FI73252C - Ozone bleaching of cellulose materials. - Google Patents

Description

1 73252

OZONE BLEACHING OF CELLULOSIC MATERIALS

This invention relates to ozone bleaching of lignocellulosic material.

It has been found that in ozone bleaching of pulp, irradiation of the pulp with ultraviolet light improves ozone efficiency and pulp bleachability.

The following definitions are used in this application.

Fiberization is the conversion of wood chips or other lignocellulosic material into a fibrous form. Chemical defibering requires the boiling of such material in solution with the chemical and this includes the partial removal of a wood-related dye such as lignin.

Bleaching is the treatment of cellulosic fibers to remove or alter the dye associated with the fibers so that the fiber can more accurately reflect white light.

The present invention relates to the decomposition of lignin, in which case the terms "lignin removal" and "bleaching" are used interchangeably.

Consistency is the weight of pulp fibers in a pulp suspension, usually expressed as a percentage. For example, one kilogram of oven-dried fibers in a 100 kilogram water-fiber mixture forms a pulp suspension with a consistency of 1%. The consistency of the pulp depends on the type of dewatering equipment used.

25 The following definitions of consistency are based on the use of these terms in "The Bleaching of Pulp", third corrected edition edited by its inventor, TAPPI Press 1979, pages 243-46.

The low consistency is typically 4 to 5%. It represents a suspension that can be pumped with a conventional centrifugal pump and can be achieved by using thickeners and filters without compression rollers.

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The average consistency is typically between 10 and 15%. Such consistency can be obtained from vacuum washers and vacuum concentrators. Above 15%, compaction rollers are required for dewatering. Medium-thick pulp can be pumped with special equipment.

The high consistency is 20-35%. This consistency is achieved by the use of presses. The high-density pulp is substantially unpumpable.

One measure of the efficiency of the bleaching process is the degree of delignification. There are many methods for measuring the degree of delignification of pulp, but most methods 15 are modifications of the permanganate test.

The permanganate test used herein gives the number of potassium permanganate solution consumed by the kiln-dry mass under specified conditions. The number of pieces is determined according to TAPPI standard method T 214 M42.

U.S. Patent 4,080,249, Kempf et al., Column 1, lines 31-46, contains a fairly extensive representation of the known literature and patent references describing the bleaching of lignocellulosic materials with ozone in the gas phase. These reference publications disclose ozone bleaching of low, medium and high density pulps. Thus, "ozone bleaching" as used herein is not limited to any particular consistency, but can be used at any desired consistency, as suitable devices are already known for all three levels. 342, a photoreactor such as that described in Veloz, following U.S. Patent 4,080,249 for the absorption of ozone carrier gas li 3,732,252 into a pulp slurry. 964 962, Carlsmith, it is contemplated that ultraviolet light emitting lamps be mounted on the walls of the reactor chamber.

According to the present invention, ultraviolet radiation increases the efficiency of ozonation. Without wishing to be bound by theory, the inventor believes that ultraviolet radiation electronically excites 10 lignin in the material to be bleached. The excited lignin can then form a high-energy complex with ozone (or possibly an oxygen atom). The complex is thought to degrade immediately to lignin degradation results. Due to its aromatic nature, lignin strongly absorbs 15 in the ultraviolet range of the spectrum. A typical lignin absorption spectrum includes a maximum at 205 nm (nanometers), a smaller peak at 280 nm, and smaller side peaks at 250, 300, and 360 nm. In the practice of the present invention, the pulp was subjected to a treatment with ozone carried by it, while being irradiated at the above-mentioned frequencies of ultraviolet light. Pulp samples weighing 0.1 g each (as determined by oven drying) were irradiated in a closed chamber of two Ia-25 plates for two hours. The cells were partially coated with a reflective film to increase the energy applied to the mass. In the following results, the decrease in kappa number (increase in whiteness) is of the order of two, which, given the low power of the spectrometer-30 photometer, must be considered significant and allow it to be converted into test equipment or commercial scale equipment.

, 73252 Sample Unit Number Untreated Mass 18,324

Ozonated mass 17,389

Ozonated and irradiated at 5 wavelength 205 nm 15.044 250 nm 15.278 280 nm 15.672 300 nm 15.586 10 36C nm 15.789. At all these wavelengths, which are characteristic of the lignin absorption spectrum, ultraviolet radiation increased the efficiency of ozonation. As observed, the greatest effect was at 205 nm with light. Although commercially available photoreactor light sources generally do not emit at a single wavelength, it is sufficient if some of the light occurs at one or more more effective frequencies. In addition, it is desirable to select wavelengths at which cellulose does not absorb. By irradiating at the absorption peaks characteristic of lignin but not cellulose, lignin can be made more susceptible to cellulose by ozone, which tends to cause ozone to react primarily with lignin and cause less cellulose spoilage.

Although the present invention has been described as a practical application of laboratory modification, it will be apparent to one of ordinary skill in the art that the invention as defined in the appended claims can be applied to various types of reactor vessels already known in the art and incorporated herein by reference.

It is thus clear that the shape of the apparatus used to carry out the invention can be modified in various ways without departing from the spirit and scope of the invention as defined in the appended claims.