A desalting and concentration system and process for waste acid in the sulfuric acid process for titanium dioxide production.

Abstract

This invention discloses a desalination and concentration system and process for waste acid in the sulfuric acid process for titanium dioxide production. The waste acid solution containing ferrous sulfite is evaporated and concentrated to a sulfuric acid concentration of 75-85%, then mixed with the waste acid from the titanium dioxide production process to a sulfuric acid concentration of 35-45%. After filtration, the ferrous sulfite solid is recovered. The waste acid after sedimentation enters a multi-effect evaporation system, where it is evaporated and concentrated to a sulfuric acid concentration of 50-65%, with most of it recycled for production. A portion of the 50-65% sulfuric acid is further concentrated to a concentration of 75-85% and mixed with the ferrous sulfite waste acid solution. Due to the common ion effect, a large amount of inorganic salts, especially sulfates, precipitate and crystallize. After crystallization and precipitation, the inorganic salts are recovered by filtration, and the supernatant enters the multi-effect evaporation system. This avoids the problem of large amounts of crystalline salt generated during the evaporation and concentration process causing blockages in system pipelines or equipment, ensuring long-term stable operation of the system.

Description

Technical Field

[0001] This invention belongs to the field of sulfuric acid process for titanium dioxide production technology, specifically relating to a desalting and concentration system and process for waste acid in the sulfuric acid process for titanium dioxide production. Background Technology

[0002] In the sulfuric acid process for producing titanium dioxide, sulfuric acid serves as an intermediate medium, participating only in the reaction and not entering the final product. Therefore, waste acid with a concentration of approximately 20-25% is discharged during the acidolysis step. This waste acid contains a large number of salt ions, including Fe... 2+ The content of MgO is 30-60 g / L, the content of TiO2 is 5-8 g / L, and it also contains a certain amount of Al. 3+ Mg 2+ and Ca 2+ Plasma.

[0003] Based on the current status of the plant, approximately 8 tons of waste acid are generated for every 1 ton of titanium dioxide produced. Due to the large volume of waste acid, direct discharge would cause significant environmental pollution and represent a serious waste of resources. To address the pollution and recycling issues of waste acid, the primary method currently is concentration followed by recycling. A typical concentration process is triple-effect evaporation. During the concentration and evaporation of waste acid, the increased acid concentration leads to supersaturation of sulfates, causing crystallization and clogging of heat exchangers and pipes. To address this clogging, many companies resort to high-pressure water jet flushing or heat exchanger replacement, significantly reducing equipment utilization and increasing maintenance costs, thus greatly increasing waste acid treatment costs. Furthermore, many companies simply stockpile the separated ferrous sulfate crystals, further wasting resources and causing environmental pollution. Summary of the Invention

[0004] To overcome the technical problem of system blockage caused by crystallization during the evaporation and concentration of by-product waste acid in the sulfuric acid process for titanium dioxide production, the present invention aims to provide a waste acid desalination and concentration system and process for the sulfuric acid process for titanium dioxide production. The system first mixes a portion of the concentrated liquid with the waste acid to increase the sulfuric acid concentration of the pre-concentrated acid. Based on the common ion effect, the solubility of ferric sulfate is reduced. After sedimentation, the solution enters the multi-effect concentration system, which can effectively prevent the problem of crystallization blockage in the concentration system.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] A desalination and concentration system for waste acid from the sulfuric acid process for titanium dioxide production includes a sedimentation system, a multi-effect evaporation system, a thin-film evaporator system, a first heater, a condenser, a first solid-liquid hydrocyclone, a second heater, and a second solid-liquid hydrocyclone.

