Method for producing zinc-based desulfurizing agent

Abstract

Provided is a zinc-based desulfurizing agent having excellent desulfurizing performance at around room temperature. This method for producing the zinc-based desulfurizing agent comprises: a neutralization step for neutralizing a zinc ion-containing aqueous solution to precipitate a zinc component; a solid-liquid separation step for subjecting the precipitated zinc component to solid-liquid separation to obtain a zinc hydroxide-containing dehydrated cake; and a drying step for drying the zinc hydroxide-containing dehydrated cake at a temperature equal to or lower than the decomposition temperature of zinc hydroxide to obtain a zinc hydroxide-containing solid.

Description

Method for manufacturing zinc-based desulfurizing agents

[0001] This invention relates to a method for producing a zinc-based desulfurizing agent.

[0002] The gases generated in various facilities such as steelmaking, petrochemicals, sewage treatment, and gas-fired power plants include hydrogen sulfide (H ２ It contains hydrogen sulfide (S), which causes foul odors and corrosion of pipes. Therefore, there is a need for technology to remove hydrogen sulfide contained in the gas.

[0003] Furthermore, such gases contain large amounts of CO ２ It often contains CO ２ From the perspective of reducing emissions, CO2 in gas ２ Technologies have been proposed to convert hydrogen sulfide into methanol and other substances through catalytic reactions for recycling. However, since the catalyst deteriorates if hydrogen sulfide is present in the gas, technologies to remove hydrogen sulfide from the gas are needed to prevent catalyst degradation.

[0004] To address the above issues, a method has been proposed in which hydrogen sulfide is adsorbed and removed using a desulfurizing agent containing a zinc compound.

[0005] For example, Patent Document 1 proposes a zinc-based desulfurizing agent manufactured by spray drying and containing zinc oxide as an active ingredient.

[0006] Furthermore, Patent Document 2 proposes a desulfurizing agent in which nickel and copper are supported on a carrier containing a silica-based component, a zinc compound, and an aluminum compound.

[0007] Japanese Patent Publication No. 2013-508527, Japanese Patent Publication No. 2017-159200

[0008] However, conventional zinc-based desulfurizing agents, such as those proposed in Patent Documents 1 and 2, had the following problems.

[0009] In other words, the desulfurizing agent proposed in Patent Document 1 is intended for use at high temperatures of 350 to 650°C. While it exhibits excellent desulfurizing performance at high temperatures, it does not have practical desulfurizing performance at or near room temperature. Therefore, this desulfurizing agent is suitable for desulfurizing high-temperature gases such as those generated in petroleum refining processes, but not suitable for desulfurizing gases at or near room temperature.

[0010] Even gases at near room temperature can be desulfurized with the above-mentioned desulfurizing agents if heated, but this requires energy and heating equipment, which results in high costs.

[0011] On the other hand, the desulfurizing agent proposed in Patent Document 2 is intended for use in desulfurizing fuel cell gases and is intended for use at relatively low temperatures. However, while it exhibits desulfurizing activity in the temperature range of 150 to 250°C, it shows almost no desulfurizing performance at or near room temperature.

[0012] Thus, no zinc-based desulfurizing agent exhibiting excellent desulfurization performance at or near room temperature has yet been found. If a desulfurizing agent with excellent desulfurization performance at or near room temperature could be provided, it would be useful for biomass power generation gases, CO2, etc. ２ This is considered extremely beneficial because it enables low-cost desulfurization in various applications where gases at or near room temperature are used, such as in efficient utilization processes and sewage treatment processes.

[0013] The present invention has been made in view of the above circumstances, and aims to provide a zinc-based desulfurizing agent that has excellent desulfurization performance at or near room temperature.

[0014] As a result of their investigation to solve the above problems, the inventors found that by using zinc hydroxide, excellent desulfurization performance can be obtained even at or near room temperature.

[0015] This invention was completed based on the above findings, and its gist is as follows.

[0016] 1. A method for producing a zinc-based desulfurizing agent, comprising: a neutralization step of neutralizing an aqueous solution containing zinc ions to precipitate zinc; a solid-liquid separation step of separating the precipitated zinc from its solid-liquid state to obtain a zinc hydroxide-containing dehydrated cake; and a drying step of drying the zinc hydroxide-containing dehydrated cake at a drying temperature of 150°C or lower to obtain a zinc hydroxide-containing solid.

[0017] 2. A method for producing a zinc-based desulfurizing agent according to claim 1, further comprising a coagulation and sedimentation step prior to the solid-liquid separation step, in which the zinc precipitated in the neutralization step is coagulated and sedimented, and the supernatant liquid is removed to obtain a zinc hydroxide-containing slurry.

