FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
As amended by the Patents (Amendment) Act, 2005
&
THE PATENTS RULES, 2003
As amended by the Patents (Amendment) Rules, 2006
COMPLETE SPECIFICATION (See section 10 and rule 13)
TITLE OF THE INVENTION
METHOD OF REDUCING FORMATION OF ELEMENTAL SULPHUR IN A VISCOSE
PROCESS
APPLICANTS ADITYA BIRLA SCIENCE AND TECHNOLOGY COMPANY PVT LTD
of address
Plot No 1 and 1-A/1, Taloja, MIDC, Taluka-Panvel, District Raigad
Maharashtra - 410208
PREAMBLE TO THE DESCRIPTION
The following specification particularly describes this invention and the manner in which it is
to be performed:

FIELD OF THE INVENTION
The invention relates to a reduction of formation of elemental sulphur in viscose process. BACKGROUND OF THE INVENTION
Viscose rayon is a fibre consisting of regenerated cellulose, typically manufactured from plant cellulose using wood based cellulose pulp or cotton linters as the feed stock. Viscose fibre is mainly prepared by using dissolving grade wood pulp as raw material, which is made up of high purity cellulose. Viscose fibres are also sometimes prepared from other cellulosic materials like cotton linters, bamboo pulp, jute pulp and recovered cellulose. In the viscose process, cellulose source is hydrolysed by caustic to form alkali cellulose or soda cellulose. This alkali cellulose, hereafter referred at alkcell, is aged to achieve desired level of depolymerization. The alkcell is then allowed to react with carbon disulphide (CS2) to form xanthate derivative of cellulose. Xanthate derivative of cellulose, also known as cellulose xanthate, is soluble form of cellulose in dilute caustic. Following is the reaction of CS2 with alkali cellulose:

Cellulose xanthate is then dissolved in dilute aqueous caustic lye to form viscose polymer solution. Viscose solution is then ripened at low temperature typically between 10°C to 25°C to achieve suitable CS2 substitution. Total ripening time vary from 6 hours to 36 hours. After achieving suitable substitution, viscose dope is extruded from spinneret and regenerated from acidic spin bath to generate cellulose fibres. Following reaction takes place during cellulose regeneration:


In this reaction, cellulose xanthate converts back into cellulose and CS2 liberates or adsorbed on the fibre surface. The CS2 also reacts with free caustic present in the system to generate different thiocompounds, majorly sodium sulphide and sodium trithiocabonate. Following reaction governs the formation of these compounds:

These reactions consume around 20-40% of the total CS2 charged in the system. These byproducts then react with sulphuric acid of spin bath to form CS2 and H2S gases along with salt. Following are the reactions for their formation.

Rate of sodium sulphide and trithiocarbonate formation is predominant from xanthation step. These byproducts continue to increase along the ripening process. And keep on continuing as long as ripening continues.
Also, there are reported reactions where sodium sulphide reacts with oxygen to form sodium thiosulfate.

The sodium thiosulfate generated from above reaction reacts with sulphuric acid in spin bath and forms elemental Sulphur liberating Sulphur di-oxide gas.

The sulphur di-oxide gas has good solubility in water, hence remains in dissolved state in spin bath and further reacts with H2S gas generated from neutralization of sodium sulphide to boost the elemental sulphur formation. Following is the reaction for the same.


This leads to significant amount of elemental sulphur formation in the viscose process. Elemental sulphur formed during spinning in spin bath solution not only contaminate the spin bath but also get carried away with fibre, spin bath and contaminate entire downstream operation, recycle loop and recovery system. This requires periodic cleaning to avoid chocking of the ducts, demanding shut-down of the manufacturing operation. As the spin bath is contaminated with elemental sulphur, the salt generated from it has traces of sulphur which significantly affect the quality of sodium sulphate salt. This reduces the premium and the applications where it can be used.
The elemental sulphur also adsorbs on the surface of fibre. Hence, chemicals are consumed for desulphurization of the fibre. This not only involves the cost but also increases demand of chemicals and utilities like water required to get desired quality of fibre. Hence, this invention relates to reducing the formation of elemental sulphur in viscose process and addressing aforesaid issues arising from it.
According to US Pat. No. 4,368,078 claims reduction of elemental sulphur by eliminating the residual vacuum in the reaction space under vacuum to remove the excess carbon sulphide by introduction of an inert gas, preferably nitrogen, to avoid the formation within the reaction space of a mixture which may be explosive. However, removing excess of CS2 during this space does not significantly reduce the by-products formation and it cannot prevent the oxidation reactions which is the major reason of elemental sulphur formation. US7850822B2 recites a wet oxidation process is used to control the concentration of reduced sulphur compounds from a viscose process. The wet oxidation process oxidizes the sulphur compounds to convert such reduced sulphur compounds to an

