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This is the first process that has to be done. The waste paper must be sorted out before processed. This is because the waste paper may contain a mixture different paper types made of mixture of different paper fibers. Paper waste from paper production or broke is used directly in the paper machine.
The examples of the paper types are Office Waste (OW), Old Magazine Paper (OMG), Old Newspaper (ONP), paperboard, corrugated fiberboard. The sorted paper is then baled and shipped to a paper mill. In order to get the required paper quality it depends on the grade of the waste paper.
The bales are opened and foreign objects are sorted out on the conveyor belt to the pulp. The twine, strapping and others can be removed by using a "ragger" from the hydropulper. Magnet can be used to remove metal straps and staples.
The pulper cuts or chops the paper into smaller pieces. During this process, water and chemicals are added. The pH is normally adjusted to 8.5-10.0.
Sodium silicate, sodium hydroxide
Hard water, lime, calcium chloride
Fatty acids, fatty acid emulsion, fatty acid soap,
There is four types of pulping machines which are mechanical pulping, chemical pulping, semi-chemical pulping, and thermo-mechanical pulping.
Mechanical pulping: the main purpose is for the production of newsprint.
is used to remove impurities such as lignin, resin, gums and any undesirables in the wood so that the pulp is mainly cellulose fibers.
These paper are more permanent, less yellowing, and fading.
Also called as CTMP.
The main purpose is to produce pulp with higher yield yet somewhat similar properties to chemical pulp.
Involved the hot pressurized refining o wood chips.
Produce pulp with a yield of more than 90% from the wood.
The pulp is strong enough for newsprint without combination of chemical pulps.
After went through the pulping process, the mixture is a slurry. The slurry is then will proceed to the screening process.
4) Cleaning and screening
Cleaning is the first step after pulping process. The pulp slurry is spinning by the centrifugal cleaning. This process remove high and medium density large particles such as rocks and dirt, nuts, bolts, nails, paper clips and other object found in waste paper.
Screening is the process to removes large ink and low density contaminants from the pulp slurry. The coarse screens fitted with holes with a size ranges from 6-20mm is used to permit the passage of cellulose fibers and liquids while holding back the large particles. The fine screens with slits 0.15-0.30mm in width is used to separate smaller contaminant particles such as unpulped paper and plastics and toner ink particles from the pulp.
5) Deinking stage:
The purpose of this stage is to remove the hydrophobic contaminants from the recycle paper. Several processes are used such as flotation, washing and dissolved air flotation.
5.1: Flotation deinking.
Diagram1: Forth flotation cell.
This process only requires air in some cases and surfactants might also be added to the mixture. The hydrophobic contaminants such as inks and glues that are still in the pulp will not dissolve in water. Therefore, by having surfactants in the pulp, air bubbles are created by blowing air through the mixture.
The ink and glues will attach to the air bubbles, lifted to the surface and form a thick froth that can be removed. Flotation deinking is useful in removing ink particles larger than about 10Î¼m. The chemical conditions for the deinking process are:
Sodium silicate, sodium hydroxide
Calcium chloride, lime, hard water
Fatty acid, fatty acid emulsion, fatty acid soap,
5.2: Washing deinking.
This process is useful and effective for removing particles smaller than 30Î¼m, such as water-based inks, fines, micro stickies, fillers, and coating particles. Wash deinking consists of a washing stage. This stage requires dispersants to remove the inks. Thickening pulp slurry will washed out the medium of the fine particles. The processing equipments are belt filters, pressure belt filters, disc filters and static filter.
Industries are usually using a single enzyme or a blend of different enzymes for deinking purposes. The main enzymes used for this process are cellulases, hemicellulases, amylase, lipases, esterases and ligninolytic enzymes. The actions of these enzymes are different either attacking on the cellulose fibers or on the surface of the inks. For examples, lipases remove oil-based inks for the surface of waste paper. Hemicellulases, cellulases, and ligninolytic enzymes alter the surface of cellulose bonds and which helps to removes inks in flotation or washing steps.
Enzymes is useful deinking process because it replace large quantity of deinking chemicals, reducing the cost of production, reduce the water pollution by chemical agents and also brightens the pulp.
