A method for concentrating ferrous sulfate heptahydrate and titanium dioxide waste acid

Abstract

The present invention relates to the field of titanium dioxide waste acid treatment, and is used to solve the problem that the type of ferrous sulfate crystallization product cannot be effectively controlled and the process is complicated in the waste acid concentration process of the prior art, and a titanium dioxide waste acid concentration system is provided, wherein the waste acid with a temperature of 45°C-55°C and a concentration of 23%-28% is heat exchanged with the circulating water of 32°C-35°C by the graphite heat exchanger to obtain the primary cooling waste acid of 38°C-41°C; the primary cooling waste acid is heat exchanged with the heat exchange medium by the first effect film heater to obtain the secondary cooling waste acid of 28°C-32°C; the secondary cooling waste acid is heat exchanged with ice water by the second effect film heater to obtain the crystallization waste acid of 4°C-6°C; the concentration of the crystallization waste acid is 35%-38%; the crystallization waste acid is added with seed crystals in the maturation tank and is taken out of the crystallization to serve as the heat exchange medium of the first effect film heater. The titanium dioxide waste acid concentration system provided by the present invention has substantially no generation of ferrous sulfate monohydrate in the process flow, thereby ensuring the fluidity of the liquid and avoiding the blockage of the heat exchange tube.

Description

Technical Field

[0001] The invention relates to the field of titanium dioxide waste acid treatment, and in particular to a method for concentrating ferrous sulfate heptahydrate and titanium dioxide waste acid. Background Art

[0002] The sulfuric acid process for producing titanium dioxide began in 1918. The process is relatively mature and mainly uses ilmenite or acid-soluble titanium slag as raw materials. Sulfuric acid is used to decompose the titanium raw materials into TiOSO 4 The solution is then cleaned and separated to finally obtain the finished titanium dioxide. In the process of producing titanium dioxide by the acid method, a large amount of waste acid is generated. In order to recycle the waste acid, it is often necessary to concentrate the waste acid and separate the inorganic salts it contains, especially the crystallization and separation of ferrous sulfate. The precipitated ferrous sulfate is mostly recycled as ferrous sulfate heptahydrate or ferrous sulfate monohydrate. Since the sulfuric acid used in the production of titanium dioxide by the acid method is highly corrosive, the heat exchange equipment used in the process all uses graphite tubes. Graphite tubes have excellent acid and alkali corrosion resistance, but the strength of graphite is relatively low. If blockage occurs, it is more easily damaged than heat exchange tubes made of other materials. During the concentration process, the temperature is generally kept high, and the crystallized product is mainly ferrous sulfate monohydrate. When the content of ferrous sulfate monohydrate increases, the viscosity of the liquid increases, which is more likely to clog the heat exchange tubes than ferrous sulfate heptahydrate. The patent with publication number CN115028207A discloses a method for concentrating waste acid of titanium dioxide by sulfuric acid process, which solves the fluidity problem in the concentration process by concentrating and separating by crystallization, so as to solve the scaling and pipe blocking. However, the crystallization process takes a certain amount of time, and the process is complicated and takes a long time by concentrating and separating by crystallization. At the same time, it cannot ensure that ferrous sulfate is mainly recovered in the form of ferrous sulfate heptahydrate, so it can only be concentrated by crystallization to ensure the good fluidity of the mother liquor. Summary of the invention

[0003] The purpose of the present invention is to provide a method for concentrating ferrous sulfate heptahydrate and titanium dioxide waste acid, which solves the problem that the type of ferrous sulfate crystallization product cannot be effectively controlled and the process is complicated in the waste acid concentration process of the prior art.

[0004] The present invention is achieved through the following technical solutions:

[0005] A method for concentrating titanium dioxide waste acid, comprising a graphite heat exchanger, a first-effect membrane heater and a second-effect membrane heater;

[0006] The graphite heat exchanger is provided with a waste acid inlet, a circulating water inlet and a waste acid outlet; the waste acid outlet of the graphite heat exchanger is connected to the waste acid inlet of the first-effect film heater; the waste acid outlet of the first-effect film heater is connected to the waste acid inlet of the second-effect film heater; the first-effect film heater and the second-effect film heater are both provided with a heat exchange medium inlet; the heat exchange medium inlet of the second-effect film heater is connected to the ice making system;

[0007] The waste acid with a temperature of 45°C-55°C and a concentration of 23%-28% is heat exchanged with circulating water at 32°C-35°C through the graphite heat exchanger to obtain primary cooling waste acid at 38°C-41°C; the primary cooling waste acid is heat exchanged with a heat exchange medium through the first-effect membrane heater to obtain secondary cooling waste acid at 28°C-32°C; the secondary cooling waste acid is heat exchanged with ice water through the second-effect membrane heater to obtain crystallized waste acid at 4°C-6°C; the concentration of the crystallized waste acid is 35%-38%; the crystallized waste acid is added with seed crystals in the maturation tank and is used as the heat exchange medium of the first-effect membrane heater after crystallization;

[0008] A stirring device is arranged in the aging tank; the stirring speed is 90r / min-140r / min.

