Dasteck ChemicalsYantai Dasteck Chemicals Co., Ltd.
Formamidinesulfinic acid (FAS) is a white crystalline, odourless reducing agent used for reductive bleaching in the pulp and paper industry. It reduces the chromophoric groups left after hydrogen peroxide bleaching without delignifying the fibre, and decomposes to urea and sodium sulphate.
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Key data at a glance
| Parameter | Typical value |
|---|---|
| FAS dosage | 0.25–1.0% on oven-dry pulp; most deinked pulp lines start at 0.3–0.6% |
| pH | Alkaline, typically 9.5–11 |
| Temperature | 60–80 °C (start at 70 °C) |
| Reaction time | 30–120 min (start at 60 min) |
| Brightness gain | Typically 1–4 ISO points; about 2 ISO points more than sodium dithionite at an equal dose |
| Reducing-equivalent ratio | 1 kg pure Na₂S₂O₄ ≡ 0.62 kg FAS |
| Placement in the sequence | Reductive post-bleach after hydrogen peroxide; never in the same bath |
Product data
| Item | Value |
|---|---|
| CAS number | 1758-73-2 |
| Formula / molecular weight | CH₄N₂O₂S / 108.12 |
| Appearance | White crystalline powder, odourless |
| Solubility in water (20 °C) | 26.7 g/L |
| Storage | Sealed, cool and dry, not above 25 °C; at least 6 months |
| Decomposition products | Urea and sodium sulphate; no heavy metals, no chlorinated compounds |
What exactly is FAS?
Formamidinesulfinic acid (FAS) is a white crystalline organic sulphur compound, CH₄N₂O₂S, molecular weight 108.12, CAS 1758-73-2, 26.7 g/L in water at 20 C, an alkali-activated reductive bleaching agent.
Why can FAS be used for deinking pulp?
The brightness ceiling in deinked pulp is set by chromophoric groups, not lignin. FAS releases sulfoxylate in alkali to reduce them without delignifying, ideal after hydrogen peroxide bleaching.
Can FAS replace Sodium Hydrosulfite?
Yes. On equal reducing equivalents 1 kg of pure sodium dithionite corresponds to about 0.62 kg FAS, with easier storage, no odour and comparable or better brightness.
How much FAS is required per ton of pulp?
A typical screening range is 0.25 to 1.0% on oven-dry pulp, with most DIP lines starting at 0.3 to 0.6%, run alkaline at 60 to 80 C for 30 to 120 minutes.
How do I convert my existing Sodium Hydrosulfite dosage to FAS?
There is no 1:1 factor. Convert on equal reducing equivalents (1 kg pure dithionite is about 0.62 kg FAS), then establish the practical ratio with a dose-response trial.
How much can FAS improve pulp brightness?
Reductive bleaching of deinked pulp typically gains 1 to 4 ISO points, and one comparison reports about 2 ISO points more than sodium dithionite at an equal dose.
Is FAS effective for colored wastepaper?
Yes, and it is commonly used for colour control. FAS breaks down conjugated chromophores in dyes and inks, lowering a* and b*, but is limited on pigment-type colorants.
Can FAS be combined with Hydrogen Peroxide?
Yes, and it is the most common combination in papermaking, but the stages must be separate: peroxide oxidation, intermediate washing, then FAS reduction. Never dose them together.
What is the recommended pH for FAS bleaching?
FAS requires alkaline conditions, typically pH 9.5 to 11 in mill trials. Alkali charge depends on the furnish, and the optimum should come from a pH-brightness response curve.
What Temperature and Reaction Time Does FAS Bleaching Require?
A typical window is 60 to 80 C for 30 to 120 minutes. Start at 70 C for 60 minutes; 60 C needs 90 to 120 minutes while 80 C can be shortened to 30 to 60 minutes.
Will FAS affect pulp strength?
Little effect under recommended conditions, because reductive bleaching does not delignify. Strength falls only with excessive temperature, excessive alkali or oxidant carryover.
Does FAS cause brightness reversion?
No worse than other reducing agents. Brightness from reductive bleaching is inherently reversible; controlling metal ions, washing thoroughly and adding a light oxidative stage all reduce reversion.
Does FAS remove ink, ash, stickies or contaminants?
These have to be separated. FAS is about colour and has a partial decolourising effect on ink, but essentially none on ash or stickies, which need flotation, washing and screening.
Are metal ions important in FAS bleaching?
Important. Iron, manganese and copper catalyse decomposition of the reducing agent and create new colour bodies. Control them at source with chelants, water quality and equipment maintenance.
What is the environmental impact of FAS?
FAS decomposes to sulphate and urea with no heavy metals or chlorinated compounds and is dosed at about 0.62 times the dithionite weight, but its nitrogen adds a total-nitrogen load.
Is FAS safer than Sodium Hydrosulfite?
Safer than hydrosulfite in handling, storage and transport because it does not self-ignite, has no rain loading restriction and is odourless, but the SDS still lists seven classifications.
Is FAS stable at high temperature?
The solid is stable when dry and sealed for at least six months, decomposes slowly above 100 C and markedly above 123 C. Thermal oxidative decomposition can release hydrogen cyanide.
Is FAS actually cheaper than Sodium Hydrosulfite?
A higher unit price but not necessarily more expensive in use: about 0.62 times the dithionite dose plus lower storage, freight and quality costs. Compare total cost per oven-dry tonne.
Do you have real paper-mill application data?
We can provide published comparative data, laboratory dose-brightness and pH-brightness curves, consented customer application conditions and COA batch data, but not unauthorised records.
Can you provide free samples and technical support for a mill trial?
Yes. New customers receive a standard free 200 g sample, existing and large accounts are agreed case by case, and support is primarily remote with site visits arranged after assessment.
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