A method for preparing ferrous sulfate by supercritical fluid-assisted purification of titanium dioxide waste acid

Abstract

The present invention discloses a method for preparing ferrous sulfate by purifying titanium dioxide waste acid with the aid of supercritical fluid, the method comprising adding a dissolving agent to the titanium dioxide waste acid, adding seed crystals to the titanium dioxide waste acid when the titanium dioxide waste acid is turbid, and obtaining a mixed system; the mixed system and the supercritical fluid are loaded into a reactor for reaction, after the reaction is completed, the reactor sprays mist, and the mist is filtered and cooled to obtain ferrous sulfate heptahydrate. The present invention controls the anti-solvent pressure, flow rate, and temperature of the supercritical fluid, and reduces the solubility of ferrous sulfate heptahydrate in the solution after mixing with the dissolving agent, thereby improving the crystallization recovery rate of ferrous sulfate heptahydrate. The present invention also adopts the technology of adding seed crystal curing and ferrous sulfate heptahydrate grain control to achieve online removal of 98% of iron impurities in titanium dioxide waste acid without reducing the concentration of sulfuric acid, and achieve zero emission in the whole process.

Description

Technical Field

[0001] The invention relates to the technical field of comprehensive utilization of industrial waste acid, and in particular to a method for preparing ferrous sulfate by purifying titanium dioxide waste acid with the assistance of supercritical fluid. Background Art

[0002] Titanium dioxide is widely used in coatings, papermaking, paints and other industries. The main production methods are sulfuric acid method, hydrochloric acid method and chlorination method. In China, the sulfuric acid method is mainly used to produce titanium dioxide. A large amount of waste acid and waste residue will be generated during the production of titanium dioxide. If not properly treated, it will cause environmental pollution. Every ton of titanium dioxide produced will produce about 8 tons of titanium dioxide waste acid. In addition to sulfuric acid with a mass fraction of about 20%, the titanium dioxide waste acid also contains 3% Fe 2+ , 0.4% Ti 4+ and Al 3+ , Ca 2+ Mg 2+ How to recover the valuable metal ions so that the waste acid can be recycled is one of the effective means to solve the waste acid pollution.

[0003] At present, titanium dioxide waste acid is mostly treated by lime neutralization method, but this method will generate gypsum waste residue containing many metal impurities, causing secondary pollution. At present, in order to treat the large amount of titanium gypsum produced by titanium dioxide waste acid, most of them are treated by slag yard storage. This method not only occupies a lot of land, but also causes great environmental pollution. There is also a method of recycling waste acid after concentration. Its technical route is to pre-evaporate the waste acid, conduct preliminary concentration, and then use water vapor vacuum evaporation to obtain 65% of the waste acid after concentration and reuse it in the acid hydrolysis process. However, the main impurity in titanium dioxide waste acid is ferrous sulfate. If the concentration is too high, it is easy to crystallize and precipitate, clogging equipment and pipelines. In addition, this method has high requirements for equipment, energy consumption and operating costs. The cost is much higher than directly using sulfuric acid for acid hydrolysis, so it is difficult to promote and use it widely.

[0004] Therefore, it is still necessary to provide a method for effectively treating titanium dioxide waste acid. Summary of the invention

[0005] In order to solve the problems existing in the prior art, the present invention adds a dissolving agent to titanium dioxide waste acid to reduce the solubility of ferrous sulfate in the titanium dioxide waste acid and adds seed crystals to control the nucleation rate; the dissolving agent is taken out of the high-pressure reactor in a gaseous state by the method and principle of supercritical fluid anti-solvent, and ferrous sulfate heptahydrate particles are precipitated at the same time.

[0006] In order to achieve the above object, the present invention provides a method for preparing ferrous sulfate by purifying titanium dioxide waste acid with the assistance of supercritical fluid, the method comprising:

[0007] adding a dissolving agent to the titanium dioxide waste acid, and adding seed crystals to the titanium dioxide waste acid when the titanium dioxide waste acid is turbid to obtain a mixed system;

[0008] The mixed system and the supercritical fluid are loaded into a reactor for reaction. After the reaction is completed, mist is sprayed out of the reactor, and the mist is filtered and cooled to obtain purified waste acid, solvent, supercritical fluid and ferrous sulfate heptahydrate product.

