A concentrated system and method of waste sulfuric acid in the production of titanium dioxide by the sulfuric acid method

Abstract

This invention discloses a system and method for concentrating waste acid from the sulfuric acid process for titanium dioxide production. The system includes a preheating device, an MVR (Medium-Voltage Reduction) heating and evaporation device, a cooling and curing device, and a vacuum device. The preheating device is connected to the MVR heating and evaporation device, and the vacuum device is also connected to the MVR heating and evaporation device. The MVR heating and evaporation device includes a heater, an evaporator, a steam compressor, and a first circulating pump. The heater is connected to the first circulating pump, the evaporator, and the cooling and curing device. The circulation ports of both the preheating device and the evaporator are connected to the first circulating pump. The evaporator is connected to the steam compressor, which is connected to the heater. The heater is connected to a condensate tank. This invention repressurizes and reheats secondary steam as a heat source for the heater, and recycles the condensate generated during production for preheating the waste acid. The entire production process is short and cost-effective, effectively achieving the production goals of energy conservation, emission reduction, cost reduction, efficiency improvement, and environmental protection.

Description

Technical Field

[0001] This invention relates to the field of sulfuric acid concentration production technology, specifically to a sulfuric acid process for concentrating waste acid from titanium dioxide production. Background Technology

[0002] The sulfuric acid process for titanium dioxide production generates waste acid with a concentration of 20%–25%, producing 6–7 tons of waste acid for every ton of titanium dioxide produced. In the titanium dioxide industry, the main methods for waste acid treatment fall into two categories: simple neutralization and concentration followed by recycling. Simple neutralization produces a large amount of gypsum, which is technically challenging to reuse, has an immature process, and low economic efficiency, resulting in resource waste and potential secondary pollution. Concentration followed by recycling commonly employs single-effect, double-effect, and multi-effect waste acid concentration technologies. However, titanium dioxide waste acid has a high water content, many impurities, and strong corrosiveness, requiring sophisticated equipment and consuming large amounts of energy. This leads to high investment and operating costs, resulting in unsatisfactory overall performance and severely hindering the further industrial application of this method. Therefore, there is an urgent need for an environmentally friendly, economically efficient, and energy-saving production process. Summary of the Invention

[0003] The main objective of this invention is to overcome the deficiencies of the prior art and provide a sulfuric acid waste acid concentration system and method for titanium dioxide production.

[0004] To achieve the above objectives, the present invention proposes a sulfuric acid process titanium dioxide waste acid concentration system, comprising a preheating device, an MVR heating and evaporation device, a cooling and ripening device, and a vacuum device. The preheating inlet of the preheating device is connected to the waste acid pipeline to be treated, the preheating outlet of the preheating device is connected to the feed inlet of the MVR heating and evaporation device, the discharge outlet of the MVR heating and evaporation device is connected to the inlet of the cooling and ripening device, and the vacuum device is connected to the MVR heating and evaporation device and provides a vacuum working environment for the MVR heating and evaporation device. The MVR heating and evaporation device includes a heater, The system comprises an evaporator, a steam compressor, and a first circulating pump. The inlet of the heater is connected to the outlet of the first circulating pump, and the outlet of the heater is connected to the inlet of the evaporator. The outlet of the evaporator is connected to the cooling and ripening equipment. The preheating outlet of the preheating equipment and the circulation port of the evaporator are both connected to the inlet of the first circulating pump. The gas-liquid outlet of the evaporator is connected to the inlet of the steam compressor. The outlet of the steam compressor and an external live steam pipeline are both connected to the heat medium inlet of the heater. The cold medium outlet of the heater is connected to the condensate tank. The secondary steam generated during the MVR heating and evaporation process is repressurized and heated by the steam compressor before being sent to the heater as a heat source for recycling, effectively reducing steam energy consumption. Producing 1 ton of 50% acid consumes only about 0.13 tons of steam, effectively reducing production costs. Forced circulation heating and evaporation via the first circulating pump solves the problem of heater clogging and effectively improves system operating efficiency.

[0005] Furthermore, the sulfuric acid process titanium dioxide waste acid concentration system also includes an alkaline washing device. This device is connected in series on the gas-liquid outlet of the evaporator and the inlet pipe of the steam compressor. The alkaline washing device includes an alkaline washing tower and a first gas-liquid separator. The cleaning inlet of the alkaline washing tower is connected to the gas-liquid outlet of the evaporator, and the cleaning outlet of the alkaline washing tower is connected to the inlet of the first gas-liquid separator. The steam outlet of the first gas-liquid separator is connected to the inlet of the steam compressor, and the liquid outlet of the first gas-liquid separator is connected to a liquid seal tank. The inlet and outlet of the cleaning liquid from the alkaline washing tower are both connected to an alkaline water tank and circulated through a spray pump for alkaline washing. The alkaline water tank is connected to the live steam pipeline. Since the secondary steam is acidic, directly recompressing and reusing it without treatment can easily cause equipment corrosion. Washing the acidic secondary steam with the alkaline washing device transforms it into neutral secondary steam, preventing equipment corrosion and extending equipment lifespan. During alkaline washing, the temperature of the washing alkaline water is maintained at a high temperature of 85–95°C, which does not affect the temperature of the secondary steam and effectively improves energy utilization.

