JPS62278119A - Production of magnesium hydroxide for exhaust gas desulfurization - Google Patents

Abstract

PURPOSE:To easily and economically produce Mg(OH)2 suitable for the desulfurization of exhaust gas and utilizable even in the form of aqueous solution, by adding an aqueous solution of a sulfate to an aqueous slurry of Mg(OH)2 containing CaCl2 as a soluble impurity and separating the resultant gypsum dihydride. CONSTITUTION:A reactor 1 is charged with seawater 21 and a Ca compound 22 such as CaO, CaO.MgO, Ca(CH)2, etc. An aqueous slurry of Mg(OH)2 produced by the reaction of the above components is passed through precipitation tanks 2, 3 settle and separate solid impurities such as CaCO3, sand, etc., a flocculant 23 is added and mixed thereto in a mixing tank 4 and the supernatant liquid is taken out of the system and concentrated. The obtained aqueous slurry of Mg(OH)2 containing CaCl2 as a soluble impurity is introduced into a reactor 6. The slurry is mixed with an aqueous solution 11 of a soluble sulfate (e.g. MaSO4) such as desulfurization waste liquid obtained by contacting an aqueous slurry 8 of Mg(OH)2 with an exhaust gas of an exhaust gas generation furnace 9 in an exhaust gas desulfurization apparatus 10. The mixture is vigorously agitated (20) and the produced gypsum dihydride is separated and removed by a wet classifier 7. The obtained aqueous slurry of Mg(OH)2 is stored in a storage tank 8.

Description

Detailed Description of the Invention 3. Detailed Description of the Invention [Industrial Field of Application] This invention is a method for removing calcium groups based on impurities from an aqueous slurry of magnesium hydroxide containing calcium chloride as a soluble impurity. The present invention relates to a method of separating and removing magnesium hydroxide suitable for exhaust gas desulfurization.

[Prior art J] In recent years, magnesium hydroxide [Mg(OH')] has been used as a desulfurizing agent for exhaust gas desulfurization to fix sulfur dioxide (SO2) in the exhaust gas as magnesium sulfite, which is then oxidized in the air. A method using water-soluble magnesium sulfate has become popular. Magnesium hydroxide is cheaper than caustic soda, and calcium hydroxide tends to cause problems with operation due to scaling.
This is because there are no such problems and it is easy to use.

By the way, since seawater contains a considerable proportion of magnesium chloride, attempts have been made for a long time to recover and use this magnesium group as magnesium hydroxide. That is, this method adds Cab, CaO-Mg to seawater.
Magnesium hydroxide is produced by adding a calcium compound such as O, Ca(OH)2 and reacting it with magnesium chloride. The magnesium hydroxide produced in this way is in the form of a water slurry, and in this slurry, in addition to insoluble impurities such as calcium carbonate, calcium chloride produced as a by-product in the above reaction and other salts contained in seawater are dissolved. Contains many sexual impurities.

Manufacturers of magnesia clinker and lightly calcined magnesia, which is used as a rubber filler, require that their final products be of particularly high purity. In addition to removing insoluble impurities such as calcium, the soluble impurities in the slurry are washed away with a large amount of water, and the thus purified high-purity water slurry is filtered and dehydrated, followed by firing, etc., to obtain the above-mentioned object. It has gained.

On the other hand, magnesium hydroxide is used as a desulfurizing agent for exhaust gas desulfurization by adding a portion of the high-purity water slurry purified by the above method to a concentration such that the concentration of magnesium hydroxide is approximately 30% by weight. It is transported to the desulfurization plant by ship or lorry for use.

[Problems to be Solved by the Invention] However, it is economically very disadvantageous to use the above-mentioned high-purity water slurry as a desulfurization agent for exhaust gas desulfurization.

This is because washing and purification using a large amount of water requires large-scale equipment, and it takes a long time to sediment and separate magnesium hydroxide during washing, and furthermore, filtration and dehydration are extremely time-consuming. This is because the product cost becomes high.

Therefore, it is an object of the present invention to obtain magnesium hydroxide for exhaust gas desulfurization by an economically advantageous method compared to the conventional method described above.

[Means for Solving the Problems] As a result of intensive studies to achieve the above object, the inventors found that magnesium hydroxide for exhaust gas desulfurization is desirable as a raw material component for magnesha clinker, etc. It does not need to be a high-purity product; as long as insoluble impurities, such as calcium carbonate, and calcium chloride, which is a soluble impurity, are removed as much as possible, it can be used for exhaust gas desulfurization without any problems. I learned that.

