AU669356B2 - Treatment of timber - Google Patents

Description

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p 000 0 0p "DRYING AND PRESERVATION OF TIMBER" This invention relates to the drying and preservation of timber.

This invention has particular but not exclusive application to heating of timber for accelerated fixing a copper-chrome-arsenic preservative solution therein and for illustrative purposes reference will be made to such application. It is to be understood however that this invention is not limited to such application and could be used in other applications such as heating of timber for drying or other purposes such as forming to shape, or treatment of timber using dyes, modifiers and the like, or preservative other than copper-chrome-arsenic.

There are two intervals within timber processing 15 schedules where improvement to the heating methods have the potential to achieve advantages in terms of quality an/or production. These intervals are timber drying and preservative fixation. However, these heating intervals have significantly different requirements.

20 For example, "drying" timber is a function of the physical removal of the moisture, whilst "fixation" is a matter of completing a chemical reaction in a solution provided by the water incorporated within the timber.

Traditional timber drying is a well established process for preparing the product for market, and the best known 2 method is the process of kiln drying where the moisture content of the timber is reduced to some 10-15% by evaporation in a heated oven or kiln. The term or kiln dried is well known in the industry and the process generally improves the dimensional stability of the finished product.

Kiln drying is a conductive heat transfer process and all knowii conductive heat transfer processes suffer from the similar problems including:inconsistent heating due to the poor conductive nature of timber; equipment installations are required to be made on a large scale, requiring furnaces, heat exchangers, insulated lines, pumps or fans and the like; in order to maintain a reasonable heat flux through the timber, and prevent overheating of the external surface :constant re-wetting with steam is needed, and :overheating, or other disruption, at the external .e surface reduces the tangential fibre adhesion and cracking is likely to occur.

The treatment of timber or lumber to prevent biological degradation of the wood is a well known process involving the placement of timber to be treated in a pressure vessel and S applying cycles of vacuum and pressure whereby a preservative solution is impregnated into the wood to react with some of the constituents in the wood in an attempt to prevent biological and other breakdown of the timber.

3 A common form of treatment is the treatment of timber with a solution of copper, chromium and arsenic salts and using the cycles of pressure and vacuum. The process is commonly known as the Bethel process or CCA process. When properly applied, the timber so treated resists both fungal and termite attack.

The copper-chrome-arsenic group is one of the few preservatives which becomes "fixed" in the timber and will not water leach over time. The mechanism of this characteristic is embodied in a complex chemical reaction, or a number of reactions, between the components of the preservative and constituents of the timber, resulting in the formation of water insoluble salts.

The rate of reaction is temperature dependent and until 15 the reaction is complete, the free preservative retains its eooe chemical identity, is water soluble and will wash out of the timber by application of water. The chemical system is critical in the consideration of environmental matters, as removal of a pack of treated timber to unprotected ground before fixation is complete will result in land contamination, particularly if it rains.

Furthermore, much research is directed towards reaction acceleration by heating the chemical solution in the timber by conventional means such as described hereinbefore, which not only suffers from the same heating problems but also, where direct heating with a liquid carrier is employed, such 4 as hot water or oil in contact with the preserved timber, pollution of the carrier occurs requiring the carrier to be purified for re-use and/or disposal.

Regulatory bodies are insisting on paved and covered "drip pads" on which material can rest out of the weather before storage in unprotected area. The size of such "drip pad" areas is a function of the plant output and reaction time, and the land area required for drip pads can be extremely large where ambient temperatures are low and reaction time long. It is for this reason that research is being directed to devising some artificial method of accelerating the reaction.

The reaction of CCA or other preservative solutions with the cellulose and/or other constituents in timber often requires a fixed period of time before the solution is properly "fixed" in the timber. The fixed reaction time is mainly dependent upon temperature and may be significantly reduced by raising the temperature of the timber.

In temperate regions where the ambient temperature averages between 25°C and 30'C, the reaction time is usually of the order of 48 hours. In cooler climates, especially where the temperature of the air is close to O°C, the reaction time may be several weeks.

Methods have been disclosed wherein the temperature of .5 timber under treatment is increased by thermal conduction heat transfer techniques such as by immersing the timber under treatment in warm or hot water, steam or heated oil baths in order to decrease the reaction time. Such techniques are limited by such factors as the poor thermal conductivity of timber, high heat capacity of the moisture in the timber, and the limited maximum temperature to which the timber may be subjected. Due to the temperature gradient in the timber, these limitations often lead to timber being treated unevenly from the outside in, wherein the reaction has been completed on the outside of the timber but for regions more distant from the outside surface of the timber, the reaction may not have been completed.

Where timber in such a condition is retailed or otherwise distributed, there is potential for unfixed preservative solution to leach from the timber with the consequential environmental problems associated therewith.

The present invention aims to alleviate one or more of the above disadvantages and to provide an improved process S" for the treatment of timber which will be reliable and efficient in use.

20 With the foregoing in view, this invention in one aspect resides broadly in a method of treating moist timber, including the steps of:enclosing the timber to be treated within a chamber and evacuating at least some of the atmosphere from said chamber; introducing a reactant into said chamber at a pressure and for a time sufficient to substantially impregnate the timber; irradiating the timber with microwave energy whereby the temperature of the moist timber is increased, and maintaining the introduction of said microwave energy into said chamber at an energy flux and for a time sufficient for said reactant to react with and/or within the timber; releasing excess said reactant, and removing the timber from said chamber. The timber to be treated may be moist as a result of being recently felled timber or it may be moist as a result of being impregnated with a treatment solution such as a CCA or other reactant.

The irradiation may be carried out in a chamber enclosing the timber and the chamber may be used for the introduction of a reactant or for facilitating removal of moisture for the purpose of drying the timber.

Preferably where moisture results from the introduction of a reactant solution, the reactant is introduced in a separate operation and the irradiation is carried out in an irradiation chamber. It is also preferred that the irradiation is continued until the reaction is substantially complete. The reactant may be a gaseous or liquid substance which modifies a property of the timber, such as a colouring agent, dye, stain, preservative or such like. Preferably, the reactant is a preservative solution of CCA and the microwave energy is introduced during a Bethel type process for CCA treatment of timber. Alternatively, the timber may be treated with a reactant whilst being irradiated with microwave energy.

Suitably, the microwave energy is of a type which substantially penetrates the timber thereby providing a substantially uniform absorption of the microwave energy by the timber. The microwave energy may be in a frequency range of from 900 MHz to 3000 MHz. It will be appreciated that 7 alternative frequencies which are absorbed by water may be utilised.

In another aspect, this invention resides broadly in a method of treating timber, said method including:impregnating the timber with a reactant; irradiating the timber with microwave energy whereby the temperature of the reactant impregnated into the timber is increased, and maintaining the irradiation of microwave energy at an energy flux and for time sufficient for the reactant to react with and/or within the timber.

The method of treatment may be achieved by enclosing the timber within a chamber, evacuating at least some of the atmosphere from the chamber, introducing microwave energy to 15 the chamber whereby the temperature of substantially all of oe o the timber is increased, introducing a reactant into the *4 chamber at a pressure and for a time sufficient to *e substantially impregnate the timber, maintaining the eo introduction of microwave energy into the chamber for a time sufficient for the reactant to react with and/or within the timber, releasing the excess reactant, and removing the timber from the chamber.

It will be appreciated that the microwave energy may be of a type which substantially penetrates the timber and excites, promotes, or initiates a reaction within the timber.

Moreover, microwave activated catalysts, initiators and the 8 like may be utilised if desired. The reactant may be a gaseous or liquid substance which modifies a property of the timber, such as a colouring agent, dye stain, preservative or such like. Preferably, the reactant is a preservative solution of CCA and microwave energy is introduced subsequent to or as a part of a Bethel type process for CCA treatment of timber.

Additionally, the microwave energy may be introduced or directed towards and within the timber at intervals, and/or at alternative sequence steps in the Bethel process. In addition thereto, microwave energy may be used to preheat timber prior to the application of the Bethel process thereto, Alternatively, or in addition to, or included within the above, the microwave energy may be introduced a varying levels, including ramped, stepped, spiked, pulsed or c modulated levels.

It is generally accepted that microwaves interact with all materials and, by their nature, will cause a response in the material dependant upon the microwave spectrum of the 20 material. Absorption of the microwave energy by absorptive *0 *..material wherever it is located in an irradiated zone results in a rise in temperature dependant further upon the strength e or power in the microwaves penetrating the material.

In the preferred embodiment, the microwave energy is of "-25 a type which is absorbed by water. It is believed that where *r such microwave energy is utilised, the timber fibers 9 themselves are not heated by the microwaves and therefore any heating of timber fibers will only occur by conduction from the water, whether such heated water is liquid or in the form of steam which may be saturated steam or superheated steam.

It is also believed that the use of microwave energy will heat the water combined in the cell walls, increasing the rate of reaction of the preservative solution where it is most effective. The method of this invention may be designated as an integral part of the treatment system, or may be extended to post treatment drying once fixa-ion of a preservative solution is complete or further as an integral part of the treatment process.

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S. It is believed that in the penetration of microwaves into a material, the intensity decays exponentially with respect to the depth of penetration. Surprisingly, however, tests have shown that the temperature gradient from the outside to the centre of a round piece of timber heated by microwaves includes an increase from the outside to the centre. Furthermore it is believed that this feature would 20 avoid the problem of crack initiation during drying.

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It is also believed that attempting to apply vacuum to a traditional kiln would result in the both the loss of heat in the material evacuated and loss of heat transfer medium and therefore the loss of thermal efficiency. However, the application of microwave energy to an evacuated kiln would not have the same disadvantage because the microwave energy will penetrate such a vacuum.

As the microwaves generate heat substantially only in the water in the timber, it is believed that removal of the water by vacuum would increase the energy efficiency as the water would be removed as water vapour, and possibly at a lower temperature, and these two methods may be used in conjunction to synergistically provide superior results.

This invention in another aspect resides in a method of drying timber, said method including:providing a substantial vacuum within which the timber may be placed, and irradiating said timber with microwave energy. It is believed that the use of microwave energy for timber drying 15 requires no heat transfer through the timber for water g removal and therefore will not damage timber fibers.

V In order that this invention may be more readily understood and put into practical effect, reference will be made to the accompanying drawing which illustrate a preferred embodiment of the invention, and wherein:- FIG. 1 is a schematic representation of a typical ~process plant for carrying out the method of the invention; FIG. 2 is a graphical representation of the drying characteristics for untreated slash pine when treated according to an example of this invention, and 11 FIG. 3 is a graphical representation of the temperature profile of a log when treated according to another example of this invention.

Referring to the FIG. i, a timber treating apparatus includes a preservative impregnation system 11 and a fixing and drying system 12. The preservative impregnation system 11 includes an impregnation chamber 11 operatively connected to a vacuum system 16 and a pressure system 13 having a preservative flood-line 14.

The fixing and drying system 12 includes an irradiation chamber 20 having an evacuation system 19 connected thereto.

The irradiation chamber also has microwave generators 21 installed in a manner which permits microwaves to be generated inside the irradiation chamber In use, a timber charge 17 is inserted into the impregnation chamber 15 and/or the irradiation chamber upon a timber trolley 18. The timber charge 17 is first impregnated with preservative in the preservation impregnation system 11 and transferred to the fixing and 20 drying system 12 immediately after treatment in the o preservative impregnation system 11.

ri.: A suitable drip pad is provided to retain and/or recycle excess preservative recovered from the preservative impregnation system 11 to the fixing and drying system 12.

Preservative may also be recovered and/or recycled from the irradiation chamber 12 In a preferred method of this invention, a log a plurality of timber logs, is prepared for processing by stacking the log or logs on a truck for ease of transport through the process plant. The charge is comprised of moist, green timber.

The charge is impregnated with CCA solution according to the well known Bethel process and transported to the irradiation chamber. Alternatively, the charge by-passes the Bethel process and is transported directly to the irradiation chamber.

The charge is then passed into the irradiation chamber on the truck and irradiated with microwave energy at a S. frequency of from 900 to 3000 MHz for a selected time whereby the temperature of the charge is raised. The charge so a 15 treated is then removed from the irradiation chamber and, in the case where the charge was impregnated with CCA solution, the charge is stored on a drip pad or CCA collection area until the CCA solution becomes fixed in the charge by the fixation reaction. The charge is then removed from the drip 20 pad and placed into storage ready for sale or distribution pu poses. In the case where the charge was not impregnated with the CCA solution, the storage of the charge on the drip pad is by-passed.

The timber drying apparatus 10 may be arranged so that successive timber charges 17 are treated simultaneously in 13 turn in each of the preservative impregnation system 11 and fixing and drying system 12.

In order that this invention may be more readily understood and put into practical effect, reference will now be made to the some examples of the process of this invention.

Samples of logs, some treated with CCA according to the well known Bethel process, and some being green, untreated logs were irradiated as described in tests following a preferred method according to the present invention. The treated logs ranged from small, being approximately 500 mm long by approximately 100 mm in diameter, to large, being approximately 590 mm long and approximately 150 mm in diameter. The untreated logs were approximately 500 mm long .,15 and approximately 100 mm in diameter. It will be appreciated that the logs provided had a diameter larger at one end than the other.

The final moisture of a sample may be determined in terms of its final mass by the following equation, 20 hereinafter referred to as Equation 1: M =W(1 M) 1 Wi 25 where: M moisture content (DB fraction),

M

i initial moisture content (DB fraction), W final mass of sample (kg) and

W

i initial mass of sample (kg).

The theoretical maximum moisture removal rate for a given amount energy may be calculated using enthalpy tables 14 for steam. Assuming that an initial water temperature of 0 C and that the water evaporates at 100 0 C, it follows from the following, hereinafter referred to as Equation 2, that: 3600 kJ/kW-hr 6 max 2676.1 62.99 kJ/kg 1.38 kg/kW-hr; where it is assumed that: hf 62.99 kJ/kg at 15°C and hg 2676.1 kJ/kg at 100°C.

From Equation 2, it follows that the process efficiency, expressed as a per centage, is: E 6/1.38 100%, hereinafter referred to as Equation 3.

Tests of the process of the present invention were conducted using a MARC 40 kW Microwave apparatus. The :.:apparatus was set up as heating/drying apparatus according to this invention and incorporating bulk chokes and the testing proper was performed using a batch process. The test samples were supported above a conveyor belt by microwave inert supports and then conveyed on the belt into the resonance cavity of the apparatus. All of the vertical sliding gates 25 on the bulk chokes were closed before the microwave power was delivered to the resonance cavity.

Whilst the samples were subjected to the microwave irradiation, the conveyor belt was slowly moved through the cavity to simulate a continuous process and to aid the distribution of energy within the samples. After a preset resonance time during which the samples were in the cavity, the irradiation was turned off and the samples removed from the cavity for inspection and measurement.

Tables 1 to 4 presented below show a typical set of test results for slash pine untreated, where the aim was to reduce the moisture content of the timber to 30%, calculated on a wet basis (WB).

Using a Sharp 1300 Watt domestic type microwave oven, a sample of untreated slash pine with a mass of 818 g 60% WB was irradiated to give the mass profile 25 shown in FIG. 2.

The untreated sample tested in the Sharp 1300 Watt microwave oven yielded a drying rate of 1.38 kg/kW-hr which was close to the theoretical maximum drying rate and corresponds to a drying energy efficiency of 98%. During the test, it was observed that most of the moisture escaped through the cut ends of the sample. After 16 minutes and seconds of resonance time, the sample started smoking, with smoke emerging mostly from the knots along the length of the S"timber sample and the centre of the cut ends, as shown at time 30 in FIG. 2.

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20 Dissection of the sample revealed that although the sample was not completely dried, the centre was ri preferentially dried due to what is believed to be the characteristic of microwave heating. The effect on this sample caused charring at the centre of the log.

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S 35 40 o f 16 TABLE 1 Run Number Initial Observations Total Power kW Microwave Power kW Magnetron Current mA 1 2 3 4 RESONANCE TIME min AMBIENT Temp °C PRODUCT TEMP Initial OC Final °C CAVITY AIR FLOWRATE kg/s PRODUCT Quantity 4 Logs Slash Pine Untreated OBSERVATIONS Log Dimensions (mm) Mass Length Dia. (kg) Initial moisture as A 502 105/105 4.300 determined in a convection B 490 96/99 3.600 convection oven at 105 0 C; C 480 93/96 3.300 150% Dry Basis D 510 90/96 2.800 Wet Basis Holes were drilled in Log B from the centre out to determine a temperature profile a.

a 35 a 40 17 TABLE 2 Run Number 1 Total Power kW 13.1 kW Microwave Power kW 6.2 kW Magnetron Current mA LEFT BANK ONLY 1 400 mA 2 400 mA 3 400 mA RESONANCE TIME min 12 minutes (Belt moving slowly) AMBIENT Temp 0 C PRODUCT TEMP Initial OC approx. Final °C 50'C to CAVITY AIR FLOWRATE kg/s 0.27 kg/s PRODUCT Quantity 4 Logs Slash Pine Untreated OBSERVATIONS Log Mass Mass Moisture Loss D.B. W.B.

Initial moisture as A 4.250 0.050 147% 59.5% determined in a convection B 3.500 0.100 143% 57.2% convection oven at 105*C; C 3.200 0.080 144% 59.0% 150% Dry Basis D 2.680 0.120 139% 55.7% 60% Wet Basis Log B Profile Centre 880C Knot 53°C 87oC 86oC Edge *5 18 TABLE 3 Run Number 2 Total Power kW 13.1 kW Microwave Power kW 6.2 kW Magnetron Current mA 1 400 mA 2 400 mA 3 400 mA 4 RESONANCE TIME min 15 minutes (Belt moving slowly) AMBIENT Temp 0 C PRODUCT TEMP Initial oC 50 0 C to Final °C 75°C to 100°C CAVITY AIR FLOWRATE kg/s 0.27 kg/s PRODUCT Quantity 4 Logs Slash Pine Untreated OBSERVATIONS Log Mass Mass Moisture Loss D.B. W.B.

Initial moisture as A 3.900 0.350 127% 55.9% determined in a convection B 3.180 0.320 120% 54.7% convection oven at 105 0 C; C 2.810 0.420 113% 53.0% 150% Dry Basis D 2.310 0.370 106% 51.5% 60% Wet Basis Log B Profile Centre 100.6°C Knot 99.8°C 99.3oC 96.5 0

C

Edge 88.9 0

C

4 5 *I S o 0 e•0 19 TABLE 4 Run Number 3 Total Power kW 13.1 kW Microwave Power kW 6.2 kW Magnetron Current mA 1 400 mA 2 400 mA 3 400 mA RESONANCE TIME min 20 minutes (Belt moving slowly) AMBIENT Temp 0 C PRODUCT TEMP Initial oC approx Final 0 C 50 0 C to 90 0

C

CAVITY AIR FLOWRATE kg/s 0.27 kg/s PRODUCT Quantity 4 Logs Slash Pine Untreated OBSERVATIONS Log Mass Mass Moisture Loss D.B. W.B.

