Process for Deinking Recycled Fibers and Use of an Acid Deinking

Abstract

The present invention relates to a process for deinking recycled fibers wherein an aqueous pulp suspension with recycled fibers, printing ink and calcium carbonate is provided. The hardness of the suspension is stabilized by causing dissolution of calcium carbonate contained in the suspension or in an aqueous fluid added to the suspension. The increased hardness suspension is subjected to flotation to separate said printing ink from the suspension. The present invention also relates to use of a weak acid for dissolving calcium carbonate and for stabilizing the hardness in a flotation process for the deinking of an aqueous pulp suspension containing said recycled fibers, printing ink and calcium carbonate.

1 . A process for deinking recycled fibers comprising
 providing an aqueous pulp suspension containing recycled fibers, printing ink and calcium carbonate;   stabilizing the hardness of said pulp suspension by causing dissolution of calcium carbonate contained in said suspension or in an aqueous fluid added to said suspension; and   subjecting the resulting pulp suspension to flotation to separate said printing ink from said suspension.   
     
     
         2 . A process according to  claim 1 , wherein solid calcium carbonate is caused to dissolve by providing a weak acid in said suspension. 
     
     
         3 . A process according to  claim 2 , wherein said weak acid is selected from carbonic acid, phosphoric acid, phosphorous acid, metaphosphoric acid, acetic acid, citric acid, formic acid and dissociated forms of these acids. 
     
     
         4 . A process according to  claim 2 , wherein said weak acid is provided by adding a weak acid into said pulp suspension. 
     
     
         5 . A process according to  claim 2 , wherein said weak acid is generated by introducing carbon dioxide into said suspension or said aqueous fluid for forming carbonic acid therein. 
     
     
         6 . A process according to  claim 2 , wherein said weak acid is initially provided in an aqueous fluid, which is subsequently added to said suspension. 
     
     
         7 . A process according to  claim 2 , wherein said weak acid is provided in an amount sufficient to reduce the pH of said pulp suspension or said aqueous fluid. 
     
     
         8 . A process according to  claim 7 , wherein the pH of said suspension or aqueous fluid is reduced by at least 0.1 pH unit, preferably 0.2 pH units or more by said weak acid. 
     
     
         9 . A process according to  claim 2 , wherein said weak acid is capable of fanning a buffer in said suspension. 
     
     
         10 . A process according to  claim 9 , wherein said weak acid is provided in an amount which is sufficient to provide a buffer stabilizing the suspension at a desired pH level for said flotation. 
     
     
         11 . A process according to  claim 10 , wherein said pH is stabilized so as to provide a Ph fluctuation in said flotation of no more than ±0.15 pH units, preferably no more than ±0.05 pH units. 
     
     
         12 . A process according to  claim 2 , wherein said weak acid is provided in an amount sufficient to cause the pH of said suspension or said aqueous fluid after said provision of said weak acid to be about pH 6 to 8, preferably about pH 6.9 to 7.5. 
     
     
         13 . A process according to  claim 2 , wherein said weak acid is provided in an amount sufficient to cause the hardness to be increased by an increased amount of calcium ions in said pulp suspension. 
     
     
         14 . A process according to  claim 2 , wherein said weak acid is provided in an amount sufficient to cause the hardness to be stabilized at a desired level for said flotation. 
     
     
         15 . A process according to  claim 2 , wherein said weak acid is provided in an amount sufficient to cause the pH and hardness to be stabilized at a desired level for said flotation. 
     
     
         16 . A process according to  claim 14 , wherein a hardness between 7 and 70°dH, preferably between 9 and 15°dH is provided in said suspension. 
     
     
         17 . A process according to  claim 15 , wherein said hardness is stabilized so as to provide a hardness fluctuation in said flotation of no more than ±1°dH units, preferably no more than ±0.5°dH unit. 
     
     
         18 . A process according to  claim 2 , wherein said weak acid is provided by carbon dioxide and wherein the amount of carbon dioxide is 0.5 to 10 kg/ton dry deinked fiber, preferably 1 to 5 kg/ton dry deinked fiber. 
     
     
         19 . A process according to  claim 1  wherein a saponifiable flotation agent is provided in said suspension for said flotation. 
     
     
         20 . A process according to  claim 1  wherein calcium carbonate is caused to dissolve to provide an amount of calcium ions which is sufficient to saponify at least part of said flotation agent. 
     
     
         21 . A process according to  claim 1  wherein solid calcium carbonate is added into said Suspension. 
     
     
         22 . A process according to  claim 1  wherein solid calcium carbonate is derived from recovered paper. 
     
     
         23 . A process according to  claim 5  wherein said carbon dioxide is introduced into said suspension during said flotation. 
     
     
         24 . A process according to  claim 5  wherein said deinking process comprises more than one loop and carbon dioxide is introduced into a loop downstream of the first loop of said process or provided in an aqueous fluid, which is added to said loop downstream of the first loop. 
     
     
         25 . A process according to  claim 2  wherein said weak acid is provided in a pulp or into an aqueous fluid upstream of said flotation. 
     
     
         26 . A process according to  claim 2  wherein said weak acid is provided in the pulp or in a washing or dilution liquid used for the pulp downstream of said flotation, and an aqueous fluid derived from said washing or dilution liquid or from said pulp is fed upstream to said pulp suspension for said flotation. 
     
     
         27 . A process according to  claim 26  wherein said weak acid is generated by introducing carbon dioxide into said pulp or said washing or dilution liquid. 
     
     
         28 . A process according to  claim 27  wherein said carbon dioxide is introduced in an amount which is effective in improving the washing, dewatering and/or filtration of said pulp. 
     
     
         29 . A process according to  claim 28  wherein said carbon dioxide is introduced into a filtrate. 
     
     
         30 . A process according to  claim 1 , wherein deinked pulp which has been subjected to said flotation is processed into paper or board. 
     
     
         31 . (canceled) 
     
     
         32 . (canceled)