DESC:NOVEL METHOD FOR REDUCING ELEMENTAL SULPHUR FORMATION IN VISCOSE PROCESS

FIELD OF THE INVENTION
This invention relates to the reduction of elemental sulphur produced in viscose process by addition of oxides, hydroxides, and salts of metals, preferably compounds of transition metals during steeping/xanthation/dissolution stage. Further viscose solution is spun to obtain viscose fibres having less elemental sulphur.

BACKGROUND OF THE INVENTION
Viscose fibre is mainly prepared by high-purity cellulose derived from dissolving grade wood pulp. Viscose fibres are also sometimes prepared from other cellulosic materials like cotton linters, bamboo pulp, jute pulp and recovered cellulose. In the viscose process, the cellulose source is hydrolysed by caustic to form alkali cellulose or soda cellulose. This alkali cellulose, hereafter referred as alkcell, is aged to achieve the desired level of depolymerization. The alkcell is then allowed to react with carbon disulphide (CS2) to form the xanthate derivative of cellulose. Following is the reaction of CS2 with alkcell:
Cell-ONa+?CS?_2 ?Cell-O?CS?_2 Na

Cellulose xanthate is then dissolved in dilute aqueous caustic lye to form a viscose polymer solution. The viscose solution is then ripened at a temperature typically between 5°C to 30°C to achieve suitable CS2 substitution. Total ripening time varies from 6 hours to 36 hours. After achieving a suitable substitution, the viscose solution is extruded from the spinneret and regenerated from an acidic regeneration bath to generate cellulose fibres. The following reaction takes place during cellulose regeneration:
2Cell-O?CS?_2 Na+H_2 ?SO?_4?2Cell-OH+?Na?_2 ?SO?_4+?2CS?_2

In this reaction, cellulose xanthate converts back into cellulose and CS2 evaporates or gets adsorbed on the fibre surface. The CS2 also reacts with the free caustic present in the system to generate different thio-compounds, majorly sodium sulphide and sodium trithiocarbonate. The following reaction governs the formation of these compounds:
2?CS?_2+6NaOH??Na?_2 ?CS?_3+??Na?_2 S +Na?_2 ?CO?_3+?3H?_2 O

These reactions consume around 20-40% of the total CS2 charged in the system. These by-products then react with the sulphuric acid of the regeneration bath to form CS2 and H2S gases along with sodium salt. Following are the reactions of their formation.
?Na?_2 ?CS?_3+H_2 ?SO?_4??CS?_2+H_2 S+ ?Na?_2 ?SO?_4
?Na?_2 S+H_2 ?SO?_4?H_2 S+?Na?_2 ?SO?_4

The rate of sodium sulphide and trithiocarbonate formation is more predominant from the 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 atmospheric oxygen to form sodium thiosulphate [1].
2?Na?_2 S+ ?2O?_2+H_2 O??Na?_2 S_2 O_3+NaOH

The sodium thiosulphate generated from above reaction reacts with sulphuric acid in regeneration bath and forms elemental Sulphur by evaporation of Sulphur di-oxide gas.
?Na?_2 S_2 O_3+H_2 ?SO?_4??Na?_2 ?SO?_4+?SO?_2+S+H_2 O

The sulphur dioxide gas has good solubility in water, hence remains dissolved in the regeneration bath and further reacts with H2S gas generated from the neutralization of sodium sulphide to further boost elemental sulphur formation. Following is the reaction to the same.
?2H?_2 S+?SO?_2??2H?_2 O+3S

This leads to a significant amount of elemental sulphur formation in the viscose process. Sulphur has limited solubility in an aqueous system and hence it tends to remain in colloidal form in a regeneration bath solution. Elemental sulphur formed during spinning not only contaminates the regeneration bath but also gets carried away with the fibre, and regeneration bath and contaminates the entire downstream operation, recycle loop and recovery system. This requires periodic cleaning to avoid choking the ducts, demanding a shutdown of the manufacturing operation. As the regeneration bath is contaminated with elemental sulphur, the salt generated from the spinning stage has traces of sulphur which significantly affects the quality of sodium sulphate salt. This reduces the quality and limits the applications where it can be used. The elemental sulphur also adsorbs on the fibre surface. Hence, chemicals are consumed for the desulphurization of the fibre. This not only involves cost but also increases demand for chemicals and utilities like water. Hence, this invention relates to reducing the formation of elemental sulphur in the viscose process and addressing the aforesaid issues arising from it.

