JP3392162B2 - White gypsum production method - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a technique for increasing the added value of gypsum obtained by a wet limestone-gypsum flue gas desulfurization process.

[0002]

2. Description of the Related Art The flue gas desulfurization process which is currently widely used in industry is the wet limestone-gypsum process. This method
By absorbing the sulfurous acid gas in the flue gas into the absorption liquid, neutralizing the generated acid in the liquid with calcium carbonate (limestone) or calcium hydroxide (slaked lime), and by air oxidation of the sulfite ion, Crystals of calcium sulfate (gypsum) (including partially unoxidized calcium sulfite, but these are collectively referred to as gypsum below) are precipitated and grown by a crystallization operation to form large-sized gypsum particles. To obtain a so-called drainage gypsum. The obtained gypsum is effectively used for various purposes. Specifically, it has been conventionally practiced to absorb sulfur dioxide gas in a gas continuous / liquid dispersion type gas absorption tower in which an absorption liquid is sprayed in the tower, and then perform oxidation, neutralization and crystallization. Recently, a liquid continuous / gas dispersion type method in which absorption, oxidation, neutralization and crystallization are carried out in the same container has been attracting attention while increasing its performance (for example, JP-B-55-372).
95).

In the wet limestone-gypsum flue gas desulfurization process, dust existing in the flue gas is mixed with the gypsum crystallization zone where gypsum is produced and crystallized, and the quality of the gypsum matrix produced is not only deteriorated. However, in the process of dehydrating and recovering gypsum from the gypsum slurry, dust adheres relatively strongly to the gypsum surface and causes deterioration of the quality of the recovered gypsum, so the use of the obtained waste gypsum is limited. There is a problem that it ends up. For this reason, conventionally, when high-quality gypsum is desired, a means of reducing the amount of dust mixed in the gypsum crystallization zone by installing a dust collector or a dedusting tower before the flue gas desulfurization device is adopted. Came. However, gypsum with higher whiteness is often desired, and in such a case, even if the above means is adopted, it is not sufficient.

Therefore, as a means for removing dust and improving the purity of gypsum, a floating method, a method utilizing insolubilization of a dissolved substance due to pH change, a method utilizing a magnetic field and the like have been proposed (Japanese Patent Laid-Open No. 53-53). -10457, JP-A-53-
112296, JP-A-54-9194, JP-A-3-2.
See each publication such as 38023 and JP-A-4-59024). However, all of these have the drawbacks that, because the dust and gypsum are not sufficiently separated, gypsum of sufficient quality cannot be obtained and the whiteness thereof is not sufficient.

Further, a surfactant is added to the gypsum slurry, and the dust aggregated in the slurry is crushed and separated to obtain a high-quality α
A method for obtaining a mold hemihydrate gypsum has also been proposed (JP-A-53-53).
No. 50092). However, this method requires a large amount of surfactant in order to add the surfactant directly to the extracted gypsum slurry, and a large amount of separation liquid containing the surfactant is generated when the gypsum slurry is concentrated. However, there is a fatal drawback that the processing must be considered separately.

Further, it has been proposed to devise a method for separating gypsum and dust in the gypsum crystallization region (see Japanese Patent Publication No. 3-59730). However, this method is not only required to make the sedimentation separation area very large, because it is expected to have a separation effect by the sedimentation separation within the gypsum crystallization area, and the separation of gypsum particles and dust is not sufficient. Therefore, it is not possible to obtain plaster of sufficient quality, and the whiteness is not sufficient.

[0007]

DISCLOSURE OF THE INVENTION The present invention provides a so-called exhaust gypsum obtained from a wet limestone-gypsum flue gas desulfurization process whose quality and whiteness are not sufficient, and is effective for solving the problem. In view of the fact that no such method has been found yet, it is an object of the present invention to effectively separate and remove dust from the gypsum particles to obtain a waste gypsum with improved quality and whiteness.

[0008]

The present invention comprises the following steps: (1) Withdrawing an absorbent containing gypsum particles and dust from a gypsum crystallization zone of a wet limestone-gypsum flue gas desulfurization process,
(2) The absorption liquid is subjected to a classification operation to separate a fraction containing large-sized gypsum particles as a main component from the remaining fraction containing dust,
(3) From an absorbent of a wet limestone-gypsum method flue gas desulfurization process, which comprises adjusting the water content of a separated fraction containing large-sized gypsum particles as a main component to a desired value to obtain flue gas desulfurization process In the method for producing the discharged gypsum, the gypsum having a high whiteness is obtained by performing the classification operation with a hydraulic classifier having a cleaning function.

