JPH0428835B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a direct causticizing method using a fluidized bed, and particularly to a method configured to increase the utilization efficiency of metal oxide used for Na recovery and reduce energy consumption.

In the pulp manufacturing process at paper mills, chemicals, mainly caustic soda (NaOH), are used to separate the wood components cellulose (fiber) and lignin (resin) and extract only the cellulose. The solution containing dissolved lignin and Na components after the cooking process is discharged as a waste liquid called black liquor, but there is a method that recovers heat by burning this black liquor and at the same time recovers and reuses NaOH. This has been practiced for a long time. This method involves precisely controlling a black liquor combustion device to combust the black liquor in the same device to react and generate Na ₂ CO ₃ , and convert this Na ₂ CO ₃ into slaked lime [Ca(OH) produced in a separate process. ) ₂ ] to recover NaOH by reacting with
The reaction process is complex and requires large equipment, and
There is a problem in that a large amount of energy is consumed to return CaCO ₃ generated during NaOH generation to Ca(OH) ₂ through a slaked process. For this reason, a NaOH recovery method called the direct causticization method has been proposed to simplify equipment and reduce energy consumption. To explain the outline of this direct causticizing method using FIG. 1, wood chips C are
Lignin is separated by NaOH, and the black liquor containing lignin and Na components undergoes the following reaction by adding Fe ₂ O ₃ as iron oxide powder in the combustion process 22.

2NaOH + CO ₂ Na ₂ CO ₃ + H ₂ O ... (1) Na ₂ CO ₃ + Fe ₂ O ₃ 2NaFeO ₂ + CO ₂ ... (2) Of these, sodium ferrate (NaFeO ₂ ) is dissolved in water W in the next dissolution step 23. is added to hydrolyze and recover NaOH.

2NaFeO ₂ + H ₂ O 2NaOH + Fe ₂ O ₃ ...(3) In other words, in the direct causticizing method, NaOH can be recovered by performing the reactions (1), (2), and (3), and this recovery This method has the advantage that the used Fe ₂ O ₃ can be recycled and reused, and equipment costs and energy consumption can be significantly reduced compared to conventional methods.

In the direct causticizing method with the above configuration, a method has been proposed in which the combustion step is performed in a fluidized bed furnace and the fluidized medium is the above-mentioned iron oxide particles. In this method, as shown in Fig. 2a, a surface layer 25 of sodium ferrate (NaFeO ₂ ) is formed on the surface of the media particles 24 during the combustion process, and the media particles thus formed are extracted from the fluidized bed furnace and then water is removed. The hydrolysis shown in formula (3) is carried out by immersion in the liquid or the like. As a result, the surface layer 5 made of NaFeO ₂ is eluted into the water, and NaOH is recovered. However, as a result of this step, the particle size of the particles is reduced by the amount of the surface layer 25, as shown in b. Further, as shown in c and d, each time the reaction and hydrolysis are carried out, the diameter of the particles becomes smaller, so that they can no longer be used as media particles and are scattered in the empty column. The scattered particles accompany the exhaust gas as dust, placing a heavy burden on the dust collector, and on the other hand, a large amount of medium particles are required to replenish the medium that has become unusable, which is uneconomical.

That is, as long as NaOH is recovered by the above method, almost the same amount of iron as Na is consumed, which is extremely uneconomical. Furthermore, Fe ₂ O ₃ , which is the medium particle from which the surface layer 25 was removed in the hydrolysis step, is 10
Since it contains ~20% moisture, when it is reused as media particles, it consumes a large amount of heat in the furnace due to evaporation of the moisture, making it thermally uneconomical.

The purpose of this invention is to eliminate the above-mentioned problems,
The aim is to significantly reduce the amount of metal oxide consumed and the amount of heat consumed during Na recovery.

In short, this invention is a method for recovering the sodium component in black liquor by burning a mixture of black liquor and metal oxide in a fluidized bed, and is a method for recovering the sodium component in black liquor, which is made of silica sand, calcined magnesium (MgO), electroformed alumina, etc. The compound of sodium component and metal formed on the surface of a chemically stable and wear-resistant fluid medium is peeled off and removed from the fluid medium by mechanical separation means, and this peeled compound is hydrolyzed to remove the sodium component. This is a direct causticizing method using a fluidized bed, which is characterized by recovering the fluid and reusing the fluidized medium.

Embodiments of the present invention will be described below with reference to the drawings.

