Process for the deresination of pulp and use of carbon dioxide or (bi) carbonate therefor

Abstract

The invention relates to a process for the deresination of pulp as well as to the use of carbon dioxide or a (bi)carbonate compound for the deresination of pulp. The process concerns the removal of wood extractives or resin from an aqueous pulp suspension in an alkaline process stage of the pulping process, such as an oxygen delignification stage, at a pH above 10. Carbonate ions are provided in the pulp at the high pH for solubilizing a significant part of the wood extractives in the pulp. The alkaline pulp is dewatered to remove solubilized extractives and for providing a deresinated pulp.

1 . A process for the deresination of pulp, characterized in
 providing an aqueous pulp suspension, which contains wood extractives in water-insoluble form;   subjecting said pulp suspension to an alkaline process stage other than a washing stage at a pH above 10;   in said alkaline stage, subjecting said pulp suspension to the action of carbonate ions for solubilizing a significant part of said wood extractives;   said carbonate ions being provided by adding a sufficient amount of a substance selected from carbonate compounds and carbonate-forming substances for causing said solubilization; and   dewatering the resulting carbonate-treated pulp suspension without lowering the pH below 10 for removing solubilized extractives from said pulp and for providing a deresinated pulp.   
   
   
       2 . A process according to  claim 1 , wherein said wood extractives comprise extractives from hardwood. 
   
   
       3 . A process according to  claim 1 , wherein said pulp is selected from chemical pulps, mechanical pulps, semi-mechanical pulps, recycled pulps and mixtures thereof. 
   
   
       4 . A process according to  claim 3 , wherein said pulp contains kraft pulp of hardwood, softwood or mixtures thereof. 
   
   
       5 . A process according to  claim 1 , wherein said alkaline process stage is selected from an oxygen delignification stage, a peroxide bleaching stage and an extraction stage. 
   
   
       6 . A process according to  claim 1 , wherein said alkaline process stage comprises an oxygen delignification stage and the pH of said aqueous pulp suspension during said process stage is maintained at a value above 10 by an alkali selected from sodium hydroxide, sodium carbonate, oxidized white liquor and mixtures thereof. 
   
   
       7 . A process according to  claim 1 , wherein said carbonate forming substance comprises carbon dioxide or a liquid containing carbonate and/or bicarbonate ions. 
   
   
       8 . A process according to  claim 7 , wherein said substance is gaseous carbon dioxide. 
   
   
       9 . A process according to  claim 7 , wherein said liquid containing carbonate and/or bicarbonate ions is produced by reaction of suitable acids and bases outside the main process. 
   
   
       10 . A process according to  claim 7 , wherein said carbon dioxide or liquid containing carbonate and/or bicarbonate is added to said alkaline process stage or to a pipe line leading to said process stage. 
   
   
       11 . A process according to  claim 7  wherein gaseous carbon dioxide is combined with a gaseous or vapour phase component used for said process stage. 
   
   
       12 . A process according to  claim 10 , wherein gaseous carbon dioxide is added in an amount which has a pH-lowering effect on the aqueous pulp suspension and said pH-lowering effect is countered by adding extra alkali to said process stage to maintain the pH at a desired level above pH 10. 
   
   
       13 . A process according to  claim 7 , wherein said carbonate forming substance comprises sodium bicarbonate brine from a tall oil recovery process. 
   
   
       14 . A process according to  claim 13 , wherein sodium bicarbonate brine is added in an amount which has a pH-lowering effect on the aqueous pulp suspension and said pH-lowering effect is countered by adding extra alkali to said process stage to maintain the pH at a desired level above pH 10. 
   
   
       15 . A process according to  claim 1 , wherein said carbonate compound or carbonate-forming compound is added in an amount sufficient for providing a carbonate ion concentration of 30 to 200 mmol/l in the aqueous phase of said pulp suspension. 
   
   
       16 . A process according to  claim 1 , wherein said alkaline process stage is operated at a temperature between 60 and 130° C. 
   
   
       17 . A process according to  claim 1 , wherein said pulp suspension is subjected to said carbonate action for a time between 5 minutes and 3 hours. 
   
   
       18 . A process according to  claim 1 , wherein said pulp suspension is subjected to said carbonate action in said alkaline process stage prior to being subjected to any stage capable of significantly saturating the unsaturated fatty acids of said wood extractives. 
   
   
       19 . A process according to  claim 1 , wherein the surface tension of the aqueous phase of said pulp suspension in said alkaline process stage is below 35 mN/n measured at 70° C. according to the Du Noüy Ring method in said process stage and in said dewatering. 
   
   
       20 . A process according to  claim 19 , wherein said surface tension is changed no more than 5% when the pulp is diluted with equal amounts of deionized water (1:1). 
   
   
       21 . A process according to  claim 1 , wherein a kraft pulp suspension containing pulp selected from softwood pulp, hardwood pulp and mixtures thereof is subjected to an oxygen delignification stage at a pH of 11 to 13 provided by the introduction of oxidized white liquor into said pulp, and gaseous carbon dioxide is injected into said pulp suspension in an amount of 0.5 to 10 kg per ton bone dry cellulose to increase the carbonate ion content of the suspension and improve the solubilization of the wood extractives, whereafter said delignified pulp suspension is dewatered at a pH of no less than 11. 
   
   
       22 . A process according to  claim 1 , wherein a kraft pulp suspension containing pulp selected from softwood pulp, hardwood pulp and mixtures thereof is subjected to an oxygen delignification stage at a pH of 11 to 13 provided by the introduction of oxidized white liquor into said pulp, and sodium bicarbonate brine is injected into said pulp suspension in an amount of 0.01 m 3  to 1 m 3  per ton bone dry cellulose to increase the carbonate ion content of the suspension and improve the solubilization of the wood extractives, whereafter said delignified pulp suspension is dewatered at a pH of no less than 11. 
   
   
       23 . A process according to any one of the preceding claims, wherein said deresinated pulp is further processed into a dried product selected from paper, board, paperboard and dry pulp. 
   
   
       24 . Use of carbon dioxide for the deresination of an aqueous pulp suspension in an alkaline process stage operating at a pH above 10. 
   
   
       25 . Use of a liquid containing carbonate and/or bicarbonate ions for deresination of an aqueous pulp suspension in an alkaline process stage operating at a pH above 10. 
   
   
       26 . The use according to  claim 25  for the deresination of an aqueous pulp suspension in an alkaline process stage operating at a pH above 10, wherein said bicarbonate is sodium bicarbonate brine from a tall oil recovery process.