JPH0436728B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a wet desulfurization method using a calcium-based absorbent, and in particular, in a wet desulfurization method using a calcium-based absorbent, a slurry-like absorbent that has come to contain gypsum is separated into solid and liquid and recovered through filtration. It concerns methods for treating and reusing water.

In a wet desulfurization device using a calcium-based absorption liquid, the calcium-based absorption liquid absorbs sulfur oxides in exhaust gas from a boiler or the like to generate calcium sulfite, and a portion of the liquid is further oxidized by oxygen in the exhaust gas to generate gypsum. After adjusting the pH of this reaction slurry in the gypsum recovery process and air oxidation, solid-liquid separation is carried out.
It is dehydrated and recovered as gypsum with less than 10% attached water. At this time, the solid-liquid separated liquid (filtrated water) is returned to the desulfurization device and reused, thereby contributing to reducing the amount of water discharged from the desulfurization device. However, gypsum is saturated in this filtered water,
If the liquid is concentrated due to pH fluctuations, cooling, or water evaporation during the dust removal process, the dissolved gypsum content in the liquid becomes supersaturated and precipitation occurs. This precipitated gypsum adheres to towers, tanks, piping, etc. and forms scale. For this reason, Na ₂ CO ₃ is added to the filtered water and fixed as Ca ²⁺ and CaCO ₃ using the formula (1) below.
The filtered water is softened by precipitation.

Ca ²⁺ SO ₄ ^2- +Na ₂ CO ₃ →CaCO ₃ +2Na ⁺ +SO ₄ ^2- ...(1) At this time, the fixed CaCo ₃ is taken out in the form of fine particles, but conventionally, as shown in Fig. 2 below, As will be explained with reference to , fine particles were sedimented and concentrated and then treated in the form of a slurry in a wastewater treatment equipment, which caused an increase in the amount of wastewater and the amount of solids processed.

Furthermore, the reduction rate of Ca ²⁺ in the filtered water by this operation is not only largely controlled by the PH of the liquid, but also, according to the analysis of the actual liquid by the present inventors, as shown in the graph of Figure 1, If the pH is below 10, the filtered water will
The Ca ²⁺ concentration greatly exceeds the saturation concentration and becomes supersaturated, and it is clear that under changes in conditions, supersaturation will precipitate and become a factor in scale formation.

Therefore, it is effective to lower the degree of supersaturation by increasing the pH to 10 or higher.

A conventional method for softening filtered water will be described below with reference to the flow sheet shown in FIG.

Exhaust gas 11 from a boiler or the like is led to the dust removal tower 1, and is cooled by the dust removal tower circulation liquid 101 at the same time as dust removal. This cooled dust-removed exhaust gas 12 is led to the absorption tower 2, and the absorption tower circulating liquid 105
As a result, sulfur oxides are absorbed and removed and are discharged as a clean desulfurized gas 13.

A part of the dust removal tower circulation liquid 100 containing soot and dust is extracted from the dust removal tower 1 according to the amount of soot and dust collected, and is sent to the wastewater treatment device A as desulfurization wastewater 102. (Also, the same method is used when removing dust with an absorption liquid.) The absorption tower 2 is supplied with limestone slurry 103 from a limestone slurry tank 3 into which limestone 14 is fed, depending on the amount of sulfur oxides absorbed. be done. Reaction in absorption tower 2,
Slurry 104 consisting of generated CaSO ₃ , CaSO ₄ and unreacted CaCO ₃ is extracted as slurry 106 according to the amount of sulfur oxide in absorption tower 2 and sent to reaction tank 4 . In the reaction tank 4, unreacted substances in the slurry 106
Make gypsum from CaCO ₃ with 15% sulfuric acid and adjust the pH of the slurry to 4.5.
~5 slurry 107 is sent to the oxidation tower 5.
In the oxidation tower 5, CaSO ₃ in the slurry 107 is converted into air 16
All the solids in the slurry become gypsum slurry 108, and Jitukuna 6
The concentrated solid slurry 109 is subjected to solid-liquid separation, and the solids are dehydrated in the dehydrator 7 to produce gypsum 110 as a by-product. The dehydrated liquid 111 is returned to the tanker 6. The supernatant water (filtered water) 112 of the Shitsukuna 6 is a liquid in which gypsum is saturated and dissolved, and if used as is, gypsum will precipitate at supersaturation due to pH fluctuations and concentration of the liquid, forming scale. Therefore, in order to lower the concentration of dissolved gypsum, Na ₂ CO ₃ (soda carbonate) 1
7 to precipitate dissolved gypsum in granular form, sedimentation,
In order to make the floc suitable for concentration, a flocculant 18 is added and the slurry 113 is tempered and sent to the softener 9 for softening treatment. Here, the solid substance mainly composed of CaCO ₃ is separated into solid and liquid, and the supernatant liquid is passed through pipe 114 as sledding water, and then sent to pipes 115 and 115 as make-up water for the dust removal tower, absorption tower, and limestone slurry tank.
16 and 117. On the other hand, the concentrated solid slurry 118 after solid-liquid separation in the softener 9 passes through the sludge tank 10 and is treated in the wastewater treatment device A as a generated solid slurry 119.

