Carbon Disulfide (CS₂) Industrial Grade: Viscose Rayon & Xanthate Synthesis

Carbon disulfide (CS₂) industrial grade is consumed in viscose rayon manufacturing predominantly as the sulfur source for converting alkali cellulose into sodium cellulose xanthate. The liquid has a boiling point of 46.3 °C at 101.3 kPa, vapour pressure of 48 kPa at 25 °C, and flash point −30 °C (small-scale closed cup, ASTM D3828). In practice, industrial-grade CS₂ is specified for xanthate synthesis with a GC-FID assay of ≥ 99.5% w/w, water content ≤ 0.005% w/w, evaporative residue ≤ 0.005% w/w, and dissolved hydrogen sulfide ≤ 0.0002% w/w. A lower assay concentrates carbonyl sulfide and sulfur oligomers, which alter the distribution of xanthate substituents along the cellulose chain and increase turbidity in the viscose dope. The liquid is unloaded under nitrogen pad into carbon steel storage tanks fitted with submerged filling lines and independent high-level interlocks.

Prior to xanthation, alkali cellulose is aged at 50–60 °C for 90–150 min to reduce the degree of polymerisation from 800–1000 to 400–600. This oxidative depolymerisation is terminated by cooling to 25 °C and transferring to the xanthator. Inconsistent ageing produces a broad molecular weight distribution that reduces viscose filterability and can cause spinneret blockages on high-speed staple lines.

What Makes Industrial-Grade Carbon Disulfide Suitable for Cellulose Xanthation?

Water content above 0.05% w/w hydrolyses CS₂ slowly to carbonyl sulfide and hydrogen sulfide, but the more immediate problem is side reaction with sodium hydroxide in the xanthator. Free water raises local NaOH activity and produces sodium thiocarbonate and sodium sulfide, which consume alkali and reduce xanthate yield. Sulfur residues in the feedstock can carry into dope filtration and block spinneret capillaries; production-scale viscose plants therefore require evaporative residue below 0.005% w/w as determined by a forced-air oven method at 105 °C.

Carbonyl sulfide is particularly objectionable because it competes for alkali cellulose hydroxyl sites and forms thiocarbonate structures that do not regenerate cellulose cleanly in the acid spinning bath. Industrial-grade CS₂ therefore has tighter carbonyl sulfide limits than reagent-grade product used in laboratory derivatisation. Bulk storage under nitrogen with tank pad pressure of 2–5 kPa and pressure-vacuum vents sized to API 2000 minimises moisture ingress and vapour release.

Sodium cellulose xanthate formation follows the stoichiometric equation: (C₆H₁₀O₅)_n + n NaOH + n CS₂ → (C₆H₉O₄OCS₂Na)_n + n H₂O. In practice, only a fraction of hydroxyl groups are substituted; the average degree of substitution is reported as gamma number. The distribution of xanthate groups at C2, C3, and C6 positions influences dope solubility; C6 substitution improves solubility but is more labile during ripening.

Reactor Conditions During Sodium Cellulose Xanthate Formation

Alkali cellulose with press ratio 2.8–3.2 and sodium hydroxide content 14–17% w/w is fed to a jacketed sigma-blade kneader or continuous paddle xanthator. Carbon disulfide is metered at 28–35 kg per 100 kg of bone-dry cellulose, corresponding to a stoichiometric excess over the target gamma number of 45–55. Reaction temperature is maintained at 24–32 °C because xanthation is exothermic; higher jacket outlet temperatures accelerate formation of dithiocarbonate byproducts and produce gel particles that survive filtration.

On production lines, excursions above 32 °C for more than 15 minutes reduce viscose filterability; the batch is downgraded or reworked at lower spinning tension. The reactor charge is mixed at 4–8 min⁻¹ in batch kneaders, while continuous xanthators use staged paddles with residence time of 45–90 min. The end of reaction is judged by colour change to translucent orange-brown crumb and by iodometric titration of xanthate sulfur. Water jacket outlet temperature is trended against agitator current because sudden viscosity increases indicate localised over-xanthation.

