JP2578199B2 - Method for removing salt and potassium salt from pulp cooking chemicals - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention is applied to a soda recovery boiler in a pulp manufacturing plant, and removes salt and potassium salts as impurities that are concentrated and accumulated in pulp cooking chemicals. On how to do it.

[Conventional technology]

There is no method for removing potassium salts from pulp cooking chemicals, which has been conventionally applied to actual machines.

As a similar technique, there is a salt removal method. That is,
This is a method in which ash is collected from the exhaust gas of a soda recovery boiler, slurried using a small amount of water and a circulating liquid, salt is dissolved in water at a temperature of 60 to 70 ° C, and solid content in the slurry is separated and recovered. In this method, the salt in the drug can be easily removed, but the solubility of the potassium salt is low, so that when trying to remove the potassium salt, Na ₂ SO ₄ is lost at the same time, so that there is a disadvantage that the loss of the drug is large.

[Problems to be solved by the invention]

Salt and potassium salts as impurities in pulp cooking chemicals mainly come from raw pulp wood. These impurities leave the chemical system along with the product pulp and lost chemicals that go out of the system, but tend to concentrate and accumulate in the chemical system as the chemicals become more closed and the chemical recovery rate increases. Occurs.

It is well known that the concentrated and accumulated salt or potassium salt not only increases the circulation load of the chemical as an inert substance and leads to a decrease in economy due to an increase in dead load, but also significantly increases the corrosiveness of the chemical recovery system.

Especially in a soda recovery boiler that burns pulp waste liquid, the presence of salt and potassium salt not only enhances the corrosiveness of the heat transfer tubes in the high temperature part of the boiler, but also significantly enhances the adhesion of combustion ash adhering to the surface of the heat transfer tubes, soda recovery. The boiler flue pressure loss will increase, which will hinder the continuous operation of the boiler.

[Means for solving the problem]

The present invention is a method for removing salt and potassium fumes accumulated and concentrated in a pulp cooking chemical system, in which collected ash collected from combustion exhaust gas of a soda recovery boiler is mixed and dispersed in low-temperature water of 20 ° C or less. After the salt and potassium salt in the collected ash are dissolved in water by holding the slurry for a certain period of time in water, the solid content in the slurry is separated and recovered, and the salt and potassium salt in the pulp cooking chemicals are collected. This is the removal method.

In the conventional salt removal method, the collected ash from the soda recovery boiler dissolves and removes the salt with a small amount of water.However, in the case of potassium, since the solubility in water is small, it is effective to add water until the required removal amount is obtained. The amount of sodium sulphate, which is a chemical, elutes, which increases the economic efficiency.

Therefore, the present invention has a main purpose of removing potassium salt,
It is intended to dissolve and remove potassium sulfate at a low temperature of 20 ° C. or less, in which the solubility of sodium sulfate is low and thus the chemical loss is smaller. This is the first feature of the present invention.

Under such low-temperature conditions, sodium sulfate becomes decahydrate, and the sludge extracted and separated with water contains a large amount of water, so that evaporation loss occurs when returned to the recovery boiler system. Therefore, in general, it can be said that dissolving and removing by water at a low temperature is disadvantageous because, in addition to requiring cooling, water is brought in by crystallization water. In the method of the present invention, the collected ash is slurried in low-temperature water, and the potassium salt is dissolved and removed in water at low temperature. The second feature of the present invention is that it has been found that the precipitation of decahydrate (Na ₂ SO ₄ 10H ₂ O) can be suppressed to less than half as compared with the method of precipitation separation, and operation is performed based on the knowledge. According to the method of the present invention, the incorporation of water of crystallization is reduced, and thus the overall economic efficiency is improved.

The third feature of the present invention is that it is necessary to separate the sludge after the collected ash is slurried in low-temperature water, but the separation is extremely good because the crystal growth is sufficiently fast even at low temperatures. That is what we found experimentally. Generally, the particle size of the collected ash is submicron, and it is extremely small and cannot be easily separated. In the conventional salt removal method, the collected ash crystals are kept in the slurry for a long time (several hours) to grow the crystals and improve the separability, but in the method of the present invention, the crystal growth is fast. It was confirmed that the residence time may be short because of good filterability.

