Tuesday, 22 December 2009

Paper recycling

Cellulosic fibres exhibit a number of properties which fulfill the requirements of papermaking. These properties are: High tensile strength, suppleness (flexibility), resistance to plastic deformation, water insoluble, hydrophilic, have a wide range of dimensions, inherent bonding ability, ability to absorb modifying additives, chemically stable and relatively colourless.       
Deinking chemicals are used for the removal of ink when recycling paper.
The deinking chemical agents are based on formulations of:
  • Sodium hydroxide
  • Sodium hydrosulfite
  • Hydrogen peroxide
  • Chelating agents
  • Surfactants
I use 1 litre of each and add them to the deinking cell in a specific order.
Deinking process can be broken down into several simple stages.    
Recycled waste paper ----> screening and cleaning ----> wash deinking or flotation----> deinking post-flotation and dewatering.------> deinked pulp    
Recycling technologies have been improved in recent years by advances in pulping, flotation deinking and cleaning/screening, resulting in the quality of paper made from secondary fibres approaching that of virgin paper. The process is a lot more eco-friendly than the virgin-papermaking process, using less energy and natural resources, produce less solid waste and fewer atmospheric emissions, and helps to preserve natural resources and landfill space.
However there are alternative ways of deinking paper which involves replacing harsh chemicals with enzymes. This is what I research.
Enzyme action is affected by the paper constituents in the de-inking condition. The chemically pulped fibres are more susceptible than mechanically pulped fibres. This is important because mechanical fibres have a lot of lignin left and hence they're much more resistant to the cellulases. Therefore, this technology works best in mills that are recycling stock paper such as office waste which is very high in chemical pulp content.
Office waste paper, unfortunately, is also high in laser and toner content, thus it has a very low value because you can't mix it with anything, and the technology for taking the toner particles out is not very good at the moment.Enzyme deinking is complemented by maceration, because the major role of cellulase is to release the toner particles from the surface of the fibre. One depends upon the mechanical action in the pulping process to release the toner particles, and physical separation remove the toner particles by flotation. 
During the past twenty years the attempt to replacing chemicals with enzymes in deinking recycled paper including cellulase, xylanase, laccase and lipase, has been pursued. Cellulases and hemicellulases have been demonstrated to dislodge inks by peeling off fibres or fines on paper surfaces. Lipases have shown some direct action on ink particles either degrading oil carriers or breaking down pigments. Lignin-degrading enzymes, such as laccase, also hold some potential for deinking, as they may selectively remove surface lignin, and hence, facilitate ink removal. Cellulolytic enzymes have shown the most promising results for deinking of mixed office paper waste. I suppose this is to be expected since paper is actually cellulose. Oddly I personally am finding that amylase is the most effective at deinking. What is frustrating though is that after the end of my experiments/deinking sessions I am finding that there is so little enzyme activity left making  it difficult to assay. Isn’t that typical of science?
Enzymatic versus chemical deinking has been widely examined for MOW (mixed office waste) and photocopy prints. This being the most difficult paper to deink. The use of enzymes could be an attractive alternative to chemicals in deinking. The application of enzymes in deinking has been studied on laboratory and pilot plant scale. This work has resulted in numerous patents. However, so far enzymatic deinking is rarely applied in commercial use although some promising applications include recycling.  The enzymatic treatment is a competitive alternative for MOW and photocopy paper deinking. However, the process requires the selection of an adequate enzymatic preparation for each paper grade. It is less costly than chemical deinking, and it is more efficient.
These 2 methods of deinking are constantly being compared and work with enzymes will continue due the benefits gained by using them. I will leave actually paper making to the experts though I have done and enjoyed that activity.

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