[0007] The multi-effect evaporation system includes a first gas-liquid separator and a second gas-liquid separator;

[0008] A thin-film evaporator system includes a thin-film evaporator;

[0009] The settling system is connected to the inlet of the first heater, the outlet of the first heater is connected to the inlet of the first gas-liquid separator, and the gas phase outlet of the first gas-liquid separator is connected to the condenser. The liquid outlet of the first gas-liquid separator is connected to the inlet of the first solid-liquid hydrocyclone, the concentrated slurry outlet of the first solid-liquid hydrocyclone is connected to the settling system, and the clear liquid outlet of the first solid-liquid hydrocyclone is divided into two paths, one connected to the heater and the other connected to the heater inlet. The heater outlet is connected to the second gas-liquid separator, the gas phase outlet of the second gas-liquid separator is connected to the inlet of the first heater, the liquid outlet of the second gas-liquid separator is connected to the inlet of the second solid-liquid hydrocyclone, the concentrated slurry outlet of the second solid-liquid hydrocyclone is connected to the settling system, and the clear liquid outlet of the second solid-liquid hydrocyclone is connected to the thin-film evaporator.

[0010] Furthermore, the liners of the first and second gas-liquid separators are made of PTFE, enamel, or graphite.

[0011] Furthermore, the first and second solid-liquid hydrocyclones are hydrocyclones for solid-liquid separation.

[0012] Furthermore, the thin-film evaporator is made of tantalum or zirconium metal, or carbon steel lined with enamel. The scrapers inside the thin-film evaporator are made of tantalum, zirconium, or PEEK.

[0013] Furthermore, a first forced circulation pump is installed between the clear liquid outlet of the first solid-liquid hydrocyclone and the heater.

[0014] Furthermore, the heater is also connected to a second forced circulation pump.

[0015] Furthermore, the heater is equipped with a heating steam inlet.

[0016] Furthermore, the thin-film evaporation system also includes a condenser, with the vapor phase outlet of the thin-film evaporator connected to the condenser.

[0017] Furthermore, the concentrate outlet of the thin-film evaporator is connected to the sedimentation system.

[0018] A method for desalting and concentrating waste acid during the sulfuric acid process for producing titanium dioxide includes the following steps:

[0019] The waste acid solution containing ferrous sulfite is evaporated and concentrated to a sulfuric acid concentration of 75-85%. Then it is mixed with the waste acid from the sulfuric acid process for titanium dioxide production to be evaporated to a sulfuric acid concentration of 35-45%. After filtration, the ferrous sulfite solid is recovered. The waste acid after settling in the precipitation system enters the multi-effect evaporation system and is evaporated and concentrated to a sulfuric acid concentration of 50-65%. Most of it is recycled for production. A portion of the sulfuric acid with a concentration of 50-65% is further concentrated to a concentration of 75-85% and mixed with the waste acid solution containing ferrous sulfite.

[0020] Compared with the prior art, the present invention has the following advantages:

[0021] This invention utilizes the high-concentration sulfuric acid obtained from multi-effect evaporation and concentration, and pretreated saline waste acid to increase the acid concentration. Based on the common ion effect, a large amount of inorganic salts, especially sulfates, precipitate and crystallize. After crystallization and precipitation, a large amount of inorganic salts settle to the bottom of the precipitation tank (pool). After filtration and recovery of inorganic salts, the supernatant enters the multi-effect evaporation system, thereby reducing the problem of system pipeline or equipment blockage caused by a large amount of crystallized salt generated during the evaporation and concentration process.

[0022] The waste acid desalination and concentration system in the sulfuric acid process for titanium dioxide production of this invention can employ double-effect, triple-effect, or multi-effect evaporation and concentration, preferably double-effect or triple-effect evaporation and concentration. To improve evaporation efficiency, after multi-effect evaporation, the sulfuric acid can be further concentrated to over 80% using a thin-film evaporator. Then, a portion of the concentrated acid solution is recycled and mixed with pretreated waste acid to increase the acid concentration. Alternatively, the concentrate from the last effect of multi-effect evaporation can be recycled. Thin-film evaporators have higher efficiency and better anti-fouling performance when concentrating higher concentrations of hydrochloric acid, and can be used to further increase the acid concentration. In this system, if the evaporation concentration is >80%, the condensate from the evaporation process is acidic. In this case, the condensate can be directly introduced into the multi-effect evaporation system to facilitate the recovery of acid from the condensate and avoid environmental pollution caused by acidic wastewater discharge. Attached Figure Description

[0023] Figure 1 This is a flowchart of the sulfuric acid process for desalting and concentrating waste acid from titanium dioxide production according to the present invention.

