Process for controlling hemicellulose concentration during the mercerization of cellulose

The disadvantages stemming from a high content of hemicellulose in recirculated sodium hydroxide solutions used for the mercerization of cellulose in the preparation of viscose are reduced or eliminated by subjecting at least a part of the spent sodium hydroxide solution to an ultrafiltration process to remove part or all of the hemicellulose therein. The filtered sodium hydroxide liquor is supplied with make-up sodium hydroxide and is returned to the mercerization step.

FIELD OF THE INVENTION 
My invention relates to methods for mercerizing cellulose as the first step 
in the viscose process for producing rayon and other such cellulose 
derivative filaments. 
BACKGROUND OF THE INVENTION 
The manufacture of cellulose derivatives in general uses as a starting 
material a high purity cellulose having an .alpha.-cellulose content in 
the range of 90-97%. The term .alpha.-cellulose is broadly used to denote 
the fraction of the dissolving cellulose pulp which is insoluble in 
aqueous caustic alkali. The soluble fraction, which is called 
hemicellulose, comprises two main fractions, .beta.- and 
.gamma.-cellulose. The .beta.-cellulose portion of the hemicellulose 
fraction may be precipitated by acidification (neutralization of the 
alkaline hemicellulose-containing solution), whereas the .gamma.-cellulose 
will remain in solution under such a treatment. In general, hemicellulose 
consists of heterogeneous polymeric carbohydrates having a degree of 
polymerization in the range of 25-50. 
The first step in the viscose process of making cellulose derivatives is 
the steeping/mercerizing of the dissolving cellulose pulp. The 
mercerization step comprises treating the cellulose with a strong alkaline 
solution usually containing more than 17%. A general account of the 
preparation of rayon (regenerated cellulose) is set forth at 17 
Kirk-Othmer Encyclopedia of Chemical Technology, Rayon, 168-209 (2d ed. 
1970); viscose preparation is discussed in detail in pages 175-180. 
After the mercerization step the excess steeping liquid is pressed out of 
the alkalized cellulose, giving a spent liquor which is diluted with 
respect to sodium hydroxide and enriched with respect to hemicellulose. 
Since the hemicellulose fraction of the cellulose pulp starting material 
is dissolved in the strong caustic solution used in the steeping step, a 
low .alpha.-cellulose content pulp will produce an increased concentration 
of hemicellulose in the spent mercerization liquid. 
After the pulp is converted to alkalized cellulose, it is shredded and aged 
by exposure to oxygen in the air, resulting in oxidative cleavage of the 
cellulose molecules to form shorter chains. After a proper period of 
aging, the alkalized cellulose is brought into contact with a proper 
amount of carbon disulfide, which converts the alkalized cellulose to 
cellulose xanthogenate which is soluble in a solution of sodium hydroxide 
and water. 
Some of the spent excess mercerization liquor is normally used for 
dissolving the cellulose xanthogenate to form viscose, which is then spun 
into filaments or staple fibers. The major part of the spent liquor, 
however, is customarily adjusted in volume and sodium hydroxide 
concentration and is reused for the steeping/mercerization of a new 
cellulose pulp. In another words, part of the mercerization liquor is 
recirculated. 
After a period of time, the concentration of hemicellulose in the 
recirculating mercerization liquor will reach an equilibrium which is 
determined by: 
the hemicellulose (.beta.- and .gamma.- cellulose) content in the 
dissolving pulp; 
the hemicellulose content of the alkalized cellulose; and 
the hemicellulose content of the spent liquor which is used for the 
subsequent dissolving of the cellulose xanthogenate. 
An excess of hemicellulose in the recirculating mercerization liquor 
produces a number of disadvantages for the viscose process, which may be 
reduced or eliminated if the concentration of hemicellulose in the 
recirculating liquor can be decreased to an acceptable level. 
It is well known that a high concentration of hemicellulose in the 
mercerizing liquor will increase the viscosity of the liquor and thus 
reduce the rate of penetration of the sodium hydroxide into the cellulose 
of the pulp, producing uneven mercerization and consequently a lower 
degree of substitution. This is described in a paper by I. Croon et al. in 
Svensk Papperstidning 1969 (No. 10) pages 336-38. 
Increased viscosity in the mercerization liquor will also produce a 
lowering of the capacity of the presses used to expel excess liquor from 
the alkalized cellulose. In order to compensate for the reduced capacity 
of the presses, it is normal to increase the temperature during the 
mercerization step, which unfortunately increases the amount of 
hemicellulose extracted from the pulp. This is described in a paper by H. 
Bruning et al. in Zellstoff und Papier, 1964/8: 225-32. This paper also 
indicates that a high hemicellulose concentration in the mercerization 
liquor increases the solubility of the hemicellulose. 