[0018] 3. A method for producing a zinc-based desulfurizing agent according to 1 or 2 above, wherein zinc-containing waste liquid is used as at least a portion of the aqueous solution containing zinc ions.

[0019] 4. The method for producing a zinc-based desulfurizing agent as described in 3 above, wherein the zinc-containing waste liquid is waste liquid generated in a zinc-based plating process.

[0020] 5. The method for producing a zinc-based desulfurizing agent according to item 4, wherein the waste liquid generated in the zinc-based plating process includes either or both of the waste liquid from the plating bath used in the electro-zinc plating process and the cleaning waste liquid after the electro-zinc plating process.

[0021] According to the present invention, a zinc-based desulfurizing agent having excellent desulfurization performance at or near room temperature can be obtained.

[0022] The present invention will be described in detail below. The following description is an example of a preferred embodiment of the present invention, and the present invention is not limited to the embodiments described below.

[0023] In one embodiment of the present invention, a method for producing a zinc-based desulfurizing agent involves sequentially neutralizing, solid-liquid separating, and drying an aqueous solution containing zinc ions to produce the zinc-based desulfurizing agent.

[0024] [Aqueous solution containing zinc ions] Any aqueous solution containing zinc ions can be used as the above-mentioned aqueous solution containing zinc ions. From the viewpoint of effectively utilizing resources, it is preferable to use waste liquid containing zinc ions (zinc-containing waste liquid) as at least a part of the aqueous solution containing zinc ions.

[0025] For example, wastewater generated during the steelmaking process at a steel mill contains a sufficient amount of zinc ions and can therefore be suitably used as the zinc-containing wastewater. In particular, it is preferable to use wastewater generated in a zinc-based plating process as the zinc-containing wastewater. The zinc-based plating process may be, for example, an electro-zinc plating process. More specifically, it is preferable that the wastewater generated in the zinc-based plating process includes either or both the wastewater from the plating bath used in the electro-zinc plating process and the cleaning wastewater after the electro-zinc plating process. Here, zinc-based plating refers to zinc plating and zinc alloy plating.

[0026] [Neutralization Step] First, the aqueous solution containing zinc ions is neutralized to precipitate the zinc (neutralization step). Although not particularly limited, typically the zinc precipitates as zinc hydroxide, as shown in equation (1) below. Zn ２＋ +2OH － → Zn(OH) ２ ↓ …(1)

[0027] In equation (1) above, the stoichiometric ratio (molar ratio) of Zn and OH is 1:2, but in actual reactions, it is not always exactly 1:2.

[0028] The pH in the neutralization step is not particularly limited. However, if the pH is excessively low, Zn ions become stable in the aqueous solution, and there may be cases where the zinc content cannot be sufficiently precipitated. Therefore, the pH is preferably 6 or higher, and more preferably 7 or higher. On the other hand, if the pH is excessively high, Zn ions also become stable in the aqueous solution, and there may be cases where the zinc content cannot be sufficiently precipitated. Therefore, the pH is preferably 12 or lower, more preferably 10 or lower, and even more preferably 9 or lower. In other words, in the neutralization step, the pH of the aqueous solution containing zinc ions is preferably 6 to 12, more preferably 6 to 10, and even more preferably 7 to 9.

[0029] In order to perform neutralization, a neutralizing agent may be added to the aqueous solution containing zinc ions. The neutralizing agent is not particularly limited, and any one can be used. However, it is preferable to use one or both of calcium carbonate (CaCO ３ 3) and calcium hydroxide (Ca(OH) ２ 2). By using one or both of calcium carbonate and calcium hydroxide as the neutralizing agent, the pH can be gently increased. As a result, it is possible to prevent the pH from becoming excessively high locally. Furthermore, since calcium carbonate and calcium hydroxide are less expensive than NaOH, the production cost of the desulfurizing agent can be reduced.

[0030] [Solid-liquid separation step] Next, the precipitated zinc content is subjected to solid-liquid separation to obtain a zinc hydroxide-containing dehydrated cake. The method for performing solid-liquid separation is not particularly limited. However, when manufacturing a desulfurizing agent on an industrial scale, it is preferable to use a filter press.

[0031] [Drying step] Thereafter, the zinc hydroxide-containing dehydrated cake is dried to obtain a zinc hydroxide-containing solid. At that time, it is important to perform the drying at a drying temperature of 150°C or lower. The reason will be explained below.