[0009] Ferrous sulfate heptahydrate is dehydrated at 56.8℃ to generate ferrous sulfate tetrahydrate, and then converted into ferrous sulfate monohydrate at 64℃-90℃. Therefore, in order to ensure that the crystallized product is ferrous sulfate heptahydrate, low-temperature freezing crystallization is used to precipitate the ferrous ions in the waste acid as ferrous sulfate heptahydrate to the greatest extent. The addition of seed crystals accelerates the crystallization speed and degree of ferrous sulfate heptahydrate. When crystallizing while stirring, stirring too fast will affect the formation of crystal nuclei. Therefore, setting a reasonable stirring speed can ensure that the particle size of ferrous sulfate heptahydrate after crystallization is uniform without affecting crystallization, and the product quality is higher.

[0010] Since the crystallized waste acid needs to be further heated and concentrated after crystallization, in order to make more reasonable use of heat, the crystallized waste acid first enters the first-effect membrane heater as a heat exchange medium, which not only helps to cool the waste acid for the first time but also increases the temperature of the crystallized waste acid, reducing the heat energy required for subsequent concentration.

[0011] After most of the ferrous sulfate in the crystallized waste acid is removed as ferrous sulfate heptahydrate, the subsequent steam heating concentration hardly generates ferrous sulfate monohydrate, which completely solves the problem of heat exchange tube blockage.

[0012] During the crystallization process of ferrous sulfate heptahydrate, water in the waste acid is taken away, which has the effect of increasing the concentration and reducing the heat consumption in the concentration process.

[0013] Preferably, the waste acid with a temperature of 50°C and a concentration of 25% is heat exchanged with circulating water at 32°C-35°C through the graphite heat exchanger to obtain the primary cooled waste acid at 40°C.

[0014] Preferably, the primary cooled waste acid exchanges heat with a heat exchange medium through the first-effect membrane heater to obtain a secondary cooled waste acid at 30°C.

[0015] Preferably, the secondary cooling waste acid is heat exchanged with ice water through the second-effect membrane heater to obtain crystalline waste acid at 5°C.

[0016] The test showed that the degree of crystallization of the waste acid was higher at 5°C, and the crystallized products were all ferrous sulfate heptahydrate.

[0017] Preferably, the concentration of the crystallized spent acid is 36%.

[0018] Preferably, the crystallized waste acid is subjected to heat exchange in a single-effect membrane heat exchanger and then further concentrated by steam heating to obtain waste acid with a concentration of 55%.

[0019] Preferably, the crystallized waste acid is converted into primary concentrated waste acid with a temperature of 18° C. and a concentration of 36% after heat exchange in a single-effect membrane heat exchanger.

[0020] Preferably, after the crystalline waste acid is fully crystallized, the crystals are removed by a plate and frame filter press.

[0021] The filtrate obtained by filtration is the 5°C waste acid after crystallization. After the filter cake is slurried, it is washed and dehydrated by a centrifuge to obtain ferrous sulfate heptahydrate. The centrifuge is rinsed with ice water, and the washing water after rinsing is used for slurrying ferrous sulfate heptahydrate. The ferrous sulfate heptahydrate obtained after centrifugation can be sold as a product.

[0022] Preferably, the ferrous sulfate heptahydrate is produced by the titanium dioxide waste acid concentration method.

[0023] The present invention has at least the following beneficial effects: substantially no ferrous sulfate monohydrate is generated in the process flow, thus ensuring the fluidity of the liquid and avoiding the blockage of the heat exchange tube; since there is no scaling and blockage phenomenon, the forced circulation pump in the process section can be stopped, thus saving about 800,000 yuan in costs per year, and reducing the time for flushing the scale surface, so that the equipment operation time can be increased by about 3 days per month; the heat of the waste acid is fully utilized and the generated ferrous sulfate heptahydrate is used to take away the water in the waste acid to achieve a concentration effect; after the improvement, the steam consumption of the waste acid concentration system of the process can be saved by 40%-50% on the original basis. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments are briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without creative work.

[0025] Figure 1 A schematic flow chart of a method for concentrating waste titanium dioxide acid provided by the present invention;

[0026] Icons: 1-graphite heat exchanger, 2-single-effect membrane heater, 3-double-effect membrane heater, 4-ice making system, 5-maturation tank, 6-plate and frame filter press, 7-centrifuge, 8-waste acid storage tank. DETAILED DESCRIPTION

[0027] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention are clearly and completely described. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. The components of the embodiments of the present invention described and shown in the drawings herein can be arranged and designed in various different configurations.