alkali sulfate and/or thiosulfate species. The alkali sulfate stream is recycled to the
viscose process.
However, the processes recited in prior art documents typically have operational and
economic limitations.
Thus, there is a need to reduce elemental sulphur emissions from viscose processes
that are effective, economical, and readily integrated into the overall flow scheme of
the viscose process.
SUMMARY OF THE INVENTION
An object of the invention is to reduce downstream challenges like sulphur scaling by
elemental sulphur.
In an aspect of the present invention, there is provided a method to reduce elemental
sulphur formation in viscose process by limiting the oxygen supply during xanthation
to ripening steps of viscose process.
In another aspect, the present invention provides a method of reducing formation of
elemental sulphur in a viscose process comprising carrying out ripening of viscose
solution under inert conditions thereby inhibiting oxidation of sodium sulphide in
viscose solution during ripening.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing summary, as well as the following detailed description of the invention will be better understood when read in conjunction with the appended drawings. For the purpose of assisting in the explanation of the invention, there are shown in the drawings embodiments which are presently preferred and considered illustrative. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown therein. In the drawings:

Figure 1 is viscose process flow sheet
Figure 2 depicts formation of sulphur under air and N2 environment after initial
dissolution under N2 for 2h.
Figure 3 depicts formation of sulphur under various conditions
DESCRIPTION OF THE INVENTION
In describing and claiming the invention, the following terminology will be used in accordance with the definitions set forth below. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are described herein. As used herein, each of the following terms has the meaning associated with it in this section. Specific and preferred values listed below for individual process parameters, substituents, and ranges are for illustration only; they do not exclude other defined values or other values falling within the preferred defined ranges.
As used herein, the singular forms "a," "an," and "the" include plural reference unless the context clearly dictates otherwise.
The terms "preferred" and "preferably" refer to embodiments of the invention that may afford certain benefits, under certain circumstances. However, other embodiments may also be preferred, under the same or other circumstances. Furthermore, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful, and is not intended to exclude other embodiments from the scope of the invention

As used herein, the terms "comprising" "including," "having," "containing,"
"involving," and the like are to be understood to be open-ended, i.e. to mean including
but not limited to.
As used herein, the term "viscose process" as used herein refers to process of
manufacturing of cellulose fibres from cellulose pulp by reaction with caustic and CS2
and regeneration of polymer solution in acidic spin bath.
As used herein, the term "xanthation" as used herein refers to process of alkcell
reaction with CS2 to form cellulose xanthate.
As used herein, the term "ripening" as used herein refers to time from the completion
of xanthation reaction to the spinning of polymer solution to extrude fibre.
In accordance with an embodiment of the invention, there is provided a method of
reducing formation of elemental sulphur in a viscose process comprising carrying out
ripening of viscose solution under inert conditions thereby inhibiting oxidation of
viscose solution during ripening.
As seen in figure 1, the viscose process typically involves treating cellulose material,
such as wood pulp, with caustic lye solution to produce a treated material, typically
referred to as alkcell. The excess caustic lye in the cellulose-caustic solution removed
by pressing alkcell.
After further processing, the alkcell undergoes xanthation where alkcell is reacted
with carbon disulphide (CS2) to form xanthate derivative of cellulose known as
cellulose xanthate.
Cellulose xanthate is subjected to then ripening at low temperature typically between
10°C to 25°C to achieve suitable CS2 substitution. After achieving suitable
substitution, viscose dope is extruded from spinneret and regenerated from acidic spin
bath to generate cellulose fibres. The cellulose fibers are further stretched and dried
for further applications.

