Method for preparing magnesium hydroxide and byproduct calcium sulfate by desulfurizing sulfur-containing flue gas by using magnesite slurry prepared from water

Abstract

The invention relates to the technical field of sulfur-containing flue gas treatment, in particular to a method for preparing magnesium hydroxide and byproduct calcium sulfate by desulfurizing sulfur-containing flue gas by using water-prepared magnesite slurry, which comprises the following steps: calcining magnesite, preparing magnesite slurry by using water, removing sulfur in the sulfur-containing flue gas, and producing calcium sulfate and magnesium hydroxide products. Pretreatment of an ammonium nitrate solution is avoided, the ammonium nitrate pretreatment cost is reduced, calcium sulfate residues poor in quality are removed in a segmented mode, calcium sulfate products high in quality are recycled, the additional value of the technology is increased, and meanwhile the technological process is more stable by combining limitation of technological parameters and technological steps. The purity of the magnesium hydroxide product reaches 98.5% or above, the sulfur-containing flue gas desulfurization additional value is increased, and the sulfur-containing flue gas desulfurization cost is reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of sulfur-containing flue gas treatment, and particularly to a method for desulfurizing sulfur-containing flue gas by using water-made magnesite slurry to prepare magnesium hydroxide as a by-product of calcium sulfate. Background Art

[0002] Sulfur-containing flue gas refers to the flue gas emitted by industries such as coal-fired power plants, smelters, and chemical production. Directly discharging sulfur-containing flue gas into the environment will not only cause serious pollution to the environment, but also lead to a large amount of sulfur resource waste. Therefore, it is necessary to desulfurize sulfur-containing flue gas before discharging the flue gas.

[0003] Currently, the main technology for removing sulfur from sulfur-containing flue gas is to use the limestone-gypsum method. This method can remove sulfur dioxide in sulfur-containing flue gas to a certain extent. After sulfur dioxide reacts with calcium ions to form calcium sulfite, it is oxidized to calcium sulfate and discharged. However, the output of the calcium sulfate product is relatively large and the quality is relatively low, resulting in an increase in the desulfurization cost of sulfur-containing flue gas and causing secondary pollution. Therefore, researching new sulfur removal technologies for sulfur-containing flue gas has become an urgent technical problem for those skilled in the art to solve.

[0004] Based on this, the research members of this research team obtained a method for preparing magnesium hydroxide and calcium sulfate products by using the desulfurization process of sulfur-containing flue gas as a medium, so that when desulfurizing sulfur-containing flue gas, chemical products with higher added value can be produced, and a patent application with the application number 202411417544.8 was filed for the method of efficiently desulfurizing coal-fired flue gas by using dolomite to produce high-quality calcium sulfate and magnesium hydroxide. However, in this method, dolomite powder is calcined and then added to ammonium nitrate solution for pretreatment to remove most of the calcium elements, and then water is used to make a slurry as the flue gas for desulfurizing sulfur-containing flue gas, resulting in a relatively high treatment cost and the quality of the obtained magnesium hydroxide being not ideal, leading to a low added value of magnesium hydroxide.

[0005] In view of this, the research team of this research continues to research the idea of preparing chemical products with high added value during the desulfurization process of sulfur-containing flue gas, introduces magnesite as a raw material, and improves the process of using the desulfurization process of sulfur-containing flue gas in the existing technology as a medium to produce chemical products such as magnesium hydroxide and calcium sulfate, so as to improve the quality of magnesium hydroxide products and reduce the desulfurization cost of sulfur-containing flue gas, and thus provides a new idea for preparing chemical products for sulfur-containing flue gas desulfurization. Summary of the Invention

[0006] Based on the above technical problems, the present invention provides a method for desulfurizing sulfur-containing flue gas by using water-made magnesite slurry to prepare magnesium hydroxide as a by-product of calcium sulfate.

