The ZDHC requirements for chemical recovery rates are categorised at three levels – Foundational, Progressive and Aspirational. This encourages continuous improvement activities at a facility to achieve higher limits of recovery of chemicals used and move towards circularity.

For viscose staple fibres and modal, the recovery of sulphur compounds due to the use of CS₂ in the process needs to be monitored. This is also linked to the level of kg of sulphur emitted to air per tonne of fibre produced.

CaCO₃: Calcium carbonate

Ca(OH)₂: Calcium hydroxide

CO₂: Carbon dioxide

CS₂: Carbon disulphide

H₂O: Water

H₂S: Hydrogen sulphide

H₂SO_4: Sulphuric acid

NaHS: Sodium hydrosulphide

Na₂S: Sodium sulphide

SO_x: Sulphur oxides

Fig– Viscose Fibre Manufacturing and S Recovery Process

Ref: https://www.birlacellulose.com/case-studies/Managing the Health and Safety aspects of CS2 in Viscose Process.pdf

Mass Balance calculation used for CS₂ Recovery & Sulphur Air emission

CS₂ Recovery = O1 + O2 + O3 + O4 + O5 + O6

Sulphur emission to air = I1 - (O1 + O2 + O3 + O4 + O5 + O6)

CS₂ recovery techniques and a mass balance calculation

In viscose manufacturing spinning frames are one of the sources of CS₂ emissions. These emissions can be avoided by the housing of spinning lines. For operations, the housing has to be equipped with leak-proof sliding windows. To avoid the accumulation of harmful and explosive gases, suction systems are installed in the housing from where CS₂ is purged to a recovery facility. CS₂ is mainly recovered by condensation and activated carbon adsorption process (CAP), but to achieve a higher recovery percentage various other techniques can also be implemented by the industry. A brief overview of the CS₂ recovery techniques is given below.

Total CS₂ recovery (Kgs/tonne of fibre)  = (CS₂ recovery from condensation route in Kgs/tonne of fibre) + (CS₂ recovery from exhaust gases through CAP & other techniques unit in Kgs/tonne of fibre)

I1) CS₂ addition to reactor including- Fresh input and CS₂ recovered from the process

01) CS₂ recovery by condensation recovery- CS₂ and steam-saturated air pass through the condenser where the gases are extracted by a water jet, in which more CS₂ is condensed by the cold water. In simple terms; the gases trapped in fibres are released by steam injection and CS₂ vapours produced are condensed in indirect condensers using soft water & chilled water.

The recovered CS₂ is of high purity, so can be used in the viscose process again with or without refining depending on the quality of CS₂ in terms of total solids present.

O2) CS₂ recovery by activated carbon adsorption (CAP)- The exhaust gases from the spinning and acid recovery section comprises of both H₂S & CS₂. To recover CS₂ through CAP it is essential to remove the H₂S from the gas stream otherwise it will damage the carbon bed. Therefore before CAP, the first step is to remove H₂S. This is achieved by either alkaline scrubbing (03) in which H₂S is converted to NaHS or Na₂S or the redox process wherein H₂S is converted into sulphur. The CS₂-rich gas is then taken to the CAP unit where gases are passed through the activated charcoal bed (adsorber) wherein CS₂ is adsorbed. Once the bed is saturated, the adsorber is isolated and taken for CS₂ recovery and other stand-by adsorber is taken back into the production line.

O3) Removal of H₂S as NaHS or Na₂S by alkaline scrubbing (wash & spray)

To avoid H₂S contamination of the gas stream in CAP, the gases are treated in a NaOH scrubber (washer) prior to CAP unit. In the scrubbing process, sodium hydroxide reacts with H₂S dissolved in an aqueous solution to form NaHS and Na₂S. This takes place in an absorption unit consisting of two stream scrubbers operating with diluted sodium hydroxide. Followed by a centrifugal scrubber, where NaOH spray fog is removed. NaHS can be valuable products.

O4) Conversion of H₂S and CS₂ into H₂SO₄ with oxidation by Wet sulphuric acid process (WSA)

With high concentrations of sulphur compounds in the exhaust air (>0.5 vol-%), there is a wider choice of techniques available. In practice, the combustion of the exhaust air by catalytic oxidation to sulphuric acid is often chosen. This process is only economically viable if the acid can be produced at desired concentrations.

