JP2014097475A - Method for treating coal gasification drainage water - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method which can efficiently treat coal gasification drainage water through metal reduction treatment and wet oxidation treatment.SOLUTION: The method comprises: bringing coal gasification drainage water into contact with a metal reductant to reduce selenium; and then subjecting the drainage water to catalytic wet oxidation treatment. The method preferably comprises: subjecting treated water in which selenium is reduced by bringing the coal gasification drainage water into contact with the metal reductant, to coagulation and solid-liquid separation treatment; and then subjecting the treated water to the catalytic wet oxidation treatment. As a metal for the metal reductant, titanium and aluminum are preferably used.

Description

  The present invention relates to a method for treating coal gasification wastewater, and more particularly, to a method for performing wet oxidation treatment of COD components, ammonia and the like after reducing selenium in coal gasification wastewater.

  Coal gasification wastewater contains selenium, COD components, ammonia and the like. As a method for treating selenium-containing wastewater, there is known a method of reducing selenium by bringing it into contact with an alloy or mixture of metal titanium and another metal and eluting a part of the other metal (Patent Documents 1 to 3). ).

  It is also known to wet-treat COD component-containing wastewater (for example, Patent Documents 4 to 6).

JP2008-30020 JP2009-11914 JP2009-11915 JP2004-17003 JP-A-9-253667 JP-A-7-265878

  When coal gasification wastewater containing selenium, COD components, etc. is first subjected to catalytic wet oxidation treatment and then metal reduction treatment, trace amounts of noble metals (metals precious than titanium) eluted from the catalyst in the catalytic wet oxidation treatment process. In the metal reduction treatment step, it is deposited on the surface of titanium, and the selenium reduction treatment reaction is inhibited.

  When two or more metals other than titanium are used as the reducing metal in the metal reduction treatment, wastewater containing noble metal further than noble metal (reduced form) is treated in the same manner as when titanium is used. Inhibition of the selenium reduction reaction occurs.

  An object of the present invention is to provide a method capable of efficiently treating coal gasification wastewater by metal reduction treatment and wet oxidation treatment.

  The method for treating coal gasification wastewater of the present invention is characterized in that the coal gasification wastewater is brought into contact with a metal reductant to reduce selenium, and then subjected to catalytic wet oxidation treatment.

  Titanium and aluminum are suitable as the metal of the metal reductant.

  In the present invention, it is preferable to subject the treated water that has been brought into contact with the metal reductant to reduce selenium to flocculation and solid-liquid separation treatment, and then to catalytic wet oxidation treatment.

  In the coal gasification wastewater treatment method of the present invention, the coal gasification wastewater is first subjected to metal reduction treatment, and then subjected to catalytic wet oxidation treatment, so there is no inhibition of selenium reduction reaction by noble metal in the metal reduction treatment step, and coal gas Chemical wastewater can be treated stably and efficiently over a long period of time.

  In addition, when using titanium and aluminum for the reduced body of a selenium process, a part of fluorine compound can be removed by the aggregation precipitation after a selenium reduction | restoration. Therefore, corrosion of the catalyst wet oxidation treatment apparatus material due to fluorine in the waste water can be reduced.

It is process drawing of a metal reduction process. It is process drawing of a catalyst wet oxidation process.

  Hereinafter, the present invention will be described in more detail.

  The present invention treats coal gasification wastewater. Coal gasification wastewater is wastewater from a coal gasification process in a coal gasification power plant or the like. Coal gasification wastewater usually contains about 0.2 to 5 mg / L of selenium, 100 to 1500 mg / L of COD components, and about 100 to 4000 mg / L of ammonia.

  In the present invention, this coal gasification wastewater is first treated with a metal reductant. As the metal reductant, it is preferable to use a first metal and a second metal that is more noble than that. In particular, it is preferable to use an alloy or a mixture of metal titanium and another metal other than metal titanium. preferable. As other metals alloyed with or mixed with titanium metal, various metals can be used, but it is preferable that the sludge composed of hydroxide generated by pH adjustment after elution of metal exhibits a white color. When the sludge is white, it is easier to dispose of the sludge than when it is colored brown or the like. Examples of the metal that generates white sludge include aluminum, zinc, tin, and copper, and any of them can be suitably used. In particular, aluminum, zinc, and tin are excellent in terms of solubility, and can be suitably used in the present invention. As the metal other than metal titanium, only one kind of metal may be used, or a mixture or alloy of two or more kinds of metals may be used.

  As an alloy of titanium metal and another metal used in the present invention, any of a solid solution, an intermetallic compound, and a covalent alloy can be used. As an alloying method, for example, a method using a difference in ionization tendency of metal, an electrolytic method, a melting method, or the like can be employed.

