JPS6055638B2 - Pulp bleaching method with ozone - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for bleaching pulp in pulp production, and in particular to a method using ozone as a bleaching agent.

Conventionally, pulp has been bleached using chlorine bleaches such as chlorine, hypochlorite, and chlorine dioxide.

However, with this method, the wastewater contains a large amount of highly corrosive substances such as chlorinated lignin and free chlorine, and the wastewater has a low solid content concentration and an extremely large amount of water. However, it was extremely difficult to handle the wastewater. Therefore, a method for treating the wastewater with a high degree of precision that does not cause the problem of contamination of the wastewater has not yet been established technically and economically.

In addition, the wastewater containing chlorine bleach is recycled by returning it to the cleaning, washing, and cooking processes, and passes it through the cooking chemical recovery process along with the cooking black liquor, converting the bleach into salt form and separating it from the white liquor. Recovery methods are also known, but in this case, the materials used for the equipment used in the screening, washing, cooking, and recovery steps must have excellent corrosion resistance to withstand the corrosive nature of the wastewater. Therefore, there is a concern that equipment costs will increase. Another disadvantage is that scale is formed from calcium and magnesium contained as impurities, often interfering with the operation of the equipment. On the other hand, ozone is a substance known as a powerful bleaching agent, but it is not commonly used when bleaching pulp because it has the disadvantage of destroying cellulose chains, lowering the degree of polymerization, and lowering the viscosity of pulp. I wasn't there.

The present invention has been made in view of the above-mentioned circumstances, and provides a valve that overcomes the above-mentioned drawbacks, exhibits an excellent bleaching effect, and is free from the risk of a decrease in the viscosity of the valve due to destruction of cellulose chains. The purpose is to provide a bleaching method.

As a result of extensive research to achieve this objective, the inventors discovered that in a valve bleaching method that uses ozone as a bleaching agent, it is necessary to carry out the process in the coexistence of recovered methanol in order to suppress the destruction of cellulose chains. We have obtained the knowledge that it is effective, and have completed this invention based on this knowledge.

That is, this invention uses ozone as a bleaching agent when manufacturing bleached bulbs, bleaches unbleached bulbs in the coexistence of recovered methanol, which is a by-product of the cooking process, in order to suppress the destruction of cellulose, and removes the bleached bulbs from the bleaching process. Contaminants separated from the unbleached pulp liquid by introducing the methanol-containing wastewater into at least one of cooking, washing, and screening processes instead of water for use,
An ozone-based valve that dissolves malodorous substances, cooking/washing black liquor, etc. into methanol-containing wastewater, recovers crude methanol from the wastewater containing these dissolved substances and methanol, and circulates the recovered methanol to the bleaching process for reuse. The gist is the bleaching method. Hereinafter, this invention will be specifically explained according to embodiments shown in the drawings.

In cooking step 1, when wood chips are cooked in a cooking liquor containing NaOH and Na2s, methanol is produced as a by-product.

The methanol-containing relief gas or blow gas condensate is led via pipe 2 to a methanol recovery step 3, where methanol is recovered by distillation. The recovered methanol obtained is led to a bleaching step 5 via a pipe 4. In step 5, ozone is supplied from the ozone generator 21 to bleach the unbleached bulbs that have passed through the cooking step 1, the washing step 6, and the selection step 7. The methanol-containing wastewater discharged from the bleaching step 5 is led through a pipe 8 to a screening step 7 and further to a washing step 6, and is also directly washed through a pipe 9. It is also led to a cleaning step 6 to bring it into countercurrent contact with the unbleached valve fluid. In this way, organic and inorganic pollutants separated from the unbleached valve liquid, malodorous substances consisting of sulfides such as H2S, CH3SH, (CH3)2S2, cooking/washing black liquor, etc. are dissolved in the methanol-containing wastewater. A portion of the waste water containing the dissolved substances and methanol is then led to the cooking step 1 through the pipe 10, dilutes the cooked bulb, and returned to the washing step 6 again. On the other hand, the remainder is led to a methanol separation step 12 through a pipe 11 to evaporate methanol and other substances,
It is further condensed and led to a methanol recovery step 3 via a pipe 13. In this step 3, contaminants and malodorous substances are removed by stripping treatment and distillation, and methanol is recovered. The recovered methanol is led to the bleaching step 5 by pipe 4, as described above. On the other hand, the concentrated black liquor left in the methanol separation step 12 is led to a cooking chemical recovery step 15 through a pipe 14, and the green liquor recovered by the boiler is led through a pipe 16 to a causticizing step 17. The causticized chemicals are then circulated through pipe 18 to the cooking stage 1 for use.The bleaching bulb from the bleaching stage 5 is transferred to the re-selection stage 1.
Lead to 9 and then use Valve Machine 20 to obtain a Craft●Valve. In order to adjust the amount of methanol in the methanol circulation system, a part of the recovered methanol may be led to the cooking chemical recovery step 15 through the pipe 22 and used as fuel for the boiler. The bulb bleaching method according to the present invention is not limited to the embodiments, but may be carried out as long as it includes a cooking process that produces methanol as a by-product.

