Methods for producing pulp and treating black liquor

Abstract

A method is provided for treating black liquor particularly derived from non-wood pulp, by heating with an alkaline earth metal oxide in a toroidal fluidised bed reactor at a temperature of above 650° C. The method may be used alone or as part of a method of converting graminaceous raw material to pulp for paper or board, said method comprising (a) digesting said raw material with a white liquor based on sodium hydroxide and further comprising calcium hydroxide in an amount effective to substantially convert silica of said raw material to calcium silicate; (b) recovering pulp and black liquor substantially free of uncombined silica; (c) heating the black liquor in a fluidized bed reactor containing calcium oxide for catalysing conversion of organic content of said black liquor to gas and for providing recovered solids including sodium values of said white liquor and calcium oxide; and regenerating said white liquor using said recovered solids. The use of the above mentioned white liquor permits treatment of wheat straw, rice straw and other high-silica materials without resulting in a black liquor that is difficult to treat.

1 . A method for treating black liquor to produce inorganic material and a synthesis gas having as components CO2, CO, H2O, and H2 together with methane and, C2+ components, said process comprising: 
 providing a reactor having a processing region provided with a mass of particulate material consisting of or containing an alkaline earth metal oxide;    supplying heated fluidizing gas to the processing region so as to induce a swirling flow of the particulate material within said processing region, the particulate material to assuming a compact band and circulating about an axis of said processing region in a turbulent manner, the fluidizing gas including a sub-stochiometric quantity of oxygen for partially oxidizing the organic material of the black liquor and for converting other organic material of the black liquor to said synthesis gas;    feeding the black liquor into said compact band of particulate material so that it is heated at a temperature of 650-725° C. and becomes gasified;    recovering said synthesis gas as off-gas from said bed containing <1% by volume of oxygen; and    recovering inorganic material from said black liquor as solids from said bed.    
   
   
       2 . The method of  claim 1 , wherein the black liquor is from the pulping of a graminaceous material selected from straw of wheat, rice, barley, oats, flax, rye, bagasse, sorghum or corn.  
   
   
       3 . The method of  claim 2 , wherein the black liquor is from a soda-type white liquor containing calcium hydroxide in an amount effective to convert harmful silica in said graminaceous material to calcium silicate.  
   
   
       4 . The method of  claim 1 , wherein the black liquor comprises 15-30% solids.  
   
   
       5 . The method of  claim 1 , which comprises heating the black liquor at 675-700° C.  
   
   
       6 . The method of  claim 1 , wherein the particulate material consists of or contains calcium oxide of mean particle size of the calcium oxide between 1 and 4 mm, the black liquor and calcium oxide being supplied to the fluidized bed so as to maintain a ratio of calcium oxide to dry solids of the black liquor of 0.2:1 to 0.4:1.  
   
   
       7 . The method of  claim 6 , in which the black liquor is pre-mixed with calcium oxide to become a granular friable material and the granular friable material is then fed into the fluidised bed reactor.  
   
   
       8 . The method of  claim 7 , in which sodium hydroxide and/or sodium carbonate and calcium hydroxide and/or calcium carbonate are produced within the fluidised bed reactor.  
   
   
       9 . The method of  claim 1 , in which the black liquor is introduced into the fluidised bed reactor by spraying the concentrated liquor into the chamber of the reactor in which a bed of fluidised material is supported.  
   
   
       10 . The method of  claim 9 , wherein the nominal fluidizing velocity of the gases supplied to said bed is >2 m/s.  
   
   
       11 . The method of  claim 1 , in which the fluidised bed overflows through a central discharge point, overflowing material is then dissolved in a dissolving tank to recover sodium hydroxide as green liquor and the green liquor is filtered to make a calcium carbonate sludge and white liquor containing sodium hydroxide for re-use in the pulping process, substantially no sodium carbonate being formed.  
   
   
       12 . A method of converting graminaceous raw material to pulp for paper or board, said method comprising: 
 digesting said raw material with a white liquor based on sodium hydroxide and further comprising calcium hydroxide in an amount effective to substantially convert silica of said raw material to calcium silicate;    recovering pulp from said digestion step;    recovering black liquor from said digestion step by washing said digested pulp and optionally also by recovering black liquor direct from said digestion step, said recovered black liquor being substantially free of soluble silicate;    heating the black liquor in a fluidized bed reactor containing calcium oxide for catalysing conversion of organic content of said black liquor to gas and for providing recovered solids including sodium values of said white liquor and calcium oxide; and    regenerating said white liquor using said recovered solids.    
   
   
       13 . The method of  claim 12 , wherein said graminaceous raw material is wheat straw, rice straw or bagasse.  
   
   
       14 . The method of  claim 12 , wherein recovery of black liquor includes combining a black liquor stream from the digestion step with a black liquor stream from the pulp washing step, and the black liquor is concentrated by evaporation to a solids content of 20-40 wt % before heating in said fluidized bed reactor.  
   
   
       15 . The method of  claim 12 , wherein the fluidizing gases contain at least stochiometric quantities of free oxygen for completely oxidizing the organic material of the black liquor.  
   
   
       16 . The method of  claim 12 , wherein the fluidizing gases contain a sub-stochiometric quantity of oxygen for partially oxidizing the organic material of the black liquor and for converting other organic material of the black liquor to a combustible off-gas and further comprising supplying fluidizing gases and black liquor so as to produce an off-gas above said bed containing <1% oxygen.  
   
   
       17 . The method of  claim 12 , comprising: 
 providing a reactor having a processing region provided with a mass of particulate material consisting of or containing calcium oxide;    supplying heated fluidizing gas to the processing region so as to generate a swirling flow of fluid within said processing region, the fluid of said swirling flow of fluid causing the particulate material to assume a compact band and circulate about an axis of said processing region in a turbulent manner, the fluidizing gas including oxygen for at least partially combusting organic material in the black liquor;    feeding the black liquor into said compact band of particulate material and treating the black liquor in said bed so as to gasify organic materials in said black liquor;    recovering organic material from said black liquor as off-gas from said bed; and    recovering inorganic material from said black liquor as solids from said bed.    
   
   
       18 . The method of  claim 17 , wherein the raw material supplied for digestion has been crushed to remove nodes therefrom and split lengthways, crushing being by means of a pair of counter rotating knurled rollers between which the raw material passes, and splitting being by means of a pair of counter rotating pinned rollers between which the crushed material passes.  
   
   
       19 . Use of calcium hydroxide as an additive in the soda process digestion of a graminaceous starting material to form pulp for inhibiting scaling during black liquor concentration and recovery when processing black liquor at least partly from pulp washing.  
   
   
       20 . The use of  claim 19 , wherein the black liquor is partly from a digester and partly from pulp washing, silica in said black liquor in the form of calcium silicate to the substantial exclusion of sodium silicate, a flocculating agent is added during washing to inhibit dispersion of calcium silicate said flocculating agent being polyacrylamide.