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General Description

Stock preparation is a process that converts raw stock into finished stock (furnish) for the production of paper. The furnish is prepared for the paper machine by blending different pulps, dilution and the addition of chemicals. The raw stocks used are the various types of chemical pulp, mechanical pulp, and paper for recycling and their mixtures. The quality of the finished stock essentially determines the properties of the paper produced. Raw stock is available in the form of bales, loose material, or, in the case of integrated mills, as suspensions.

Stock preparation consists of several process steps that are adapted to one another and include fibre disintegration, cleaning, fibre modification and storage and mixing. These systems differ considerably depending on the raw stock used and on the quality of furnish required. For instance, in the case of pulp being pumped directly from the pulp mill, the slushing and deflaking stages are omitted.

Stock preparation removes impurities and air and conditions the strength properties of the fibres (refining). Chemicals are added to the process to affect the final quality of the paper sheet (resins, wet strength agents, colours, fillers). In non-integrated mills the fibres are normally received with around a 10 % moisture content. They are suspended in a pulper to create a suspension that can be pumped. Undissolved impurities are removed from the slurry by screening (screens) and cleaning (centrifugal cleaners). The objective of screening is the removal of interfering substances from the fibres. The acceptable fibre suspension is passed through a screen with apertures in the form of slots or round holes, while the impurities are separated and rejected by the screen. Furthermore, the fibre suspension is cleaned in a centrifugal cleaner (hydrocyclones). A distinction is made between heavy-particle and light-particle cleaners, depending on the purpose of separation. Most cleaners are multistage systems (up to five stages).

To improve the bonding ability of the individual fibres in order to strengthen the paper, refining is usually carried out. Refining is also done to condition the fibres to create the required properties of the finished product. Refining is carried out in refiners equipped, for example, with rotating disks (or rotating disks combined with stationary disks) between which the stock is treated. The space between the disks can be adjusted, depending on the degree of refining desired and to control the fibre properties. The electrical energy consumed in refining as part of the papermaking process is usually in the range of 10 kWh/t to 500 kWh/t for most papers but can be up to 3 000 kWh/t for speciality papers. Thus for a non-integrated paper mill using chemical pulp, refining represents the largest use of electrical energy (drying being the largest use of heat). Practically all of the energy input to this refining will be turned into heat (heating the process water) and there is no option here for energy recovery, although this heat generated contributes to the elevated temperature sought in the process.

Complete stock preparation for a paper machine usually consists of several lines where different raw stocks are prepared. The processing of broke from the paper machine is also part of this process. Finally, the pulp is pumped to the storage chests or mixing chests. These chests serve as a buffer between the stock preparation and the actual paper machine, to secure process continuity. In mixing chests, prepared stocks are mixed in proportions for the particular grade of paper to be made, the required additives are added and the required fibre consistency is adjusted. Storage chests or mixing chests might be substituted by advanced stock tower controls and in-line mixing.

Source: Best Available Techniques (BAT), Reference Document for the Production of Pulp, Paper and Board (2015)

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