In this flow, Na ₂ CO ₃ is
By adding (soda carbonate) 17, the pH of the filtered water increases, CaCO ₃ is precipitated, and the Ca ²⁺ concentration in the liquid decreases. However, at a pH of 10 or less, Ca ²⁺ exists above its saturation solubility, and depending on changes in conditions such as pH and flow rate, it may
CaCO ₃ on the inner walls of 14, 115, 116, 117
Since this crystallizes and accumulates as scale, countermeasures were needed. That is, the methods according to the prior art had such drawbacks.

Furthermore, as can be seen from the above explanation, the conventional method not only increases the volume of desulfurization wastewater, but also increases the amount of solids treated in the wastewater treatment equipment. .

An object of the present invention is to provide a method for reusing filtrate of desulfurization equipment using a wet desulfurization method using a calcium-based absorption liquid, which eliminates the drawbacks and problems of the prior art described above.

The feature of the reusing method for desulfurization equipment filtrate of the present invention is that the desulfurization equipment filtrate water generated when recovering gypsum is reused in a wet desulfurization equipment using a calcium-based absorption liquid. In the water softening treatment, Na ₂ CO ₃ and
The method includes a process of adding NaOH simultaneously or one after another. In this case, the treatment is preferably carried out so that the pH of the treatment liquid is 10 or more, more preferably about 11.

In this way, by adding NaOH, which is a strong alkali, in addition to Na ₂ CO ₃ during the softening treatment to raise the pH to 10 or more, preferably to about 11,
By further lowering the Ca ²⁺ concentration to below the saturation solubility and at the same time reducing the concentration of scale components such as Mg ²⁺ , the softened water treated in this way can be used as make-up water for the desulfurization equipment, resulting in stable This has the effect of making it possible to drive.

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

FIG. 3 is a flow sheet showing an example of the method for reusing filtrate of desulfurization equipment of the present invention. In the figure, by-product gypsum is separated. The desulfurization device filtrate water 112, which is the supernatant liquid of Shitukuna, is led to the filtrate water adjustment tank 81 of the softening treatment device, where soda carbonate (Na ₂ CO ₃ ) 17 is added. Then, 800 ml of caustic soda (NaOH), which is another essential component in the present invention, is added to the tank at the same time. Note that the same effect can be obtained by adding caustic soda 800 in a separate tank after adding soda carbonate 17.

Here, the reason why soda carbonate is added instead of adjusting the pH using strong alkali alone is because calcium carbonate (CaCO ₃ ) is produced as a reaction product.
It is designed to lower the solubility of Ca ²⁺ in the liquid. If only caustic soda is used, calcium hydroxide is produced, which has higher solubility than calcium carbonate, and cannot lower the concentration of Ca ²⁺ .
Also, it is difficult to raise the pH to 10 or higher with only soda carbonate, but by adding caustic soda,
The pH can be raised to 10 or higher, and at this time, heavy metals such as Mg ²⁺ also become hydroxides, promoting precipitation.
Can significantly reduce Ca ²⁺ concentration.

The filtrate treated in the filtrate adjustment tank 81 is led to the flocculation tank 82 through a pipe 801, where a flocculant 18 is added to form a floc suitable for flocculation and precipitation, and the resulting floc is sent through a pipe 802 to a flocculation sedimentation tank 83. . Here, the solid-liquid is separated and the supernatant liquid 114
is stored as treated water in a treated water tank 84,
The treated water is supplied to the desulfurization equipment through supply run B. On the other hand, the concentrated slurry 118 after solid-liquid separation of the coagulation-sedimentation filter 83 is sent to the wastewater treatment device A and treated. In order to raise the pH to 10 or more, it is possible to use calcium hydroxide instead of caustic soda, but this would increase the amount of concentrated solid slurry 118 from the flocculation sedimentation system 83 and increase the amount of sludge in wastewater treatment equipment A. There is a drawback that it does.