[0024] In the figure, 1 is a settling tank, 2 is the first forced circulation pump, 3 is the first heater, 4 is the first gas-liquid separator, 5 is a condenser, 6 is the first solid-liquid hydrocyclone, 7 is the second forced circulation pump, 8 is the second heater, 9 is the second gas-liquid separator, 10 is the second solid-liquid hydrocyclone, 11 is the thin film evaporator, and 12 is the condenser. Detailed Implementation

[0025] To facilitate understanding of the present invention, a more complete description will be given below with reference to the accompanying drawings. Preferred embodiments of the invention are shown in the drawings. However, the invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of the invention.

[0026] Furthermore, elements in this invention are referred to as being "fixed to" or "set on" another element, which may be directly on the other element or may also include an intervening element. When an element is considered to be "connected" to another element, it may be directly connected to the other element or may also include an intervening element. The terms "vertical," "horizontal," "left," "right," and similar expressions used herein are for illustrative purposes only and do not represent the only possible implementations.

[0027] The multi-effect evaporation system used in the desalination and concentration process of waste acid in the sulfuric acid process for producing titanium dioxide according to the present invention is illustrated by taking double-effect evaporation as an example.

[0028] See Figure 1 A desalting and concentration system for waste acid in the sulfuric acid process for producing titanium dioxide includes a sedimentation system, a multi-effect evaporation system, a thin-film evaporator system, a first heater 3, a condenser 5, a first solid-liquid hydrocyclone 6, a second heater 8, and a second solid-liquid hydrocyclone 10.

[0029] The settling system includes settling tank 1;

[0030] The multi-effect evaporation system includes a first gas-liquid separator 4 and a second gas-liquid separator 9; the first gas-liquid separator 4 and the second gas-liquid separator 9 are lined with PTFE or enamel, or have a graphite structure to prevent acid corrosion.

[0031] Each effect of the multi-effect evaporation system employs a forced circulation pump to increase the flow rate within the heater heat exchange tubes, preventing salt crystallization. Each effect is equipped with a hydrocyclone to facilitate the separation of small amounts of crystallized material produced during the evaporation process. The separated concentrate enters a settling tank for crystallization and sedimentation, while the clear liquid from the hydrocyclone flows into the next effect. After multi-effect evaporation, the waste acid is finally concentrated to 50–65% (wt), achieving reuse. The condensate from the multi-effect evaporation can be used to generate process water or makeup water for the circulating water system.

[0032] The first solid-liquid hydrocyclone 6 and the second solid-liquid hydrocyclone 10 are hydrocyclones for solid-liquid separation.

[0033] The thin-film evaporator system includes a thin-film evaporator 11, a condenser 12, a first forced circulation pump 2, and a second forced circulation pump 7. A portion of the sulfuric acid concentrated by the multi-effect evaporation system enters the thin-film evaporator system to further increase the concentration of the concentrated acid to 75-85%, with the aim of increasing the concentration of the acid to be concentrated.

[0034] The multi-effect evaporation heater of the multi-effect evaporation system uses a graphite heat exchanger and silicon carbide material.

[0035] The thin-film evaporator 11 is made of tantalum metal or zirconium metal, or lined with enamel. The scraper 3 used in the internal components of the thin-film evaporator 11 is made of tantalum metal, zirconium metal, PEEK, or other sulfuric acid resistant materials.

