JPH05247867A - Method for treating coal mud slurry - Google Patents

Abstract

PURPOSE:To prevent sand-like foreign matter from accumulating on the bottom of the vat of a filter by-separating a coarse particle part in a coal mud slurry and subsequently spraying the coarse particle part on the cake layer of the filter from the upward direction. CONSTITUTION:A coal mud slurry highly containing coarse particles is taken out from the bottom of a coarse particle precipitation-facilitating reaction tank among caustification reaction tanks and subsequently fed into a cyclone. The coal mud slurry in the cyclone is carried from the fine particle-discharging port at the upper part of the cyclone into the vat of the belt filter. The coal mud slurry taken out from the coarse particle-precipitating tank at the lower part of the cyclone is carried into the shower pipe of the belt filter and subsequently sprayed on the coal mud cake of the belt filtration cloth of the filter.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in a method for dewatering and concentrating white liquor from a lime mud slurry generated in a causticizing step of kraft pulp production.

[0002]

2. Description of the Related Art Kraft pulp (hereinafter referred to as "KP")
In the cooking method, an aqueous solution containing caustic soda and sodium sulfide as main components (this liquid is generally called "white liquor") is added to wood chips and steamed to obtain pulp. At this time, lignin, resin, a part of cellulose, etc. in the wood are decomposed or eluted in the white liquor to become cooking effluent (this liquor is generally called "black liquor").

In the KP method, soda and sulfur are recovered from this black liquor and white liquor is produced again. That is, the black liquor is concentrated and burned, and the smelt produced (inorganic high temperature melt) is dissolved in a weak liquid (a dilute alkaline aqueous solution generated in the washing step described later) (sodium carbonate and sodium sulfide). To produce an aqueous solution).

When quicklime is added to this green liquor to cause a causticizing reaction, sodium carbonate becomes caustic soda and quicklime becomes calcium carbonate.

[0005] The lime mud containing calcium carbonate as a main component separates white liquor and is discharged and washed (a weak liquor is generated here).
After concentrating, it is calcined to form quicklime again, which is used in the causticizing step.

[0006] Thus, although lime is recycled and used, it has some runoff, and in order to compensate for this, natural limestone finely ground to a diameter of several mm is constantly replenished in the lime mud during the firing process. There is a need.

However, this limestone, even if it has the same composition as calcium carbonate, has a high density, so that it is difficult to sufficiently sinter the inside even if it is fired. As a result, sand-like uncalcined calcium carbonate particles are generated in the causticizing step following the calcining step. Therefore, the particle size is larger than that of the lime mud concentrated in the causticizing step, which is 50 μm or more, and the larger is 50.
Those of about 0 μm are also included.

Usually, the causticizing step is composed of a plurality of reaction tanks, and the lime mud slurry successively passes through these reaction tanks to allow the causticizing reaction to proceed sufficiently over a residence time of several hours.

In the reaction tank, a paddle type agitator is installed and agitated at 30 to 60 RPM to prevent even relatively coarse particles from accumulating at the bottom of the reaction tank. Then, the slurry is sent from the final reaction tank to the vat in which the drum type filter is immersed, and the white liquor is dehydrated and separated by the drum type filter to concentrate the lime mud slurry.

As a device for dehydrating white liquor from lime mud slurry, a drum type filter has been used for a long time. In this device, the lower part of the drum-shaped cylinder is immersed in the slurry of lime mud, and while the drum is rotating, the lime mud slurry is scooped up to dewater the white liquor, and at the same time, the concentrated cake layer of lime mud is drummed. It is formed on a wire or filter cloth stretched around the surface.

In such a method for treating a lime mud slurry, when the lime mud slurry contains coarse particles of 50 μm or more as described above, the coarse particles are gradually deposited on the bottom of the vat into which the drum filter is dipped. This impedes the rotation of the drum filter, which increases the load and causes a problem that the operation cannot be continued.

Usually, an agitator is also installed in the vat in which the drum type filter is immersed, but the driving is gentle so that it does not hinder the formation of the cake tank on the drum surface. However, it is difficult to avoid that coarse particles settle and deposit on the bottom of the vat of the filter.

By the way, the inventors of the present invention have investigated and found that the normal particle size distribution of calcium carbonate in the causticizing step is generally 50 μm or less, but when natural limestone added for supplementation is used. Inevitably 50μ
Sandy coarse particles exceeding m are generated, and in large ones,
It was found that it could reach about 500 μm.

When crushing natural limestone for use,
It is difficult to prevent the generation of such coarse particles, so the inventors of the present invention have made various investigations and developed a countermeasure method according to the present invention.

[0015]

SUMMARY OF THE INVENTION The present invention prevents coarse particles of calcium carbonate from gradually accumulating on the bottom of a vat in which a filter is immersed, which prevents the rotating function of the drum from being hindered. The present invention provides a method that enables long-term continuous operation of the filter.

[0016]

According to the present invention, in the causticizing step of kraft pulp production, calcium carbonate-based lime mud slurry produced by the reaction between calcium oxide and green liquor is filtered to remove white liquor and concentrated. In the method, a coarse particle portion in the lime mud slurry is separated and is dispersed on the surface of a drum forming a filter, which is a method for treating the lime mud slurry.

[0017]

In the present invention, it is achieved by using the lime mud slurry separated in the previous step of the filter or the lime mud slurry extracted from the bottom of the vat of the filter as the slurry of the coarse particles scattered on the surface of the drum. To be done.

