JP7110128B2 - Method for removing mercury in smoke washing wastewater - Google Patents

Description

The present invention relates to a method for removing mercury in smoke washing wastewater.

In recent years, there has been a growing movement to regulate atmospheric emissions of mercury, including revisions to the Air Pollution Control Law following the entry into force of the Minamata Convention on Mercury, which aims to protect human health and the environment. The Air Pollution Control Law stipulates mercury emission standards for non-ferrous metal (copper, lead, zinc and industrial gold) manufacturing facilities, waste incineration facilities and other facilities that emit mercury. Therefore, such mercury-emitting facilities are required to remove mercury contained in the exhaust gas generated in the smelting process, the roasting process, the incineration process, and the like, in order to meet the standards.

In this regard, a method of removing mercury by cleaning exhaust gas using a wet scrubber is known. In the wet scrubber, toxic substances such as mercury and sulfur oxides (SOx) contained in the exhaust gas are recovered in the water by bringing the exhaust gas into contact with water circulating in the scrubber. However, this time, it is desired to remove the mercury contained in the smoke washing waste water, which is the waste water after cleaning the exhaust gas.

Here, a method has been proposed to adsorb and remove mercury in wastewater using a chelating agent, activated carbon, or the like. For example, Patent Document 1 (Japanese Patent Application Laid-Open No. 2015-128754) discloses a technique for immobilizing mercury by adding a chelating agent to desulfurization effluent used to remove sulfur oxides in exhaust gas. there is

By the way, regarding the recycling of zinc, it is common to prepare crude zinc oxide by heating, reducing and volatilizing waste such as electric furnace dust, and recovering metallic zinc from the obtained crude zinc oxide. For example, in Patent Document 2 (Japanese Patent No. 3727232), a raw material containing zinc oxide and iron oxide is heated and reduced, and crude zinc oxide produced by reoxidation of the produced metallic zinc vapor and metallic iron produced A technique is disclosed in which the contents are respectively collected, mixed and heated to condense metallic zinc vapor produced, and metallic zinc is collected.

JP 2015-128754 A Japanese Patent No. 3727232

However, since the chelating agent and activated carbon disclosed in Patent Document 1 are expensive, the cost required to remove mercury from the smoke washing wastewater is high.

The present inventors have recently found that mercury in smoke washing wastewater can be easily and inexpensively removed by using crude zinc oxide derived from zinc-containing waste materials as an adsorbent.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a method for removing mercury from smoke washing wastewater simply and inexpensively.

According to one aspect of the present invention, a method for removing mercury in smoke washing wastewater, comprising:
a step of preparing crude zinc oxide obtained through heat-reduction volatilization of zinc-containing waste material and smoke washing wastewater containing mercury;
a step of mixing the smoke washing wastewater with the crude zinc oxide to remove mercury in the smoke washing wastewater by adsorption to the crude zinc oxide;
A method is provided, comprising:

1 is a schematic diagram of a waste treatment apparatus used to produce crude zinc oxide in Example 1. FIG.

The method for removing mercury in smoke washing wastewater according to the present invention comprises the steps of preparing crude zinc oxide obtained through thermal reduction volatilization of zinc-containing waste materials and mercury-containing smoke washing wastewater; and a step of removing mercury in the flue scrubbing wastewater by mixing it with zinc oxide to cause the crude zinc oxide to adsorb and remove mercury.

Hereinafter, the method of the present invention will be described as follows: (1) preparation of crude zinc oxide, (2) preparation of smoke washing wastewater, (3) mixing of crude zinc oxide and smoke washing wastewater, and (4) optionally in crude zinc oxide Each operation of separation of mercury will be explained in order.

