Wood preservation facilities, 2013 technical recommendations: chapter 2
2. Overview of Wood Preservation Facilities
2.1 The Canadian Preservation Industry
There were 55 wood preservation facilities operating in Canada in 2011. All facilities had pressure treatment plants and three facilities employed both pressure and thermal treatments. Many CCA facilities migrated to ACQ or CA-B when CCA was withdrawn from use in residential application in 2003. In 2011, 30 facilities used CCA and only 16 used it as their sole preservative. ACQ and/or CA-B were used by 33 facilities, while 11 facilities used them as their sole preservative (10). Only 10 facilities were using oil-borne preservatives, creosote and/or PCP. In 2011, 23 plants were involved in multi-preservative operations, compared to 13 in 2000 (10).
Treatment facilities exist in all provinces except Prince Edward Island and Labrador. The early facilities were conveniently located to serve the railways. However, newer facilities are concentrated in areas where there is great demand for consumer lumber, which represents more than 50% of the total industry output (11). In 2011, the provinces with the most active facilities are British Columbia with 14, Ontario with 12 and Quebec with 11 (10).
2.2 Description of Current Plant Designs
2.2.1 General Plant Designs
Wood preservation facilities generally consist of four components (11):
- yards for storage of untreated and treated wood;
- wood processing facilities (peelers, framing lines, kilns, etc.);
- impregnation facilities; and
- offices, laboratory space.
The size of storage yards can vary significantly depending on the facilities’ treatment capacity and the manner of drying the wood. Air seasoning requires a large storage space. However, facilities that process wood, particularly for the residential market, may employ kiln drying, which requires less white wood inventory space. The storage cycle of treated wood is generally short, necessitating only a limited yard or shed area. Facilities that provide storage for their customers--for example, the major railways and utilities--are an exception.
Wood processing equipments may include pole peelers, saws, framing lines, sorting tables, incisors, kilns and stackers. Railway tie plants are equipped with special boring and incising machines.
The designs of impregnation facilities are specific to the treatment process employed and the preservatives used. A more detailed description can be found in the relevant preservative-specific sections.
2.2.2 Preservation Processes
Preservation processes are aimed at injecting requisite amounts of preservative liquids deep into the wood to provide long-term protection against wood destroyers. In North America, the majority of preserved wood is treated by pressure impregnation processes. Thermal treatments are of secondary importance.
The applied treatment parameters for all processes are limited by the directions for use on the registered pesticide labels. The CAN/CSA O80 (12) also has retention and process standards to ensure effective treatments for specific uses without damage to the wood. The pesticide label is the legal document and should be consider as such in the event of discrepancy between the standards.
Also, special requirements are contained in the Best Management Practices for the Use of Treated Wood in Aquatic and Other Sensitive Environments issued by Western Wood Preservers Institute, Wood Preservation Canada, the Southern Pressure Treaters’ Association and the Southern Forest Products Association. The purpose of the Best Management Practices (BMP) is to place enough preservative into a product to provide the needed level of protection while also minimizing use of the preservative above the required minimum industry standard to reduce the amount potentially available for movement in the environment (13). These BMPs were developed in order to reduce the amount potentially available for movement into the environment.
Wood Conditioning
Before wood can be successfully impregnated with preservatives, the bark has to be removed and the moisture content reduced by a process involving drying or conditioning. This may be achieved by air seasoning, kiln drying or by a process carried out in the treatment cylinder, for example, a steam/vacuum process or boiling-under-vacuum (Boultonizing) in the presence of the treating solution. The method chosen depends on the wood product, specifications, the available equipment, desired moisture levels and the preservative used. For example, kiln drying is most common for lumber destined for the residential market; air seasoning is most economical for large commodities, such as ties, timbers and poles; a steam/vacuum process is preferred for poles to be treated with PCP/oil; and Boultonizing is common with ties and marine pilings to be treated with creosote or creosote/oil solutions.
Sawn wood, which generally exposes refractory heartwood, requires “incising” to enable good preservative penetration. Incising is a process whereby the wood surfaces are punctured by toothed rollers. Various incising patterns are available to ensure good penetration without causing undue structural damage. Individual pieces are generally cut to final size and shape prior to treating to ensure good preservation of all exposed faces. Machining after treatment may expose untreated wood, in which case subsequent field treatments must be applied. Even properly applied field preservation cannot protect such exposed wood as effectively as either pressure or thermal treatments.
Full-cell (Bethell) Process
The full-cell process was introduced in 1838. It is the only process employed for treatments with CCA and the other water-borne preservatives, as well as for creosote (Figure 1).
