Process for producing a pulp

Abstract

The invention relates to a process for producing a dissolving pulp from a cellulosic starting material using the kraft process, comprising the step of cooking the starting material with a cooking liquor. The process according to the invention characterized in that the starting material is exposed to a steam treatment prior to cooking and that the pulp obtained by cooking is subjected to cold caustic extraction (CCE) in the course of further processing.

What is claimed is: 
     
         1 . A process for producing a dissolving pulp from a cellulosic starting material using the kraft process, comprising the step of cooking the starting material with a cooking liquor, wherein the starting material is exposed to a steam treatment prior to cooking and wherein the pulp obtained by cooking is subjected to cold caustic extraction (CCE) in the course of further processing. 
     
     
         2 . The process according to  claim 1 , wherein the steam treatment is carried out at a temperature of 120° C. or more and a P-factor of 50 or less. 
     
     
         3 . The process according to  claim 1  or  2 , wherein, for the steam treatment, the starting material is treated with medium-pressure steam at a temperature of from 150 to 180° C. 
     
     
         4 . The process according to any of the preceding claims, wherein the starting material is subjected to an impregnation prior to cooking. 
     
     
         5 . The process according to any of the preceding claims, wherein the cooking, the steam treatment and optionally the impregnation of the starting material are carried out in the same cooker. 
     
     
         6 . The process according to any of the preceding claims, wherein the cooking is carried out in the form of continuous batch cooking (CBC). 
     
     
         7 . The process according to any of the preceding claims, wherein at least a portion of the press liquor forming in the cold caustic extraction stage is purified using a membrane separation process, preferably nano- or ultrafiltration. 
     
     
         8 . The process according to  claim 7 , wherein the membrane separation process is carried out in two stages, with a dilution of the retentate of the first stage with water being performed between the two separation stages. 
     
     
         9 . The process according to any of  claims 7  and  8 , wherein from at least a portion of the retentate of the membrane separation process the hemicelluloses contained therein, in particular xylans, are obtained. 
     
     
         10 . The process according to  claim 9 , wherein the precipitable hemicelluloses, in particular xylans, are obtained from the retentate of the membrane separation process by reverse precipitation. 
     
     
         11 . The process according to  claim 10 , wherein, for the reverse precipitation, an agent selected from the group consisting of mineral acids, CO 2  and monovalent or polyvalent alcohols is used. 
     
     
         12 . The process according to any of  claims 9  to  11 , wherein the xylans obtained are processed at least partially into xylo-oligosaccharides. 
     
     
         13 . The process according to  claim 12 , wherein, for producing the xylo-oligosaccharides, a process selected from the group consisting of hydrothermolysis and enzymatic hydrolysis is carried out. 
     
     
         14 . The process according to any of the preceding claims, wherein it is linked to a process for producing a paper pulp and at least a portion of the press liquor forming in the cold caustic extraction stage and/or at least a portion of the retentate of a membrane separation process performed on the press liquor is/are supplied to the cooking liquor of a cooking process carried out for the production of paper pulp. 
     
     
         15 . The process according to any of the preceding claims, wherein the pulp obtained by cooking in the course of further processing, in addition to a bleaching step which has optionally been carried out, is subjected to a treatment for adjusting the degree of polymerization (DP). 
     
     
         16 . The process according to  claim 15 , wherein a treatment selected from the group consisting of oxidative bleaching methods, acid hydrolytic methods and electron irradiation methods is carried out. 
     
     
         17 . The process according to  claim 16 , wherein an electron irradiation of the pulp is performed wherein the pulp is present in a stock density of at least 35%, preferably of 40-50%, and in a layer thickness of from 2 to 6 mm, preferably from 3 to 4 mm, and an accelerating voltage of less than 2.5 MeV is applied. 
     
     
         18 . The process according to any of  claims 15  to  17 , wherein an alkaline-oxidative treatment, preferably an alkaline peroxide treatment, is carried out after the treatment for adjusting the degree of polymerization. 
     
     
         19 . The process according to any of the preceding claims, wherein the cooking step of the pulp is carried out in two stages, and wherein, in the first stage, a delignification using a first cooking liquor to a kappa number of from 35 to 70 in case of softwoods as the starting material and to a kappa number of from 30 to 60 in case of hardwoods as the starting material is carried out and, in the second stage, the cooking is continued to the desired degree of purity of the pulp, using a second cooking liquor. 
     
     
         20 . The process according to  claim 19 , wherein the proportion of the H-factor of the first cooking stage amounts to 40% to 80%, based on the H-factor of both cooking stages together, in case of softwoods as the starting material and, respectively, amounts to 20% to 60% in case of hardwoods as the starting material. 
     
     
         21 . The process according to  claim 19  or  20 , wherein, during the transition from the first to the second cooking stage, the first cooking liquor is replaced by the second cooking liquor. 
     
     
         22 . A process according to any of  claims 19  to  21 , characterized in that the first cooking liquor has an EA-concentration of from 10 g/L to 30 g/L, preferably from 15 g/L to 25 g/L, particularly preferably of 20 g/L, as well as a sulfidity of from 50% to 120%, preferably of 100%. 
     
     
         23 . The process according to any of  claims 19  to  22 , wherein the composition of the second cooking liquor differs from that of the first cooking liquor. 
     
     
         24 . The process according to  claim 23 , wherein the second cooking liquor is enriched with a liquor selected from the group consisting of the filtrate of a subsequent washing stage, a white liquor from caustification, the press liquor from a cold caustic extraction stage and the retentate of a membrane separation process performed on a press liquor from a cold caustic extraction stage. 
     
     
         25 . The process according to any of  claims 19  to  24 , wherein both cooking stages are carried out in the form of continuous batch cooking (CBC). 
     
     
         26 . The use of a dissolving pulp produced according to a process according to any of  claims 1  to  25  for the production of Viscose and Lyocell fibres. 
     
     
         27 . A Lyocell fibre obtainable by the spinning of a dissolving pulp produced according to a process according to any of  claims 1  to  25 . 
     
     
         28 . The Lyocell fibre according to  claim 27 , wherein its content H of hemicellulose amounts to more than 3.5% by weight and its yield of mercerization is 90% or more. 
     
     
         29 . The Lyocell fibre according to  claim 28 , wherein the content H amounts to less than 5% by weight. 
     
     
         30 . The Lyocell fibre according to  claim 28  or  29 , wherein its strength amounts to 37 cN/tex or more in the conditioned state. 
     
     
         31 . The Lyocell fibre according to any of  claims 28  to  30 , wherein the average molar mass of the xylans isolable from the fibre amounts to 20 kDa or more. 
     
     
         32 . A Viscose fibre obtainable by the spinning of a dissolving pulp produced according to a process according to any of  claims 1  to  25 . 
     
     
         33 . The Viscose fibre according to  claim 32 , wherein its content H of hemicellulose amounts to more than 2.0% by weight and its yield of mercerization is 90% or more. 
     
     
         34 . The Viscose fibre according to  claim 32  or  33 , wherein its strength amounts to 23 cN/tex or more in the conditioned state and its elongation amounts to 18% or more in the conditioned state.