Another disadvantage discovered in the course of my own unpublished 
investigations with respect to raw pulp consumption is in accordance with 
these views, namely, that an increased content of hemicellulose in the 
alkalized cellulose makes it necessary to increase the amount of carbon 
disulfide used during xanthation in order to achieve the same level of 
quality viscose as is obtained when using a starting pulp material with a 
high .alpha.-cellulose content. High strength/modulus fibers cannot be 
prepared from cellulose materials having .alpha.-cellulose contents in the 
lower portion of the 90-97% range without simultaneously removing the 
hemicellulose that is dissolved in the recirculating spent mercerization 
liquor. 
Accordingly, several methods have been devised in the prior art to reduce 
the hemicellulose content in the recirculated mercerization liquor. The 
Cerini process, the most well known presently available process, requires 
dilution of the spent liquor to approximately 6-8% before being subjected 
to continuous dialysis. As a result, it is necessary to concentrate the 
liquor by evaporation before it can be reused. At present, only a few 
viscose producers use this method, as it is relatively costly with respect 
to maintenance and energy consumption. 
According to German Offlegungsschrift No. 24 33 235 and a paper by H. 
Sihtola et al. at Tappi 57: 73-75 (July 1974), hemicellulose in the 
recirculated mercerization liquor can be precipitated by the addition of 
ethanol or methanol. The precipitation agent must have a lower boiling 
point than that of water, so that the precipitation agent can be recovered 
by simple distillation. The precipitated mixture is separated by 
centrifugation or filtration, the centrifugate or filtrate then being 
distilled in order to recover the precipitating agent. The purified liquor 
may be reused for mercerization after being adjusted with respect to the 
sodium hydroxide concentration. This process is complicated and costly 
both with respect to the capital investment required and to production 
costs. The use of methanol and ethanol also poses environmental 
difficulties. So far as I know, this process has not been used on an 
industrial scale. 
Of course, hemicellulose may also be removed by the conventional process of 
evaporating and reconstituting the mercerization liquor. This common 
process does not help with the .gamma.-cellulose fraction of the 
mercerization liquor, however. 
SUMMARY OF THE INVENTION 
I have invented an improved method to reduce the hemicellulose 
concentration of the recirculated mercerization liquor. I accomplished 
this goal by continuously removing polymer carbohydrates from the 
recirculating mercerization liquor by submitting the same to 
ultrafiltration. The spent liquor is thus divided into two fractions, one 
depleted with respect to hemicellulose and one concentrated with respect 
to dissolved hemicellulose. 
In particular, my invention relates to a continuous process for the 
mercerization of cellulose preparatory to making viscose, which comprises 
treating cellulose with a strong aqueous solution of sodium hydroxide, 
removing the excess liquor containing dissolved hemicellulose from the 
alkalized cellulose so produced, passing that excess liquor through 
ultrafiltration to remove all or part of the dissolved hemicellulose, and 
returning the portion of the sodium hydroxide liquor depleted of 
hemicellulose back to the original mercerization step and removing for 
other uses the ultrafiltrate portion which is high in hemicellulose 
content. The ultrafiltration may be carried out using an alkali-resistant 
membrane having a molecular weight separating limit of about 10,000. 
According to my invention, the hemicellulose content of the mercerization 
solution may be controlled by bleeding off a predetermined amount of the 
excess liquor before ultrafiltration has taken place and returning the 
measured amount of liquor to the original mercerization treatment step. 
The portion of the ultrafiltrate high in hemicellulose content produced in 
accordance with my invention may be used to dissolve cellulose 
xanthogenate further on in the viscose process. 
The advantages of my invention are apparent, being simplicity of operation, 
low capital investment and production costs, easy availability of 
materials necessary to carry out the process and the efficient reuse of 
the entirety of the excess mercerization liquor. These features and other 
advantages of my invention will be apparent to persons skilled in this art 
from reading this specification and the claims appended hereto.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 is a simplified flow diagram illustrating my invention. Into 1 is 
introduced a cellulose slurry comprising 42% cellulose and 58% water. 
Simultaneously a steeping liquor comprising an aqueous solution of sodium 
hydroxide containing 240 g NaOH/l as well as some dissolved hemicellulose 
is introduced into 1. After the steeping/mercerization step the product is 
pressed at 2 and the alkalized cellulose produced thereby, containing 33% 
cellulose and 16% NaOH, is passed to step 4. The spent excess liquor 
withdrawn at 3 is then passed to ultrafiltration step 5. In the prior art 
processes, the amount of hemicellulose in the recirculating stream of 
liquor could be adjusted by varying the amount of liquor bled off and 
returned to the mercerization step 1 as a function of the volume of fresh 
steeping liquor supplied. Also in the prior art process a minor part of 
the spent liquor could also be diverted to step 4, into which the pressed 
alkali cellulose and carbon disulfide are introduced and in which the 
cellulose xanthogenate is formed and dissolved, forming the viscose which 
is further processed in a conventional manner. 