As a concrete means for separating the coarse particle lime mud slurry in the previous step of the filter, the coarse particles easily precipitate in the reaction vessel, for example, a method of taking out from the bottom of the first reaction vessel, Alternatively, by lowering the rotation of any agitator in the reaction tank or changing the position or shape of the blades, the function is weakened to accelerate the sedimentation of coarse particles, and a method of extracting the coarse particles from the bottom, or a causticizing reaction tank From the bottom of the lime mud slurry to the filter vat, a sedimentation tank or a heavy foreign matter separator such as a cyclone may be installed, and coarse particles of the lime mud slurry may be extracted from the bottom.

Further, it is not necessary to continuously perform such a withdrawing operation, and it is more effective to intermittently withdraw by a timer.

[0020]

EXAMPLE FIG. 1 shows that there are three causticizing reaction tanks, the agitator of the first and second reaction tanks is 40 RPM, and the agitator of the third reaction tank is decelerated to 20 RPM. The third reaction tank is provided with an outlet (2) for the lime mud slurry containing a large amount of coarse particles from the bottom thereof, in addition to the ordinary outlet (1) for taking out the lime mud slurry. The lime mud slurry containing a large amount of coarse particles enters the cyclone (3) tangentially from the take-out port (2), and the lime mud slurry that comes out of the fine particle take-out port (4) at the upper part of the cyclone enters the vat of the belt filter. Sent.

On the other hand, the lime mud slurry taken out from the lower part of the settling tank (5) for coarse particles provided at the lower part of the cyclone by intermittent operation at a rate of 1 minute for 10 minutes is sent to the shower pipe of the belt filter, It is sprinkled on the lime mud cake of the belt filter cloth. The coarse particles thus dispersed are concentrated with a filter together with normal fine particles of lime mud, and then sent to a washing and firing step.

In this case, the content of particles having a particle size of 50 μm or more is% by weight, and 0.1% is contained in the usual outlet (1) of the reaction tank.
Hereinafter, about 12% was contained in the settling tank (5) for coarse particles. The amount of natural limestone used at this time was 3% by dry weight with respect to the lime mud.

The sand deposition phenomenon inside the filter vat in such equipment was hardly seen even after continuous operation for 4 months.

In FIG. 2, there are three causticizing reaction tanks, and the agitator of each tank is 50 RPM. The lime mud slurry is
It is sent to the vat of the belt filter through the usual outlet (6) of the reaction tank.

At the bottom of the vat, there is provided an outlet (7) for the slurry of lime mud containing coarse particles. From this take-out port (7), lime mud slurry is taken out by intermittent operation at a rate of 1 minute for 30 minutes, sent to the shower pipe of the belt filter, and sprinkled on the lime mud cake of the belt filter cloth of the filter. ..

The coarse particles thus dispersed are concentrated together with normal fine particles of lime mud by a filter, and then sent to a washing and firing step.

In this case, the content of particles having a particle size of 50 μm or more is% by weight, the normal lime mud slurry take-out port (6) is about 0.6% in the reaction tank, and the lime mud slurry take-out port containing coarse particles at the bottom of the vat. About 7% was included in (7).
The amount of natural limestone used at this time was 3% by dry weight with respect to the lime mud.

Almost no sand accumulation phenomenon inside the filter vat in such equipment was observed even after 4 months of continuous operation.

[0029]

[Comparative Example] In the same equipment as in Example 2, the use of the line for the lime mud slurry extraction port (7) containing coarse particles was stopped, and the comparison was performed. In this case, a large amount of sand was accumulated inside the vat in about 30 days, and the load on the drum increased, which made the operation impossible. The operation was stopped and the sand was removed. The amount of sand discharged was about 2.5 ton dry weight. The sand was analyzed and found to be calcium carbonate. The amount of natural limestone used at this time was 3% by dry weight with respect to the lime mud.

[0030]

[Effect] According to the present invention, since coarse particles are forcibly sent over the cake layer of the filter and dispersed, the coarse particles are removed from the system of the causticizing reaction tank and the filter vat together with the cake of normal lime mud. Since it is carried out and re-fired in the subsequent lime firing process, the accumulation of sandy foreign matter on the bottom of the filter vat is eliminated, and the filter can be continuously operated for a long period of time.

Moreover, it has been found that such sandy coarse particles can be converted into fine lime without problems by re-firing.
Therefore, conventionally, such sand accumulated inside the vat had to be regularly discharged out of the system and treated as industrial waste, but the present invention eliminates the need.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing an embodiment of the present invention.

FIG. 2 is an explanatory diagram showing another embodiment of the present invention.

[Explanation of symbols]

 1 Normal take-out port 2 Slurry take-out port containing a lot of coarse particles 3 Cyclone 4 Fine particle take-out port 5 Coarse particle settling tank 6 Normal take-out port 7 Slurry take-out port containing a lot of coarse particles

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[Procedure amendment]

[Submission date] March 9, 1992

[Procedure Amendment 1]

[Document name to be corrected] Drawing

[Name of item to be corrected] Figure 2

[Correction method] Change

[Correction content]

[Fig. 2]

Claims (3)

[Claims] 1. A method for filtering a lime mud slurry mainly composed of calcium carbonate produced by a reaction between calcium oxide and green liquor in a causticizing step of kraft pulp production to remove white liquor and concentrate the lime mud slurry. The method for treating a lime mud slurry, characterized in that the coarse particles of the above are separated and sprayed on the surface of a drum constituting a filter. 2. The method for treating a lime mud slurry according to claim 1, wherein the slurry of coarse particles scattered on the surface of the drum is a lime mud slurry separated in the preceding step of the filter. 3. The method for treating lime mud slurry according to claim 1, wherein the slurry of coarse particles scattered on the surface of the drum is lime mud slurry extracted from the bottom of the vat of the filter.