HRP930460A2 - A process for purifying aqueous solutions of n-methyl-morpholine n-oxide - Google Patents

Description

The invention relates to a process for cleaning the aqueous N-methylmorpholine-N-oxide (NMMO) solution, especially the solution of the spinning bath in the preparation of cellulose products.

It is known that cellulose is introduced into aqueous NMMO solutions and a homogeneous cellulose solution is prepared for spinning. By settling these solutions in water, foils, fibers or shaped products based on cellulose are obtained, i.e. objects, which today are produced on a large scale using the viscose fiber process. Spinning pulp solutions in aqueous NMMO have an excellent advantage over viscose in terms of environmental portability, as the NMMO from the spinning bath can be recovered and, in addition, no sulfur-containing emissions occur.

Since the NMMO from the spent bath can be used to re-prepare cellulose spinning solutions, the spinning bath solution needs to be cleaned and concentrated.

Complete cleaning must include the following stages:

A) Color removal:

Evaporation of water due to the concentration of NMMO from diluted aqueous NMMO solutions results in a strong red to brown coloration due to the reaction of NMMO with cellulose degradation products.

Pigment compounds are created, first of all, from polyhydroxyphenol, from the decomposition products of cellulose itself and from NMMO-stabilizers, which usually have to be added to the solution.

Due to the growing coloration, NMMO cellulose semi-products can no longer be bleached to the desired degree of whiteness.

B) Removal of transition metals:

Transitional metals, especially iron, are introduced into the process cycle, on the one hand, with the used tissue, and, on the other hand, with corrosion. However, the content of transition metal ions is critical in that it lowers the initial temperature for deflagration of the spinning mass. If gallic acid propyl ester is used as a stabilizer, anionic metal complexes are formed, which can be removed with anion exchangers. However, if, for example, rutin is used as a stabilizer, iron complexes are formed that can no longer be removed with ion exchangers. Therefore, without cleaning the NMMO in the lead cycle, there would be an inevitable accumulation of iron in the process and thus an increase in the safety risk. Therefore, the removal of iron and other transition metal ions from the process is absolutely necessary.

C) Removal of nitrosamines:

Nitrosamines may also be present in fresh NMMO - depending on its preparation; they can cause a number of different toxic effects, such as, for example, acute liver damage, gene toxicity both in vitro and in germ cells, cancerous diseases in somatic cells, etc. Based on the general effect of nitrosamines, that they promote tumors, due to work safety, insist on their complete removal.

D) Removal of substances that cause turbidity:

In addition to the coloring of the spinning bath, a precipitate may also form, which mainly consists of the finest cellulose substance. In addition, there are also alkaline and alkaline earth salts. These substances, which cause turbidity, and which accumulate during repeated use of solvents, cannot be filtered without an auxiliary means; they affect the quality of the product, lead to disturbances, for example, in continuous color measurement and must therefore be removed.

With the mentioned stages of cleaning, it should be additionally warned that the loss of NMMO should be avoided as much as possible.

The cleaning procedures known so far use both of the following methods with fundamental disadvantages:

a) Cleaning with anion exchanger:

According to this method, color removal is limited to ionic colored complexes: iron or transition metals can only be removed if they are in ionic form, which, among other things, depends on the stabilizer system; nitrosamines cannot be removed; it is also not possible to remove the fine cellulose sediment worth mentioning; relatively large amounts of chemicals are required for regeneration.

b) Recrystallization from acetone:

This method requires a lot of time and energy; in addition the NMMO recovery is only at most 85%.

In terms of the invention, these defects can be avoided by bringing the solution into contact with absorbents and then subjecting it to filtration. In this case, it can be of particular advantage if aluminum oxide, silicon dioxide and/or coal are used as absorbent.

The process according to the invention achieves at least 70% decolorization of the solution, practically quantitative removal of transition metals, complete removal of nitrosamine, as well as removal of a fine cellulose precipitate.

The purified solution is furthermore completely free of any turbidity-causing substance.

Furthermore, a significant advantage of the process according to the invention is that there is practically no loss of amine oxide. The following warnings serve to clarify the procedure:

i) We use Al2O3 type "C" from DEGUSSA. The amount used calculated at 20% of the spinning bath is approx. 1%. The retention time is a few minutes. The absorbent can easily be removed by filtration together with the turbidity-causing substances. By subsequent washing of the filter cake, NMMO is completely recovered.

ii) We use flint from the company DEGUSSA with the type mark "FK 700". The amount used calculated on 20% aqueous NMMO is 1%. The retention time is a few minutes and the separation of SiO2 is done by filtration together with substances that cause turbidity.

iii) We use powdered coal (from brown or black coal) with an average particle size of 0.15 mm. In this case, the particle size of the coal used and thus available for cleaning is decisive.

