JPH0929201A - Method for removal of salt and potassium salt from collected ash of sode recovery boiler - Google Patents

Abstract

PROBLEM TO BE SOLVED: To recover effectively salt and potassium salt by a method wherein a collected ash collected from an exhaust gas of a soda recovery boiler is mixed with water to make slurry, salt in the collected ash is dissolved in water, and when a solid content is separated from the liquid after depositing the solid content, temperature and pH of the slurry are respectively specified. SOLUTION: When salt and potassium salt are removed from a collected ash collected from an exhaust gas in a pulp manufacturing plant, at first, the collected ash of an electric static precipitator and water are fed to a first agitating bath 1 to make slurry. In this case, temperature of the slurry is kept at 20 deg.C or above with a heat exchanger 4, and pH of the slurry is adjusted at 10 or above, by feeding diluted sulfuric acid 12 controlled with a pH adjustor 15. after keeping this state for 1-2h, the slurry is fed to a second agitating bath 6, water or ice is fed therein to keep temperature at 20 deg.C or below, and crystallization of Na2 SO4 is accelerated. Then, after separating a solid content in the slurry with a decanter 10, sludge is dissolved again in black liquor before concentration, and the solid content is recovered by returning it to a front courrent of a black liquor concentrator.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is applied to a soda recovery boiler in a pulp manufacturing plant, and collects salt and potassium salts as impurities that are concentrated and accumulated in pulp cooking chemicals from combustion exhaust gas. It relates to a method of removing ash.

[0002]

2. Description of the Related Art Removal of potassium salts in pulp cooking chemicals
There is no method that has been applied to actual equipment as a conventional method, but it is similar
There is a salt removal method as the technique. That is, soda times
Uses a small amount of water and circulating liquid to collect the collected ash from the exhaust gas of the collecting boiler.
To make it into a slurry and dissolve the salt in water at a temperature of 60-70 ℃.
It is a method to separate and recover the solid content in the slurry.
However, this method can easily remove the salt in the drug.
However, since the solubility of potassium salt is low, potassium salt is removed.
Na is an effective drug when trying to leave_TwoSO _FourBut at the same time
However, it has a drawback that the chemical loss becomes large.

[0003]

In pulp cooking chemicals, salt and potassium salts as impurities are mainly brought from pulp logs. These impurities leave the chemical recovery system along with the pulp that is the product and the lost chemical that goes out of the system, but as the chemical recovery becomes closed and the chemical recovery rate increases, it is concentrated and accumulated in the chemical recovery system. There is a tendency to

It is well known that concentrated and accumulated salt or potassium salt, as an inert substance, not only increases the circulation load of chemicals and leads to a decrease in economic efficiency due to an increase in dead load, but also significantly increases the corrosiveness of the chemical recovery system. ing. Especially in soda recovery boilers that burn pulp waste liquid,
The presence of sodium chloride and potassium salt not only enhances the corrosiveness of the heat transfer tube in the high temperature part of the boiler, but also significantly increases the adhesion of the combustion ash adhering to the surface of the heat transfer tube, increasing the flue pressure loss of the soda recovery boiler and continuing the boiler. Leading to disruption of operations.

The present invention solves the above problems in the prior art, removes salt and potassium salt from the chemical recovery system, and is capable of efficiently recovering chemicals. A method for removing potassium salt is provided.

[0006]

According to the present invention, the collected ash collected from the combustion exhaust gas of a soda recovery boiler is mixed with water to form a slurry, and the pH of the slurry is adjusted to 10 or less by adding sulfuric acid, and the temperature is set to 20. After adjusting the temperature to ℃ or more and maintaining it for a certain period of time to dissolve the salt and potassium salt in the collected ash in water and cooling the slurry to a temperature of less than 20 ° C. to precipitate the solid content, the solid content and liquid The liquid is discarded to the outside of the system, the solid content is redissolved in the black liquor before concentration, and the solid content is recovered by returning the black liquor to the upstream of the black liquor concentrator. It is a method of removing salt and potassium salts from the collected ash of a soda recovery boiler.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION As described above, in the conventional salt removal method, the ash collected from the soda recovery boiler is contacted with a small amount of water at 60 to 70 ° C. to remove the salt. Therefore, if water is added until the required removal amount is obtained, the amount of sodium sulfate, which is an effective chemical, is eluted and the economy is deteriorated.