[0006] Furthermore, the vacuum equipment includes a vacuum pump, a vacuum tank, a second gas-liquid separator, and a condenser. The suction port of the vacuum pump is connected to the outlet of the vacuum tank, the inlet of the vacuum tank is connected to the gas outlet of the second gas-liquid separator, the liquid outlet of the second gas-liquid separator is connected to the liquid seal tank, the inlet of the second gas-liquid separator is connected to the gas outlet of the condenser, the gas inlet of the condenser is connected to both the heater and the condensate tank, and the liquid outlet of the condenser is connected to the liquid seal tank. The vacuum equipment maintains the vacuum level of the entire system within the range of 50–80 kPa, and vacuum balance ensures the stability of system operation.

[0007] Furthermore, the preheating equipment includes a hot water tank, a preheating tank, a hot water preheater, a steam preheater, and a second circulation pump. The inlet of the hot water preheater is connected to the pipeline containing the waste acid to be treated, the outlet of the hot water preheater is connected to the inlet of the preheating tank, the circulation port of the preheating tank is connected to the inlet of the second circulation pump, the outlet of the second circulation pump is connected to the inlet of the steam preheater, the outlet of the steam preheater is connected to the inlet of the preheating tank, the heat medium inlet of the steam preheater is connected to the live steam pipeline, the condensate outlet of the steam preheater and the outlet of the condensate tank are both connected to the hot water tank, and the outlet of the hot water tank is connected to the heat medium inlet of the hot water preheater. By collecting the condensate generated during system operation into the hot water tank and using it as a heat source for the hot water preheater to preheat the waste acid, energy utilization efficiency is increased and energy consumption is reduced.

[0008] Furthermore, a mixing tank is connected to the pipeline containing the waste acid to be treated, and the outlet of the evaporator is connected to the mixing tank via a circulation pipeline. By diverting the concentrated high-temperature 50% waste acid into the mixing tank and mixing it with 20%–25% waste acid, the concentration of the waste acid before entering the preheating equipment is increased. This allows ferrous sulfate monohydrate and calcium sulfate to precipitate as large solid crystals before entering the MVR heating evaporation equipment, preventing the slow precipitation of metal salts in the waste acid and their adhesion to the inner wall of the hot water preheater pipeline, thus avoiding scale formation and affecting heat exchange efficiency.

[0009] Furthermore, a branch pipe is connected to the circulation pipeline, which leads to the preheating tank. The bottom of the preheating tank is connected to a filtration device. After the waste acid enters the preheating tank, under high temperature and high concentration conditions, a large amount of ferrous sulfate monohydrate and calcium sulfate are further precipitated, along with metatitanic acid crystals. The crystalline solid particles settle at the bottom of the preheating tank and are then extracted to the filtration device for solid-liquid separation. In the continuous concentration process of waste acid, scaling on the inner wall of the pipeline can be effectively avoided, eliminating the need for shutdown for descaling and maintenance, and effectively improving operating efficiency.

[0010] This invention also proposes a method for concentrating waste acid from the sulfuric acid process for titanium dioxide production. The method utilizes the aforementioned waste acid concentration system for the sulfuric acid process to concentrate 20%–25% of the waste acid, comprising the following steps:

[0011] S1. Start the MVR heating and evaporation equipment and vacuum equipment. Pass the waste acid to be treated into the hot water preheater and steam preheater of the preheating equipment in sequence for preheating. The waste acid is preheated by circulating in the steam preheater and preheating tank through the second circulation pump until the preheating temperature reaches the set temperature.

[0012] S2. The waste acid that has reached the set temperature is sent to the heater for heating through the discharge pump. The heated waste acid is then sent to the evaporator for evaporation and concentration. During evaporation and concentration, the waste acid is circulated and heated through the first circulation pump until the concentration of the waste acid reaches 50-55% of the process requirements. Then it is sent to the cooling and maturation equipment to cool the waste acid at a temperature of 100-105℃ to below 55℃. The matured and cooled waste acid is then sent back to the titanium dioxide production leaching process for recycling.

[0013] S3. In the initial stage of start-up and operation, the steam preheater and heater need to continuously supply saturated live steam through the live steam pipeline to heat the waste acid. The condensate produced by the steam preheater and heater enters the hot water tank and is used to preheat the waste acid in the hot water preheater.

[0014] S4. During the start-up and operation, the secondary acidic steam generated by the evaporator is introduced into the alkaline washing tower and sprayed with alkaline water at a temperature of 85-95℃ and a pH value of 8-12. After being separated by the first gas-liquid separator, the resulting neutral steam enters the steam compressor for pressurization and heating, and is then sent to the heater to heat the waste acid.

[0015] S5. During the start-up process, the non-condensable gas generated by the heater and the condensate tank is drawn to the condenser for cooling. The cooled gas enters the second gas-liquid separator and the vacuum tank in sequence and is discharged through the exhaust port of the vacuum pump. The condensate generated in the condenser, the second gas-liquid separator and the first gas-liquid separator enters the liquid seal tank.

