JPH0368152B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of burning a waste liquid called black liquor produced in a pulp manufacturing process to recover chemicals and heat recovery, and is particularly suitable for direct causticization. Regarding.

In the pulp manufacturing process at paper mills, chemicals, mainly caustic soda (NaOH), are used to separate the wood components cellulose (fiber) and glinin (resin) and extract only the cellulose. The solution containing dissolved lignin and Na components after the cooking 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. This method involves precisely controlling a black liquor combustion device to combust black liquor within the device to react and generate _{Na 2 CO 3 .}
NaOH is recovered by reacting it with slaked lime (Ca(OH) ₂ ) produced in another process.The reaction process is complicated and requires large equipment, and the CaCO ₃ produced during NaOH production is recovered. There is a problem in that a large amount of energy is consumed to return Ca(OH) ₂ through the slaked process. For this reason, a NaOH recovery method called the direct causticization method has been proposed to simplify equipment and reduce energy consumption. To explain the outline of this direct causticizing method using Figure 1, lignin is separated from wood chips using NaOH in the cooking process 1, and the black liquor containing lignin and Na components is converted into iron oxide powder in the combustion process 2. By adding ₂ O ₃ , the following reaction is carried out.

2NaOH＋OC ₂ →Na ₂ CO ₃ +H ₂ O ...(1) Na ₂ CO ₃ +Fe ₂ O ₃ →2NaFeO ₂ +CO ₂ ...(2) Of these, sodium ferrate (NaFeO ₂ ) is added in the next stage of dissolution process 3. It is hydrolyzed and NaOH is recovered.

2NaFeO ₂ +H ₂ O → 2NaOH + Fe ₂ O ₃ ...(3) In other words, in the direct causticization method, NaOH can be recovered by performing the reactions (1), (2), and (3), and this recovery This method has the advantage of being able to recycle and reuse the Fe ₂ O ₃ used in the process, significantly reducing equipment costs and energy consumption compared to conventional methods.

However, in this method, it is difficult to mix black liquor and Fe ₂ O ₃ before combustion for the following reasons. In other words, the viscosity of black liquor is quite high at room temperature, and in order to mix it with Fe ₂ O ₃ , it is necessary to heat the black liquor to lower its viscosity, and a considerable amount of the heat recovered from black liquor combustion is consumed. is consumed for heating the black liquor, which is uneconomical. Further, a special device must be installed to mix the two, and in addition to equipment costs, power costs for operating this device are also required, which is also uneconomical.

An object of the present invention is to provide a black liquor combustion method that eliminates the need for mixing black liquor and Fe ₂ O ₃ and simplifies the process.

In short, this invention separately supplies black liquor and oxidized metal powder to a combustion furnace, maintains the temperature inside the furnace at 1100 degrees Celsius or less and burns it, and transports the sodium ferrate powder produced in the furnace with combustion exhaust gas. Collected by a dust collector, caustic soda and oxidized metal powder are recovered, and this is dissolved in a melting tank to separate metal powder and caustic soda. The oxidized metal powder is sent to the combustion furnace, and the caustic soda is recycled and reused in the pulp manufacturing process. This black liquor combustion method is characterized by the following.

Embodiments of the present invention will be described below with reference to the drawings.

