Pulp Bleaching Processes

Abstract

The present invention aims to further advance the acid treatment or irradiation technology for pulp to develop a bleaching process using smaller amounts of chlorine chemicals with higher efficiency as compared with conventional bleaching processes. It also aims to provide an excellent high brightness pulp having a low environmental impact and no discoloration as well as a paper containing it. The present invention provides a chlorine-free bleaching process with very high efficiency by irradiating a pulp washed after an acid treatment with UV light and / or visible light at a wavelength of 100-400 nm under alkaline conditions, preferably in a pH range of 10-13. The acid treatment can be performed under conditions of pH 1-6 and a temperature of 80° C. or more. The irradiation treatment can be performed in the presence of at least one compound selected from the group consisting of reducing agents, peroxides, and hydrogen-donating organic compounds. The present invention also provides a high brightness chemical pulp having an ISO brightness of 88% or more and a brightness loss of 1.0% or less in a specific fading test.

Description

TECHNICAL FIELD [0001] The present invention relates to a bleaching process for pulp characterized in that a pulp washed after an acid treatment is irradiated with UV light and / or visible light under alkaline conditions; a bleaching process characterized in that a pulp washed after an acid treatment is treated by a combination of irradiation with UV light and / or visible light and ozone feeding, thereby promoting pulp bleaching; and a bleaching process for chemical pulp by which a high brightness chemical pulp can be obtained (herein collectively referred to as the first invention); as well as a high brightness chemical pulp with greatly improved discoloration and a paper containing it, more specifically a novel high brightness chemical pulp with greatly improved discoloration obtained by further treating a bleached chemical pulp with UV light and a paper containing it (herein collectively referred to as the second invention). BACKGROUND ART [0002] The background art of the first inv...

AI Technical Summary

Benefits of technology

[0068] The second aspect of the first invention is characterized in that the irradiation treatment with UV light and / or visible light at a wavelength of 100-400 nm is performed in the presence of ozone. In the case of irradiation with light in a wavelength range of 135-242 nm, ozone is normally generated due to the presence of air in the gas layer around the light source. In the present invention, air is continuously supplied to the surroundings of the light source while continuously extracting the generated ozone and injecting it into the irradiation target material, whereby the ozone can be used as a reaction promoter without supplying ozone from any external source. In addition, a larger amount of ozone can be obtained by supplying air to the gas layer around the light source. Naturally, the generated ozone can be used not only as a promoter during the irradiation reaction but also for regular ozone bleaching. Thus, another great advantage of the present invention is that the ozone by-produced in the irradiation reactor can be used. Ozone can also be generated by supplying such air or oxygen to the pulp to be treated but not to the surroundings of the light source. Moreover, ozone obtained from an external source by a method not relying on such effect of UV light can also be used as a reaction promoter.

[0069] The concentration of the ozone by-produced in the irradiation reactor is 0.5-100 ppm, depending on the manner of supplying air or oxygen or the concentration of oxygen. A major feature of the second aspect of the first invention is that a high bleaching efficiency can be obtained with even such low-concentration ozone by combination with the irradiation treatment.

[0070] This concept can be further expanded to select and use multiple light sources having a characteristic wavelength range within 100-400 nm and different characteristic wavelengths as light sources in the irradiation reactor. Specifically, a light source having a narrow wavelength characteristic of 135-242 nm providing a high ozone-generating efficiency and a light source having an evenly distributed wavelength range of 100-400 nm suitable for irradiation reaction can be combined, for example, whereby a further higher bleaching efficiency can be obtained.

[0071] In the irradiation treatment of the present invention, the irradiation reaction efficiency can be increased by using a combination of additives such as reducing agents (e.g., NaBH4, hydrazine, hydrogen), oxidizing agents (e.g., oxygen, ozone), peroxides (e.g., hydrogen peroxide, peracetic acid, Na percarbonate, Na perborate), hydrogen-donating organic compounds (e.g., alcohols; linear amines such as ethyl amine, diethylamine; cyclic amines such as tetramethyl piperidine), acetyl-containing organic compounds (e.g., a-acetyl-γ-butyrolactone, acetol, acetone).

[0072] In the third aspect of the first invention, ozone bleaching, hydrogen peroxide bleaching and the like can be used alone or in combination as TCF bleaching for attaining an ISO brightness of 70-75% before the light bleaching stage.

[0073] In the third aspect of the first invention, ozone bleaching can be performed under normal ozone bleaching conditions. That is, ozone bleaching conditions may include using ozone gas at an ozone concentration of 1-20% by weight at pH 1-8, a pulp consistency of 0.1-50% by weight, and a temperature of 25-95° C. The pressure during ozone bleaching is not specifically limited from a negative pressure state to a pressurized state.

Problems solved by technology

Under such circumstances, there is a tendency to use limited chemicals such as chlorine dioxide, hydrogen peroxide, oxygen and ozone in ECF bleaching or TCF bleaching.

However, pulp quality, especially brightness obtained by bleaching with only such chemicals is limited even if they are used in combination, or large amounts of expensive chemicals must be used to attain sufficient quality.

It has been known that various metals from pulp promote decomposition of oxygen-based bleaching chemicals, thereby wasting the oxygen-based bleaching chemicals.

Recently, water pollution caused by industrial wastewater or domestic wastewater is worsening and water environment pollution has become social concerns.

However, it was difficult to avoid brightness loss with time or so-called discoloration phenomenon in high brightness pulps obtained by the conventional processes because very small amounts of potential coloring matters liable to be darkened by heat or UV light remain in them.

Images