Purification of alkaline washing liquid

Abstract

A method for washing sulphate cellulose pulp in the washing department of a sulphate cellulose mill, in which method a first pulp flow ( 1 ) is conducted to a first washer ( 101 ), where a washing liquid ( 12 ) is combined with the first pulp flow ( 1 ). A second pulp flow ( 2 ) is conducted from the first washer ( 101 ) to a second washer ( 102 ), where a washing liquid ( 3 ) is combined with the second pulp flow ( 2 ). The washed pulp flow ( 5 ) is proceeded from the second washer ( 102 ) in the process and the filtrate ( 4 ) displaced by the washing liquid ( 3 ) is recycled to the process so that the filtrate ( 4 ) displaced by the washing liquid ( 3 ) in the second washer ( 102 ) is divided into a first part flow ( 6 ) and a second part flow ( 7 ) and the pH of the second part flow ( 7 ) is dropped by combining an acidifier ( 8 ) with the second part flow ( 7 ). The acidified part flow ( 9 ) is conducted to a process unit ( 103 ), where sludge ( 10 ) containing extractives and metals is separated from the part flow ( 9 ) and the entire process and the part flow ( 11 ) treated in the process unit ( 103 ) is combined with the first part flow ( 6 ) and is recycled to the process for use as the washing liquid ( 12 ) in the first washer ( 101 ).

1 . A method for washing an alkaline liquid in the washing department of a sulphate cellulose mill, the method comprising the following steps in the order below: 
 conducting a first pulp flow ( 1 ) to a first washer ( 101 ), where a washing liquid ( 12 ) is combined with the first pulp flow ( 1 ),    conducting a second pulp flow ( 2 ) from the first washer ( 101 ) to a second washer ( 102 ), where a washing liquid ( 3 ) is combined with the second pulp flow ( 2 ),    proceeding the washed pulp flow ( 5 ) from the second washer ( 102 ) in the process and recycling the filtrate ( 4 ) displaced by the washing agent ( 3 ) to the process, characterised in    dividing the filtrate ( 4 ) displaced by the washing agent ( 3 ) from the washer ( 102 ) into a first part flow ( 6 ) and a second part flow ( 7 ),    decreasing the pH of the second part flow ( 7 ) by combining an acidifier ( 8 ) with the second part flow ( 7 )    conducting the acidified part flow ( 9 ) to the process unit ( 103 ), where the sludge ( 10 ) containing extractives and metals is removed from the part flow ( 9 ) and the entire process    combining the part flow ( 11 ) treated in the process unit ( 103 ) with the first part flow ( 6 ) and recycling it to the process for use as the washing liquid ( 12 ) of the first washer ( 101 ).    
   
   
       2 . A method as defined in  claim 1 , characterised in that the filtrate ( 4 ) from the washer ( 102 ) is divided into a first part flow ( 6 ) and a second part flow ( 7 ) in the ratio 1:1, 2:1, 3:1, 4:1 or 5:1.  
   
   
       3 . A method as defined in  claim 2 , characterised in that the pH of the second part flow ( 7 ) of the filtrate ( 4 ) is decreased to the range 6 to 8.  
   
   
       4 . A method as defined in  claim 2 , characterised in that the pH of the second part flow ( 7 ) of the filtrate ( 4 ) is decreased to the range 6.5 to 7.5.  
   
   
       5 . A method as defined in  claim 3  or  4 , characterised in that carbon dioxide, mineral acids or other similar substances are used as an acidifier ( 8 ).  
   
   
       6 . A method as defined in  claim 5 , characterised in that the washer ( 101 ) is the one before the last and the washer ( 102 ) is the last washer.  
   
   
       7 . A method as defined in  claim 6 , characterised in that the extractives and metals are removed from the process unit ( 103 ) by coagulation, flocculation and flotation or any similar methods.