Process for recovering inorganic material from deinking sludge

Abstract

A process for recovering inorganic material from a deinking sludge obtained as reject material in the recovery of recycled fiber from waste paper. In accordance with the invention the process comprises the following steps: (a) the proportion of organic material in the form of fiber material in the deinking sludge is 15-40% (dry solids content), (b) the water content of the sludge is adjusted so that the dry solids content becomes 10-35%, step (a) and step (b) being so adapted to each other that the sludge is pumpable, (c) the pressure of the sludge is increased to at least 22 MPa, (d) the temperature is increased to supercritical or near supercritical temperature, (e) a medium containing oxygen is added to the sludge to oxidize all organic material at supercritical temperature and pressure in a reactor, (f) energy released at the oxidation of the organic material is recovered, and (g) the flow of material from which energy has been recovered and which comprises said inorganic material, gases and water, is subjected to material separation for recovery of the inorganic material.

1 . A process for continuously recovering inorganic material from a deinking sludge obtained as reject material in the recovery of recycled fiber from waste paper, characterized in that it comprises the following steps: 
 (a) controlling and/or adjusting the proportion of organic material in the form of fiber material in the deinking sludge so that it is or becomes 15-40%, calculated on the dry solids content,    (b) adjusting the water content of the deinking sludge so that the dry solids content becomes 10-35%, step (a) and step (b) being so adjusted to each other that the deinking sludge is pumpable,    (c) increasing the pressure of the pumpable deinking sludge to at least 22 MPa,    (d) increasing the temperature of the pumpable deinking sludge to supercritical or almost supercritical temperature,    (e) adding a medium containing oxygen to the deinking sludge in a sufficient quantity to oxidize all organic material, which takes place at supercritical temperature and pressure in a reactor,    (f) recovering energy released at the oxidation of the organic material in a suitable form, and    (g) subjecting the flow of material from which energy has been recovered and which comprises said inorganic material, gases and water, to material separation comprising recovery of the valuable inorganic material.    
     
     
         2 . A process as claimed in  claim 1 , characterized in that the quantity of organic material in said step (a) is 15-30%.  
     
     
         3 . A process as claimed in  claim 1  or  claim 2 , characterized in that said step (a) and step (b) are adapted to each other so that, after said step (b) the deinking sludge has a COD content within the interval 40-200 g/l, preferably 60-150 g/l.  
     
     
         4 . A process as claimed in any one of claims  1 - 3 , characterized in that in said step (d) the temperature is increased to at most 425° C.  
     
     
         5 . A process as claimed in any one of claims  1 - 4 , characterized in that the temperature in the reactor after said step (e) is at most 650° C.  
     
     
         6 . A process as claimed in  claim 5 , characterized in that the temperature in the reactor is between 500° C. and 600° C.  
     
     
         7 . A process as claimed in any one of claims  1 - 6 , characterized in that gaseous reaction products are separated from the flow of material in said step (g) and are released into the atmosphere or collected.  
     
     
         8 . A process as claimed in  claim 7 , characterized in that carbon dioxide is recovered from the gases collected.  
     
     
         9 . A process as claimed in any one of claims  1 - 8 , characterized in that the deinking sludge is subjected to washing prior to said step (c).  
     
     
         10 . A process as claimed in  claim 9  characterized in that the deinking sludge has a salt content of at most 1000 ppm after said washing.  
     
     
         11 . A process as claimed in any one of claims  1 - 10 , characteriz d in that the oxidation time in the reactor is about 10 minutes or less, preferably about 5 minutes or less and most preferably about 1 minute.  
     
     
         12 . A process as claimed in any one of claims  1 - 11 , characterized in that the fiber material is removed from the deinking sludge in order to achieve a content within the interval in step (a) by means of flotation, sedimentation, vortex cleaning, screening, centrifuging or a combination of two or more of these techniques.  
     
     
         13 . A process as claimed in any one of claims  1 - 11 , characterized in that the fiber material is removed from the deinking sludge in order to achieve a content within the interval in step (a) by means of flotation, sedimentation, vortex cleaning, screening, centrifuging, or a combination of two or more of these techniques being integrated into the deinking process.  
     
     
         14 . A process as claimed in  claim 12  or  claim 13 , characterized in that recycled fiber is recovered from the fiber material removed.  
     
     
         15 . A process as claimed in any one of claims  1 - 11 , characterized in that the fiber material is removed from the deinking sludge in order to achieve a content within the interval in step (a) by means of biological decomposition (aerobic or anaerobic) of a part of the fiber material, converting a part of the fiber material to ethanol through enzymatic or some other type of hydrolysis followed by fermentation, or a combination of two or more of these techniques.  
     
     
         16 . A process as claimed in any one of claims  1 - 15 , characterized in that the water content is adjusted in step (b) by means of sedimentation, vortex cleaning, centrifuging, pressing in a suitable press, or by a combination of two or more of these techniques.  
     
     
         17 . A process as claimed in any one of claims  1 - 16 , characterized in that the inorganic material obtained in step (g) has a brightness of at least 68% ISO.  
     
     
         18 . A process as claimed in any one of claims  1 - 17 , characterized in that the inorganic material obtained in step (g) is subjected to after-treatment in order to increase the brightness by 2 or more % ISO.  
     
     
         19 . A process as claimed in  claim 18 , characterized in that the after-treatment comprises cleaning to remove brightness-reducing metal oxides and/or bleaching with a bleaching agent.  
     
     
         20 . A process as claimed in any one of claims  1 - 19 , characterized in that the valuable inorganic material recovered also contains pigment having a smaller particle size than the particle size of the filler, the inorganic material being divided into a filler fraction and a pigment fraction.  
     
     
         21 . A process as claimed in any one of claims  1 - 20 , characterized in that the energy in step (f) is recovered in the form of vapour and/or hot water.  
     
     
         22 . A process as claimed in any one of claims  1 - 21 , characterized in that the incoming deinking sludge has a dry solids content that is normally less than 8% prior to dewatering and contains organic material in a quantity that is normally over 35%, the remainder being inorganic material, calculated on the dry solids content.