Dasteck ChemicalsYantai Dasteck Chemicals Co., Ltd.
Direct answer
Designing the reductive bleaching stage for deinked pulp comes down to four things: where it sits in the sequence, how much FAS to use, what pH, temperature and time to run, and which measures to accept. Starting point: FAS 0.3 to 0.6% on oven-dry pulp, pH 9.5 to 11, 70 °C for 60 minutes.
1. Where it sits
The standard sequence is hydrogen peroxide oxidative bleaching, intermediate washing, then FAS reductive post-bleaching.
The oxidative stage lifts brightness and removes part of the colour; the reductive stage reduces the chromophoric groups left behind and corrects the shade by lowering b*. The two must not share a bath - oxidant and reductant simply consume each other. Washing or thorough draining between stages is essential.
2. Dose and supporting parameters
| Parameter | Starting value | Note |
|---|---|---|
| FAS dose | 0.3 to 0.6% on oven-dry pulp | Conventional DIP; up to 1.0% for coloured pulp |
| pH | 9.5 to 11 | Alkali charge set by the buffering capacity of the furnish |
| Temperature | 70 °C | Range 60 to 80 °C |
| Time | 60 min | Range 30 to 120 min |
| Consistency | Existing line | Mass transfer is slower at high consistency; allow more time |
3. What to measure
4. How to fix the dose
Run a dose gradient with at least four points (for example 0.2 / 0.3 / 0.5 / 0.8%) and plot a dose-brightness curve. The knee of that curve is the economic dose; adding more rarely pays. Do not copy someone else's recipe - furnish mix and ink system vary too widely.
5. Three common mistakes
1. Running it in the same bath as peroxide: both chemicals are wasted and brightness fluctuates
2. No intermediate washing: residual peroxide keeps working during the hot reductive stage and consumes the reducing agent
3. Chasing brightness without cost: past the knee, the cost of each extra ISO point rises steeply
Sources
Parameters are the recommended ranges from our technical data sheet and published literature; confirm by bench trial on your own furnish.