JPS6137394B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for closing the process of reducing sodium silicate from the effluent when pulp is bleached using peroxide in the presence of sodium silicate, and further relates to Specifically, by adding calcium oxide to the pulp slurry after bleaching the pulp with peroxide in the presence of sodium silicate, most of the sodium silicate is converted to calcium-containing silicate and made insoluble. Moreover, by adsorbing it to pulp and obtaining a bleaching liquid containing almost no silicic acid, it is possible to close the bleaching process using peroxide, which was previously impossible due to problems caused by scale formation. The present invention relates to a new method for treating peroxide bleach wastewater that has been successfully made possible.

In order to make the pulp bleaching process pollution-free, a so-called closed system that completely treats the wastewater and does not discharge it outside the system was strongly desired in the paper and pulp industry, and for this reason peroxide bleaching that does not use chlorine was developed. The law has been attracting attention.

Peroxide bleaching, generally bleaching with hydrogen peroxide in the presence of an alkali, is used to bleach pulp, especially groundwood pulp, refined mechanical pulp,
It is a method widely used for bleaching high-yield pulp such as thermomechanical pulp and chemi-mechanical pulp, and can reach a relatively high degree of whiteness with almost no elution of pulp components. It has the characteristic that there is little color reversion. However, since peroxide is unstable, especially under alkaline conditions, unless some stabilization measures are taken, the whiteness obtained by self-decomposition of peroxide will be low.
Moreover, not only the fluctuation is severe, but also there is a risk of explosion when mixing peroxide with alkali to prepare a bleaching solution, which is extremely dangerous. Therefore, at present, peroxide bleaching is generally carried out by adding sodium silicate as a peroxide stabilizer.

Methods to stabilize peroxide without adding sodium silicate include treatment with an aminopolycarboxylic acid chelating agent before bleaching, or bleaching in the presence of a chelating agent. A method is known in which heavy metals, which are catalysts for decomposition of peroxides, are removed from the system or inactivated. However, these methods are economically disadvantageous because even if a large amount of an expensive chelating agent is used, the effect is not as good as when sodium silicate is added.

For the above reasons, it is essential to add sodium silicate in peroxide bleaching, but since this sodium silicate is present in the bleaching liquid, it is difficult to close the peroxide bleaching process. Attempts to solve wastewater treatment problems and save water use lead to serious problems such as scale formation in the wastewater circulation system and concentrated combustion system, so up until now the peroxide bleaching process has been closed. That was impossible.

However, it should not be allowed to dispose of the wastewater as it is without collecting it due to environmental issues, so it is necessary to perform external treatment such as coagulation sedimentation treatment. The current situation is that the treatment effect is small and it cannot be completely treated even if a large amount of money is spent.

As mentioned above, with the conventional peroxide bleaching method, it is impossible to close the bleaching process, and on the other hand, a method for completely and economically treating wastewater has not yet been established, so it is not possible to achieve a closed bleaching process in the paper and pulp industry. There has been a strong desire for a method to make the oxide bleaching process pollution-free.

The inventors of the present invention have conducted intensive research on a method that can make the peroxide bleaching process a closed process. It was discovered that most of the sodium silicate is converted into calcium-containing silicate and becomes insolubilized, and that the sodium silicate itself is easily adsorbed to the pulp.

That is, in the method of the present invention, pulp slurry that has been bleached with peroxide in the presence of sodium silicate contains at least 30% of the silicic acid content in the sodium silicate.
A method for treating peroxide bleaching effluent, which is characterized by adding 200% or less of calcium oxide to convert most of the sodium silicate into calcium-containing silicate, insolubilizing it, and adsorbing it to pulp. This method provides a completely closed bleaching process.

The reason why the water-soluble calcium compound added in the present invention is limited to calcium oxide is that calcium chloride, which can be considered as another water-soluble calcium compound, is undesirable because it causes problems such as corrosion due to the presence of chlorine during concentration and combustion. This is because the peroxide bleaching method loses its significance as a bleaching method that does not use peroxide bleaching.

The amount of calcium oxide added is determined depending on the amount of sodium silicate contained in the waste liquid after treatment.