The H₂S gas is burnt at 350^oC on a noble metal catalyst to form SO_2, Then is oxidised to SO₃ in one step on a wet catalyst (V₂O₅). The gases containing SO₃ are condensed at 250^oC, which leads to an approximately 88 % sulphuric acid.

O5) Conversion of H₂S and CS₂ into SOx by exhaust gas incineration/boiler followed by scrubbing of flue gases by lime to produce gypsum

Calculation method for O5 (gypsum) 

Incineration in a coal-fired boiler 

Sulphur from the viscose process and coal is fed to a boiler or incinerator and gets converted into SOx. The SOx produced is scrubbed (removal of impurities from a gas by chemical means) by lime to make gypsum. The flue gas from the boiler will have some remaining unscrubbed sulphur in SOx form. The purity of gypsum varies depending on the flue gas desulphurization process applied. 

Explanation about gypsum on scrubbing of flue gases from Captive Power Plant (CPP )Boilers (O5)

O5 calculation : sulphur in (A+B) - sulphur out (C+D)

Sulphur in: 

A: Exhaust flow rate from VSF in M³/hr x equivalent sulphur mg/m³ (from CS₂ and  H₂S)

B: % sulphur in coal + quantity of coal burnt in boilers in metric tonne (MT)

Sulphur out:

C:  SO₂ in CPP boiler stack flue gas flow rate Nm³/hr x SO₂ mg/Nm³)x32/64

D: (gypsum produced MT) x (32/120)

Reactions:

Dry scrubbing

CaCO_3(s) + SO_2(g) → CaSO_3(s) + CO_2(g)

Wet Scrubbing

Ca(OH)_2(s) + SO_2(g) → CaSO_3(s) + H₂O_(l)

^{S=solid, l=liquid, g=gas}

O6) Conversion H₂S, CS₂ or both to sulphur by biological or catalytic processes or redox process

Biological Process

Example of Biological Process:

Currently, in the Claus, Stretford or other processes, hydrogen sulphide is removed from process gas streams by a series of reactions at high temperatures to produce elemental sulphur. These physicochemical processes have high investment and operating costs, as well as often being restricted by contaminants, due to not effectively removing all the H₂S. As an alternative, the anaerobic, photosynthetic bacterium, chlorobium thiosulfatophilum, has been demonstrated to convert hydrogen sulphide to elemental sulphur in a single step at atmospheric conditions. The autotrophic bacterium uses CO₂ as the carbon source. Energy for cell metabolism is provided by incandescent light and the oxidation of H₂S. A pilot study has been performed in a continuous stirred tank reactor (CSTR) equipped with a sulphur separator. Optimum process conditions have been achieved to maximise cell growth and elemental sulphur production. Near total conversion of H₂S is achieved in a retention time of a few minutes. High concentrations of H₂S or organics do not affect the culture. Sulphur recovery by settling is very efficient and near theoretical yields of sulphur are achieved. To reduce the sulphur emissions these advanced technologies might need to be adopted by the industry in the near future.

_{Ref: Rajendra Nath Basu, Biological Conversion of Hydrogen Sulfide into Elemental Sulfur, Environmental Progress 15(4):234 - 238, July 2006, DOI:10.1002/ep.670150412}

Recovery of sulphate from spinning baths

During the viscose fibre manufacturing process, sodium sulphate produced as a by-product can be crystallised as Glauber salt. The spinning bath solution containing sodium sulphate is led to multiple-stage thickeners to raise the concentration of sodium sulphate. There, water is vaporised to saturation point and crystallisation of sodium sulphate takes place. By recrystallising in melting pots and evaporation of the crystal water in crystallisers, a pulp of sodium sulphate is obtained.

This pulp is centrifuged and dried at 450 °C in a drying tower by direct heating with a natural gas burner. Another technique for drying is the usage of a tumble dryer and a subsequent separation in a cyclone.

Calculation of sodium sulphate recovery

Sodium sulphate recovery (%) =[ (quantity of sodium sulphate produced as by-product (tonne)/ viscose or modal fibres produced (tonne)]*100