As soon as the level in the paper machine's white water tower alarms, foam pours out of the sight glass, the vacuum at the wire section fluctuates, and the sheet carries foam spots and pinholes. On site people often keep topping up antifoam in the white water tank; the foam disappears but oil spots appear on the sheet and sizing degree drops. Foam sources in a papermaking system are fairly concentrated: the sizing agent, wet strength agent and dispersant in the stock are themselves surface-active substances, and their concentration keeps building as white water is closed and recirculated; air entrained during sheet forming is sheared fine by the stock pump. Using the wrong antifoam type or dosing at the wrong point creates more trouble than the foam itself.
Whether the Foam Is in the Wet End or the Dry End Tells You Which Section to Treat
Foam concentrated in the white water tower and the wire pit and rather coarse in texture belongs to wet-end foaming, mostly from additives and stock. Foam that reaches the press and dryer sections and leaves foam spots on the sheet still originates upstream, carried over from residual wet-end foam. Foam that turns brown and carries stock flocs is often accompanied by lower retention, with fillers and fines lost along with the foam. When white water consistency and foam volume rise together, it is basically the result of surface-active substances accumulating under closed circulation.
Order of Handling: Adjust Additives First, Then the White Water, Use Antifoam Last
First check the wet-end additive ratios and dosing points; excess sizing agent and wet strength agent raise the foaming tendency, and lowering the concentration often reduces foam. Then adjust fresh water make-up and discharge in the white water system; long high-closure operation lets foaming substances build up, and moderate replacement eases it. Only then comes the antifoam, dosed at the inlet of the white water tower reuse pump for the wet end, and at the wire section or before the press for the dry end. Position matters more than dosage; dosing into a dead zone or waiting until foam has formed both reduce the effect.
Selection Points: Wet End Looks at Compatibility, Dry End at Deairing
For wet-end antifoam, look first at compatibility with the sizing system: grades that break emulsion and float oil will drop the sizing degree, cause oil spots on the sheet and foul the felts. For dry-end or stock-section use, look at deairing ability: foam that is fine and mixed into the stock must be deaired in the stock before it reaches the forming wire. Also look at temperature and shear resistance; the shear from stock pumps and screens is not light, and grades that break down quickly have poor persistence.
Chemitech's Solution
For the wet end, white water circulation and coating sections of papermaking, Chemitech offers silicone, silicone-free and powder antifoams that can be selected by process position.
Compared with silicone types, silicone-free types have better compatibility in the sizing system, are less likely to affect sizing degree and sheet cleanliness, and suit continuous dosing at the wet end; powder types are easier to disperse in the stock section where dilution and make-up are needed; silicone types defoam faster in the white water tower where foaming is quick and foam is coarse. The three differ markedly in compatibility and applicable position.
When white water closure is high and additives have clearly accumulated, antifoam alone can only suppress the foam temporarily; the root still has to be controlled through additive ratios and white water replacement. Which type to choose must be judged in light of the system conditions.
Describe the stock type, wet-end additive system, degree of white water closure and where the foam appears, and our technical team will recommend a type by operating condition and provide samples. Paper Industry Antifoam Solutions and Papermaking Dispersants are worth a look first.
Summary
Recurring white water foam is mostly the result of closed circulation pushing additive concentrations to high levels, and antifoam alone cannot suppress it. Bring additive ratios back to a reasonable range and leave room for white water replacement, and the basis for foaming drops. Antifoam here is a backstop, and it must be selected separately for the wet end and the dry end, compatibility at the wet end and deairing at the dry end. Chemitech offers silicone, silicone-free and powder products that can be matched by process position. If you have a stock and additive list, our engineers can go through it with you.