scrub SO 2  gas and typically comprises solids materials such as fly ash. 
     BACKGROUND OF THE INVENTION 
     A common problem faced in industry today is the removal of sulfur dioxide, an environmental pollutant formed by the oxidation of sulfur or sulfur-containing substances, from industrial exhaust gases. This pollutant is found as a component in various waste gases such as blast furnace gases, emission gases from certain chemical factories, and flue gases from coal or oil-burning furnaces used in utility plants. For example, in U.S. Pat. No. 3,918,521, to Snavely et al, a steam injection oil recovery system is disclosed wherein the flue gas from an oil-burning steam generator contains sulfur dioxide. 
     One widely accepted method for attacking this problem of sulfur dioxide removal is the use of wet scrubber systems in which sulfur dioxide-containing gases are intimately contacted with a scrubbing liquor. The scrubbing liquor is selective for sulfur dioxide due to the addition of certain chemicals such as lime, limestone or magnesium oxide. 
     When the sulfur dioxide-containing gas is contacted with the scrubbing liquor, these noted chemicals react with the sulfur dioxide to form a sulfite-containing reaction product which remains with the liquor, permitting the resulting relatively sulfur dioxide-free exhaust gas to pass on through the process stream. The thus spent scrubbing liquor is then circulated through a regeneration sidestream in which, as the name implies, the sulfur dioxide-selective chemicals are regenerated or replaced; and the thus replenished scrubbing liquor is recirculated into the wet scrubber system. It is due to the oxidation of this sulfite-containing reaction product in the spent scrubbing liquor that many problems arise, as will be illustrated below. 
     One major type of wet scrubber system for removing sulfur dioxide is known as the &#34;limestone slurry system&#34; in which the scrubbing liquor contains limestone as a sulfur dioxide-selective chemical. Upon contacting the sulfur dioxide-containing gas with the limestone slurry, sulfur dioxide is removed according to the reaction: 
     
         CaCO.sub.3 +SO.sub.2 +1/2H.sub.2 O→CaSO.sub.3 1/2H.sub.2 O+CO.sub.2. (1) 
    
     A major problem experienced with these systems is related to a secondary reaction in which the aqueous oxidation of sulfite to sulfate in the scrubbing liquor occurs. In the pH range of most scrubbers, the reaction is: 
     
         HSO.sub.3.sup.- +1/2O.sub.2 →SO.sub.4.sup.-- +H.sup.+, or (2) 
    
     
         SO.sub.3.sup.-- +1/2O.sub.2 →SO.sub.4.sup.--.       (3) 
    
     The negative consequences of this oxidation are several-fold as follows: 
     1. Calcium sulfate, a most tenacious scale, is formed and precipitates or crystallizes on various surfaces throughout the system. In contrast, the formation of calcium sulfite precipitate can be controlled in the scrubber by relying on the usually lower pH and hence favoring the formation of the more soluble bisulfite species: 
     
         CaSO.sub.3.1/2H.sub.2 O or CaSO.sub.3 (solid)+H.sub.2 SO.sub.3 →Ca.sup.++ +2HSO.sub.3.sup.- +1/2H.sub.2 O.        (4) 
    
     2. The pH of the scrubbing liquor drops (Equation 2), thus reducing the scrubber efficiency. 
     3. Examination of scale surfaces in scrubbers occasionally shows calcium sulfate to be the initial depositing species with other constituents forming on the calcium sulfate deposit. 
     A second major type of wet scrubber system for removing sulfur dioxide from a gas is known as the &#34;double-alkali system&#34; which contains a scrubbing loop and a separate precipitation loop. This system utilizes a sodium based scrubbing loop in which sulfur dioxide is removed from exhaust gas according to: 
     
         Na.sub.2 SO.sub.3 +SO.sub.2 +H.sub.2 O→2NaHSO.sub.3. (5) 
    
     If sodium hydroxide is also used, sulfur dioxide is removed according to: 
     
         NaOH+SO.sub.2 →NaHSO.sub.3.                         (6) 
    
     In the precipitation loop, the spent scrubbing liquor is regenerated by treatment with lime to precipitate the sulfite reaction product according to: 
     
         2NaHSO.sub.3 +Ca(OH).sub.2 →CaSO.sub.3 +Na.sub.2 SO.sub.3 +2H.sub.2 O.                                                        (7) 
    