[0007] The specific technical solution is as follows:

[0008] A method for desulfurizing sulfur-containing flue gas by using water-made magnesite slurry to prepare magnesium hydroxide as a by-product of calcium sulfate, comprising the following steps:

[0009] (1) Calcined magnesite powder is calcined to prepare calcined powder, and water is added to the calcined powder to prepare magnesite slurry.

[0010] (2) The magnesite slurry is pumped into a desulfurization tower, and the sulfur-containing flue gas is treated at 100 - 200 °C. After solid-liquid separation, calcium sulfate slag and magnesium sulfate-containing solution are obtained; the gas-liquid mass ratio of the magnesite slurry to the sulfur-containing flue gas is 3 - 10 L / m 3 ; the contact reaction time of the magnesite slurry with the sulfur-containing flue gas is 10 - 30 min;

[0011] (3) Calcium nitrate solution is added to the magnesium sulfate-containing solution, and after filtration, the filter residue is washed and dried to obtain calcium sulfate product; the filtrate is a solution containing magnesium nitrate; before adding the calcium nitrate solution, the magnesium sulfate-containing solution is heated to 60 - 70 °C, and after the calcium nitrate solution is added, the temperature is raised to 70 - 98 °C for reaction for 60 - 90 min; the addition amount of the calcium nitrate solution is 1.1 times the theoretical mass of magnesium sulfate in the magnesium sulfate-containing solution; the concentration of the calcium nitrate solution is 8 - 12% in terms of calcium oxide;

[0012] (4) Barium nitrate solution is added to the solution containing magnesium nitrate and reacted for 30 - 60 min. After filtration, the filter residue is barium sulfate, and the filtrate is a purified magnesium nitrate solution; the addition amount of the barium nitrate solution is 0.9 - 1.1 times the theoretical mass of barium sulfate produced, and the barium nitrate solution is a solution saturated by dissolving barium nitrate at 80 - 95 °C;

[0013] (5) Ammonia water is added to the purified magnesium nitrate solution, and after filtration, the filter residue is washed and dried to obtain magnesium hydroxide; the filtrate is a solution containing ammonium nitrate. Before adding ammonia water to the purified magnesium nitrate solution, the temperature of the purified magnesium nitrate solution is adjusted to 50 - 80 °C, and the reaction time after adding ammonia water is 1 h. The addition amount of ammonia water is 2 - 3.5 times the theoretical mass of magnesium hydroxide.

[0014] After calcining magnesite and preparing it into magnesite slurry with water, it is used to remove sulfur from sulfur-containing flue gas and produce calcium sulfate and magnesium hydroxide products. This not only avoids the pretreatment with ammonium nitrate solution and reduces the cost of ammonium nitrate pretreatment, but also realizes the staged removal of calcium sulfate slag with poor quality and the recovery of calcium sulfate products with high quality, increasing the added value of the process. At the same time, with the limitation of process parameters and process steps, the purity of the magnesium hydroxide product reaches more than 98.5%, improving the desulfurization added value of sulfur-containing flue gas and reducing the desulfurization cost of sulfur-containing flue gas.

[0015] In order to achieve recycling, reduce the discharge of waste liquid and waste gas, and reduce the raw material cost, preferably, lime is added to the ammonium nitrate-containing solution obtained in step (5) to obtain a calcium nitrate solution and an escaping gas; the calcium nitrate solution is returned to step (3) for recycling, and the escaping gas is absorbed by water and then returned to step (5). This enables the addition of ammonia water in a combined manner of subsequent recycling and supplementation after the first addition of ammonia water, reducing the usage amount of ammonia water; at the same time, it can also achieve the recycling of nitrate ions and reduce the usage cost of calcium nitrate raw materials.

[0016] In order to enable the magnesite to be calcined and decomposed sufficiently, preferably, the calcination temperature is 800 - 1200 °C, and the calcination time is 1 - 3 h. More preferably, the calcination temperature is 900 °C, and the calcination time is 2 h.