(1) Preparation of Crude Zinc Oxide Crude zinc oxide used in the method of the present invention is obtained through thermal reduction volatilization of zinc-containing waste materials. That is, by heating the zinc-containing waste material together with a reducing agent such as coke or coal, zinc oxide and the like in the zinc-containing waste material are reduced and volatilized. Zinc oxide can be obtained. Such crude zinc oxide is a low purity zinc oxide that is widely produced and used in the zinc smelting industry as a zinc concentrate replacement raw material for zinc smelting and is therefore inexpensively available. Crude zinc oxide typically contains Zn in an amount of 40% by weight or more and 80% by weight or less, more typically 50% by weight or more and 75% by weight or less, and even more typically Specifically, it contains 60% by weight or more and 70% by weight or less. Therefore, it can be said that crude zinc oxide contains Zn in an amount within the above range, and the remainder is unavoidable impurities derived from zinc-containing waste materials. Zinc-containing waste is waste containing zinc and/or zinc compounds. The zinc-containing waste material is preferably a raw material for zinc recycling. Examples of raw materials for zinc recycling include molten fly ash, steelmaking flue ash, sludge, incineration fly ash, waste dry batteries, smelting residue, and the like. The zinc-containing waste material may contain metal components other than zinc (for example, lead, copper, etc.). Also, zinc-containing waste typically contains mercury as an unavoidable impurity.

The heat reduction volatilization of the zinc-containing waste material may be performed using known equipment capable of producing crude zinc oxide, and is not particularly limited. For example, facilities such as a blast furnace and a rotary kiln can be preferably used. The temperature at which the heat reduction volatilization is performed is preferably 1000° C. or higher and 1500° C. or lower, more preferably 1100° C. or higher and 1400° C. or lower, and still more preferably 1150° C. or higher and 1350° C. or lower. By doing so, it becomes possible to reduce and volatilize the zinc-containing waste material more reliably.

Prior to thermal reduction volatilization, the zinc-containing waste material may be pretreated, and an example of such a pretreatment is a flocculation treatment. Fulfilling involves mixing zinc-containing waste materials with fluxes such as silica stone and iron oxide, reducing agents such as coal, binders such as pulp waste liquid, etc., followed by drying and pulverizing, followed by compression molding using a framing machine. It is a process to create briquettes by Stable operation becomes possible by heat-treating zinc-containing waste materials in the form of briquettes. When the zinc-containing waste material contains molten fly ash, the molten fly ash may be dehalogenated by a known method.

The zinc-containing waste material is melted by heating and reduction and separated into three phases: slag, matte and metal. Among them, the zinc in the zinc-containing waste material is mainly present in the metal, and the metal volatilizes in a high-temperature atmosphere. Most of the metal volatilized by heating is oxidized during cooling to become a solid content containing zinc oxide and, if present, other metal oxides (such as lead oxide and lead chloride). . Therefore, crude zinc oxide can be obtained by collecting this solid content. Preferred examples of collection methods include filtering, electrostatic precipitator, cyclone, gravity sedimentation, wet scrubber, and the like. When the zinc-containing waste material contains mercury, part of it is contained in the crude zinc oxide, but the rest is contained in the exhaust gas after collecting and removing the crude zinc oxide.

Since the crude zinc oxide thus obtained usually contains halogens such as chlorine and fluorine, it is generally subjected to dehalogenation treatment by wet cleaning using industrial water or the like. In this respect, in the present invention, the smoke washing waste water is mixed with the crude zinc oxide, so that the chlorines in the crude zinc oxide can be removed by the smoke washing waste water. Thus, according to the present invention, it is possible not only to remove mercury in the smoke washing wastewater, but also to dehalogenate crude zinc oxide at the same time. Therefore, the crude zinc oxide prepared in the present invention can be one that has not undergone dehalogenation. In other words, the crude zinc oxide used in the present invention may contain halogens, for example chlorine.

(2) Preparation of smoke washing wastewater The smoke washing wastewater used in the method of the present invention is not particularly limited as long as it contains mercury to be removed. This smoke washing wastewater is typically a cleaning liquid used for cleaning exhaust gas containing mercury. A method for cleaning the exhaust gas is not particularly limited, and a known method can be adopted. For example, a flue gas desulfurization facility such as a wet scrubber can be used to wash the flue gas, which is obtained by washing the flue gas. Specifically, by bringing the circulating water (cleaning liquid) into contact with the exhaust gas, the mercury contained in the exhaust gas is recovered in the circulating water, and the circulating water is discharged or collected to be used as smoke washing wastewater. The concentration of mercury contained in the smoke washing wastewater is not particularly limited, but is typically 0.001 mg/L or more and 100 mg/L or less, more typically 0.01 mg/L or more and 10 mg/L or less, More typically, it is 0.1 mg/L or more and 1 mg/L or less.

