Pulp & paper

Measuring pH in paper production and process extractions  

The quality of your product depends on quality of your pH measurement

Refining of Tall-oil

Extraction of tall oil is a major cost-effective part of pulp and paper plant operations. Crude tall oil is used to produce a tall oil rosin and tall oil fatty acids, small amounts of which are used as the foaming agent in the beneficiation of ores. Purified crude tall oil is used in the paint industry to produce drying oil, mastic, and paints.

The tall oil is separated from the black liquor by skimming and sedimentation of residues. An addition of H2SO4 separates tall oil into fatty and resin acids. pH is essential in monitoring this process and getting the right output, as improper control may lead to worse quality and quantity in the tall oil.

  • pH control is important for the recirculation line to secure the chemical reaction and optimize reagent consumption
  • Reliable pH-measurement in tall-oils requires automatic cleaning

Bleaching towers

The process of bleaching highly depends on pH value, which means that proper control and reliable pH measurements are very important.  Pulp is mixed together with Chlorine Dioxide typically as a ditching agent; bleached pulp is put in a new reactor to increase pH, so as to remove the lignin. Next, another tower is used to reduce the pH with acid, and then increase it again, cyclically. The quality (cleanliness) of the product is directly related to the quality of pH measurements.

  • Chlorine dioxide allows efficient bleaching at pH 3.5 – 4, without degrading the cellulose’s components. Extraction stages are conducted at pH 10-11 to dissolve and remove the reacted lignin.
  • pH control is important to secure process efficiency
    and minimize reagent consumption.
  • ORP is used to verify complete ClO2washing


In the Pulp and Paper industry, the scrubbers are commonly used to wash out and neutralize gases such as SO2 and HCl, and reduce environmental damages. Pollutants are absorbed by the chemicals in the liquid, which is collected in a holding tank at the bottom of the scrubber. In almost all gas scrubbers you will find some pH-measuring loops that control the pH of the scrubbing liquid. They are needed for several reasons – primarily to increase scrubbers’ work efficiency by minimizing the amount of chemicals used and securing maximal absorbance of pollutants. An incorrect reading of only 0.5 pH above the true value leads to a significant cost increase and waste of excess chemicals. To remove sulphur dioxide (SO2), a simple chemical reaction is used: by adding lime, limestone or hydrated lime and fresh air, the sulphur dioxide turns into calcium sulphate, better known as plaster. The ideal pH for this scrubber process is between 5.5 to 6.0. If the pH falls outside these ranges, serious issues can occur. A pH above 6 will result in wasting scrubbing chemicals and in clogging, due to lower extraction of calcium sulphite and reduced SO2 scrubbing efficiency. In the worst case scenario, the complete scrubber gets blocked, which would require days of work to fix. A pH below 5 results into corrosion of the scrubber, and the potential release of pollutants into the environment.

Paper Machine Wet End

pH affects production yield, paper quality and reagent consumption. For pulp to bind properly and form the paper sheet, pH measurement in the headbox is significant. pH is controlled in the headbox and adjusted with Alun (or H2SO4) to around 4.5 for proper binding of fibres into paper sheets.  Incorrect pH can also affect the printing qualities or cause discoloration when printing.


In addition, pH must be measured in and after the wire pit. The pH value of the water removed (white-water) must be examined to evaluate and preserve the strengths and quality of the paper; this will differentiate thin toilet paper from strong printing paper.