CN113952909B - Method for preparing magnesium sulfate from waste sulfuric acid - Google Patents
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Abstract
A method for preparing magnesium sulfate from waste sulfuric acid comprises the steps of carrying out photocatalytic oxidation treatment and neutralization decolorization on the waste sulfuric acid to prepare magnesium sulfate; in the photocatalytic oxidation treatment of the waste sulfuric acid, the carbon point modified titanium dioxide microspheres are utilized to catalyze and degrade organic matters in the waste sulfuric acid under visible light; neutralizing and decolorizing to obtain magnesium sulfate, neutralizing sulfuric acid obtained after degrading organic matters with magnesium oxide, decolorizing with thiourea dioxide, and crystallizing to obtain magnesium sulfate crystal. The method uses visible light to catalyze, oxidize and degrade organic matters in the waste sulfuric acid, the cost is low, the efficiency is high, and COD amplitude reduction can reach 95.7-96.2% before and after waste acid treatment; the photocatalyst carbon point modified titanium dioxide microsphere can be reused; the magnesium sulfate crystal with the purity of 97.3-99.5% can be obtained through one-step neutralization and chemical reaction decoloration.
Description
Technical Field
The invention relates to a method for preparing magnesium sulfate from waste sulfuric acid, belonging to the field of fine chemical industry.
Background
Sulfuric acid is one of the most basic chemical raw materials, and is widely applied to a plurality of industries such as petrochemical industry, coal chemical industry, material chemical industry, metallurgy, pesticide fertilizer production, medical synthesis, dye synthesis and the like. In these application scenarios, the sulfuric acid no longer meets the use requirements due to impurity mixing, concentration reduction or other reasons, and then a large amount of waste sulfuric acid is generated. For example, sulfuric acid alkylation processes in the petrochemical industry produce dark viscous, irritating odors, waste sulfuric acid containing olefins, sulfate esters, alkyl sulfonic acids, and sulfide impurities. Further, in the production of cation exchange resins, sulfonation produces spent sulfuric acid containing a large amount of tiny carbides, which are difficult to physically separate and remove. How to treat the waste sulfuric acid with low cost is not only related to the economic benefit of the sulfuric acid using enterprises, but also has profound significance for promoting the social and economic development and environmental protection.
Chinese patent CN100537416C, CN204917971U discloses a method for recovering sulfuric acid by treating waste sulfuric acid by pyrolysis (temperature above 1000 ℃) and related equipment, although the recovery rate of sulfuric acid by this method can reach above 90%, the equipment has strict requirements on high temperature corrosion resistance, large investment, high running cost and high recovery cost, and is not bearable by most enterprises. The method for producing magnesium sulfate by utilizing colored waste sulfuric acid in the production of ion exchange resin is disclosed in China patent CN107473436A, and comprises the steps of adding a compound coagulant, flocculating and precipitating an oligomer and a low-molecular-weight polyphenyl sulfonic acid copolymer in the colored waste sulfuric acid, and then reacting with a magnesium-containing mineral to produce a colorless or white magnesium sulfate product, wherein COD (chemical oxygen demand) amplitude reduction in waste acid can reach 80%, but organic matters precipitated by the compound coagulant still exist as solid waste, and cannot cause secondary pollution.
Disclosure of Invention
Aiming at the defects of the prior art, the invention provides a method for preparing magnesium sulfate from waste sulfuric acid, which realizes the following aims: the waste sulfuric acid is treated with low cost, COD amplitude reduction before and after the waste acid treatment is improved, organic matters in the waste sulfuric acid are thoroughly degraded, and secondary pollution is avoided.
In order to achieve the aim of the invention, the invention adopts the following technical scheme:
a method for preparing magnesium sulfate from waste sulfuric acid, which is characterized in that: comprises two steps of waste sulfuric acid photocatalytic oxidation treatment and neutralization decoloration to prepare magnesium sulfate; in the photocatalytic oxidation treatment of the waste sulfuric acid, carbon-point-modified titanium dioxide microspheres are utilized to catalyze and degrade organic matters in the waste sulfuric acid under visible light; the neutralization and decoloration are carried out to obtain magnesium sulfate, sulfuric acid obtained after degrading organic matters is neutralized by magnesium oxide, and then is decolored by thiourea dioxide and crystallized to obtain magnesium sulfate crystals.