Figure 1 The Full Cell (Bethell) Pressure Treatment Process
Text description
Shematic representation of the full cell pressure treatment process named the Bethell process. The representation includes a graphic explaining the 9 steps sequence of pressure and vacuum applied through a 180min elapsed time. The 9 process steps are:
- Initial vacuum
- Fill with preservative
- Pressure increase
- Pressure impregnation cycle
- Pressure release
- Preservative pump-out
- Final vacuum
- Vacuum release
- Treated wood discharge
The figure 1 also includes a schematic representation of treatment in the vessel being filled and pumped out of preservatives.
After a wood charge is placed into the pressure cylinder, the treatment process commences with the application of an initial vacuum for 30 minutes to an hour. The preservative solution is then admitted to the cylinder, while maintaining the vacuum. In case of the water-borne preservatives, the solution is at ambient temperature, whereas oil-borne preservatives are heated (70 to 90°C). After the cylinder is filled, pressure is applied, usually to a maximum of 1040 kPa, and held until a predetermined amount of preservative has been injected into the wood. This pressure cycle may take from 30 minutes to several hours. At that point the pressure is released and the excess preservative is returned to a storage tank for use on subsequent treatments. The impregnation stage is usually followed by a final vacuum in the case of CCA and the other water-borne preservatives or an expansion bath and a final vacuum in the case of creosote. These processes remove excess preservative from wood subsurfaces and are aimed at rendering the product surfaces as dry as possible.
Empty-cell Processes
This category includes two processes, the Rueping and the Lowry, both of which are used with creosote and pentachlorophenol for treatment of utility poles, railway ties, posts and construction lumber, and timber. The processes are designed to give deep penetration, while minimizing preservative retention (Figure 2).
Figure 2 The Empty-Cell (Rüeping) Pressure Impregnating Process
Text description
Shematic representation of the empty cell pressure treatment process named the Rueping process. The representation includes a graphic explaining the 9 steps sequence of pressure and vacuum applied through a 200min elapsed time. The 9 process steps are:
- Initial pressure
- Fill with preservative
- Pressure increase
- Pressure impregnation cycle
- Pressure release
- Preservative pump-out
- Final vacuum
- Vacuum release
- Treated wood discharge
The Rueping process applies an initial air pressure (200-500 kPa for 15 minutes) to the wood charge in the cylinder prior to admitting the preservative. The pressure compresses the air inside the wood. Hot preservative is then admitted to the wood without releasing the air pressure. The pressure is increased to a typical maximum of 1040 kPa and held until predetermined solution absorption has been achieved. When the pressure is released at the completion of the impregnation cycle, the compressed air in the wood expands and expels excess preservative. This effect, which is called the “kickback,” is usually enhanced by a quick final vacuum. Excess preservative is returned to storage for use in subsequent treatments.
The Lowry process is similar to the Rueping process, except that no initial air is applied and the preservative is admitted at atmospheric pressure. The remainder of the process continues in the same manner as the Rueping process. There is usually a smaller amount of preservative recovered by the kickback in a Lowry process.
Thermal Treatment Process
This process is applied with PCP/oil solutions for the pole butt treatment of dry utility poles.
A pressure vessel is not required to carry out the process. The lower ends of poles (butts) are impregnated with preservative in upright, open-top tanks. During the cycle, dry wood is first immersed in hot preservative (88 to 113°C) for a minimum of six hours (hot bath). Thereafter, the hot preservative is quickly replaced by cooler preservative for at least two hours (cold bath). Excess preservative is returned to the storage tank.
After-impregnation Processes
Treatment cycles are followed by a final vacuum, which equilibrates internal pressure, removes air and preservative from the surface fibres of wood and, in the case of oil-borne treatments that use elevated temperatures, cools the wood. For creosote and PCP, a final vacuum may not be adequate to create clean surfaces. In these cases, the impregnation cycle may be followed by an expansion bath or a final steam cycle, both of which add a final vacuum step. These processes can be quite effective, but the final steam cycle creates large volumes of contaminated water that must be treated to meet all discharge criteria.
Storage After Treatment
Treated wood, removed from the cylinder, is stored on a drip pad until preservative drippage has stopped. The duration of this storage may vary from hours to days. Alternatively, most CCA treatment plants now carry out an accelerated fixation process to ensure that the preservative chemicals are highly leach resistant. Such a process entails a heating cycle, usually in the presence of high humidity. Fixation chambers are employed or the process may be carried out in drying kilns (14). When fixation has been verified, the treated wood may be transferred to a designated yard area for storage until shipment or it may be directly loaded for immediate shipment.
