Method for recovery of pulping chemicals in an alkaline sulphite pulping process and for production of steam

Abstract

A method of recovery of alkaline sulphite pulping chemicals and for production of steam is disclosed. The method comprises gasification of evaporated spent cooking liquor at conditions resulting in a hydrogen sulphide containing gas and a solid residue. The gas is combusted in a steam boiler, where the hydrogen sulphide is converted into sulphur dioxide and steam is produced. The solid residue is recovered in a leaching process, preferably a two-stage leaching process, where process-foreign substances are removed and the rest of the contents is divided into substantially pure sodium carbonate and a mixture of sodium carbonate, sodium sulphate and sodium sulphide. The substantially pure sodium carbonate is used for absorption of sulphur dioxide from the steam boiler. The mixture of sodium carbonate, sodium sulphate and sodium sulphide is causticized and the resulting sodium hydroxide containing solution can optionally be mixed with the substantially pure sodium carbonate after the absorption of sulphur dioxide in order to produce a fresh cooking liquor.

1 . A method for recovery of pulping chemicals in an alkaline sulphite pulping process and for production of steam, wherein 
 a) an evaporated spent liquor is fed into a gasification reactor in which said liquor is decomposed into a hydrogen sulphide (H 2 S)-containing gas and a solid residue containing sodium carbonate (Na 2 CO 3 ) and a controlled concentration of sodium sulphide (Na 2 S);    b) the solid residue from the gasification reactor in step a) is leached in a counter-current leaching process which involves at least two stages wherein    in a first leaching stage b′) soluble process-foreign elements such as K and Cl are dissolved and separated, and    in a second leaching stage b″) sodium sulphide (Na 2 S), sodium sulphate (Na 2 SO 4 ) and a portion of the sodium carbonate (Na 2 CO 3 ) present are dissolved and separated;    c) the remaining solid residue from the leaching process in step b) is dissolved in water;    d) the solution resulting from step c) is filtered to remove non-soluble process-foreign elements and any possible carbon residue and is treated in an oxidizing stage to give a substantially pure sodium carbonate (Na 2 CO 3 ) solution;    e) the solution containing the sodium sulphide (Na 2 S), sodium sulphate (Na 2 SO 4 ) and sodium carbonate (Na 2 CO 3 ) that was separated in the second leaching stage of step b) is causticized, resulting in a sodium hydroxide (NaOH)-containing solution;    f) the hydrogen sulphide (H 2 S)-containing gas from the gasification reactor in step a) is led through a dust collector and is combusted in a steam boiler, producing steam;    g) the sulphur dioxide (SO 2 ) generated in the steam boiler in step f) is absorbed in the substantially pure sodium carbonate (Na 2 CO 3 ) solution resulting from the oxidizing stage in step d), and    h) carbon dioxide is removed from the solution of step g) in a desorption stage to form a substantially pure sodium sulphite (Na 2 SO 3 ) solution.    
     
     
         2 . A method according to  claim 1 , wherein the leaching liquor of the first leaching stage b′) is a part of the saturated leaching liquor that is separated from the second leaching stage b″), and the leaching liquor of the second stage b″) is a part of the sodium carbonate (Na 2 CO 3 )-containing solution from step c).  
     
     
         3 . A method according to  claim 2 , wherein the proportion of the resulting leaching liquor from the second leaching stage of step b) taken out for use as leaching liquor of the first leaching stage is less than 5%.  
     
     
         4 . A method according to  claim 3 , wherein the proportion taken out is 1-2%.  
     
     
         5 . A method according to  claim 1 , further comprising a step i) for mixing the substantially pure sodium sulphite (Na 2 SO 3 ) solution resulting from step h) with a predetermined amount of the sodium hydroxide (NaOH)-containing solution resulting from the causticizing step d) thus giving a fresh sulphite cooking liquor containing a controlled amount of sodium sulphide (Na 2 S).  
     
     
         6 . A method according to  claim 1 , wherein the evaporated spent liquor in step a) is gasified at a temperature of 650-750° C.  
     
     
         7 . A method according to  claim 6 , wherein the evaporated spent liquor in step a) is gasified at a temperature of 700-750° C.  
     
     
         8 . A method according to  claim 1 , wherein air is added to the gasification reactor of step a) in an amount of 25-75% of the total amount required for complete combustion of the evaporated spent liquor.  
     
     
         9 . A method according to  claim 8 , wherein the amount of air added to the gasification reactor is 30-50% of the total amount required for complete combustion of the evaporated spent liquor.  
     
     
         10 . A method according to  claim 1 , wherein said evaporated spent liquor is gasified in step a) at a pressure of at most 5 bar.  
     
     
         11 . A method according to  claim 10 , wherein said liquor is gasified at a pressure of at most 2 bar.  
     
     
         12 . A method according to  claim 1 , wherein the gasification reactor of step a) is a fluidized bed reactor.  
     
     
         13 . A method according to  claim 1  wherein the gas from the gasification reactor in step a) is lead to the dust collector of step f) without intermediate cooling.  
     
     
         14 . A method according to  claim 1 , wherein the solution containing sodium sulphide (Na 2 S), sodium sulphate (Na 2 SO 4 ) and sodium carbonate (Na 2 CO 3 ) that is separated in the second leaching stage of step b) contains 20-60% of the total amount of the sodium salts that were fed into the second leaching stage.  
     
     
         15 . A method according to  claim 14 , wherein the solution containing sodium sulphide (Na 2 S), sodium sulphate (Na 2 SO 4 ) and sodium carbonate (Na 2 CO 3 ) that is separated in the second leaching stage of step b) contains 40-50% of the total amount of the sodium salts that were fed to the second leaching stage.  
     
     
         16 . A method according to  claim 1 , wherein the temperature of the second leaching stage of step b) is 20-100° C.  
     
     
         17 . A method according to  claim 16 , wherein the temperature is 50-70° C.  
     
     
         18 . A method according to  claim 1 , wherein the steam boiler of step f) has a working pressure of 60-120 bars and a super-heating temperature of 450-560° C.  
     
     
         19 . A method according to  claim 1 , further comprising a step j) for production of sulphuric acid (H 2 SO 4 ) from a part of the sulphur dioxide (SO 2 )-containing gas from the steam boiler of step f).  
     
     
         20 . A method according to  claim 1 , further comprising a causticizing step k) for production of sodium hydroxide (NaOH) from a part of the substantially pure sodium carbonate (Na 2 CO 3 ) solution from step d).  
     
     
         21 . A method according to  claim 19 , wherein the sulphuric acid (H 2 SO 4 ) produced in step j) and/or the sodium hydroxide (NaOH) produced in step k) and/or the sodium carbonate (Na 2 CO 3 ) from step d) are (is) used for bleaching of pulp, resulting in a closed alkaline sulphite pulping and bleaching process.