The following is a further improvement of the above technical scheme:
step (1) waste sulfuric acid photocatalytic oxidation treatment
Uniformly fixing carbon-point modified titanium dioxide microspheres on the bottom of a waste sulfuric acid insolation tank by using a 300-mesh glass fiber net, then introducing the waste sulfuric acid with solid particles filtered out into the insolation tank for photocatalytic oxidation treatment, controlling the depth of the waste sulfuric acid in the insolation tank to be 2-6 cm, and controlling the average illuminance to be 5 multiplied by 10 3 ~1×10 5 And exposing for 1-2 hours under lx condition to obtain the photocatalytic oxidation waste sulfuric acid.
The preparation method of the carbon dot modified titanium dioxide microsphere comprises the following steps:
adding 5-8% of triethanolamine and 10-15% of glycerol into 20-40% of oxalic acid aqueous solution at a stirring rate of 500-1200 rpm, stirring and mixing uniformly, and then adjusting pH=7-9 with 10-20% of ammonia water, and marking as A solution for later use; adding sodium fluosilicate accounting for 5-15% of the total mass of the titanyl sulfate aqueous solution into 30-40wt% of the titanyl sulfate aqueous solution, and marking as solution B after the solution is completely dissolved; pouring the solution A into the solution B, stirring for 10-16 hours at the speed of 1000-4000 rpm, transferring the fully stirred and mixed solution into a polytetrafluoroethylene hydrothermal reaction kettle, heating to 200-260 ℃ at a speed of 4-8 ℃/min, reacting at a constant temperature for 5-9 hours, cooling to room temperature after the reaction is completed, washing the obtained solid powder by using absolute ethyl alcohol and deionized water for three times respectively, and drying for 10-14 hours at the temperature of 80-100 ℃ to obtain carbon-point modified titanium dioxide microspheres with the particle size of 750-900 micrometers; and pouring the solution A into the solution B, wherein the mass ratio of the solution A to the solution B is 1-2:1.
Neutralizing and decoloring to obtain magnesium sulfate
Adding the photocatalytic oxidation waste sulfuric acid obtained in the step (1) into a reaction kettle, adding magnesium oxide to neutralize until the pH=5-6 to obtain a magnesium sulfate solution, adding thiourea dioxide accounting for 0.1-1% of the total mass of the magnesium sulfate solution to perform decolorization treatment, stirring at a speed of 1000-2000 rpm in a decolorization process, decolorizing at a constant temperature of 60-90 ℃ for 5-9 hours, cooling to room temperature, filtering to obtain a decolorized magnesium sulfate solution, distilling, concentrating until the density is 1.35g/ml, stopping distilling, crystallizing at a constant temperature, and performing pressure filtration to obtain magnesium sulfate crystals.
The preferable technical scheme is as follows:
the step (1) of the photocatalytic oxidation treatment of the waste sulfuric acid, wherein the depth of the waste sulfuric acid is controlled to be 4 cm, and the average illuminance is 8 multiplied by 10 4 lx, exposing for 1.5 hours to obtain waste sulfuric acid of photocatalytic oxidation;
preparing a solution A, 30wt% of oxalic acid aqueous solution, adding triethanolamine accounting for 7% of the total mass of the oxalic acid aqueous solution and glycerol accounting for 12% of the total mass of the oxalic acid aqueous solution at 800 revolutions per minute, and adjusting pH=8 by using 15wt% of ammonia water; 35wt% of titanyl sulfate aqueous solution, 10% of sodium fluosilicate by mass of the aqueous solution is added to dissolve the aqueous solution into solution B; A. mixing the solution B, stirring for 13 hours at 2500 rpm, transferring into a polytetrafluoroethylene hydrothermal reaction kettle, heating to 230 ℃ at a speed of 6 ℃/min, reacting at a constant temperature for 7 hours, washing solid powder obtained by filtering after the reaction, and drying at 90 ℃ for 12 hours to obtain carbon-point modified titanium dioxide microspheres with a particle size of 800 microns; and A, B liquid is mixed, and the mass ratio of the liquid A to the liquid B is 1.5:1.
And (3) neutralizing and decoloring to obtain magnesium sulfate, adding the waste sulfuric acid from the photocatalytic oxidation into magnesium oxide to neutralize until the pH=5.5 to obtain a magnesium sulfate solution, adding thiourea dioxide accounting for 0.5% of the total mass of the magnesium sulfate solution, stirring at a speed of 1500 rpm, and adsorbing and decoloring at 80 ℃ for 7 hours.