     There are, indeed, drawbacks in the double-alkali system related to the oxidation of sulfite to sulfate in the scrubbing liquor. As opposed to the bisulfite ion, the sulfate ion is no longer regenerable and is of no further use in the process. This necessitates purging of the sulfate from the scrubbing liquor to avoid calcium sulfate scale. This purging results in the loss of sodium compounds from the scrubbing liquor, which compounds must be replaced at considerable expense by the addition of soda ash, usually to the scrubbing loop. 
     To inhibit the oxidation of the sulfite species, certain antioxidant agents have been added to the sulfite containing aqueous scrubber medium. For instance, in U.S. patent application Ser. No. 25,304, filed Mar. 30, 1979 (of common assignment herewith), there is disclosed the utilization of polyethyleneamine compounds, to achieve the desired sulfite antioxidation result. Japanese Pat. No. Sho 49-43893, published Apr. 25, 1974, discloses the use of certain aromatic amine sulfite antioxidant additives to effect the desired result. 
     Despite the advent and use of the above noted sulfite antioxidant additives, one problem that has been encountered in this procedure is that, in those instances wherein the aqueous scrubber medium contains solids, such as fly ash, the effectiveness of the antioxidant additives is reduced, probably due to adsorption of the antioxidant onto the solids particles. Also, the metal surfaces of the scrubber may act to reduce the activity of the antioxidant when used by itself. 
     Accordingly, it is apparent that there is a need in the art for a method and means for improving the efficacy of sulfite antioxidant agents, especially in those instances wherein solids matter, typically fly ash or the like particulate matter, is contained within the aqueous scrubber medium, or when the metal surfaces of the scrubber tend to reduce the activity of the antioxidant. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The above and other problems inherent in the art are solved by the present invention. It has been surprisingly found that the addition of a phosphorus containing &#34;topping&#34; agent to the aqueous scrubber medium significantly enhances the performance of the sulfite antioxidant agent. 
     Typically, the &#34;topping&#34; agent is added to the aqueous medium in a range of about 1-100 parts &#34;topping&#34; agent per 1 million parts of said medium, depending upon the solids content of the aqueous medium. A preferred &#34;topping&#34; agent addition range, with respect to normally encountered scrubber aqueous mediums, is from about 0.1 to 25 parts &#34;topping&#34; agent to 1 million parts of the medium. In the most preferred range, the antioxidant agents are added to the aqueous medium in an amount of 0.5-10 ppm. 
     Although applicants are not to be bound by any particular theory of operation, it is believed that the phosphorus containing &#34;topping&#34; agent preferentially adsorbs onto the solids particles in the aqueous scrubber medium, leaving the antioxidant free to inhibit oxidation. 
     Preferably, separate feed points should be used for the &#34;topping&#34; agent and the antioxidant. The &#34;topping&#34; agent should be injected into the scrubber system at a point prior to and sufficiently removed from the antioxidant injection point. This sequential addition provides ample time for the &#34;topping&#34; agent to adsorb onto the solids and/or scrubber metal surfaces and hinder antioxidant deactivation by adsorption. 
     Sulfite antioxidants such as those disclosed in U.S. patent application Ser. No. 25,304, filed Mar. 30, 1979 (of common assignment herewith) may be utilized in combination with the novel &#34;topping&#34; agents herein disclosed. The entire content of the above commonly assigned U.S. patent application is hereby incorporated by reference. 
     Typically, the antioxidant is added to the aqueous scrubber medium in an amount as low as about 0.5 parts (weight) per million parts of the medium. The preferred lower limit is about 1 ppm. On the other hand, the upper limit could be as high as about 100 parts antioxidant per million parts of aqueous medium. Based on economic factors, 10 ppm is considered to be the most preferred upper limit. Of course, the amount of antioxidant added to a particular aqueous medium depends on known factors such as the nature and severity of the problem being encountered. 
     Specifically, the sulfite antioxidant agents may include linear, water soluble polyethyleneamines having the formula: 
     
         NH.sub.2 (CH.sub.2 CH.sub.2 NH).sub.X H 
    
     wherein X is greater than 1, and is preferably 2 to about 10. The following specific sulfite antioxidant compounds may be mentioned as exemplary: 
     diethylenetriamine 
     triethylenetetramine 
     tetraethylenepentamine 
     pentaethylenehexamine 
     It is to be understood that the term polyethyleneamine as used herein shall also refer to any of the well known water soluble salts of these sulfite antioxidant compounds. 
     Other suitable antioxidant agents include aromatic amines, and water soluble salts thereof, having the formula: ##STR1## wherein R 1  =OH, NHR 5 , lower alkyl having 1-3 carbon atoms, or, ##STR2## R 2  =H, or OH with the proviso that R 1  =OH when R 2  =OH, R 3  =H, or lower alkyl having 1 to 3 carbon atoms, 
     R 4  =H or NHR 5 , 
     R 5  =H, or lower alkyl having 1 to 3 carbon atoms. 
     Exemplary aromatic amine sulfite antioxidant agents include: 
     N,N dimethyl-p-phenylenediamine sulfate 
     N,N&#39; di-sec-butyl-p-phenylenediamine 
     N,N&#39; di-isopropyl-p-phenylenediamine 
     p-aminophenyl 
     p-aminobiphenyl 
     N-methylaniline 
     p-methylaniline 
     3,4-dihydroxyaniline 
     As the &#34;topping&#34; agent which is to be added to the aqueous scrubber medium in addition to the sulfite antioxidant agent, water soluble polyphosphates, and organo-phosphonic acid compounds are efficacious. 
     The water soluble polyphosphate compounds which are operable for the present purposes generally include the sodium polyphosphates, the potassium polyphosphates, the lithium polyphosphates and ammonium polyphosphates. The following specific compounds may be listed as being exemplary: 
     Na 5  P 3  O 10  sodium tripolyphosphate 
     Na 4  P 2  O 7  tetrasodium pyrophosphate 
     Na 2  H 2  P 2  O 7  disodium pyrophosphate 
     (NaPO 3 ) 6  sodium hexametaphosphate 
     K 4  P 2  O 7  tetrapotassium pyrophosphate 
     Na 2  (NH 4 ) 2  P 2  O 7  sodium ammonium pyrophosphate 
     Na 4  HP 3  O 10 .1.5H 2  O sodium acid tripolyphosphate 
     K 5  P 3  O 10  potassium tripolyphosphate 
     Na 6  P 4  O 13  sodium tetrapolyphosphate 
     Na 3  P 3  O 9  sodium trimetaphosphate 
     Na 4  P 4  O 12  sodium tetrametaphosphate 
     (NaPO 3 ) 7  sodium heptametaphosphate 
     (NaPO 3 ) 8  sodium octametaphosphate 
     Additional polyphosphates which are utilizable under the present concept are set forth in the publication entitled &#34;Phosphorus and its Compounds&#34;, J. R. Van Wazer, Volume 1, Interscience Publishers, New York, 1958. 
     The organo-phosphonic acid compounds, useful in conjunction with the present invention, are those having a carbon to phosphorus bond, i.e., ##STR3## 
     Compounds within the scope of the above description generally are included in one of perhaps 3 categories which are respectively expressed by the following general formulas (8, 9, 10): ##STR4## where R is a lower alkyl having from about one to six carbon atoms, e.g., methyl, ethyl, butyl, propyl, isopropyl, pentyl, isopentyl and hexyl, substituted lower alkyl of from one to six carbon atoms, e.g., hydroxyl and amino-substituted alkyls; a mononuclear aromatic (aryl) radical, e.g., phenyl, benzene, etc., or a substituted mononuclear aromatic compound, e.g., hydroxyl, amino, lower alkyl substituted aromatic, e.g., benzylphosphonic acid; and M is a water-soluble cation, e.g., sodium, potassium, ammonium, lithium, etc. or hydrogen. 
     Specific examples of compounds which are encompassed by this formula include: 
     
