Alkali recycle in cellulose spinning process
The present invention concerns a method for the recovery of solvent in a process for preparation of regenerated cellulosic fibers using sodium hydroxide as solvent for cellulose dissolving in the manufacturing of a cellulose spinning dope wherein sodium hydroxide present in the spinning dope is at least partially recovered and recycled as sodium hydroxide from a cellulose fiber regeneration or cellulose coagulation step and wherein said cellulose fiber regeneration or cellulose coagulation step consists of a bath into which cellulose spinning dope is injected. Recovered sodium hydroxide may be directly or indirectly recycled from a cellulose fiber regeneration or cellulose coagulation step to a cellulose dissolving step.
1. Method for the recovery of sodium hydroxide in a process for preparation of regenerated cellulosic fibers using aqueous sodium hydroxide as solvent for cellulose or cellulose derivatives dissolving in the manufacture of a cellulosic spinning dope, wherein sodium hydroxide present in the cellulosic spinning dope is at least partially recovered as sodium hydroxide from a coagulation bath into which the cellulosic spinning dope is injected, and wherein die-swell of filaments and coagulation rate of cellulosic moieties in the coagulation bath are controlled by adjusting a spinneret hole diameter (D) and land length (L) to a L/D ratio below 4.
2. The method according to claim 1 , wherein the sodium hydroxide recovered from the coagulation bath is directly, or indirectly, recycled to a cellulose dissolving step.
3. The method according to claim 1 , wherein the cellulose in the spinning dope is substantially composed of alpha cellulose, or derivatised cellulose.
4. The method according to claim 3 , wherein the derivatised cellulose includes one or more of cellulose xanthate, cellulose carbamate and cellulose carbonate.
5. The method according to claim 1 , wherein the pH in the coagulation bath is kept above about 6.
6. The method according to claim 5 , wherein alkalinity of the coagulation bath is maintained by causticizing with lime, borates or titanates.
7. The method according to claim 1 , wherein the coagulation bath comprises sodium carbonate.
8. The method according to claim 7 , wherein sodium carbonate in the coagulation bath is separated from a sodium hydroxide present in the cellulosic spinning dope injected into the coagulation bath, the separated sodium carbonate being at least partially recovered and recycled to the coagulation bath.
9. The method according to claim 7 , wherein the average concentration of sodium carbonate in the coagulation bath is in the range of 15 to 45% (by weight).
10. The method according to claim 1 , wherein the coagulation bath comprises sodium sulphate.
11. The method according to claim 10 , wherein sodium sulphate in the coagulation bath is separated from a sodium hydroxide present in the cellulosic spinning dope injected into the coagulation bath, the separated sodium sulphate being at least partially recovered and recycled to the coagulation bath.
12. The method according to claim 1 , wherein the coagulation bath comprises an organic solvent.
13. The method according to claim 1 , wherein the coagulation bath comprises one or more salts.
14. The method according to claim 1 , wherein the average concentration of sodium hydroxide in the coagulation bath is in the range of 2 to 12% (by weight).
15. The method according to claim 1 , further comprising washing the regenerated cellulosic fibers with a washing liquid, wherein alkalinity of the washing liquid used to wash the regenerated cellulosic fibers is restored by causticizing with lime, borates or titanates.
16. The method according to claim 1 , comprising the step of adding sodium carbonate and/or sodium sulphate to the coagulation bath.
17. The method according to claim 1 , wherein the L/D ratio is larger than 0.5.
18. The method according to claim 1 , wherein the coagulation bath comprises one or more of aluminum salts, potassium salts, zinc salts and zinc oxide.