JPS5988989A - Recovery of chemicals from black liquor - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for recovering chemicals, mainly caustic soda, from a waste liquid called black liquor generated in a pulp manufacturing process.

In the valve manufacturing process at paper mills, chemicals such as caustic soda (NaOH) are used to completely separate the wood components nocellulose (fiber) and lignin (Ml fat) and extract only cellulose. The solution containing dissolved lignin and NaOH that has passed through the fish waste 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.

FIG. 1 is a system diagram showing a conventional black liquor combustion and N a OH recovery method. In the figure, water conservancy chips are in the cooking process 1.
The rigenin contained in it is removed by NaOH,
The mixed solution of NaOH and lignin is discharged as black liquor, and combustion 2a is performed in a dedicated combustion device such as a black liquor recovery boiler to recover heat and the reaction shown in the following equation takes place. The reaction that should occur takes place.

Na,, O+ SOz+20x -□ Na, 5o
4-(1) Nat O+ Co2NazOOs...
・(2) Also, at the bottom of the furnace, due to the combustion of char, the following equation (3)
.

The reaction shown in (4) is mainly carried out.

NazS○4 + 20 →Na2S + 2002
...(3) Na2Co3Naz O-1-002-
(4) In other words, in the conventional method, the above-mentioned complex reactions must be carried out in the combustion device, and in order to make these reactions occur well, the combustion air supplied to the combustion device is also primary,
It is necessary to separate the air into secondary and tertiary air, and to precisely control these air types. Next, Na2C generated in combustion
o, water is added in the dissolution step 3a and sent to the causticization step 4. In this step, as shown in the following formula (5)G, it reacts with slaked lime [Ca(OH)2] to form Na, OH
Collect and reuse.

Na200s + Ca (OH)2-p-2NaOH
+ 0aOOx 45) However, 0a(OH)2 used in the double equation (5) is 0
Heating aOOs and adding water to it produces aa(oH)
To generate 2, 1/) must go through a complicated process,
The problems of increased equipment costs and increased energy consumption are widespread throughout the world.

That is, in the calcination step 5, a calcination device usually called a lime kiln is used, and the following reaction is carried out by consuming a large amount of energy such as petroleum.

C! acos-)-C! ao +CO2・=(6)
Furthermore, the quicklime (OaO) produced by this is removed from the slaked step 6.
In this step, the lime becomes slaked lime as shown in the following formula (7) and is ready for use in the causticizing process.

Oa, O+H20--Sea Oa (OH)2 ・
(7) As mentioned above, the reaction process of the conventional method is extremely duplex, and therefore, it is difficult to control and requires a more complicated setup, which is uneconomical.

Furthermore, the calcination process consumes a large amount of energy as fuel, making it extremely difficult to achieve energy conservation, which is a social requirement.

In view of the above points, the inventors have previously proposed a chemical recovery method called a direct causticization method. The greatest advantage of this method is that it does not require a calcination step that consumes a large amount of energy, as will be described later. In this direct causticizing method, Fe2O3 is added as iron oxide powder to the black liquor to generate NaFeO2, which is reacted with the NaOH contained in the black liquor.
This method further hydrolyzes this NaFeO2 to recover NaOH. Therefore, in this method, NaFeO
The quality of the hydrolysis efficiency of No. 2 directly affects the NaOH recovery efficiency. However, the direct causticization method itself has not yet become an established technology, and the actual situation is that the efficiency of the above-mentioned hydrolysis is still at a low level.

An object of the present invention is to provide a direct causticizing method that is constructed in view of the above-mentioned problems and allows for easy handling of recovered chemicals and high efficiency in recovering chemicals.

In short, this invention makes it easier to handle the recovered NaOH by maintaining the reaction humidity above the melting point of NaOH to be recovered when hydrolyzing NaFe0z, and also enables hydrolysis under these humidity conditions. In order to
In some cases, the H20 used is converted into high-temperature steam, and the firing humidity of NaFe0z is lowered as much as possible.
This is a direct causticizing method designed to increase the recovery efficiency of shaft OH.

Examples of the present invention will be described below.

First, the outline of the direct causticization method, which is the premise of the method according to the present invention, will be explained using FIG.

In the figure, wood chips are treated with NaOH in cooking step 1.
The lignin is separated into lignin and NaO.
Black liquor containing H undergoes the following reaction during combustion by adding Fe2Og as iron oxide powder in combustion process 2 or in the preceding stage.