In FIG. 2, 10 is a fluidized bed boiler (fluidized bed furnace) which is a black liquor combustion device (also serving as a reaction device), and an intrabed heat exchanger tube 10b is arranged in a fluidized bed 10a formed in the boiler. The temperature inside the bed is controlled by recovering the heat generated by black liquor combustion and adjusting the amount of feed water _W1 passing through. here,
The softening point of NaFeO ₂ produced by (1) and (2) above is 1100
℃ or higher, the melting point is 1180℃ or higher, and NaFeO ₂ can be easily recovered if it is powdered, so the temperature inside the layer is
The water supply _W1 is adjusted by the valve 4 to control the amount of black liquor supplied so that the temperature becomes 1100°C or less. The NaFeO ₂ produced by the above control is in the form of fine powder particles with a particle size of 100μ or less, and is discharged from the furnace together with the combustion exhaust gas rising inside the furnace, reaching the dust collector (the one shown is a cyclone) 12. NaFeO ₂ is recovered and exhaust gas G is discharged outside the system. In this case, the fluidized bed furnace 10
Most of the NaFeO ₂ generated in the process flows out of the furnace together with the exhaust gas G, and in the case of a cyclone type dust collector,
The minimum particle size that can be collected is approximately 10μ, so
In the end, almost all of the NaFeO ₂ generated in the fluidized bed furnace 10 is collected by the dust collector 12. The collected NaFeO ₂ reaches the hopper 13 and dissolves into dissolved water W ₂
The feed amount is adjusted by the rotary feeder 14 in accordance with the feed amount, and the feed amount reaches the dissolution tank 16. Here, the hydrolysis shown in formula (3) above is performed, and NaOH and
Fe ₂ O ₃ is produced. In this case, heating the solution in the dissolution tank 16 will accelerate the reaction. As the heating medium, hot water or steam generated in the fluidized bed boiler 10 and supplied through the pipe 40 is used.

A mixed solution of hydrolyzed NaOH and Fe ₂ O ₃ is supplied to a sedimentation tank 19 through a pipe 17, where it is sedimented and separated.
Fe ₂ O ₃ is supplied to the dehydrator 20 via a pipe 39.

On the other hand, NaOH overflowing from the settling tank 19 flows into the overflow tank 23, passes through the pipe line 24, is stored in the NaOH storage tank 27, and passes through the pipe line 28 to be used in the sequential cooking process. Note that the liquid (HaOH) discharged from the dehydrator 20 passes through a pipe 26 to the NaOH storage tank 2.
7.

On the other hand, the Fe ₂ O ₃ dehydrated cake discharged from the dehydrator 20 is transferred to a dryer 3 which also serves as a storage tank by a conveyor 30.
1, is completely dried by a heating medium 33 such as hot air, and stored as a powder. stored
A predetermined amount of Fe ₂ O ₃ is supplied to the ejector 34 via the feeder 32, and the air is transported to the pipe line 3 by the air transporting air A.
5 to the fluidized tank 10a of the combustion device 10. On the other hand, the black liquor BL is supplied to the fluidization tank 10a through another pipe line 36. Note that since the black liquor and Fe ₂ O ₃ supplied separately need to be well mixed in the fluidized bed 10a, it is preferable that the nozzles 37 and 38 for injecting both are arranged close to each other.

By implementing this invention, a device for mixing black liquor and Fe ₂ O ₃ in advance becomes unnecessary, and the black liquor and oxidized metal powder can be mixed well in the fluidized bed, reducing equipment costs and power costs. Economical.

Furthermore, since the black liquor only needs to have a viscosity sufficient to be injected into the combustion device, the number of heat sources for heating the black liquor may be smaller than when mixing and stirring with Fe ₂ O ₃ is performed.

[Brief explanation of drawings]

FIG. 1 is a system diagram showing an outline of the direct causticizing method, and FIG. 2 is a system diagram of a plant implementing the method according to the present invention. DESCRIPTION OF SYMBOLS 10... Fluidized bed combustion device, 35... Fe ₂ O ₃ supply pipe, 36... Black liquor supply pipe, A... Air for air flow transport, BL... Black liquor.

Claims (1)

[Claims] 1. Separately supply black liquor and oxidized metal powder to the combustion furnace,
The temperature inside the furnace is kept below 1100℃ and the combustion is carried out, and the combustion exhaust gas transports the sodium ferrate powder generated in the furnace, which is collected by a dust collector, and the caustic soda and metal oxide powder are recovered, which are then transferred to the melting tank. A black liquor combustion method characterized by dissolving metal powder and separating it into caustic soda.The oxidized metal powder is sent to a combustion furnace, and the caustic soda is recycled and reused in the pulp manufacturing process.