[Action]

FIG. 1 shows the solubility of a Na ₂ SO ₄ —K ₂ SO ₄ mixed system with respect to temperature, and explains the first feature of the present invention. In Fig. 1, Na ₂ SO ₄ (mixed system), K ₂ SO ₄ (mixed system)
Are the solubility curves of Na ₂ SO ₄ and K ₂ SO _{4 in} the Na ₂ SO ₄ -K ₂ SO ₄ mixture, and Na ₂ SO ₄ (alone) and K ₂ SO ₄ (alone)
₃ shows solubility curves of Na ₂ SO ₄ and K ₂ SO ₄ . Na ₂ SO ₄ .10H ₂ O (single / mixed) is almost the same as anhydrous salt and decahydrate, and thus has a single solubility curve.

In FIG. 1, the solubility of K ₂ SO ₄ has little change with respect to temperature, but the effect of temperature is large in the case of Na ₂ SO ₄ , and the solubility is significantly reduced at low temperatures.

As salt removal, compared to the case of extraction with 60 to 70 ° C., when subjected to extraction for example at 10 ° C., the same K ₂ SO ₄ removal amount, the loss due to dissolution of Na ₂ SO ₄ is low About 1/4
become. At the same Na ₂ SO ₄ recovery rate, the K ₂ SO ₄ elution amount is 4
Can be doubled. This is the hardening of the extraction at low temperature and is the gist of the present invention.

The suppression of decahydrate, which is a feature of FIG. 2, will be specifically described in Examples, but the effect is to supply excessively collected ash beyond the solubility into the originally low-temperature water. This is because solid Na ₂ SO ₄ can remain as a solid in the slurry.

The reason why the sludge filterability is improved at low temperature, which is a characteristic of FIG. 3, is that the solid particles, which were initially a maximum of 2-3 microns in the slurry, grow into particles of about 100 microns and are coarsened. It is clear from the fact that was confirmed.

〔Example〕

Na ₂ SO ₄ : 85.9%, NaCl: 3.0%, K ₂ SO ₄ : 11.1%, collected ash 100
kg was mixed and slurried in 207.6 kg of water at 10 ° C., kept at that temperature for 8 hours, and then the crystals were filtered off. Na in sludge
₂ SO ₄ gained 74 kg. Na ₂ SO ₄ in the separated liquid is 11.8 kg, K ₂ SO ₄
Was 11.0 kg. This is approximately equal to the saturation solubility,
It can be said that the purpose of extraction with water was sufficiently achieved.

The recovery rate of sodium sulfate was 86%, indicating that the method was practical. The removal rate of salt was 80% or more, and the removal rate of K ₂ SO ₄ was 70% or more.

In the above embodiment, the collected ash was held in water for 8 hours, but the holding time is not limited to this, and may be in the range of 1 to 2 hours.

〔The invention's effect〕

1 By removing salt and potassium salt from the collected ash of the exhaust gas from the recovery boiler, the concentration of chlorine and potassium in the chemical system can be reduced. As a result, the adhesion of combustion ash to the recovery boiler flue is improved, and conventionally, continuous operation for about three months has been carried out for six months to one month by removing salt and potassium salts.
Could be extended to two years.

2 At the same time, the amount of steam consumed by the suit blower was significantly reduced due to the removal of ash attached to the heat transfer surface.

(3) The removal of the salt and the potassium salt prevented the decrease in the melting point of the attached ash and improved the corrosiveness. 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.

[Brief description of the drawings]

FIG. 1 is a table showing the solubility of Na ₂ SO ₄ and K ₂ SO _{4 in} a Na ₂ SO ₄ -K ₂ SO ₄ mixed system and the solubility of Na ₂ SO ₄ alone and K ₂ SO ₄ alone. .

 ──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Takayuki Maeda 1-1, Akunouramachi, Nagasaki City, Nagasaki Prefecture Mitsubishi Heavy Industries, Ltd. Nagasaki Shipyard (56) References JP 50-116701 (JP, A) JP JP-A-52-12303 (JP, A) JP-A-62-272005 (JP, A) JP-A-56-7620 (JP, A) JP-A-49-14701 (JP, A) JP-A-62-177293 (JP, A) , A)

Claims (1)

(57) [Claims] 1. The ash collected from the flue gas of a soda recovery boiler is mixed and dispersed in low-temperature water of 20 ° C. or less to form a slurry, and the slurry is retained for a certain period of time to remove the salt and potassium salt in the ash collected. A method for removing salt and potassium in pulp cooking chemicals, comprising separating and recovering a solid content in a slurry after dissolving in water.