The use of waste or recycled paper as the raw material for pulp production has increased during the last several decades, and some paper plants depend almost completely on waste paper. In some countries, waste paper is separated from other household waste at the source before it is collected. In other countries separation by grade (e.g., corrugated board, newsprint, high-grade paper, mixed) takes place in special recycling plants.
Recycled paper can be repulped in a relatively mild process which uses water and sometimes NaOH. Small metal pieces and plastics may be separated during and/or after repulping, using a debris rope, cyclones or centrifugation. Filling agents, glues and resins are removed in a cleaning stage by blowing air through the pulp slurry, sometimes with the addition of flocculating agents. The foam contains the unwanted chemicals and is removed. The pulp can be de-inked using a series of washing steps which may or may not include the use of chemicals (i.e., surfactant fatty acid derivatives) to dissolve remaining impurities, and bleaching agents to whiten the pulp. Bleaching has the disadvantage that it may reduce fibre length and therefore lessen final paper quality. The bleaching chemicals used in recycled pulp production are usually similar to those used in brightening operations for mechanical pulps. After the repulping and de-inking operations, sheet production follows in a manner very similar to that using virgin fibre pulp.