[0028] Embodiment 1:

[0029] ① Pre-cooling: The waste acid with a concentration of 25% and 50°C is heat exchanged with 34°C circulating water through the graphite heat exchanger 1 to obtain the initially cooled waste acid at 40°C.

[0030] ② Primary heat exchange: the initially cooled waste acid enters the primary membrane heater 2, and exchanges heat with the 5°C crystallized waste acid after crystallization filtration to obtain 30°C secondary cooled waste acid; the temperature of the 5°C crystallized waste acid is increased to 18°C, with a concentration of 36%, and then conventional steam heating is used for concentration to obtain a waste acid with a concentration of 55%.

[0031] ③ Cooling crystallization: The 30°C secondary cooling waste acid enters the second-effect membrane heater 3, exchanges heat with the ice water of the ice making system 4, and obtains 5°C crystallized waste acid. The ice water after heat exchange is returned to the ice making system 4 for recycling.

[0032] ④ Crystallization separation: 5°C crystallized waste acid is sent to the maturation tank 5, seed crystals are added to the maturation tank 5, and stirring is performed at a speed of 100r / min. After maturation and crystallization, the waste acid is filtered by a plate and frame filter press 6; the filtrate obtained by filtering is the 5°C waste acid after crystallization, which is stored in the waste acid storage tank 8 and then returned to the first-effect membrane heater 2 as a heat exchange medium; after pulping the filter cake, it is washed and dehydrated by a centrifuge 7 to obtain ferrous sulfate heptahydrate, and the centrifuge 7 is rinsed with ice water. The washing water after rinsing is used for pulping ferrous sulfate heptahydrate, and the ferrous sulfate heptahydrate obtained after centrifugation can be sold as a product.

[0033] Embodiment 2:

[0034] ① Pre-cooling: The waste acid with a concentration of 28% and 55°C is heat exchanged with 32°C circulating water through the graphite heat exchanger 1 to obtain the initially cooled waste acid at 41°C.

[0035] ② Primary heat exchange: the initially cooled waste acid enters the primary membrane heater 2, and exchanges heat with the 5°C crystallized waste acid after crystallization filtration to obtain 32°C secondary cooled waste acid; the 5°C crystallized waste acid is heated to 18°C, with a concentration of 38%, and then concentrated by conventional steam heating to obtain a waste acid with a concentration of 55%.

[0036] ③ Cooling crystallization: The secondary cooling waste acid at 32°C enters the second-effect membrane heater 3, exchanges heat with the ice water of the ice-making system to obtain 5°C crystallized waste acid, and the ice water after heat exchange is returned to the ice-making system for recycling.

[0037] ④ Crystallization separation: 5°C crystallized waste acid is sent to the maturation tank 5, seed crystals are added to the maturation tank 5, and stirring is performed at a speed of 140r / min. After maturation and crystallization, the waste acid is filtered by a plate and frame filter press; the filtrate obtained by the filter press is the 5°C waste acid after crystallization, which is returned to the first-effect membrane heater 2 as a heat exchange medium; after the filter cake is slurried, it is washed and dehydrated by a centrifuge 7 to obtain ferrous sulfate heptahydrate, the centrifuge 7 is rinsed with ice water, and the washing water after rinsing is used for slurrying of ferrous sulfate heptahydrate, and the ferrous sulfate heptahydrate obtained after centrifugation can be sold as a product.

[0038] Embodiment 3:

[0039] ① Pre-cooling: The waste acid with a concentration of 23% and 45°C is heat exchanged with 32°C circulating water through the graphite heat exchanger 1 to obtain the initially cooled waste acid at 38°C.

[0040] ② Primary heat exchange: the initially cooled waste acid enters the primary membrane heater 2, and exchanges heat with the 5°C crystallized waste acid after crystallization filtration to obtain a secondary cooled waste acid at 28°C; the temperature of the 5°C crystallized waste acid is increased to 18°C, with a concentration of 36%, and then conventional steam heating is used for concentration to obtain a waste acid with a concentration of 55%.

[0041] ③ Cooling crystallization: The secondary cooling waste acid at 28°C enters the second-effect membrane heater 3, exchanges heat with the ice water of the ice-making system to obtain 5°C crystallized waste acid, and the ice water after heat exchange is returned to the ice-making system for recycling.