         ______________________________________ methylphosphonic acidCH.sub.3 PO.sub.3 H.sub.2ethylphosphonic acidCH.sub.3 CH.sub.2 PO.sub.3 H.sub.22-hydroxyethylphosphonic acid ##STR5##2-aminoethylphosphonic acid ##STR6##isopropylphosphonic acid ##STR7##benzene phosphonic acidC.sub.6 H.sub.5 PO.sub.3 H.sub.2benzylphosphonic acidC.sub.6 H.sub.5 CH.sub.2 PO.sub.3 H.sub.2 ##STR8##                      (9)______________________________________ 
    
     where R 1  is an alkylene having from about one to about 12 carbon atoms or a substituted alkylene having from about 1 to about 12 carbon atoms, e.g., hydroxyl, amino, etc. substituted alkylenes, and M is as earlier defined for (8) above. It is to be noted that oligomers, especially ester condensate oligomers, of the structure shown above in formula (9) are also useful for the present purposes. These oligomers are detailed in U.S. Pat. No. 4,237,005 (Becker). 
     Specific exemplary compounds and their respective formulas which are encompassed by the above formula (9) are as follows: 
     
         ______________________________________ methylene diphosphonic acidH.sub.2 O.sub.3 PCH.sub.2 PO.sub.3 H.sub.2ethylidene diphosphonic acidH.sub.2 O.sub.3 PCH(CH.sub.3)PO.sub.3 H.sub.2isopropylidene diphosphonic acid(CH.sub.3).sub.2 C(PO.sub.3 H.sub.2).sub.21-hydroxyethylidene 1,1-diphosphonic acid (HEDP) ##STR9##hexamethylene diphosphonic acidH.sub.2 O.sub.3 PCH.sub.2 (CH.sub.2).sub.4 CH.sub.2 PO.sub.3 H.sub.2trimethylene diphosphonic acidH.sub.2 O.sub.3 P(CH.sub.2).sub.3 PO.sub.3 H.sub.2decamethylene diphosphonic acidH.sub.2 O.sub.3 P(CH.sub.2).sub.10 PO.sub.3 H.sub.21-hydroxypropylidene 1,1-diphosphonic acidCH.sub.3 CH.sub.2 C(OH)(PO.sub.3 H.sub.2).sub.21,6-dihydroxy-1,6-dimethylhexamethylene-1,6-diphosphonic acidH.sub.2 O.sub.3 PC(CH.sub.3)(OH)(CH.sub.2).sub.4 C(CH.sub.3)(OH)PO.sub.3H.sub.21,2-dihydroxy-1,2-diethylethylene-1,2-diphosphonic acidH.sub.2 O.sub.3 PC(OH)(C.sub.2 H.sub.5)(OH)(C.sub.2 H.sub.5)CPO.sub.3H.sub.2 ##STR10##                    (10)______________________________________ 
    
     where R 2  is a lower alkylene having from about one to about four carbon atoms, or an amine or hydroxy substituted lower alkylene; R 3  is [R 2  -PO 3  M 2  ], H, OH, NH 2 , substituted amino, an alkyl having from one to six carbon atoms, a substituted alkyl of from one to six carbon atoms (e.g., OH, NH 2  substituted), a mononuclear aromatic radical and a substituted mononuclear aromatic radical (e.g., OH, NH 2  substituted); R 4  is R 3  or the group represented by the formula ##STR11## where R 5  and R 6  are each hydrogen, lower alkyl of from about one to six carbon atoms, a substituted lower alkyl (e.g., OH, NH 2  substituted), hydrogen, hydroxyl, amino group, substituted amino group, a mononuclear aromatic radical, and a substituted mononuclear aromatic radical (e.g., OH and amine substituted); R 7  is R 5 , R 6 , or the group R 2  -PO 3  M 2  (R 2  is as defined above); n is a number of from 1 through about 15; y is a number of from about 1 through about 14; and M is as earlier defined. 
     Compounds or formulas therefore which can be considered exemplary for the above formulas are as follows: 
     