[0016] S6. During startup and operation, once the system's feed rate, moisture evaporation rate, and discharge rate are balanced, and the system's pressure, temperature, and discharge concentration are stable, gradually close the steam valves that allow saturated live steam to enter the steam preheater and heater through the live steam pipeline.

[0017] Further, in step S6, the system's waste acid feed rate is 5-6 t / h, water evaporation rate is 3 t / h, waste acid discharge rate is 2.5-3.0 t / h, vacuum degree is 50-80 kPa, waste acid discharge concentration is 50%-55%, waste acid feed temperature is 25℃, waste acid discharge temperature is below 55℃, hot water preheating temperature is 40℃-55℃, preheating tank control temperature is 75℃-85℃, and the operating liquid temperature of the MVR heating evaporation equipment is 100℃-105℃.

[0018] Furthermore, during the start-up process, a portion of the waste acid with a concentration of 50% to 55% that comes out of the evaporator is sent back to the mixing tank through a circulation pipeline to mix with the waste acid with a concentration of 20% to 25%, so that the concentration of waste acid is adjusted to 33%, and some calcium sulfate and ferrous sulfate monohydrate solid large particle crystals are precipitated. The solid large particle crystals mixed with the waste acid are sent to the hot water preheater for preheating.

[0019] Furthermore, during the start-up process, waste acid with a concentration of 50% to 55% in the circulation pipeline is sent into the preheating tank through the diversion pipeline. The concentration of waste acid in the preheating tank is adjusted to 38%, and the precipitate at the bottom of the preheating tank is led out to the filtration equipment to filter out large solid crystals.

[0020] The beneficial effects of this invention include: by circulating the secondary steam generated during the heating and evaporation process within the MVR heating and evaporation equipment, which is then pumped to a steam compressor for repressurization and reheating, it is recycled as a heat source for the heater. The forced circulation action of the first circulation pump effectively solves the problem of heater blockage. By recovering the condensate generated during the production process of the MVR heating and evaporation equipment and using it to preheat waste acid, the steam consumption is effectively reduced, with only about 0.13 tons of steam consumed per ton of 50% acid produced. Moreover, the entire system has a short process flow, low power consumption, and low production cost, effectively achieving the goals of energy conservation, emission reduction, cost reduction, efficiency improvement, and environmental protection. Attached Figure Description

[0021] Figure 1 This is a process flow diagram of the sulfuric acid waste acid concentration system for titanium dioxide production in this embodiment of the invention.

[0022] Figure 2 This is a flowchart of the sulfuric acid process for concentrating waste acid from titanium dioxide in an embodiment of the present invention.

[0023] Figure reference numerals: 1 Cooling and maturation equipment; 2 Waste acid pipeline to be treated; 3 Heater; 4 Evaporator; 5 Steam compressor; 6 First circulation pump; 7 Live steam pipeline; 8 Condensate tank; 9 Alkali washing tower; 10 First gas-liquid separator; 11 Liquid seal tank; 12 Alkali water tank; 13 Spray pump; 14 Vacuum pump; 15 Vacuum tank; 16 Second gas-liquid separator; 17 Condenser; 18 Hot water tank; 19 Preheating tank; 20 Hot water preheater; 21 Steam preheater; 22 Second circulation pump; 23 Circulation pipeline; 24 Filtration equipment; 25 Diversion pipeline; 26 Mixing tank; 27 Hot water pump; 28 Feed pump; 29 Discharge pump; 30 High-pressure pump; 31 Finished product pump. Detailed Implementation

[0024] To make the technical problems, technical solutions, and beneficial effects of the embodiments of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the present invention and are not intended to limit the present invention.

[0025] It should be noted that when a component is referred to as "fixed to" or "set on" another component, it can be directly on or indirectly on that other component. When a component is referred to as "connected to" another component, it can be directly connected to or indirectly connected to that other component. Furthermore, a connection can be for both fixing and circuit connection purposes.

[0026] It should be understood that the terms "length", "width", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", and "outer" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing the embodiments of the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the present invention.

[0027] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of embodiments of the present invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0028] Example 1