Therefore, in the present invention, the main purpose is to remove potassium salts, and the collected ash is slurried with water and neutralized with sulfuric acid to adjust the pH of the slurry to 10 or less (10 to 7). Most of the carbonates contained in the collected ash mainly composed of salt are also sulfates. In this way, potassium and sodium contained in the collected ash are eluted as potassium sulfate and sodium sulfate to generate sensible heat, heat of dissolution, heat of neutralization, etc. of the collected ash, and then the slurry is washed with industrial water or the like. The temperature of 20
° C. or higher (preferably Na ₂ SO 30 ° C. before and after reduction in solubility is less in ₄₎ is adjusted to. The slurry is then treated at 20 ° C., where the solubility of sodium sulfate is low and therefore chemical loss is lower.
At a low temperature of less than (generally less than 20 ° C. and 10 ° C. or higher), sodium sulfate is precipitated, separated and recovered, and potassium sulfate is dissolved / removed. By cooling the aqueous solution of 20 ° C. or more (preferably around 30 ° C.) in which the collected ash is dissolved to a temperature of less than 20 ° C., the degree of supersaturation of the solution can be increased and the precipitation of coarse crystals can be promoted. Since the precipitated crystal grains have a large particle size, subsequent separation / recovery becomes easy.

The solid content thus separated and recovered is N
Since it contains a ₂ SO ₄ · 10H ₂ O and water adhering during solid-liquid separation, the water content in the solid content reaches about 60%. When this solid content is returned to the concentrated black liquor immediately before the recovery boiler, the amount of water brought into the recovery boiler increases and the boiler efficiency decreases.
Therefore, this solid content is dissolved in the black liquor before concentration, returned to the upstream of the black liquor concentrator, and concentrated in the black liquor concentrator. As a result, the amount of water brought into the recovery boiler does not increase, and the decrease in boiler efficiency due to solid content recovery can be eliminated.

(Operation) FIG. 1 shows Na_TwoSO_Four-K_TwoSO_Four
Is a plot of the solubility of a mixed system against temperature.
You. 1, Na_TwoSO_Four(Mixed system), K_TwoSO_Four(Mixed
Combined system) is Na _TwoSO_Four-K_TwoSO_FourNa in mixed system_Two
SO_Four, K_TwoSO_FourSolubility curve, Na _TwoSO_Four(single
Germany), K_TwoSO_Four(Single) means each individual Na_TwoS
O_Four, K_TwoSO_FourShows the solubility curve of K in FIG.
_TwoSO_FourThe solubility of Na changes little with temperature, but
_TwoSO_FourThe solubility of is highly dependent on temperature,
The solubility is significantly reduced.

Using a fixed amount of the collected ash of the same composition, 80
Make slurries at 10 ° C and 10 ° C. In this case, hydrogenation of both
Na that melts on the water side by changing the amount added_TwoSO_FourThe same amount of
Slurry Then, in the case of 80 ℃ slurry, it melts
Na_TwoSO_FourAbout 1/4 amount of K_TwoSO_FourOnly melted
No, but at 10 ° C, Na_TwoSO_FourThe same amount of K _Two
SO_FourDissolves. Therefore, K that remains in the solid matter in the slurry
_TwoSO_FourIs Na_TwoSO _FourAgainst K_TwoSO_FourMelts a lot
Solids in the slurry at 10 ° C are reduced as less is discharged
K inside_TwoSO_FourIs about 1 / compared to that at 80 ℃
It becomes 4.

However, when the collected ash of the recovery boiler is brought into contact with low temperature water from the beginning, undissolved coarse particles are likely to remain, and slurry handling such as stirring and pumping is difficult. On the other hand, as in the present invention, when the slurry is previously adjusted to 20 ° C. or higher (preferably around 30 ° C. in which the decrease in the solubility of Na ₂ SO ₄ is small) and subsequently kept at a low temperature of less than 20 ° C. Since the ash is once in solution, it is easy to handle.

In the above operation, the dust collecting ash (electrostatic dust collecting ash) is once dissolved in water and then crystallized. During this dissolution, the dust collecting ash (hereinafter referred to as EP ash) is dissolved. salt,
Particularly, Na ₂ CO _{3 and the} like generate a large heat of solution as shown in the following equation.

