How should FAS be dosed in a paper mill?

Direct answer

Dosing FAS in a paper mill comes down to getting three things right: dissolving, metering and the addition point. FAS should be dissolved into solution before it is added, and the addition point should be where mixing is good and the temperature and pH are already in range. Dropping dry powder into a hot stock line tends to produce localised lumps and incomplete dissolution.

1. Three common ways to dose

2. Making up the solution

• Use cold or warm water, never hot: heat accelerates decomposition
• Alkali-free solution only is stable: an FAS solution keeps for about 6 hours at room temperature without alkali; add the alkali only shortly before dosing
• Make up as needed: standing solution gradually loses reducing power
• Keep the concentration moderate: too strong makes metering and mixing harder

3. Four requirements for the addition point

1. Good mixing: choose a point with agitation or high shear

2. Temperature in range: FAS releases fully only above 70 C

3. pH already alkaline: the alkali goes in shortly before the FAS

4. Enough retention time: design for 30 to 120 minutes depending on the process

4. Four easy mistakes

• Dry powder into a hot stock line: lumps form on the outside while the inside stays undissolved, and brightness fluctuates
• Leaving the alkaline solution overnight: an alkaline solution is unstable and has already lost reducing power
• Choosing an addition point in a mixing dead zone: local over-concentration with under-dosed areas elsewhere
• Estimating the dose from pump flow alone: check with a flow meter regularly to catch metering drift

Sources

Solution stability and temperature requirements come from our technical data sheet. The dosing method must be set according to your own equipment and process layout.