Preparation of the paste
The bales of waste paper are picked up from the yard, the metal wires (or other fastenings) are undone and is put on a conveyor belt which transports the paper into the high density pulper. Once the water has been added, the paper is crushed as much as possible using a mechanical action which lasts about 20 mins per batch. Each batch uses about 2 tonnes of waste paper and 18 tonnes or water which comes from the clear-flocculation cycle (or from the source during the start up phase of the mill)
During this stage the cellulose pulp fibres are saturated and separated from the contaminants, a good number of which, however, stay suspended inside the mass.
Separation of the plastic
After the pulping the paste has to go through a dense paste purifier, especially for waste paper. In fact, after having been in the pulper, the paper, with the added water and other products that turn it into pulp, is then treated to eliminate the impurities, such as small pieces of iron and waste material with a high specific weight. This process improves the quality of the paste. The polymeric contaminants (essentially various types of plastics in the waste paper, such as additives, impurities, stationary items etc.) are removed by using another machine which is called the “Separ Plaster”, which separates the matter thanks to a physical principal (spinning).
Once the treatment has been done, the paste that has been obtained can be used, in the following stages of the production process, as in any other paper mill.
The finished product obtained at this stage is called “suspension” and is made up of fibres suspended in a watery liquid at about 4%.
The paste that is made by the pulper must undergo another mechanical process which is called refinement.
The aim of refinement is to increase the links between the fibres to make the sheet solid and resistant.
A sheet of paper that has been made with refining the fibres generally has scarce mechanical properties, is very bulky and has an irregular and cracked surface. Other parameters which can be altered thanks to refinement are: dullness, porosity, permeability, printability etc.
These variations in the paper’s properties are to be found in the changes that each fibre undertakes during the refinement process.
During this stage, the fibres are squashed and then cut, which is aimed at making them more plastic, flexible, and moreover the fibre’s surface often has thin fibres showing which means they are fibril. This makes the fibres come into closer contact between one another and the characteristics of machine resistance will be exalted in the following stage of the sheet formation.