[0017] In order to improve the desulfurization effect and ensure the quality of the magnesium hydroxide product, preferably, the sulfur dioxide content in the sulfur-containing flue gas is 2000 - 15000 mg / m 3 ; and the magnesite slurry has a mass concentration of 8 - 12% calculated as magnesium oxide. More preferably, the sulfur dioxide content in the sulfur-containing flue gas is 7000 - 10000 mg / m 3 ; and the magnesite slurry has a mass concentration of 8 - 12% calculated as magnesium oxide. Further preferably, the sulfur dioxide content in the sulfur-containing flue gas is 7000 - 10000 mg / m 3 ; and the magnesite slurry has a mass concentration of 10% calculated as magnesium oxide. Still further preferably, the sulfur dioxide content in the sulfur-containing flue gas is 7000 mg / m 3 ; and the magnesite slurry has a mass concentration of 10% calculated as magnesium oxide.

[0018] Preferably, step (2) is to treat the sulfur-containing flue gas at 150 °C.

[0019] In order to enable calcium sulfite to be rapidly oxidized and converted into calcium sulfate, preferably, in step (2), 0.5 - 1% of hydrogen peroxide based on the mass of the magnesite slurry is also added when treating the sulfur-containing flue gas with the magnesite slurry.

[0020] Compared with the prior art, the technical effects of the present invention are reflected in:

[0021] The present invention prepares magnesite slurry by calcining magnesite and adding water, and then desulfurizes sulfur-containing flue gas with a specific sulfur dioxide content at a specific concentration to prepare calcium sulfate and magnesium hydroxide products, avoiding the step of pre-treating with ammonium nitrate solution, shortening the process flow, and improving the desulfurization efficiency.

[0022] The present invention utilizes the magnesite calcined and then added with water to prepare a slurry as a desulfurizer to treat sulfur-containing flue gas, realizing the production of relatively high-purity magnesium hydroxide and calcium sulfate products by taking the desulfurization process as a medium, improving the process added value of the sulfur-containing flue gas desulfurization process, and reducing the cost of sulfur-containing flue gas desulfurization. It is known through experiments that the present invention can make the purity of the magnesium hydroxide product reach more than 98.5%, even up to 99.24%, and the whiteness reach more than 97%; moreover, it can also make the purity of the calcium sulfate product reach more than 98% and the whiteness reach more than 97%.

[0023] The technological process of the present invention is simple, easy to operate and control, and suitable for industrialized popularization and application. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to enable those skilled in the art to fully understand the technical solution of the present invention, the following description is made on the provided drawings in combination with the content of the technical solution and the provided drawings.

[0025] Figure 1 is the technological process flow chart of the present invention.

[0026] Figure 2 is Figure 1 the technological process flow chart of another embodiment. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0027] In order to facilitate those skilled in the art to correctly understand the present invention and enable those skilled in the art to fully understand the technical content of the present invention, the following further elaborates on the technical solution of the present invention in combination with specific embodiments and drawings, but this elaboration does not limit the scope of protection required by the present invention. Those skilled in the art's protection scope of the present invention cannot be limited only to the following elaboration content. Any equivalent substitution or change made by those skilled in the art or those familiar with the art in the art based on the technical solution of the present invention and its inventive concept should be covered within the protection scope of the present invention.

[0028] In some embodiments, as Figure 1 shown, the method for desulfurizing sulfur-containing flue gas with water-made magnesite slurry to prepare magnesium hydroxide as a by-product of calcium sulfate includes the following steps:

[0029] (1) Calcinate magnesite powder to obtain calcined powder, and add water to the calcined powder to prepare magnesite slurry; the calcination temperature is 800 - 1200 °C, and the calcination time is 1 - 3 h. For example: calcine at 800 °C for 1 h, 900 °C for 1 h, 1000 °C for 1 h, 1100 °C for 1 h, 1200 °C for 1 h, 800 °C for 2 h, 900 °C for 2 h, 1000 °C for 2 h, 1100 °C for 2 h, 1200 °C for 2 h, 800 °C for 3 h, 900 °C for 3 h, 1000 °C for 3 h, 1100 °C for 3 h, or 1200 °C for 3 h. In the specific implementation of the present invention, calcination is carried out at 900 °C for 2 h.