After xanthation reaches the target gamma number, the sodium cellulose xanthate is discharged into a high-shear dissolver containing dilute NaOH at 2–3% w/w and cooled to 8–12 °C. Dissolution produces viscose dope with a cellulose content of 8.5–9.5% w/w and total alkali of 5.0–6.0% w/w. The dope is then filtered through plate-and-frame presses with cotton/polypropylene media and deaerated under vacuum to remove trapped air and residual CS₂. Ripening proceeds for 18–24 h at 18–22 °C until the falling-ball viscosity decreases to 50–90 seconds and gamma number falls by 5–10 units, a window that balances spinnability against coagulation rate.

When Carbon Disulfide, Sulfuric Acid, and Zinc Sulfate Meet in the Spinning Bath

Viscose is extruded through spinnerets with hole diameters between 40 μm and 80 μm into a bath containing 100–130 g/L H₂SO₄, 250–350 g/L Na₂SO₄, and 10–15 g/L ZnSO₄. The bath temperature is controlled at 45–55 °C. Regeneration releases CS₂, H₂S, and COS from the filaments before the fibre passes over godets and is cut into staple. In a typical continuous spinning line, spent bath is recirculated through a flash tank under 10–15 kPa absolute pressure to recover carbon disulfide, while the remaining gas stream passes through activated carbon beds. Recovery efficiency across condensation and carbon adsorption is commonly reported at 90–95% of evolved CS₂, though published data for each installed configuration remain plant-specific.

Filament stretching at 1.6–2.2 draw ratio occurs in a second acid bath before cutting. Washing trains use countercurrent hot water at 60–70 °C and dilute sodium carbonate to reduce residual sulfur to ≤ 0.15% w/w in staple fibre. Desulfurisation using 2–5 g/L NaOH at 70–80 °C removes chemically bound sulfur and improves fibre whiteness. Process water from washing is then sent to anaerobic/aerobic treatment because spent liquors contain 20–80 mg/L sulfide and 300–800 mg/L sulfate.

Explosion Hazard Controls for Carbon Disulfide Vapour

Carbon disulfide vapour has a flammable range of 1.3–50.0 vol% in air and an autoignition temperature of 90 °C; its vapour density is 2.63 relative to air. In basement trenches and pump pits, CS₂ accumulates because it is heavier than air. Electrical classification for process areas follows Zone 1 or Class I Division 1 boundaries, and bulk CS₂ is managed as a Class IB flammable liquid under NFPA 30. All storage tanks are inerted with nitrogen to maintain oxygen concentration below 3 vol%. Pumps handling CS₂ use double mechanical seals with buffer liquid, because leakage must be prevented before it reaches hot surfaces or electrical equipment. Static discharge is controlled by bonding and grounding with resistance below 10 Ω and by limiting pipe velocity to 1 m/s during initial tank filling.

Routine quality control in viscose plants includes Fourier transform infrared spectroscopy of alkali cellulose prior to xanthation, iodometric titration of xanthate sulfur in the dope, and GC-FID analysis of CS₂ headspace during spinning. Acceptable gamma number for staple viscose is 45–55; for high-tenacity cord, the target is 30–40 to shift the coagulation profile. The latter requires more controlled ageing and lower CS₂ addition because over-xanthation produces a soft filament that collapses on the godet. Compliance limits for occupational exposure are shown below.

Control	Standard/regulation	Value
8-hour TWA	NIOSH REL	1 ppm (3 mg/m³)
Short-term exposure limit	NIOSH REL	10 ppm (30 mg/m³)
Permissible exposure limit	OSHA 29 CFR 1910.1000 Table Z-2	20 ppm
Ceiling	OSHA 29 CFR 1910.1000 Table Z-2	30 ppm
Maximum peak	OSHA 29 CFR 1910.1000 Table Z-2	100 ppm for 30 minutes
Immediately dangerous to life or health	NIOSH	500 ppm

Environmental permits for viscose rayon facilities require continuous monitoring of total reduced sulfur at stack outlets and limit hydrogen sulfide and CS₂ concentrations in exhaust gas. Scrubbing with alkaline hypochlorite or catalytic oxidation removes residual CS₂ before discharge, but the spent scrubber liquor must be neutralised and tested for sulfate before wastewater release. Liquid effluent discharge is normally controlled by site-specific permits rather than uniform national limits; however, the receiving stream is typically monitored for biochemical oxygen demand, sulfide, and sulfate. Plant operation is only stable when the carbon disulfide recovery, tail-gas oxidation, and aerobic wastewater treatment trains are run as a single mass-balance system.