[0009]

[Function] Generally, in the wet limestone-gypsum flue gas desulfurization process, the absorption liquid containing gypsum particles is extracted from the gypsum crystallization area at the bottom of the desulfurization device, and after the gypsum particles are separated, the absorption liquid is sent to the desulfurization device. Will be returned. Alternatively, the absorption liquid containing gypsum particles may be extracted from a region in the gypsum crystallization region (or provided separately) that selectively contains gypsum particles of particularly large particle size. The term "gypsum crystallization region" in the present invention means any of the above-mentioned regions from which the absorbing liquid containing gypsum particles is usually extracted.

The absorbing liquid extracted from the gypsum crystallization region contains the gypsum particles that have been precipitated and the dust taken in from the flue gas. This dust is mainly carbon dust in the case of oil cooking, and mainly fly ash in the case of coal cooking. The amount of dust contained in the absorbing liquid settles to a value at which a material balance is established between the amount of dust taken in from the flue gas and the amount of dust removed from the absorbing liquid circulation path. Therefore, if dust is removed in advance by providing dust removing means such as an electrostatic precipitator or a cooling dust removing tower in the preceding stage of the flue gas desulfurization device, the amount of dust in the absorbing liquid is reduced, and the whiteness of the gypsum obtained by the method of the present invention is reduced. Is expected to improve further. However, even if the dust is not preliminarily removed, according to the method of the present invention, a whiteness higher than that of the conventionally obtained waste gypsum plaster can be obtained.

Next, a classification operation is performed on the extracted absorption liquid containing gypsum and dust. It is important to note that the dust-containing fraction may contain some gypsum particles, but the fraction containing large gypsum particles as the main component should be divided to contain almost no dust. It is to be. Therefore, as a means for this, it is necessary to have high classification accuracy on the large particle size side. The present invention is based on the finding that a hydraulic classifier having a cleaning function is effective as a classifying device for this purpose. Here, the washing function means, for example, as shown in FIG. 1, that an ascending flow is opposed to a sedimented mixed particle group, and fine particles having a sedimentation speed smaller than the ascending speed are washed out from the sedimented coarse particle group. The action of separating. Generally, a hydraulic classifier has a function of separating a group of particles having a large sedimentation velocity and a group of particles having a small sedimentation velocity with respect to the ascending velocity of an ascending flow. Some fine particles may be mixed in the coarse particle fraction due to the generation of the downward flow, and in order to almost completely separate the fine particles from the coarse particle fraction, the upward flow should be uniform or the particles should be uniform. It is necessary to devise such as dispersing. Therefore, not all hydraulic classifiers have the cleaning action referred to in the present invention. In the case of sedimentation separation using only gravity, it is not possible to say that it has a cleaning action because the fine particles existing in the lower part of the sedimentation tank from the beginning cannot be separated from the coarse particle group.

As a mode of performing the classification operation using a hydraulic classifier having a cleaning function, there are those shown in FIGS. 2 (a) to 2 (c). FIG. 2 (a) shows that only one stage of the hydraulic classifier A having a cleaning function divides the fraction containing large-sized gypsum particles as the main component and the remaining fraction containing dust. FIG. 2 (b) shows that the first step is pre-divided into a fraction mainly containing large-sized gypsum particles and a remaining fraction containing dust by a relatively simple means such as a hydrocyclone. Is further treated in the second stage with a hydraulic classifier having a washing function. In this method, the amount of fine particles and dust to be separated and removed by the hydraulic classifier A having a cleaning function is reduced, and the load on the hydraulic classifier is reduced,
Finally, the amount of fine particles and dust mixed in the large particle gypsum fraction can be further reduced. The overhead fraction from the second-stage hydraulic classifier may be combined with the dust-containing fraction from the first-stage hydrocyclone for treatment as shown in the figure. FIG. 2 (c) shows that the latter fraction of the first stage in FIG. 2 (b), that is, the fraction containing fine particles and dust is further treated by a hydraulic power classifier provided separately from the hydraulic classifier. Is. In this method, the bottom fraction from the hydraulic classifier that processed the latter fraction can be added to the feed to the second-stage hydraulic classifier A that processes the former fraction. Thereby, the recovery rate of large-sized gypsum particles can be increased.