[0030] (2) Pump the magnesite slurry into the desulfurization tower, and treat the sulfur-containing flue gas at 100 - 200 °C, for example: 100 °C, 110 °C, 140 °C, 150 °C, 180 °C, or 200 °C, etc. Perform solid-liquid separation to obtain calcium sulfate slag and magnesium sulfate-containing solution; the gas-liquid mass ratio of the magnesite slurry to the sulfur-containing flue gas is 3 - 10 L / m 3 , for example: 3 L / m 3 , 4 L / m 3 , 5 L / m 3 , 6 L / m 3 , 7 L / m 3 , 8 L / m 3 , 9 L / m 3 or 10 L / m 3 ; the contact reaction time between the magnesite slurry and the sulfur-containing flue gas is 10 - 30 min, for example: 10 min, 15 min, 20 min, 25 min, or 30 min, etc.;

[0031] (3) Add calcium nitrate solution to the magnesium sulfate-containing solution, filter, and after washing and drying the filter residue, obtain calcium sulfate product; the filtrate is a solution containing magnesium nitrate; before adding the calcium nitrate solution, heat the magnesium sulfate-containing solution to 60 - 70 °C, for example: 60 °C, 65 °C, 70 °C, etc. After adding the calcium nitrate solution, raise the temperature to 70 - 98 °C, for example: 70 °C, 80 °C, 90 °C, or 98 °C, etc. and react for 60 - 90 min, for example: 60 min, 70 min, 80 min, 90 min, etc.; the addition amount of the calcium nitrate solution is 1.1 times the theoretical mass of magnesium sulfate in the magnesium sulfate-containing solution; the concentration of the calcium nitrate solution is 8 - 12% in terms of calcium oxide, for example: 8%, 9%, 10%, 11%, 12%, etc.;

[0032] (4) Add barium nitrate solution to the solution containing magnesium nitrate and react for 30 - 60 min, for example: 30 min, 40 min, 50 min, 60 min, etc. Filter, the filter residue is barium sulfate, and the filtrate is the purified magnesium nitrate solution; the addition amount of the barium nitrate solution is 0.9 - 1.1 times the theoretical mass of barium sulfate produced (accounting for 0.08 - 0.14% of the mass of the solution containing magnesium nitrate), for example: 0.9 times, 1 time, 1.1 times, etc. The barium nitrate solution is the solution when barium nitrate is dissolved to saturation at 80 - 95 °C; for example, barium nitrate is dissolved to saturation with water at 80 °C, barium nitrate is dissolved to saturation with water at 85 °C, barium nitrate is dissolved to saturation with water at 90 °C, or barium nitrate is dissolved to saturation with water at 85 °C, etc.

[0033] (5) Add ammonia water to the purified magnesium nitrate solution, filter, wash and dry the filter residue to obtain magnesium hydroxide; the filtrate is the solution containing ammonium nitrate. Before adding ammonia water to the purified magnesium nitrate solution, adjust the temperature of the purified magnesium nitrate solution to 50 - 80 °C, for example: 50 °C, 60 °C, 70 °C, 80 °C, etc. The reaction time after adding ammonia water is 1 h, and the addition amount of the ammonia water is 2 - 3.5 times the theoretical mass of magnesium hydroxide, for example 2 times, 2.3 times, 2.5 times, 2.9 times, 3 times, or 3.5 times.

[0034] As Figure 2 shown, in some embodiments, add lime to the solution containing ammonium nitrate obtained in step (5) to obtain a calcium nitrate solution and an escaping gas; the calcium nitrate solution returns to step (3) for recycling, and the escaping gas is absorbed by water and then returns to step (5). It is possible to add ammonia water through a combination of subsequent recycling and supplementation, reducing the usage amount of ammonia water; at the same time, it is also possible to achieve the recycling of nitrate ions and reduce the usage cost of calcium nitrate raw materials.

