At a Canadian mill, a new chip handling facility operates reliably in adverse weather to provide quality chips

Canadian Mill Installs Chip Facility For Reliable, Quality Fiber Supply

In addition, the mill closed its woodroom and converted to a 100% purchased wood chip fiber supply. Due to this, the mill became totally dependent upon a wide variety of sawmill operations in both local and remote locations. Because of the varying chip sources, the new wood chip handling facility was required to handle and process the expected swings in fiber quality coming from the different sources, as well as those due to seasonal changes.

Further, the new chip storage site had to be sufficiently large to allow for anticipated interruptions or reduction in wood chip delivery rates during winter storm periods or during the spring “break-up” period. Finally, the new system was required to deliver wood chips to the pulp mill reliably and with consistent quality.

The new chip handling operation has been in operation since 1996 and has experienced a wide range of operating parameters in adverse weather conditions. To date, the system seems to have met mill expectations in terms of the expected high level of reliability in feeding the TMP plant with a consistent chip quality, as well as in terms of fiber conservation and reduced operating and maintenance costs.

CHALLENGING MILL LOCATION. The mill is located in a region that is subject to strong seasonal winds, as well as heavy snowfalls with subzero temperatures during the winter months. Also, spring provides frequent fluctuating freeze/thaw periods with occasional freezing rain.

The new facility was to be located on a stretch of real estate adjoining the mill. This land was developed from dredged reclaim land, with a water table approximately three feet below nominal grade elevation. For this reason, the mill eventually chose a chip reclaiming system that would be installed totally above grade, without the need for below grade pits.

An aerial view of the mill’s total chip handling system, including chip receiving, stacking and reclaiming, and intermediate chip bin.

EVALUATION OF TECHNOLOGIES. The stacker/reclaimer system was one of the key elements in the mill’s new wood chip handling facility that required particular consideration and evaluation of available technologies. During the initial design phase, several viable chip storage and reclaim concepts were evaluated. These concepts included single or multiple screw re-claimers, either in open piles (circular and rectangular) or in silos (cylindrical and A-frame type). A radial stacker with a surface pile reclaimer was also examined.

Based on the size of chip pile required (90-ft-high by 440-ft-dia.) and the site specific operating conditions (both physical and climatological), the specific radial stacker/reclaimer system ultimately selected had already demonstrated reliable consistent operation in other Northern pulp and paper mill operations.

The reclaimer section of the new unit comes equipped with a patented feature enabling the removal and processing of frozen chip lumps (Figure 1). This feature consists of a series of hydraulically driven coarse disc screen shafts and lump breaker rolls.

In addition, the initial analysis confirmed that the selected system provided the most economical solution for this particular mill application, both from the point of view of capital investment and operating costs. The new chip storage and reclaim system would operate continuously, both in the stacking and reclaiming modes, without the requirement for mobile equipment intervention (100% live storage volume). Also, it would require mostly remote operator supervision with only periodic on-board operator control when moving the reclaimer to a new pile sector every three or four days.

TABLE 1: The following is a summary of the major equipment and its specifications.

FIGURE 1: Section A simply separates accept chips from frozen material. Through a series of disc screen shafts, Section B gently breaks the major lumps into smaller, acceptable pieces.

Finally, the interactive sequencing of the stacking and reclaiming operation, coupled with the more gentle reclaiming method from the surface of the pile, would minimize chip damage while presenting a consistently homogeneous mixture of reclaimed chips at the new screening facility.

The new truck unloading system would have to be designed to handle B-train rigs (approximately 5,000 ft³ volume) with coupled tractor configuration. It also had to be capable of unloading up to six trucks per hour. These facts established the requirement for a “drive-through” truck dumper installation with retractable backstop and a hydraulic pit bridge.

The volume of the receiving hopper/pit would also have to be sized for at least 1.5 trucks. The system would be capable of handling the high extracting rates with potentially frozen chips/lumps in the chip furnish, without blockages and/or interruptions to the unloading operations during the very demanding winter months. It was felt that the hydraulic stoker technology had been successfully demonstrated at other similar Northern mill installations on high capacity applications with a good maintenance history.

Finally, the mill opted for a state-of-the-art chip thickness screening system incorporating an amply sized (6.5-ft by 20-ft) and efficient fines screen that would be capable of handling the periodic high swings in fines and snow without blinding, given the sources of supply and weather conditions. For obvious reasons, all belt conveyors were designed to be installed in tubular truss-type enclosed galleries.

Table 1 describes in detail the equipment chosen by the Canadian mill for its woodyard upgrade. Except for the belt conveyors and galleries, all equipment was supplied by Rader Canada, a division of Rader Companies Inc.

A close up of the drive-through truck dumper, followed by two conveyors going to the radial stacker/reclaimer.

FLOW OF CHIPS. At the mill, trucks are unloaded at 34,000 ft³/hour Chips are transferred via an elevating chain conveyor followed by a belt conveyor with a cantilevered section that feeds the radial stacker. At the first transfer tower, a diverting chute can bypass wood chips from the truck unloading operation directly to the reclaim transfer belt conveyor that goes to the screening system at the TMP plant. The hydraulic stoker system at the truck unloading facility has the capability of reducing the extracting rate, in this bypass mode, to match that of the screening facility.

A close-up view of the radial stacker/reclaimer with infeed and transfer belt conveyors

The reclaim belt system as it travels to the building housing a scalping screen and intermediate surge bin. This is followed by a conveyor going to the thickness screening system located at the TMP plant.

The radial pile surface reclaimer system is designed to unload at rates up to a maximum of 12,000 ft³/hour. A belt conveyor transfers reclaimed wood chips to a transfer building where they are processed across a scalping disc screen for removal of minor frozen lumps, oversized wood, and contaminants. The processed chips are then stored in an intermediate cylindrical storage bin with 20 minutes to 30 minutes of retention capacity.

The bin floor extracting system features a unique full coverage live-bottom radial stoker mechanism identical to two other stainless steel pre-steaming bins supplied in the TMP plant by Rader. Wood chips are discharged from the intermediate bin at a remotely controlled rate by a variable speed metering screw conveyor located below the bin floor. They are then transferred by a belt conveyor gallery to the new TMP plant where the thickness screening system is located.

The screening facility consists of a DynaGage primary bar screen, a flexible mat Liwell screen, an air density separator, and a chip slicer. The accepts flow from the screening system and are fed to the atmospheric pre-steaming (APS) bin, APS #1, followed by the chip washing and pre-steaming processes in the TMP plant. The fines, or rejects from the screening system, are collected and pneumatically conveyed to the bark storage area of the mill.

To date, the system seems to have met mill expectations in terms of the expected high level of reliability in feeding the TMP plant with a consistent chip quality. Comparable results in operating history have been experienced with this wood chip handling equipment at other Northern installations, such as in Thunderbay, Ont.; Dryden,Ont.; and in Espanola,Ont. This may well establish the trend for the future of open chip storage pile systems that need to address 100% live storage capability in adverse weather conditions. Indeed, both for capital investment reasons and operating cost savings related to fiber conservation and reduced manpower and maintenance costs, this solution would seem very viable for larger storage pile applications both sides of the border.

Gordon Murray is design mgr. for the Materials Handling Corp. at Ajami, Bedard, Gagnon, and Sexton Inc. (ABGS) in Montreal, Que. Richard Goncalves is pres. and GM of Rader Canada in Montreal, Que., a subsidiary of Rader Companies.