OBJECT OF THE INVENTION
The object of the invention is to reduce elemental sulphur formation in the viscose process.

SUMMARY OF THE INVENTION
The object of the invention is achieved by a method for reducing elemental sulphur formation in viscose process. Firstly, a cellulose pulp is mercerized in 17-22 percent aqueous sodium hydroxide. Thereafter, excess caustic is removed to provide an alkcell. Further, alkcell is shredded to obtain the fluffier mass of alkcell. Fourthly, fluffier mass of alkcell is depolymerized in a controlled environment to provide an alkcell of polymerization degree of 250-350 and followed by xanthation of the same under vacuum. Thereafter, solution of a metal compound and lye is added to the xanthate and a viscose solution is obtained, which is further filtered, ripened and de-aerated. After desired ripening, spinning of the viscose solution in a regeneration bath solution is carried out, and it is regenerated in the form of fibres.

According to one embodiment, wherein the mercerizing temperature is 40-55o C for 60-240 minutes.

According to another embodiment of the method, wherein xanthation is carried out at 25-35°C.

According to yet another embodiment of the method, wherein metal compounds are metal oxide, metal carbonates, or metal hydroxides, or combination thereof, wherein metal oxide are of calcium, iron, zinc, manganese, or nickel, and metal carbonates are of sodium, calcium, iron, zinc, manganese, nickel, and metal hydroxides are of magnesium, calcium, iron, zinc, manganese, nickel.

According to one embodiment of the method, wherein the concentration of metal oxide is 0.01 w/w% - 1 w/w % with respect to that of cellulose.

According to another embodiment of the method, wherein metal compound is added during dissolution, steeping or xanthation.

According to yet another embodiment of the method, wherein sulphur content of the viscose solution is measured at 2 hr, 4 hr and 16 hr.

BRIEF DESCRIPTION OF DRAWINGS
The novel features and characteristics of the disclosure are outlined in the description. The disclosure itself, however, as well as a preferred mode of use, further objectives, and advantages thereof, will best be understood by reference to the following description of an illustrative embodiment when read in conjunction with the accompanying drawings. One or more embodiments are now described, by way of example only, concerning the accompanying drawings wherein like reference numerals represent like elements and in which:
Fig. 1 shows flow sheet of viscose process with the stage at which addition of metal oxide is proposed.
DESCRIPTION OF THE INVENTION
For promoting an understanding of the principles of the invention, reference will now be made to the embodiment illustrated in the figures and specific language will be used to describe them. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended. Such alterations and further modifications in the illustrated system, and such further applications of the principles of the invention as would normally occur to those skilled in the art are to be construed as being within the scope of the present invention.

It will be understood by those skilled in the art that the foregoing general description and the following detailed description are exemplary and explanatory of the invention and are not intended to be restrictive thereof.

The terms "comprises", "comprising", or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a process or method that comprises a list of steps does not include only those steps but may include other steps not expressly listed or inherent to such a process or method. Similarly, one or more sub-systems or elements or structures or components preceded by "comprises... a" does not, without more constraints, preclude the existence of other, sub-systems, elements, structures, components, additional sub-systems, additional elements, additional structures, or additional components. Appearances of the phrase "in an embodiment", "in another embodiment" and similar language throughout this specification may, but not necessarily do, all refer to the same embodiment.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which this invention belongs. The system, methods, and examples provided herein are only illustrative and not intended to be limiting.

Embodiments of the present invention will be described below in detail with reference to the accompanying figures.

The invention relates to the reduction of elemental sulphur formation during viscose spinning by the addition of metal oxides, metal carbonates, metal hydroxides and other metal compounds, in the concentration of 0.01 w/w% - 1 w/w % with respect to that of cellulose, preferably compounds of transition metals. The addition of metal compounds preferably takes place during dissolution. However, it can be added during steeping or xanthation as well. The addition of metal oxides effectively controls the formation of oxidation products such as sodium thiosulphate. During the spinning process, sodium thiosulphate reacts with sulphuric acid in the regeneration bath to yield elemental sulphur and sulphur dioxide gas. Sulphur dioxide gas is highly soluble in water and remains dissolved in the regeneration bath solution. Hydrogen sulphide generated from the neutralization of sodium sulphide reacts with dissolved sulphur dioxide in the regeneration bath to form elemental sulphur further.

