JPH0665797B2 - Soda recovery method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to a direct causticizing fluidized bed system for recovering caustic soda from pulp cooking waste liquor (also referred to as black liquor) using iron oxide as a causticizing agent to remove impurities. More specifically, the present invention relates to a soda recovery method in which NaCl is not accumulated in the system by blowing the collected matter of the dust collector to the outside of the system.

[Conventional technology]

Conventionally, when recovering caustic soda from pulp cooking waste liquid (hereinafter referred to as pulp waste liquid) generated when wood chips are digested, the lime method has been put into practical use. However, this lime method has drawbacks in that the process is complicated, waste is generated, and measures against odor and drainage are required, and the causticizing rate is low.

In order to eliminate these drawbacks, as shown in Japanese Patent Publication No. 51-12724, the alkaline waste liquid containing substantially no sulfur compounds discharged from the cooking and bleaching steps of the fibrin substance is concentrated and iron oxide is concentrated. A method has been proposed in which sodium hydroxide is added and burned, and the obtained sodium ferrate (sodium ferrate) is put into water to directly recover caustic soda as an extraction aqueous solution, and the iron oxide obtained as an extraction residue is circulated and reused. ing. In this method, caustic soda is produced from the alkaline waste liquid, and the organic matter contained in the waste liquid is probably decomposed by combustion of the waste liquid, and the sodium carbonate and / or sodium oxide obtained as the product is expressed by the following formula (1). And as in formula (2), reacts with iron oxide to produce sodium ferrate,
It is considered that by treating this in water, caustic soda and iron oxide can be obtained as shown in formula (3).

Na ₂ CO ₃ + Fe ₂ O ₃ → Na ₂ Fe ₂ O ₄ + CO ₂ (1) Na ₂ O + Fe ₂ O ₃ → Na ₂ Fe ₂ O ₄ (2) Na ₂ Fe ₂ O ₄ + H ₂ O → 2 NaOH + Fe ₂ O ₃ (3) However, when applying the above method to a fluidized bed furnace, in the direct causticizing method, iron oxide (iron ore) is generally recycled and used, and only the carryover amount of a dust collector or the like is supplied. Yes (supplement rate is at most a few percent). On the other hand, granular iron ore, which is both a causticizing agent and a fluidized bed maintainer, increases the pulverization in the furnace as the number of times of re-use increases (about 10% pulverization), and other feeders Some pulverization is also seen in the handling of iron ore, causticizing process, etc. In addition, since the fluidized bed system basically uses a granular causticizing agent, the reaction molar ratio Fe / Na cannot be set to near 1.0 (equal molar ratio as shown in reaction formulas (1) and (2)). On the other hand, there is a problem that the powdery causticizing agent jumps out of the furnace immediately and the reaction cannot be expected to proceed sufficiently.

In order to solve the above-mentioned problems, the present inventors causticize powdered sodium ferrate collected by a fluidized bed boiler, a cyclone, and an electric dust collector, and then dry the dried powdered iron oxide. We developed a technology to granulate a part or all of the product using black liquor as a binder and supply it as granules to a fluidized bed furnace again, and filed a patent application as Japanese Patent Application No. 58-167927 (Japanese Patent Application Laid-Open No. 60-59190). ing.

[Problems to be solved by the invention]

The above-mentioned direct causticizing method has an excellent process in which the lime kiln is omitted, the process is simplified and the energy is saved as compared with the KP method, but there is a problem in that it is characterized by being closed.

That is, in the papermaking process, impurities other than the chemical liquid necessary for cooking enter due to wood chips, industrial water, and the like. In the KP method, impurities such as dregs and sludge are discarded outside the system as precipitates, whereas in the direct causticization method, the system is closed, so impurities that have entered from outside the system do not enter the system. It tends to be gradually concentrated. Of these, NaCl adversely affects the iron soda formation reaction (reacts reaction) and easily forms a eutectic mixture that melts in the low temperature range with Na ₂ CO ₃ , which adversely affects the direct causticizing fluidized bed system. That is a concern.

The present invention has been made in view of the above points, without accumulating NaCl in the system, the concentration is controlled below a certain value,
It is intended to provide a method capable of efficiently collecting caustic soda.

[Means and Actions for Solving Problems]

The soda recovery method of the first invention of the present application is to burn a pulp waste liquid containing NaCl in a fluidized bed furnace using iron oxide as a causticizing agent and a fluidizing medium, and then add the product sodium ferrate to water. In this method, the caustic soda and iron oxide are recovered, and the exhaust gas from the fluidized bed furnace is recovered by heat in a boiler, and then introduced into a cyclone or an electrostatic precipitator to collect dust from the electrostatic precipitator outside the system. It is characterized by doing.

That is, as a dust collector after the boiler, a cyclone such as a multi-cyclone, an electric dust collector, etc. are used, and among the flue trapped powdery sodium ferrate, the dust collector with the highest NaCl content is used. By discarding the collected material out of the system, the fluidized bed system can be suitably operated without accumulating NaCl in the system.

Further, the soda recovery method of the second invention of the present application is to blow all the collected matter of the electrostatic precipitator having a high NaCl content and a part or all of the collected matter of the cyclone having a high NaCl content to the outside of the system. Therefore, the fluidized bed system can be suitably operated without accumulating NaCl in the system.

