Method and a system for precipitation of calcium carbonate and a product comprising calcium carbonate

Abstract

A method for precipitating calcium carbonate, in which method calcium hydroxide is introduced to a fiber suspension containing cellulose-containing fibers, the calcium hydroxide is allowed to attach onto the surfaces of fibers for a pre-determined time, and the aqueous suspension of calcium hydroxide and fibers is brought into contact with carbon dioxide for precipitating the calcium carbonate. A corresponding system for precipitating calcium carbonate in connection with cellulose-containing fibers and a fibrous product, which comprises pulp material produced by the method and comprising calcium carbonate, are also set forth.

1 . A method for precipitating calcium carbonate onto surfaces of cellulose-containing fibers in a refining process, comprising:
 introducing calcium hydroxide before refining to a fiber suspension comprising cellulose-containing fibers,   allowing calcium hydroxide to attach onto the surfaces of the fibers for a pre-determined time period,   refining the fibers, and   bringing the aqueous suspension of calcium hydroxide and fibers into contact with carbon dioxide after refining in order to precipitate calcium carbonate.   
     
     
         2 . The method according to  claim 1 , wherein the pre-determined time period is a time period it takes for fiber pulp to travel from a refining feeding tank through at least one refiner to a pulp feeding tank. 
     
     
         3 . The method according to  claim 1 , wherein the pre-determined time period is at least 1 minute. 
     
     
         4 . The method according to  claim 1 , wherein the pre-determined time period is at least 1.5 minutes. 
     
     
         5 . The method according to  claim 1 , wherein the pre-determined time period is at least 5 minutes. 
     
     
         6 . The method according to  claim 1 , wherein the pre-determined time period is at least 15 minutes. 
     
     
         7 . The method according to  claim 1 , wherein the precipitation of calcium carbonate is performed by flowing the aqueous suspension of calcium hydroxide and fibers through a tube or line and injecting carbon dioxide into the flowing aqueous suspension of calcium hydroxide and fibers. 
     
     
         8 . The method according  claim 1 , wherein the fibers comprise hardwood pulp. 
     
     
         9 . The method according to  claim 1 , wherein the fibers comprise softwood pulp. 
     
     
         10 . The method according to  claim 1 , wherein the median particle size of the precipitated calcium carbonate is less than 2.0 μm. 
     
     
         11 . The method according to  claim 1 , wherein the median particle size of the precipitated calcium carbonate is less than 1.7 μm. 
     
     
         12 . The method according to  claim 1 , wherein an aqueous suspension of calcium hydroxide is introduced to the fiber suspension. 
     
     
         13 . The method according to  claim 1 , wherein the precipitation of calcium carbonate is carried out continuously. 
     
     
         14 . The method according to  claim 1 , wherein the precipitation of calcium carbonate is carried out in a continuous tube reactor. 
     
     
         15 . A system for precipitating calcium carbonate in connection with cellulose-containing fibers, the system comprising:
 at least one refiner,   first adding elements for introducing calcium hydroxide before the at least one refiner to the cellulose-containing fibers,   means for arranging a pre-determined time period after the calcium hydroxide introduction, and   second adding elements for introducing carbon dioxide to the fiber-calcium hydroxide mixture after the at least one refiner and after the pre-determined delay time for precipitating calcium carbonate.   
     
     
         16 . The system of  claim 15  wherein the precipitation of calcium carbonate is to be carried out continuously. 
     
     
         17 . The system of  claim 15 , wherein the system comprises a continuous tube reactor for precipitating calcium carbonate. 
     
     
         18 . The system of  claim 15 , wherein the pre-determined time period is the time period it takes for fiber pulp to travel from a refining feeding tank through the at least one refiner to a pulp feeding tank. 
     
     
         19 . The system of  claim 15 , wherein the pre-determined time period is at least a minute. 
     
     
         20 . The system of  claim 15 , wherein the pre-determined time period is at least 1.5 minutes. 
     
     
         21 . The system of  claim 15 , wherein the pre-determined time period is at least 5 minutes. 
     
     
         22 . The system of  claim 15 , wherein the pre-determined time period is at least 15 minutes. 
     
     
         23 . The system of  claim 15 , comprising a tank preceding the at least one refiner for introducing calcium hydroxide. 
     
     
         24 . The system of  claim 15 , comprising a tank following the at least one refiner for introducing carbon dioxide.