Enzyme for Deinking Process

The use of recycled fibres in newsprint, tissue and currently in higher quality paper grades has significantly increased over the last 20 years, particularly as a result of consumers’ attitude and the successful development of deinking processes. Deinking and contaminant removal are critical major barriers in paper recycling. Toners are particularly difficult to remove. These are resilient, plastic polymers that are fused to fibers. Unlike dispersible inks that occur in newsprint or offset printing, laser and xerographic inks don’t disperse. The residual ink decreases the brightness, and the toner particles create a conspicuous background.

The most common deinking techniques are wash and flotation deinking. The removal of ink called deinking is based on two steps: ink detachment from the surface of the disintegrated fibres which is performed during pulping (slushing), followed by the removal of the detached ink particles from the pulp slurry by washing or flotation.

The use of enzymes could be an attractive alternative to chemicals in deinking: enzymes could improve the de-inkability of inks difficult to detach with the conventional deinking process and since enzymes are used in neutral conditions, they could be supposed to produce a lower COD concentration than conventional alkaline deinking.

One of the advantages of enzymatic deinking is the avoidance of alkaline deinking chemicals. Deinking at a neutral pH prevents alkaline yellowing of the recycled fibre pulp and simplifies deinking chemistry. In an industrial operation, the use of enzymes as deinking agents could thus contribute to lower chemical costs and might decreased environmental impacts. Enzymatic deinking also changes the ink particle size distribution, apparently reducing the average particle size. In addition to ink removal, enzymatic deinking may contribute to improved strength properties of the paper sheets and freeness and reduced fines content of the recycled fibre pulp.

Cellulases could work in several possible ways to enhance deinking. First they might reduce the hydrodynamic drag to increase the filtration and flotation rates. We know that cellulases enhance drainage rates. If they enhance drainage rates, they can enhance any separation process, such as filtration or flotation.

They might also decrease the specific surface area of the fibers and thereby reduce interaction with contaminants. That is to say there might be microfibrils on the surface of. these very frazzled, recycled fibers which could be trapping the ink particles, and by giving the fibers a “haircut”, we reduce their adhesion.

Our results indicate that the most important factor with respect to toner removal is the increased flotation efficiencies imparted by cellulase activities. The biggest increase in toner removal that we observed occurred during the flotation stage. During flotation, air bubbles rise to the surface of the flotation tank through a relatively dilute pulp stock, approximately 1% consistency. The surfaces of air bubbles are relatively hydrophobic and they carry the toner particles to the top where they are removed by a skimming action.

Generally there are three different approaches available for the use of enzymes in deinking:

  • The fibre surface can be attacked by enzymes
  • The starch-based coating can be solubilised by enzymes
  • Vegetable oil-based binders of the printing inks can be hydrolysed

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