HK1025339B - Method for viscose production - Google Patents

Description

Viscose production method

The present invention relates to an improved process for producing viscose.

Methods for producing viscose from cellulose-containing raw materials are well known. A detailed description of this process can be found in Chapter "Viscoseyon" -spinoser "-schw mme", for example, K.G * tze, Chemiefa ch dem Viscoseverfahren, published by Spring-Verlag, 3 rd edition (1967) and Ullmanns Enzyklopadie der technischen Chemie, Ullmann's encyclopedia of Industrial chemistry, Vol.18, Vol.131-.

The individual steps of the viscose process can be briefly summarized below:

chemical pulp or other suitable cellulose-containing material is treated with an aqueous solution of caustic soda (typically at a concentration of 18-22%). To obtain so-called alkali cellulose. Nowadays alkalization is mainly carried out by the impregnation process, in which the cellulose-containing raw material is dispersed in an aqueous solution of caustic soda in a vessel or a vessel system, which is equipped with suitable dispersing equipment, to form alkali cellulose. The resulting dispersion, often also referred to as impregnation liquor, generally contains a solids content of 2 to 6%. Excess liquid is removed with a roll press or belt press to separate the alkali cellulose formed. The alkali cellulose comprises about 30-36% cellulosic material, about 15-20% NaOH, and moisture. The removed caustic soda solution is then recycled to the impregnation step. The composition in the caustic soda solution circulation system is constantly maintained at the desired value by removing a portion of the liquid stream and adding fresh caustic soda.

The separated alkali cellulose is aged, i.e., exposed to the action of air, to accomplish some oxidative degradation. Oxidative degradation depolymerizes the cellulose molecules to suitable chain lengths for use in the remaining steps of the viscose process. The aging step requires a residence time of up to 1.5 days.

The alkalization of the cellulose can also be carried out by impregnating the electron-treated cellulose-containing starting material with an alkali. The electron-treatment of cellulose-containing raw materials is described in detail, for example, in DE-A-2,941,624. This patent document proposes that the pulp be treated with an electron beam of 1-30KGy before being used for viscose production. This results in better process economics because the electronically treated cellulose-containing feedstock can be alkalized with a more dilute caustic soda solution (less than 18% strength) than used with untreated cellulose, resulting in significantly less caustic soda solution consumption while also improving the carbon disulfide reactivity of the cellulose. If the electron treatment is controlled correctly, the chain length of the cellulose can be adjusted to a value at which a further ageing step becomes superfluous. The increase in reactivity opens up the prospect of reducing carbon disulphide consumption.

The alkali cellulose is then reacted with carbon disulfide, and this xanthation converts the alkali cellulose to cellulose xanthate. The reaction is typically carried out at about 25-30 c for 3 hours, typically with an amount of carbon disulfide of 28-32% by weight of the cellulose-containing material present in the alkali cellulose. The resulting cellulose xanthate is a brittle yellow-orange mass.

The cellulose xanthate is then dissolved in dilute caustic soda solution. The solution of cellulose xanthate in this caustic soda solution is known as viscose.

The viscose was left at ambient temperature for several hours to age. During this time, the xanthate groups are distributed along the cellulose chain. Other steps such as filtration and degassing are also performed during the maturation period.

In the production of shaped structures, such as fibers, the cured viscose is forced through small holes into a spinning bath. The spin bath is generally acidic and typically contains sulfuric acid (about 10%), sodium sulfate (about 20%), and also small amounts of zinc sulfate (about 1%). The viscose is coagulated in a spinning bath to form fibers. At the same time as the coagulation, the cellulose is regenerated from the cellulose xanthate. The regenerated fibrous cellulose product is then cut, washed to remove accompaniments and impurities, and dried.

The properties of the product can be influenced within certain limits by changes in the viscose production and spinning process parameters.

A very significant cost factor in producing products consisting of viscose is the consumption of chemicals, such as carbon disulphide, caustic soda solution and sulphuric acid. However, the reduction in chemical consumption is technically limited. If the amount of carbon disulphide and/or caustic soda solution used is below a certain limit, the filterability of the viscose deteriorates, whereas good viscose filterability is absolutely necessary for the economic feasibility of the whole process.

Repeated attempts have been made in the past to improve the economics of the viscose process by reducing the consumption of chemicals. One possible approach is to use an electronically treated pulp.