[0036] Waste acid is fed directly into settling tank 1. Settling tank 1 is connected to the inlet of first heater 3. The supernatant from settling tank 1 is discharged into first heater 3. The outlet of first heater 3 is connected to the inlet of first gas-liquid separator 4. The gas phase outlet of first gas-liquid separator 4 is connected to condenser 5. The liquid outlet of the first gas-liquid separator 4 is connected to the inlet of the first solid-liquid hydrocyclone 6. The slurry outlet of the first solid-liquid hydrocyclone 6 is connected to the settling tank 1. The clear liquid outlet of the first solid-liquid hydrocyclone 6 is divided into two paths: one path is connected to the heater 3 via the first forced circulation pump 2, and the other path is connected to the inlet of the heater 8. The second forced circulation pump 7 is also connected to the inlet of the heater 8. The heater 8 is equipped with a heating steam inlet. The outlet of the heater 8 is connected to the second gas-liquid separator 9. The gas phase outlet of the second gas-liquid separator 9 is connected to the shell inlet of the first heater 3. The liquid outlet of the second gas-liquid separator 9 is connected to the inlet of the second solid-liquid hydrocyclone 10. The slurry outlet of the second solid-liquid hydrocyclone 10 is connected to the settling tank 1 via a valve. The shell side of the second heater 8 uses steam from the utility as the heating source. The clear liquid outlet of the second solid-liquid hydrocyclone 10 is connected to the thin film evaporator 11. One stream of the clear liquid from the second solid-liquid hydrocyclone 10 is recycled as sulfuric acid in the production process, and the other stream enters the thin film evaporator 11 for further concentration, concentrating the sulfuric acid to a mass concentration of 75-85%. The concentrated liquid outlet of the thin film evaporator 11 is connected to the settling tank 1. The concentrated sulfuric acid is directly returned to the settling tank 1 and mixed with the waste acid to be concentrated. After the waste acid to be concentrated and the concentrated sulfuric acid are mixed, their concentration is ensured to be 35-45% (wt) so as to minimize the solubility of inorganic salts in the waste acid. The gas phase outlet of the thin film evaporator 11 is connected to the condenser 12.

[0037] The multi-effect evaporation system used in this invention can be an MVR evaporation system. MVR evaporation also uses a forced circulation evaporation system. After discharge, the concentrate is equipped with a solid-liquid hydrocyclone to separate the solid and liquid components of the concentrate.

[0038] A multi-effect evaporation system can employ either co-current or counter-current heating. A multi-effect counter-current evaporation system is preferred, where steam enters from the last-effect heater, and the evaporated steam is condensed in the first-effect condenser.

[0039] The present invention discloses a desalting and concentration process for waste acid in the sulfuric acid process for producing titanium dioxide, comprising the following steps:

[0040] 1) First, the waste acid from titanium dioxide is evaporated and concentrated to a sulfuric acid concentration of 80%. Then, it is mixed in a settling tank 1 until the sulfuric acid concentration is 35-45% to minimize the solubility of inorganic salts in the waste acid. Experimental studies show that when the sulfuric acid concentration is greater than 35%, the solubility of ferric sulfate and other impurities in the waste acid is less than 5%, and the solubility of ferric sulfate and other impurities decreases significantly with increasing temperature and sulfuric acid concentration. Therefore, after mixing, the waste acid solution is settled in settling tank 1 and then filtered to recover the ferrous sulfite solids. The waste acid after settling in settling tank 1 enters a double-effect evaporation device (comprising a first gas-liquid separator 4 and a second gas-liquid separator 9) for further evaporation and concentration. Since a large number of inorganic salt ions in the waste acid solution have been removed by sedimentation, even if a small amount of inorganic salt crystallizes during the subsequent concentration process, it will not cause crystallization in the pipeline.

[0041] 2) The double-effect evaporation process uses highly efficient concentration of concentrated sulfuric acid entering the double-effect evaporation. During the evaporation and concentration process, a small amount of inorganic salt ions crystallize and precipitate. Each effect is equipped with a bottom gas-liquid separator to discharge a small amount of concentrated slurry into the settling tank 1, which can effectively prevent the evaporation and concentration system from being blocked due to the crystallization and accumulation of a small amount of crystals in the system.

[0042] 3) By using double-effect evaporation, the mass concentration of waste acid can be increased to 50-65%, which can be reused in the production process and realize resource utilization.