Compared with the prior art, the invention has the following beneficial effects:
1. the visible light catalytic oxidation degradation of organic matters in the waste sulfuric acid has low cost and high efficiency, and COD (chemical oxygen demand) amplitude reduction can reach 95.7-96.2% before and after waste acid treatment;
2. the photocatalyst carbon point modified titanium dioxide microsphere can be reused;
3. the magnesium sulfate crystal with the purity of 97.3-99.5% can be obtained through one-step neutralization and chemical reaction decoloration.
Detailed Description
The following description of the preferred embodiments of the present invention is provided for the purpose of illustration and explanation only and is not intended to limit the present invention.
Example 1: method for preparing magnesium sulfate from waste sulfuric acid
The method comprises the following steps:
1. photocatalytic oxidation treatment of waste sulfuric acid
Uniformly (laying 0.8 kg of carbon dot modified titanium dioxide microspheres on a single layer per square meter) and fixing the microspheres on the bottom of a insolation tank by using a 300-mesh glass fiber net, then introducing raw waste sulfuric acid with solid particles filtered out into the insolation tank for photocatalytic oxidation treatment, controlling the depth of the waste sulfuric acid in the insolation tank to be 4 cm, and controlling the average illuminance to be 8 multiplied by 10 4 And exposing for 1.5 hours under lx condition to obtain the photocatalytic oxidation waste sulfuric acid.
COD12898mg/L of the raw waste sulfuric acid and chromaticity of the raw waste sulfuric acid are 1052;
COD of the photocatalytic oxidation waste sulfuric acid is 490 mg/L, chromaticity is 369, and the reduction of COD after treatment is 96.2%;
the carbon point modified titanium dioxide microsphere is prepared by the following steps: adding 1000 kg of 30wt% oxalic acid aqueous solution into a reaction kettle, adding 70 kg of triethanolamine and 120 kg of glycerol at a stirring rate of 800 revolutions per minute, stirring and mixing uniformly, and then regulating pH to be 8 by using 15wt% ammonia water to obtain 1246 kg of A solution; 755 kg of 35wt% titanyl sulfate aqueous solution is added with 75.5 kg of sodium fluosilicate, and the solution is marked as solution B after complete dissolution; and pouring 1246 kg of A solution into 830.5 kg of B solution, stirring at 2500 rpm for 13 hours, transferring the fully stirred and mixed solution into a polytetrafluoroethylene hydrothermal reaction kettle, heating to 230 ℃ at a speed of 6 ℃/min, reacting at a constant temperature for 7 hours, cooling to room temperature after the reaction is finished, washing the obtained solid powder by absolute ethyl alcohol and deionized water for three times respectively, and drying at 90 ℃ for 12 hours to obtain carbon dot modified titanium dioxide microspheres with a particle size of 800 microns.
2. Neutralizing and decolorizing to obtain magnesium sulfate
460 kg of the photocatalytic oxidation waste sulfuric acid solution obtained in the step (1) is added into a reaction kettle, 20.5 kg of magnesium oxide is added for neutralization until the pH=5.5, then 2.4 kg of thiourea dioxide is added for decolorization treatment, the stirring speed in the decolorization process is 1500 rpm, the temperature is kept at 80 ℃ for adsorption decolorization for 7 hours, the temperature is reduced to room temperature, filtration is carried out to obtain a decolorized magnesium sulfate solution, distillation is carried out, when the decolorized magnesium sulfate solution is concentrated to a density of 1.35g/ml, distillation is stopped, and the distilled solution is subjected to constant-temperature crystallization and pressure filtration to obtain magnesium sulfate crystals.
The chromaticity 411 of the magnesium sulfate solution and the chromaticity 24 of the decolored magnesium sulfate solution;
COD test of waste sulfuric acid liquid before and after photocatalytic oxidation treatment is carried out according to HJ 828-2017;
chroma testing was performed with reference to GB 11903-1989;
example 1 COD was reduced by 96.2% before and after photocatalytic oxidation of spent sulfuric acid to give 68.9 kg of magnesium sulfate crystals with a purity of 99.5%.
Example 2:
1. photocatalytic oxidation treatment of waste sulfuric acid
Uniformly (laying 0.8 kg of carbon dot modified titanium dioxide microspheres on a single layer per square meter) and fixing the microspheres on the bottom of a insolation tank by using a 300-mesh glass fiber net, then introducing raw waste sulfuric acid with solid particles filtered out into the insolation tank for photocatalytic oxidation treatment, controlling the depth of the waste sulfuric acid in the insolation tank to be 2 cm, and controlling the average illuminance to be 5 multiplied by 10 3 And exposing for 1 hour under lx condition to obtain the waste sulfuric acid by photocatalytic oxidation.