The conversion of sodium cellulose xanthate into regenerated cellulose can also
produce byproducts such as sulphides, thiocarbonates, thiosulphates and
polysulphides sulphur compounds which is not desirable in the final product. This
sulphur contamination is usually forms CS2 and H2S gases and are evolved from the
system, however non-gaseous Sulphur compounds removed from viscose fibers
through desulphurization process.
Accordingly, in order to achieve the object of the invention, there is a provided a
method of reducing formation of elemental sulphur in a viscose process comprising
carrying out ripening of viscose solution under inert conditions thereby inhibiting
oxidation of viscose solution during ripening. The inert conditions comprise of
vacuum and/or in presence of nitrogen gas.
In an embodiment, said inert conditions is achieved by carrying out ripening process
under nitrogen atmosphere.
In an embodiment, said inert conditions is achieved by carrying out ripening process
under vacuum.
In an embodiment, said inert conditions is achieved by carrying out only the
dissolution step of ripening process under nitrogen atmosphere.
The ripening of cellulose xanthate is achieved in three steps. In the first step, cellulose
xanthate solution is dissolved in a dilute aqueous caustic lye to form viscose polymer
solution. The viscose polymer solution is further subjected to filtration to obtain a
filtrate and a supernatant. The third step in ripening is the de-aeration of the filtrate
obtained in (b).
In an embodiment, duration of ripening is in the range of 2 to 24 hours and the
temperature during ripening is in the range of 5°C to 30°C.
The method as claimed in claim 1, further comprising the step of spinning/extrusion
the ripened viscose solution to form cellulose fibers.

The method as claimed in claim 1, wherein reduction of elemental sulphur in viscose process is in the range of 60% to 80%.
By carrying out the ripening process under limited supply of air/oxygen oxidation of viscose dope is reduced. Reducing the contact with air/oxygen will reduce oxidation products of sulphide compounds, especially sodium thiosulphate, which further reacts with sulphuric acid in spin bath to yield elemental sulphur and sulphur dioxide gas.
WORKING EXAMPLES
The following specific examples are illustrative and explanatory of the present invention but are not to be construed as limiting the scope of the invention. EXAMPLE 1:
This example illustrates a process of the present invention for reducing elemental sulphur formation during spinning process by nitrogen blanketing during entire ripening process. The process till xanthation remains same in all the examples. A dissolving grade cellulose pulp was mercerized in 18 percent aqueous sodium hydroxide at a temperature of 40-55°C for 2-20 minutes, following which excess caustic was removed to provide an alkcell. The alkcell was then shredded to obtain fluffier mass of alkcell and depolymerized (matured) in a controlled environment to provide an alkcell of polymerization degree of 250-350. Xanthation was carried out under vacuum at 25-35°C. After ensuring complete xanthation, desired quantity of lye was added to obtain viscose solution with a composition of 6-10 percent cellulose, 4-6 percent sodium hydroxide (or Modal with 4-7 percent cellulose, 4-7 percent sodium hydroxide).
Lye addition was done under vacuum. After lye addition viscose mixture was discharged to dissolver having positive pressure of nitrogen. The entire dissolution process was achieved under nitrogen blanketing/ vacuum/ limited supply of air at a

temperature of 10-20 °C. The viscose solution was then sent further for filtration, ripening and de-aeration. This entire process was done under nitrogen blanketing/ vacuum/ limited supply of air at a temperature of 15-25 °C.
Ensuring no/ minimum contact of viscose solution with air/ oxygen. The viscose solution after desired ripening sent for spinning by regenerating in the form of fibres in spin bath solution. The sulphur content in viscose solution was measured after regenerating viscose solution in 2 hr, 4 hr and 16 hr.

Time (in hr) S in ppm (under N2) S in ppm (in air)
2 19.81 38.94
4 22.27 65.56
16 24.02 88.97
Table 1: Sulphur content in viscose solution kept under nitrogen atmosphere for 16 hr
The formation of elemental sulphur in viscose solution kept under nitrogen blanketing/vacuum was 73% less than viscose dope ripened in air or in oxygen environment after 16 hr of ripening. Other properties of viscose solution observed to be unaffected by these modifications in the process.
EXAMPLE 2:
This example illustrates a process of the present invention for reducing elemental sulphur formation during spinning process by application of vacuum throughout the ripening process. The process till xanthation remains same as mentioned in example 1. After ensuring complete xanthation, desired quantity of lye is added to obtain viscose solution with a composition of 4-10 percent cellulose, 4-6 percent sodium

hydroxide (or Modal with 4-7 percent cellulose, 4-7 percent sodium hydroxide). Lye addition was done under vacuum.
The entire dissolution process was achieved under vacuum at a temperature of 10-20 °C. It takes around 30-120 min to achieve complete dissolution of cellulose xanthate to viscose solution. The viscose solution was then sent further for filtration, ripening and de-aeration. The entire ripening operation was carried out under vacuum at 15-25 °C. The viscose solution after desired ripening sent for spinning by removing the vacuum and then regenerating in the form of fibres in spin bath solution. The sulphur content in viscose solution is measured in 2 hr, 4 hr and 16 hr.

