1. Mechanism of Action
FAS is a powerful reductive bleaching agent. Under weakly alkaline conditions, it gradually releases reducing capacity.
Reductive decolorization: It destroys quinoid chromophoric groups on the surface of carbon black in printing ink and converts colored substances into colorless matters with no damage to pulp fibers.
Dye removal: It delivers far better decolorizing performance on common basic dyes and direct dyes in newsprint than hydrogen peroxide.
Fluorescent whitening agent protection: It does not degrade fluorescent whitening agents, ensuring stable brightness of pulp.
Metal ion chelation: It reduces and complexes ferric ions, cupric ions and other metal ions in pulp to mitigate coloration, and meanwhile prevent ineffective decomposition of hydrogen peroxide.
In short, hydrogen peroxide mainly targets lignin, while FAS works on ink and dyes. The combined use of the two achieves the optimal brightness for newsprint deinked pulp (DIP).
2. Standard Process Parameters for Newsprint DIP
² Single-stage FAS Bleaching (Commonly Used)
² Two-stage Bleaching (H₂O₂ → FAS, Mainstream for Mills)
3. Position in Newsprint Deinking Line
Pulping → High-consistency cleaning → Coarse screening → Flotation deinking → Washing → FAS bleaching / Two-stage bleaching with H₂O₂ & FAS → Thickening → Paper making
FAS is applied after flotation deinking. With most ink removed in advance, FAS focuses on decolorization and brightness improvement for higher efficiency.
4. Core Advantages over Sodium Dithionite (Hydrosulfite)
Excellent stability: Solid FAS features good stability; its aqueous solution decomposes slowly. It is non-flammable and odorless, and does not produce hydrogen sulfide.
Higher brightness: Under the same dosage, the pulp brightness is 2%–4% ISO higher than that with sodium dithionite.
Lower dosage: 0.3% FAS is equivalent to 0.8% sodium dithionite, cutting overall chemical cost.
Low equipment corrosion: Barely corrosive to production equipment.
Eco-friendliness: Chlorine-free, no AOX and dioxin generation, easy wastewater treatment.
5. Key Notes
Temperature shall not exceed 80 °C, otherwise FAS will decompose rapidly and lose efficacy.
pH value shall not be higher than 11, to avoid fiber strength loss and accelerated yellowing reversion.
Minimize long-term contact between pulp and copper/iron materials which will catalyze FAS decomposition. Carbon steel or 304 stainless steel is applicable for short-term use.
Never mix with chlorine-based bleaching agents, as toxic gas will be generated.
Control yellowing reversion: Wash and process the pulp into paper immediately after FAS bleaching to reduce oxidation caused by air contact.
6. Actual Production Performance Data (Newsprint DIP)
- Brightness of original deinked pulp: 50% ISO
- Single-stage FAS bleaching (0.4% dosage, 60 °C, 45 min): 55%–56% ISO
- Two-stage H₂O₂ & FAS bleaching: 59%–61% ISO (qualified for newsprint production)