Advantageously, the cleaning liquid used for cleaning the exhaust gas generated in the crude zinc oxide production process can be used as the fume scrubbing wastewater. Therefore, the smoke washing wastewater is preferably the washing liquid used for washing the exhaust gas after collecting and removing the crude zinc oxide from the zinc-containing waste material which has undergone heating, reduction and volatilization. That is, since zinc-containing waste materials (for example, molten fly ash) typically contain mercury, the exhaust gas generated in the crude zinc oxide production process typically also contains mercury. In this regard, mercury in the cleaning solution can be removed by mixing the crude zinc oxide collected in the same process with the cleaning solution used for cleaning the exhaust gas after collecting and removing the crude zinc oxide. Since it can be adsorbed on crude zinc oxide, it is possible to recover mercury very efficiently. The cleaning liquid preferably contains an oxidizing agent, and preferred examples of such oxidizing agents include sodium hypochlorite, oxygen, ozone, hydrogen peroxide, halogen molecules (e.g. fluorine, chlorine, bromine or iodine), hypochlorite. Halogenates, halites, halides, perhalogenates, permanganates, dichromates, chromates and combinations thereof, and sodium hypochlorite is particularly preferred. . Here, the mercury contained in the exhaust gas is considered to exist in two forms of water-soluble divalent mercury and water-insoluble zero-valent mercury. In this regard, when the cleaning liquid contains an oxidizing agent, the zero-valent mercury can be oxidized into water-soluble divalent mercury, and more mercury can be absorbed into the cleaning liquid. As a result, the concentration of mercury in the smoke washing wastewater increases, and more mercury can be adsorbed onto the crude zinc oxide. Needless to say, the smoke washing wastewater may be a washing liquid used for washing exhaust gas generated in a process unrelated to the production of crude zinc oxide.

(3) Mixing Crude Zinc Oxide and Smoke Washing Wastewater Smoke washing wastewater is mixed with crude zinc oxide, and mercury in the smoke washing wastewater is absorbed by the crude zinc oxide and removed. The mixing method is not particularly limited, and a known method using a commercially available stirrer or the like may be used. The mixing time is not particularly limited, but is preferably 1 minute or longer and 24 hours or shorter, more preferably 10 minutes or longer and 12 hours or shorter, and still more preferably 30 minutes or longer and 6 hours or shorter. The amount of crude zinc oxide added is preferably 50 g/L or more and 400 g/L or less, more preferably 100 g/L or more and 300 g/L or less, and still more preferably 150 g/L or more and 200 g/L or less as a slurry concentration. . The amount to be added may be appropriately changed according to the expected mercury concentration in the smoke washing wastewater. The pH during mixing is preferably 6 or more and 13 or less, more preferably 7 or more and 12 or less, and still more preferably 8 or more and 11 or less. Caustic soda or the like can be used as a chemical used for pH adjustment.

In the method of the present invention, mercury present in smoke washing waste water can be effectively adsorbed onto crude zinc oxide by a very simple technique of mixing smoke washing waste water with crude zinc oxide. In this respect, conventionally proposed methods of removing mercury from smoke washing wastewater using chelating agents, activated carbon, etc., require the preparation of expensive chelating agents, activated carbon, etc., which is costly to remove mercury. was difficult to keep low. On the other hand, crude zinc oxide used in the method of the present invention is obtained in the process of recycling zinc, which is a valuable resource, from waste, and is generally available at extremely low cost compared to chelating agents and activated carbon. can be done. As described above, according to the present invention, by using crude zinc oxide derived from zinc-containing waste materials as an adsorbent, it is possible to easily and inexpensively remove mercury from smoke washing wastewater.