[0043] 4) In order to reduce the amount of recycling and meet different production needs, it is necessary to further concentrate some of the concentrated sulfuric acid with a mass concentration of 50-65% to a mass concentration of 75-85%. The concentrated sulfuric acid can be reused in the production process, and part of it is used for recycling and mixing with waste acid with a mass concentration of about 20% to ensure that the concentration of the mixed sulfuric acid is 35-45%.

[0044] The multi-effect evaporation process used in this invention employs a forced circulation evaporation process. A solid-liquid hydrocyclone 10 is installed before the forced circulation pump to separate the small amount of salt crystallized from each effect. This salt is then discharged to the pretreatment sedimentation tank 1 (or sedimentation pond) and crystallized and precipitated together with the inorganic salts in the original pretreatment waste acid.

[0045] In the multi-effect evaporation system of this invention, any material that comes into contact with acid can be made of corrosion-resistant silicon carbide or graphite, or metal tan or metal zirconium.

[0046] The innovations of this invention are as follows:

[0047] 1. By mixing partially refluxed concentrated sulfuric acid with sulfuric acid to be concentrated, the concentration of sulfuric acid is increased to more than 35%, which crystallizes and separates most of the salt ions in the waste acid, preventing blockage of heat exchange tubes and pipes caused by salt ions during the subsequent concentration process in multi-effect heat exchangers.

[0048] 2. The multi-effect evaporation of this invention uses a forced circulation pump to avoid blockage of the heater heat exchange tubes due to crystallization. In addition, a solid-liquid hydrocyclone is added to each effect to separate the small amount of inorganic salt particles crystallized in each effect, so as not to affect the subsequent crystallization evaporation system.

[0049] 3. In order to reduce the circulation volume of the mixed acid after the multi-effect evaporation system, the acid concentration needs to be further increased. Due to the increased boiling point and the presence of salt ions under high-concentration acid conditions, this system uses a thin-film evaporator 11 for concentration in order to achieve better concentration. The thin-film evaporator 11 uses a scraper, which effectively prevents the problem of low efficiency and easy crystallization and blockage of high-concentration acid due to inorganic salts using ordinary heat exchangers while performing forced evaporation.

[0050] 4. In this invention, waste acid is first mixed with a portion of the concentrated solution to increase the sulfuric acid concentration of the pre-concentrated waste acid. Based on the common ion effect, the solubility of inorganic salts (mainly ferrous sulfite) is reduced. The inorganic salts crystallize and precipitate in the settling tank. After further settling and separation, the supernatant enters the multi-effect concentration system. Due to the large amount of inorganic salts undergoing crystallization, a small amount of salt ions are concentrated into crystal particles. In each crystallization stage, a cyclone solid-liquid separator is installed after a transfer pump to return the small amount of crystals from each stage to the settling tank for further settling and separation. A portion of the finally concentrated sulfuric acid is then returned to the thin-film evaporator for further concentration to 75-85%. The concentrated solution is directly recycled back to the pretreatment system to increase the concentration of the waste acid to be concentrated. This process effectively prevents crystallization blockage in the concentration system, ensuring long-term stable operation of the system.

[0051] The above description is only of the preferred embodiment of the present invention and should not be construed as limiting the scope of the claims. The present invention is not limited to the above embodiments, and variations in its specific structure are permitted. All variations made within the scope of the independent claims of the present invention are also within the scope of protection of the present invention.

