Method of Recovering Aqueous N-Methylmorpholine-N-Oxide Solution Used in Production of Lyocell Fiber

Abstract

A method of recovering aqueous N-Methylmorpholine-N-Oxide solution used in production of Lyocell fiber comprises following steps. Bleach means for decoloring coloration in aqueous NMMO solution via alternate blow-mixing adsorption mode and static suspending adsorption mode reiteration. Filtration means for purifying the activated carbon powder and impurities by two filtering stages of first coarse filtering stage and second fine filtering stage. Concentration means for intensifying aqueous NMMO solution to obtain a condensed aqueous solution without NMMO solvent and a concentrated aqueous solution with NMMO solvent respectively by a sequential multi-stage evaporating system. Refinement means for purifying aqueous NMMO solution with promoting purity of concentrated aqueous solution to obtain required recovered aqueous solution by adding suitable agents in the redox reactions involved. Owing to streamlining and simplicity, the method not only has better competitiveness from promoted recovery cost, efficiency and quality but also meets regulations of environmental protection.

What is claimed is: 
     
         1 . A method of recovering aqueous N-Methylmorpholine-N-Oxide (NMMO) solution used in production of Lyocell fiber comprises steps of bleach (step 1), filtration (step 2), condensation (step 3) and refinement (step 4), wherein:
 The bleach in step 1 means for decoloring the aqueous NMMO solution:   Firstly, the aqueous NMMO solution to be recovered is loaded into a bleaching tank, and activated carbon powder featuring with good adsorbability and suspendability is added into the aqueous NMMO solution therein; secondly, the activated carbon powder and the aqueous NMMO solution are mixed together by using an agitation blower; and finally, the agitation blower is intermittently energized so that an alternate blow-mixing adsorption mode and static suspending adsorption mode reiterates to have activated carbon powder fully contacted with the aqueous NMMO solution thoroughly in an energy-efficient manner; thereby, the adsorption efficiency of the activated carbon powder is essentially increased; preferably, the added dosage of the activated carbon powder is in range of 0.05 wt %-0.10 wt % on the basis of total weight for the aqueous NMMO solution; wherein, the blow-mixing adsorption mode means for blowing ambient air into the bleaching tank by impeller rotation of the agitation blower to have activated carbon powder fully contacted with the aqueous NMMO solution thoroughly to facilitate the speed of the bleach process while the static suspending adsorption mode means for keeping the aqueous NMMO solution in stationary manner to let the aqueous NMMO solution precipitate automatically to save related energy; moreover, by means of a timer switch acting on the agitation blower, the blow-mixing adsorption mode and static suspending adsorption mode are intermittently alternated in reiterative fashion so that overall bleach efficiency is substantially increased; a duration ratio for the blow-mixing adsorption mode and static suspending adsorption mode is in range of 1:3-1:6; besides, total time in the bleach process of the aqueous NMMO solution is not longer than 8 hours; thus, by means of blow-mixing adsorption mode, not only the activated carbon powder can be fully contacted with the aqueous NMMO solution thoroughly but also the speed of the bleach process can be facilitated while by means of static suspending adsorption mode, not only the processing energy can be essentially saved but also the efficiency of the bleach process can be enhanced;   The filtration in step 2 means for purifying the aqueous NMMO solution:   Two filtering stages of first coarse filtering stage and successive second fine filtering stage (ultrafiltration UF) are orderly adopted so as to remove the activated carbon powder and impurities from the aqueous NMMO solution  1 , which has been decolored in previous bleach process; for the first coarse filtering stage, a filter cartridge having filtering material with pore size in range of 1 μm-100 μm (1 .mu-100 .mu.m) is used so that the activated carbon powder and the impurities of large particle size appeared in previous bleach process can be removed; for the second fine filtering stage (ultrafiltration UF), a filter material with pore size in range of 0.01 μm-1 μm (0.01 .mu.m-1 .mu.m) is used so that the tiny impurities of small particle size can be removed; wherein, the cartridge filter used in the first coarse filtering stage, over the surface thereof is beforehand pre-coated a filter aid, which is made from mixture of diatomaceous earth and cellulose with weight ratio of the diatomaceous earth to the cellulose is 4:1 preferably so that it prevents the activated carbon powder from accumulating on the surface thereof in hindering the filtering speed; besides, the filter aid is also added into the bleached aqueous NMMO solution with quantity in range of 0.03-0.05 wt % to increase the valid filtering area; wherein, some filtering dregs, which contain residual filter aid and a larger quantity of residual activated carbon powder mostly accumulated on the surface portion, are created after the first coarse filtering stage; after the filtering dregs is scraped off, the residual filter aid therein can be recovered and reused in the first coarse filtering stage step;   The concentration in step 3 means for intensifying the aqueous NMMO solution:   A sequential