Control of pH levels in the stock preparation section of a paper mill is essential to the process of effective papermaking. Although pH is also controlled upstream of the headbox, the last opportunity to control the pH of the pulp slurry occurs at this location.
Raw fiber from the pulp mill has poor cohesion, low bursting and tensile strengths, and a tendency to flocculate. Therefore, the fiber is unsuitable for paper manufacturing and must undergo further processing in refineries, cleaners, and chests prior to delivery to the headbox. During this processing, additives such as resins, dyes, fillers, sizers, and chelating agents are blended with the fiber to provide better fiber binding, strength, smoothness, color, and opacity.
The headbox receives the processed pulp and serves to smooth out any outstanding variability in the pulp flow. Figure 1 illustrates the role of the headbox in this part of the process. Uniform flow velocity and composition are required for the paper machine to smoothly produce good quality paper. The head pressure in the headbox determines how quickly the headbox discharges onto the forming wire. Pressure is varied depending on the speed of the wire (throughput) and the type of paper being produced.