  Moreover, in this invention, metal titanium and another metal can also be used as a mixture, without alloying. Metal titanium in the form of powder, granule, fiber, etc. and other metal in the form of powder, granule, fiber, etc., mixed in the same form or different Forms can be mixed into a mixture.

  The shape of the alloy or mixture of titanium and another metal is preferably one having a large surface area. For example, it is used as a powdery material, a granular material, a fibrous material, a fine thin film, or the like having a particle size of about 10 μm to 5 mm.

  There is no restriction | limiting in the method of contacting a selenium containing waste_water | drain with the alloy or mixture of titanium and another metal, It can contact with arbitrary types of reduction reactors. The reduction reactor may be, for example, a reduction reaction tank in which selenium-containing wastewater is introduced into the reaction tank and a powdered, fine-grained alloy or mixture is added. It is also possible to use a packed tower in which waste water is passed through the packed bed.

  When the selenium-containing wastewater is brought into contact with an alloy or mixture of titanium metal and another metal, the other metal is eluted and dissolved in the wastewater. When the metal elutes into ions, a strong reducing action occurs, and selenium in the wastewater is reduced. Since this metal elution is neutral and takes a long time, it is preferable to promote acid elution by adding an acid to the selenium-containing wastewater. Examples of the acid to be added include hydrochloric acid and sulfuric acid. The amount of acid added is preferably set according to the amount of metal to be eluted. The metal elution amount is generally proportional to the acid addition amount, and the acid addition amount can be determined by a relational expression obtained in advance by experiments. Further, the elution amount of the metal can be set according to the hexavalent selenium concentration to be reduced.

  Metals other than selenium eluted in the selenium-containing wastewater, such as aluminum and zinc, are considered to react with selenate ions as shown below to reduce selenium.

2Al ⁰ + SeO ₄ ²⁻ + 8H ⁺ → 2Al ³⁺ + Se ⁰ + 4H ₂ O
3Zn ⁰ + SeO ₄ ²⁻ + 8H ⁺ → 3Zn ²⁺ + Se ⁰ + 4H ₂ O
In the present invention, when contact is made with wastewater under the coexistence state by alloying or mixing of metal titanium and another metal, the selenium reduction treatment performance is greatly improved as compared with the treatment by single contact of metal titanium or other metal. The reason is that metals other than titanium, such as aluminum and zinc, dissolve, electrons move through titanium that hardly dissolves even in the presence of an acid, and selenium is reduced on the titanium surface. At that time, there is a possibility that some electrical effect is manifested.

  The ratio T / M between the volume T of metallic titanium and the volume M of other metals is preferably ¼ or more, particularly preferably ½ or more. When the ratio T / M is 1/4 or more, the selenium reduction treatment performance is improved. The reason for this is that since the proportion of titanium metal is high, the amount of electrons generated during the dissolution of other metals moves to the surface of the titanium metal, and the amount of selenium reduced on the surface of the metallic titanium increases. It is thought that it is to do. On the other hand, when the ratio T / M is less than ¼, the ratio of metal titanium is low, so among the electrons generated when other metals are dissolved, they are emitted from the surface of the other metals. It is considered that the performance of selenium reduction treatment decreases because the amount of electrons that increase and the amount of electrons that move to the metal titanium surface and contribute to the reduction of selenium decreases.

  When the filling capacity of other metals is constant and the ratio T / M is increased, a good reduction treatment performance is maintained, but a large amount of titanium metal is required, and these other metals and metals The volume of the titanium filling device needs to be increased. For this reason, this ratio T / M is preferably 4/1 or less, particularly 2/1 or less. More preferably, this ratio T / M is 1/4 to 4/1, in particular 1/2 to 2/1.

  In the present invention, the other metal is preferably aluminum. Also, when the volume of titanium metal is T and the volume of aluminum is A, when T / A is 1/4 to 4/1, particularly 1/2 to 2/1, selenium is removed very efficiently. Can do.

  Reduced selenium, for example hexavalent selenium, becomes mostly zero-valent selenium, which is deposited on the titanium surface of the alloy or mixture and removed from the waste water. Residual selenium is reduced from hexavalent to low valent selenium, for example, tetravalent selenium, and is easily precipitated by agglomeration.

  In the present invention, it is preferable to subject the selenium-containing wastewater to a reduction treatment and then a reduction treatment of the reduction-treated water. The agglomeration treatment is performed by adjusting the pH of the reduction treatment water, precipitating the eluted metal as an insoluble compound such as hydroxide, and solid-liquid separation of the precipitated metal compound.

