Dasteck ChemicalsYantai Dasteck Chemicals Co., Ltd.
Direct answer
In more than nine cases out of ten the cause is temperature or alkali charge not being adjusted along with the dose. Thiourea dioxide only develops its full reducing effect above 70 °C and in the presence of alkali; carrying over a low-temperature hydrosulfite recipe will underperform. Work through the list below in order.
Troubleshooting order (by hit rate)
| # | Check | Criterion and fix |
|---|---|---|
| 1 | Temperature | Release is slow below 70 °C; reduction clearing runs at 70-95 °C, stripping and machine cleaning at the boil |
| 2 | Alkali charge | Alkali must be added before the product; without it there is no reduction, and it cannot be cut in step with the reducing agent |
| 3 | Order of addition | Alkali first, then the product. Reversed order oxidises the product in weakly acidic conditions |
| 4 | Solution make-up | Cold water, prepare and use immediately; hot water or storing with alkali consumes activity |
| 5 | Time | Release is controlled and normally needs 20-30 minutes; too short means incomplete reaction |
| 6 | Air ingress | An exposed liquor surface, vigorous agitation and continuous dosing all consume the reducing agent |
| 7 | Metal ions | Iron, manganese and copper catalyse decomposition; add a chelant where needed |
| 8 | Dye system | Black and grey shades strip poorly; blend with hydrosulfite at roughly 1:10 |
The most commonly missed point
The alkali charge cannot be cut along with the reducing agent. Many mills cut the dose to one third and the caustic soda to one third as well, so the alkali is insufficient and the reducing agent is never activated. This is the single most common reason for "the dose is right but nothing happens".
How to confirm
Holding temperature, time and liquor ratio constant, run one trial at a higher alkali charge. If performance improves clearly, the problem is alkali; if not, adjust temperature and time in turn.
Sources
Our product technical data sheet and the process data in the 40 published technical Q&As.