[0032] In conventional zinc-based desulfurizing agents, it was common to form zinc oxide (ZnO) by heating at a high temperature during the manufacturing process. For example, in the invention described in Patent Document 1, Zn(OH)２ A sample containing a zinc compound such as is heated at 300 °C or higher to form ZnO crystals. This reaction is typically represented by the following formula (2). Zn(OH) ２ → ZnO + H ２ O … (2)

[0033] Although the zinc oxide obtained in this way has excellent desulfurization performance in the high-temperature range, it has poor desulfurization performance near room temperature.

[0034] On the other hand, as described above, the inventors of the present invention have found that zinc hydroxide exhibits high desulfurization performance even near room temperature. The reaction when hydrogen sulfide is brought into contact with zinc hydroxide is represented by the following formula (3). Zn(OH) ２ + H ２ S → ZnS + 2H ２ O … (3)

[0035] Hydrogen sulfide H ２ S is adsorbed on the surface of zinc hydroxide by interaction with the OH groups on the surface of zinc hydroxide, and finally converted into zinc sulfide and water by the reaction of the above formula (3). Therefore, unlike ZnO, zinc hydroxide can maintain high hydrogen sulfide removal performance even at room temperature.

[0036] However, zinc hydroxide is easily decomposed by heating. That is, when zinc hydroxide is heated above the decomposition temperature, it changes into zinc oxide (ZnO) by the reaction of the following formula (4). Zn(OH) ２ → ZnO + H ２ O … (4)

[0037] Therefore, in the drying step of the present invention, it is important to dry the zinc hydroxide-containing dewatered cake at a drying temperature of 150 °C or lower. Thereby, decomposition of zinc hydroxide can be prevented, and a zinc-based desulfurizer having excellent performance at room temperature can be obtained.

[0038] Generally, zinc hydroxide as a compound is considered to thermally decompose at 125 °C or higher. However, since moisture remains in the dewatered cake obtained by solid-liquid separation of the precipitated zinc component, the internal temperature of the dewatered cake is lower than the drying temperature (atmospheric temperature). Therefore, in the present invention, the drying temperature may be 150 °C or lower.

[0039] In one embodiment of the present invention, from the viewpoint of further accelerating drying, the drying temperature may be set to 125°C or higher, which is the decomposition temperature of zinc hydroxide. That is, the drying temperature may be 125 to 150°C.

[0040] On the other hand, from the viewpoint of further suppressing the decomposition of zinc hydroxide, the drying temperature in the drying step is preferably 120°C or lower, more preferably 100°C or lower, even more preferably 80°C or lower, and most preferably 60°C or lower. On the other hand, the lower limit of the drying temperature is not particularly limited, but if it is too low, drying will take a long time, so it is preferably 35°C or higher, more preferably 40°C or higher, and even more preferably 45°C or higher. In one embodiment of the present invention, the drying temperature is preferably 120 to 35°C, more preferably 100 to 40°C, even more preferably 80 to 40°C, and most preferably 60 to 45°C.

[0041] The drying time is not particularly limited and can be appropriately determined depending on the amount of zinc hydroxide-containing dehydrated cake to be dried and the drying temperature. For example, under typical conditions, from the viewpoint of sufficiently removing moisture, it is preferable to dry for 5 hours or more, more preferably 10 hours or more, and even more preferably 15 hours or more. On the other hand, if the drying time is excessively long, the moisture inside the dehydrated cake will be depleted, and as a result the temperature inside the dehydrated cake will rise, decomposition of zinc hydroxide may occur. Therefore, depending on the drying temperature, it is preferable to dry for, for example, 100 hours or less, more preferably 50 hours or less, and even more preferably 30 hours or less. It is also preferable to measure the weight of the zinc hydroxide-containing dehydrated cake and dry it until there is no change in weight. In one embodiment of the present invention, the drying time is preferably 5 to 100 hours, more preferably 10 to 50 hours, and even more preferably 15 to 30 hours.

[0042] [Coagulation and Sedimentation Process] In another embodiment of the present invention, prior to the solid-liquid separation process, the zinc precipitated in the neutralization process can be coagulated and precipitated, and the supernatant liquid can be removed to obtain a zinc hydroxide-containing slurry (coagulation and sedimentation process). The effects of performing coagulation and sedimentation will be explained below.

[0043] To separate zinc components such as zinc hydroxide precipitated during the neutralization process into solid and liquid phases, gravity sedimentation is generally considered. If coagulation sedimentation is not performed, the particles need to be enlarged by crystal growth in an aqueous solution in order to achieve gravity sedimentation. The zinc components obtained in this way are mainly crystalline and coarse particles, resulting in a small specific surface area.