  The pH of the reduced water is usually adjusted by adding an alkali such as sodium hydroxide, potassium hydroxide or slaked lime. When the metal used with the metal titanium is aluminum, an alkali is added to the reduction-treated water, and the dissolved aluminum is precipitated as aluminum hydroxide. It is preferable to adjust the pH to 5 to 8 by adding an alkali, and aluminum hydroxide dissolves at pH 4 or lower or pH 9 or higher, which is not suitable. When the metal used together with titanium metal is zinc, the pH is adjusted to 9 to 10, and when it is tin, it is precipitated as a hydroxide by adjusting the pH to around 8.

  When or after the metal compound is precipitated by adjusting the pH, an organic flocculant (polymer flocculant) and / or an inorganic flocculant can be added to improve the solid-liquid separation.

  In order to separate the precipitated metal compound from water, a solid-liquid separation operation is performed. For solid-liquid separation, any commonly used method can be adopted, and separation into treated water and sludge composed of an insoluble metal compound is performed by precipitation, filtration, centrifugation, membrane separation, or the like.

  The metal eluted during the reduction treatment is insolubilized by pH adjustment and solid-liquid separation of the reduced treated water, separated from the water, and discharged as treated water containing no metal. Further, when the eluted metal is precipitated as an insoluble compound, for example, aluminum hydroxide, reduced low-valent selenium remaining in the water is also adsorbed on the aluminum hydroxide floc and can be removed by a coprecipitation phenomenon.

  In addition, when fluorine and / or boron coexist in the selenium-containing wastewater, when aluminum is used as the metal to be used with the metal titanium, when aluminum hydroxide is precipitated by pH adjustment after the reduction treatment, fluorine and / or boron is also present. Can be removed by coprecipitation phenomenon.

  When the coal gasification wastewater contains turbidity, heavy metals other than selenium, etc., before the metal reduction treatment, for example, a pretreatment device (not shown) such as an agglomeration device, a filtration device, a membrane separation device, or an activated carbon adsorption device. The coexisting pollutants may be removed.

  Hereinafter, a metal reduction treatment apparatus for selenium-containing wastewater will be described with reference to FIG.

  Water to be treated consisting of coal gasification wastewater or pretreated water pretreated as described above is added with acid in the pH adjustment tank 1 and then fed by the pump 2, and the acid is added to the line as necessary. After being heated by the heat exchanger 3 and further heated by the heater 4, the water is passed through the reduction tower 5 and comes into contact with the metal reductant to reduce selenium. The water flowing out from the reduction tower 5 is heat-exchanged with the water to be treated in the heat exchanger 3 and cooled, and then introduced into the reaction tank 6 and added with an alkali such as NaOH, stirred, and pH 6-8, in particular. It is about 7. Next, the polymer flocculant is introduced into the flocculation tank 7 and added and stirred. Then, it is introduced into the precipitation tank 8 and separated into solid and liquid, and the supernatant is taken out as metal reduction treated water. Discharged outside.

  In the present invention, this metal reduction treated water is then subjected to catalytic wet oxidation treatment. In the catalytic wet oxidation treatment, an oxygen-containing gas such as air is mixed into the water to be treated, and is oxidized with a catalyst.

  Examples of the catalyst include Fe, Co, Ni, Ru, Rh, Pd, Ir, Pt, Cu, Au, and W, and water-insoluble or hardly soluble compounds of these metals. More specific examples of such compounds include oxides (such as cobalt oxide and iron oxide), chlorides (such as ruthenium dichloride and platinum dichloride), and sulfides (such as ruthenium sulfide and rhodium sulfide). It is done. These metals and their compounds may be used alone or in combination of two or more. These catalytically active components are used in a state where they are supported on a known metal oxide carrier or metal carrier according to a conventional method. The metal oxide carrier and the metal carrier are not particularly limited, and those used as known catalyst carriers can be used as they are. Examples of metal oxide carriers include alumina, silica, zirconia, titania, composite metal oxides or mixed metal oxides containing these metal oxides (alumina-silica, alumina-silica-zirconia, titania-zirconia, etc.), and these metal oxides. And metal oxide-based carriers mainly composed of mixed metal oxides or mixed metal oxides, and examples of the metal carriers include iron and aluminum. Among these carriers, zirconia, titania and titania-zirconia having excellent durability are more preferable.

The shape of the supported catalyst is not particularly limited, and examples thereof include a spherical shape, a pellet shape, a cylindrical shape, a crushed piece shape, a powder shape, and a honeycomb shape. In the case of using such a supported catalyst, the reactor volume is such that, in the case of a fixed bed, the space velocity of the liquid is about 0.5 to 10 hr ⁻¹ , more preferably about ¹ to 5 hr ^−1. Good to do. The size of the supported catalyst used in the fixed bed is spherical, pellet-shaped, cylindrical, or fragmented. In the case of powder or the like, it is usually about 1 to 50 mm, more preferably about 3 to 25 mm. In addition, as a honeycomb structure when the catalyst is supported on a honeycomb-shaped carrier, an opening having an arbitrary shape such as a square, a hexagon, or a circle is used. The area per unit volume, the aperture ratio, and the like are not particularly limited, but usually those having an area per unit volume of about 200 to 800 m ² / m ³ and an aperture ratio of about 40 to 80% are used. Examples of the material of the honeycomb structure include metal oxides and metals similar to those described above, and zirconia, titania, and titania-zirconia, which are also excellent in durability, are more preferable.