(4) Separation of Mercury in Crude Zinc Oxide If desired, the mercury-adsorbed crude zinc oxide may be heated to volatilize and separate mercury. This heating can be carried out, for example, using existing zinc smelter facilities. The heat treatment temperature is preferably 700° C. or higher and 1500° C. or lower, more preferably 800° C. or higher and 1300° C. or lower, and still more preferably 900° C. or higher and 1200° C. or lower. Equipment for the heat treatment is not particularly limited, but a preferred example is a sintering furnace used in a dry zinc smelter or the like. By heat-treating crude zinc oxide with mercury adsorbed thereon in such equipment, the crude zinc oxide can be supplied as a raw material for smelting while volatilizing and separating mercury. The volatilized mercury may be recovered by cooling and condensing the mercury, or by dissolving the mercury in a solution such as concentrated sulfuric acid. All of these recovering operations can be performed using existing facilities such as zinc smelters. Thus, by using the method of the present invention, it is possible to treat mercury at a low cost by utilizing existing facilities.

The invention is further illustrated by the following examples.

Example 1 : Treatment of smoke washing wastewater using crude zinc oxide Smoke washing wastewater containing mercury and crude zinc oxide were mixed to confirm the presence or absence of adsorption of mercury. Specifically, it is as follows.

(1) Preparation of Crude Zinc Oxide A zinc-containing waste material containing steelmaking flue ash and molten fly ash (dechlorinated in advance) was prepared. This zinc-containing waste material, flux (silica and iron oxide), and reducing agent (coal) were mixed. The resulting mixture was dried and pulverized, and then compression-molded with a briquette machine to obtain briquettes. Mercury derived from raw materials is contained in this briquette as an unavoidable impurity. The produced briquette was subjected to treatment including heat reduction volatilization using the waste treatment apparatus 10 shown in FIG. The waste treatment system 10 includes a blast furnace 12 , a boiler 14 , a cooling tower 16 , a bag filter 18 , a fan 20 , a desulfurization tower 22 and a chimney 24 . First, briquettes were charged into the blast furnace 12 . The blast furnace 12 is heated after the briquette is charged, and preheated air, oxygen and/or oxygen-enriched air is blown through the tuyeres provided in the blast furnace 12 to maintain a high temperature of 1200° C. or higher to maintain the molten state of the briquette. formed a reaction zone for The briquettes charged into the blast furnace 12 are melted in the reaction zone to produce slag mainly composed of oxides of iron, silicon and calcium, matte containing copper sulfides and zinc produced by reduction in the reaction zone. and a metal mainly composed of lead. While the slag and matte are continuously extracted from the outlet located at the bottom of the blast furnace 12, the zinc and lead that constitute the metal are volatilized by the high-temperature atmosphere of the blast furnace 12 and then oxidized in the oxidation zone above the reaction zone. Zinc and lead oxide were discharged from the blast furnace 12 together with the exhaust gas. At this time, the mercury contained in the briquettes was similarly volatilized and discharged from the blast furnace 12 together with the exhaust gas. After the exhaust gas discharged from the blast furnace 12 is sent to the boiler 14 to recover waste heat, it is rapidly cooled to 200° C. or less by spraying water in the cooling tower 16 to prevent the resynthesis of dioxins. By passing the quenched exhaust gas through the bag filter 18, solids such as zinc oxide and lead oxide were filtered and collected. Solids such as zinc oxide and lead oxide filtered and collected by the bag filter 18 were collected as crude zinc oxide. The concentration of mercury in the crude zinc oxide was 3 mg/kg when measured by the vaporization atomic absorption method using a mercury measuring device (manufactured by Nippon Instruments Co., Ltd., MA-3000).

The exhaust gas from which the solid matter has been collected and removed is fed through the fan 20 into the desulfurization tower 22 and washed with a washing liquid (aqueous solution containing caustic soda). After the mercury in the flue gas was thus recovered in the cleaning liquid, the flue gas from which the mercury had been removed was discharged from the chimney 24 .

(2) Preparation of smoke-washing wastewater In the above (1), the cleaning liquid used for washing the exhaust gas in the desulfurization tower 22 was collected and used as smoke-washing wastewater. The mercury concentration of this smoke washing wastewater was measured by reduction vaporization atomic absorption spectrometry using a mercury measuring device (manufactured by Nippon Instruments Co., Ltd., MA-3000) and found to be 0.32 mg/L.