The depleted filtrate 6 from the ultrafiltration step 5, which is low in 
hemicellulose content, is returned to the mercerization step after the 
addition of make-up liquor. The concentrated filtrate stream 7 is 
exhausted through a pipe line, denoted 8, and may either be used for 
dissolving the cellulose xanthogenate produced in step 4 by being 
forwarded through pipe 9 or may be diverted out of the system altogether 
for other alternative uses of the hemicellulose concentrate. It is of 
course possible to use a part of the depleted filtrate stream 6 for 
dissolving the cellulose xanthogenate in step 4 as well as the 
concentrated filtrate, if so desired. 
The level of hemicellulose content of the steeping liquor recirculated to 
the mercerization step 1 may be determined by regulating the amount if any 
of spent excess liquor bled off through by pass line 10. This may be 
accomplished by conventional metering means or other means known to those 
skilled in this art. 
The ultrafiltration process used according to my invention is described in 
U.S. Pat. No. 4,151,207, the disclosure of which is hereby incorporated by 
reference. That patent describes method in which spent sulfite liquor from 
paper making and the like can be enriched in high molecular weight 
fractions such as lignosulfonates so that the enriched fraction may be 
oxidized under alkaline conditions with molecular oxygen in the presence 
in a suitable catalyst in order to prepare vanillin (3-methoxy-4- 
hydroxybenzaldehyde). For the purposes of my invention, alkali-resistant 
membranes having a molecular weight separation limit of approximately 
10,000 will remove a substantial part of the unwanted hemicellulose in the 
spent excess mercerization liquor. These membranes are available from De 
Danske Sukkerfabrikker, Denmark. 
I illustrate my invention by the following examples in which 
ultrafiltration in accordance with my invention was applied to sheet 
mercerization and slurry mercerization processes respectively. The 
filtration unit was of the same general type described in U.S. Pat. No. 
4,151,207, and the filtration members were of an alkali resistant type 
stated to have a molecular weight separation limit of approximately 
10,000. 
EXAMPLE 1--SHEET MERCERIZATION 
Cellulose sheets having an .alpha.-cellulose content of 93.4% were dipped 
in a sodium hydroxide solution containing 19% by weight of NaOH and 10.7 
g/l hemicellulose for 60 minutes at 25.degree. C. after mercerization, the 
excess steeping liquor was pressed off and was determined to contain 14.4% 
by weight of NaOH, while the hemicellulose content had increased to 23.3 
g/l. Approximately 80% of the spent liquor was subjected to 
ultrafiltration, and the depleted filtrate was combined with the remaining 
20% of the spent liquor. This was admixed with an appropriate amount of 
make-up liquor (43% by weight NaOH) and water to give a final sodium 
hydroxide concentration of 19% by weight. The hemicellulose concentration 
of this made-up recirculated liquor was determined to be 10.7 g/l, which 
is the same as for the original mercerization liquor. This liquor was then 
reused in a new steeping process. The concentrated ultrafiltrate was 
determined to have a hemicellulose content of 42.4 g/l, and this liquor 
was used for dissolving cellulose xanthogenate in step 4 of FIG. 1. 
However, of course, other uses of this hemicellulose-enriched liquor are 
possible. 
EXAMPLE 2--SLURRY MERCERIZATION 
Dissolving quality cellulose containing 90% .alpha.-cellulose was 
continuously mercerized in the form of a slurry containing 4% cellulose 
and 96% aqueous caustic alkali solution containing 18% by weight NaOH and 
46 g/l hemicellulose. The equilibrium concentration of hemicellulose in 
the steeping liquor was 46 g/l when the spent liquor was not submitted to 
ultrafiltration. However, by subjecting just 8% of this spent liquor to 
ultrafiltration, the equilibrium concentration could be reduced to 30 g/l. 
The concentrated filtrate stream from the ultrafiltration step was found 
to contain 76.3 g/l hemicellulose. In this case, as well, the concentrated 
filtrate was used to dissolve cellulose xanthogenate. In general, I have 
observed that the equilibrium concentration of hemicellulose in the 
mercerization liquor can be reduced to any desired level by subjecting a 
larger portion of the spent liquor than the 8% disclosed in this example 
to ultrafiltration. 
The foregoing description of my invention has been directed to particular 
embodiments in accordance with the requirements of the Patent Act and for 
purposes of explanation and illustration. It will be apparent, however, to 
those skilled in this art that many modifications and changes may be made 
without departing from the scope and spirit of my invention. For example, 
it is apparent from the disclosure that modifications may be made which 
are necessary to satisfy the needs of any particular application, such as 
using different concentrations of NaOH, cellulose or hemicellulose, and 
which are within the state of the art. It is my intention in the following 
claims to cover all such modifications and variations, whether 
specifically set forth herein or not, as fall within the true scope and 
spirit of my invention.