Embedded image

[0014] Figure 2 but shows the pH and concentration of Na ₂ CO ₃ and Na ₂ SO ₄ in the separation liquid of the slurry separation liquid, the slurry when Na ₂ CO ₃ in EP ash increases Since Na ₂ CO ₃ dissolves in the solution as the pH rises, the Na recovery rate decreases overall. In order to prevent this, as shown in FIG. 2, it is necessary to neutralize the pH of the slurry to 10 or less, but during this neutralization, a large heat of neutralization is generated as in the following equation.

Embedded image Na ₂ CO ₃ + H ₂ SO ₄ → Na ₂ SO ₄ + H
₂ O + CO ₂ -61.6 Kcal / kg And EP ash is one collected in a temperature range around an exhaust gas temperature of about 200 ° C., and its own sensible heat is also large.

Therefore, when the crystals are suddenly deposited into a slurry of less than 20 ° C., all of the sensible heat, the heat of dissolution, the heat of crystallization, the heat of crystallization, etc. of the EP ash are all consumed by the expensive cold heat cooled to less than 20 ° C. in the cooler. Need to be cooled, which increases the cooling cost.

On the other hand, as in the method of the present invention, EP ash is first dissolved and slurried in the first step, and then neutralized at 20 ° C.
Above (preferably around 30 ℃), 20 ℃ in the second stage
The mixture is cooled to a temperature below the temperature to precipitate crystals, and solid-liquid separation is performed. In this case, the sensible heat, the heat of dissolution, the heat of neutralization, etc. of the EP ash in the first stage can be cooled by heat exchange with ordinary cooling water, and the crystallization heat in the second stage can be mainly cooled by using a cooler. It will be cooled below ℃. Thus, the cooling cost can be greatly reduced by cooling the sensible heat, the heat of dissolution, the heat of neutralization, etc. of the EP ash by cooling with cooling water such as ordinary industrial water.

The method of the present invention is then applied to Na ₂ C in EP ash.
The higher the O ₃ concentration, the greater the effect. In addition,
Since the aqueous solution of the collected ash which has been heated to 20 ° C. or higher in advance is cooled to below 20 ° C. in the next step, the supersaturation degree of the solution is increased and N
Precipitation of coarse crystals of a ₂ SO ₄ can be promoted.

Solid matter having a high water content of about 60% obtained by precipitating solid matter in this way and separating and recovering it is not returned to the dark black liquor before charging the recovery boiler, but before concentration. By returning to the black liquor and concentrating it with the black liquor concentrator, it is possible to eliminate an increase in the amount of water brought into the recovery boiler, and the efficiency of the boiler is not lowered.

[0019]

EXAMPLE A concrete example of the present invention will be described with reference to FIG.
Will be explained. Collected ash from the electrostatic precipitator 1.0 in the first stirring tank 1.
Tons / hour (The composition of the collected ash is wt% NaCl: 9.7
%, Na_TwoSO_Four: 67.2%, Na_TwoCO_Three: 10.
1%, KCl: 1.5%, K _TwoSO_Four: 9.9%, K_Two
CO_Three: 1.6%) continuously supplied from conveyor 2.
In parallel with that, water: 1.0 ton / in the first stirring tank 1
Supply time. Strongly stir the stirrer 3 in the first stirring tank 1.
Then, the collected ash and water are thoroughly mixed to form a slurry. On this occasion,
With the heat exchanger 4 using cooling water, the slurry in the first stirring tank 1 is
Keep the temperature at 25 ° C. At this time, the slurry
pH is Na_TwoCO_ThreeSince it shows 10 or more, dilute sulfuric acid
Dilute the sulfuric acid 12 in the tank 11 with the instruction of the pH meter 14
The H regulator 15 controls the chemical injection pump 13 to control the first stirring tank 1
And the pH of the slurry is adjusted to 9.
As a result, Na in the slurry_TwoCO_ThreeMost of the Na_TwoS
O _FourConvert to In this state, stay for 1-2 hours,
Approximately 2.0 tons / hour of slurry is supplied to the second stirring tank 6 by the rear pump 5.
Transfer.

Next, water or ice is supplied to the second stirring tank 6 in a total amount of about 1 ton / hour and gently stirred by the stirrer 7.
8 hours allowed to stay, encourage crystal growth of Na ₂ SO _4. At this time, the temperature of the slurry in the second stirring tank 6 is kept at 15 ° C. by the supply of ice and the cooler 8.

