Abstract:
A method of controlling micro-organisms during peroxide bleaching of cellulose pulp comprising adding to the pulp an effective micro-organism controlling amount of  
     a) an aqueous biocide solution comprising one or more organic peracids; and  
     b) an aqueous solution comprising one or more chelating agents.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates to a method of controlling microorganisms responsible for degrading hydrogen peroxide in hydrogen peroxide pulp bleaching processes using a synergistic combination of one or more organic peracid biocides and one or more chelating agents.  
         BACKGROUND OF THE INVENTION  
         [0002]    One of the factors which can reduce the efficiency of hydrogen peroxide pulp bleaching processes is the presence of the enzyme catalase in the process liquors. The micro-organisms which produce these enzymes are commonly found in all areas of pulp and paper mills. During respiration, various toxic oxygen derivatives are produced within the bacterial cell. To destroy these toxic substances, bacteria produce enzymes, the most common being catalase, which breaks down hydrogen peroxide to oxygen and water. The destruction of hydrogen peroxide by catalase can lower bleaching efficiency and decrease the brightness levels in finished paper. The problems of inefficient hydrogen peroxide usage caused by the presence of catalase are particularly prevalent where the paper mill is producing paper from recycled waste paper. Accordingly, there exists an ongoing need for treatments that reduce the number of catalase producing microorganisms present in the process liquors.  
           [0003]    Various chelating agents are known to enhance brighness, an example of which is hydroxylamine, an established product known for its brightening effects and degradation of enzymes that can affect the brightening process. See, for example, GB-A-2 269 191 and GB-A-846 079, EP 686 216 and U.S. Pat. No. 4,752,354. However, hydroxylamine alone does not reduce bacteria populations.  
           [0004]    Peracetic acid based biocides are known to be effective for controlling microbiological populations in industrial water systems, including papermaking process water. See GB 2,269,191 and U.S. Pat. No. 5,494,588. Furthermore, a process for bleaching cellulose by means of an organic peracid in the acid region followed by peroxide in the alkaline region is disclosed in U.S. Pat. No. 4,400,237. None of these references, however, disclose a dual treatment program comprising hydroxylamine and an organic peracid biocide.  
         SUMMARY OF THE INVENTION  
         [0005]    We have unexpectedly discovered a synergistic effect between a hydroxylamine based product and peracetic acid based biocides, with higher reductions in bacterial total counts than with either biocide or hydroxylamine alone. The result is less biofilm build-up in the de-inking mill, which is known to harbor very high levels of micro-organisms and associated enzymes together with a perceptible brightening effect, which could result in either reduced peroxide or sulfite usage for bleaching.  
           [0006]    Accordingly, this invention is a method of controlling micro-organisms during peroxide bleaching of cellulose pulp comprising adding to the pulp an effective micro-organism controlling amount of  
           [0007]    a) an aqueous biocide solution comprising one or more organic peracids, and  
           [0008]    b) an aqueous solution comprising one or more chelating agents. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0009]    The method of this invention is suitable for controlling microbiological populations in programs for bleaching all manner of cellulose pulps, including pulp made from recycled paper, pulps from sulfite or sulfate cooking, mechanical pulp, thermomechanical pulp and chemothermomechanical pulp. The method is especially suitable for controlling microbiological populations responsible for degrading hydrogen peroxide during the bleaching process in de-inking plants.  
         [0010]    As used herein “controlling” encompasses both reducing microbiological populations and inhibiting the growth of microbiological populations.  
         [0011]    The aqueous biocide solution of this invention typically comprises about 5 to about 60 percent by weight of one or more organic peracids, about 5 to about 60 percent by weight of the corresponding organic carboxylic acids and about 10 to about 20 percent by weight hydrogen peroxide. The aqueous biocides solution may also contain stabilizers to prolong the storage stability of the peracid. Representative stabilizers include ethyleneaminopolymethylenephosphonic acids, hydroxyethylidene diphosphonic acid or salts thereof, and heterocyclic carboxylic acids such as dipicolinic acid, quinolinic acid, and the like.  
         [0012]    “Organic peracid” means a compound of formula RC(O)OOH where R is straight or branched C 1 -C 6  alkyl or phenyl. Representative organic peracids include peracetic acid, perpropionic acid, perbenzoic acid, and the like.  
         [0013]    In a preferred aspect of this invention, the organic peracid is peracetic acid.  
         [0014]    As used herein “corresponding organic carboxylic acid” means an acid of formula RCO 2 H where R is a defined above and the same as the R group of the percarboxylic acid. By way of example, acetic acid is the corresponding carboxylic acid of peracetic acid.  
         [0015]    The aqueous biocide solution may be prepared by mixing the corresponding carboxylic acid and hydrogen peroxide in aqueous solution in the presence of any desired stabilizers. Suitable aqueous biocide solutions are available commercially from several sources including Ondeo Nalco Company, Naperville, Ill.  
