JPH0711114B2 - 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 collected in the dust collector is extracted with cold water and the NaCl-containing waste liquid is discharged to the outside of the system to prevent NaCl from accumulating in 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 disclosed 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 oxidized. A method is proposed in which iron is added and burned, the resulting 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. Has been done. In this method, caustic soda is produced from an alkaline waste liquid, organic matter contained in the waste liquid is decomposed by combustion of the waste liquid, and sodium carbonate and / or sodium oxide obtained as a 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 → 2NaOH + Fe ₂ O ₃ (3) However, when applying the above method to a fluidized bed furnace, it is a general principle to recycle iron oxide (iron ore) in the direct causticizing method. Replenish only for carryover such as (the replenishment rate is at the order of several% at most). 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. Further, in the fluidized bed system, since a granular causticizing agent is basically used, the reaction molar ratio Fe / Na cannot be set 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 problems, the present inventors causticize powdered sodium iron oxide collected by a fluidized bed boiler, a cyclone, and an electrostatic precipitator, and then dry the dried powdered iron oxide. We developed a technology to granulate a part or all of the product using a black liquor as a binder and then supply it to the fluidized bed furnace as granules again. ing.

[Problems to be solved by the invention]

Compared to the KP method, the direct causticizing method is an excellent process aiming at simplifying the process and energy saving by omitting the lime kiln, but has a problem in that it is 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 as a precipitate out of the system, whereas in the direct causticization method, the system is closed. It tends to be gradually concentrated. Of these, NaCl has a negative effect on the sodium ferrate formation reaction (restricts the reaction), and it is easy to form a eutectic mixture that melts with Na ₂ CO ₃ in the low temperature range, which adversely affects the direct causticizing fluidized bed system. give.

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. Then, the caustic soda and iron oxide are recovered, the fluidized bed furnace exhaust gas is heat-recovered by the boiler, and then the cyclone is introduced into an electric dust collector to collect dust, and the collected electricity dust collector is extracted with cold water. The feature is that the NaCl-containing waste liquid is discharged to the outside of the system, and the powdery iron oxide is introduced into the causticizing process or the drying process.

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 extracting the collected matter with cold water and discarding the NaCl-containing waste liquid to the outside of the system, it is possible to suitably operate the fluidized bed system without accumulating NaCl in the system.

The second invention soda recovery method of the present application, cold water extraction of all of the electrostatic precipitator trap with high NaCl content, and part or all of the cyclone trap with high NaCl content,
It is characterized by discharging the NaCl-containing waste liquid to the outside of the system and introducing the powdered iron oxide into the causticizing process or the drying process, which allows the fluidized bed system to operate properly without accumulating NaCl in the system. be able to.

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 ore, reduced pellets, and granulated iron-making dust are used. To be 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 a part or all of the collected substances of the cyclone (17) are supplied to the cold water extraction device (20) for cold water extraction, and NaCl is extracted.
The contained waste liquid is discharged to the outside of the system to prevent NaCl from accumulating in the system, and the fine iron oxide after cold water extraction is introduced into the second causticizing device 15 or the second drying device 16.

Thus, the whole amount of the electrostatic precipitator trap having the highest NaCl content is extracted with cold water. There are cases where the whole cyclone is extracted with cold water, a case where a part is extracted with cold water, and a case where the whole is not extracted with cold water.

The portion of the cyclone collected product that is not extracted with cold water is directly supplied to the second causticizing device 15 or 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. After collecting and recovering the heat of the fluidized bed furnace exhaust gas with a boiler, in a method of introducing dust into a cyclone and an electric dust collector, the collected matter of the electric dust collector is extracted with cold water,
A soda recovery method comprising discharging a NaCl-containing waste liquid to the outside of the system and introducing powdered iron oxide into a causticizing step or a drying step. 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 put 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 Extract all with cold water and add NaCl
A soda recovery method comprising discharging the contained waste liquid to the outside of the system and introducing the powdered iron oxide into a causticizing step or a drying step.