Wood may arrive at a pulp mill woodyard in the form of raw logs or as chips from a lumber mill. Some pulp mill operations have on-site sawmills (often called “woodrooms”) which produce both marketable lumber and stock for the pulp mill. Sawmilling is discussed in detail in the chapter Lumber. This article discusses those elements of wood preparation which are specific to pulp mill operations.
The wood preparation area of a pulp mill has several basic functions: to receive and meter the wood supply to the pulping process at the rate demanded by the mill; to prepare the wood so that it meets the mill’s feed specifications for species, cleanliness and dimensions; and to collect any material rejected by the previous operations and send it to final disposal. Wood is converted into chips or logs suitable for pulping in a series of steps which may include debarking, sawing, chipping and screening.
Logs are debarked because bark contains little fibre, has a high extractives content, is dark, and often carries large quantities of grit. Debarking can be done hydraulically with high-pressure water jets, or mechanically by rubbing logs against each other or with metal cutting tools. Hydraulic debarkers may be used in coastal areas; however, the effluent generated is difficult to treat and contributes to water pollution.
Debarked logs may be sawn into short lengths (1 to 6 metres) for stone groundwood pulping or chipped for refiner mechanical or chemical pulping methods. Chippers tend to produce chips with a considerable size range, but pulping requires chips of very specific dimensions to ensure constant flow through refiners and uniform cooking in digesters. Chips are therefore passed over a series of screens whose function is to separate chips on the basis of length or thickness. Oversized chips are rechipped, while undersized chips are either used as waste fuel or are metered back into the chip flow.
The requirements of the particular pulping process and chip conditions will dictate the duration of chip storage (figure 1; note the different types of chips available for pulping). Depending on fibre supply and mill demand, a mill will maintain a 2 to 6 week unscreened chip inventory, usually in large outdoor chip piles. Chips may degrade through auto-oxidation and hydrolysis reactions or fungal attack of the wood components. In order to avoid contamination, short-term inventories (hours to days) of screened chips are stored in chip silos or bins. Chips for sulphite pulping may be stored outside for several months to allow volatilization of extractives which may cause problems in subsequent operations. Chips used in kraft mills where turpentine and tall oil are recovered as commercial products typically proceed directly to pulping.
Figure 1. Chip storage area with front end loaders