multi-stage evaporating system is adopted so as to intensify the aqueous NMMO solution, which has been purified in previous filtration process so that a condensed aqueous solution without NMMO solvent and a concentrated aqueous solution with NMMO solvent are respectively obtained; the sequential multi-stage evaporating system mainly comprises a first evaporating vessel with a first steam tank, a second evaporating vessel with a second steam tank and a third evaporating vessel with a third steam tank, wherein: the first evaporating vessel and the first steam tank are connected by a first steam inlet pipe while the first steam tank and the second evaporating vessel are connected by a first steam outlet pipe such that the first steam tank is connected to a first vacuum pump; moreover, the first evaporating vessel and second evaporating vessel are connected by a first solution recovering pipe, on which a first concentration meter and a first suction pump are disposed respectively; the second evaporating vessel and the second steam tank are connected by a second steam inlet pipe while the second steam tank and the third evaporating vessel are connected by a second steam outlet pipe such that the second steam tank is connected to a second vacuum pump; moreover, the second evaporating vessel and third evaporating vessel are connected by a second solution recovering pip, on which a second concentration meter and a second suction pump are disposed respectively; the third evaporating vessel and the third steam tank are connected by a third steam inlet pipe while the third steam tank is connected to a third vacuum pump and a third steam outlet pipe, which is further connected to a separating tank for steam and aqueous solution; moreover, the third evaporating vessel and a second suction pump are connected by a third solution recovering pipe, on which a third concentration meter and a third suction pump are disposed respectively; wherein, the aqueous solution outlet of the separating tank is connected to the third solution recovering pipe while the steam outlet of the separating tank is connected to a mechanical steam compressor, which is further connected to a first evaporating vessel via a steam recovering pipe; for the first evaporating vessel, other than the recovered steam being fed via the steam recovering pipe, a primary steam from a steam boiler is supplied via an input pipe for steam, and an aqueous NMMO solution, which has been purified in previous filtration process, is also supplied by an input pipe for aqueous NMMO solution, which orderly passes through the third steam tank, second steam tank and first evaporating vessel as well as a heat exchanger and a input pump for aqueous NMMO solution; by controlling the concentration of the recovered aqueous solution at the outlet of the first evaporating vessel in range of 10-20 wt % and the concentration of the recovered aqueous solution at the outlet of the second evaporating vessel in range of 22-38 wt % as well as feeding the steam evaporated by the recovered aqueous solution at the outlet of the third evaporating vessel back to the first evaporating vessel as supplementary steam source via the steam recovering pipe after it has been orderly processed by the third steam tank, separating tank and steam compressor, the overall recovered quantity of the concentrated aqueous NMMO solution under the same consumed quantity of the primary steam source can be substantially increased so that the goal of promoting recovery efficiency can be achieved; similarly, the condensed aqueous solution collected by the cold condensed water pipe from the first evaporating vessel, second evaporating vessel and third evaporating vessel can also be recovered for reusing in the rinse process of the Lyocell fiber production to remove the solvent and impurities attached on the raw filaments; and   The refinement in step 4 means for purifying the aqueous NMMO solution:   Before the concentrated aqueous NMMO solution is subjected to the refinement step, a bit of residual N-methylmorpholine (NMM) that arises from decomposition of NMMO, which is caused by heating during the dissolution step in the production of Lyocell fiber; the quantity of residual NMM in the concentrated aqueous NMMO solution is in range of 0.1-0.3 wt %; to perform the refinement step here, an oxidizer (namely oxidizing agent) is added into the concentrated aqueous NMMO solution processed by previous refinement process (step 3) so that the residual N-methylmorpholine (NMM) is oxidized into N-methylmorpholine-N-oxide (NMMO) via oxidation reaction by the oxidizer under reaction temperature being 80±2° C. (80.+/−0.2.degree of Celsius); after the oxidation reaction aforesaid, some residual oxidizer becomes redundant impurity, which should be completely removed anyhow; accordingly, a reducer (namely neutralizing agent) is added into the concentrated aqueous NMMO solution processed by previous oxidation reaction process aforesaid to neutralize the residual oxidizer via reduction reaction by the reducer to an quantity in range less than 0.06 wt % so that a recovered aqueous NMMO solution of high purity is obtained; wherein, the oxidizer applied is H 2 O 2  (hydrogen peroxide), and the reducer applied is N 2 H 4 H 2 O (hydrazine hydrate); moreover, the final applied quantities for the oxidizer and the reducer are decided by the testing result of the concentrated aqueous NMMO solution processed by foregoing redox reaction (namely reduction reaction and oxidation reaction) via potentiometric titration.