The use of coal in terms of the degree of contamination of the spinning baths, the desired cleaning effect and the size of the active surface of the coal is between 0.1% and 1% with respect to the amount of spinning baths. The average residence time is a few minutes.

Normal filtration for the separation of coated coal is adversely affected, firstly, by small traces of present cellulose sediment, because they greatly increase the pressure loss on the filter even after the shortest duration of filtration, and, secondly, by substances present in the spinning bath, which cause turbidity. Therefore, we suggest the following methods for coal separation:

- filtration with tissue,

- flood filtration or

- fast microfiltration (QMF).

In addition, special attention should be paid to the following:

Filtration with tissue as a filter aid

Since even the smallest coal particles drastically reduce the whiteness of cellulose semi-products, the separation of coated coal must be complete. It is also a guarantee that most of the substances that cause turbidity have been removed.

The fine cellulose sediment in the NMMO spinning bath as well as the smallest coal particles lead to a large pressure loss on the filter even after a very short period of filtration. For this reason, a porous filter layer must be sought which is normally permeable to aqueous NMMO, and retains the fine substances described above.

This task was solved by breaking the leaf tissue with a blender in water (to obtain fibers) and then floating it on a relatively coarse metal sieve. When we get a layer of cells about 1 cm thick, we can completely remove the coal from the suspension. By subsequent washing with VE-water (i.e. completely demineralized water) NMMO can be washed from the filter layer without loss.

Alluvial filtration

A porous filter layer is obtained if coal in the form of a dense coal/water suspension is floated directly onto, for example, a standing filter column. The solution of the spinning bath, which needs to be cleaned, can then be freed from the coal through this layer, where care must be taken to avoid clouding caused by the smallest coal particles in this way of guiding. Since the carbon cleaning effect has worn off, with subsequent rinsing with VE-water, we completely wash the amine oxide from the coal layer. The standing filter column also offers the advantage that, due to the minimization of the amount of water for rinsing, we can release the amine oxide before the rinsing cycle and thus prevent the formation of a mixed zone during coal rinsing. Before drying by blowing coal to increase the calorific value, let's drain the water from washing again. If the pressure difference between the carbon layers outside and the filter column inside is maintained when the liquid is discharged, this enables the carbon layer to be maintained also when the medium is changed.

Quick microfiltration (QMF)

First, we put the charcoal/spinning bath suspension into the installed container of the rapid microfiltration (QMF) device. Then there is a continuous separation of the cleaned spinning bath as QMF permeate. The coal suspension with a highly concentrated volume (retentate) is then dried over chamber filter presses. Then we wash the coal with VE-water in a chamber filter press, so that it is free of NMMO. By blowing air, we dry the coal even further, to increase its calorific value. We can either heat the coal or regenerate it again for a new use.

For regeneration, for example, the following regeneration chemicals are suitable: sodium hydroxide/ethanol, ammonia/methanol, ammonia/propanol-2 and/or ammonia/acetone.

The coal, which needs to be regenerated, is suspended in the regeneration solution after complete elution of NMMO and then filtered. After the neutral rinse, it can then be used again to clean the aqueous NMMO solution.

We use the following analytical methods to test the cleaning effect of individual variants of the procedure:

- discoloration: measurement of extinction at 470 nm with a Perkin-Elmer photometer,

- iron content: atomic absorption and X-ray fluorescence measurement,

- turbidity (caused by fine cellulose sediment): turbidity measuring device TRM-L by DROTT,

- nitrosamines: after gas chromatographic separation, it is proven with the THERMO ELECTRON TEA detector; the previous experiment was performed with N-nitrosomorpholine and dimethylnitrosamine.

Further details of the procedure in terms of the invention are visible from the following examples.

Example 1

Use of aluminum oxide as an absorbent

Mix 50 ml of NMMO spinning bath with 0.5 g of aluminum oxide (ie 0.1% based on the spinning bath) in a glass beaker and then let it stand for 30 minutes. Then we filter through a filter with a blue band and analyze the filtrate.

Decolorization effect is 98%, removal is 94%. The turbidity reduction is 98%.

Example 2

The use of silicon dioxide as an absorbent

Mix 50 ml of the spinning bath with 0.5 g of silica and filter after 30 minutes through a filter with a blue strip. The filtrate is completely clear, decolorized to 72%, the iron content is reduced by approx. 70%.

Examples 3 to 8

The use of brown coal as an absorbent

Example 3

2 g of brown coal coking powder suspended for 2 minutes in a 100 ml spinning bath. The suspension is filtered through glass frit no. 3 (15 cm2 filter area), on which a filter with a blue strip is laid, and measure the extrinsicity at 470 nm.