Time (in hr) S in ppm (in vacuum)
2 12.73
4 14.07
16 30.61
Table 2: Sulphur content in viscose solution kept under vacuum during ripening process
[53] The formation of elemental sulphur in viscose solution kept under vacuum shows 65% reduction in elemental sulphur formation than viscose solution ripened in air or in oxygen environment after 16 hr of ripening. Other properties of viscose solution seen to be unaffected by these modifications in the process.
EXAMPLE 3
[54] This example illustrates a process of the present invention for reducing elemental sulphur formation during spinning process by nitrogen blanketing only during dissolution process. The process till xanthation remains same as mentioned in example 1. After ensuring complete xanthation, desired quantity of lye is added to obtain viscose solution with a composition of 4-10 percent cellulose, 4-6 percent sodium hydroxide (or Modal with 4-7 percent cellulose, 4-7 percent sodium hydroxide).

Lye addition is done under vacuum. After lye addition viscose mixture is discharged to dissolver having positive pressure of nitrogen/under vacuum. The entire dissolution process is achieved under nitrogen blanketing at a temperature of 10-20 °C. It takes around 30-120 min to achieve complete dissolution of cellulose xanthate to viscose solution. The viscose solution is then sent further for filtration, ripening and de-aeration. The operation after dissolution is conducted in presence of air/by conventional route at 15-25 °C. The viscose solution after desired ripening sent for spinning by regenerating in the form of fibres in spin bath solution. The sulphur content in viscose solution which was deprived of oxygen only for first 2 hrs of operation was measured after regenerating dope solution in 2 hr, 4 hr and 16 hr and comparing it with viscose solution prepared without nitrogen blanketing/ blanketing under other inert gases/vacuum/limited supply of air.
Table 3: Sulphur content in viscose solution kept under nitrogen atmosphere for 2 hr
The formation of elemental sulphur in viscose solution kept under nitrogen blanketing/ vacuum only for 2 hr during dissolution process shows 62% reduction in elemental sulphur formation than viscose solution ripened in air or in oxygen environment after 16 hr of ripening. Other properties of viscose solution seen to be unaffected by these modifications in the process.
Hence, nearly similar effect of elemental sulphur reduction was observed by keeping the viscose solution for only 2 hr under nitrogen blanketing. This will save huge amount of money for connecting entire assembly under nitrogen/power utilization for attaining vacuum at the same time equally effective.

The elemental sulphur reduction is found to be around 73% when entire ripening operation conducted under nitrogen blanketing. The formation of elemental sulphur in viscose can be significantly reduced (-62%) by performing only dissolution under nitrogen blanketing. This invention has potential to reduce downstream challenges like spin bath contamination, sulphur scaling, chocking of ducts, reduction of desulphurization chemicals and quality improvement of salt and fibre. The above description of the present disclosure is provided for the purpose of illustration, and it would be understood by those skilled in the art that various changes and modifications may be made without changing technical conception and essential features of the present disclosure. Thus, it is clear that the above described examples are illustrative in all aspects and do not limit the present disclosure.

We Claim:
1) A method of reducing formation of elemental sulphur in a viscose process comprising carrying out ripening of viscose solution under inert conditions thereby inhibiting oxidation of viscose solution during ripening.
2) The method as claimed in claim 1, wherein said inert conditions comprises of vacuum and/or in presence of nitrogen gas.
3) The method as claimed in claim 1, wherein duration of ripening is in the range of 2 to 24 hours.
4) The method as claimed in claim 1, wherein temperature during ripening is in the range of 5oC to 30oC.
5) The method as claimed in claim 1, wherein said ripening comprises the steps of from cellulose xanthate dissolution till fibre spinning which comprises

a) dissolution of cellulose xanthate in lye solution;
b) filtration of the resultant solution of (a); and
c) de-aeration of the filtrate obtained in (b);

6) The method as claimed in claim 1, further comprising the step of spinning/extrusion the ripened viscose solution to form cellulose fibers.
7) The method as claimed in claim 1, wherein reduction of elemental sulphur in viscose process is in the range of 60% to 80%.