Next, about 3.0 tons / hour of the slurry in the second stirring tank 6 was transferred to the decanter 10 by the pump 9 to separate the solid content in the slurry. As a result, 2.1 ton / hour of sludge and 0.9 ton / hour of the separated liquid were obtained. The separated liquid 0.9 ton / hour was diluted with water and then sent to the wastewater treatment device in the factory. The 2.1 ton / hour sludge is composed of 0.9 ton / hour of solid content, 1.2 ton / hour of water of crystallization and adhering liquid. This sludge was sent to a dissolution tank 16 equipped with a stirrer 17, dissolved in the black liquor before concentration, and returned to the black liquor in the front stream of the black liquor concentrator. In this case as well, it was confirmed that the black liquor concentration at the outlet of the black liquor concentrator could be concentrated to 75% as the solid content concentration, as in the case where sludge was not returned.

The Na ₂ SO ₄ recovery in this example is about 8
5%, the removal rate of potassium components such as KCl and K ₂ SO ₄ was about 95%, and the removal rate of NaCl was about 97%.

Next, 1 ton of EP ash containing 15% of Na ₂ CO ₃ at 150 ° C. (assuming the rest to be Na ₂ SO ₄ : 85%) was slurried with 2 tons of water, and this was all cooled. Table 1 shows a comparison between the amount of cooling heat when cooled to 15.degree. C. and the amount of cooling heat of the preferred method of the present invention using cooling water and a cooler. In Table 1, the values (measured values) shown in Table 2 were used.

[0024]

[Table 1]

[0025]

[Table 2]

As is clear from Table 1, the method using the cooling water and the cooler is about half of the total cooling heat amount (59155/1).
It is understood that the cooling heat amount of (27855 × 100 = 46.3%) is cooled by inexpensive cooling water, and the rest is cooled by the cooler, whereby the cooling cost can be greatly reduced. And the method of returning sludge (solid content) that has been separated and recovered to the black liquor before concentration and concentrating it with the black liquor concentrator, which is the method of the present invention, does not increase the amount of water brought into the recovery boiler and reduces boiler efficiency. There is nothing to do.

[0027]

【The invention's effect】

1. By removing salt and potassium salt from the collected ash of the recovered boiler exhaust gas, it is possible to reduce the concentration of chlorine and potassium in the chemical system. As a result, the adhesion of the combustion ash of the recovery boiler flue was reduced, and the conventional continuous operation of about 3 months could be extended to 6 months to 1 year. 2. At the same time, the amount of steam consumed in the suit blow for removing the ash adhering to the heat transfer surface was significantly reduced. 3. By removing the salt and potassium salt, the melting point of the adhered ash was not lowered, and the corrosion of the heat transfer tube in the high temperature part of the boiler was reduced. The corrosiveness of each part of the recovery boiler has been improved, for example, the service life of the smelt spout has been extended about three times. 4. Cooling with cooling water cuts the power of the cooler in half, making it possible to greatly reduce cooling costs. 5. Chemicals can be efficiently recovered by returning the separated and recovered sludge (solid content) to the black liquor in the upstream of the black liquor concentrator. According to this method, the amount of water brought into the recovery boiler is not increased, and the boiler efficiency is not reduced unlike the method of returning sludge to black liquor before the boiler is charged.

[Brief description of drawings]

FIG. 1 Na ₂ SO ₄ mixed system of Na ₂ SO ₄ —K ₂ SO _4
4 , solubility of K ₂ SO ₄ and Na ₂ SO ₄ alone system, K ₂
Sole system diagram showing the solubility of each of the SO _4.

FIG. 2 pH of slurry separation liquid and Na ₂ S in the separation liquid
O _4, table showing the relationship between the concentration of Na ₂ CO _3.

FIG. 3 is a schematic view of a process for explaining one embodiment of the present invention.

Claims (1)

[Claims] 1. The collected ash collected from the combustion exhaust gas of a soda recovery boiler is mixed with water to form a slurry, and p of the slurry is mixed.
H was adjusted to 10 or less by adding sulfuric acid and the temperature was adjusted to 20 ° C. or more, and the salt and potassium salt in the collected ash were dissolved in water by maintaining for a certain time, and the slurry was cooled to a temperature of less than 20 ° C. After precipitating the solid content, it is separated into the solid content and the liquid, the liquid is discarded outside the system, and the solid content is redissolved in the black liquor before concentration,
A method for removing salt and potassium salt from the collected ash of a soda recovery boiler, which comprises recovering the solid content by returning the black liquor to the upstream of the black liquor concentrator.