         ______________________________________nitrilo-tri(methylene phosphonic acid)N(CH.sub.2 PO.sub.3 H.sub.2).sub.3imino-di(methylene phosphonic acid)NH(CH.sub.2 PO.sub.3 H.sub.2).sub.2n-butylamino-N,N-di(methylene phosphonic acid)C.sub.4 H.sub.9 N(CH.sub.2 PO.sub.3 H.sub.2).sub.2decylamino-N,N-di(methylene phosphonic acid)C.sub.10 H.sub.21 N(CH.sub.2 PO.sub.3 H.sub.2).sub.2trisodiumpentadecylamino-N,N-dimethylene phosphonateC.sub.15 H.sub.31 N(CH.sub.2 PO.sub.3 HNa)(CH.sub.2 PO.sub.3 Na.sub.2)n-butylamino-N,N-di(ethylene phosphonic acid)C.sub.4 H.sub.9 N(CH.sub.2 CH.sub.2 PO.sub.3 H.sub.2).sub.2tetrasodium-n-butylamino-N,N-di(methylene phosphonate)C.sub.4 H.sub.9 N(CH.sub.2 PO.sub.3 Na.sub.2).sub.2triammonium tetradecylamino-N,N-di(methylene phosphonate)C.sub.14 H.sub.29 N(CH.sub.2 PO.sub.3 (NH.sub.4).sub.2)CH.sub.2 PO.sub.3HNH.sub.4phenylaminodi-N,N-(methylene phosphonic acid)C.sub.6 H.sub.5 N(CH.sub.2 PO.sub.3 H.sub.2).sub.24-hydroxyphenylamino-N,N-di(methylene phosphonic acid)HOC.sub.6 H.sub.4 N(CH.sub.2 PO.sub.3 H.sub.2).sub.2N-propylphenylamino-N,N-di(methylene phosphonic acid)C.sub.6 H.sub.5 (CH.sub.2).sub.3 N(CH.sub.2 PO.sub.3 H.sub.2).sub.2tetrasodiummethylphenyl-N,N-aminodi(methylene phosphonicacid)C.sub.6 H.sub.5 (CH.sub.2).sub.2 N(CH.sub.2 PO.sub.3 Na.sub.2).sub.2ethylene diamine-N,N,N&#39;,N&#39;-tetra(methylene phosphonic acid)(H.sub.2 O.sub.3 PCH.sub.2).sub.2 N(CH.sub.2).sub.2 N(CH.sub.2 PO.sub.3H.sub.2).sub.2trimethylene diamine-N,N,N&#39;,N&#39;-tetra(methylene phosphonic acid)(H.sub.2 O.sub.3 PCH.sub.2).sub.2 N(CH.sub.2).sub.3 N(CH.sub.2 PO.sub.3H.sub.2).sub.2heptamethylene diamine-N,N,N&#39;,N&#39;-tetra(methylene phosphonicacid)(H.sub.2 O.sub.3 PCH.sub.2).sub.2 N(CH.sub.2).sub.7 N(CH.sub.2 PO.sub.3H.sub.2).sub.2decamethylene diamine-N,N,N&#39;,N&#39;-tetra(methylene phosphonicacid)(H.sub.2 O.sub.3 PCH.sub.2).sub.2 N(CH.sub.2).sub.10 N(CH.sub.2 PO.sub.3H.sub.2).sub.2hexamethylene diamine-N,N,N&#39;,N&#39;-tetra(methylene phosphonicacid)(H.sub.2 O.sub.3 PCH.sub.2).sub.2 N(CH.sub.2).sub.6 N(CH.sub.2 PO.sub.3H.sub.2).sub.2tetradecamethylenediamine-N,N,N&#39;,N&#39;-tetra(methylene phosphonicacid)(H.sub.2 O.sub.3 PCH.sub.2).sub.2 N(CH.sub.2).sub.14 N(CH.sub.2 PO.sub.3H.sub.2).sub.2ethylenediamine-N,N,N&#39;-tri(methylene phosphonic acid)(H.sub.2 O.sub.3 PCH.sub.2).sub.2 N(CH.sub.2).sub.2 NHCH.sub.2 PO.sub.3H.sub.2ethylenediamine-N,N&#39;-di(methylene phosphonic acid)H.sub.2 O.sub.3 PCH.sub.2 NH(CH.sub.2).sub.2 NHCH.sub.2 PO.sub.3 H.sub.2n-hexylamine-N,N-di(methylene phosphonic acid)C.sub.6 H.sub.13 N(CH.sub.2 PO.sub.3 H.sub.2).sub.2diethylenetriamine-N,N,N&#39;,N&#34;,N&#34;-penta(methylene phosphonicacid)(H.sub.2 O.sub.3 PCH.sub.2).sub.2 N(CH.sub.2).sub.2 N(CH.sub.2 PO.sub.3H.sub.2)(CH.sub.2).sub.2 N--(CH.sub.2 PO.sub.3 H.sub.2).sub.2ethanolamine-N,N-di(methylene phosphonic acid)HO(CH.sub.2).sub.2 N(CH.sub.2 PO.sub.3 H.sub.2).sub.2n-hexylamino-N-(isopropylidene phosphonic acid)-N-methylene-phosphonic acidC.sub.6 H.sub.13 N(C(CH.sub.3).sub.2 PO.sub.3 H.sub.2)(CH.sub.2 PO.sub.3H.sub.2)trihydroxymethylmethylamino-N,N-di(methylene phosphonicacid)(HOCH.sub.2).sub.3 CN(CH.sub.2 PO.sub.3 H.sub.2).sub.2triethylenetetraamine-N,N,N&#39;,N&#34;,N&#39;&#34;,N&#39;&#34;-hexa(methylenephosphonic acid)(H.sub.2 O.sub.3 PCH.sub.2).sub.2 N(CH.sub.2).sub.2 N(CH.sub.2 PO.sub.3H.sub.2)(CH.sub.2).sub.2 N--(CH.sub.2 PO.sub.3 H.sub.2)(CH.sub.2).sub.2 N(CH.sub.2 PO.sub.3 H.sub.2).sub.2N-monoethanoldiethylenetriamine-N,N&#34;,N&#34;-tri(methylenephosphonic acid)HOCH.sub.2 CH.sub.2 N(CH.sub.2 PO.sub.3 H.sub.2)(CH.sub.2).sub.2 NH(CH.sub.2).sub.2 N--(CH.sub.2 PO.sub.3 H.sub.2).sub.2chlorethyleneamine-N,N-di(methylene phosphonic acid)ClCH.sub.2 CH.sub.2 N(CH.sub.2 PO(OH).sub.2).sub.2______________________________________ 
    
