Unit operations and equipment in recycled fibre processing

Paper for Recycling (PfR) today is a mixture of different paper grades and each piece of paper in this mixture consists of different fibres. lt may also contain pigments or fillers, binder or additives and other components. ln addition, the paper products in the recovered paper mixture may have been coated, printed or upgraded by other means. The PfR may also contain non-paper products. Consequently, the mixture has to be processed in various ways to meet the quality criteria for the paper to be produced. Unlike the production of virgin-fibre pulps, in which wood is transformed into a fibre suspension, in recovered paper processing the aim is to separate the components that are not suitable for papermaking, and sometimes to restore or improve the quality of the fibres. Recycled fibre processing is therefore significantly more complex than virgin fibre production because the processes have to handle different types of fibre and also different types of contaminants and other substances detrimental for paper production.

Processing of PfR starts with the recovered paper delivered to the paper mill and ends with the pulp suspension ready for paper production stored in a chest. Between these two end points unit operations are arranged and the complexity of such systems depends on the quality requirements for the final pulp. Simpler systems as used for instance for corrugated material production where only particles larger than fibres have to be separated. In systems for the production of graphic or hygiene paper grades also fine particles like printing inks and filler material have to be separated and in many cases the pulp need to be bleached. While the unit operations are discussed in this section information about combining the unit operations to systems can be found in part Design of Processes. Figure 1 shows the basic layout of a so-called stock preparation system, which is the combination of unit operations to a process for PfR processing

Figure 1 shows the main product stream, only. Real system are much more complex since some of the unit operations are installed several fold in cascaded or feed forward arrangements, water is circulated back in order to reduce water consumption and secondary processes for water treatment can be installed. These aspects are discussed in section Design of processes.

The basic steps of a stock preparation system
Entry inspection	Most paper mills check incoming raw materials by weighing the full and empty trucks. The quality of incoming paper for recycling is checked visually and some mills also have installed near infrared sensors for analysing the composition of the PfR.
Storage and Mixing	Stores the suspension in tanks and prevents components from demixing or mixes them together.
Dewiring/ Opening of Bales	If PfR is delivered in bales, the bales have to be opened by cutting the wires. Depending on the device used for pulping wires have to be removed.
Repulping	Breaks down the recovered paper first into pumpable condition enabling coarse separation that is normally included in this stage and then by deflaking if necessary.
Coarse Cleaning	Acceleration in centrifugal field and separation of heavy weight from light weight fraction.
Coarse Screening	Passage through apertures (typically holes) and separation particles from retained particles.
Cleaning	Acceleration in centrifugal field and separation of heavy weight from light weight fraction.
Screening	Passage through apertures (typically slots in the range of 100 µm to 250 µm) and separation of passing particles from retained particles.
Flotation	Attachment of hydrophobic particles to air bubbles and separation of foam.
Thickening/ Dewatering	Removes water from the suspension making certain processes possible such as dispersing or more cost-effective such as bleaching or storage (drainage also separates the water loops for ionic trash and temperature control).
(Washing)	Dilution to very low consistency and passing of fine particles through small apertures (typically wire mesh).
Dispersion (kneading)	Makes residual dirt specks and stickies smaller or floatable, detaches ink still adhering to the fibres, mixes in bleaching agents, and conditions the fibres technically.
Bleaching	Endows yellowed or coloured fibres with the required brightness or luminance by application of oxidative or reductive bleaching additives.
Refining	Modifies the fibre morphology and surface characteristics to ensure the required bonding properties and paper quality.

After the re-pulping step all unit operations are carried out with pulp fibre suspensions in a consistency range between about 0.8 % and 20 %. Therefore, some basic knowledge about the fluid mechanics of such fibre suspensions is important for understanding mechanisms in PfR processing. An introduction is given in article Fluid mechanics of pulp suspensions.

Source https://forestbiofacts.com/