Main Steps in Paper and Board Recycling
Paper recycling includes several stages to convert into secondary fibres, as shown in Figure 4.2. Pre-processing and post-processing stages are described and some useful explanations are clarified below.
4.2.6.1 Collection
The process begins with collection of waste paper and board from individuals and enterprises (so-called private and industrial collection) and then they are delivered to mills. Collecting step is one of the most expensive in paper recycling (Bajpai 2014). One of the main problems here is that usually waste paper is a mixture of many fibre types in different paper and board products, and it has a low value, since mixed papers will not normally provide the properties required for a specific end use, such
FIGURE 4.2 Main steps in paper recycling.
as necessary strength requirements in packaging, contrast in printing papers, or absorbency in tissue (Technology of paper recycling 1995). Hence, mixtures of fibre types can only be used to produce the lowest grades of paper and board.
4.2.6.2 Sorting into Grades
Once waste paper and board have been sorted into grades, then they have a higher value than mixed paper; how'ever, sorting requires time and additional expenses that vary from country to country. There is a huge number of grades, according to classifications CEPI EN 643-2013 in Europe and ISRI PS-2018 in North America, that makes the task of sorting complicated. As a result, the most part of w'aste paper is processed without sorting into the low'est grades, namely cardboard and tissue.
4.2.6.3 Repulping
When w'aste paper and board have been collected and sorted into grades (or not sorted), they undergo the processing at recycling facilities. The first step of the processing is repulping. The paper is soaked in large vats, reducing the paper into fibres. Repulping is followed with several additional stages such as the removal of contaminants when washing, screening, or cleaning are used for each type of contaminant. Then selective purification is done by fractionation w'hen the separation of short and long fibres occurs.
4.2.6.4 Deinking, Removal of the Contaminants, and Bleaching
The big share of waste paper is black and white or multicoloured printed paper and packaging board. In order to isolate useful secondary fibres from printed paper, ink usually must be removed during the deinking process, although, paper w'ith a greyish tone without deinking step may be used for some applications. When paper swells during repulping, ink particles separate from the cellulose fibre. Firstly, the pulp is cleaned several times with heat and chemicals. During this procedure, some ink and additives are removed. Then, to withdraw residual ink from the pulp, the flotation process is implemented. Small colloidal particles of contaminants are flocculated and driven to the surface with air bubbles w'here it can be skimmed away. After the deinking process, the pulp is ready to be manufactured into paper and related products in a similar manner to that by which paper is produced from wood pulp (Bajpai 2014).
The fibres undergo commonly by bleaching to increase brightness. For this purpose hydrogen peroxide (oxidative bleaching) or sodium hydrosulfite (reductive bleaching) are commonly used.
As mentioned earlier, one more problem in the recycling process is that waste paper keeps non-fibrous materials. In general, the rate of formation of such residues is between 5% and 40% depending on the recovered paper grade processed and the paper grade produced (Bajpai 2014). For example, many packaging boards or printing papers contain up to 5% by weight of starch. Printing papers contain about 20-30 w't.% of mineral fillers, while coated papers have added coating fillers so that they contain up to 40 wt.% of mineral fillers and coating binders. As a result, all waste paper has a need for some contaminants’ removal (Technology of paper recycling 1995). Most non-fibrous materials reduce fibre web strength. As particle size increases, this effect increases, so large particles cause weak spots in the fibre web.
Removal is difficult due to the diversity of materials, organic and inorganic, natural and synthetic, etc. (Technology of paper recycling 1995). However, not always all of these non-fibrous materials are considered as contaminants. For example, while in tissue, coating and filler clays are regarded as contaminants, for printing and writing paper they are not due to different demands to these types of paper. Aside from this, the non-fibrous residuals can be used as a raw material in other industries. While some treatments of the remainders include the production of fuels or energy recovery via incineration, other techniques are more eco-friendly due to the real reuse of non- fibrous residuals. For example, Pitroda (2016) showed that so-called hypo sludge which contains useless low-quality paper fibres, as well as calcium oxide, calcium chloride, magnesium oxide, and silicon dioxide can be used in concrete formulations as a supplementary cementitious material. Tofani et al. used recycled fillers (e.g., CaCO,, kaolin) from deinking paper sludge for newspaper production. Deinking paper sludge was incinerated, subsequently bleached, and then reused for papermaking (Tofani et al. 2020). Also, the construction industry is a field for the second life of non-fibrous wastes from paper recycling where they are transformed into building panels or insulation, as well as in such processes as the conversion of plastic and making composite semi-finished products (COST Action 2009).
Thus, besides valuable cellulosic fibres, paper contains non-fibrous impurities, which usually should be removed. Removal impacts the loss of time and chemicals during additional stages of processing, and as a result, residues are generated. However, non-fibrous waste can be used in other manufacturing, which partly makes closed-loop processing and raise the sustainability of paper recycling.
In general, the paper recycling process is a number of several steps, each of them has its own drawbacks; however, all in all, secondary fibres that are extracted during this process are the valuable raw materials for different functionalization.