     The above listed compounds are included for illustration purposes and are not intended to be a complete listing of the compounds which are operable within the confines of the present invention. 
    
    
     SPECIFIC EMBODIMENTS OF THE INVENTION 
     The invention is illustrated by the following examples which are intended merely for purpose of illustration and are not to be regarded as limiting the scope of the invention or the manner in which it may be practiced. 
     Test Procedure 
     300 mls. of 0.02 M Na 2  SO 3  solution were placed into a 500 ml Erlenmeyer flask. The solution was then stirred by means of a magnetic stirring device. In certain &#34;runs&#34;, the resulting solution was heated to about 50° C. to simulate scrubber conditions. The pH of the solution was then adjusted to about 6.5 by the addition of 1.0 N HCl. 
     Fly ash solids, if required, were added to the solution in the desired amounts to result in a sulfite containing aqueous medium having the constituency proportions indicated in the tables hereinbelow. In certain &#34;runs&#34; metallic coupons were added to the resulting solutions so as to better approximate the physical conditions usually encountered within a scrubber. The antioxidant and &#34;topping&#34; agents were then added, when required, in the amounts indicated in the tables. 
     To determine the initial amount of SO 3   --  present in each solution, a 15 ml aliquot of the solution was removed from the flask and was filtered through a 0.2μ millipore filter. Ten mls of the filtrate were then placed in an Erlenmeyer flask. Titrimetric determination of the sulfite content was then undertaken utilizing potassium iodide/iodate solution as is detailed in Betz &#34;Handbook of Industrial Water Conditioning&#34;, Eighth Edition; 1980, pages 411-12. 
     Immediately after the initial aliquot was removed, a gas bubbler, with 1.5 l./min. of CO 2  free air passing through it, was placed in the sulfite solution. At predetermined time intervals (as shown in the tables hereinbelow), additional aliquot samples of the sulfite containing solution were taken and sulfite content determination made therefor. For each sample, the following calculations were made: ##EQU1## 
     EXAMPLES 
     In accordance with the above general testing procedures, the following examples and comparative examples were performed. The results and example parameters are set forth in the following tables. 
     
                       TABLE I______________________________________0.1% Fly Ash Solids     ppm                    PercentTreatment Oxidized  Percent      Antioxidantand Treat-     After     Oxidation After                            Activity Af-ment Level     30 Minutes               30 Minutes   ter 30 Minutes______________________________________Control   434/396   31.20/30.00  0/05 ppm OP-2     229       18.39        39.902 ppm TEPA     418       30.66        02 ppm TEPA/1 ppm OP-2      93       7.36         75.942 ppm TEPA/5 ppm OP-2      55       4.35         85.78______________________________________ 
    
     
                       TABLE II______________________________________1.0% Fly Ash Solids                             Percent       ppm        Percent    Antioxidant       Oxidized   Oxidation  ActivityTreatment and       After      After      AfterTreatment Level       30 Minutes 30 Minutes 30 Minutes______________________________________Control     410/480    36.68/37.06                             0/0 5 ppm TEPA/10 ppm OP-2 326        29.69      19.4710 ppm TEPA/10 ppm OP-2 195        18.93      48.6515 ppm TEPA/ 5 ppm OP-2 138        11.59      68.4015 ppm TEPA/10 ppm OP-2 120        10.04      72.63______________________________________ OP-2 = a phosphonate oligomer of 1hydroxypropane-1,1-diphosphonic acid structure: ##STR12## TEPA = tetraethylenepentamine   As shown in Tables I and II, combinations of tetraethylenepentamine and phosphonate are effective sulfite antioxidants when fly ash solids are present. 
    