Although electronically treated pulps can be used throughout the production, the manufacturer sometimes has a need to use normal untreated pulps. As described above, the alkali cellulose prepared from the electron-treated cellulose is no longer required to be aged, and therefore it cannot be mixed with the alkali cellulose prepared from the untreated cellulose until after the aging thereof. In some cases, it may be useful or necessary to keep the two types of alkali cellulose products separate until the final product. In both cases, however, it is economically advantageous to carry out the alkalization with a common lye system. This is not only because of the simpler equipment, but also the simpler processing of the solution is an advantageous further aspect. However, if the electronically treated pulp is alkalized with the same caustic soda solution used to alkalize the untreated pulp, the incompatibility becomes very significant because the filterability of the viscose produced is significantly impaired

Example 1 may provide proof.

Therefore, there is a need for a process for producing viscose, in which an electron-treated cellulose-containing raw material can be alkalized with a lye used for alkalizing untreated cellulose, and which does not give rise to the above-mentioned incompatibility. The use of a common lye system for both raw materials constitutes a further improvement in the economics of viscose production, since separate treatment of the lye for alkalization can be dispensed with.

It has now surprisingly been found that the above-mentioned incompatibility of the lye system can be avoided if the lye obtained from alkalizing untreated cellulose is treated so that the dispersed solids present in the lye are substantially removed before it is used for alkalizing the electron-treated cellulose. The viscose thus produced has a filtration value acceptable for industrial production.

Accordingly, the present invention provides a process for producing viscose, comprising the steps of:

a) the electron-treated cellulose-containing raw material is dispersed in an aqueous solution of caustic soda in a conventional manner,

b) excess caustic soda solution is removed from the obtained alkali cellulose by a conventional method,

c) xanthating the alkali cellulose obtained in step b) by reacting with carbon disulfide according to a conventional method,

d) the cellulose xanthate formed in step c) is dissolved in aqueous alkali solution in a conventional manner,

wherein the caustic soda solution used in step a) has been used in advance in the alkalization of electron-untreated cellulose and has been treated so that the solids dispersed therein do not exceed 0.16 g/l.

Dispersed solids in the sense of the present invention are undissolved impurities present in the caustic soda solution resulting from alkalization of electronic untreated cellulose. The caustic soda solution used in the alkalization of electron-untreated cellulose in advance is also referred to as plant lye. If desired, it can be adjusted to the desired concentration before step a) by dilution or addition of fresh caustic soda solution. The measurement of the dispersed solids in the plant lye is carried out by filtering a certain amount of the plant lye through a G3 frit and then quantitatively measuring the filter cake.

The electron-treated cellulose-containing material used in step a) is preferably a cellulose-containing material treated with an electron beam in a dose of 1 to 30 KGy. The electronically treated cellulose preferably comprises at least 85 wt.%, particularly 90-98 wt.% alpha-cellulose.

The electron-treated cellulose-containing feedstock is alkalized by reaction with aqueous caustic soda solution used to alkalize the electron-untreated cellulose. It is important that the aqueous caustic soda solution be treated so that the amount of solids dispersed in the caustic soda solution does not exceed 0.16g/l, which can be accomplished by techniques well known to those skilled in the art. Suitable methods include: such as filtration, centrifugation, sedimentation or equivalent solid/liquid separation techniques. The concentration of the caustic soda solution is generally 14 to 20 wt% NaOH, in particular 15 to 17 wt% NaOH. The concentration of the aqueous caustic soda solution used is adjusted by adding water and/or caustic soda.

Subsequent removal of excess caustic soda may be accomplished, for example, by pressurization, centrifugation, or other suitable means. The alkali cellulose of step b) generally comprises 28 to 38% by weight, preferably 30 to 35% by weight, of cellulose and 12 to 18% by weight, preferably 14 to 17% by weight, of caustic soda (NaOH). The Degree of Polymerization (DP) of the alkali cellulose is 200-500, preferably 250-400. The degree of polymerization is determined viscometrically on a solution of alkali cellulose in an aqueous metal/amine complex according to TAPPI Skandard T206.

The xanthation (step C) of the alkali cellulose is carried out by reacting it with 15-35% by weight, preferably 15-25% by weight, of carbon disulphide, the xanthation generally taking place at room temperature or at a slightly elevated temperature.

Thereafter, the cellulose xanthate formed is dissolved with an aqueous alkali solution. The preferred aqueous alkali solution is caustic soda solution and the resulting alkaline solution of cellulose xanthate, known as viscose, usually contains 7-11 wt%, preferably 8-10 wt% cellulose and 3-8 wt%, preferably 3.8-5 wt% NaOH. In addition to these components, viscose may also comprise additives to improve its spinnability. Mention may be made here in particular of ethylene glycol-like and amine-containing modifiers.