[0009] Furthermore, the dissolving agent is added to the titanium dioxide waste acid, and when the titanium dioxide waste acid is turbid, seed crystals are added to the titanium dioxide waste acid to grow crystals, to obtain a mixed system comprising:

[0010] 5-20 g of the dissolving agent is added to 0.8-1.2 kg of titanium dioxide waste acid. The state of the titanium dioxide waste acid is observed during the addition. When the titanium dioxide waste acid becomes turbid, ferrous sulfate heptahydrate is added to the titanium dioxide waste acid to grow crystals to obtain a mixed system.

[0011] Furthermore, the ferrous sulfate heptahydrate accounts for 0.1-2% of the mass of the titanium dioxide waste acid.

[0012] Furthermore, the crystal growing time is 30-240 minutes.

[0013] Furthermore, the mixed system and the supercritical fluid are loaded into a reactor for reaction. After the reaction is completed, the reactor sprays out mist, and the mist is filtered and cooled to obtain ferrous sulfate heptahydrate, including:

[0014] The supercritical fluid is loaded into the reactor at an injection rate of 0.1-1.0 mL / min for reaction. During the reaction, the pressure of the reactor is set to 8-20 MPa and the temperature is set to 35-70° C.;

[0015] After the reaction is completed, mist is sprayed out from the reactor, and the mist is filtered and cooled to obtain ferrous sulfate heptahydrate.

[0016] Furthermore, the reaction time is 3-30 min.

[0017] Further, the mixed system and the supercritical fluid are loaded into a reactor for reaction. After the reaction is completed, the reactor sprays mist, and the mist is filtered and cooled to obtain the purified waste acid, solvent, supercritical fluid and ferrous sulfate heptahydrate product, which also includes,

[0018] The purified waste acid, solvent and supercritical fluid are recycled.

[0019] Furthermore, the solvent is at least one of ethanol, isopropanol, ethylenediamine, ethylene glycol, ethyl ether, 1,2-propylene glycol, pyridine and diethylene glycol.

[0020] Furthermore, the supercritical fluid is supercritical CO2.

[0021] Furthermore, the titanium dioxide waste acid contains 1-50% by mass of sulfuric acid.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] (1) The present invention develops a novel online titanium dioxide waste acid recovery process, which prepares high-quality ferrous sulfate heptahydrate crystals while deeply removing iron, removes part of the water in the titanium dioxide waste acid, plays a role in concentrating the waste acid to a certain extent, so that the acidity of the waste acid can be maintained, and can prevent the waste acid from clogging equipment and pipelines when it is recycled;

[0024] (2) The present invention uses the anti-solvent effect of supercritical fluid to transform the solvent into a gas phase and separate it from the waste acid, thereby solving the problem that the traditional solvent requires complex post-treatment (distillation, flash evaporation, etc.) to be completely removed; the diffusion coefficient of the supercritical anti-solvent fluid is two orders of magnitude higher than that of the liquid, the reaction speed is fast, and the ferrous sulfate heptahydrate grains are uniform and controllable;

[0025] (3) The present invention can realize zero emission in the whole process of waste acid recycling, solvent recycling and CO2 recycling, reduce the cost of titanium dioxide waste acid recovery, improve economic benefits, help enterprises transform and upgrade, reduce environmental pollution, and has good application prospects.

[0026] Other features and advantages of the present invention will be described in the following description, and partly become apparent from the description, or understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained by the steps or processes indicated in the description and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0028] Figure 1 A flow chart of a method for preparing ferrous sulfate by supercritical fluid-assisted purification of titanium dioxide waste acid in Example 3 of the present invention is shown. DETAILED DESCRIPTION

[0029] The endpoints and any values ​​of the ranges disclosed in the present invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be regarded as specifically disclosed in the present invention.

[0030] Supercritical antisolvent technology has unique advantages in the preparation of ultrafine particles, and has been widely used in the production of organic substances such as drugs, superconductors, pigments, explosives and polymers, but is less used in inorganic particles. The present invention combines the advantages of supercritical antisolvent technology, controls the pressure, flow rate and temperature of the supercritical fluid antisolvent, and reduces the solubility of ferrous sulfate heptahydrate in the solution after mixing with the solvent, thereby improving the crystallization recovery rate of ferrous sulfate heptahydrate; utilizes the gaseous excited state of the supercritical antisolvent to separate the solvent from the waste acid in gaseous form.

[0031] The present invention provides a method for preparing ferrous sulfate by purifying titanium dioxide waste acid with the assistance of supercritical fluid. The method comprises the following steps:

[0032] S1. Adding a dissolving agent to the titanium dioxide waste acid, and adding seed crystals to the titanium dioxide waste acid when the titanium dioxide waste acid is turbid, to obtain a mixed system.