Working of the invention:

Example 1:
This example illustrates a process of the present invention for reducing elemental sulphur formation during the spinning process by the addition of a solution of metal oxides made in mixed charge lye during the dissolution process. The process till xanthation remains the same in all the examples.

A dissolving grade cellulose pulp was mercerized in 18 per cent aqueous sodium hydroxide at a temperature of 40-55°C for 60-240 minutes, following which excess caustic was removed to provide an alkcell. The alkcell is then shredded to obtain the 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, the desired quantity of lye is made, and metal oxide (concentration 0.01-1.0 w/w%) is added to it. The mixture is stirred to get a homogeneous solution.

This solution is added to xanthate to obtain a viscose solution with a composition of 6-10 per cent cellulose, 4-6 percent sodium hydroxide (or Modal with 4-7 percent cellulose, 4-7 per cent sodium hydroxide). Lye addition is done under vacuum/atmospheric pressure. The viscose solution is then sent further for filtration, ripening and de-aeration. The viscose solution after desired ripening sent for spinning by regenerating in the form of fibres in the regeneration bath solution. The sulphur content in the viscose solution was measured after regenerating the viscose solution for 2 hr, 4 hr and 16 hr.

Table 1: Sulphur content in viscose solution by addition of metal oxide in dissolution lye
Time (in hr) S in ppm (conventional route) S in ppm (with addition of metal oxide)
2 221.8 155.3
4 360.2 266.4
16 423.3 304.7

The formation of elemental sulphur in viscose solution having metal oxide shows 20-40% less Sulphur than the viscose solution prepared without the addition of metal compounds after 16 hr of ripening. Other properties of the viscose solution were 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 the spinning process by the addition of metal hydroxide during the dissolution process. The process till xanthation remains the same as mentioned in example 1. After ensuring complete xanthation, the desired quantity of lye having 0.1-1% (w/w of cellulose) of metal hydroxide is added to obtain a viscose solution with a composition of 4-10 per cent cellulose, 4-6 per cent sodium hydroxide (or Modal with 4-7 per cent cellulose, 4-7 per cent sodium hydroxide). The entire dissolution process is done 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 ripening temperature is maintained at 15-25 °C. The viscose solution after desired ripening sent for spinning and then regenerating in the form of fibres in the regeneration bath solution. The sulphur content in viscose solution is measured in 2 hr, 4 hr and 16 hr.

Table 2: Sulphur content in viscose solution by addition of metal hydroxide in dissolution lye
Time (in hr) S in ppm (conventional route) S in ppm (with addition of metal hydroxide)
2 221.8 132.3
4 360.2 245.7
16 423.3 293.4

The formation of elemental sulphur in viscose solution having transition metal hydroxide solution shows a 25-35% reduction in elemental sulphur formation than viscose solution made without the addition of any additive after 16 hr of ripening. Other properties of viscose solution are seen to be unaffected by these modifications in the process.

Example 3:
This example illustrates a process of the present invention for reducing elemental sulphur formation by the addition of metal oxide in the steeping process. In this process the metal oxide is dissolved in 18% lye solution, maintaining the concentration in alkcell at 0.1-1.0% w/w of the cellulose. The remaining process remains the same till xanthation as mentioned in example 1. After ensuring complete xanthation, the desired quantity of lye is added to obtain a viscose solution with a composition of 4-10 per cent cellulose, 4-6 per cent sodium hydroxide (or Modal with 4-7 per cent cellulose, 4-7 per cent sodium hydroxide). Lye addition is done under a vacuum. After lye addition viscose mixture is discharged to the dissolver using the conventional method of releasing vacuum by lye addition. The entire dissolution process is done 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 a conventional route at 15-25 °C. The viscose solution after desired ripening sent for spinning by regenerating in the form of fibres in the regeneration bath solution. The sulphur content in viscose solution was measured after regenerating the solution in 2 hr, 4 hr and 16 hr and comparing it with viscose prepared from the conventional route.