COD12898mg/L of the raw waste sulfuric acid and chromaticity of the raw waste sulfuric acid are 1052;
COD of the photocatalytic oxidation waste sulfuric acid is 538mg/L, chromaticity is 387, and the reduction of COD after treatment is 95.8%;
the carbon point modified titanium dioxide microsphere is prepared by the following steps: adding 1000 kg of 20wt% oxalic acid aqueous solution into a reaction kettle, adding 50 kg of triethanolamine and 100 kg of glycerol at a stirring rate of 500 rpm, stirring and mixing uniformly, and then regulating pH to be 7 by using 10wt% ammonia water to obtain 1197 kg of A solution; 1140 kg of 30wt% titanyl sulfate aqueous solution is added with 57 kg of sodium fluosilicate, and the solution is marked as solution B after complete dissolution; pouring 1197 kg of A solution into 1197 kg of B solution, stirring at the speed of 1000 rpm for 10 hours, transferring the fully stirred and mixed solution into a polytetrafluoroethylene hydrothermal reaction kettle, heating to 200 ℃ at a speed of 4 ℃/min, reacting at a constant temperature for 5 hours, cooling to room temperature after the reaction is completed, respectively washing the obtained solid powder by using absolute ethyl alcohol and deionized water for three times, and drying at 80 ℃ for 10 hours to obtain carbon dot modified titanium dioxide microspheres with the particle size of 750 micrometers.
2. Neutralizing and decolorizing to obtain magnesium sulfate
460 kg of the photocatalytic oxidation waste sulfuric acid solution obtained in the step (1) is added into a reaction kettle, 18.4 kg of magnesium oxide is added for neutralization until the pH=5, then 0.48 kg of thiourea dioxide is added for decolorization treatment, the stirring speed in the decolorization process is 1000 r/min, the temperature is kept at 60 ℃ for adsorption decolorization for 5 hours, the temperature is reduced to room temperature, filtration is carried out to obtain a decolorized magnesium sulfate solution, distillation is carried out, when the decolorized magnesium sulfate solution is concentrated to a density of 1.35g/ml, distillation is stopped, and the distilled solution is subjected to constant-temperature crystallization and pressure filtration to obtain magnesium sulfate crystals.
The chromaticity 397 of the magnesium sulfate solution and the chromaticity 22 of the decolored magnesium sulfate solution;
COD test of waste sulfuric acid liquid before and after photocatalytic oxidation treatment is carried out according to HJ 828-2017;
chroma testing was performed with reference to GB 11903-1989;
example 2 COD was reduced by 95.8% before and after photocatalytic oxidation of spent sulfuric acid to give 63.6 kg of magnesium sulfate crystals with a purity of 98.2%.
Example 3:
1. photocatalytic oxidation treatment of waste sulfuric acid
Uniformly (laying 0.8 kg of carbon dot modified titanium dioxide microspheres on a single layer per square meter) and fixing the microspheres on the bottom of a insolation tank by using a 300-mesh glass fiber net, then introducing raw waste sulfuric acid with solid particles filtered out into the insolation tank for photocatalytic oxidation treatment, controlling the depth of the waste sulfuric acid in the insolation tank to be 6 cm, and controlling the average illuminance to be 1 multiplied by 10 5 And exposing for 2 hours under lx condition to obtain the photocatalytic oxidation waste sulfuric acid.
COD12898mg/L of the raw waste sulfuric acid and chromaticity of the raw waste sulfuric acid are 1052;
COD of the photocatalytic oxidation waste sulfuric acid is 559 mg/L, chromaticity is 406, and the reduction of COD after treatment is 95.7%;
the carbon point modified titanium dioxide microsphere is prepared by the following steps: adding 1000 kg of 40wt% oxalic acid aqueous solution into a reaction kettle, adding 80 kg of triethanolamine and 150 kg of glycerol under the stirring rate of 1200 rpm, and after stirring and mixing uniformly, regulating the pH to be 9 by using 20wt% ammonia water to obtain 1313 kg of A solution; 2283.5 kg of 40wt% titanyl sulfate aqueous solution is added with 342.5 kg of sodium fluosilicate, and the solution is marked as solution B after the solution is completely dissolved; and (3) pouring 2626 kg of A solution into 1313 kg of B solution, stirring for 16 hours at 4000 rpm, transferring the fully stirred and mixed solution into a polytetrafluoroethylene hydrothermal reaction kettle, heating to 260 ℃ at 8 ℃/min, keeping the temperature for reaction for 9 hours, cooling to room temperature after the reaction is completed, respectively washing the solid powder obtained by filtration with absolute ethyl alcohol and deionized water for three times, and drying at 100 ℃ for 14 hours to obtain carbon dot modified titanium dioxide microspheres with the particle size of 900 micrometers.