[0033] In step S1 of the present invention, 5-20 g of a dissolving agent is added to 0.8-1.2 kg of titanium dioxide waste acid, and the state of the titanium dioxide waste acid is observed during the addition. When the titanium dioxide waste acid becomes turbid, ferrous sulfate heptahydrate is added to the titanium dioxide waste acid for crystal growth to obtain a mixed system. The ferrous sulfate heptahydrate is 0.1-2% of the mass of the titanium dioxide waste acid, and the crystal growth time is 30-240 min.

[0034] Preferably, the mass ratio of the dissolving agent to the titanium dioxide waste acid is 5g:1kg, 10g:1kg and 20g:1kg.

[0035] Preferably, the crystal growing time is 30, 50 and 60 minutes.

[0036] Preferably, the ferrous sulfate heptahydrate is 0.1, 0.5 and 2% of the mass of the titanium dioxide waste acid.

[0037] Preferably, the solvent is at least one of ethanol, isopropanol, ethylenediamine, ethylene glycol, diethyl ether, 1,2-propylene glycol, pyridine and diethylene glycol.

[0038] S2. Loading the mixed system and the supercritical fluid into a reactor for reaction. After the reaction is completed, the reactor sprays out mist, and the mist is filtered and cooled to obtain purified waste acid, solvent, supercritical fluid and ferrous sulfate heptahydrate product.

[0039] In step S2 of the present invention, the supercritical fluid is loaded into a reactor at an injection rate of 0.1-1.0 mL / min for reaction, and the pressure of the reactor is set to 8-20 MPa and the temperature is set to 35-70° C. After the reaction is completed, the reactor sprays mist, and the mist is filtered and cooled to obtain a ferrous sulfate heptahydrate product.

[0040] Preferably, the reactor pressure is 8, 15 and 20 MPa.

[0041] Preferably, the reactor temperature is 35, 45 and 70°C.

[0042] Preferably, the reaction time is 3-30 min, more preferably 3, 15 and 30 min.

[0043] Preferably, the supercritical fluid is supercritical CO2.

[0044] After step S2 of the present invention, the purified waste acid, solvent and supercritical fluid may be recycled.

[0045] It should be noted that the injection rate of supercritical CO2 should not be too large, preferably 0.1, 0.5 and 1.0 mL / min. This is because when the CO2 flow rate is fixed, as the injection rate of the titanium dioxide waste acid solution increases, the volume ratio of CO2 to the solvent decreases, which weakens the mass transfer effect of CO2 and anhydrous ethanol, resulting in a decrease in the supersaturation coefficient, and at the same time, the droplets become larger, resulting in an increase in the particle size.

[0046] The present invention also provides the application of the above-mentioned method for purifying titanium dioxide waste acid to prepare ferrous sulfate with the assistance of supercritical fluid, which is applied to the purification of titanium dioxide waste acid and the preparation of ferrous sulfate.

[0047] It should be noted that in the present invention, the waste titanium dioxide acid contains 1-50% by mass of sulfuric acid. Preferably, the waste titanium dioxide acid contains 21% by mass of sulfuric acid.

[0048] The technical solution of the present invention will be clearly and completely described below in conjunction with the specific embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0049] Example 1

[0050] A method for preparing ferrous sulfate by purifying titanium dioxide waste acid with the assistance of supercritical fluid, the method comprising the following steps:

[0051] Step 1: At 25°C and a stirring speed of 200 r / min, 5 g of ethanol was slowly added dropwise to 1 kg of titanium dioxide waste acid, wherein the mass fraction of H2SO4 in the titanium dioxide waste acid was 21%, and the mass fraction of Fe 2+ The content is 50.27 g / L. During the dropwise addition process, when the titanium dioxide waste acid becomes turbid, 1 g of ferrous sulfate heptahydrate is added to the titanium dioxide waste acid for crystal growth. The crystal growth is performed for 30 minutes to obtain a mixed system.

[0052] Step 2, cooling the supercritical CO2 in an ice-water bath and using a high-pressure column pressure pump, and then spraying it into a high-pressure reactor at an injection rate of 0.1 mL / min, and at the same time pumping the mixed system into the high-pressure reactor through a high-performance liquid phase pump; setting the reactor pressure to 8 MPa and the temperature to 35°C, reacting the mixed system with the supercritical CO2 for 3 minutes, and then spraying out mist, and separating the mist by filtration and condensation to obtain purified waste acid, ferrous sulfate heptahydrate product, CO2 and ethanol, respectively.