Table 3: Sulphur content in viscose solution on addition of metal oxide during steeping
Time (in hr) S in ppm (conventional route) S in ppm (addition of metal oxide in steep lye)
2 221.8 150.2
4 360.2 270.2
16 423.3 326.9

The formation of elemental sulphur in viscose solution by the addition of metal hydroxide shows a 25-35% reduction in elemental sulphur formation than viscose prepared by conventional route after 16 hr of ripening. Other properties of viscose solution are seen to be unaffected by these modifications in the process.

Example 4:
This example illustrates a process of the present invention for reducing elemental sulphur formation by the addition of metal oxide in the dissolution process along with nitrogen blanketing during xanthation & dissolution. The process till xanthation remains the same as mentioned in example 1. After ensuring complete xanthation, the vacuum is cut through nitrogen gas and the desired quantity of lye having 0.1-1% (w/w of cellulose) of metal oxide/hydroxide is added to obtain a viscose solution with a composition of 4-10 per cent cellulose, 4-6 per cent sodium hydroxide (or Modal with 4-7 per cent cellulose, 4-7 per cent sodium hydroxide). After lye addition, viscose mixture is discharged to the 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 a conventional route at 15-25 °C. The viscose solution is then sent further for filtration, ripening and de-aeration. The ripening temperature is maintained at 15-25 °C. The viscose solution after desired ripening sent for spinning and then regenerating in the form of fibres in the regeneration bath solution. The sulphur content in viscose solution is measured in 2 hr, 4 hr and 16 hr.

Table 4: Sulphur content in viscose solution on addition of metal oxide/hydroxide along with nitrogen blanketing/vacuum
Time (in hr) S in ppm (conventional route) S in ppm (addition of metal oxide + N2 blanketing)
2 221.8 66.2
4 360.2 82.0
16 423.3 97.3

The formation of elemental sulphur in viscose solution by combined treatment of the addition of metal hydroxide and nitrogen purging shows an 70-90% reduction in elemental sulphur formation than viscose prepared by conventional route after 16 hr of ripening. Other properties of viscose solution are seen to be unaffected by these modifications in the process.
,CLAIMS:I/ We claim
1. The method for reducing elemental sulphur formation in viscose process comprising:
- mercerizing a cellulose pulp in 18 percent aqueous sodium hydroxide;
- removing excess caustic to provide an alkcell;
- shredding alkcell to obtain the fluffier mass of alkcell;
- depolymerizing fluffier mass of alkcell in a controlled environment to provide an alkcell of polymerization degree of 250-350, and carrying out xanthation on the same under vacuum;
- adding solution of a metal compound and lye to the xanthate and obtaining a viscose solution;
- filtering, ripening and de-aerating the viscose solution;
- after desired ripening, spinning of the viscose solution tin a regeneration bath solution, and regenerating it in the form of fibres.
.
2. The method for reducing elemental sulphur formation as claimed in claim 1, wherein the mercerizing temperature is 40-55o C for 60-240 minutes.

3. The Method for reducing elemental sulphur formation as claimed in claim 1, wherein xanthation is carried out at 25-35°C.

4. The Method for reducing elemental sulphur formation as claimed in claim 1, wherein metal compounds are metal oxide, metal carbonates, or metal hydroxides, or combination thereof, wherein metal oxide are of calcium, iron, zinc, manganese, or nickel, and metal carbonates are of sodium, calcium, iron, zinc, manganese, nickel, and metal hydroxides are of magnesium, calcium, iron, zinc, manganese, nickel.

5. The Method for reducing elemental sulphur formation as claimed in claim 1, wherein the concentration of metal oxide is 0.01 w/w% - 1 w/w % with respect to that of cellulose.

6. The Method for reducing elemental sulphur formation as claimed in claim 1, wherein metal compound is added during dissolution, steeping or xanthation.

7. The Method for reducing elemental sulphur formation as claimed in claim 1, wherein sulphur content of the viscose solution is measured for 2 hr, 4 hr and 16 hr.