As a hydraulic classifier having a cleaning function, one generally known as an elitorator can be preferably used. In this case, it is advantageous to use an inclined straightening plate inside the selection cylinder in order to improve the classification accuracy. FIG. 3 (a) shows an example of such a hydraulic classifier, and FIG. 3 (b) shows the state of the flow of liquid and particles in the straightening plate region. When the inclined rectifying plate is provided in this way, the inside of the selection cylinder 5 is divided into several zones by the rectifying plate 6, and the downward flow that may occur in each zone is blocked by the rectifying plate, so that the downward flow is reduced. It is effective in preventing the fine particles and dust carried on the board from finally mixing as a coarse particle portion into the large particle gypsum fraction discharged from the bottom of the tower. In FIG. 3, the absorbing liquid is uniformly introduced into the sorting cylinder by the distributor 7.

Further, the hydraulic classifier is divided into a region having a relatively high flow velocity and a region having a relatively low flow velocity, and the absorption liquid extracted from the gypsum crystallization region is first supplied to the region having a high flow velocity to provide a large particle gypsum as a main component. May be separated as a coarse particle portion, and then the remaining fraction containing dust may be supplied to a region having a low flow rate to separate medium particle gypsum from fine particle gypsum and dust. In this way, the action of removing fine gypsum and dust from the large gypsum is enhanced. Alternatively, the absorption liquid is divided into a fraction containing large-particle gypsum as a main component and a fraction containing fine particles as a main component in advance, and the former is supplied to a high flow velocity region and the latter is supplied to a low flow velocity region. May be. Examples of these are shown in FIGS. FIG. 4A shows the former case in which the absorbing liquid is supplied all at once to the region where the flow velocity is high, and FIG. 4B shows the latter case in which the absorbing liquid is divided in advance and supplied separately.

The fraction containing large-sized gypsum particles as the main component is separated and removed from the absorbing solution, and the water content is adjusted to a desired value to obtain white gypsum as a product. The water content is usually 15% or less. As means for this purpose, for example, a device generally classified as a dehydrator such as a filter press, a centrifuge, a vacuum filter, a belt press or the like is preferably used. The separated absorption liquid may be returned to the absorption tower of the flue gas desulfurization device for reuse, or may be used as pressure water for hydraulic classification as described later.

On the other hand, the dust-containing fraction separated by the above classification operation can be directly returned to the absorption tower, but it is preferably subjected to solid-liquid separation to obtain a fraction containing dust and gypsum particles as main components. And the fraction composed of the absorption liquid, and only the fraction composed of the absorption liquid is returned to the absorption tower, or used as pressure water for hydraulic classification as described later. As the solid-liquid separator for this purpose, a device generally classified as a dehydrator is preferably used, as in the case of adjusting the water content of a fraction containing large-sized gypsum particles as a main component. When a cooling tower is provided separately from the flue gas discharge device, a solid-liquid separation device that processes a part of the circulating flow can be used. In the case of having a dry dust collector, the dust-containing fraction may be collected in the form of a slurry, or may be sprayed in the upstream of the device while appropriately reducing the water content and collected as a dry solid. If this method is applied in the case of coal cooking, since an appropriate amount of fly ash and gypsum are contained in the dry solid, ettringite can be produced and solidified by adding a small amount of slaked lime. Therefore, it is possible to easily prevent the harmful substances from being eluted from the dry solid. Also in the case of cooking oil, the same effect can be obtained by adding a substance equivalent to fly ash, such as clay. Thus the process can be drained.

In order to generate an upward flow necessary for hydraulic classification, it is necessary to introduce pressure water from the bottom of the hydraulic classifier or in the vicinity thereof.
(1) Separation water generated when the water content of the fraction containing large-sized gypsum particles as the main component is adjusted to a predetermined value, (2) Occurring when the remaining fraction containing dust is subjected to solid-liquid separation The separated water or (3) makeup water, or two or more of them can be used in combination.

The liquid continuous / gas dispersion type wet limestone-gypsum flue gas desulfurization process is characterized in that the particle size of gypsum particles produced is larger than that of the gas continuous / liquid dispersion type. This facilitates the classification of gypsum and dust and contributes greatly to improving the purity and whiteness of the resulting gypsum.