[0035] In some embodiments, the sulfur dioxide content in the sulfur-containing flue gas is 2000 - 15000 mg / m 3 ; and the mass concentration of the magnesite slurry in terms of magnesium oxide is 8 - 12%. The purity of the magnesium hydroxide product reaches 98.14 - 99.24%, and the desulfurization efficiency reaches 95.2 - 99.1%.

[0036] In some embodiments, the sulfur dioxide content in the sulfur-containing flue gas is 7000 - 10000 mg / m 3 ; and the mass concentration of the magnesite slurry in terms of magnesium oxide is 8 - 12%. The purity of the magnesium hydroxide product reaches 98.31 - 99.24%, and the desulfurization efficiency reaches 96.2 - 99.1%.

[0037] In some embodiments, the sulfur dioxide content in the sulfur-containing flue gas is 7000 - 10000 mg / m 3The mass concentration of the magnesite slurry is 10% based on magnesium oxide, so that the purity of the magnesium hydroxide product reaches more than 99%, and the desulfurization efficiency reaches more than 98.4%.

[0038] In some embodiments, the sulfur dioxide content in the sulfur-containing flue gas is 7000 mg / m 3 The mass concentration of the magnesite slurry is 10% based on magnesium oxide, so that the purity of the magnesium hydroxide product reaches more than 99.24% and the desulfurization efficiency reaches 98.4%.

[0039] In some embodiments, in step (2), when treating sulfur-containing flue gas with magnesite slurry, hydrogen peroxide accounting for 0.5-1% by weight of the magnesite slurry, for example, 0.5%, 0.8%, 0.9% or 1%, etc. is added. Adding hydrogen peroxide only increases the rate and efficiency of converting sulfite into sulfate, and has little effect on the quality of the product.

[0040] In order to better verify the technical effects that the invention can bring, the research team of the invention carried out the following experimental research:

[0041] Experiment 1: Study on the effect of different magnesite slurries and sulfur-containing flue gas with different sulfur dioxide concentrations at different temperatures on the quality of magnesium hydroxide and calcium sulfate

[0042] The magnesite powder was calcined at 900°C for 2 hours to obtain calcined powder; water was added to the calcined powder according to the mass concentration of magnesium oxide to prepare a magnesite slurry of a certain concentration; the magnesite slurry was added to the sulfur-containing flue gas at a gas-liquid mass ratio of 5L / m 3 The product was pumped to a desulfurization tower, and the sulfur-containing flue gas was treated at a reaction temperature for 30 minutes. The solid-liquid separation was performed to obtain calcium sulfate residue and a magnesium sulfate solution. The calcium sulfate residue was discharged as waste residue. A calcium nitrate solution (mass concentration of 10%) accounting for 1.1 times the theoretical mass of magnesium sulfate in the magnesium sulfate solution was added to the magnesium sulfate solution at a temperature of 65°C. The temperature was adjusted to 80°C for reaction for 30 minutes. The product was filtered to obtain a filter residue and a filtrate containing magnesium nitrate. The filter residue was washed with water for 3 times and dried to a constant weight to obtain a calcium sulfate product. The product was heated to a temperature between 60 and 80°C. A barium nitrate solution is added to the filtrate containing magnesium nitrate at 0°C (barium nitrate is dissolved in water at 85°C to reach saturation), the amount of the barium nitrate solution added is 1 times the theoretical mass of barium sulfate produced (accounting for 0.1% of the mass of the filtrate containing magnesium nitrate), and after reacting for 30 minutes, the solution is filtered to obtain barium sulfate and a purified magnesium nitrate solution; ammonia water is added to the purified magnesium nitrate solution at a temperature of 70°C for 1 hour, the amount of ammonia water added is 3 times the theoretical mass of magnesium hydroxide, and the solution is filtered to obtain a solid and a filtrate containing ammonium nitrate; the solid is washed three times with water and dried to obtain magnesium hydroxide.

