It is well known that appropriately sized chips are quite important in the production of wood pulp. Briefly, in the pulping process, a digester, with the use of chemicals and elevated pressures and temperatures, breaks down wood chips into their constituent elements, basically lignin and cellulose (wood fibers). The cellulose is then processed to produce pulp.
Screening systems of various kinds have been used to correctly size the inflow of wood chips. Undersized chips, referred to as "fines" may be overcooked in the digester, which results in a lower pulp yield and the weakening of the pulp, while oversized (particularly overthick) chips are not broken down completely in the digester, and the remaining particles from the overthick chips must be removed at a later point from the pulp, increasing the expense of the process and reducing the overall pulp yield.
In the past, the sizing of wood chips has typically been based on the length and width dimensions of the chip, primarily width. However, the thickness dimension of the chip is currently regarded to be the most important dimensional consideration. Therefore, the chip screening process has been developed to separate chips based somewhat upon traditional length and width criteria, but primarily on thickness. Generally, for the purposes of this application, the term "sizing" will refer to the separation of chips based on thickness. The separation of chips according to size is also referred to hereinafter as fractionation, i.e. separating chip inflow into 1) chips within an acceptable predetermined size range (accepts), 2) chips which are smaller than the predetermined size range (fines), and 3) chips which are thicker than the predetermined range (overthick).
The publication of E. Christensen, in the May 1976 TAPPI Journal, Vol. 59, No. 5, discloses a chip sizing system which includes a gyratory screen in combination with a disk screen. The gyratory screen typically is a sheet member with openings therethrough of a particular size, while the disk screen comprises a number of parallel rows of interleaved, shafted-mounted spaced disks. The spacing of the disks primarily determines the size of the chip that will fall through the disk screen. The majority of the material which remains atop the disk screen is overthick. The disk screen has been found to be particularly useful in sorting chips according to thickness. In a typical situation, the predetermined chip thickness range is 2 mm to 10 mm, and for hardwood chips 2 mm to 8 mm.
An improvement to Christensen's system is described in U.S. Pat. No. 4,376,042 to Brown, titled "Chip Sizing Process", which is assigned to the same assignee as the present invention. In the '042 patent, a two-deck gyratory screen forming a first screening station is used to produce three fractions. At least 30% to 60% of the total chip flow is screened at a second screening station, comprising a disk screen, resulting in efficient processing of the chip inflow and a reduction in the capital cost of the overall system. The invention also permits process changes to be accomplished in a simple manner and at a relatively low cost.
Both the Christensen publication and U.S. Pat. No. 4,376,042 are hereby incorporated by reference. However, the system of U.S. Pat. No. 4,376,042 did from time-to-time result in an overrun of the capability of the disk screen, and in those systems involving a retrofit, the total capital expense of the system was still relatively high. Hence, there is a continuing need in the chip sizing portion of the pulping process for improved efficiency and capital cost reduction.