  The temperature in the catalytic wet oxidation reactor is usually about 100 ° C. or higher, more preferably about 150 to 370 ° C.

  The amount of oxygen added to the object to be processed is not less than the required theoretical oxygen amount, more preferably about 1.05 to 1.5 times the theoretical oxygen amount.

  FIG. 2 shows a catalytic wet oxidation apparatus. Water to be treated consisting of treated water from the above-mentioned metal reduction treatment step is received in the storage tank 11, fed by the pump 12, heated by the heat exchanger 13 and the heater 14, then passed through the catalyst packed tower 15, and the catalyst Wet oxidation treatment. The effluent water from the catalyst packed tower 15 is heat-exchanged with the water to be treated in the heat exchanger 13 and cooled down, and then taken out as catalyst wet oxidation treated water through the pressure regulating valve 16.

  Hereinafter, the present invention will be described with reference to examples and comparative examples. However, the present invention is not limited to the following examples unless it exceeds the gist.

Example 1
A metal mixture (Table 1) in which 20 mL (14 g) of metal titanium having a particle size of 2 to 5 mm and a purity of 99% or more and 10 mL (5.9 g) of metal aluminum having a particle size of 1 to 2 mm and a purity of 99.5% were uniformly mixed. (Indicated as Al / Ti.) Was prepared and packed in a column having an inner diameter of 11 mm. The volume ratio T / A of metal titanium and metal aluminum of this metal mixture is 2/1.

2500 mg / L of sulfuric acid was added to the following water quality coal gasification wastewater, and this column was passed through an upward flow at a flow rate of SV5 (hr ⁻¹ ) while being heated to 65 ° C. for reduction treatment. Next, sodium hydroxide is added to the column outlet water to adjust the pH to around 7, and then 2 mg / L of Cryfarm PA893 (Kurita Kogyo Co., Ltd.) is added as a polymer flocculant, and then in a precipitation tank (250 mL). Solid-liquid separation was performed, and the water quality of the supernatant was measured. The results are shown in Table 1.

<Water quality of coal gasification wastewater>
Selenium: 3.3 mg / L
NH ₄ -N: 200 mg / L
COD _Mn : 210 mg / L
Fluorine compound: 15 mg / L

Next, this supernatant water was passed through a 50 mL capacity catalyst packed tower under the following conditions to carry out catalytic wet oxidation treatment.
Water flow: Upflow catalyst: 1 wt% platinum-supported titania catalyst Water flow temperature: 160 ° C
Pressure: 0.9 MPa
Water flow rate: SV = 2 hr ⁻¹
PH during water flow: 11
Oxygen content: 1.2 times equivalent to ammonia and COD

  Table 1 shows the measurement results of the quality of the treated water.

Comparative Example 1
The treatment was performed under the same conditions as in Example 1 except that the metal reduction treatment step was after the catalyst wet oxidation treatment step. Table 1 shows the water quality measurement results.

  As is apparent from Table 1, in Example 1, the selenium concentration in the treated water is sufficiently low even after 24 days, while in Comparative Example 1, the selenium concentration increases after 12 days. From Example 1 and Comparative Example 1, when the waste water containing selenium and COD and / or ammonia is treated by combining the metal reductant method and the catalytic wet oxidation method, first, the metal reductant method is performed. It was confirmed that by performing the catalytic wet oxidation method, inhibition of the selenium reduction reaction can be suppressed, and a long-term stable treatment becomes possible.

DESCRIPTION OF SYMBOLS 1 pH adjustment tank 3,13 Heat exchanger 4,14 Heater 5 Reduction tower 15 Catalyst packed tower 16 Pressure regulation valve

Claims (3)

  A method for treating coal gasification wastewater, wherein the coal gasification wastewater is brought into contact with a metal reductant to reduce selenium, followed by catalytic wet oxidation treatment.   2. The method for treating coal gasification waste water according to claim 1, wherein the metal of the metal reductant is titanium and aluminum.   The method for treating coal gasification wastewater according to claim 1 or 2, wherein the treated water obtained by bringing the selenium into contact with a metal reductant is subjected to coagulation and solid-liquid separation treatment, followed by catalytic wet oxidation treatment.

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