Application Note

pH Measurement In Deinking Mills (Secondary Fiber)

Source: Emerson

Deinking is a process that turns recycled paper products (also called secondary fiber) into raw pulp for production of refined paper products. Lower pulp cost for the pulp mill and reduced burden on landfills are primary motivations for the use of deinking. The net cost of deinked pulp can fluctuate widely, however it is generally lower than wood pulping. Repulped fibers are not as strong as virgin fibers, so they are often blended to meet product requirements.

Deinking is named for the part of the recycling process that removes coatings and inks from paper before refining the pulp into a finished product.

The steps involved are:

  • Pulping or fibering
  • Cleaning and screening
  • Washing (or floating) out contaminants
  • Thickening or dewatering
  • Bleaching (if necessary)

The secondary fiber is combined with pulping chemicals and water in the pulper. The pulper can be operated in batch mode, typically used for smaller loads or continuous mode, when a steady feed of raw material is available. Three types of surfactants are used; detergents remove the ink from the fiber, dispersants prevent redeposition onto the fibers, and foaming agents collect the dispersed ink in the form of foam. Other chemicals such as caustic, sodium silicate, borax, and peroxide may also be used. The pH is generally alkaline. Consistency in the pulper is relatively high at 5–8 % solids.