[0029] Please see Figure 1 and Figure 2The sulfuric acid process titanium dioxide waste acid concentration system disclosed in this invention includes a preheating device, an MVR heating and evaporation device, a cooling and ripening device 1, and a vacuum device. The preheating inlet of the preheating device is connected to the waste acid pipeline 2 to be treated, and is used to pump 20% to 25% of the waste acid into the preheating device at a certain flow rate for preheating. The waste acid with a feed temperature of 25°C reaches a temperature of 75°C to 85°C after two stages of preheating. The preheating outlet of the preheating device is connected to the feed inlet of the MVR heating and evaporation device, and the preheated waste acid is sent to the MVR heating and evaporation device for heating and evaporation for concentration. The discharge outlet of the MVR heating and evaporation device is connected to the inlet of the cooling and ripening device 1, and the concentrated waste acid is sent to the cooling and ripening device 1 for cooling and ripening before being sent back to the titanium dioxide production leaching process. The process involves recycling and reuse. The vacuum equipment is connected to the MVR heating and evaporation equipment and provides a vacuum working environment of 50-80 kPa for the MVR heating and evaporation equipment. The MVR heating and evaporation equipment includes a heater 3, an evaporator 4, a steam compressor 5, and a first circulation pump 6. The inlet of the heater 3 is connected to the outlet of the first circulation pump 6, and the outlet of the heater 3 is connected to the inlet of the evaporator 4. The outlet of the evaporator 4 is connected to the cooling and ripening equipment 1. The preheating outlet of the preheating equipment and the circulation port of the evaporator 4 are both connected to the inlet of the first circulation pump 6. The gas-liquid outlet of the evaporator 4 is connected to the inlet of the steam compressor 5. The outlet of the steam compressor 5 and the external live steam pipeline 7 are both connected to the heat medium inlet of the heater 3. The cold medium outlet of heater 3 is connected to the condensate tank 8. Specifically, the vacuum equipment includes a vacuum pump 14, a vacuum tank 15, a second gas-liquid separator 16, and a condenser 17. The suction port of the vacuum pump 14 is connected to the outlet of the vacuum tank 15, the inlet of the vacuum tank 15 is connected to the gas outlet of the second gas-liquid separator 16, the liquid outlet of the second gas-liquid separator 16 is connected to the liquid seal tank 11, the inlet of the second gas-liquid separator 16 is connected to the gas outlet of the condenser 17, the gas inlet of the condenser 17 is connected to both heater 3 and condensate tank 8, and the liquid outlet of the condenser 17 is connected to the liquid seal tank 11. The non-condensable gases generated in heater 3 and condensate tank 8 are cooled by the condenser 17 and then separated by the second gas-liquid separator 16, allowing the waste acid to reach its target temperature. Upon reaching boiling point, the gas enters the vacuum tank 15 for buffering and then exits through the exhaust port of the vacuum pump 14. During this process, to ensure system vacuum balance, the generated condensate enters the liquid seal tank 11, ensuring the system vacuum level remains stable within the range of 50–80 kPa. The preheating equipment includes a hot water tank 18, a preheating tank 19, a hot water preheater 20, a steam preheater 21, and a second circulation pump 22. The inlet of the hot water preheater 20 is connected to the waste acid pipeline 2 to be treated, and the outlet of the hot water preheater 20 is connected to the inlet of the preheating tank 19. The circulation port of the preheating tank 19 is connected to the inlet of the second circulation pump 22, the outlet of the second circulation pump 22 is connected to the inlet of the steam preheater 21, and the outlet of the steam preheater 21 is connected to the inlet of the preheating tank 19.The heat medium inlet of the steam preheater 21 is connected to the live steam pipeline 7. The condensate outlet of the steam preheater 21 and the condensate outlet of the condensate tank 8 are both connected to the hot water tank 18. The outlet of the hot water tank 18 is connected to the heat medium inlet of the hot water preheater 20. The hot water tank 18 collects the condensed hot water generated during the heating of waste acid by the steam preheater 21 and the heater 3. The collected condensed hot water has a temperature of 60-70℃. Then, the hot water is pumped into the hot water preheater 20 by the hot water pump 27 to perform the first stage of preheating of the waste acid. After the waste acid is heated to 40-50℃ in the hot water preheater 20, it enters the preheating tank 19. The waste acid in the preheating tank 19 is pumped into the steam preheater 21 by the second circulation pump 22 to perform the second stage of preheating of the waste acid. When the temperature of the liquid reaches close to the boiling point of 50% dilute sulfuric acid, it is pumped into the inlet of the first circulation pump 6 of the MVR heating and evaporation equipment by the feed pump 28 and enters the heater 3 for further heating. In this process, the waste acid is heated and evaporated through heater 3 and evaporator 4 under the forced circulation action of the first circulation pump 6 until the concentration of the liquid reaches 50%. Then, it is pumped into the cooling and ripening equipment 1 through the discharge pump 29. In this embodiment, the secondary steam generated during the circulating heating and evaporation process of the waste acid in the MVR heating and evaporation equipment is drawn to the steam compressor 5 for repressurization and heating, and then used as the heat source for heater 3 for recycling. The steam compressor 5 is a three-stage steam compressor; after pressurizing and heating the secondary steam, it is used as the heat source for heater 3 for recycling. Only about 0.13 tons of steam are consumed for producing 1 ton of 50% acid. The forced circulation action of the first circulation pump 6 effectively solves the problem of easy clogging of heater 3. Moreover, the entire system has a short process flow, low power consumption, and low production cost, effectively achieving the goals of energy saving, emission reduction, cost reduction and efficiency improvement, and environmental protection.