         [0016]    A preferred aqueous biocide solution comprises about 5 to about 15 percent by weight of peracetic acid, about 10 to about 20 percent by weight hydrogen peroxide and about 8 to about 35 percent by weight of acetic acid.  
         [0017]    The aqueous biocide solution is used in conjunction with an aqueous solution comprising about 10 to about 50 weight percent of one or more chelating agents in order to control microbiological populations.  
         [0018]    “Chelant” and “chelating agent” mean an agent capable of complexing metals such as iron and manganese. Preferred chelants include hydroxylamine compounds, phosphonic acids and polyhydroxycarboxylic acids.  
         [0019]    Representative phosphonic acids include N,N-bis-(carboxymethyl)-1-aminoethane-1,1-diphosphonic acid, N-2-carboxyethyl-1-aminoethane-1,1-diphosphonic acid, N,N-bis-(hydroxymethly)-1-aminoethane-1,1-diphosphonic acid, 1,2,1-tricarboxybutane-2-phosphonic acid, diethylenetriamine-pentamethylenephosphonic acid (DTPMP), hydroxyethanediphosphonic acid (HEDP) and aminotrismethylenephosphonic acid (ATMP), and the like and salts thereof.  
         [0020]    Representative polyhydroxycarboxylic acids include gluconic acid, citric acid, N,N-dihydroxyethyleneglycine, diethylenetriamine-pentaacetic acid (DTPA), ethylenediamine-tetraacetic acid (EDTA), nitrilotriacetic acid (NTA), and the like and salts thereof.  
         [0021]    “Hydroxylamine compound” means hydroxylamine and alkyl hydroxylamine and salts thereof. Alkyl groups are straight chain or branched C 1 -C 10  alkyl. A representative alkyl hydroxylamine is N-methylhydroxylamine. Representative hydroxylamine salts include hydroxylamine hydrochloride, hydroxylamine sulfate and hydroxylamine salts of ammonium thiocyanate, salts of organic acids such as formic acid, ascorbic acid, salicylic acid, and the like and salts of nitrites such as sodium nitrite, potassium nitrite, calcium nitrite, magnesium nitrite, and the like.  
         [0022]    In a preferred aspect of this invention, the hydroxylamine compound is hydroxylamine sulfate.  
         [0023]    In another preferred aspect, the chelating agent is a mixture of one or more hydroxylamine compounds and one or more phosphonic acids or polyhydroxycarboxylic acids.  
         [0024]    In another preferred aspect, the chelating agent is a mixture of hydroxylamine sulfate and one or more phosphonic acids or polyhydroxycarboxylic acids.  
         [0025]    In another preferred aspect, the chelating agent is a mixture of hydroxylamine sulfate and diethylenetriamine-pentaacetic acid.  
         [0026]    In another preferred aspect, the organic peracid is peracetic acid and the chelating agent is a mixture of hydroxylamine sulfate and diethylenetriamine-pentaacetic acid.  
         [0027]    The aqueous biocide solution and the aqueous hydroxylamine solution can be added anywhere in the pulp bleaching process including to the pulp prior the bleaching step. In particular, the aqueous biocide solution and aqueous hydroxylamine solution may be added at the mixing screw before the bleaching tower, at the flotation, at the pulper and in the incoming white water from the press and in the white water tanks. Preferably, the solutions are added to pulper fill water.  
         [0028]    The amount of aqueous biocide solution and aqueous hydroxylamine solution are determined by measuring residual hydrogen peroxide in the process water and pulp and with regard to the brightness of the pulp. The brightness depends on the pH, temperature, to what extent the process water is recirculated and the used pulp, especially when recycled paper is used, because the pulp can contain varying amounts of microorganisms depending on the conditions under which it is stored.  
         [0029]    Typical doses of aqueous biocide solution and aqueous hydroxylamine solution are about 50 ppm to about 200 ppm based on active organic peracid and hydroxylamine compound. Both solutions may be added continuously or intermittently, preferably at the same point of addition.  
         [0030]    Preferably, the aqueous hydroxylamine solution is added continuously to the pulper fill water at a dose of about 50 ppm and the aqueous biocide solution is added to the pulper fill water at 4-hour intervals at a dose of about 75 ppm.  
         [0031]    The foregoing may be better understood by reference to the following Example, which is presented for purposes of illustration and is not intended to limit the scope of this invention.  
       EXAMPLE 1  
       [0032]    Pulper Fill Tank water from a de-inking plant is allowed to stand overnight to allow any residual hydrogen peroxide to be degraded and allow the existing microbiological populations to proliferate. The pH of the water is 8.5 and the ORP (Oxidative-Reductive Potential) is around +100 mV (slightly oxidative conditions).  
