Organic compounds -- part of the class 532-570 series – Organic compounds – Carbohydrates or derivatives
Reexamination Certificate
1999-08-12
2002-05-21
Geist, Gary (Department: 1623)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carbohydrates or derivatives
C536S060000, C536S124000, C536S127000
Reexamination Certificate
active
06392033
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an improved process for producing viscose.
2. Description of Related Art
The production of viscose from cellulosic materials is well known. An extensive description of this technology may be found, for example, in K. Götze, Chemiefasern nach dem Viskoseverfahren, Springer-Verlag, 3rd edition (1967) and in the chapter “Viscosereyon, -spinnfasern, -schwämme” in Ullman's Enzyklopädie der technischen Chemie, Volume 18, pages 131-177, and Volume 9, pages 213-222.
The individual steps of the viscose process can briefly be summarized as follows:
Chemical pulp or some other suitable cellulosic raw material is treated with an aqueous solution of caustic soda whose concentration is customarily between 18 and 22% to obtain alkali cellulose. Today, the alkalization is predominantly carried out by steeping. The cellulosic raw material is dispersed in the aqueous caustic soda solution in a vessel or vessel system equipped with suitable dispersing means. Alkali cellulose forms. The dispersion formed is frequently also referred to as the steep and generally has a solids content of 2-6%. The excess liquid is removed in roll or belt presses to isolate the alkali cellulose formed. Alkali cellulose comprises about 30-36% of cellulosic material, about 15-20% of NaOH and water. The removed caustic soda is then recycled into the steeping step. The addition of fresh caustic soda and the removal of a bleed stream maintain the composition in this caustic recycle system consistently at the desired values.
The isolated alkali cellulose is aged, i.e., exposed to the action of air, to achieve some oxidative degradation which depolymerizes the cellulose molecules to a suitable chain length for the rest of the process. This aging step requires a residence time of up to 1.5 days.
The foregoing alkalization of the cellulose can also be effected by steeping electron-treated cellulosic material in alkali. This electron treatment of the cellulosic material is described in detail, for example, in DE-A-2,941,624. There it is proposed that pulp be treated with 1-30 kGy electron beams before use for viscose production. This leads to better processing economics, since electron-treated cellulosic material can be alkalized by means of more dilute caustic soda (concentrations of less than 18%) than is possible in the case of untreated cellulose, so that caustic soda consumption distinctly diminished. At the same time, the carbon disulfide reactivity of the cellulose improves. With correct control of the electron treatment, the chain length of the cellulose can be adjusted to a value at which a further aging step becomes superfluous. The increased reactivity opens up the prospect of reducing the consumption of carbon disulfide.
The alkali cellulose is then reacted with carbon disulfide. This xanthation turns the alkali cellulose into a cellulose xanthate. The reaction typically involves 3 hours at temperatures of about 25 to 30° C., and a carbon disulfide quantity which typically amounts to 28 to 32% by weight of the cellulosic material present in the alkali cellulose. The resulting cellulose xanthate is obtained as a crumbly yellowish orange mass.
The cellulose xanthate is subsequently dissolved in dilute caustic soda. It is this solution of cellulose xanthate in caustic soda which is known as viscose.
The viscose is allowed to stand at about ambient temperature for several hours to ripen. During this time, the xanthate groups become distributed along the cellulose chains. During the ripening time, other process steps such as filtration and deaeration are carried out as well.
To produce shaped structures, for example fiber, the ripened viscose is forced through small orifices into a spin bath. The spin bath is typically acid in character and customarily comprises sulfuric acid (about 10%), sodium sulfate (about 20%) and also small amounts of zinc sulfate (about 1%). The spin bath coagulates the viscose to form fiber. 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 concomitants and impurities, and dried.
The properties of the products can be influenced within limits through variation of the process parameters of viscose production and of spinning.
A very significant cost factor for making products composed of viscose is the consumption of chemicals such as carbon disulfide, caustic soda and sulfuric acid. However, there are technical limits to reducing the consumption of chemicals. If the amount of carbon disulfide and/or caustic soda used drops below certain limits, the filterability of the viscose deteriorates, but good viscose filterability is an absolute must for the entire process to be economically feasible.
In the past, there have been repeated attempts to improve the economics of the viscose process by reducing the consumption of chemicals. One possible way is to use electron-treated pulp.
Although electron-treated pulp may be used for the entire production, a production plant may be faced with the need to use normal, untreated pulp as well to some extent. As explained above, alkali cellulose prepared from electron-treated cellulose no longer has to be aged, so that it cannot be mixed with alkali cellulose prepared from untreated cellulose until after the aging thereof.
In certain cases, it can be useful or necessary to keep the two types of alkali cellulose product separate through to the finished product. In both cases, however, it is economically advantageous to carry out the alkalization with a common caustic system. Not only is the equipment simpler, but the simpler processing procedure is another point in favor of this solution. However, if electron-treated pulp is alkalized using the same caustic previously used to alkalize the untreated pulp, incompatibility becomes apparent as a dramatic deterioration in the filterability of the viscose produced. This is documented by Example 1 described below.
There is therefore a need for a process for producing viscose in which electron-treated cellulosic material can be alkalized using a caustic previously used in the alkalization of untreated cellulose and which does not give rise to the above-described incompatibility. The use of a common caustic system for both sources of raw material constitutes a further improvement in the economics of viscose production, since separate handling of the alkalizing caustic can be dispensed with.
SUMMARY OF THE INVENTION
It has now been found that, surprisingly, the above-described incompatibility of the caustic system can be avoided if the caustic obtained from the alkalization of the untreated cellulose is subjected to a treatment whereby dispersed solids present in the caustic are substantially removed before use in the alkalization of electron-treated cellulose. A viscose thus produced has an acceptable filter value for industrial production.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The present invention accordingly provides a process for producing viscose, which comprises the steps of:
a) dispersing electron-treated cellulosic material in an aqueous solution of caustic soda in a conventional manner,
b) removing excess caustic soda from the resulting alkali cellulose in a conventional manner,
c) xanthating the alkali cellulose obtained as per step b) by reaction with carbon disulfide in a conventional manner,
d) dissolving the cellulose xanthate formed as per step c) in an aqueous solution of a lye in a conventional manner,
wherein the caustic soda used in step a) was previously used in the alkalization of electron-untreated cellulose and treated in such a way that the level of dispersed solids in the caustic does not exceed 0.16 g/l.
Dispersed solids for the purposes of the present invention are undissolved impurities present in the caustic soda which result from the alkalization of electron-untreated cellulose. The caustic soda previously used in the alkalization of electron-untreated cellulose is also known as pl
Hidasi Geza
Poggi Tatjana
Acordis Kelheim GmbH
Geist Gary
White Everett
LandOfFree
Method for viscose production does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for viscose production, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for viscose production will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2911715