Process for adjusting the ph of a cellulosic pulp, production of pulp or paper from cellulosic pulp and use of (bi)carbonate removal for reducing the consumption of acid

Abstract

A process for adjusting the pH of a cellulosic pulp in a pulp and/or paper mill, and to the production of pulp, paper or board from so treated cellulosic pulp is disclosed. In the process, the pH of an alkaline cellulosic pulp is adjusted at least twice with carbon dioxide before a neutral treatment and at least once with a strong acid before a subsequent acidic treatment. Between the first and the second carbon dioxide induced pH adjustment there is provided a (bi)carbonate removal step, which reduces the buffering capacity of the pulp. Also provided for is the use of (bi)carbonate removal for reducing the consumption of acid in the treatment of a cellulosic pulp.

1 . A process for adjusting the pH of a cellulosic pulp, characterized in
 a) providing an alkaline cellulosic pulp, wherein said pulp has been produced by a chemical, mechanical or repulping treatment and has been subsequently subjected to washing and optional oxygen delignification;   b) adjusting the pH of the alkaline cellulosic pulp so obtained at least twice with carbon dioxide before a neutral treatment and at least once with a strong acid before a subsequent acidic treatment; and   c) providing a (bi)carbonate removal step between the first and the second carbon dioxide induced pH adjustment.   
   
   
       2 . The process according to  claim 1 , wherein said carbon dioxide induced pH adjustment(s) is/are performed by adding gaseous carbon dioxide to the pulp and/or to water used for diluting and/or washing the pulp. 
   
   
       3 . The process according to  claim 1 , wherein the first carbon dioxide induced pH adjustment is performed in connection with pulp washing. 
   
   
       4 . The process according to  claim 1 , wherein the second carbon dioxide induced pH adjustment is performed as a step separate from pulp washing prior to said neutral treatment to adjust the pH of said pulp to suit said neutral treatment. 
   
   
       5 . The process according to  claim 1 , wherein the pH adjustment with a strong acid is performed after said neutral treatment and before said acidic treatment and the pH of the pulp is adjusted with said strong acid to suit said acidic treatment. 
   
   
       6 . The process according to  claim 5 , wherein there is a second bicarbonate removal step after said neutral treatment and before said pH adjustment with a strong acid. 
   
   
       7 . The process according to  claim 1 , wherein said (bi)carbonate removal step comprises using water for displacing and/or washing (bi)carbonate ions from the pulp. 
   
   
       8 . The process according to  claim 7 , wherein said water comprises shower water applied to the pulp in a filter. 
   
   
       9 . The process according to  claim 1 , wherein said (bi)carbonate removal step comprises dewatering and/or compression of said pH-adjusted pulp and subsequent dilution with water. 
   
   
       10 . The process according to  claim 7 , wherein said water used for displacing, washing and/or dilution has a lower content of (bi)carbonate ions than the pulp being subjected to said (bi)carbonate removal step. 
   
   
       11 . The process according to  claim 10 , wherein said water is selected from fresh water, condensate, clear filtrate and washing water/filtrate from a downstream pulp washing. 
   
   
       12 . The process according to  1 , wherein said (bi)carbonate removal step comprises at least two (bi)carbonate removal modes selected from washing, filtration, displacement, dewatering and compression of the pulp. 
   
   
       13 . The process according to  claim 1 , wherein said pulp comprises a Kraft pulp which has been subjected to oxygen delignification and said (bi)carbonate removal step comprises at least one step of washing the pulp. 
   
   
       14 . The process according to  claim 1 , wherein the initial pH of said alkaline cellulosic pulp is between pH 8 and 13, the pH is adjusted to between pH 7 and 12 in the first carbon dioxide induced pH adjustment, the pH is adjusted to between pH 6 and 9 in the second carbon dioxide induced pH adjustment and the pH is adjusted to between pH 2 and 7 with the strong acid. 
   
   
       15 . The process according to  claim 1 , wherein said strong acid is selected from the group consisting of sulfuric acid, sulfurous acid, bisulfite, chlorine dioxide, waste acid from chlorine dioxide production and alum. 
   
   
       16 . The process according to  claim 1 , wherein, said neutral treatment is selected from an enzyme stage, a chelating stage, a flotation stage, a refining stage and a dithionite bleaching stage. 
   
   
       17 . The process according to  claim 1 , wherein said neutral treatment comprises an enzyme treatment after the second pH adjustment with carbon dioxide. 
   
   
       18 . The process according to  claim 1 , wherein said acidic treatment comprises an acidic bleaching treatment with a bleaching chemical selected from chlorine dioxide, ozone, peroxyacid and dithionite. 
   
   
       19 . The process according to  claim 1 , wherein said (bi)carbonate removal step removes at least 25% of the (bi)carbonate ions created by the first carbon dioxide induced pH adjustment. 
   
   
       20 . A process for the production of pulp, board or paper from cellulosic pulp, characterized in
 a) providing an alkaline cellulosic pulp in a pulp and/or paper mill, wherein said pulp has been produced by a chemical, mechanical and/or repulping treatment and has been subsequently subjected to washing and optional oxygen delignification;   b) adjusting the pH of the alkaline cellulosic pulp so obtained at least twice with carbon dioxide before a neutral treatment and at least once with a strong acid before a subsequent acidic treatment;   c) providing a (bi)carbonate removal step between the first and the second carbon dioxide induced pH adjustment; and   d) processing the pulp after the acidic treatment in a pulp or paper making process to form a dried product selected from pulp, board and paper.   
   
   
       21 . A process for reducing the consumption of acid in the acidification of an alkaline cellulosic pulp by (bi)carbonate removal. 
   
   
       22 . The process according to  claim 21 , wherein the pH of said pulp is reduced twice with carbon dioxide, and (bi)carbonate ions created in the first pH reduction are removed before the second pH reduction. 
   
   
       23 . The process according to  claim 21 , wherein said acidification comprises pH reduction of a (bi)carbonate containing pulp with a strong acid. 
   
   
       24 . The process according to  claim 22 , wherein said pH adjustment twice with carbon dioxide is followed by an enzyme treatment and said acidification with said acid is followed by acidic bleaching.