How do I convert my existing Sodium Hydrosulfite dosage to FAS?

Direct Answer

Direct Answer There is no universal 1:1 conversion factor, but there is a reliable two-step method.

Step 1: convert on equal reducing equivalents

On a pure-product, equal-reducing-equivalent basis, 1 kg of pure sodium hydrosulfite corresponds to roughly 0.62 kg of formamidinesulfinic acid (FAS). If the commercial product you use is of a lower assay, convert once more on the actual content. This step gives the theoretical floor, not a production dose.

Step 2: run a dose-response trial

Do not move the theoretical figure straight onto the line. Run parallel trials on the following gradients:

Then compare ISO brightness, brightness gain, colour reduction, brightness stability, pulp strength and chemical cost per oven-dry tonne.

Why a gradient rather than a single point

The loss rate of sodium hydrosulfite in hot alkali and in contact with air varies with equipment and operating practice, so the same nominal dose corresponds to very different effective reducing power at different mills. A gradient measures that difference, and only then does the substitution ratio mean anything.

Bottom line

Convert at 0.62 to get the theoretical floor, then establish the practical ratio through a five-point dose gradient.

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