The viscose can then be used in a conventional manner to produce shaped structures, such as fibers or films, which are likewise part of the subject matter of the invention. Where cellulose is regenerated from cellulose xanthate.

The viscose is filtered before the forming process, and in case a dispersed solids amount of more than 0.16g/l is used, the filtration will show the above-mentioned incompatibility, i.e. the filtration value of the viscose is more than 700.

The following examples illustrate the invention:

example 1

300 g of the electron-treated viscose pulp are impregnated with an alkalizing lye (200g/l NaOH) from an industrial-scale production plant (use of untreated cellulose/plant lye), and then the composition is adjusted to 32.5% cellulose and 15.2% NaOH by centrifugation. The alkali cellulose obtained was xanthated with 28% carbon disulphide (calculated on the basis of the cellulose content). Dissolution with dilute caustic solution gave the viscose properties shown in the column "shop lye" in the table below.

A new prepared alkalizing lye with the same concentration is used to produce a comparative viscose in the same way, the properties of which are shown in the column "fresh lye".

	Workshop alkali liquor	New-made lye
			Cellulose in viscose (%)	9.0	9.0
NaOH in viscose (%)	4.0	4.0
			Filter value	2040	152

The filter values used to characterize the filterability herein show the following plant filterability:

filter value	Filterability in workshop
		＜400	Is very good
400-500	Good taste
		500-600	Medium and high grade
600-700	Not good
		＞700	Can not filter

The results show that the use of a workshop soda lye, i.e. a lye used for alkalizing untreated cellulose and which is subsequently not filtered, leads to the production of viscose which cannot be filtered. The process of the present invention provides a solution for the use of an alkalizing system for both untreated and electron-treated pulps.

It was surprisingly found that the incompatibility demonstrated above with example 1 is due to dispersed solids in the plant alkalizing lye, which dispersed solids can be removed, for example, by mechanical filtration. The viscose produced can be filtered more and more effectively if the plant alkalization lye is filtered more rigorously before the alkalization of the electron-treated pulp.

Example 2

In each test 300 g of the electron-treated viscose pulp were impregnated with alkalinizing lye (190g/l NaOH) from an industrial scale plant (untreated cellulose/plant lye was used) which was filtered to different degrees before use (thus leaving different concentrations of dispersed solids in the lye). The resulting alkali cellulose was then centrifuged until the composition was 31.8-32.2% cellulose and 16.2-16.4% NaOH. Xanthation was then carried out with 22% carbon disulphide (calculated on the basis of the cellulose content). Dissolving with dilute caustic soda solution to obtain viscose with cellulose content of 8.9-9.0% and NaOH content of 4.0-4.1%. The degree of filtration isMeasured as the concentration of the mixture which can still be removed from the alkalising lye using the G3 frit.

Experimental number	Mixture (g/l) in alkaline solution for alkalization	Filter value
			1	1.25	5680
2	0.46	945
			3	0.28	739
4	0.16	498
			5	0.03	247
6	0.008	193
			7	0.002	196
8	0.000	170

The filterability of the viscose can be judged according to the criteria given in example 1.

The results show that, when the lye used for alkalization contains dispersed solids in an amount of 0.28g/l, the viscose produced is no longer filterable on an industrial scale. Good filtration properties can only be achieved if the filterable solids content does not exceed 0.16 g/l.

Claims (5)

1. A method for producing viscose, the method comprising the steps of:

a) the electron-treated cellulose-containing raw material is dispersed in an aqueous solution of caustic soda in a conventional manner,

b) excess caustic soda solution is removed from the obtained alkali cellulose by a conventional method,

c) xanthating the alkali cellulose obtained in step b) by reacting with carbon disulfide according to a conventional method,

d) the cellulose xanthate formed in step c) is dissolved in aqueous alkali solution in a conventional manner,

characterized in that the caustic soda solution used in step a) has been used in advance in the alkalization of electron-untreated cellulose and has been treated so that the solids content dispersed in the caustic soda solution does not exceed 0.16 g/l.

2. The method according to claim 1, characterized in that the caustic soda concentration in step a) corresponds to 14-20 wt% NaOH.

3. The method according to claim 1, wherein the amount of dispersed solids in the caustic soda solution used in step a) is determined by filtration through a G3 frit.

4. The process according to claim 1, characterized in that the caustic soda solution used in step a) contains not more than 0.03g/l of dispersed solids.

5. The method according to claim 1, characterized in that the caustic soda solution used in step a) is treated by filtration and/or centrifugation.