[0053] The method of this embodiment is used to measure the Fe content in the purified waste acid. 2+ The content is 3.49 g / L. After recovering ethanol, the iron recovery rate is calculated to be 88.45%.

[0054] Example 2

[0055] A method for preparing ferrous sulfate by purifying titanium dioxide waste acid with the assistance of supercritical fluid, the method comprising the following steps:

[0056] Step 1: At 25°C and a stirring speed of 200 r / min, 10 g of ethylene glycol is slowly added dropwise to 1 kg of titanium dioxide waste acid, wherein the mass fraction of H2SO4 in the titanium dioxide waste acid is 21%, and the mass fraction of Fe 2+ The content is 50.27 g / L. During the dropwise addition process, when the titanium dioxide waste acid becomes turbid, 5 g of ferrous sulfate heptahydrate is added to the titanium dioxide waste acid for crystal growth. The crystal growth is performed for 50 minutes to obtain a mixed system.

[0057] Step 2, cooling the supercritical CO2 in an ice-water bath and using a high-pressure column pressure pump, and then spraying it into a high-pressure reactor at an injection rate of 0.5 mL / min, and at the same time pumping the mixed system into the high-pressure reactor through a high-performance liquid phase pump; setting the reactor pressure to 15 MPa and the temperature to 45° C., reacting the mixed system with the supercritical CO2 for 15 minutes, and then spraying out mist, and separating the mist by filtration and condensation to obtain purified waste acid, ferrous sulfate heptahydrate product, CO2 and ethylene glycol, respectively.

[0058] The method of this embodiment is used to measure the Fe content in the purified waste acid.2+ The content is 0.94 g / L. After distillation to recover ethylene glycol, the iron recovery rate is calculated to be 95.34%.

[0059] Example 3

[0060] A method for preparing ferrous sulfate by purifying titanium dioxide waste acid with the assistance of supercritical fluid, the method comprising the following steps:

[0061] Step 1: At 25°C and a stirring speed of 200 r / min, 20 g of diethylene glycol was slowly added dropwise to 1 kg of titanium dioxide waste acid, wherein the mass fraction of H2SO4 in the titanium dioxide waste acid was 21%, and the mass fraction of Fe 2+ The content is 50.27 g / L. During the dropwise addition process, when the titanium dioxide waste acid becomes turbid, 20 g of ferrous sulfate heptahydrate is added to the titanium dioxide waste acid for crystal growth. The crystal growth is performed for 60 minutes to obtain a mixed system.

[0062] Step 2, cooling the supercritical CO2 in an ice-water bath and using a high-pressure column pressure pump, and then spraying it into a high-pressure reactor at an injection rate of 1.0 mL / min, and at the same time pumping the mixed system into the high-pressure reactor through a high-performance liquid phase pump; setting the reactor pressure to 20 MPa and the temperature to 70°C, reacting the mixed system with the supercritical CO2 for 30 minutes, and then spraying out mist, and separating the mist by filtration and condensation to obtain purified waste acid, ferrous sulfate heptahydrate product, CO2 and diethylene glycol, respectively.

[0063] The method of this embodiment is used to measure the Fe content in the purified waste acid. 2+ The content is 0.61 g / L. After distillation to recover diethylene glycol, the iron recovery rate is calculated to be 98.78%.

[0064] In addition to the steps mentioned in this embodiment, the distillation recovery of diethylene glycol, CO2 and the purified waste acid can also be included for recycling, which can achieve zero emissions in the entire process, reduce the cost of titanium dioxide waste acid recovery, and improve economic benefits.

[0065] like Figure 1 As shown, the operation steps of a method for preparing ferrous sulfate by supercritical fluid-assisted purification of titanium dioxide waste acid in Example 3 are summarized. Figure 1 The green vitriol in it is ferrous sulfate heptahydrate.

