Process for producing a pulp
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.
1. A process for producing a lyocell fiber from a dissolving pulp, comprising the step of cooking a cellulosic starting material with a cooking liquor in a kraft process to obtain the dissolving pulp, wherein the cellulosic starting material is exposed to a steam treatment prior to cooking, wherein the steam treatment consists of applying a temperature of 120° C. or more and a P-factor of 50 or less, wherein the dissolving pulp obtained by cooking is subjected to cold caustic extraction (CCE) in the course of further processing whereby a CCE treated dissolving pulp is formed, and wherein the lyocell fiber having a content H of hemicellulose of more than 3.5% by weight and less than 5% by weight is produced from the CCE treated dissolving pulp.
2. The process according to claim 1 , further comprising treating the dissolving pulp obtained by cooking to adjust the degree of polymerization (DP).
3. The process according to claim 1 , wherein, for the steam treatment, the cellulosic starting material is treated with medium-pressure steam at a temperature of from 150 to 180° C.
4. The process according to claim 1 , wherein the cellulosic starting material is subjected to an impregnation prior to cooking.
5. The process according to claim 1 , wherein the cooking, the steam treatment and optionally an impregnation of the cellulosic starting material are carried out in the same cooker.
6. The process according to claim 1 , wherein the cooking is carried out in a form of continuous batch cooking (CBC).
7. The process according to claim 1 , wherein at least a portion of a press liquor formed in the cold caustic extraction is purified using a membrane separation process.
8. The process according to claim 7 , wherein the membrane separation process is nanofiltration.
9. The process according to claim 7 , wherein the membrane separation process is an ultrafiltration.
10. The process according to claim 1 , wherein at least one of a portion of a press liquor formed in the cold caustic extraction and a portion of a retentate of a membrane separation process performed on the press liquor is supplied to the cooking liquor of a linked cooking process carried out for the production of paper pulp.
11. The process according to claim 1 , further comprising bleaching the pulp obtained by cooking.
12. The process according to claim 1 , wherein the cooking step of the pulp is carried out in two stages, and wherein, in a first stage, a delignification using a first cooking liquor to a kappa number of from 35 to 70 in case of softwoods as the cellulosic starting material and to a kappa number of from 30 to 60 in case of hardwoods as the cellulosic starting material is carried out and, in a second stage, the cooking is continued to the desired degree of purity of the pulp, using a second cooking liquor.