6) Washing and dewatering:
Washing and dewatering is a filtration process where small pieces which are less than 5Î¼m are removed. This process can be done by passing water through the pulps (Klungness and Doshi 1992).
There are 3 washing stages in order to prepare the clean pulp. The two stages are with screw presses and one stage is with twin wire presses.
Dewatering process is done in twin wire presses. They consist of two travelling wires which are used to form a pulp mat. Then, it is pressed through a series of pressure loaded rolls. This process force out the water through the wires and the system as well. The dewatered pulp is transferred to the flash dryers while the filtrate is returned to the process.
Diagram2: Pulp washing.
Diagram3: Pulp dewatering.
7) Dispersion or kneading:
This process is a mechanical action that is applied to fragment contaminant particles. Chemicals like chelants maybe added to prepare for peroxide bleaching.
The goal of this process is to improve the cleanliness and brightness of the pulp. The other main goals are to improve the brightness stability and to reduce pitch content. The process involves removing or brightening the colored substances existed in the pulp. Residual lignin is a main component in color that must be removed or brightened.
The washed substances are then put into a weak bath containing bleaching powder. After 6 to 12 hours, they are washed and boiled with soda carbonate for 2 to 4 hours. Some of the fibers are hard, therefore the hard fibers will be treated with sulfuric acid, Hâ‚‚SOâ‚„ and well drained before boiling with the soda. Then they are finally placed in a bath containing bleaching powder and soda for 4 to 6 hours.
After that, the fibers are passed through an alkaline chlorine bath containing excess of caustic alkali for jute and other substances different to bleach. The temperature for this process should not exceed 50°C. The process from deinking through to bleaching process is usually done twice or more to create a cleaner and brighter pulp depending on the condition whether the papers have more inks, other contaminants such as glue and gloss coating.
There are two typical bleaching sequences. First, the wood pulp is bleached with chlorine in solutions (C stage), extracted with alkali (E stage), and then treated with chlorine dioxide (D stage). The second typical bleaching sequence is written as C-E-H-H where C is chlorination (C stage), E is alkali extraction (E stage) and H-H are the two hypochlorite bleaching (H stage). (Casey, page 669)
There are two major categories for chemicals used for bleaching process which are oxidative and reductive.
Formamidine sulfuric acid, FAS
When the pulp achieves the desired color and strength, it is passed to a vessel where the filtrate of the mixture is discarded, including the large amounts of water and other chemicals used in previous processes. The semi-dried pulp is then ready to enter the paper machine.
10) Paper making:
Fourdrinier paper machine is a machine to produce paper. This machine comprises of 4 sections which are wet end section, press section, drier section and calendar section.
Diagram4: The fourdrinier paper machine.
10.1: Wet end section.
The wet end section is the important sub process to produce high quality and consistent end product. The refined pulp which is spilling from the head box section (stuff box) is designed to line up its fibers across the width of the wire or screen of the wet end. After that, the process of aligning fibers starts in the machine running direction. The water also will be removed from the refined pulp.
10.2: Press section.
The press section can have two or more presses. The wet web of paper is then transferred onto the felt located at the couch roll and it will be transmitted through the presses on the felts. The main purpose of this process is to remove water from the bottom sides of the paper web.
10.3: Drier section.
There is a series of driers normally used is steam-heated cylinders. The paper web is then transported against the driers. First, the felt side or top side are pressed against the heated surface of the drier and followed by the wire side or bottom side of the web paper.
10.4: Calendar section.
This is the final section for this process. This section consists of calendar stacks with a winding mechanism which used to collect the paper web onto a roll once it leaves the end of the paper machine. The main function of the calendar section is to smooth the paper. Water or starch may be used to obtain specific surface of the final product thus improving printing and other application characteristics.
Once this section is finished, the paper web is spun into a master machine roll where it can be eliminated from the end of the paper machine. The paper web is then sent to a converting station. The paper will be either slit into a smaller rolls or it may be layered for use in offset printing.
The unusable materials or sludge such as inks, plastics, fillers and short fibers is buried in a landfill. The sludge will be burned to generate energy at the paper mill or it may be used as fertilizers for local farmers.