The ratio of NaCl in the cyclone collected matter and the electric dust collector collected is usually 10 to 70% by weight of the total soda, and in the electric dust collector collected, 50% or more of Na is NaCl. It is contained in the form of.

〔Example〕

Hereinafter, embodiments of the present invention will be illustratively described in detail with reference to the drawings. However, the arrangement of the constituent devices described in this embodiment is not limited to them unless otherwise specified, and is merely an example of description.

The diluted black liquor is introduced into the thin film evaporator 1, evaporated and concentrated to 50-60% black liquor, and then introduced into the disc evaporator 2 to obtain 70-80% dark liquor. It is supplied to the fluidized bed furnace 3. On the other hand, granular iron ore storage tank 4 supplies granular iron oxide to fluidized bed furnace 3. Bed temperature of fluidized bed furnace 3 is 700-1
The temperature is 100 ° C., preferably 900 to 1000 ° C., and the Fe / Na is 1.0 or more. As the fluidized bed forming material, granular iron-based granular compounds such as pure iron, iron ore (Fe ₂ O ₃ , Fe ₃ O ₄ ) sinter, reduced pellets, and granulated iron-making dust are used. . In this example, iron ore is used as iron oxide, and the replenishment is performed with granular iron ore. 5 is a fluidized bed, 6 is an air dispersion plate, and 7 is a wind box.

Sodium ferrate from the fluidized bed furnace 3 is introduced into the first causticizing device 8 to extract and separate into a caustic soda aqueous solution and recovered iron oxide,
After the recovered iron oxide is dried by combustion exhaust gas in the first drying device 10, it is classified into granular iron oxide and powdered iron oxide by the classifying device 11, and the granular iron oxide is charged into the granular iron ore storage tank 4, while The powdery iron oxide is supplied to the granulating device 12 and the powdery iron oxide storage tank 13. As this granulating device, a compression granulating device is preferable.

On the other hand, the powdery collected matter collected by the boiler 14 is used as the second causticizing device.
Introduced into 15 and extracted into aqueous caustic soda solution and recovered iron oxide,
The separated iron oxide is separated and dried in the second drying device 16 by combustion exhaust gas or the like, and then, in the granulating device 12, the black liquor is granulated as a binder to form granular iron oxide into the iron ore storage tank 4.

All of the collected matter of the electric dust collector 18 or the electric dust collector 18
All of the collected substances of (1) and some or all of the collected substances of cyclone 17 are blown out of the system to prevent the accumulation of NaCl in the system.

In this way, the total amount of the collected matter of the electrostatic precipitator having the highest NaCl content is blown out of the system and discharged. There are cases where the whole amount of the cyclone collected is blown out of the system, a case where a part thereof is blown out of the system, and a case where the whole amount is not blown out of the system.

The part of the cyclone collected that is not blown out of the system is supplied to the second causticizing device 15 or the second drying device 16, but is introduced into the cold water extracting device 20 to remove and extract NaCl, and then the second causticizing device 15
Alternatively, it may be supplied to the second drying device 16.

〔The invention's effect〕

As described above, according to the method of the present invention, NaCl is not accumulated in the system, its concentration is controlled to a certain level or less, and the causticizing reaction is appropriately advanced, and iron oxide and sodium ferrate are allowed to flow. The fluidized bed used as the fluidizing medium can maintain an excellent fluidized state.

[Brief description of drawings]

The drawings are flow sheets showing an example of an apparatus for carrying out the soda recovery method of the present invention. 1 ... Thin film evaporator, 2 ... Disk evaporator, 3 ... Fluidized bed furnace, 4 ... Granular iron ore storage tank, 5 ... Fluidized bed, 6 ... Air dispersion plate, 7 ... Wind box, 8 ... 1st causticizing device, 10 ... 1st drying device, 11 ... Classifying device, 12 ...
Granulator, 13 …… Powdery iron oxide storage tank, 14 …… Boiler, 15…
… Second causticizer, 16 …… Second dryer, 17 …… Cyclone, 18 …… Electrostatic precipitator, 20 …… Cold water extractor

Claims (2)

[Claims] 1. A pulp waste liquor containing NaCl is combusted in a fluidized bed furnace using iron oxide as a causticizing agent and a fluidizing medium, and then the product sodium ferrate is put into water to remove caustic soda and iron oxide. In the method of collecting and collecting the heat of the fluidized bed furnace exhaust gas with a boiler, and introducing it to a cyclone or an electric dust collector to collect dust, a soda characterized by blowing the collected electricity dust collector to the outside of the system Recovery method. 2. A pulp waste liquor containing NaCl is combusted in a fluidized bed furnace using iron oxide as a causticizing agent and a fluidizing medium, and then the product sodium ferrate is charged into water to remove caustic soda and iron oxide. In the method of recovering and collecting the heat of the fluidized bed furnace exhaust gas with a boiler, and then introducing it into a cyclone or an electric dust collector to collect dust, all of the collected matter of the electric dust collector and a part of the collected matter of the cyclone or A soda recovery method characterized by blowing all of it out of the system.