[0043] Referring to the operation steps of the above embodiments, the mass concentration (A) of magnesite slurry, the sulfur dioxide content (B) in sulfur-containing flue gas, and the reaction temperature (C) are adjusted as follows to prepare magnesium hydroxide products and by-product calcium sulfate products by desulfurizing sulfur-containing flue gas, and the purity, whiteness (D) of magnesium hydroxide products, the purity and whiteness (E) of calcium sulfate products, and the desulfurization efficiency (F) of sulfur-containing flue gas are detected and calculated (the average value is taken three times), and the results are shown in Table 1 below:

[0044] Table 1

[0045]

[0046]

[0047] Note: In this experiment, the sulfur dioxide concentration in sulfur-containing flue gas was adjusted by diluting with nitrogen or supplementing the concentration by injecting sulfur dioxide. In this experiment, the desulfurization efficiency was the mass percentage of the mass difference of sulfur dioxide contained in the gas at the inlet and outlet of the desulfurization tower to the sulfur dioxide content in the gas at the inlet of the desulfurization tower.

[0048] It can be seen from Table 1 that: ① The present invention can achieve a sulfur removal efficiency of more than 95% for sulfur in sulfur-containing flue gas, reduce the sulfur dioxide emission in sulfur-containing flue gas, and reduce the ability to pollute the environment; at the same time, during the desulfurization treatment of sulfur-containing flue gas, magnesium hydroxide products with a purity of more than 98.5% and a whiteness of more than 97% can be obtained, and calcium sulfate products with a purity of more than 98% and a whiteness of more than 97% can also be obtained, greatly reducing the desulfurization cost of sulfur-containing flue gas, increasing the process added value, and reducing the desulfurization cost of sulfur-containing flue gas. ② The quality of magnesium hydroxide products and calcium sulfate products obtained by the process of the present invention is closely related to the magnesite slurry concentration, the sulfur dioxide content in sulfur-containing flue gas, and the reaction temperature of magnesite slurry and sulfur-containing flue gas. When the magnesite slurry concentration is 8%-12%, and the sulfur dioxide concentration in sulfur-containing flue gas is 2000-15000 mg / m 3 , and the reaction is carried out at 100-200 °C, the quality of magnesium hydroxide products and calcium sulfate products can be better. ③ The present invention preferably selects a magnesite slurry concentration of 10%, and the sulfur dioxide concentration in sulfur-containing flue gas is 7000-10000 mg / m 3 , and reacting at 150 °C can obtain better magnesium hydroxide products, making its purity reach 99.24%.

[0049] Experiment 2: Study on the influence of different temperatures of magnesium sulfate solution on the quality of calcium sulfate products

[0050] On the basis of Example 8, the magnesium sulfate-containing solution before adding the calcium nitrate solution was temperature-adjusted as shown in Table 2 below, and the other operations were the same as those in Example 8. The purity and whiteness of the obtained calcium sulfate product were detected (the average value was taken three times), and the results are shown in Table 2 below.

[0051] Table 2

[0052]

[0053] As can be seen from Table 1 and Table 2: Controlling the temperature of the magnesium sulfate-containing solution at 60-70°C and then adding the calcium nitrate solution to remove sulfate ions will help improve the purity and whiteness of the calcium sulfate product and enhance the quality of the calcium sulfate product.

[0054] Experiment 3: Research on the influence of the process of recycling the ammonium nitrate solution discharged from the process on the quality of magnesium hydroxide and calcium sulfate

[0055] On the basis of Example 8, the filtrate containing ammonium nitrate produced in Example 8 was mixed with quicklime, and the ammonia gas generated by the mixture was absorbed by water and returned to the magnesium precipitation step for magnesium precipitation treatment. The calcium nitrate solution generated by the mixture was returned to the sulfate removal step to remove sulfate ions. The other steps and parameters were operated according to Example 8. The purity and whiteness of the calcium sulfate product and magnesium hydroxide product obtained after recycling three times respectively were detected (the average value was taken three times), and the results are shown in Table 3 below.