2. Neutralizing and decolorizing to obtain magnesium sulfate
460 kg of the photocatalytic oxidation waste sulfuric acid solution obtained in the step (1) is added into a reaction kettle, 23.2 kg of magnesium oxide is added for neutralization until the pH=6, then 4.84 kg of thiourea dioxide is added for decolorization treatment, the stirring speed in the decolorization process is 2000 r/min, the temperature is kept at 90 ℃ for adsorption decolorization for 9 hours, the temperature is reduced to room temperature, filtration is carried out to obtain a decolorized magnesium sulfate solution, distillation is carried out, when the decolorized magnesium sulfate solution is concentrated to a density of 1.35g/ml, distillation is stopped, and the distilled solution is subjected to constant-temperature crystallization and pressure filtration to obtain magnesium sulfate crystals.
The chromaticity 446 of the magnesium sulfate solution and the chromaticity 25 of the decolored magnesium sulfate solution;
COD test of waste sulfuric acid liquid before and after photocatalytic oxidation treatment is carried out according to HJ 828-2017;
chroma testing was performed with reference to GB 11903-1989;
example 3 COD was reduced by 95.7% before and after photocatalytic oxidation of spent sulfuric acid to give 62.1 kg of magnesium sulfate crystals with a purity of 97.3%.
Comparative example 1:
in the catalytic oxidation treatment process of the waste sulfuric acid, the titanium dioxide microspheres are not subjected to carbon point modification, and the waste sulfuric acid is only catalytically oxidized by the titanium dioxide microspheres under visible light.
1. Photocatalytic oxidation treatment of waste sulfuric acid
Uniformly (laying 0.8 kg per square meter of single layer) titanium dioxide microspheres on the bottom of a insolation tank by using a 300-mesh glass fiber net, then introducing raw waste sulfuric acid with solid particles filtered out into the insolation tank for photocatalytic oxidation treatment, controlling the depth of the waste sulfuric acid in the insolation tank to be 4 cm, and controlling the average illumination to be 8 multiplied by 10 4 And exposing for 1.5 hours under lx condition to obtain the photocatalytic oxidation waste sulfuric acid.
Raw waste sulfuric acid COD12898mg/L, chromaticity 1052, photocatalytic oxidation waste sulfuric acid COD12817mg/L, chromaticity 1089, COD width reduction before and after treatment is 0.6%;
the preparation method of the titanium dioxide microsphere comprises the following steps: 750 kg of 35wt% titanyl sulfate aqueous solution is added with 75 kg of sodium fluosilicate, after complete dissolution, the mixture is transferred into a polytetrafluoroethylene hydrothermal reaction kettle, the temperature is raised to 230 ℃ at 6 ℃/min, the reaction is carried out for 7 hours at constant temperature, after the reaction is completed and the temperature is reduced to room temperature, solid powder obtained by filtration is respectively washed three times by absolute ethyl alcohol and deionized water, and then dried for 12 hours at 90 ℃ to obtain the titanium dioxide microsphere with the particle size of 800 microns.
Comparative example 2:
in the catalytic oxidation treatment process of the waste sulfuric acid, the titanium dioxide microspheres are not subjected to carbon point modification, and the waste sulfuric acid is only catalytically oxidized by the titanium dioxide microspheres under ultraviolet light.
1. Photocatalytic oxidation treatment of waste sulfuric acid
Uniformly (laying 0.8 kg per square meter of single layer) titanium dioxide microspheres on the bottom of a insolation tank by using a 300-mesh glass fiber net, then introducing raw waste sulfuric acid with solid particles filtered out into the insolation tank for ultraviolet catalytic oxidation treatment, selecting an ultraviolet high-pressure mercury lamp with the power of 500W and the wavelength of 365nm, fixing the distance between a light source and the liquid level of the waste sulfuric acid to be 15 cm, controlling the depth of the waste sulfuric acid in the insolation tank to be 4 cm, and controlling the average irradiation intensity to 8300uw/cm 2 And (3) irradiating the waste sulfuric acid for 1.5 hours under the condition of ultraviolet irradiation to obtain ultraviolet catalytic oxidation waste sulfuric acid.
