RECYCLING OF WASTE PAPER
With the progress of civilization, the paper and board consumption has increased tremendously over the years resulting in increased waste papers called "urban forests", making the recycling technology more and more complex and important. Public attitudes have also been stimulated by a growing awareness for control of environmental pollution and have as a result changed more towards conservation of natural resources. There is now an awareness that recycling conserves natural resources and reduces the solid waste burden. In the developing and third world countries, recycling of waste paper becomes an absolute necessity due to scarcity of raw materials.
Paper is one of the major components of urban solid waste (household and commercial waste) and has a potential resource value when collected and reused. The recycling and reuse of paper may be for various reasons viz.
- Recycling of the waste paper has been a practice that has prevailed in the paper industry since its inception and therefore continues.
- Use of the organic content in solid waste for its energy values.
- Production of non-paper products with extended life, (e.g.) building materials.
- Chemical or biological conversion, new compounds such as protein.
The use of waste paper in non-paper fibres products has a relatively limited application. Some of them are- particleboards, bricks, insulation, pipe, hardboard, and simulated wood and cement panels. Expansion of markets for products currently made from recycled fibres will provide only a fraction of the desired recycling increase. This is the reason why the development of new products which can be made from reclaimed fibres is so essential.
The reuse of waste paper in the manufacture of paper and board is not new and the first recorded report of recycling of waste paper dates back to 1695, at the George athasar IIIy Mill in Denmark. Over 100 years later, a British patent was granted in the name of Mathias Koops on a process for the utilisation of waste paper.
There are three major factors which favour utilization of waste paper for recycling to paper products.
- Worldwide shortage of raw materials.
- Increasing consciousness for cleaner environment and control of "Solid pollution".
- Valuable benefits of recovering a ready-made fibre from waste.
At the same time, there are factors which discourage the use of waste papers beyond certain levels. They are-
- Virgin fibres is a more versatile raw material than waste paper and is preferred due to its uniform characteristics. Waste Paper is heterogenous, its market is fluctuating and complex in terms of processing due to the presence of contaminants and has limitations of availability in very large quantities.
- Due to this reason, larger production units with economic production of virgin pulp are favoured to smaller waste paper mills.
- Forest resources may be owned and managed to provide long-term stability; which is difficult to realise in waste paper reutilization.
- End-uses in which recycled fibre has been used widely have not grown at the desired rate so as to consume all the waste paper generated with some exceptions such as newsprint and corrugating medium. Competitive materials also have displaced some of these products.
Traditionally, it has been a practice to use the waste paper in boards and corrugating medium. But presently, many other products apart from these two are manufactured using varying proportions of waste paper. They are newsprint, printing and writing papers, tissues, etc. Newsprint from recycled newsprint has become technically feasible, so is printing and writing paper with 50 to 80% recycled fibre. In tissues manufacture, there are no technical limitations to the amount of recycled fibre used.
The economics and technology of recycling differs from country to country. The various factors that influence the economics are-
- Virgin material availability.
- Cost of virgin raw materials (Vs.) waste paper and
- Market values of virgin papers and waste recycled papers.
In some countries, paper collection and recycling is engaging the attention of Governments too.
Sweden has established a 'Paper Collection Act of 1975' by which collection is organised by the local authorities under the municipality. Separation is done at the source. Except for offices, shops and other firms, waste paper can be removed only by those specially engaged for this purpose.
The European Economic Community (EEC) has formulated multinational research projects and funding is from all the countries of EEC. Each area is headed by one country and the Chairmanship for 'Recycling' is given to Belgium.
WASTE PAPER SOURCES
Usually waste paper is available as-
- Pre-consumer wastes like press cuttings, converting wastes, etc.
- Post consumer wastes like books, stationary, office and shop waste, household waste paper, street sweepings, etc,.
Ideally paper manufacturers would prefer to have the wastes which are clean and uniform in characteristics. In practice, however, they contain many undesirable elements other than fibres which are called 'contaminants' and contraries' Additives (fillers, dyes, wet strength resins) printing inks, varnish, wax, etc. are added during paper making or in converting. Many other contaminants like metals, strings, wood chips dirt, etc., enter the steam during collection, packaging and transportation. All these have to be removed in order to obtain a clean pulp. An initial sorting either at collection point (source) or at the mill removes heavily contaminated wastes and impurities. Many processing steps like screening, cleaning, washing and some special treatments are given to remove most of the contaminants. Each contaminant depending on its physical and chemical nature, requires special technology and equipment.