[0056] Table 3

[0057]

[0058]

[0059] As can be seen from Table 1 and Table 3, the ammonium nitrate solution generated in the process steps can be recycled after reacting with quicklime, and it can ensure the quality of the obtained calcium sulfate and magnesium hydroxide products, which helps to reduce the discharge of waste liquid and reduce the discharge of the three wastes in the process. Of course, the ammonium nitrate solution generated in the present invention can also be sent to the production of ammonium nitrate fertilizers.

[0060] Experiment 4: Research on the influence of adding hydrogen peroxide on the desulfurization efficiency of sulfur-containing flue gas

[0061] On the basis of Experiment 8, when the magnesite slurry was pumped into the desulfurization tower, hydrogen peroxide was also added at 0.5%, 0.8%, and 1% of the mass of the magnesite slurry, and the others were the same as those in Example 8. The purity of magnesium hydroxide and calcium sulfate after adding hydrogen peroxide was detected (the average value was taken three times), and the results are shown in Table 4 below:

[0062] Table 4

[0063]

[0064] As can be seen from Table 1 and Table 4, adding hydrogen peroxide can help improve the purity of calcium sulfate products and magnesium hydroxide purity.

[0065] Experiment Five: Influence Study on the Preparation of Purified Magnesium Nitrate Solution by Adding Barium Nitrate Solution

[0066] On the basis of Example 8, add the amount of barium nitrate solution (percentage by mass of the filtrate containing magnesium nitrate) according to the amounts in Table 5 below. Other conditions are the same as in Example 8, and detect the purity and whiteness of the obtained magnesium hydroxide product (take the average value three times). The results are shown in Table 5 below.

[0067] Table 5

[0068]

[0069] As can be seen from Table 1 and Table 5, adding barium nitrate solution to deeply purify the solution containing magnesium nitrate to prepare purified magnesium nitrate solution will help improve the quality of magnesium hydroxide products, increase the purity and whiteness of magnesium hydroxide products. However, as the amount of barium nitrate solution added increases, when it reaches more than 0.12% of the mass of the solution containing magnesium nitrate, the change in the quality of magnesium hydroxide products is no longer significant.

[0070] For other matters not covered in this invention, reference can be made to the prior art or common general knowledge well-known to those skilled in the art, and they can be realized by conventional technical means. For example, the drying of magnesium hydroxide, the drying of calcium sulfate, etc. can all be carried out with reference to the drying treatment steps in the prior art.

Claims

1. A method for preparing magnesium hydroxide and producing calcium sulfate as a by-product by desulfurizing sulfur-containing flue gas using water-made magnesite slurry, characterized in that: The following steps are involved: (1) calcining magnesite powder to prepare calcined powder, and adding water to the calcined powder to prepare magnesite slurry; (2) Pumping the magnesite slurry into the desulfurization tower, treating the sulfur-containing flue gas at 100-200° C., separating the solid and the liquid, and obtaining calcium sulfate slag and a magnesium sulfate solution; the gas-liquid mass ratio of the magnesite slurry to the sulfur-containing flue gas is 3-10 L / m 3 ; The contact reaction time between the magnesite slurry and the sulfur-containing flue gas is 10-30 minutes; (3) adding a calcium nitrate solution to a magnesium sulfate solution, filtering, washing and drying the filter residue to obtain a calcium sulfate product; the filtrate is a solution containing magnesium nitrate; before adding the calcium nitrate solution, the magnesium sulfate solution is heated to 60-70° C., and after the calcium nitrate solution is added, the temperature is raised to 70-98° C. and reacted for 60-90 minutes; the amount of the calcium nitrate solution added is 1.1 times the theoretical mass of magnesium sulfate in the magnesium sulfate solution; the concentration of the calcium nitrate solution is 8-12% in terms of calcium oxide; (4) adding a barium nitrate solution to a solution containing magnesium nitrate, filtering, the filter residue is barium sulfate, and the filtrate is a purified magnesium nitrate solution; the amount of the barium nitrate solution added is 0.9-1.1 times the theoretical mass of barium sulfate produced, and the barium nitrate solution is a solution obtained by dissolving barium nitrate at 80-95° C. to reach saturation; (5) adding ammonia water to the purified magnesium nitrate solution, filtering, washing and drying the filter residue to obtain magnesium hydroxide; the filtrate is a solution containing ammonium nitrate. Before adding ammonia water to the purified magnesium nitrate solution, the temperature of the purified magnesium nitrate solution is adjusted to 50-80° C., the reaction time after adding ammonia water is 1 hour, and the amount of ammonia water added is 2-3.5 times the theoretical mass of magnesium hydroxide.