Raw waste sulfuric acid COD12898mg/L, chromaticity 1052, ultraviolet light catalytic oxidation waste sulfuric acid COD8396mg/L, chromaticity 857, COD width reduction 34.9% before and after treatment;
the preparation method of the titanium dioxide microsphere comprises the following steps: 750 kg of 35wt% titanyl sulfate aqueous solution is added with 75 kg of sodium fluosilicate, after complete dissolution, the mixture is transferred into a polytetrafluoroethylene hydrothermal reaction kettle, the temperature is raised to 230 ℃ at 6 ℃/min, the reaction is carried out for 7 hours at constant temperature, after the reaction is completed and the temperature is reduced to room temperature, solid powder obtained by filtration is respectively washed three times by absolute ethyl alcohol and deionized water, and then dried for 12 hours at 90 ℃ to obtain the titanium dioxide microsphere with the particle size of 800 microns.
Claims (2)
1. A method for preparing magnesium sulfate from waste sulfuric acid, which is characterized by comprising the steps of carrying out photocatalytic oxidation treatment and neutralization decolorization on the waste sulfuric acid to prepare magnesium sulfate;
in the photocatalytic oxidation treatment of the waste sulfuric acid, carbon-point-modified titanium dioxide microspheres are utilized to catalyze and degrade organic matters in the waste sulfuric acid under visible light;
the neutralization and decoloration are carried out to prepare magnesium sulfate, sulfuric acid obtained after organic matter degradation is neutralized by magnesium oxide, and then crystal is obtained after decoloration by thiourea dioxide;
the waste sulfuric acid photocatalytic oxidation treatment specifically comprises the following steps: uniformly fixing carbon point modified titanium dioxide microspheres on the bottom of a waste sulfuric acid insolating tank by using a 300-mesh glass fiber net, then introducing the waste sulfuric acid with solid particles filtered out into the insolating tank for photocatalytic oxidation treatment, wherein the depth of the waste sulfuric acid in the tank is controlled to be 2-6 cm, and the average illuminance is 5 multiplied by 10 3 -1×10 5 Exposing for 1-2 hours under lx condition to obtain waste sulfuric acid of photocatalytic oxidation;
the carbon point modified titanium dioxide microsphere comprises 20-40wt% of oxalic acid aqueous solution, 5-8% of triethanolamine and 10-15% of glycerol, based on the total mass of the oxalic acid aqueous solution, at the stirring rate of 500-1200 rpm, and after stirring and mixing uniformly, regulating the pH value to be 7-9 by using 10-20wt% of ammonia water, and marking the mixture as solution A for later use; adding sodium fluosilicate accounting for 5-15% of the total mass of the titanyl sulfate aqueous solution into the titanyl sulfate aqueous solution accounting for 30-40wt%, and marking the solution as solution B after the solution is completely dissolved; stirring the solution A and the solution B for 10-16 hours at the speed of 1000-4000 rpm according to the mass ratio of 1-2:1, transferring into a polytetrafluoroethylene hydrothermal reaction kettle for hydrothermal reaction, and performing post-treatment to obtain carbon point modified titanium dioxide microspheres with the particle size of 750-900 micrometers;
the hydrothermal reaction condition is that the temperature is increased to 200-260 ℃ at 4-8 ℃/min and the reaction is carried out for 5-9 hours at constant temperature; the post-treatment is to cool the reaction to room temperature, filter, wash the obtained solid powder with absolute ethyl alcohol and deionized water for three times respectively, and dry for 10-14 hours at 80-100 ℃.
2. A process for preparing magnesium sulfate from spent sulfuric acid according to claim 1, characterized in that: and (3) neutralizing and decoloring to obtain magnesium sulfate, adding the waste sulfuric acid subjected to photocatalytic oxidation into magnesium oxide to neutralize to pH=5-6 to obtain a magnesium sulfate solution, adding thiourea dioxide accounting for 0.1-1% of the total mass of the magnesium sulfate solution, stirring at a speed of 1000-2000 r/min, decoloring at a constant temperature of 60-90 ℃ for 5-9 hours, cooling, filtering, distilling and concentrating the filtrate until the density is 1.35g/ml, stopping distilling, crystallizing at a constant temperature, and press-filtering to obtain magnesium sulfate crystals.
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