[0030] Example 2

[0031] Please continue reading. Figure 1 and Figure 2Since the secondary steam generated during the internal circulation heating and evaporation process of the MVR heating and evaporation equipment is acidic steam, in order to avoid the acidic secondary steam corroding the steam compressor 5 and affecting the working efficiency of the steam compressor 5, this embodiment adds an alkaline washing device based on embodiment 1. The alkaline washing device is connected in series on the gas-liquid outlet of the evaporator 4 and the inlet pipe of the steam compressor 5. The alkaline washing device includes an alkaline washing tower 9 and a first gas-liquid separator 10. The cleaning inlet of the alkaline washing tower 9 is connected to the gas-liquid outlet of the evaporator 4, the cleaning outlet of the alkaline washing tower 9 is connected to the inlet of the first gas-liquid separator 10, the steam outlet of the first gas-liquid separator 10 is connected to the inlet of the steam compressor 5, and the liquid outlet of the first gas-liquid separator 10 is connected to the liquid seal tank 11. The cleaning liquid inlet and outlet of the alkaline washing tower 9 are both connected to the alkaline water tank 12 and are circulated through the spray pump 13 for alkaline washing. The alkaline water tank 12 is connected to the live steam pipeline 7. The temperature and pH of the alkaline water tank 12 are controlled by an interlocking system to ensure that the temperature of the liquid alkali is controlled at 85-95℃ and the pH is controlled at 8-12. Then, the alkaline water is pumped into the alkaline washing tower 9 by the spray pump 13 to wash the acidic secondary steam. Since the temperature of the alkaline water is maintained at a high temperature of 85-95℃, the temperature loss of the neutral secondary steam obtained after washing is not significant. The secondary steam enters the steam compressor 5 at a high temperature for efficient pressurization and heating, and is then recycled to the heater 3, effectively reducing the amount of live steam used and improving energy utilization.

[0032] Example 3

[0033] Please continue reading. Figure 1 and Figure 2To further avoid blockages in the hot water preheater 20, steam preheater 21, heater 3, and system pipes, this embodiment adds a mixing tank 26, a circulation pipe 23, and a diversion pipe 25 to the system based on embodiment 2. The mixing tank 26 is connected to the waste acid pipeline 2 to be treated. The outlet of the evaporator 4 is connected to the mixing tank 26 through the circulation pipe 23. The diversion pipe 25 is connected to the circulation pipe 23 and is connected to the preheating tank 19. The bottom of the preheating tank 19 is connected to a filter device 24, which is a membrane filter, a filter press, or a cyclone separator, or any one or more of these components in combination. By diverting a portion of the concentrated, high-temperature 50% waste acid (without cooling) into mixing tank 26 to mix with 20%–25% of the waste acid, the concentration of the waste acid before entering the preheating equipment is increased. As the concentration of the waste acid increases, the sulfate ion concentration gradually increases, the common ion effect is enhanced, and the dissolution equilibrium of ferrous sulfate and calcium sulfate shifts in the opposite direction. The solubility of ferrous sulfate monohydrate and calcium sulfate decreases, and some ferrous sulfate monohydrate and calcium sulfate precipitate as large solid crystals. The large solid crystals mixed with the waste acid are pumped into the hot water preheater 20 by high-pressure pump 30 to exchange heat with the hot water in the hot water tank 18. Large solid crystals flow at high speed through the inner wall of the pipes in the hot water preheater 20 without adhering. To fully utilize the heat from the hot water in the hot water tank 18, a hot water preheating circulation pipe can be added to the outlet of the hot water preheater 20 to return a portion of the preheated liquid to the mixing tank 26 for further preheating. Since the metal salts in the waste acid in the mixing tank 26 have already partially crystallized at their current concentration, the maximum heat exchange temperature of the hot water preheater 20 is 55℃. Under this maximum temperature condition, although the calcium sulfate concentration will continue to decrease and crystals will precipitate, the gradient of change is relatively small (calcium sulfate is greatly affected by both sulfuric acid concentration and temperature). The concentration of ferrous sulfate changes less (although temperature affects the concentration of ferrous sulfate, the concentration has a greater impact), while the concentration of titanium oxysulfate remains unchanged (temperature does not affect the hydrolysis of titanium oxysulfate to form metatitanic acid or its saturation and precipitation), thus avoiding the slow precipitation of metal salts from waste acid that adhere to the inner wall of the pipe and form scale. After the large solid particles of ferrous sulfate monohydrate and calcium sulfate precipitate and the waste acid enter the preheating tank 19, with the continuous increase in temperature (75℃-85℃), and the concentrated high-temperature 50% waste acid portion directly entering the preheating tank 19 through the diversion pipe 25, the concentration of waste acid continues to increase under high temperature and high concentration conditions. Under these conditions, most of the ferrous sulfate monohydrate and calcium sulfate in the waste acid continuously precipitate out in large quantities. At the same time, the crystals of ferrous sulfate monohydrate and calcium sulfate act as seed crystals, lowering the hydrolysis temperature of titanium oxysulfate and increasing the hydrolysis rate of titanium oxysulfate. This causes titanium oxysulfate to hydrolyze and generate metatitanic acid crystals, which precipitate out. The precipitated solid particles settle at the bottom of the preheating tank 19, which has a vertical flow sedimentation tank structure. The solid particles settle at the bottom and are then extracted to the filtration equipment 24 for solid-liquid separation. In the continuous concentration process of waste acid, scaling on the inner wall of the pipeline can be effectively avoided, eliminating the need for shutdown for descaling and maintenance, and effectively improving operating efficiency.

