Are metal ions important in FAS bleaching?
Direct Answer
Direct Answer Important, and one of the most easily overlooked variables in day-to-day production.
Why metal ions matter
- Transition metal ions such as iron, manganese and copper catalyse the oxidative decomposition of the reducing agent, consuming the active species that should be doing the reduction
- They also form complexes with chromophoric groups in the stock, creating new colour bodies and holding brightness down
- Some of them catalyse the useless decomposition of hydrogen peroxide, cutting the efficiency of the preceding oxidative stage
Where they come from
- The recovered paper itself: iron-containing inks, laminated paper, packaging with metal foil
- Process water, particularly in closed loop systems
- Iron carried in by equipment wear
- Coating pigments and fillers in recovered fibre
How to deal with them
| Measure | Comment |
|---|---|
| Chelating agent | EDTA, DTPA or phosphonate types; adding before the oxidative stage works better |
| Water control | Monitor iron and manganese in process water and pretreat where needed |
| Equipment maintenance | Reduce corrosion of pipework and pump bodies |
| Increasing the formamidinesulfinic acid (FAS) dose | A passive compensation that raises cost, and should be the last resort |
Bottom line
Important. Metal ions both consume the reducing agent and create colour, so they should be controlled at source with a chelant rather than absorbed by pushing more FAS into the process.
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