2. The method for preparing magnesium hydroxide and producing calcium sulfate as a by-product by desulfurizing sulfur-containing flue gas using water-made magnesite slurry as claimed in claim 1, characterized in that: Lime is added to the ammonium nitrate solution obtained in step (5) to obtain a calcium nitrate solution and escaping gas; the calcium nitrate solution is returned to step (3) for circulation, and the escaping gas is absorbed by water and then returned to step (5).

3. The method for preparing magnesium hydroxide and producing calcium sulfate as a by-product by desulfurizing sulfur-containing flue gas using water-made magnesite slurry as claimed in claim 1, characterized in that: The calcination temperature is 800-1200° C., and the calcination time is 1-3 hours.

4. The method for preparing magnesium hydroxide and producing calcium sulfate as a by-product by desulfurizing sulfur-containing flue gas using water-made magnesite slurry as claimed in claim 1 or 3, characterized in that: The calcination temperature is 900° C. and the calcination time is 2 hours.

5. The method for preparing magnesium hydroxide and producing calcium sulfate as a by-product by desulfurizing sulfur-containing flue gas using water-made magnesite slurry as claimed in claim 1, characterized in that: The sulfur dioxide content in the sulfur-containing flue gas is 2000-15000 mg / m 3 ; and the mass concentration of the magnesite slurry is 8-12% based on magnesium oxide.

6. The method for preparing magnesium hydroxide and producing calcium sulfate as a by-product by desulfurizing sulfur-containing flue gas using water-made magnesite slurry as claimed in claim 1 or 5, characterized in that: The sulfur dioxide content in the sulfur-containing flue gas is 7000-10000 mg / m 3 ; and the mass concentration of the magnesite slurry is 8-12% based on magnesium oxide.

7. The method for preparing magnesium hydroxide and producing calcium sulfate as a by-product by desulfurizing sulfur-containing flue gas using water-made magnesite slurry as claimed in claim 1 or 5, characterized in that: The sulfur dioxide content in the sulfur-containing flue gas is 7000-10000 mg / m 3 ; and the mass concentration of the magnesite slurry is 10% based on magnesium oxide.

8. The method for preparing magnesium hydroxide and producing calcium sulfate as a by-product by desulfurizing sulfur-containing flue gas using water-made magnesite slurry as claimed in claim 1 or 5, characterized in that: The sulfur dioxide content in the sulfur-containing flue gas is 7000 mg / m 3 ; and the mass concentration of the magnesite slurry is 10% based on magnesium oxide.

9. The method for preparing magnesium hydroxide and producing calcium sulfate as a by-product by desulfurizing sulfur-containing flue gas using water-made magnesite slurry as claimed in claim 1, characterized in that: The step (2) is to treat the sulfur-containing flue gas at 150°C.

10. The method for preparing magnesium hydroxide and producing calcium sulfate as a by-product by desulfurizing sulfur-containing flue gas using water-made magnesite slurry as claimed in claim 1, characterized in that: In the step (2), when the sulfur-containing flue gas is treated with the magnesite slurry, 0.5-1% of the mass of the magnesite slurry of hydrogen peroxide is added.

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