Method and apparatus for detecting and controlling the caustic in paper pulp bleaching

A method of regulating the flow of a caustic compound into a chlorinated aqueous slurry of paper pulp comprising the steps of:

• A. injecting a substantially continuous flow stream of caustic into a flowing slurry of fibrous cellulose and lignin suspended in a chlorinated aqueous solution to combine with chlorinated lignin compounds therein, said caustic being injected at a rate sufficient to maintain a caustic residual concentration range of substantially 0 to 100 grams of caustic measured as sodium oxide per 100 liters of solution and an alkalinity range of 7 to 11 pH;
• B. mixing said slurry to homogenously distribute said caustic within said slurry;
• C. heating the causticized slurry to accelerate the combination rate of said caustic with said chlorinated lignin compounds;
• D. measuring the voltaic response of a first electrolytic cell which comprises the heated, causticized slurry as an electrolyte and a ferro-metallic wall of a vessel confining said slurry as one of at least two cell poles, the other cell pole being of a metallic material having a different tendency to ionize in said causticized slurry than iron and also a consistent voltaic response in said causticized slurry over said entire residual concentration and alkalinity range;
• E. generating a first signal proportional to voltaic response measurements of said first cell;
• F. retaining said causticized slurry as a reactive system for at least thirty minutes of flow transit time following said caustic injecting;
• G. measuring the voltaic response of a second electrolytic cell comprising said causticized slurry after said retaining time as an electrolyte, a ferro-metallic wall of a vessel confining said causticized slurry as one cell pole and a metallic material having a different tendency to ionize in said causticized slurry than iron and also a consistent voltaic response in said causticized slurry over said entire residual concentration and alkalinity range as another cell pole;
• H. generating a second signal proportional to voltaic response measurements of said second cell;
• I. comparing said second signal to a set-point reference signal to derive a first error signal;
• J. comparing said first signal to said first error signal to derive a second error signal;
• and,
• K. controlling the flow rate of caustic into said flowing slurry as a function of said second error signal.

Apparatus for detecting the concentration of residual caustic in solution with a chlorinated aqueous slurry of paper pulp wherein said solution is an electrolytic cell electrolyte having a caustic residual concentration within the range of 0 to 100 grams caustic measured as sodium oxide per 100 liters of solution and an alkalinity within the range of 7 to 11 pH, said apparatus comprising:

• A. a vessel having ferro-metallic containment walls contacting said causticized slurry electrolyte;
• B. cathode means comprising an alloy of substantially equal proportions of lead and tin in contact with said electrolyte;
• and
• C. means to measure between said ferro-metallic walls and said cathode means the voltaic response thereof to changes in the concentration of residual caustic within said solution.

An apparatus for regulating the flow of caustic compound into solution with a chlorinated aqueous slurry of paper pulp comprising:

• A. a ferro-metallic conduit for a flow stream of said slurry;
• B. a conduit for delivering a caustic compound into solution with said slurry flow stream at an injection point and at a rate sufficient to maintain within said solution a caustic residual concentration within the range of 0 to 100 grams of caustic measured as sodium oxide per 100 liters of solution and an alkalinity range of 7 to 11 pH.
• C. means for heating the causticized slurry above 170.degree. F;
• D. a first electrolytic cell means comprising said heated, causticized slurry as an electrolyte, said ferro-metallic conduit as a first cell pole and a second cell pole of metallic material having a different tendency to ionize in said causticized slurry than iron and also a consistent voltaic response in said heated, causticized slurry over said entire residual concentration and alkalinity range;
• E. means for measuring the voltaic response of said first cell to changes in caustic residual concentration within said heated, causticized slurry;
• F. first signal means for generating a first signal porportional to said voltaic response of said first cell;
• G. flow stream retention means for retaining said causticized slurry as a reactive system for at least 30 minutes following said caustic injection;
• H. a second electrolytic cell means comprising said causticized slurry at a flow point subsequent to said retention means as an electrolyte, said ferro-metallic conduit as a first cell pole and a second cell pole of metallic material having a different tendency to ionize in said causticized slurry than iron and also a consistent voltaic response to said causticized slurry over said entire residual concentration and alkalinity range;
• I. means for measuring the voltaic response of said second cell to changes in caustic residual concentration within said causticized slurry;
• J. second signal means for generating a second signal proportional to said voltaic response of said second cell;
• K. means to compare said second signal to a setpoint reference signal to derive a first error signal;
• L. means to compare said first signal to said first error signal to derive a second error signal;
• and,
• M. means to control the flow rate of caustic into said flowing slurry as a function of said second error signal.