How a paper machine line works
1. Raw materials processing
Paper making starts with chemically or mechanically processed wood pulp. The first part of the paper machine is the wet end. This is where fillers and additives are added to the pulp to achieve the properties required for the paper.
2. Raw water treatment
The degree to which raw water is treated varies depending on its use and the purity requirements. Normally, raw water is purified mechanically, i.e. sand and other debris is removed. Several different methods are used to remove impurities. If the quality of the raw water is adequate, mechanical purification is enough for most processes. If necessary, the water is purified also chemically.
3. Waste water processing
Paper making produces waste water, most of which is treated in biological treatment plants. The aim is to use the mill’s process water as many times as possible in order to minimize the amount of raw water intake and the amount of water to be biologically treated.
4. Headbox and wire section
A paper machine’s web forming section consists of a headbox and wire section. This is where many of the important paper properties are determined. The headbox dispenses pulp evenly onto the wire mesh for the entire width of the machine. The most common wire section types are the fourdrinier, hybrid former and gap former. The wire section is where the water from the forming web is removed by drainage and thickening.
5. Press section
The press section removes as much water as possible from the paper web and compresses it. Felt is used to convey the web through the press nip formed between the press rolls. Wet pressing has a significant affect on paper properties. When the paper’s wet strength is good, the paper web can withstand the move to the dryer section without breaking.
6. Dryer section
In the dryer section, water is removed from the paper web by evaporation. The aim is for evaporation to occur efficiently, economically, evenly, and without compromising paper quality. Methods commonly used in drying paper and board are contact or cylinder-drying, blow drying, and radiation drying.
7. Air systems
Air systems are used to manage the necessary air circulation (like ventilation and exhaust) in the paper machine and in the building that houses the machine. In the dryer section, the evaporated water is removed from the process using air. Air is also needed for a paper machine’s runnability systems, which enable the machines to run at high speeds and improve paper quality. Efficient noise attenuation and the removal of moisture and mist prevent environmental impacts at the paper mill. Good air quality at the mill not only contributes to a pleasant working environment, it is also an important factor in improving work safety.
8. Coating color and recycling
Coating improves the printability properties of paper. Different types of paper need different coatings, which are prepared in the coating color kitchen. The raw materials for the coating, typically clay, calcium carbonate and talc, are cleaned and stored in tanks. From there, they are pumped into the mixer. The ready coating color remains in the tanks until it is pumped to the coating machine. The chemicals used in preparing coating color are also recycled, i.e. they are recovered and reused.
The coating of paper and board improves the look and printing properties of the product. The coating color made in the coating color kitchen is spread on the surface of the paper during this phase of the process.
Calendering means the pressing that is done to modify the thickness, roughness and glossiness of the paper. Pressing the paper between the rolls gives it a glossy surface, and thus improves its printability properties.
The purpose of reeling is to transfer the planiform paper or board to a big machine reel for easier handling when winding.
A machine reel can be more than ten meters wide and can contain tens of kilometers of paper. A winder cuts the reels into smaller rolls according to customer specifications. A printing house can order the rolls they need according to diameter, weight or paper length.
13. Roll handling
The rolls are packaged in the appropriate wrapping paper to keep them clean, and then they are marked with an identifying code to facilitate transportation from the mill to the correct end user.