[0044] On the other hand, when agglomeration and precipitation occur, most of the zinc hydroxide precipitates in an amorphous state before crystal growth can occur. The desulfurizing agent obtained in this way contains a large amount of zinc hydroxide, which has a large specific surface area, and therefore has even better desulfurizing performance.

[0045] The method for coagulation and precipitation is not particularly limited, but typically, a coagulant can be added to the aqueous solution after neutralization.

[0046] The aforementioned flocculant is not particularly limited and any such agent can be used, but it is preferable to use an anionic polymer. That is, the zinc hydroxide precipitated in the neutralization step exists as positively charged colloidal particles and is dispersed in the aqueous solution due to the repulsive force between the positive charges. Therefore, by adding an anionic polymer, the negative charge of the anionic polymer cancels out the electrostatic repulsive force acting between the colloidal particles, and flocculation and precipitation can be effectively achieved.

[0047] Furthermore, when manufacturing desulfurizing agents on an industrial scale, it is preferable to use a coagulation and sedimentation apparatus (thickener) to perform gravity precipitation.

[0048] [Zinc-based desulfurizing agent] By the above method, a zinc-based desulfurizing agent having excellent desulfurizing performance at room temperature can be produced. As described above, the desulfurizing agent obtained by this method contains zinc hydroxide. The zinc hydroxide content in the desulfurizing agent is not particularly limited, but it is preferably 60 to 100% by mass.

[0049] The remainder of the zinc-based desulfurizing agent can be any, without particular limitation. Typically, the remainder consists of metal oxides, metal salts, and SiO ２ It is at least one of the following. Examples of the metal oxide include at least one oxide selected from the group consisting of Mg, Al, Zn, Ca, Mn, Cu, Na, Fe, Co, Ni, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W. Examples of the metal salt include at least one salt selected from the group consisting of Mg, Al, Zn, Ca, Mn, Cu, Na, Fe, Co, Ni, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W. However, the remaining components are not limited to these and may include, for example, any components derived from the aqueous solution containing zinc ions used.

[0050] The specific surface area of ​​the above zinc-based desulfurizing agent is not particularly limited. However, from the viewpoint of further improving desulfurization performance, a specific surface area of ​​50 m² is desirable. ２ It is preferable that it be / g, and 80m ２ It is even more preferable that the amount is greater than or equal to the amount of H ２ This is because the number of adsorption points that S can come into contact with increases. On the other hand, there is no particular upper limit to the specific surface area, but it is typically 120 m. ２ It may be less than or equal to / g, and 100m ２ It may be less than or equal to / g. Here, the specific surface area refers to the BET specific surface area measured using nitrogen gas. In one embodiment of the present invention, the specific surface area is 50 to 120 m². ２ It is preferable that the value be / g, and 80 to 100m ２ It is more preferable that it be / g.

[0051] Next, the present invention will be described in more detail based on examples. The following examples illustrate preferred embodiments of the present invention, and the present invention is not limited in any way by these examples.

[0052] A zinc-based desulfurizing agent was manufactured using the following procedure, and its properties were evaluated.

[0053] (Neutralization Process) First, an aqueous solution containing zinc ions was neutralized to precipitate zinc. The aqueous solution used was wastewater generated in the electrogalvanizing process of a steel mill. The wastewater included both the wastewater from the plating bath generated in the electrogalvanizing process and the wastewater from washing the electrogalvanized steel sheet obtained in the plating process. Note that in Invention Examples 2 to 4 and Comparative Example 1, wastewater of the same composition collected from the same location was used. On the other hand, in Invention Example 1, wastewater collected from a different location was used. Therefore, the composition of the wastewater used in Invention Example 1 is slightly different from the composition of the wastewater used in the other examples.

[0054] By adding calcium hydroxide to the waste liquid so that the pH was between 7 and 9, Zn in the waste liquid was precipitated as zinc hydroxide.

[0055] (Coagulation and Sedimentation Process) Next, a polymer flocculant was added to the waste liquid to coagulate and precipitate the precipitated zinc hydroxide, forming a slurry. As the polymer flocculant, Kurita Water Industries Ltd.'s polymer flocculant "Cliffloc" was used.

[0056] (Solid-liquid separation process) The slurry was then compressed and dewatered using a filter press to perform solid-liquid separation and obtain a zinc hydroxide-containing dehydrated cake.

[0057] Furthermore, the state of Zn contained in the zinc hydroxide-containing dehydrated cake before drying was confirmed by X-ray diffraction measurement. As a result, in both the inventive example and the comparative example, no zinc oxide peak was detected, and only the zinc hydroxide peak was detected. Therefore, it can be concluded that all the Zn contained in the zinc hydroxide-containing dehydrated cake exists as zinc hydroxide.