[0042] ④ Crystallization separation: 5°C crystallized waste acid is sent to the maturation tank 5, seed crystals are added to the maturation tank 5, and stirring is performed at a speed of 90r / min. After maturation and crystallization, the waste acid is filtered by a plate and frame filter press; the filtrate obtained by the filter press is the 5°C waste acid after crystallization, which is returned to the first-effect membrane heater 2 as a heat exchange medium; after the filter cake is slurried, it is washed and dehydrated by a centrifuge 7 to obtain ferrous sulfate heptahydrate, the centrifuge 7 is rinsed with ice water, and the washing water after rinsing is used for slurrying of ferrous sulfate heptahydrate, and the ferrous sulfate heptahydrate obtained after centrifugation can be sold as a product.

[0043] Comparative Example 1:

[0044] Except that no seed crystals are added, other technical means are the same as those in Example 1.

[0045] Comparative Example 2:

[0046] Except that the aging tank is not stirred, the other technical means are the same as those in Example 1.

[0047] Comparative Example 3:

[0048] Except that the crystallization temperature is 20°C, the other technical means are the same as those in Example 1.

[0049] The ferrous sulfate content in the waste acid after crystallization of Example 1-3 and Comparative Example 1-3 was measured, and the removal rate was calculated; at the same time, the water content of the ferrous sulfate heptahydrate product was measured.

[0050] Table 1 Ferrous sulfate removal rate and water content of ferrous sulfate heptahydrate product

[0051]

[0052] As can be seen from Table 1, the production process provided by the present application has removed more than 80% of ferrous sulfate in the form of seven crystal waters during crystallization, and no ferrous sulfate monohydrate is generated in the subsequent waste acid vapor concentration process, which completely solves the problem of blockage caused by ferrous sulfate monohydrate. The lower the water content of ferrous sulfate heptahydrate, the larger the crystals are, and the lower the adhesion to water. The larger the crystals are, the less likely they are to adhere to the pipe wall, causing scaling and blockage.

[0053] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for concentrating titanium dioxide waste acid, characterized in that: It includes a graphite heat exchanger, a single-effect film heater and a double-effect film heater; The graphite heat exchanger is provided with a waste acid inlet, a circulating water inlet and a waste acid outlet; the waste acid outlet of the graphite heat exchanger is connected to the waste acid inlet of the first-effect film heater; the waste acid outlet of the first-effect film heater is connected to the waste acid inlet of the second-effect film heater; the first-effect film heater and the second-effect film heater are both provided with a heat exchange medium inlet; the heat exchange medium inlet of the second-effect film heater is connected to the ice making system; The waste acid with a temperature of 45°C-55°C and a concentration of 23%-28% is heat exchanged with circulating water at 32°C-35°C through the graphite heat exchanger to obtain primary cooling waste acid at 38°C-41°C; the primary cooling waste acid is heat exchanged with a heat exchange medium through the first-effect membrane heater to obtain secondary cooling waste acid at 28°C-32°C; the secondary cooling waste acid is heat exchanged with ice water through the second-effect membrane heater to obtain crystallized waste acid at 4°C-6°C; the concentration of the crystallized waste acid is 35%-38%; the crystallized waste acid is added with seed crystals in the maturation tank and is used as the heat exchange medium of the first-effect membrane heater after crystallization; The aging tank is provided with a stirring device; the stirring speed is 90r / min-140r / min; The secondary cooling waste acid is heat exchanged with ice water by the second-effect membrane heater to obtain crystalline waste acid at 5°C.

2. The method for concentrating titanium dioxide waste acid according to claim 1, characterized in that: The waste acid with a temperature of 50° C. and a concentration of 25% is heat exchanged with circulating water at 32° C.-35° C. through the graphite heat exchanger to obtain the primary cooled waste acid at 40° C.

3. The method for concentrating titanium dioxide waste acid according to claim 1, characterized in that: The primary cooling waste acid exchanges heat with the heat exchange medium through the first-effect membrane heater to obtain the secondary cooling waste acid at 30°C.

4. The method for concentrating titanium dioxide waste acid according to claim 1, characterized in that: The concentration of the crystallized spent acid was 36%.

5. The method for concentrating titanium dioxide waste acid according to any one of claims 1 to 4, characterized in that: The crystallized waste acid is subjected to heat exchange in a single-effect membrane heat exchanger and then further concentrated by steam heating to obtain waste acid with a concentration of 55%.

6. The method for concentrating titanium dioxide waste acid according to claim 5, characterized in that: The crystallized waste acid is converted into primary concentrated waste acid with a temperature of 18° C. and a concentration of 36% after heat exchange in a single-effect membrane heat exchanger.

7. The method for concentrating titanium dioxide waste acid according to claim 5, characterized in that: After the crystalline waste acid is fully crystallized, the crystals are removed by a plate and frame filter press.

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