The absorbing liquid containing gypsum particles and dust may be subjected to a high shearing force before the classification operation. This shearing force is effective for peeling the dust adhering to the gypsum particle surface by acting between the gypsum particle surface and the absorbing liquid, while dust is attached to the gypsum surface in the gypsum crystallization area. To work. Therefore, it is important to provide a sufficient relative velocity between the absorbing liquid and the gypsum particle surface. Also,
This high shear force disperses large particles of dust and agglomerates of dust to a particle size that is sufficiently smaller than gypsum particles and prevents dust from entering the fraction containing large gypsum particles in subsequent classification operations. It also works. From the above,
Suitable means include a homomixer, a rotary disc type homogenizer, a homogenizer type pump, and jet injection into a liquid.

Further, before applying a high shearing force, the absorbing liquid extracted is preliminarily divided into a fraction having a low dust content and a fraction having a high dust content by using a classifying means such as a hydrocyclone. Alternatively, only the fraction having a low dust content may be the target to which the high shearing force is applied.

[0021]

EXAMPLE FIG. 5 shows an example of an apparatus for carrying out the method of the present invention. An absorbent containing gypsum particles and dust is extracted from the gypsum crystallization area of the absorption tower 1 of the wet limestone-gypsum method flue gas desulfurization apparatus, and then used as a classification means 2 as shown in FIG.
It divides into the fraction which has a large-sized gypsum particle as a main component and the fraction which contains dust using the hydraulic classifier shown in. Thereafter, the former fraction is separated and removed by the water content adjusting means 3 to obtain white gypsum as a product, and the latter fraction is similarly separated and removed by the solid-liquid separating means 4 to contain dust. Use as low quality gypsum or discard. Either of the separated absorption liquids are returned to the absorption tower or used as a cleaning liquid (pressure water) for a hydraulic classifier.

Example 1 From the gypsum crystallization area at the bottom of the absorption part of the wet limestone-gypsum method flue gas desulfurization device (type in which absorption, oxidation, neutralization and crystallization are carried out in the same vessel) of a heavy oil combustion boiler The absorbing liquid containing particles and carbon dust was supplied to the hydraulic classifier shown in FIG. In FIG. 6, the selection cylinder 8 has an inner diameter of 50 mm.
Using a transparent vinyl chloride cylinder with both ends plugged,
A 1 L glass filter bottle was used as the coarse particle fraction container 9, and this was placed in the fine particle fraction container 10. The absorption liquid has a slurry concentration of 20 wt% and a carbon concentration of 660 pp.
It was m. The absorption liquid is introduced from the slurry storage tank 11 into the sorting cylinder by the slurry feeder 12, while the cleaning liquid storage tank 1
The cleaning liquid was pressure-injected from 3 through the suction port of the glass filter bottle. The length of the classification zone from the lower end of the slurry feeder to the lower end of the sorting cylinder is 330 mm. The fine particle fraction was allowed to overflow from the hole provided in the stopper at the upper end. Regarding the operating conditions of this hydraulic classifier, the supply rate of the absorbing solution was 100 cc / min, and the supply rate of the gypsum saturated solution as a cleaning solution was 350 cc / min. At this time, the rising speed of the cleaning liquid inside the hydraulic classifier is 3
mm / sec. After 20 minutes of operation, the large particle gypsum at the bottom of the coarse particle fraction container was taken out and dried, and its whiteness was measured with a white meter (Minolta color white meter CR-210 type), and the L value was 80.1. Got The carbon concentration in the large particle gypsum was 40 ppm.

Comparative Example 1 The gypsum was obtained by extracting the absorbing liquid under the same conditions as in Example 1, leaving it to stand still to remove the upper liquid, and then drying the precipitated solid content. When the whiteness was measured in the same manner as in Example 1, it was 75.6, and the carbon concentration in the large particle gypsum was 150 ppm.

[0024]

According to the method of the present invention, since the dust in the absorbing liquid is effectively removed and the mixture with the large-sized gypsum particles is prevented, the waste gypsum having extremely high quality and whiteness can be obtained. .

[Brief description of drawings]

FIG. 1 shows an example of classification operation by a hydraulic classifier having a cleaning function.

2 (a) to (c) each show a mode in which a hydraulic classifier having a cleaning function is used.

FIG. 3 (a) shows an example of a hydraulic classifier having an inclined rectifying plate inside, and FIG. 3 (b) shows the flow of liquid and particles in the rectifying plate region.