[0058] (Drying process) The obtained zinc hydroxide-containing dehydrated cake was dried at the drying temperature shown in Table 1 to serve as a desulfurizing agent. In all cases, since there was no change in weight after 24 hours, it was determined that the material was sufficiently dried and the drying process was terminated.

[0059] Subsequently, the state of Zn contained in the obtained desulfurizing agent was confirmed by X-ray diffraction measurement. As a result, in the comparative example where the drying temperature was 200°C, peaks of both zinc oxide and zinc hydroxide were detected. On the other hand, in the inventive example where the drying temperature was 150°C or lower, no peak of zinc oxide was detected, and only a peak of zinc hydroxide was detected.

[0060] (Specific Surface Area) The specific surface area of ​​the obtained desulfurizing agent was measured by nitrogen adsorption / desorption spectroscopy. Specifically, first, the agent was degassed under vacuum at room temperature for 12 hours before measurement. Then, adsorption / desorption spectroscopy was performed using nitrogen gas to obtain an adsorption isotherm. The temperature during measurement was -196°C. Next, the surface area per unit weight (specific surface area) was calculated by Brunauer-Emmett-Teller (BET) analysis of the obtained adsorption isotherm. The measurement results are shown in Table 1.

[0061] (Adsorption Capacity) Next, the amount of sulfur adsorbed per unit weight of the obtained desulfurizing agent (adsorption capacity) was evaluated by the following method. First, the obtained desulfurizing agent was sieved to obtain a sample with a particle size of 0.5 to 1.0 mm. Next, the sample was placed in a reaction tube with an inner diameter of 4 mm for 1 cm. ３ The reaction tube was filled, and its temperature was controlled to 30°C using a constant temperature bath. N1 was introduced from the inlet of the reaction tube. ２ 7000 ppm H diluted ２ S is flowed through the reaction tube, and the gas coming out of the outlet is collected every 15 minutes. The H in the gas is then analyzed using a gas chromatograph. ２ The S concentration was measured.

[0062] For a while after the start of the test, no hydrogen sulfide was detected in the outlet gas because all of it was adsorbed within the reaction tube. However, after a certain amount of time, the concentration of hydrogen sulfide in the outlet gas increased rapidly. This is because the desulfurizing agent in the reaction tube could no longer adsorb all of the hydrogen sulfide. Therefore, the mass (g) of sulfur atoms adsorbed on the desulfurizing agent was calculated from the amount of gas that flowed through the reaction tube until hydrogen sulfide began to be detected in the outlet gas. Then, the adsorption capacity (wt%) of the desulfurizing agent was determined from the mass of sulfur atoms. Here, the adsorption capacity is the percentage of the mass (g) of adsorbed sulfur atoms relative to the initial weight (g) of the desulfurizing agent. The measurement results are shown in Table 1.

[0063] As can be seen from the results shown in Table 1, the zinc-based desulfurizing agent produced by the method of the present invention showed excellent desulfurization performance even at near room temperature. In contrast, the zinc-based desulfurizing agent obtained in the comparative example had significantly inferior adsorption capacity at near room temperature compared to the inventive example.

[0064]

Claims

1. A method for producing a zinc-based desulfurizing agent, comprising: a neutralization step of neutralizing an aqueous solution containing zinc ions to precipitate zinc; a solid-liquid separation step of separating the precipitated zinc from its solid-liquid state to obtain a zinc hydroxide-containing dehydrated cake; and a drying step of drying the zinc hydroxide-containing dehydrated cake at a drying temperature of 150°C or lower to obtain a zinc hydroxide-containing solid.

2. The method for producing a zinc-based desulfurizing agent according to claim 1, further comprising a coagulation and sedimentation step prior to the solid-liquid separation step, in which the zinc precipitated in the neutralization step is coagulated and sedimented, and the supernatant liquid is removed to obtain a zinc hydroxide-containing slurry.

3. A method for producing a zinc-based desulfurizing agent according to claim 1 or 2, wherein at least a portion of the aqueous solution containing zinc ions is used, which is a zinc-containing waste liquid.

4. The method for producing a zinc-based desulfurizing agent according to claim 3, wherein the zinc-containing waste liquid is waste liquid generated in a zinc-based plating process.

5. The method for producing a zinc-based desulfurizing agent according to claim 4, wherein the waste liquid generated in the zinc-based plating process includes either or both of the waste liquid from the plating bath used in the electro-zinc plating process and the cleaning waste liquid after the electro-zinc plating process.

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