[0034] Example 4

[0035] Please continue reading. Figure 1 and Figure 2 The sulfuric acid waste acid concentration method for titanium dioxide production disclosed in this embodiment utilizes the sulfuric acid waste acid concentration system of Examples 1 to 3 to concentrate 20% to 25% of the waste acid, including the following steps:

[0036] S1. Start the MVR heating and evaporation equipment and vacuum equipment. The waste acid to be treated is sequentially fed into the hot water preheater 20 and steam preheater 21 of the preheating equipment for preheating. The waste acid is circulated and preheated within the steam preheater 21 and preheating tank 19 by the second circulation pump 22 until the preheating temperature reaches the set temperature (75℃-85℃). During the preheating start-up process, a portion of the 50%–55% concentration waste acid from the evaporator 4 is sent back to the mixing tank 26 through the circulation pipe 23 to mix with the 20%–25% concentration waste acid in the waste acid pipeline 2. The waste acid concentration is adjusted to 33%. In the mixing tank 26, at a lower temperature, as the concentration of concentrated waste acid increases, the sulfate ion concentration gradually increases, enhancing the common ion effect. The dissolution equilibrium of ferrous sulfate and calcium sulfate shifts in the reverse direction, reducing the solubility of ferrous sulfate monohydrate and calcium sulfate. Some ferrous sulfate monohydrate and calcium sulfate monohydrate precipitate as large solid crystals. The solid phase formed by the aggregation of these large solid crystals is suspended in the solution and pumped into the hot water preheater 20 by the high-pressure pump 30 to exchange heat with the hot water in the hot water tank 18. The large solid crystals flow at high speed through the pipes of the hot water preheater 20. The waste acid does not adhere to the inner wall of the pipe. After preheating by the hot water preheater 20, part of the waste acid enters the preheating tank 19, and part of it continuously circulates between the hot water preheater 20 and the mixing tank 26. The waste acid with a concentration of 50% to 55% in the circulation pipe 23 is sent into the preheating tank 19 through the diversion pipe 25, adjusting the concentration of waste acid in the preheating tank 19 to 38%. The second circulation pump 22 continuously circulates and heats the waste acid between the preheating tank 19 and the steam preheater 21, controlling the temperature in the preheating tank 19 to 75℃-85℃. Most of the ferrous sulfate monohydrate and calcium sulfate in the waste acid continuously precipitate out in large quantities, while ferrous sulfate monohydrate... Iron and calcium sulfate crystals act as seed crystals, lowering the hydrolysis temperature of titanium oxysulfate and increasing its hydrolysis rate. This causes the hydrolysis of titanium oxysulfate to produce metatitanic acid crystals, which precipitate at the bottom of the preheating tank 19 and are then extracted to the filtration device 24 for solid-liquid separation. The solid titanium dioxide contained in the original waste acid, under high temperature and high concentration, causes the metatitanic acid particles generated by the hydrolysis of soluble titanium. These particles easily aggregate with the crystallized ferrous sulfate and calcium sulfate during system operation or start-up and shutdown, forming solid phases. The density difference between the solid phases and the solution causes them to suspend in the solution, leading to blockage of some tubes. Therefore, before the waste acid enters the MVR heating evaporation equipment, it is necessary to remove solid titanium dioxide and other sulfates from the waste acid to the greatest extent possible. This effectively avoids scaling on the inner walls of the pipes during the continuous concentration process of waste acid, eliminating the need for shutdown for descaling and maintenance, and effectively improving operating efficiency.

[0037] S2. The waste acid that has reached the set temperature is sent to the heater 3 for heating via the discharge pump. The heated waste acid is then sent to the evaporator 4 for evaporation and concentration. During evaporation and concentration, the first circulation pump 6 is used for forced circulation heating and evaporation to prevent the heater 3 from easily clogging. When the waste acid concentration reaches 50-55% of the process requirements, it is sent to the cooling and ripening equipment 1 to cool the waste acid at a temperature of 100-105℃ to below 55℃. The ripened and cooled waste acid is then sent back to the titanium dioxide production leaching process for recycling via the finished product pump 31.

[0038] S3. In the initial stage of start-up operation, the steam preheater 21 and heater 3 need to continuously supply saturated live steam through the live steam pipeline 7 to heat the waste acid. The condensate generated by the steam preheater 21 and heater 3 enters the hot water tank 18 and is used to preheat the waste acid in the hot water preheater 20.

[0039] S4. During start-up and operation, the secondary acidic steam generated by the evaporator 4 is introduced into the alkaline washing tower 9 for spray cleaning with alkaline water at a temperature of 85-95℃ and a pH value of 8-12. After separation by the first gas-liquid separator 10, the resulting neutral steam enters the steam compressor 5 for pressurization and heating, and is then sent to the heater 3 to heat the waste acid. The alkaline washing process transforms the secondary acidic steam into neutral secondary steam, preventing the acidic steam from corroding the steam compressor 5. The temperature of the alkaline water used for alkaline washing is maintained at 85-95℃, and the pH value is within the range of 8-12, preventing the alkaline washing process from affecting the secondary steam temperature and causing a decrease in the inlet temperature of the steam compressor 5. The secondary steam is pressurized and heated by the three-stage steam compressor 5 and then used as the heat source for the heater 3. The steam is recycled, and only about 0.13 tons of steam are consumed to produce 1 ton of 50% acid, effectively reducing production costs.