[0066] In summary, a method for preparing ferrous sulfate by supercritical fluid-assisted purification of titanium dioxide waste acid of the present invention utilizes supercritical antisolvent principle, with supercritical fluid as stripping agent, ferrous sulfate heptahydrate solid matter and dissolution agent are limitedly mutually soluble, and the solubility in supercritical fluid is very small, when supercritical fluid is dissolved in solution, solution dilution and expansion are made, the solubility of original solvent to solute is reduced, and a larger supersaturation is formed in a short time and solute crystallization is made to separate out, forming ferrous sulfate heptahydrate of fine particles with high purity and uniform particle size distribution. In the reaction process of the present invention, when high-pressure airflow passes through the solution injection port, gas can expand rapidly and crush ferrous sulfate heptahydrate crystal seeds into small droplets, and then droplets are dried to form microparticles, so the increase of pressure enhances the ability of gas expansion solvent, and ferrous sulfate heptahydrate is broken more broken at the same time, and gas-liquid contact area is also increased, and ferrous sulfate heptahydrate micropowder particles can be formed faster. When pressure is relatively low, gas-liquid mixing effect is not good, resulting in poor iron removal effect, so that ferrous sulfate heptahydrate particles are reunited. The present invention also utilizes the property that the solvent is miscible with water but immiscible with ferrous sulfate heptahydrate, reduces the solubility of ferrous sulfate heptahydrate in titanium dioxide waste acid, controls the process conditions, and precipitates ferrous sulfate heptahydrate from the titanium dioxide waste acid in the form of large-particle crystals, and obtains purified waste acid and ferrous sulfate heptahydrate after solid-liquid separation.

[0067] Finally, it should be noted that the above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, it is still possible for those skilled in the art to modify the technical solutions described in the aforementioned embodiments or to make equivalent substitutions for some of the technical features therein. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing ferrous sulfate by supercritical fluid-assisted purification of titanium dioxide waste acid, characterized in that: The method comprises, adding a dissolving agent to the titanium dioxide waste acid, and adding seed crystals to the titanium dioxide waste acid when the titanium dioxide waste acid is turbid to obtain a mixed system; The mixed system and the supercritical fluid are loaded into a reactor for reaction. After the reaction is completed, the reactor sprays out mist, and the mist is filtered and cooled to obtain purified waste acid, a solvent, a supercritical fluid and a ferrous sulfate heptahydrate product; The dissolving agent is added to the titanium dioxide waste acid, and when the titanium dioxide waste acid is turbid, seed crystals are added to the titanium dioxide waste acid to grow crystals, to obtain a mixed system comprising: 5-20 g of a dissolving agent is added to 0.8-1.2 kg of titanium dioxide waste acid, and the state of the titanium dioxide waste acid is observed during the addition. When the titanium dioxide waste acid becomes turbid, ferrous sulfate heptahydrate is added to the titanium dioxide waste acid to grow crystals, thereby obtaining a mixed system; the ferrous sulfate heptahydrate is 0.1-2% of the mass of the titanium dioxide waste acid; The mixed system and the supercritical fluid are loaded into a reactor for reaction. After the reaction is completed, the reactor sprays out mist, and the mist is filtered and cooled to obtain ferrous sulfate heptahydrate, including: The supercritical fluid is loaded into the reactor at an injection rate of 0.1-1.0 mL / min for reaction. During the reaction, the pressure of the reactor is set to 8-20 MPa and the temperature is set to 35-70° C.; The mixed system and the supercritical fluid are loaded into a reactor for reaction. After the reaction is completed, the reactor sprays out mist, and the mist is filtered and cooled to obtain purified waste acid, solvent, supercritical fluid and ferrous sulfate heptahydrate product, which also includes: The purified waste acid, solvent and supercritical fluid are recycled.

2. The method for preparing ferrous sulfate by supercritical fluid-assisted purification of titanium dioxide waste acid according to claim 1, characterized in that: The crystal growing time is 30-240 minutes.

3. The method for preparing ferrous sulfate by supercritical fluid-assisted purification of titanium dioxide waste acid according to claim 1, characterized in that: After the reaction is completed, mist is sprayed out from the reactor, and the mist is filtered and cooled to obtain ferrous sulfate heptahydrate.

4. The method for preparing ferrous sulfate by supercritical fluid-assisted purification of titanium dioxide waste acid according to claim 3, characterized in that: The reaction time is 3-30 min.

5. The method for preparing ferrous sulfate by supercritical fluid-assisted purification of titanium dioxide waste acid according to any one of claims 1 to 4, characterized in that: The solvent is at least one of ethanol, isopropanol, ethylenediamine, ethylene glycol, ethyl ether, 1,2-propylene glycol, pyridine and diethylene glycol.

6. The method for preparing ferrous sulfate by supercritical fluid-assisted purification of titanium dioxide waste acid according to any one of claims 1 to 4, characterized in that: The supercritical fluid is supercritical CO2.

7. The method for preparing ferrous sulfate by supercritical fluid-assisted purification of titanium dioxide waste acid according to any one of claims 1 to 4, characterized in that: The titanium dioxide waste acid contains 1-50% by mass of sulfuric acid.

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