2NaOH-1-(EOI-Il-Na2COs +H
20=(8)Na2COs+Fe 20$ -2NaF
eCb + CO2- (9) Of these, sodium ferrate (
NaFe02) is hydrolyzed in the next dissolution step 7 to recover NaOH.

2NaFe02-1- H2C1→2NaOH+Fe2
es ・H+ (10) In other words, in the direct causticization method shown in Figure 2, it is possible to recover all NaOH by performing the reactions (8L (9) and (10)), and the Fe20s used for this recovery can be recycled. can be used.

Here, Figure 3 shows NaFeO2 fired at different temperatures.
FIG. 2 is a diagram showing the relationship between the hydrolysis reaction time and the caustic sorter (Na, OH) recovery rate when the inventors conducted a NaOH recovery experiment using the NaOH sorter. Note that the hydrolysis was performed at a temperature of 50°C. As is clear from this diagram, NaF
The lower the eO2 calcination temperature, the higher the caustic soda recovery rate, for example 4112o.

The recovery rate was low for those calcined at temperatures higher than °C, and a high recovery rate could not be obtained even if the reaction time was extended. Therefore, the firing temperature of NaFeO2 is preferably], OO
The temperature is preferably 0°C or lower.

Figure 4 shows a direct injection diagram constructed with the above points in mind.

Figure 2 shows a systematic diagram of pulp production including the sexualization method. In the figure, water conservancy chips, which are the raw materials for Noilub, are supplied to the Noilbu from line ], 0, and Na and OH, which are cooking chemicals, are supplied to the Noilbu from the line 20, and cooking 1 is performed.

When the cooking is completed, the contents of the mulberry tube pass through the line 30 to the blow tank 12, and further via the line 40 to the washer 13. Here, the noggle is discharged from the system via line 50 and sent to the paper manufacturing process. On the other hand, NaOH
The waste liquid containing black liquor passes through line 60 to concentrator 1
4, and is supplied to a combustion device 15 such as a black liquor recycling Jfi boiler via a line 70, where it is combusted. When the black liquor is supplied to the combustion device, Fe2O8 is mixed into the black liquor through the line 130, and the reactions (8) and (9) described above are carried out in the combustion device 15.

In this case, as described above, it is desirable that the reaction humidity during NaFeO2 production be as low as possible.

Exhaust gas generated by combustion is discharged to the outside of the system through line 90. N2LFθ02 generated in the combustion device 15
is fed to reactor 16 via line 80.

In this case, if the humidity of 14aFθ02 is 1200'c or more, this NaFeO2 is in a molten or semi-molten state. In this reactor, water is added to carry out hydrolysis, and if the reaction is carried out above the melting point of NaOH (318.4°C), the recovered NaOH can be easily handled. For this reason, it is effective to supply steam at a temperature higher than the above temperature as N20 for hydrolysis through the line 100, for example. Completely hydrolyzed Fex Os and Na
The OH reaches the decomposition tank 17 via the line] and 10, and the separated FθWOS is mixed into the black liquor as described above, while the N
The aOH is stored in the caustic soda tank 18 via line 120 and is supplied to the cooking process 1 via line 20 as appropriate.

According to the present invention, it becomes possible to carry out the direct causticization method effectively, and specifically, since the chemical can be recovered in a molten state, it is easier to handle compared to the case where the chemical is recovered as a solid substance.

In addition, by lowering the temperature at which the NaFeO2 reaction forms, the recovery efficiency of 1JaOH can be increased.

[Brief explanation of drawings]

Figure 1 is a system diagram showing the conventional chemical recovery method, Figure 2 is a schematic diagram showing the direct causticization method, and Figure 3 is the relationship between Nal1'θ02 hydrolysis reaction time and caustic soda recovery rate at each calcination temperature. FIG. 4 is a system diagram of a direct causticizing method illustrating the method according to the present invention. 15...Black liquor combustion device 16...Reactor attorney OJ Gobe

Claims (1)

[Claims] 1. Adding F6B On to black liquor to make NaFeO
A method for recovering chemicals in black liquor, characterized in that the reaction temperature of the hydrolysis is set to be higher than the melting point of NaOH, in a method of recovering NaOH by reacting and producing NaFeO2 and hydrolyzing the NaFeO2. 2. The method for recovering chemicals in black liquor according to claim 1, characterized in that the H2O used in the hydrolysis is high-temperature steam. 3. The reaction formation temperature of Na and FeOz is set to 1000°.
3. The method for recovering chemicals in black liquor according to claim 1 or 2, characterized in that the concentration is less than or equal to C.