         [0033]    Varying amounts of an aqueous biocide solution (Composition A) and an aqueous hydroxylamine solution (Composition B) are then added to aliquots of the pulper fill water and the samples are left to stand for one hour. After this time, the waters are tested for various parameters as described below.  
         [0034]    In this Example, Composition A is 12% active by weight peracetic acid blended with hydrogen peroxide and acetic acid. Composition B is a mixture of hydroxylamine sulphate (12% by weight) and inorganic salts with aminocarboxylic acids. Both compositions are available from Ondeo Nalco Company, Naperville, Ill.  
         [0035]    Toxicity is measured using the Ondeo-Nalco Tra-cide™ system, a diagnostic tool, which measures TOX (toxicity) and ATP (adenosine-tri-phosphate). Toxicity is a test based on the response of luminescent bacteria to toxic compounds and is measured in RTU (Relative Toxicity Units). As the toxic compounds kill or inhibit the luminescent bacteria, the light output decreases. Therefore, a high RTU reading indicates high toxicity.  
         [0036]    Total viable count is measured by diluting the sample and plating out onto Total Aerobic Petrifilms™ (available from 3M, 3M House, Loughborough, Leicestershire, UK). The resulting count is multiplied by the dilution factor applied.  
         [0037]    To define true synergy the following equation is applied:  
         
       Q=A 
       c 
       /A 
       a 
       +B 
       c 
       /B 
       a  
     
         [0038]    Where:  
         [0039]    Q=synergy, must be less than 1 for synergy and the lower the figure, the greater the synergy  
         [0040]    A c =value of endpoint (concentration) for compound A when combined with compound B  
         [0041]    A a =value of endpoint for compound A when used alone  
         [0042]    B c =value of endpoint (concentration) for compound B when combined with compound A  
         [0043]    B a =value of endpoint for compound B when used alone  
                                                           TABLE 1                           Toxicity Measurement                Product           CFU           Concentration   Toxicity   ATP   ml-1                            Control   0.16   33738   1.00E+07           Product A           50   137.71   9722   3.90E+05           100   235.00   2730   3.60E+05           200   358.32   1934   3.40E+04           Product B           50   0.02   22460   1.43E+06           100   0.10   19868   1.29E+06           200   0.29   15706   1.23E+06           “A” + 50 ppm “B”           50   138.92   6723   2.90E+05           100   262.13   2245   9.70E+04           200   436.19   1534   1.02E+04           “A” + 100 ppm “B”           50   139.68   2438   2.30E+05           100   421.89   1954   4.30E+04           200   466.33   1042   5.40E+03           “A” + 200 ppm “B”           50   193.45   1934   1.60E+05           100   301.27   1120   1.20E+04           200   498.23   902   1.30E+03           “B” + 50 ppm “A”           50   138.23   6812   2.80E+05           100   142.21   2496   2.27E+05           200   197.23   1968   1.61E+05           “B” + 100 ppm “A”           50   136.42   2558   6.00E+04           100   420.08   2160   3.40E+04           200   471.17   1666   3.06E+04           “B” + 200 ppm “A”           50   440.21   1512   1.10E+04           100   467.59   1002   4.90E+03           200   501.16   898   1.20E+03                      
 
         [0044]    As shown in Table 1, treatment with both Composition A and Composition B shows a higher toxicity than treatment with either composition alone. The data in Table 1 also demonstrate synergy in that the measured toxicity for the combination treatment is greater than would be expected from calculating the arithmetic mean toxicity based on each component individually.  
         [0045]    Tables 2 and 3 show the synergy achieved for defined endpoints, 3-log 10  reduction in total viable count and reduction in ATP to less than 2000 RLU, respectively, for the combination treatment of this invention.  
                                                                                                                                                         TABLE 2                           Synergy where the Endpoint is a 3-log 10  Reduction in Total Viable Count:            A c     A a     A c /A a     B c     B a     B c /B a     Synergy                    100 ppm “A” + 100 ppm “B”            100   200   0.50   100   1000   0.10   0.60            50 ppm “A” + 200 ppm “B”            50   200   0.25   200   1000   0.20   0.45            100 ppm “A” + 200 ppm “B”            100   200   0.50   200   1000   0.20   0.70                  
 
         [0046]    [0046]                                                                                                                                                         TABLE 3                           Synergy where the Endpoint is a       Reduction in ATP to less than 2000 RLU            A c     A a     A c /A a     B c     B a     B c /B a     Synergy                    100 ppm “A” + 50 ppm “B”            100   200   0.50   50   1000   0.05   0.55            100 ppm “A” + 100 ppm “B”            100   200   0.50   100   1000   0.10   0.60            100 ppm “A” + 200 ppm “B”            100   200   0.50   200   1000   0.20   0.70                    
         [0047]    Changes can be made in the composition, operation and arrangement of the method of the present invention described herein without departing from the concept and scope of the invention as defined in the following claims: