Microwave pretreatment of logs for use in making paper and other wood products

Abstract

A method of producing pulp for use in making paper products using microwave radiation to pretreat the source of pulp prior to further processing. Practicing the method of the invention results in substantial energy savings while decreasing environmental impact and improving paper qualities.

Description

CROSS REFERENCE TO RELATED APPLICATIONS [0001] This application is a continuation of U.S. application Ser. No. 10 / 494,396 filed May 3, 2004 which is a filing under 35 U.S.C. § 371 based on International Application No. PCT / US02 / 36443 filed Nov. 12, 2002 which claims the benefit under 35 USC § 119(e) of U.S. Provisional Application No. 60 / 347,818 filed Nov. 9, 2001, the applications being incorporated herein by reference, in their entirety.BACKGROUND OF THE INVENTION [0002] In the manufacture of paper from wood, the wood is first reduced to an intermediate stage in which the wood fibers are separated from their natural environment and transformed into a viscous liquid suspension known as a pulp. There are several classes of techniques which are known, and in general commercial use, for the production of pulp from various types of wood. The simplest in concept of these techniques is the so-called refiner mechanical pulping (RMP) method, in which the input wood is simply ground or abra...

AI Technical Summary

Benefits of technology

[0059] Thus, an additional advantage of the invention is a reduction in amounts of bleaching chemicals required during bleaching. This, in turn, increases the opacity of the resulting paper and reduces the effluent treatment costs associated with paper production. TABLE 2Brightness responseBrightnessTreatments(% ISO)Control1Initial Brightness59.61.5% Hydrogen Peroxide + 1.5% Sodium Hydroxide71.42% Hydrogen Peroxide + 2% Sodium Hydroxide73.6Treatment2Initial Brightness55.51.5% Hydrogen Peroxide + 1.5% Sodium Hydroxide73.4

[0060] Objective: To achieve electrical energy savings and improvements in paper strength by microwaving pine logs prior to mechanical pulping.

[0061] Materials: Pine logs were received from a mill specializing in the production of light weight coated paper. Logs were microwaved at Microdry in Louisville, Ky. Logs were debarked and chipped to a nominal size of 6-14 mm. Chips were placed in plastic freezer bags and frozen to prevent the growth of contaminating microorganisms. Log discs were cut before debarking and chipping that was approximately 3 centimeters thick. Moisture content varies from approximately 50%-56% depending on the microwave treatment time.

[0062] Microwave Treatments: Logs were subject to three microwaving conditions. Logs were microwaved at 50 kW for 5 minutes (50 / 5), 20 kW for 6 minutes (20 / 6), and 20 kW for 8 minutes (20 / 8).

[0063] Chip fiberization, pulp refining and handsheet production: Microwaved wood chips were fiberized in a Sprout-Waldron Model D2202 single rotating 300 mm diameter disk refiner. Energy consumption was measured using an Ohio Semitronic Model WH 30-11195 integrating Wattmeter attached to the power supply side of the 44.8 kW electric motor. Feed rate through the refiner was between 10 kW and 15 kW. Energy reported in WH / kg. Refiner plate settings were 0.025 inch, 0.014 inch, 0.010 inch, and 0.008 inch. Pulp was collected at each pass as hot water slurry. Between the passes the pulp slurry was dewatered to approximately 25% solids in a porous bag by vacuum. Dilution water at 85 degrees Celsius was then added each time as the pulp was fed into the refiner. Samples of the pulp were taken and tested for the Canadian Standard Freeness (CSF). Samples refined to 100 CSF. Handsheets were prepared and tested using TAPPI standard testing methods.

[0064] Results: See Table 3. TABLE 3Pine TreatmentsSample IdentificationBurstTearEnergy SavingsIncluding Log Size(kN / g)(mN−m{circumflex over ( )}2 / g)(%)Control0.471.99—50 / 50.562.4311.720 / 60.532.206.920 / 80.522.149.1Example 4 Microwave Pretreatment of Aspen Logs

Problems solved by technology

This volume is expected to increase in the future as raw materials become more difficult to obtain.

However, mechanical pulping is electrical energy-intensive and yields paper with lower strength than chemical pulps.

Kraft pulp is often added to mechanical pulp to impart strength, but it is much more expensive than mechanical pulp.

These disadvantages limit the use of mechanical pulp in many grades of paper.

However, chemical pulps are expensive to produce and fiber yields are generally very low (about 50%).

On the other hand, mechanical pulps have fiber yields in excess of 90%, but pulp quality is degraded because fiberization is sometimes not complete and fibers can be severely damaged.

However, faced with the reality of more restrictive environmental regulations, increased energy costs, competitive pricing, and a more diverse raw wood resource, papermakers are being forced to be more creative in selecting furnish components.

If one exceeds the pressure above 30 lbs during presteaming, then the lignin will be melted and deposited on the surface of fibers and the fiber flexibility will be lost resulting in poor quality fibers that resemble the fibers produced during medium density fiber board production.

The drawback of the TMP process is that it takes significantly higher amounts of energy compared to the RMP process.

The steam pressure also results in the darkening of pulp.

In particular, high energy inputs are generally required to obtain fiber separation in woods rich in lignin as such woods typically call for extended refining periods and high temperatures and / or pressures.

Studies have also suggested that even thermal or chemical softening treatments of such woods does not guarantee a lower total energy consumption in the production of pulp.

This is because unprocessed fibers that are only mildly separated by the thermal or chemical treatments are difficult to fibrillate during the refining mechanical process.

As a result, high energy consumption in TMP and CTMP processes has been generally necessary in current pulping practices.

However, the economics of the process is highly dependent on the treatment time and processing costs such as those associated with ventilation of the pile for two weeks to remove metabolic heat generated by the fungus.

However, it is known that direct application of isolated enzymes on wood chips does not yield results similar to those obtained with fungal pretreatment because these enzymes cannot penetrate the wood due to their larger size compared to the pore size in the wood.

However, technical difficulties have been reported in applying enzymes after primary stage refining because pulp after primary stage refining enters into secondary stage refining within seconds.

Thus, to date, energy savings due to enzymes have been insignificant.

Currently, it is not possible for high molecular weight compounds to penetrate the logs or the lumber for certain applications, such as wood hardening.

In addition to the above-described efforts to increase pulp yield, decrease energy consumption and enhance paper quality, another issue concerning the pulp and paper industry is pitch content.

It causes a number of problems in wood processing, including at least deposits on tile and metal surfaces, plugging of drains, discoloration of felt, tears and other defects in paper and downtime for cleaning.

However, both traditional and biotechnological methods of pitch control fail to remove all traces of pitch from most wood species and thus only alleviate, but do not eliminate the problems associated with pitch in wood processing.

Yet another dilemma faced by the pulp and paper industry is blue staining of wood.

This problem occurs when freshly cut logs are stored for a long period of time in wood yards prior to debarking and chipping.

More bleach chemicals are therefore needed to overcome the loss of brightness which, in turn, results in increased costs for effluent treatment.

This is a serious problem in the southern parts of the U.S. where the logs are exposed to high temperature and humidity, which tend to exacerbate the problem.

It has been shown that treatment with CARTAPIP™ also controls unwanted colored blue stain microorganisms that lead to increased costs in the purchase of bleach chemicals.

However, as with pitch reduction methods, efforts to reduce blue staining have not been completely successful.

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