Extrinsic: spinning bath output: 0.608

cleaned spinning bath: 0.095

Decolorization effect: 85%

Turbidity: Spinning bath output: 16.3 FTU

cleaned spinning bath: 0.2 FTU

Haze reduction: 98.8%

(FTU = formazin turbidity unit; formazin is a sedative.)

Example 4

We filter over 2.5 g of powdered coke in a 100 ml spinning bath (extinction 0.413) and determine the extinction of the filtrate.

The table shows the relationship between the amount of coal used and its discoloration effect.

[image]

The extinction reduction in all examples is greater than 95%, although the duration of filtration in a series of experiments increased tenfold.

Example 5

200 ml of the spinning bath (20.6% NMMO) is filtered over 27.37 g (=50 ml) of dry coke powder. In doing so, we get 48.52 g of wet coke powder; this corresponds to an amount of NMMO of 4.45 g.

We subsequently wash the wet coal four times, each time with 50 ml of VE-water, and determine the NMMO value of the individual fractions of the washing water.

[image]

Example 6

We add FeCl3 6H2O to a 20% aqueous solution of NMMO and then measure the removal of iron at different amounts of coal. The extinction of the starting substance at 470 nm is 0.682, the iron content is 33.5 ppm, the turbidity is 20.3 FTU.

[image]

Example 7

Disperse 5 kg of coconut powder for 5 minutes in a 200 ml disposable spinning bath (20.7% NMMO). To separate coal, we use a 5 μ GAF-filter (5 1). The first filtrate is colored black due to the smallest particles, although with increasing duration of filtration it becomes lighter and lighter, until finally it becomes as clear as water.

Discoloration: 93%

Haze reduction: 97.5%.

Example 8

200 g of coal is suspended in 1800 ml of VE-water and floated on a standing filtration column (filter area 0.012 m2) of the company Dr. M. Marke Fundapack. Together we clean 44 l, the specific flow at that moment drops from 1250 1/m2 to 910 1/m2. The decolorization effect is 96.4%. Turbidity reduction is 99%, iron reduction is 96%. To rinse the NMMO, use 3 1 VE-water, while washing the NMMO completely from the coal. By blowing in 7 1 air, we reach a dry coal content of 62%.

Example 9

The use of activated carbon as an absorbent

Mix 200 l of the spinning bath (20% NMMO) in the installed container of the rapid microfiltration device with 0.05% activated carbon from Chemviron, type BL, and preheat to 50°C. To separate coal and the smallest cellulose particles (starting material: 12 FTU), we use a Teflon membrane from the company Purolator.

Membrane overflow: 2 m/s.

Pressure difference: 0.2 bar.

Permeate flow: 16600 1/m2/h, which decreases to 1000 1/m2/h.

Permeate turbidity: 0.2 FTU.

No concentration of NMMO occurs (NMMO-concentration of the starting material is identical to that of permeate and retentate).

We evaporate the coal suspension to 9 l, which corresponds to a concentration of 1:22. Then we dry the dense coal suspension in a chamber filter press (flood pressure towards the end of 10 bar) and wash the coal cake with VE-water, so that it is free of NMMO. Then, by blowing in air, we achieve a dry content in the coal of 59.6%.

[image]

Example 10

Regeneration of used coal in the sense of example 9

We explain in more detail the regeneration of activated carbon, which is carried out primarily with sodium hydroxide in combination with an organic solvent, especially acetone; at the same time, we manage to keep the loss of capacity after regeneration under 2%.

We use a single-use spinning bath with a concentration of 19.8% and active carbon from Chemviron. For each coating, we suspend activated carbon by intensive mixing in the spinning bath. Coal filtration is done through a membrane filter (type PA or Versapor). The filter cake is washed with VE-water until neutral and processed in small portions with the regeneration solution. After rinsing until neutral, scrape the carbon from the membrane and use it again.

Comparison of coal capacity after regeneration with different regeneration solutions:

(Quantity of coal with respect to NMMO: 0.5%.)

Coal capacity (% of zero value)

[image]

Claims (6)

1. Process for cleaning the aqueous N-methylmorpholine-N-oxide solution, especially the solution of the spinning bath, indicated by bringing the solution into contact with absorbents and then subjecting it to filtration. 2. The method according to claim 1, characterized in that aluminum oxide is used as an absorbent. 3. The method according to claim 1, characterized in that silicon dioxide is used as an absorbent. 4. The method according to claim 1, characterized in that coal is used as an absorbent. 5. The method according to one of claims 1 to 4, characterized in that the absorbent has a particle size of <0.15 nm 6. The method according to claim 4, characterized by the fact that the filtration is carried out with cellulose as an auxiliary filter agent, or that the filtration is flood filtration, or rapid microfiltration.