[0040] S5. During startup, the non-condensable gas generated by heater 3 and condensate tank 8 is drawn to condenser 17 for cooling. The cooled gas enters the second gas-liquid separator 16 and vacuum tank 15 in sequence and is discharged through the exhaust port of vacuum pump 14. The condensate generated in condenser 17, second gas-liquid separator 16 and first gas-liquid separator 10 enters liquid seal tank 11 to ensure system vacuum pressure balance.

[0041] S6. During startup and operation, once the system's feed rate, moisture evaporation rate, and discharge rate are balanced, and the system's pressure, temperature, and discharge concentration are stable, gradually close the steam valves that allow saturated live steam to enter the steam preheater 21 and heater 3 through the live steam pipeline 7. The system's waste acid feed rate is 5-6 t / h, moisture evaporation rate is 3 t / h, waste acid discharge rate is 2.5-3.0 t / h, vacuum degree is 50-80 kPa, waste acid discharge concentration is 50%-55%, waste acid feed temperature is 25℃, waste acid discharge temperature is below 55℃, hot water preheating temperature is 40℃-55℃, preheating tank 19 temperature is 75℃-85℃, and the operating liquid temperature of the MVR heating and evaporation equipment is 100℃-105℃.

[0042] The applicant completed and put into operation a sulfuric acid process for titanium dioxide waste acid concentration in November 2023. The system's designed capacity is: 160 t / d of 23% waste acid consumption and 74 t / d of 50% waste acid production. The actual production capacity until June 2024 is shown in the table below:

[0043]

[0044] The system has been running continuously for 7 months, with an actual operating efficiency of about 90%, basically meeting the design requirements.

[0045] The above description provides a further detailed explanation of the present invention in conjunction with specific / preferred embodiments, and it should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, various substitutions or modifications can be made to these described embodiments without departing from the concept of the present invention, and all such substitutions or modifications should be considered within the scope of protection of the present invention. In the description of this specification, the reference to terms such as "an embodiment," "some embodiments," "preferred embodiment," "example," "specific example," or "some examples," etc., indicates that the specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in any suitable manner in one or more embodiments or examples. Without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification and the features of different embodiments or examples. Although the embodiments of the present invention and their advantages have been described in detail, it should be understood that various changes, substitutions, and modifications can be made herein without departing from the scope of protection of the patent application.

Claims

1. A method for concentrating waste sulfuric acid in the production of titanium dioxide by the sulfuric acid process, characterized by: A sulfuric acid process titanium dioxide waste acid concentration system is used to concentrate 20%–25% of waste acid. The system includes a preheating unit, a multi-phase vapor refrigerant (MVR) heating and evaporation unit, a cooling and curing unit, and a vacuum unit. The preheating inlet of the preheating unit is connected to the waste acid pipeline, and the preheating outlet is connected to the inlet of the MVR heating and evaporation unit. The outlet of the MVR heating and evaporation unit is connected to the inlet of the cooling and curing unit. The vacuum unit is connected to the MVR heating and evaporation unit and provides a vacuum working environment. The MVR heating and evaporation unit includes a heater, an evaporator, a steam compressor, and a first circulation pump. The inlet of the heater is connected to the outlet of the first circulation pump, and the outlet of the heater is connected to the inlet of the evaporator. The outlet of the evaporator is connected to the cooling and curing unit. The preheating outlet of the preheating unit and the circulation port of the evaporator are both connected to the inlet of the first circulation pump. The gas-liquid outlet of the evaporator is connected to the inlet of the steam compressor. Both the external and internal live steam pipelines are connected to the heat medium inlet of the heater, and the cold medium outlet of the heater is connected to the condensate tank. The preheating equipment includes a hot water tank, a preheating tank, a hot water preheater, a steam preheater, and a second circulation pump. The inlet of the hot water preheater is connected to the waste acid pipeline to be treated, and the outlet of the hot water preheater is connected to the inlet of the preheating tank. The circulation port of the preheating tank is connected to the inlet of the second circulation pump, and the outlet of the second circulation pump is connected to the inlet of the steam preheater. The discharge port is connected to the inlet of the preheating tank; the heat medium inlet of the steam preheater is connected to the live steam pipeline; the condensate outlet of the steam preheater and the outlet of the condensate tank are both connected to the hot water tank; the outlet of the hot water tank is connected to the heat medium inlet of the hot water preheater; a mixing tank is connected to the waste acid pipeline to be treated; the discharge port of the evaporator is connected to the mixing tank through a circulation pipeline; a diversion pipeline is connected to the circulation pipeline; the diversion pipeline is connected to the preheating tank; and the bottom of the preheating tank is connected to the filtration equipment. The method for concentrating 20%–25% of waste acid using the sulfuric acid process titanium dioxide waste acid concentration system includes the following steps: S1. Start the MVR heating and evaporation equipment and vacuum equipment. Pass the waste acid to be treated into the hot water preheater and steam preheater of the preheating equipment in sequence for preheating. The waste acid is preheated by circulating in the steam preheater and preheating tank through the second circulation pump until the preheating temperature reaches the set temperature. S2. The waste acid that has reached the set temperature is sent to the heater for heating through the discharge pump. The heated waste acid is then sent to the evaporator for evaporation and concentration. During evaporation and concentration, the waste acid is circulated and heated through the first circulation pump until the concentration of the waste acid reaches 50-55% of the process requirements. Then it is sent to the cooling and maturation equipment to cool the waste acid at a temperature of 100-105℃ to below 55℃. The matured and cooled waste acid is then sent back to the titanium dioxide production leaching process for recycling. S3. In the initial stage of start-up and operation, the steam preheater and heater need to continuously supply saturated live steam through the live steam pipeline to heat the waste acid. The condensate produced by the steam preheater and heater enters the hot water tank and is used to preheat the waste acid in the hot water preheater. S4. During the start-up process, a portion of the waste acid with a concentration of 50% to 55% that comes out of the evaporator will be sent back to the mixing tank through the circulation pipeline to mix with the waste acid with a concentration of 20% to 25%, so that the concentration of waste acid is adjusted to 33%, and some calcium sulfate and ferrous sulfate monohydrate solid large particle crystals will precipitate. The solid large particle crystals mixed with the waste acid will be sent to the hot water preheater for preheating. S5. During the start-up process, waste acid with a concentration of 50% to 55% in the circulation pipeline is sent into the preheating tank through the diversion pipeline. The concentration of waste acid in the preheating tank is adjusted to 38%. The precipitate at the bottom of the preheating tank is led out to the filtration equipment to filter out large solid crystals. S6. During startup and operation, once the system's feed rate, moisture evaporation rate, and discharge rate are balanced, and the system's pressure, temperature, and discharge concentration are stable, gradually close the steam valves that allow saturated live steam to enter the steam preheater and heater through the live steam pipeline.