     
                       TABLE III______________________________________No Solids                          %                          AntioxidantTreatment.sup.+     % Oxidation                          Activity______________________________________Control             30.76*     01 ppm Dequest 2054  25.70      16.450.5 ppm TEPA        23.26      24.381.0 ppm TEPA        5.63*      81.701 ppm TEPA/1 ppm Dequest 2054               7.35       76.100.5 ppm AO-23       30.62      0.451.0 ppm AO-23       1.60*      94.801 ppm AO-23/1 ppm Dequest 2054               2.95       90.4125 ppm TKPP         24.60      20.03______________________________________ .sup.+ All treatment levels are ppm active. *These numbers represent the average of two or more experiments. ##STR13## 36% potassium salt solution (25% active acid) available Monsanto ##STR14## active ingredient = 50% (weight) solvent =  50% (weight) 40/60 mixture of anhydrous methanol and isopropanol available  DuPont. TKPP = Tetrapotassium pyrophosphate 
    
     
                       TABLE IV______________________________________Stainless Steel Coupon              %         % AntioxidantTreatment.sup.+    Oxidation Activity______________________________________Control            37.15     01 ppm Dequest 2054 26.01     29.991 ppm TKPP         32.60     12.251 ppm TEPA         28.78*    22.531 ppm TEPA/1 ppm Dequest 2054              7.81*     78.981 ppm AO-23        30.40     18.171 ppm AO-23/1 ppm Dequest 2054              0.89      97.601 ppm AO-23/1 ppm TKPP              8.69      76.61______________________________________ .sup.+ All treatment levels are ppm active. *These numbers represent the average of two or more experiments. 
    
     
                       TABLE V______________________________________0.1% Fly Ash              %         % AntioxidantTreatment.sup.+    Oxidation Activity______________________________________Control            46.46*    05 ppm Dequest 2054 27.23     41.392 ppm TEPA         40.29     13.285 ppm TEPA         8.73      81.212 ppm TEPA/2 ppm Dequest 2054              12.79     72.472 ppm TEPA/5 ppm Dequest 2054              4.61      90.082 ppm TEPA/5 ppm STPP              8.80      81.062 ppm TEPA/5 ppm TKPP              9.32      79.94______________________________________ .sup.+ All treatment levels are ppm active. *These numbers represent the average of two or more experiments. STPP = Sodium Tripolyphosphate 
    
     
                       TABLE VI______________________________________1.0% Fly Ash                          %                 %        AntioxidantTreatment.sup.+       Oxidation                          Activity______________________________________Control               43.88*   05 ppm Dequest 2054    32.75    25.3650 ppm TKPP           22.64    48.401 ppm TEPA            44.92    05 ppm TEPA            45.29    020 ppm TEPA           16.32    62.8150 ppm TEPA           14.10    67.87100 ppm TEPA          12.25*   72.081 ppm TEPA/1 ppm Dequest 2054                 47.98    02 ppm TEPA/2 ppm Dequest 2054                 44.25    05 ppm TEPA/5 ppm Dequest 2054                 34.38    21.652 ppm TEPA/50 ppm STPP                 46.87    05 ppm TEPA/100 ppm STPP                 36.63    16.521 ppm AO-23           42.08    4.105 ppm AO-23           21.84    50.231 ppm AO-23/1 ppm Dequest 2054                 38.00    13.402 ppm AO-23/2 ppm Dequest 2054                 27.63    16.255 ppm AO-23/5 ppm Dequest 2054                 18.44    57.982 ppm AO-23/25 ppm TKPP                 33.95    22.632 ppm AO-23/50 ppm TKPP                 9.00     79.491 ppm AO-23/100 ppm STPP                 17.12    60.982 ppm AO-23/50 ppm TKPP                 12.42    71.702 ppm AO-23/50 ppm TKPP                 17.60    59.89Control (50° C.)                 99.20    02 ppm AO-23/50 ppm TKPP (50° C.)                 34.37    65.352 ppm AO-23/100 ppm TKPP (50° C.)                 32.9     69.475 ppm AO-27/50 ppm TKPP (50° C.)                 31.94    67.80______________________________________ .sup.+ All treatment levels are ppm active. *These numbers represent the average of two or more experiments. 
    
