Electrochemical method for determinating lignin content of pulp

Abstract

This invention describes a new method to measure kappa number or lignin content of kraft pulps based on the voltammetric measurement of catalytic reactions involving lignin and redox mediators. This method comprises measuring the rate of regeneration of a lignin redox catalyst in the presence of pulp following its oxidation at a voltammetric electrode. The intensity of the catalytic current generated at the electrode surface is shown to be proportional to the amount of lignin present in a pulp. Within pulp types (softwood, hardwood or oxygen delignified) linear relationships can be obtained between the intensities of the generated current and a range of kappa numbers. The redox characteristics and concentration of the mediator, and the voltage sweep rate of the voltammetric procedure, are among parameters that can be tuned to obtained linear relationships with kappa number of various pulps. Automation of this method can lead to development of a novel on-line electro-kappa analyzer to be used for either pulping or bleaching process control.

We claim:  
     
         1 . A method of determining kappa number or lignin content of a pulp comprising: 
 i) contacting a sample of the pulp with a voltammetric electrode in an electrochemical cell, in the presence of a redox mediator, in an oxidized state effective to oxidize lignin in the pulp, with formation of a reduced state of said redox mediator and a current discharge at said electrode which re-oxidizes the redox mediator,    ii) measuring the peak intensity of the current discharge voltammetrically at said electrode, and    iii) determining the kappa number or lignin content of the pulp sample from the measured peak intensity in ii).    
     
     
         2 . A method according to  claim 1 , wherein said redox mediator is a reversible redox mediator and forms a redox couple at a potential between 0.3 and 1.2 volts, when measured against an Ag/AgCl reference electrode.  
     
     
         3 . A method according to  claim 2 , wherein said redox mediator is water soluble.  
     
     
         4 . A method according to  claim 1 , wherein said redox mediator is 2,2′-azinobis(3-ethylbenzthiazoline-5-sulphonate).  
     
     
         5 . A method according to  claim 1 , wherein said redox mediator is potassium octacyanomolybdate.  
     
     
         6 . A method for analyzing the lignin content of a pulp comprising: 
 a) mixing a sample of the pulp with a redox mediator    b) placing the resulting sample in contact with a working electrode in a cyclic voltammetry apparatus,    c) measuring the intensity of the peak current generated at the electrode, and    d) converting the peak current to kappa number from calibration data determined with pulps of known lignin content.    
     
     
         7 . A method according to  claim 6 , where the redox mediator is an organic or inorganic compound which can form a redox couple at a potential between 0.3 to 1.2 volt when measured against an Ag/AgCl reference electrode (RE).  
     
     
         8 . A method according to  claim 6 , wherein said redox mediator is water soluble and is a reversible redox mediator.  
     
     
         9 . A method according to  claim 7 , wherein said redox mediator is water soluble and is a reversible redox mediator.  
     
     
         10 . A method according to  claim 6 , wherein the mediator is ABTS, and a ratio of two peak currents measured at 0.52 V ant 0.92V is used to generate information on the oxidation state of lignin in the pulp.  
     
     
         11 . A method according to  claim 7 , wherein the mediator is ABTS, and a ratio of two peak currents measured at 0.52 V ant 0.92V is used to generate information on the oxidation state of lignin in the pulp.  
     
     
         12 . A method according to  claim 8 , wherein the mediator is ABTS, and a ratio of two peak currents measured at 0.52 V ant 0.92V is used to generate information on the oxidation state of lignin in the pulp.  
     
     
         13 . A method according to  claim 9 , wherein the mediator is ABTS, and a ratio of two peak currents measured at 0.52 V ant 0.92V is used to generate information on the oxidation state of lignin in the pulp.  
     
     
         14 . A method according to  claim 1 , wherein said redox mediator is 2,2′-azinobis(3-ethylbenzthiazoline-5-sulphonate).  
     
     
         15 . A method according to  claim 1 , wherein said redox mediator is potassium octacyanomolybdate.  
     
     
         16 . An on-line automated procedure for kappa number determination based on the principles described in  claim 1 , combined with established technology for pulp sampling and washing.