[0052] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

Claims

1. A desalting and concentration system for waste acid during the sulfuric acid process for titanium dioxide production, characterized in that, It includes a sedimentation system, a multi-effect evaporation system, a thin-film evaporator system, a first heater (3), a first condenser (5), a first solid-liquid cyclone separator (6), a second heater (8), and a second solid-liquid cyclone separator (10); The multi-effect evaporation system includes a first gas-liquid separator (4) and a second gas-liquid separator (9); The thin-film evaporator system includes a thin-film evaporator (11); The settling system is connected to the inlet of the first heater (3), the outlet of the first heater (3) is connected to the inlet of the first gas-liquid separator (4), and the gas phase outlet of the first gas-liquid separator (4) is connected to the first condenser (5). The liquid outlet of the first gas-liquid separator (4) is connected to the inlet of the first solid-liquid hydrocyclone (6). The slurry outlet of the first solid-liquid hydrocyclone (6) is connected to the sedimentation system. The clear liquid outlet of the first solid-liquid hydrocyclone (6) is divided into two paths: one path is connected to the first heater (3), and the other path is connected to the inlet of the second heater (8). The outlet of the second heater (8) is connected to the second gas-liquid separator (9). The gas phase outlet of the second gas-liquid separator (9) is connected to the inlet of the first heater (3). The liquid outlet of the second gas-liquid separator (9) is connected to the inlet of the second solid-liquid hydrocyclone (10). The slurry outlet of the second solid-liquid hydrocyclone (10) is connected to the sedimentation system. The clear liquid outlet of the second solid-liquid hydrocyclone (10) is connected to the thin film evaporator (11), and the concentrated liquid outlet of the thin film evaporator (11) is connected to the sedimentation system. The titanium dioxide waste acid solution containing ferrous sulfate is evaporated and concentrated to a sulfuric acid mass concentration of 75-85% through a multi-effect evaporation system and a thin-film evaporator system. Then, it is mixed with the titanium dioxide waste acid to be evaporated in a sedimentation system until the sulfuric acid mass concentration is 35-45%. After filtration, the ferrous sulfate solid is recovered.

2. The desalting and concentration system for waste acid in the sulfuric acid process for titanium dioxide production according to claim 1, characterized in that, The liners of the first gas-liquid separator (4) and the second gas-liquid separator (9) are polytetrafluoroethylene, enamel or graphite.

3. The desalting and concentration system for waste acid in the sulfuric acid process for titanium dioxide production according to claim 1, characterized in that, The thin film evaporator (11) is made of tantalum metal, zirconium metal, or carbon steel lined with enamel. The scraper inside the thin film evaporator (11) is made of tantalum, zirconium, or PEEK.

4. The desalting and concentration system for waste acid in the sulfuric acid process for titanium dioxide production according to claim 1, characterized in that, A first forced circulation pump (2) is provided between the clear liquid outlet of the first solid-liquid hydrocyclone (6) and the first heater (3).

5. The desalting and concentration system for waste acid in the sulfuric acid process for titanium dioxide production according to claim 1, characterized in that, The second heater (8) is also connected to a second forced circulation pump (7).

6. The desalting and concentration system for waste acid in the sulfuric acid process for titanium dioxide production according to claim 1, characterized in that, The second heater (8) is provided with a heating steam inlet.

7. The desalting and concentration system for waste acid in the sulfuric acid process for titanium dioxide production according to claim 1, characterized in that, The thin-film evaporator system also includes a second condenser (12), with the vapor phase outlet of the thin-film evaporator (11) connected to the second condenser (12).

8. A method for desalting and concentrating waste acid in the sulfuric acid process for producing titanium dioxide based on the desalting and concentration system of claim 1, characterized in that, Includes the following steps: The waste acid solution containing ferrous sulfate from titanium dioxide is evaporated and concentrated to a sulfuric acid concentration of 75-85% using a multi-effect evaporator system and a thin-film evaporator system. Then, it is mixed with the waste acid from titanium dioxide to be evaporated in a sedimentation system until the sulfuric acid concentration is 35-45%. After filtration, the ferrous sulfate solid is recovered. The waste acid after sedimentation in the sedimentation system enters the multi-effect evaporator system for evaporation and concentration to a sulfuric acid concentration of 50-65%. Most of it is recycled for production. A portion of the sulfuric acid with a concentration of 50-65% is sent to the thin-film evaporator system for further concentration to a concentration of 75-85%, and then mixed with the waste acid solution containing ferrous sulfate from titanium dioxide.

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