THE WASTE PAPER RECYCLING SYSTEM
Some of the general processing steps in any waste paper recycling system are:
Screening (Removal of heavy and large size contaminants)
Cleaning (Removal of grit and heavy small size contaminants)
Special methods of removal of contraries (Deinking, etc,.)
Dispersion of some contraries.
Bleaching or Whitening.
Beating or Refining.
Additives (Size, alum, dyes, etc.)
More steps may be added depending on the waste stock and final product. Seven basic parameters have been defined for the evaluation of the waste paper system.
- End Product Quality
- Defibering index (%)
- Classification of fibres
- Freeness (oSR)
- Ash Content (%)
- Efficiency of removal of impurities (%)
- Optical properties
- Strength properties
- Yield %
- Cost of Installation
- Operating Cost
- Power consumption
- Water consumption
- Steam consumption
- Cost of chemical additives
- Quality of Effluents
- Runnability of technological cycle.
- Reliability of the Machinery
IMPORTANT TECHNOLOGISTS AND SYSTEMS DEVELOPED FOR WASTE PAPER RECYCLING
1) Dispersion of Contaminants
Inks, colours, wax, bitumen, etc., are dispersed finely and thoroughly so that they do not give specks and patches on the paper. Except for hotmelts like wax, bitumen, varnish, etc., pulping at slightly higher temperatures with chemicals and a mild refining disperses most of the contaminants. The hot-melts or stickies are very problematic. They have either to be removed or dispersed finely. If not they tend to stick to paper machine felts, wires, etc., and cause operational problems. These are effectively dispersed by refining a high consistency stock at temperatures above 100Â°C with some pressure. Some of the popular hot dispersion systems are available from Voith, Beloit-Jones, Maulay, etc.
2) Removal of Contaminants
This is a better alternative for dispersion, as the product becomes cleaner in quality, since otherwise of impurities have a cumulative effect in the waste paper market.
a) Printing Inks are removed by three progressive stages of the deinking process-
- Ink removal from fibre (with chemical addition)
- Ink removal from pulp (washing or froth flotation
- Ink removal from effluent (clarification)
Ink must be first removed from the fibres and dispersed. This is achieved by shear forces encountered in pulping and by the addition of deinking chemicals. Once separated from the fibres, ink has to be removed from the pulp slurry. There are two basic approaches to this: Dilution washing is a mechanical process of rinsing ink particles from the pulp. Froth flotation utilizes the different behaviour of fibres and ink pigments when wetted with water. When flotation chemicals and finely dispersed air bubbles are introduced into the suspension of ink and fibres, the ink pigments adhere to the bubbles and rise to the surface and form a layer of froth that is removed by skimmers.
Clarification of the inky effluent using flocculation is very important in any commercial deinking process.
b) Chemical Treatment for contaminant removal is not too plentiful, but these are becoming increasingly important. e. g., Treatment of plastics and wax coated carton stock using trichloroethane to dissolve these materials leaving white paper.
c) Mechanical Treatments
- Pulp-Polyethylene suspension is diluted to allow floating of the pieces of polyethylene, which are mechanically raised out of the suspension by way of a conveyor.
- One method describes the removal of bitumen from waste paper slurries by agitating the slurry in the presence of plastic materials at a temperature above the softening point of the bitumen. The bitumen adheres to the plastic and the bitumen-coated-plastic is then screened out of the waste paper stock at the end. Addition of surfactants enhances the adhesion of bitumen to plastic.
A FEW IMPORTANT AND NOVEL TECHNIQUES/SYSTEMS
1) Reverse Cleaning
In a conventional forward centri-cleaner, heavy contaminants like grit and coarse fibres are rejected through the apex end. Only 5% of the flow and 15% of the solids are rejected. In a reverse cleaner, the proportions are somewhat changed by different orifice size and/or value action. 45% of the flow and 85% of the solids (mostly good fibre) are accepted through the apex and the rejected (lightweight contaminants like hotmelts, wax, plastic bits and fibre fines) via, the vortex end.