In addition, the above calcium carbonate and calcium chloride react with sulfur dioxide gas in the desulfurization equipment, and after this reaction, they are oxidized to become trihydrate (CaSO4.2H2O), which becomes hard scale and adheres to the inside of the equipment. This results in problems with the operation of the drainage device.

Here, insoluble impurities such as calcium carbonate are coarser and more likely to settle than magnesium hydroxide, and therefore are relatively easily separated from the aqueous slurry of magnesium hydroxide. On the other hand, since calcium chloride is water-soluble, it is not easy to remove it from the water slurry (and washing with a large amount of evening water as in the conventional method causes economical problems).

Therefore, the present inventors obtained the water slurry of magnesium hydroxide obtained by adding a calcium compound such as CaO to seawater and reacting it, and after this reflection, insoluble impurities such as calcium carbonate were removed. As a result of further studies on how to economically separate and remove calcium chloride, it was found that the above objective could be achieved by using a water-soluble sulfate such as magnesium sulfate as a calcium chloride removal agent. understood.

That is, when an aqueous solution containing a water-soluble sulfate is added to an aqueous slurry of magnesium hydroxide containing calcium chloride, gypsum trihydrate is produced according to the following reaction formula, for example.

CaCl24Mg5O, -MgC12'', Ca5O,
・2H2O This trihydrate is a coarse particle and can be easily taken out of the reaction system, while by-product magnesium chloride is mixed into the water slurry, but even if it is introduced into the exhaust gas desulfurization equipment. There is no problem of scaling, and therefore the water slurry from which the two plasters have been removed can be used as is as a desulfurization agent for exhaust gas desulfurization. Therefore, the water washing process using a large amount of water as in the conventional method is not necessary, and a water slurry of magnesium hydroxide for exhaust gas desulfurization can be obtained economically and easily.

This invention was made based on the above findings, and its gist is that an aqueous solution containing a water-soluble sulfate is added to an aqueous slurry of magnesium hydroxide containing calcium chloride as a soluble impurity. The present invention provides a method for producing magnesium hydroxide for exhaust gas desulfurization, characterized in that after the reaction, two gypsum as a reaction product is separated and removed from the water slurry.

[Structure and operation of the invention] Hereinafter, the method of the invention will be explained in detail with reference to the drawings.

In the figure, 1 is a reaction tank for producing a water slurry of magnesium hydroxide, 2 and 3 are settling tanks for separating and removing insoluble solid impurities in the water slurry, and 4 is a tank for separating and removing the solid impurities. 5 is a mixing tank in which a flocculant is added to the water slurry to facilitate coagulation and sedimentation of magnesium hydroxide; 5 is a concentration tank in which the water slurry to which the flocculant has been added is allowed to stand; 6 is a concentration tank in which the magnesium hydroxide is precipitated; A reaction tank for adding an aqueous solution containing magnesium sulfate to the concentrated magnesium hydroxide water slurry to cause the calcium chloride and magnesium sulfate in the water slurry to react; 7 is a wet classifier such as a wet cyclone; 8 is the above-mentioned This is a storage tank for aqueous slurry of magnesium hydroxide for exhaust gas desulfurization after the classifier 7 separates and removes a reaction product of calcium chloride and magnesium sulfate.

9 is an exhaust gas generating furnace such as a heavy oil-fired combustion furnace or boiler, 1
0 is an exhaust gas desulfurization device; 11 is a storage tank for an aqueous solution containing magnesium sulfate generated in the device 10, that is, a desulfurization waste liquid; 12, 13, 14, 15, 16, and 17 are pumps;
8 and 19.2O are stirring blades 18a, respectively.

19a, a stirrer equipped with 2Oa, 21 a seawater tank, 22 a water slurry tank for a calcium compound such as Ca0%CaO-Mg0%Ca(OH), and 23 a flocculant tank.

In the apparatus configured as described above, first, the above compound is added from the calcium compound water slurry tank 22 to the seawater supplied from the seawater tank 21 in the reaction tank 1, and the above compound is strongly stirred by the stirrer 18, whereby the magnesium chloride in the seawater and the above described compound are added. The reaction with the compound produces a slurry of magnesium hydroxide in water. As a result of this reaction, calcium chloride, which is a water-soluble impurity, is mixed into the water slurry as a by-product.

In addition to the above reaction, calcium bicarbonate [Ca
(HCO3)2 ] and Calcilla hydroxide to produce calcium carbonate as a solid impurity, which is mixed into the water slurry.

