Bleaching process with at least one extraction stage

Abstract

An improved bleaching process for bleaching pulp having at least one Eop stage or Ep stage which comprises subjecting the pulp to a Q stage in which the pulp is treated with a chelating agent at a pH of from greater than 2.5 to less than 7 followed by interstage washing prior to treating the pulp in the Eop or Ep stage. Q stage washing can be at any stage in the process provided that Q stage occurs prior to Eop or Ep stage. For example, Q stage washing may occur after brownstock washing at prior to any delignification stage. Q stage washing can also occur after earlier delignification stages or an oxygen delignification stage or a Do delignification stage. Q stage washing immediately preceding the Eop or Ep stage, or can be separated from these stages by one or more other stage.

1 . A bleaching process for bleaching a pulp having at least one Eop stage or Ep stage, the bleaching process comprising:
 subjecting the pulp to a Q stage in which the pulp is treated with a chelating agent at a pH of from greater than 2.5 to less than 7 followed by a washing stage prior to treating the pulp in the Eop or Ep stage.   
     
     
         2 . The bleaching process of  claim 1  comprising a bleaching sequence selected from the group of the formula:
 QEop or QEp or a combination thereof, with interstage washing after the Q stage and wherein:   Q is a stage in which a pulp is treated with a chelating agent at a pH of from greater than 2.5 to less than 7,   Eop is an extraction stage in which pulp is treated with a composition comprising oxygen, base, and peroxide; and   Ep is an extraction stage in which pulp is treated with a composition comprising base and peroxide.   
     
     
         3 . The bleaching process of  claim 1  comprising a bleaching sequence selected from the group of the formula: DoQEop, DoQEp or combination thereof, without interstage washing between Do and Q stages and with interstage washing after the Q stage and wherein Do is a delignification stage in which pulp is treated with an agent comprising chlorine dioxide without elemental chlorine dioxide (ECF). 
     
     
         4 . The bleaching process of  claim 1  wherein the chelating agent is selected from the group consisting of diethylene triamine pentaacetic acid (DTPA), to aminopolycarboxylic acids (APCA), ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), phosphoric acids, ethylenediaminetetramethylene-phosphonic acid (EDTMP), diethylenetriaminepentamethylenephosphonic acid (DTPMP), nitrilotrimethylenephosphonic acid (NTMP), polycarboxylic acids, gluconates, citrates, polyacrylates, and polyaspartates or any combination of two or more thereof. 
     
     
         5 . The bleaching process of  claim 1  wherein the chelating agent is EDTA or DTPA. 
     
     
         6 . The bleaching process of  claim 1  wherein the chelating agent is DTPA. 
     
     
         7 . The bleaching process of  claim 1  wherein the pulp pH is from about 5.0 to about 5.50. 
     
     
         8 . The bleaching process of  claim 1  wherein the pulp consistency in the Q stage is from about 6.0% to about 10%. 
     
     
         9 . The bleaching process of  claim 1  wherein the retention time is from about 10 min. to about 300 min. 
     
     
         10 . The bleaching process of  claim 1  wherein the temperature is from about 25° C. to about 120° C. 
     
     
         11 . The bleaching process of  claim 1  wherein the pulp washing stage after the Q stage removes at least about 50% of the chelated metals. 
     
     
         12 . The bleaching process of  claim 1  wherein the pulp washing stage after the Q stage removes at least about 75% of the chelated metals. 
     
     
         13 . The bleaching process of  claim 1  wherein the pulp washing stage after the Q stage removes substantially all of the chelated metals. 
     
     
         14 . The bleaching process of  claim 1  wherein an amount of hydrogen peroxide (H 2 O 2 ) less than an amount of hydrogen peroxide (H 2 O 2 ) used in the same or substantially the same bleaching processes without the Q stage followed by a washing stage to obtain the same or substantially level of pulp brightness in the Do stage. 
     
     
         15 . The bleaching process of  claim 1  wherein the reduction in the amount of AOX is at least about 20% less than the amount of AOX produced in the same or substantially the same bleaching processes without the Q stage to obtain the same or substantially level of pulp brightness in the Eop and/or Ep stages. 
     
     
         16 . The bleaching process of  claim 1  wherein pulp viscosity is at least about 5% greater than the viscosity of the pulp made by the same or substantially the same bleaching processes without the Q stage. 
     
     
         17 . The bleaching process of  claim 1  wherein pulp brightness is at least about 7% greater than the brightness of the pulp made by the same or substantially the same bleaching processes without the Q stage.