4 (a) and 4 (b) show examples of hydraulic classifiers having regions with different rising velocities.

FIG. 5 shows an example of a device used in a preferred implementation of the method of the invention.

FIG. 6 shows an experimental apparatus of the hydraulic classifier used in the examples.

[Explanation of symbols]

1 absorption tower 2 classification means 3 Water content adjustment means 4 Solid-liquid separation means 5 sorting cylinder 6 current plate 7 distributor 8 sorting tubes 9 Coarse particle fraction container 10 Fine particle fraction receiving tank 11 Slurry storage tank 12 Slurry feeder 13 Cleaning liquid storage tank

─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP 62-114630 (JP, A) JP 61-197021 (JP, A) JP 54-99074 (JP, A) JP 49- 110574 (JP, A) JP 6-115933 (JP, A) JP 52-6370 (JP, A) Chemical Engineering Association, Chemical Engineering Handbook, Maruzen Co., Ltd., May 10, 1968, full revision Revised 3rd edition, page 882, "b. Hydropower classifier" item (58) Fields investigated (Int.Cl. ⁷ , DB name) C01F 11/46 B01D 53/34

Claims (12)

(57) [Claims] 1. The following steps: (1) Withdrawing an absorption liquid containing gypsum particles and dust from a gypsum crystallization area of a wet limestone-gypsum flue gas desulfurization process, and (2) performing a classification operation on the absorption liquid. The fraction containing the large-sized gypsum particles as the main component is separated from the remaining fraction containing dust, and (3) the water content of the separated fraction containing the large-sized gypsum particles as the main component is adjusted to a desired value. In the method for producing exhaust desorption gypsum from the absorption liquid of the wet lime-gypsum method flue gas desulfurization process, the classification operation is performed by a hydraulic classifier having a plurality of rectifying plates inclined inside. A method characterized by obtaining gypsum with high whiteness by carrying out. 2. The method of claim 1 further comprising the step of dehydrating the remaining fraction containing dust. 3. The method according to claim 2, wherein the remaining dust-containing fraction is mixed with a solid-liquid separator in which a part of the cooling tower circulation stream is treated for dehydration. 4. The pressure water to be introduced into the hydraulic classifier includes: (1) separation water generated when the water content of a fraction containing large-sized gypsum particles as a main component is adjusted to a predetermined value; 4. The method according to any one of claims 1 to 3, wherein at least one selected from the group consisting of: (3) separation water generated when dehydrating the remaining fraction containing dust, and (3) makeup water. 5. The separating step preliminarily divides a fraction containing large-sized gypsum particles as a main component by another classification means provided in the preceding stage of the hydraulic classifier, and then the fraction is tilted inward. 5. A method according to any one of claims 1 to 4, further comprising treatment with the hydraulic classifier having a plurality of straightening vanes. 6. The method according to claim 5, wherein the remaining fraction containing dust separated by the another classifying means is further treated by another separating means. 7. The hydraulic classifier has a region having a relatively high flow velocity and a region having a relatively low flow velocity therein, and the absorbing liquid containing gypsum particles and dust is first introduced into the region having a high flow velocity, Therefore, it is separated into a fraction containing large-particle gypsum as a main component and a fraction containing dust, and then a fraction containing dust is introduced into a region having a low flow rate to obtain a fraction containing medium-particle gypsum as a main component. The method according to any one of claims 1 to 6, wherein the method is separated into a particulate gypsum and a fraction containing dust as a main component. 8. The method according to claim 1, wherein a high shearing force is first applied to the extracted absorption liquid, and then a classification operation is performed. 9. The method according to claim 8, wherein a homomixer, a rotary disk type homogenizer, a homogenizer type pump or jet injection into a liquid is used as the means for applying a high shearing force. 10. The method according to claim 1, wherein the wet limestone-gypsum flue gas desulfurization process is liquid continuous / gas dispersion type. 11. A dry dust collector is provided before the wet limestone-gypsum flue gas desulfurization process, and the remaining fraction containing the dust is sprayed into the flue gas sent to the dry dust collector to collect the dust. The method according to any one of claims 1 to 10, wherein the method is carried out in a dry solid state. 12. The wet limestone-gypsum flue gas desulfurization process treats flue gas after being discharged from a heavy oil combustion boiler and sequentially processed by a dry type electrostatic precipitator and a dust removing tower. The method described in either.