2.2.3 Current Treatment Plant Designs
Water-borne Preservation Plant Designs
Water-borne preservation plants, like CCA or ACQ, are housed within a heated building (11). Figure 3 is a schematic view of a typical CCA plant. The major difference from other water-borne preservation plants, as shown in the figure, is the presence of a fixation chamber. The pressure vessel, also called a retort or cylinder, is commonly 1.8 m in diameter and 24.4 m long. The wood is normally charged and discharged through a single door by means of trams that run on tracks. Other designs use conveyors to move wood in and out of the cylinder and may involve doors at either end to insert and remove the wood. Pumps are provided to apply process conditions (i.e. vacuum or pressure) as well as to transfer liquids from and to the cylinder and between tanks. A tank farm typically includes a concentrate tank, one or more tanks for working solutions, and an effluent recovery tank or makeup water tank. The process controls and instrumentation vary in sophistication, depending on the degree of automation. Most water-borne preservation plants have systems that are fully automated to control the impregnation process parameters. A number of plants have heated storage areas for recently treated wood and facilities for accelerating the fixation or stabilization of the preservative components in the treated wood (14, 15).
The full-cell treatment process, used to apply the preservative in water-borne treatment plants, consists of the following steps:
- application of an initial vacuum to remove air from the wood cells;
- flooding with working solution and pressurization (up to 1040 kPa) until the target retention level is achieved;
- draining of the excess working solution (to the working tank for reuse with subsequent charges); and
- application of a final vacuum.
The specific treatment times and pressures are dictated by the species of wood, the type of wood product (e.g. plywood or poles) and the moisture content of the wood. A predetermined range of process parameters is defined by the applicable treatment standards (12), and quality control tests are carried out to ensure that the product meets the minimum quality standard. Once the treated wood is withdrawn from the treating cylinder, it is either subjected to a fixation process or stored on-site for periods that generally range from days to months for stabilization. The fixation and stabilisation process requires a sealed drip pad and a roofed area. This is essential, since the preservatives are water soluble and would leach at various concentrations when exposed to precipitation with the exception of CCA that is leach resistant due to the fixation process
Paved or concrete flooring often have roofs over all or portions of the treated wood storage area to reduce or eliminate exposure to the elements.
Figure 3 Conceptual Drawing of a CCATreatment Facility
Text description
Conceptual drawing of a typical CCA pressure treatment facility including the delivery, the storage of the CCA, the charging area, the retort cylinder, the fixation chamber and the treated wood storage area.
Creosote and PCPPlant Designs
Creosote and PCP/oil solutions are often used interchangeably in the same treatment facility. Hence, plants using these preservatives have a similar layout (Figure 4). The pressure cylinders are usually somewhat larger than those used in water-borne preservation plants (2.1 m in diameter and 36.5 m in length). Tank farms are generally placed outdoors, and tanks are equipped with internal heating devices. The production equipment, including the cylinder, pumps, condensers, controls and effluent treatment systems, is housed within a treating house. Facilities treating with PCP or creosote solutions require a heat source for warming the preservative and to carry out specific processes, such as steam conditioning. When treating with PCP, either an autoclave or a designated mix tank is used to dissolve the solid preservative in a suitable oil solvent. Effluent treatment facilities may consist of an oil/water separator, a flocculation system and carbon filtration. An air filtration system to collect exhausts from treatment vessels, vacuum systems and tank vents may also be part of the installations. The vacuum systems are equipped with condensers and condensate collection tanks.
Thermal Butt Treatment Plant Designs
As Figure 5 shows, thermal butt treating plant facilities have less sophisticated impregnation equipment and process controls than pressure treatment facilities. The treatment vessel is a vertical open-top tank, which should be provided with spill containment. The tank farm contains a PCP/oil mix tank, as well as oil storage and hot and cold preservative solution storage tanks. Transfer pumps are used to remove solution from the treatment vessel and transfer it to the storage tanks. The effluent treatment system may involve oil/water separation, flocculation and carbon filtration.
Figure 4 Conceptual Drawing of an Oil-Borne Pressure Treatment Facility
Text description
Conceptual drawing of a typical oil-borne pressure treatment facility including the delivery, the storage of the preservative, the charging area, the retort cylinder and the treated wood storage area.
Figure 5 Conceptual Drawing of an Oil-Borne Thermal Treatment Facility
Text description
Conceptual drawing of a typical oil-borne thermal treatment facility including the storage of the preservative, the treatment tank and the curbed drip pad for freshly treated wood storage area.
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