Non-scaling chip conditioning system for energy reduction in mechanical pulping

Abstract

A method for pulping a fibrous lignocellulosic material includes a pretreatment step wherein the lignocellulosic material is reacted with a chelating agent selected from the group consisting of amino acids, monocarboxylic acids and polycarboxylic acids preferably having from two to six carbon atoms. The chelating agent reacts with calcium ions in the material to form stable, water-soluble calcium complexes so as to avoid scaling, and the treated material requires a significantly lower energy input to undergo refining using either a mechanical or thermomechanical pulping method.

1 . A method for pulping a fibrous lignocellulosic material, the method comprising:
 (a) reducing the material to a size appropriate for pulping;   (b) contacting the material with at least one chelating agent selected from the group consisting of amino acids, monocarboxylic acids and polycarboxylic acids, wherein the chelating agent contains from two to six carbon atoms;   (c) reacting the material with the chelating agent at a temperature and for a time sufficient such that the chelating agent reacts with calcium ions in the material to form stable, water-soluble calcium complexes; and   (d) mechanically refining the material so as to produce a pulp suspension;   wherein a lower energy input is required to pulp the material, after the material is reacted with the chelating agent, than if the material was refined without first being contacted with and reacted with the chelating agent; and   wherein the calcium complexes are sufficiently soluble throughout the entire pulping process so as to substantially avoid the formation of scale on process equipment and piping.   
   
   
       2 . The method of  claim 1 , wherein the amino acids are selected from the group consisting of glycine, alanine, valine, serine, threonine, cysteine, asparagine, glutamine, aspartic acid, glutamic acid and gamma-aminobutyric acid. 
   
   
       3 . The method of  claim 1 , wherein the monocarboxylic and polycarboxylic acids are selected from the group consisting of citric acid, succinic acid, fumaric acid, malic acid, oxaloacetic acid, propionic acid, valeric acid, acrylic acid, butyric acid, pyruvic acid, malonic acid, glutaric acid, lactic acid and tartaric acid. 
   
   
       4 . The method of  claim 1 , wherein the chelating agent is citric acid. 
   
   
       5 . The method of  claim 1 , wherein the chelating agent is added in combination with one or more additional chelating agents selected from ethylenediamine tetraacetate (EDTA) and diethylenetriaminepentaacetic acid (DTPA). 
   
   
       6 . The method of  claim 1 , wherein the chelating agent is contacted with the lignocellulosic material in the form of an aqueous solution. 
   
   
       7 . The method of  claim 1 , wherein step (b) is conducted in a first vessel and wherein step (c) is conducted in a second vessel. 
   
   
       8 . The method of  claim 7 , wherein the chelating agent is added to both the first and second vessels. 
   
   
       9 . The method of  claim 1 , wherein the solubility of the calcium complexes is at least one order of magnitude greater than the solubility of calcium oxalate. 
   
   
       10 . The method of  claim 1 , wherein the calcium complexes remain substantially dissolved in the pulp suspension throughout a pH range of from about 5 to about 9. 
   
   
       11 . The method of  claim 1 , wherein the chelating agent and the lignocellulosic material are reacted at a temperature in the range from about 50 to about 150 degrees Celsius. 
   
   
       12 . The method of  claim 1 , wherein the chelating agent and the lignocellulosic material are reacted for a time of about 10 to 20 minutes. 
   
   
       13 . The method of  claim 1 , wherein the chelating agent and the lignocellulosic material are reacted at an elevated pressure and temperature. 
   
   
       14 . The method of  claim 7 , further comprising the step of reacting the lignocellulosic material at an elevated pressure in a third vessel, wherein the third vessel comprises a pressure vessel which is pressurized by compressed air and steam. 
   
   
       15 . The method of  claim 1 , wherein the lignocellulosic material is in the form of wood chips. 
   
   
       16 . The method of  claim 1 , wherein the lignocellulosic material is refined using either a mechanical pulping method or a thermo-mechanical pulping method. 
   
   
       17 . The method of  claim 1 , wherein the energy input required to pulp the material, after the material is reacted with the chelating agent, is at least 20 percent lower than if the material was refined without first being contacted with and reacted with the chelating agent.