Initial moisture as A 3.250 0.650 89% 47% determined in a convection B 2.470 0.710 72% 42% convection oven at 105 0 C; C 2.310 0.500 75% 43% 150% Dry Basis D 1.700 0.610 52% 34% 60% Wet Basis Log B Profile Centre 91 0 C Knot 78°C 94°C 95oC 90 0

C

Edge The drying curve 30 shown in FIG. 2 illustrates that the majority of the water is free moisture.

The samples tested in the 40 kW microwave oven yielded the results shown in TABLES 1 to 4. The heaviest log, log A, dried the least whilst the lightest log, log D, dried the most. However, each lost substantially the same amount of water.

Log A lost 1.050 kg water, log B lost 1.130 kg water, log C lost 0.990 kg water and log D lost 1.100 kg water.

Run 1 in TABLE 2 exhibited the highest temperature at the centre of the log. On completion of Run 2, the centre of the log reached 100°C. It is believed that at that point, the temperature and pressure gradients were most favorable for drying of the log. After Run 3, the highest temperature 15 of the log was midway between the centre and the outer circumference. This would suggest that the centre of the log V.."was drier than the outer part of the log.

S' A second test was conducted in the 40 kW drier using two large treated logs. A reduced drying rate was observed due 20 to the increased log diameter. Similar drying curves were observed for each sample tested. Reduced microwave penetration to the centre of the logs was observed for the larger diameter logs. In reference to FIG. 3, a temperature profile 36 of one of the larger treated logs 35 shows that the temperature did not increase all of the way into the centre of the log. The temperature profile 36 shows the 21 temperature for the log 35 taken at various points from within the log 35. The temperature at 51 is at the centre 41, the temperature at 50 is at an off-centre 40 beyond the centre 41 from a mid-point 42 and the edge 43 which correspond to the temperatures 52 and 53 respectively. The temperature 54 at knot 44 in the log 35 was also measured.

After a three day period where the samples were stored in ambient air, no form of checking or drying cracks were observed on the surfaces of the samples. Some minimal checking was observed on the edges of some the samples.

Moreover, a higher power level of microwave irradiation did not produce any differences in the amount of checking of the ool samples.

It will of course be realised that while the above has 15 been given by way of illustrative example of this invention, all such and other modifications and variations thereto as would be apparent to persons skilled in the art are deemed to fall within the broad scope and ambit of this invention as set forth in the following claims.

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Claims (6)

1. 2. A method as claimed in claim i, wherein the reactant is introduced in a separate operation and the irradiation is SS carried out in an irradiation chamber.
2. 3. A method as claimed in claim 1 or claim 2, wherein said microwave irradiation is continued until the fixation reaction between the reactant and the timber is substantially complete. 23
3. 4. A method as claimed in claim 1 or claim 2, wherein said reactant is a CCA solution which undergoes a fixation reaction with the timber and sufficient irradiation introduced to raise the temperature of the timber such that said fixation reaction is substantially complete within a desired time. A method as claimed in any one of the preceding claims wherein at least some of said microwave energy heats water water combined in cell walls of the timber.
4. 6. A method as claimed in any one of the preceding claims, :wherein said microwave energy is introduced during a Bethel type process for CCA treatment of timber. .i7. A method as claimed in any one of the preceding claims, wherein said microwave energy is in the frequency range of 0 M t 7 A method as claimed in any one of the preceding claims, wherein the absorption of said microwave energy is in the frequencrgy range of :900 MHz to 3000 MHz. S
5. 8. A method as claimed in any one of the preceding claims, :..wherein the absorption of said microwave energy provides a temperature gradient inside the timber which includes an increase from the outside of the timber to the centre of the timber.
6. 9. A method as claimed in any one of the preceding claims and including post treatment drying of the timber once fixation of said reactant is complete. A method as claimed in any one of the preceding claims wherein said method includes drying of the moist timber. DATED THIS TWENTY-NINTH DAY OF FEBRUARY 1996. LOGFORM INDUSTRIES PTY. LTD, ACN 010 212 049 by PIZZEYS 0 e *0 0 0 *o*