In Fig. 3, 1 is a fluidized bed furnace, and inside this furnace there is a metal oxide which has high wear resistance and which is different from metal oxide (hereinafter explained using "Fe ₂ O ₃ " as an example), which is contained in black liquor. A fluidized bed is formed using a chemically stable medium that does not react with the Na content. Note that silica sand or the like is suitable as the above-mentioned medium, and more preferably calcined MgO or electroformed alumina (A1 ₂ O ₃ ). 16 is a mixer, in which the black liquor 14 and Fe ₂ O ₃ are mixed, and the mixture 5 is combusted in the fluidized bed furnace 1 . During the combustion stage, NaOH in the black liquor reacts as shown in equations (1) and (2) above to form a layer of NaFeO ₂ around the fluidized medium. The medium particles 6 with _two layers of NaFeO formed on their surfaces are extracted from the fluidized bed and supplied to the cooler 2. In this cooler 2, air 1 is used as a cooling medium.
1 is supplied, and the heated air 14 is returned to the fluidized bed furnace 1 as combustion air. Note that the cooling medium to be supplied to the cooler is not limited to air; for example, water may be used, and the heat may be recovered as hot water or steam. The cooled media particles 7 are then fed to the separator 3. In this separator, for example in a pad mixer, the media particles are stirred and the particles adhere to the surface.
The NaFeO ₂ layer is mechanically peeled off from the media particle surface. In this case, since the media particles have high wear resistance, only a small amount is crushed during the exfoliation removal operation, and most of them can be reused as the fluidized media 10 and returned to the fluidized bed furnace 1. and used.

On the other hand, the peeled NaFeO ₂ layer becomes a powder 8 which is separated and collected and sent to a hydrolysis step 17. Here, NaFeO ₂ is hydrolyzed as shown in formula (3).

2NaFeO ₂ +H ₂ O → 2NaOH + Fe ₂ O ₃ ...(3) As a result, Fe ₂ O ₃ is re-supplied to the mixer 16 for mixing with the black liquor 14, while being recovered.
NaOH will be reused in the pulp manufacturing process.

Apart from this powder, the NaFeO ₂ layer formed on the surface of the medium in the fluidized bed furnace 1 is fluidized, so before the medium is sent to the separator 3, a part of it is caused by mutual friction and collision between the media. The particles are separated from the medium surface and scattered outside the furnace together with the combustion gas 12. scattered
NaFeO ₂ is collected in the dust collector 4 and sent to the hydrolysis step 17 as a collected powder 18,
We are trying to improve the recovery efficiency of NaOH.

In addition, in the above process, the bed height of the fluidized bed is increased to increase opportunities for contact and friction between media particles, and the NaFeO ₂ layer formed on the media surface is actively exfoliated and removed in this fluidized bed furnace 1 for combustion. It may also be collected from within the gas 12. In this way, the load on the separator 3 can be reduced.

Since this invention is configured as described above, Na
The fluidized medium does not become smaller in diameter when the compound of metal and metal is removed from the surface of the medium, and the fluidized medium can be used repeatedly over a long period of time.

Further, the compound removal and hydrolysis steps are separated, and water is not used in compound removal, so the fluidized medium that is resupplied does not contain water and does not take away heat from the furnace, resulting in high thermal efficiency.

[Brief explanation of drawings]

Figure 1 is a process diagram of a conventional direct causticizing method, Figure 2 is a cross-sectional view of fluidized medium particles made of Fe ₂ O ₃ , and Figure 3 is a process diagram of a direct causticizing method showing an embodiment of the present invention. be. 1... Fluidized bed furnace, 3... Separator, 14... Black liquor, 16... Mixer.

Claims (1)

[Scope of Claims] 1. A method for recovering sodium components in black liquor by burning a mixture of black liquor and metal oxide in a fluidized bed, the method comprising: silica sand, calcined magnesium (MgO), electroformed alumina, etc. The compound of the sodium component and the metal formed on the surface of the chemically stable and wear-resistant fluid medium is peeled off and removed from the fluid medium by mechanical separation means, and this peeled compound is separated by hydrolysis. A direct causticizing method using a fluidized bed, characterized in that the sodium component is recovered and the fluidized medium is reused. 2. The fluidized fluid according to claim 1, characterized in that part of the step of stripping off the sodium and metal compound is carried out in a fluidized bed, and the stripped compound is collected from combustion gas and hydrolyzed. Direct causticization method using layers.