Fig. 1. Forward (Contentional) and Reverse-Flow Centrifugal Cleaners.
2) Ahlstrom - Fibre flow System
Waste paper is wetted to a consistency of about 15% with water and some wetting and swelling chemicals and repeatedly dropped on to a hard surface. This action fibrizes the wet paper, but does not break up bits of plastic, string, etc. A rubbing effect is added when a rolling motion is applied between drops. Ink, size, laminates and hotmelt substances are loosened from the fibres.
CROSS SECTION OF THE DRUM
Fig. 2. Ahlstrom Fibreflow system.
After the high consistency zone, the stock is diluted and individual fibres pass through the perforated drum. The rejects (wires, plastics, etc.) eventually find their way to the end of the drum and are removed.
Major principle of the system is that treatment of fibres at low consistency should be minimized, as it creates a lot of fines which increase the drainage resistance. High consistency treatment utilizes the energy more efficiently and leads to increased temperature of the pulp which is necessary for dispersion of hotmelts like wax, bitumen, etc. The 'Frotapulper' consists of two intermeshing screws rotating synchronously against each other. The paper flakes are sheared between the flanks of the screws, which leads to a complete separation of fibres; plastic coating (Polyethylene) is rolled into pirals which can be easily removed by screening.
Fig. 3. Working zone of the frotapulper.
Dry-sorting- is based on a differential mechanical fragmentation an screening of feed material into 3 fractions-Mechanical fibre enriched, intermediate, and chemical fibre-enriched. The quality of separation is dependent upon the differences in strength properties of the furnish components as well as upon process operating variables. The strength of a sheet of paper or board is indicative of the quality of its consistent fibres. The sorter consists of a longcylindrical rotor with parallel blades extending radially from its surface. The rotor is either partially or entirely covered by a screen made by heavy wiremesh and spaced some distance from the rotor. Waste paper is fed from one side, subjected to intensive mechanical treatment by the blades as they rotate. Weak components breakdown first and pass through the screen. The fractions passing through the last screen sections are made of the strongest fibre in a given mix.
THE INDIAN WASTE PAPER RECYCLING INDUSTRY
In India also, waste paper is recycled extensively and more than 30% of our paper and board production is from waste paper. Even household waste gets sorted and ends up in a board mill as "street sweepings".
While the waste paper dealers and collectors find good profits in the trade, environmental protection aspect also gets satisfied indirectly. Moreover, our acute raw material shortage makes recycling a necessity.
Our end-uses of waste paper differ much from those of other countries. The good quality waste papers like imported kraft waste, press trimming, cuttings, etc. end as kraft, liners for duplex boards and in printing and writing papers. The inferior quality wastes like duplex and grey board cuttings, office waste, street sweepings, etc., are mainly used for corrugating medium and filleqs in duplex and grey boards.
The technology and equipment required for upgrading these wastes for the manufacture of higher-grade products is expensive. Moreover, the waste papers available in the market are highly heterogeneous. There is considerable variation between consignments from the same supplier or between suppliers on account of various grades of paper and board manufactured in the country and the numerous agents supplying waste to the wholesaler. Table 2 gives the present approximate prices of a few varieties of waste papers, virgin pulp and raw materials.
Sorting is done both at collection points and at the mills, but more so at the mill site. A good and thorough sorting is very costly and after a certain limit becomes prohibitive. Inspite of visual sorting sometimes mistakes happen.
Bitumen and wax embedded between layers of kraft board, plastic in between layers of paper in duplex cuttings are some of the hard to sort out cases. Our processing has to overcome such mistakes and guarantee clean products. For the range of products so far manufactured in India, the following sequence would ensure good pulp at accep Table prices.
- Rapid and efficient difiberation to ensure complete disintegration of the fibres;
- Good dispersion of contaminants like inks, colours, waxes and varnish.
- Screening and (d) cleaning.
These basic steps are necessary for all mills-big or small. Government aid by means of subsidies would encourage the recyclers to produce better quality products from waste papers. This will ultimately increase the quality of wastes in the country. Government can also control the quality of waste papers and products by standardization and price limitation.
Research in our country should be aimed at improving the present processes without additional machinery or higher processing costs. On the other hand, simple and economic technologies and equipment should be developed for upgrading the waste for the manufacture of better grades.