Thermal Treatment of Western Red Cedar with Copper Naphthenate (cont)
Materials and Methods
Pole Material and Conditioning
Western red cedar (WRC) was selected as the specie of wood for thermal treatment since it is currently used in commercial thermal production with Penta in oil. Test specimens were cut from the top sections of WRC poles in the seasoned inventory yard, as well as from fresh material coming through the pole peeling operation. The samples ranged from about 7 to 9.5 inches in diameter and were cut to 8 feet in length.
To simulate the commercial butt treatment preparation, incising was performed on a section 1 foot from the butt end and extending 2 feet along the length of the pole to a height of 3 feet. Due to the small diameter and short run of the pattern necessary, this operation was performed with a hand-incisor hammer. This provided more control than would have been possible with the commercial incisor. These knife slits did adequately perforate the surface to a depth of 0.5 to 0.75 inch.
To condition the partially air-seasoned specimens additional drying was performed by loading these samples into a tunnel kiln with commercial poles to facilitate the completion of the drying process and provide a more consistent "dry pole" moisture level. These samples were identified as the dry condition material having an average oven dry moisture at 1 inch of less than 35%. Pole sections taken from the freshly peeled or incoming shipments recorded moisture contents ranging from 44-147% on an oven dry basis. This was considered to be representative of the normal, expected condition of non-kiln dried material that is typically acceptable for the plant's thermal process.
The kiln-dried poles were individually weighed for the first set of treatment samples, however the additional weights of the non-dry and combined (wet and dry) bundles were over the capacity of the scale, therefore all other sets were weighed as bundle units on the truck scale. The dry and wet bundles contained 7 pole sections in each set and the combined bundles contained 3 dry and 3 wet poles. All samples were identified according to a pole number and treatment letter code with metal tags nailed to the top end grain and face of each pole.
Preservative Solutions
Three different types of P-9 base oils were included in the test to evaluate the differences in processing, penetration and retention with the copper naphthenate preservative in this non-pressure process. All of these oil carriers met the AWPA P9-01 Type A requirements for solvent used in copper naphthenate solutions with oils A and C also meeting the required P9 penta solvency. Oil A was a commercially available wood treating oil having an aromatic content over 70%. Test oil B, a standard diesel fuel that is readily available for off-highway plant equipment, had a mid-range aromatic content of approximately 40%. This carrier oil did not meet the penta solvency, but that that is not a requirement for a solvent used in blending copper naphthenate solutions. Test oil C contained approximately 30% aromatics.
The copper naphthenate was a commercial 8% concentrate meeting the specified requirements of the AWPA P8-01 Standard for Oil-Borne Preservatives. The working solutions were blended by volume using the specific gravity of the oils to approximate a 1% copper metal concentration, by weight, in the mixture for each of the three oils. The solutions were mixed by adding the copper naphthenate concentrate to the oil in the hot tank and then heating and circulating with the pump for 1 hour. The temperature of the solutions at this point was between 140 and 160ºF. The transfer pump was used to circulate the solution between the two tanks to achieve a consistent mixture in both. Each treating solution was freshly prepared prior to treatment of that particular set of samples. The volume of treating solution for each batch was approximately 900-1000 gallons.
Samples of the treating solutions were taken from each of the bath vats prior to and following treatment to allow tracking of the copper content and identification of any preferential absorption or other unexpected changes in the solution. Sludging in the tank bottom was not monitored until completion of the treating with each candidate solution. Visual inspections could then be easily performed when removing the solutions from each tank. The solution analyses were conducted according to AWPA A5-00 Method 12 for % Cu, A13-96 for Total Acid Number (TAN) using potentiometric titration, and % sludge using BWLM #14.
Historical plant treating cycle information, as well as operator experience, were taken into consideration when defining the treating times and bath temperatures for the test material. Based on the retention and penetration data available from oil Penta thermal treatments, it was expected that the absorption would be in the range of 14-16 pcf (pounds per cubic foot) of solution in the assay zone for Western red cedar. Calculating a gauge retention level on this absorption rate and the 1% copper concentration in the solution, the AWPA 0.15 pcf Cu as metal assay retention could be obtained.
The pilot-scale thermal plant unit used for this test is illustrated in Figure 1. This unit was designed and constructed at the Arlington plant specifically for this project. The ability to safely transfer the poles into each of the tanks and an equally safe and quick placement into the cold bath to complete the cycle were critical aspects of its design. An electric winch-style lift was installed on an overhead beam to allow the bundle of poles to slide directly over both tanks and back to the loading/unloading area.