2. The method for concentrating waste sulfuric acid in a sulfuric acid process for producing titanium dioxide according to claim 1, characterized by: The sulfuric acid process titanium dioxide waste acid concentration system also includes an alkaline washing device. This device is connected in series on the gas-liquid outlet of the evaporator and the inlet pipe of the steam compressor. The alkaline washing device includes an alkaline washing tower and a first gas-liquid separator. The cleaning inlet of the alkaline washing tower is connected to the gas-liquid outlet of the evaporator, and the cleaning outlet of the alkaline washing tower is connected to the inlet of the first gas-liquid separator. The steam outlet of the first gas-liquid separator is connected to the inlet of the steam compressor, and the liquid outlet of the first gas-liquid separator is connected to a liquid seal tank. The cleaning liquid inlet and outlet of the alkaline washing tower are both connected to an alkaline water tank and circulated through a spray pump for alkaline washing. The alkaline water tank is connected to the live steam pipeline. During start-up and operation, the secondary acidic steam generated by the evaporator is introduced into the alkaline washing tower and sprayed with alkaline water at a temperature of 85–95°C and a pH of 8–12. After separation by the first gas-liquid separator, the resulting neutral steam enters the steam compressor for pressurization and heating before being sent to a heater to heat the waste acid.

3. The method for concentrating spent sulfuric acid of a sulfuric acid process titanium white according to claim 2, characterized by: The vacuum equipment includes a vacuum pump, a vacuum tank, a second gas-liquid separator, and a condenser. The suction port of the vacuum pump is connected to the outlet of the vacuum tank, the inlet of the vacuum tank is connected to the gas outlet of the second gas-liquid separator, the liquid outlet of the second gas-liquid separator is connected to the liquid seal tank, the inlet of the second gas-liquid separator is connected to the gas outlet of the condenser, the gas inlet of the condenser is connected to the heater and the condensate tank, and the liquid outlet of the condenser is connected to the liquid seal tank. During startup, the non-condensable gas generated by the heater and the condensate tank is drawn to the condenser for cooling. The cooled gas sequentially enters the second gas-liquid separator and the vacuum tank and is discharged through the exhaust port of the vacuum pump. The condensate generated in the condenser, the second gas-liquid separator, and the first gas-liquid separator enters the liquid seal tank.

4. The method of concentrating spent sulfuric acid from a sulfuric acid process for producing titanium dioxide according to claim 3, characterized in that: In step S6, the system has the following parameters: waste acid feed rate of 5-6 t / h, water evaporation rate of 3 t / h, waste acid discharge rate of 2.5-3.0 t / h, vacuum degree of 50-80 kPa, waste acid discharge concentration of 50%-55%, waste acid feed temperature of 25°C, waste acid discharge temperature below 55°C, hot water preheating temperature of 40°C-55°C, preheating tank control temperature of 75°C-85°C, and MVR heating evaporation equipment operating liquid temperature of 100°C-105°C.

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