The water slurry containing such impurities is sent to settling tanks 2 and 3.
solid impurities such as calcium carbonate and sand contained in the seawater are sedimented and separated, and are taken out of the yarn by the pump 13. The water slurry from which the solid impurities have been separated and removed in this way is then led to the mixing tank 4, where a flocculant such as aluminum, supplied from the flocculant tank 23, is added to the water slurry at a concentration of about 1 ppm by the agitator 19. Stir vigorously to mix.

By introducing this mixture into the concentration tank 5 and allowing it to stand still, the magnesium hydroxide in the water slurry is precipitated, and the supernatant liquid is taken out of the system. The removed supernatant liquid is returned to the sea as drained seawater with a pH adjusted to about 7 to 8. On the other hand, a water slurry of magnesium hydroxide concentrated by sedimentation is introduced into the reaction tank 6 by a pump 14. The concentration of magnesium hydroxide in the slurry at this time is about 7 to 10% by weight.

In the reaction tank 6, 3 to 1 parts of magnesium sulfate is added from the storage tank 11.
When an aqueous solution containing about 0% by weight is supplied and strongly stirred by the stirrer 2O, calcium chloride as a soluble impurity contained in the water slurry reacts with magnesium sulfate, and dihydric gypsum is produced. Since these two plasters have coarse particles and are easily separated by sedimentation, they are easily separated by supplying the reaction liquid to the wet classifier using the pump 15, and the separated two plasters are separated from the sedimentation tank 3. A small amount of the water slurry of magnesium hydroxide that is introduced into the slurry and mixed therein is recovered and taken out of the system together with the solid impurities such as calcium carbonate mentioned above by the pump 13.

The water slurry of magnesium hydroxide from which the two plasters have been separated and removed in this way can be used without any problem as a desulfurization agent for exhaust gas desulfurization, and can be used in the exhaust gas desulfurization equipment 10.
The desulfurizing agent is stored in a storage tank 8 as a desulfurizing agent to be supplied to the water.

In the exhaust gas desulfurization device 10, the exhaust gas from the exhaust gas generation furnace 9 is introduced and the water slurry from the storage tank 8 is supplied to it, so that the sulfur dioxide gas in the exhaust gas and magnesium hydroxide react, and after this reaction, oxidized This produces magnesium sulfate. This aqueous solution containing magnesium sulfate, that is, the desulfurization waste liquid, is taken out of the device by the pump 17 when the magnesium sulfate reaches the above concentration and is stored in the storage tank 11. A part of it is dumped into the sea as waste liquid, and the rest is used as a reaction reagent to be supplied to the reaction tank 6.

Although magnesium sulfate is used as the water-soluble sulfate in the above example, any other water-soluble sulfate such as sodium sulfate can be used in the same manner as above. Also,
In the above example, an aqueous slurry of magnesium hydroxide containing calcium chloride as a soluble impurity is obtained by adding and reacting calcium compounds such as CaO, CaO・MgO, and Ca(OH)2 to seawater. However, the water slurry may be obtained by other methods.

That is, since this invention is characterized by a means for separating and removing calcium chloride contained in the water slurry, the water slurry is not limited to the one produced from seawater as described above, but can also be produced by other methods. It can also be something. For example, the desulfurization waste obtained by desulfurizing coal combustion exhaust gas with a water slurry of magnesium hydroxide contains a considerable proportion of magnesium chloride as well as magnesium sulfate. Magnesium oxide is produced, and by separating and removing the two plasters, an aqueous slurry of magnesium hydroxide is obtained. However, since calcium chloride produced by the reaction between magnesium chloride and calcium hydroxide is mixed into this water slurry, it cannot be used as it is as a desulfurizing agent for exhaust gas desulfurization, so the method of this invention is not applied. By removing the above-mentioned calcium chloride, it becomes possible to use it as a desulfurization agent.

[Effects of the Invention] As described above, in this invention, a water-soluble removal agent is used as a removing agent for separating and removing calcium groups based on impurities from an aqueous slurry of magnesium hydroxide containing calcium chloride as a soluble impurity. By using an aqueous solution containing a chemical sulfate, it is possible to economically and easily produce magnesium hydroxide, which can be used without any problems as a desulfurizing agent for exhaust gas desulfurization. In addition, when magnesium sulfate is used as the water-soluble sulfate,
Since the desulfurization waste liquid obtained from the exhaust gas desulfurization apparatus can be used as it is as an aqueous solution containing magnesium sulfate, improvement in economic efficiency is also expected from this point of view.