PH and Ca ²⁺ of softened water according to the embodiment of the present invention
The relationship between concentrations is shown in the graph of FIG. After adding sodium carbonate to filtered water to make the pH 8, caustic soda is added. At pH 11 or higher, Ca ²⁺ becomes CaCO ₃
It has decreased to near saturated dissolution.

In contrast to the method of the present invention as described above, in the method according to the prior art, only adding soda carbonate,
The filtered water had been softened and the objective of removing Ca ²⁺ had been achieved, but due to changes in the conditions of the filtered water, scale had been deposited on the tank and piping, and this had to be dealt with exclusively. This was dealt with by mechanically removing the scale.

As can be seen from the above explanation, in the present invention, by treating filtered water,
Not only does this reduce the Ca ²⁺ concentration, preventing supersaturation and prevent CaCO ₃ scale from adhering to ovens, tanks, and piping, but it also removes heavy metal components such as Mg ²⁺ as hydroxides at the same time. , Furthermore, it also has a great effect on preventing scaling. Furthermore, even if the softened water is returned to the desulfurization equipment and used, stable operation can be achieved regardless of conditions such as pH and flow rate.

[Brief explanation of drawings]

FIG. 1 is a graph showing the relationship between PH and Ca ²⁺ concentration of desulfurization equipment filtrate water. FIG. 2 is a flow sheet of a desulfurization device having a filtered water softening device in a conventional method. FIG. 3 is a flow sheet of a desulfurization apparatus showing the softening treatment of filtrate water exemplified in the examples of the present invention. FIG. 4 is a graph showing the Ca ²⁺ concentration in the treated water when caustic soda was added after adding sodium carbonate to the filtered water to adjust the pH to 8. 1... Dust removal tower, 2... Absorption tower, 3... Limestone slurry tank, 4... Reaction tank, 5... Oxidation tower, 6...
Shitsukuna, 7... Dehydrator, 8... Softening treatment device,
9...Softening treatment material, 10...Sludge tank, 11...Exhaust gas, 12...Dust removal gas, 13
... Desulfurization treatment gas, 14 ... Limestone, 15 ... Sulfuric acid, 16 ... Air, 17 ... Soda carbonate, 18 ...
...Flocculant, 81...Filtered water adjustment tank, 82...
Flocculation generation tank, 83... Coagulation sedimentation tank, 84... Treated water tank, 110... By-product gypsum, 112... Filtered water (supernatant water), 113... Tempered slurry, 114... Supernatant liquid piping, 118...
...Concentrated solid slurry, 119...Produced solid slurry, 800...Caustic soda, 801,802...
Piping.

Claims (1)

[Claims] 1. Sulfur oxide-containing exhaust gas is brought into contact with a calcium compound slurry absorption liquid, and the sulfur oxides in the exhaust gas are absorbed and removed by the absorption liquid, and calcium sulfite generated in the absorption liquid is oxidized. In the method of recovering gypsum by solid-liquid separation of the slurry absorption liquid containing the gypsum and reusing the filtrate of the desulfurization device after the gypsum separation, sodium carbonate and caustic soda are added to the filtrate at the same time, Or add them one after the other to reduce the calcium ion concentration in the filtered water.
A method for reusing filtrated water of a desulfurization device, characterized by using this filtrated water. 2. When the treatment is performed by adding soda carbonate and caustic soda to the filtered water of the desulfurization device at the same time or one after the other, the PH value of the treatment liquid is adjusted to substantially 10 to 11. A method for reusing filtrate water from a desulfurization device according to claim 1. 3. Treated water that has been treated by adding soda carbonate and caustic soda to the filtered water of the desulfurization equipment at the same time or one after the other is used as evaporation make-up water or mist eliminator in the dust removal tower of the wet desulfurization equipment. 3. A method for reusing filtrated water from a desulfurization apparatus according to claim 1 or 2, wherein the water is reused as cleaning water for a demister (also referred to as a demister) or make-up water for dissolving limestone.