The poles (6 or 7 in a banded bundle) were lifted from the fork truck with a cable choker; moved into position over the hot bath; and then lowered until they were situated on the grating above the coils. The choker was then used to bind the bundle of wood to the side of the tank to prevent it from floating during the hot cycle. At this time any final adjustment in the depth of solution in the hot tank was made by adding or removing the preservative until the line of solution level was at 3 feet above the butt end of the pieces.
The processing cycle established by the plant required the hot tank temperature to be above 215º F before the time count could begin for the hot bath. The steam pressure lines were fully opened to the coils in order to begin a ramp up of the temperature toward this targeted 215º F minimum temperature. The amount of ramp up time varied, but was considerably longer for the first charges of each new preservative solution, due to the initial heating that was necessary to raise the temperature from the ambient and mixing levels. The first cycle had a preheating ramp-up time of 17 hours, but this cycle was effected by a high demand on the boiler steam from other larger systems of the plant that were heating at the same time. Subsequent hot bath temperatures were reached in as little as 2 or 3 hours, emphasizing the need to have the solution heated as closely as practical to the final treating temperature prior to addition of the poles.
The hot bath cycle was maintained between 215- 230º F for periods of 20-26 hours depending on the size and moisture content of the material being treated. At the completion of the prescribed hot cycle, the bundle was freed from the anchor binder; hoisted from the hot tank; moved horizontally over the cold bath and lowered into the cold bath solution. The bottom of the cold tank did not contain a grate, as there were no coils to protect, so the bundle sat directly on the tank bottom. The binder was used again to keep the material from floating in the bath and the solution depth was corrected to the previously marked depth of 3 feet from the butts.
The cold bath cycle was consistent among all treatments with a soaking time of 8 hours. The solution temperatures ranged from about 80-120º F at the onset, depending on the amount of hot oil transferred initially to correct the oil level, to a fairly consistent 60-85º F temperature at completion. The pole bundles were raised out of the cold bath and then suspended above the tank to allow drainage of any excess preservative from the surface and seasoning checks. Once the solution drips had stopped, the material was moved to the loading area, placed on the fork lift and yarded to allow penetration and retention cores to be taken.
One charge of poles was subjected to an expansion bath following the cold bath in an attempt to improve the dryness of the surface after treatment. The P9 high aromatic oil was selected for this experiment because of the high comfort level with the physical characteristics of this carrier based on past plant experience with its use in the penta treatments. This expansion bath step involved placing the poles back in the hot bath at a temperature of 190-210º F for 4 hours suspending the poles to drain the drips, and then yarding the material as was done with the other treatments.
Treating solutions were pumped into a tote for storage upon completion of the test charges. The tank bottoms could then be examined for any residue that may have formed or collected and cleaned if necessary prior to blending the next fresh solution.
Sampling After Treatment
Each pole had two cores taken in the incised zone in quadrants at 90 degrees from one another. This was done to meet the assay requirement in C35 of having no less than 12 cores from a charge, and to collect a representative sample from all poles in each treated set. It also provided a better evaluation for the consistency of penetration and amount of sapwood present in the treated material. The cores were measured for penetration and then cut into the 0.0 - 0.5 inch assay zone to determine the retention using various methods of analyses.
AWPA Standard A9-01, "Standard Method for Analysis of Treated Wood and Treating Solutions by X-ray Spectroscopy" were conducted using the settings for copper on the ACZA preservative treated wood calibration. The cores were dried under a heat lamp for 30 minutes, ground twice through a Wiley mill using a 10 mesh screen and analyzed using an ASOMA 8620 XRF.
These samples of prepared wood dust were also analyzed at the Merichem Laboratory in Alabama using XRF before being evaluated at the Houston using ICP (AWPA A21-00 Inductively Coupled Plasma Emission Spectrometry) and EDTA thiosulfate titration (AWPA A5-00, Method 12). The EDTA method was also used in the analyses of treating solutions for their copper concentrations.
A few selected poles from each test bundle were also cut through their cross sections in the incised and non-incised zones to view the penetration characteristics of each area of the pole. The remaining samples were placed on pole skids and yarded to observe any changes in the surface color or to identify preservative bleeding that might occur over time in storage.
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