The method of the present invention will be explained below based on detailed experiments conducted by the present inventors.

First, it has been found that in order to effectively carry out the method of the present invention, it is necessary to carefully select the amount of calcium oxide added. In other words, when the amount of calcium oxide is less than 30% of the silicate content in sodium silicate, the silicate removal efficiency is extremely low, and when the amount of calcium oxide is 200% or more, the silicate removal effect is poor. The level of calcium decreases and a significant amount of calcium remains in the waste liquid, which not only causes problems when concentrating and burning the waste liquid, but also causes the pH of the pulp slurry to become extremely high, leading to coloration of the bleached pulp.

Therefore, the amount of calcium oxide added in the method of the present invention should generally be 30% or more and 200% or less, preferably 60% or more and 150% or less, based on the silicic acid content in sodium silicate. is selected.

Next, it became clear that the pulp concentration and temperature at the time of calcium oxide addition had almost no effect on the silicic acid removal effect. That is, in the method of the present invention, the pulp concentration at the time of addition of calcium oxide only needs to be 3 to 25% and the temperature should be 20 to 90°C. It can be applied to anything. Further, there is no effect on the time of calcium oxide treatment, and generally 1 to 120 minutes is sufficient. Examples of peroxides include hydrogen peroxide, hydroperoxides such as t-butyl hydroperoxide or salts thereof, and percarboxylic acids such as peracetic acid or salts thereof, but hydrogen peroxide is the most common. .

In the method of the present invention, if the bleached pulp concentration is relatively low, calcium oxide may be added directly to the pulp slurry after bleaching, or if bleaching is carried out at a relatively high concentration, calcium oxide may be added directly to the pulp slurry after bleaching. The calcium chloride may be diluted appropriately so that the calcium chloride is uniformly dispersed. Note that it is also possible to once separate the waste liquid from the pulp slurry after bleaching, add calcium oxide to the waste liquid to generate a calcium-containing silicate, and then mix it with the pulp again.

In the method of the present invention, calcium oxide is added in the form of a solid or an aqueous solution, preferably in the form of an aqueous solution.

The method of the invention is applicable to all types of pulp, and since calcium-containing silicates are strongly adsorbed to the pulp, there is no need to reduce the water content of the pulp to increase the surface area of the fibers. Note that as a device for separating the waste liquid from the pulp slurry after the calcium oxide treatment, a commonly used separation device such as a drum-type vacuum filter is sufficient.

In addition, since the amount of calcium-containing silicate adsorbed to the pulp by the method of the present invention is small,
It does not interfere with the subsequent paper-making process in any way, and the strength of the paper does not decrease at all.

As detailed above, the method of the present invention makes it possible to close the peroxide bleaching process and greatly contributes to the development of the pulp and paper industry.

Next, the present invention will be explained in more detail with reference to Examples.

Example Sulfite-based chemical mechanical pulp produced from hardwood chips (water content 640 ml CSF) was added with hydrogen peroxide amount of 1% and sodium hydroxide amount of 0.8.
%, sodium silicate (No. 3 water glass) addition amount 5
% (all percentages are bone dry pulp), pulp density 20
%, bleached at 80°C for 3 hours. A saturated aqueous calcium hydroxide solution was added to the pulp slurry after bleaching so that the amount of calcium oxide relative to the silicate content in the sodium silicate was 133%, and then treated at 30°C for 15 minutes at a pulp concentration of 4.3%. I spent my time with paper. The concentration of silicic acid in a solution obtained by concentrating this solution twice was only 74 ppm. on the other hand,
For comparison, the pulp slurry after bleaching was diluted with water to give a pulp concentration of 4.3%, filtered through paper, and the resulting solution was concentrated twice. In this case, the silicic acid concentration of the liquid was 1340 ppm.

Claims (1)

[Claims] 1 The pulp slurry after bleaching the pulp with peroxide in the presence of sodium silicate contains at least 30% of the silicic acid content in the sodium silicate.
% or less of calcium oxide is added to convert most of the sodium silicate into calcium-containing silicate, making it insolubilized, adsorbed to pulp, and concentrating and burning the waste liquid without discharging it outside the system. Closed treatment method for peroxide bleaching wastewater.