Examples of the cooking process include a sulfate method, a sulfite method, an alkali method, an oxygen-alkali method, and the like. Further, in the bulb bleaching method according to the present invention, a non-chlorine bleaching agent such as peroxide or oxygen may be used in combination with ozone as a bleaching agent.

The remainder of the recovered methanol may be used as fuel once the amount necessary for the bleaching process is obtained, or it may be stored for use in starting up the device.

Since the present invention is constructed as described above, there is no possibility that the problem caused by chlorine bleach as described at the beginning will occur.

In addition, the bleached bulb can be effectively bleached without destroying the cellulose chains. In addition, since methanol by-produced in the cooking process is used, there is no need to introduce methanol from outside the system, and since the methanol-containing wastewater from the bleaching process is introduced into at least one of the cooking, washing, and screening processes, The amount of water supplied from outside the system for cleaning etc. can be significantly reduced. moreover,
By recovering crude methanol from waste water containing dissolved substances and methanol, and recycling the recovered methanol to the bleaching process for reuse, methanol can be used effectively and production costs can be significantly reduced. Reference Example Experiment 1 Whiteness 28.8%, viscosity? 5.6cm●Poise, unbleached NUKP (Douglas Fur 65) with a kappa value of 24.1
%, other 35%) in advance for 1 hour in a dilute sulfuric acid aqueous solution with a pH of 3, washed with water, and dried to prepare a sample bulb.

Take an amount of this sample bulb to give an absolute dry weight of 5y and immerse it in 100ml of commercially available methanol. A sample is taken out of the methanol solution and the water content is adjusted so that the water content in the sample is 45%. The amount of methanol impregnated is determined by measuring the amount of increase in the weight of the sample. A valve impregnated with methanol is placed in the flask, oxygen containing 1% by volume of ozone is passed through the flask, and the gas leaving the flask is 0.0% by volume.
The ozone is absorbed by a collection pin containing 5M potassium iodide solution. Pour the bleached valve into 350ml of distilled water.
Mix with water, filter and wash. Experiment 2 Other operations are performed as in Experiment 1, using the same sample valve as in Experiment 1 and without using any methanol.

Experiment 3 Other operations were performed in the same manner as in Experiment 1, using the same sample valve as in Experiment 1 and using recovered methanol from the methanol recovery process of the contaminant example.

The results of each experiment are shown below.

Whiteness is measured with a Hander colorimeter, and viscosity is measured with a copper ethylenediamine pipette method.

The amount of ozone supplied is calculated from the ozone concentration immediately before passing ozone-containing oxygen into the flask, the aeration rate, and the bleaching time.
The amount of ozone consumed is calculated by subtracting the amount of ozone absorbed into the 0.5M potassium iodide aqueous solution from the amount of ozone supplied. )
These experiments show that recovered methanol has the same effect as commercially available methanol in suppressing the destruction of cellulose.

[Brief explanation of the drawing]

The drawings are process diagrams showing one embodiment of the present invention. 1... Cooking process, 3... Methanol recovery process, 5...
... Bleaching process, 6... Washing process, 7... Selection process,
12... Methanol separation process, 15... Cooking chemical recovery process, 17... Causticizing process, 19... Re-selection process, 20... Valve machine, 21... Ozone generator.

Claims (1)

[Claims] 1. When producing bleached pulp, ozone is used as a bleaching agent, and in order to suppress the destruction of cellulose, unbleached pulp is bleached in the coexistence of recovered methanol, which is a by-product of the cooking process, and methanol-containing wastewater from the bleaching process is treated. , the contaminants, malodorous substances, cooking and
A pulp bleaching method using ozone that consists of dissolving cleaning black liquor etc. in methanol-containing wastewater, recovering crude methanol from these dissolved substances from the methanol-containing wastewater, and recycling the recovered methanol to the bleaching process for reuse.