PH varying

1. Sudden PH changes are most frequently due to an influx of fresh water.
— the system must be examined, and material and flow balances run to determine the cause.
— appropriate system changes should be made to ensure that uncontrolled and sudden influxes of fresh water do not occur.
2. The washing of chemical pulps should be checked for carry-over or anionic trash.
3. The PH is best controlled as far back in the system as possible to a value about 0.2 – 0.3 units above that finally required.
— a trimming pH adjustment, under automatic control, can then be made at the fan pump. With such a system, the pH will automatically be maintained.
— if automatic control is not provided, then the amount of alum, acid or alkali added at the fan pump can be manually adjusted. However, over adjustment often occurs with manual control.
4. Sudden pH changes are most detrimental to good papermaking because:
(a) the likelihood of pitch deposition is increased.
— there is also a greater tendency for machine clothing to fill up.
(b) the adhesion to press rolls changes, and the draw at an open draw couch will change.
(c) the shade or color may change according to the dyes used.
— most dyestuffs are indicators and change color with PH.
(d) saveall operation can be affected.
(e) drainage on the forming fabric changes.
— this also affects drying rate.
(f) has large negative effect on sizing.
(g) contributes to picking, crumbs, etc.
(h) can degrade most strength properties if too low.
(i) can reduce adhesion of size press coatings to fiber or reduce penetration of size press solution into web.
(j) more energy is needed to refine stock at lower PH range.
(k) generally has a negative effect on machine runnability.
(l) can cause changes in Yankee coating and adhesion.
5. Low pH is corrosive to pumps and pipes.
— may also break loose scale and rust which results in dirt in system.
See also Acidity.