     When no solids were present (Table III), combinations of antioxidant and phosphonate were no more effective than the antioxidant alone. However, when solids were present, the antioxidants alone showed almost no activity except at extremely high treatment levels (Table VI). 
     Also examined were combinations of phosphates (TKPP and STPP) and antioxidants (Tables IV, V, VI). The results were comparable to those attained by use of the phosphonate/antioxidant combination. To a slight extent, the antioxidants exhibited better activity when combined with the phosphonates instead of the phosphates. However, since the phosphates are less costly, the use of high feed rates for products containing phosphate/antioxidant combinations may still be a cost-effective method of controlling sulfite oxidation in medium to high solids scrubber systems. 
     In Table III wherein no solids were present, 1 ppm of either N,N&#39;-diisopropyl-p-phenylenediamine or tetraethylenepentamine exhibited good antioxidant activity. 1 ppm of &#34;Dequest&#34; 2054 and 25 ppm TKPP, by themselves, showed only slight antioxidant activity. 
     When a stainless steel coupon was introduced into the system (Table IV), both antioxidants lost their activity. The combination of 1 ppm Dequest 2054 and 1 ppm of either antioxidant increased the antioxidant activity significantly in the presence of the stainless steel coupon. A combination of 1 ppm AO-23/1 ppm TKPP also worked, but to a slightly lesser extent. 
     The fly ash used in the Table III-Table VI examples and comparative examples was obtained from a midwestern public service company. It was first washed to remove a majority of the soluble surface ions. With 0.1% fly ash (Table V), 2 ppm TEPA shows almost no antioxidant activity. However, a combination of 5 ppm Dequest 2054 and 2 ppm TEPA increased the antioxidant activity significantly. Additional examples using 2 ppm TEPA/5 ppm TKPP and 2 ppm TEPA/5 ppm STPP were also undertaken (Table V). Both of these combined treatments exhibited substantial increases in antioxidant activity relative to TEPA alone. 
     With 1% fly ash present (Table VI), typical feedrates of 2 ppm antioxidant and 50 ppm phosphate were needed to effectively inhibit oxidation. Here, it is noted that AO-23 showed better antioxidant activity than TEPA. Also, TKPP was observed to be a better &#34;topping&#34; agent than STPP. 
     
                       TABLE VII______________________________________No Solids                              Percent      ppm         Percent     Antioxidant      Oxidized    Oxidation   ActivityTreatment and      After 30    After 30    After 30Treatment Level      Minutes     Minutes     Minutes______________________________________Control    323/342/371 =                  24.41/26.47/                              0/0/0      -x = 345    27.00       -x = 0                  25.96 = -x3 ppm DETA 361         26.25       .sup.]06 ppm DETA 163         13.22       49.082 ppm PEHA  13          0.99       96.19______________________________________ DETA = diethylenetriamine PEHA = pentaethylenehexamine 
    
     
                       TABLE VIII______________________________________0.170% Fly Ash                              Percent       ppm        Percent     Antioxidant       Oxidized   Oxidation   ActivityTreatment and       After      After       AfterTreatment Level       30 Minutes 30 Minutes  30 Minutes______________________________________Control     393/511    36.39/42.65/                              0/0/0/0/0       558/460       500 -x = 484                  42.44/39.48 -x = 0                  41.56 -x = 40.516 ppm DETA  475        38.15       5.8315 ppm DETA 215        19.03       53.026 ppm DETA/5 ppm Deq. 2054       16         1.49        96.326 ppm DETA/25 ppm TKPP 151        12.58       68.942 ppm Deq. 2054.sup.+       260        24.35       39.445 ppm STPP.sup.+       337        32.88       18.835 ppm TKPP.sup.+       258        24.27       40.092 ppm AO-23 158        13.88       65.742 ppm AO-23/2 ppm Deq. 2054       0          0           100.002 ppm AO-23/5 ppm Deq. 2054       0          0           100.005 ppm. Deq. 2000.sup.+       180        16.36       59.615 ppm Deq. 2010.sup.+       257        22.16       45.292 ppm TEPA/5 ppm Deq. 2000       153        14.32       64.652 ppm TEPA/5 ppm Deq. 2010       25         2.34        94.225 ppm TEPA/5 ppm Deq. 2000       0          0           100.002 ppm PEHA  420        37.83       6.665 ppm PEHA  79         6.13        84.862 ppm PEHA/25 ppm TKPP 165        14.28       64.752 ppm PEHA/5 ppm Deq. 2054       137        10.38       74.37______________________________________ .sup.+ this slight activity may be due to the phosphonate/phosphate adsorbing onto the solids and deactivating some of the catalytic surface sites for sulfite oxidation. &#34;Dequest 2000&#34; = 50% active acid solution of ##STR15## &#34;Dequest 2010&#34; = 60% active solution of ##STR16## available Monsanto. 
    
     
                       TABLE IX______________________________________0.5% Fly Ash      ppm       Percent   Percent      Oxidized  Oxidation AntioxidantTreatment and      After 30  After 30  ActivityTreatment Level      Minutes   Minutes   After 30 Minutes______________________________________Control    417/416   37.60/35.40                          0/0      -x = 417  -x = 36.40                          -x = 05 ppm TEPA 356       37.32      0.005 ppm Deq. 2054      338       27.93     23.475 ppm TEPA/5 ppm Deq. 2054      317       29.94     20.7110 ppm TEPA      160       17.00     53.4210 ppm TEPA/5 ppm Deq. 2054      162       14.18     61.15______________________________________ 
    