Furthermore, when the exhaust gas desulfurization equipment is installed near the coast, a water slurry of magnesium hydroxide containing calcium chloride as a soluble impurity is obtained by reacting seawater with calcium compounds such as CaO, and this water slurry By reacting the desulfurization waste liquid discharged from the above apparatus, a desulfurizing agent that can be supplied into the above apparatus can be obtained.In other words, by putting the method of the present invention into practical use in an exhaust gas desulfurization factory as described above, This eliminates the hassle of transporting the magnesium hydroxide water slurry as an intermediate raw material produced by manufacturers of magnesha clinkers and other products to exhaust gas desulfurization plants across Japan, and eliminates costs such as transportation costs. This will further improve economic efficiency.

[Example 1] Examples of the present invention will be described below to explain more specifically.

Example Using the apparatus shown in the drawings, seawater was introduced into the reaction tank 1 at a rate of 18 tons/hour, and a water slurry containing 7% by weight of CaO was added thereto so that the pH would be in the range of 10.4 to 10.6. supplied. The reaction solution was supplied to settling tanks 2 and 3, and solid impurities such as calcium carbonate and sand were separated and removed from the lower part of settling tank 3. In the mixing tank 4, an aluminum-based coagulant (trade name: PAC manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.) is added to the water slurry of magnesium hydroxide overflowing from the settling tank 2.
After adding the solution to a concentration of ppm, it was allowed to stand in the concentration tank 5 to precipitate magnesium hydroxide.

50 ml of water slug IJ- containing precipitated magnesium hydroxide
It was introduced into the reaction tank 6 at a rate of 0 kg/hour. Note that the concentration of magnesium hydroxide in the water slurry was 7% by weight. 40% of an aqueous solution (desulfurization waste liquid) containing magnesium sulfate at a concentration of 3% by weight was added to the reaction tank 6 from the storage tank 11.
It was fed and reacted at a rate of h/h. The reaction solution contained magnesium hydroxide and gypsum trihydrate at a weight ratio of 97:3, and was led to a wet classifier (wet cyclone) 7 to separate the gypsum trihydrate. The water slurry of magnesium hydroxide from which the gypsum was separated was allowed to overflow from the classifier 7 and stored in a storage tank 8.

500 kg of this water slurry of magnesium hydroxide was introduced into a desulfurization equipment in which exhaust gas (sulfur oxides: 1,500 ppm, oxygen content: 4%) from the exhaust gas generating furnace 9 was introduced at a rate of 9,00 Nm/hour. /hour to carry out the desulfurization reaction. The desulfurization waste liquid is led to the storage tank 11 as an aqueous solution containing 3% by weight of magnesium sulfate, where the PH
After adjusting the amount, a part of the amount was continuously supplied to the reaction tank 6.

By performing continuous operation in this way, the desulfurization equipment 1
Desulfurization treatment could be carried out stably over a long period of time without causing problems such as scaling within the range of 0.

[Brief explanation of the drawing]

The drawing is a schematic diagram showing an example of a device configuration to which the method of the present invention is applied. 5. Concentrating tank for magnesium hydroxide water slurry, 6.
・Reaction tank, 7... Wet classifier for separation of gypsum trihydrate, 8... Storage tank for water slurry of magnesium hydroxide for desulfurization, 11... Storage tank for aqueous solution containing magnesium sulfate Patent applicant Ken Nakagawa - 1) Jalco

Claims (4)

[Claims] (1) After adding an aqueous solution containing a water-soluble sulfate to an aqueous slurry of magnesium hydroxide containing calcium chloride as a soluble impurity and causing a reaction, gypsum dihydrate (CaSO_4.2H_2O) is produced as a reaction product. A method for producing magnesium hydroxide for exhaust gas desulfurization, which comprises separating and removing from the water slurry. (2) Water slurry of magnesium hydroxide containing calcium chloride as a soluble impurity is added to seawater with CaO,
A method for producing magnesium hydroxide for exhaust gas desulfurization according to claim (1), which comprises a water slurry obtained by adding and reacting calcium compounds such as CaO.MgO and Ca(OH)_2. (3) The method for producing magnesium hydroxide for exhaust gas desulfurization according to claim 1 or (2), wherein the water-soluble sulfate is magnesium sulfate. (4) The method for producing magnesium hydroxide for exhaust gas desulfurization according to claim (3), wherein the aqueous solution containing magnesium sulfate is a desulfurization waste obtained by contacting exhaust gas with an aqueous slurry of magnesium hydroxide.