(3) Smoke-washing wastewater treatment test 200 mL of smoke-washing wastewater was taken in a beaker, and 20 g of crude zinc oxide was added thereto and thoroughly mixed to obtain a slurry. After the slurry was stirred for 30 minutes, it was filtered with filter paper to separate solid and liquid into a filtrate and crude zinc oxide. Using a mercury measuring device (manufactured by Nippon Instruments Co., Ltd., MA-3000), the mercury concentration in the filtrate was measured by reduction vaporization atomic absorption spectroscopy, and the mercury concentration in the crude zinc oxide after filtration was measured by heat vaporization atomic absorption spectroscopy. As a result, it was as shown in Table 1.

Example 2 (Comparative): Smoke washing wastewater treatment using purified zinc oxide Commercially available purified high-purity zinc oxide (manufactured by Kanto Kagaku Co., Ltd., grade 1, purity greater than 99.0%) instead of crude zinc oxide The treatment of the smoke washing wastewater and the measurement of the mercury concentration were carried out in the same manner as in Example 1, except that the was used. The results were as shown in Table 1.

As shown in Table 1, in Example 1 using crude zinc oxide, it can be seen that mercury in the smoke washing wastewater was adsorbed by the crude zinc oxide. On the other hand, in Example 2, in which commercially available zinc oxide was used instead of crude zinc oxide, most of the mercury in the smoke washing wastewater remained in the liquid.

Example 3 : Heating Test of Crude Zinc Oxide In order to confirm that mercury can be separated from crude zinc oxide, a test of heating crude zinc oxide with mercury adsorbed thereon was carried out as follows. First, crude zinc oxide to which mercury was adsorbed was prepared separately by the same procedure as in Example 1 (3), and used as a sample for a heating test. When the mercury concentration of this heating test sample was measured in the same manner as in Example 1, it was 6 mg/kg. A 5 g sample for heating test was placed in each of five sample pans, and these sample pans were heated at different temperatures shown in Table 2 for 30 minutes using an electric furnace. Air was introduced as the heating atmosphere, and the mercury concentration in the exhaust gas was measured using an exhaust gas mercury continuous measuring device (manufactured by Nippon Instruments Co., Ltd., EMP-2). Since the concentration of mercury in the flue gas fluctuates and is not stable, the approximate maximum concentration was used as the measured value. Also, the mercury concentration of the heating test sample after heating was measured in the same manner as described above. The results were as shown in Table 2.

As shown in Table 2, it was found that by heating the crude zinc oxide that had adsorbed mercury, the mercury volatilized and was discharged together with the exhaust gas. Moreover, there was a tendency that the higher the heating temperature, the easier the demercury progressed.

10 waste treatment equipment 12 blast furnace 14 boiler 16 cooling tower 18 bag filter 20 fan 22 desulfurization tower 24 chimney

Claims (5)

A method for removing mercury in smoke washing wastewater, comprising:
a step of preparing crude zinc oxide obtained through heat-reduction volatilization of zinc-containing waste material and smoke washing wastewater containing mercury;
a step of mixing the smoke washing wastewater with the crude zinc oxide to remove mercury in the smoke washing wastewater by adsorption to the crude zinc oxide;
A method, including 2. The method according to claim 1, wherein said smoke washing wastewater is a washing liquid used for washing exhaust gas after capturing and removing crude zinc oxide that has undergone thermal reduction volatilization of zinc-containing waste materials. 3. The method of claim 2, wherein the cleaning liquid comprises an oxidizing agent. The oxidizing agent is sodium hypochlorite, oxygen, ozone, hydrogen peroxide, halogen molecules, hypohalite, halite, halide, perhalate, permanganate, dichromic acid 4. The method of claim 3, wherein the at least one is selected from the group consisting of salts and chromates. The method according to any one of claims 1 to 4, further comprising a step of heating the crude zinc oxide with the adsorbed mercury to volatilize and separate the mercury.

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