     
                       TABLE X______________________________________1.0% Fly Ash                             Percent      ppm         Percent    Antioxidant      Oxidized    Oxidation  ActivityTreatment and      After 30    After 30   After 30Treatment Level      Minutes     Minutes    Minutes______________________________________Control    494/404/560 40.99/35.78/                             0/0/0/0/0      474/358     43.44      -x = 0      -x = 458    43.11/37.10                  -x = 40.072 ppm AO-23      419         34.95      12.7025 ppm TKPP      446         37.29      6.9450 ppm STPP      424         39.74      0.8210 ppm TEPA      498         45.77      0.0015 ppm TEPA      196         16.00      60.0010 ppm Deq. 2054      494         43.18      0.0010 ppm TEPA/5 ppm Deq. 2054      433         39.36      1.7710 ppm TEPA/10 ppm Deq. 2054      405         34.32      14.3515 ppm TEPA/5 ppm Deq. 2054      325         26.32      34.3115 ppm TEPA/10 ppm Deq. 2054      85          8.47       78.865 ppm AO-23/10 ppm Deq. 2054      120         10.36      74.1510 ppm AO-23/10 ppm Deq. 2054      107         8.52       78.735 ppm AO-23/25 ppm TKPP      96          8.64       78.4410 ppm AO-23/25 ppm TKPP      24          2.32       94.21______________________________________ 
    
     
                       TABLE XI______________________________________0.1% Fly Ash       ppm       Percent   Percent       Oxidized  Oxidation AntioxidantTreatment and       After 30  After 30  ActivityTreatment Level       Minutes   Minutes   After 30 Minutes______________________________________5 ppm Combina-tionA           126       9.55      76.43B           18        1.35      96.66C           80        6.22      84.642 ppm Combina-tionD           0         0         100.00E           0         0         100.00F           89        6.03      85.11______________________________________ Combination A = 6.6% Dequest 2000, 6.6% TEPA Combination B = 6.6% Dequest 2010, 6.6% TEPA Combination C = 6.6% Dequest 2054, 6.6% TEPA Combination D = 5.0% Dequest 2000, 5% AO23 Combination E = 5.0% Dequest 2010, 5% AO23 Combination F = 5.0% Dequest 2054, 5% AO23 
    
     With respect to Tables VII-XI herein, it is noted that the phosphonate (Dequest) materials showed only slight antioxidant activity when used alone. However, when used in combination with antioxidants and with fly ash solids present, these phosphonates significantly increased the activity of the antioxidants (See Table VIII). 
     The DETA and PEHA performed very much like TEPA. When no solids were present these compounds exhibited sulfite antioxidant activity. The DETA at comparable levels did not work as well as TEPA, while PEHA at comparable levels worked better than TEPA. With fly ash solids added to the system, the performance of these compounds deteriorated significantly. When phosphonates or phosphates were used in combination with these compounds their performance improved markedly (See Table VIII). 
     Several combination products were formulated. Their compositions are listed below Table XI. These compositions remained stable for 24 hours with no obvious signs of precipitation or phase separation. These &#34;combined&#34; compositions appear to be only slightly less active than when fed as dual products. 
     
                       TABLE XII______________________________________No Solids                              Percent                   Percent    Antioxidant      ppm          Oxidation  ActivityTreatment and      Oxidized     After 30   After 30Treatment Level      After 30 Minutes                   Minutes    Minutes______________________________________Control    323/342/371 =                   24.41/26.47/                              0/0/0      -x = 345     27.00      -x = 0                   -x = 25.961 ppm AO-22      0            0          100.002 ppm DMPDS      61           4.39       82.85______________________________________ AO-22 is a chloride salt of N,N&#39; Disec-butyl-p-phenylenediamine, availabl DuPont. DMPDS = N,Ndimethyl-p-phenylenediamine sulfate 
    
     
                       TABLE XIII______________________________________0.1% Fly Ash Solids                             Percent      ppm         Percent    Antioxidant      Oxidized    Oxidation  ActivityTreatment and      After 30    After 30   After 30Treatment Level      Minutes     Minutes    Minutes______________________________________Control    393/511/558/                  36.39/42.65/                             0/0/0      460/500     42.44/39.48/                             0/0      -x = 484    41.56      -x = 0                  -x = 40.511 ppm AO-22      370         28.24      30.283 ppm AO-22      0           0          100.005 ppm Deq. 2054/1 ppm AO-22      30          2.33       94.2425 ppm TKPP/1 ppm AO-22      28          2.35       94.202 ppm DMPDS      361         28.99      28.445 ppm DMPDS      0           0          100.002 ppm DMPDS/5 ppm Deq. 2054      0           0          100.002 ppm DMPDS/25 ppm TKPP      45          3.53       91.29______________________________________ 
    
     Tables XII and XIII show that, when no fly ash solids are present in the aqueous gas scrubber medium, the antioxidants (AO-22, DMPDS) show excellent activity. However, the antioxidant activity of these antioxidants deteriorates when solids (fly ash) are present. When the polyphosphates and phosphonates were added in addition to the antioxidants, in the presence of fly ash solids, the antioxidant activity increased significantly. 
     While certain features of this invention have been described in detail with respect to various embodiments thereof, it will, of course, be apparent that other modifications can be made within the spirit and scope of this invention and it is not intended to limit the invention to the exact details shown above except insofar as they are defined in the appended claims.