What is the recommended pH for FAS bleaching?

Direct Answer

Direct Answer Alkaline, but a single number should not be quoted as the answer.

Why it has to be alkaline

formamidinesulfinic acid (FAS) only decomposes to release sulfoxylate, the species that actually does the reducing, under alkaline conditions. In acidic or neutral conditions it performs no reduction and is instead oxidised to formamidine sulfonic acid and loses its activity.

Recommended range

In practical pulp trials the pH is usually held between 9.5 and 11. The alkali charge should be set from the buffering capacity and starting pH of the furnish rather than fixed at one number.

The optimum depends on

• Furnish mix and buffering capacity
• FAS concentration
• Temperature
• Reaction time
• Alkali charge
• Metal ion concentration

The right approach

A single fixed pH should not be handed to every mill. Instead, build a pH-brightness response curve in the laboratory: hold everything else constant, run three or four pH points, and measure brightness, colour and pulp strength. The optimum is the point with the highest brightness at an acceptable fibre loss.

Two common mistakes

• Too low: the reducing agent is never activated, brightness does not move, and the product is wrongly judged ineffective
• Too high: short-term brightness may look better, but fibre degradation and effluent load rise, which does not pay over time

Bottom line

Work between pH 9.5 and 11, and use a pH-brightness curve to find the best point for your furnish.

Sources

1. US5958184A, Process for producing thiourea dioxide（还原当量质量比、碱中水解式、木浆与脱墨浆等当量漂白实施例），https://patents.google.com/patent/US5958184A/en
2. The effect of bleaching reagents on bleachabilities of DIP and environmental loads，KTAPPI 47(1), 2015，DOI 10.7584/ktappi.2015.47.1.066