How much can FAS improve pulp brightness?

Direct Answer

Direct Answer It depends on the furnish, the raw material and the starting brightness, so there is no single figure.

Four factors that move the gain

• Raw material: newsprint grades usually gain more than office grades because they start darker with more chromophoric groups
• Starting brightness: the lower the starting point, the more room there is; where brightness is already high the gain narrows quickly
• Dose: within a sensible range brightness rises with dose, but beyond a certain point the marginal return falls away fast
• Process conditions: if pH, temperature, time and consistency are mismatched, the gain can be half of what was expected

Reference levels

Industry experience puts the gain from a reductive stage on deinked pulp at roughly 1 to 4 ISO points. Low-brightness furnishes sit near the top of that band, high-brightness furnishes near the bottom.

A separate comparative study reported that at an equal dose formamidinesulfinic acid (FAS) delivered about 2 ISO points more brightness than sodium dithionite.

How to pin the number down

The only reliable route is a dose gradient plotted as a dose-brightness curve. The knee of the curve is the economic dose; adding more beyond it rarely pays.

Bottom line

A typical gain of 1 to 4 ISO points, with the actual figure established from your own furnish via a dose-brightness curve.

Sources

1. US3384534A, Bleaching of wood pulps with thiourea dioxide（对绝干浆用量范围与白度提升数据），https://patents.google.com/patent/US3384534A/en
2. 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
3. The effect of bleaching reagents on bleachabilities of DIP and environmental loads，KTAPPI 47(1), 2015，DOI 10.7584/ktappi.2015.47.1.066