A cleaning method for a concentrated heat exchanger of sulfuric acid method titanium dioxide waste acid

Abstract

The application provides a cleaning method of a waste acid concentration heat exchanger in a sulfuric acid method titanium white, which comprises the following steps: 1) circulating matured acid into the inner pipeline of the waste acid concentration heat exchanger for a predetermined time, then discharging the matured acid; circulating process water into the inner pipeline of the waste acid concentration heat exchanger for a predetermined time, then discharging the process water; repeating the above steps until the inner pipeline of the waste acid concentration heat exchanger is cleaned; 2) circulating mixed acid of low-concentration HF and HCl into the outer pipeline of the waste acid concentration heat exchanger for a predetermined time, then discharging the mixed acid; repeating the above steps until the outer pipeline of the waste acid concentration heat exchanger is cleaned; circulating process water into the outer pipeline of the waste acid concentration heat exchanger to clean the residual mixed acid of low-concentration HF and HCl. The method greatly reduces the cleaning labor intensity and cleaning cost of the heat exchanger.

Description

Technical Field

[0001] This invention relates to the field of waste acid concentration in the sulfuric acid process for titanium dioxide production, and specifically to a cleaning method for a heat exchanger used for waste acid concentration in the sulfuric acid process for titanium dioxide production. Background Technology

[0002] Titanium dioxide, as an important chemical raw material, is widely used in industries such as coatings, plastics, and inks. Currently, titanium dioxide production processes are mainly divided into the sulfate process and the chloride process. Due to limitations in titanium resources, technology, and equipment, most domestic titanium dioxide enterprises use the sulfate process to produce titanium dioxide powder, accounting for approximately 90% of production capacity. However, for every ton of titanium dioxide produced using the sulfate process, 4-8 tons of waste acid (20% nitrate) are generated. Currently, how to recycle or comprehensively utilize this waste acid is the biggest challenge restricting the clean production of titanium dioxide using the sulfate process. In existing technologies, the mainstream and economical method for recycling waste acid is to concentrate it to a certain concentration and then reuse it in the acidolysis process.

[0003] Domestic titanium dioxide producers using the sulfuric acid process primarily use titanium concentrate from the Panzhihua-Xichang region or titanium slag obtained from its smelting as their main titanium raw material. Compared to foreign titanium raw materials, these materials have higher calcium and silicon content. For example, the CaO and SiO2 content of titanium concentrate from QIT in Canada is approximately 0.1% and 0.6%, respectively, while that from RBM in South Africa is approximately 0.05% and 0.6%. In contrast, the CaO and SiO2 content of titanium concentrate from the Panzhihua-Xichang region ranges from 0.8% to 1.5% and 2.5% to 4.3%, respectively. Furthermore, the CaO and SiO2 content in the titanium slag obtained from the smelting of titanium concentrate increases significantly. This results in a substantial increase in the content of calcium- and silicon-containing colloidal substances in the waste acid produced as a byproduct of titanium dioxide production. During the waste acid concentration process, these substances easily adsorb onto the heat exchanger tube walls, gradually increasing in concentration and ultimately affecting the efficiency and operating cycle of the waste acid concentration process. In addition, some titanium dioxide sulfuric acid manufacturers use the secondary steam generated in the primary vacuum evaporation chamber as the heat source for the next stage concentration heat exchanger in order to improve heat utilization efficiency. Inevitably, this secondary steam will carry a small amount of scale, which will gradually accumulate over time. Once it reaches a certain level, it will seriously affect the heat exchange effect, operating cycle and service life of the waste acid concentration heat exchanger.

[0004] Theoretical calculations show that a 1mm layer of calcium sulfate scale will reduce the heat exchange efficiency of a graphite heat exchanger by approximately 60%. Chemical analysis of the scale reveals that calcium sulfate accounts for more than one-third of the composition of the inner wall of the graphite tubes, while it accounts for about 5% of the outer wall. Furthermore, after about 10 days of stable operation of the waste acid concentration system, the scale thickness on the inner wall of the graphite tubes can reach 1-2mm, while the outer wall reaches 0.5mm after six months of stable operation. Therefore, the scale material has a decisive impact on heat exchange performance. To address the scaling problem of graphite tubes in heat exchangers, it is generally necessary to disassemble the heat exchanger and use chemical methods for thorough cleaning. However, this cleaning method is time-consuming, costly, and labor-intensive.

[0005] In order to effectively solve the technical problems of low heat exchange efficiency and poor stability of waste acid concentration system caused by heat exchanger scaling during the waste acid concentration process, it is necessary to develop a simple and effective cleaning method to reduce cleaning labor intensity and cleaning cost, and ultimately achieve efficient operation of waste acid concentration system. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention aims to provide a cleaning method for a waste acid concentration heat exchanger used in the sulfuric acid process for titanium dioxide production.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] This invention provides a cleaning method for a waste acid concentration heat exchanger used in the sulfuric acid process for titanium dioxide production, comprising the following steps:

[0009] 1) Clean the pipes inside the waste acid concentrate heat exchanger.

[0010] A hydrated acid is introduced into the pipes inside the waste acid concentration heat exchanger to circulate and clean the pipes inside the waste acid concentration heat exchanger for a predetermined time, and then the hydrated acid is discharged.

[0011] Process water is introduced into the pipes inside the waste acid concentration heat exchanger and circulated to clean the pipes inside the waste acid concentration heat exchanger for a predetermined time, and then the process water is discharged.

[0012] Repeat the above steps until the pipes inside the waste acid concentration heat exchanger are thoroughly cleaned.

[0013] 2) Clean the external pipes of the waste acid concentrate heat exchanger.

[0014] A low-concentration mixture of HF and HCl is introduced into the external pipe of the waste acid concentration heat exchanger to soak the external pipe for a predetermined time, and then the mixture is discharged.

[0015] Repeat the above steps until the external pipes of the waste acid concentration heat exchanger are thoroughly cleaned.

[0016] Process water is introduced into the external pipe of the waste acid concentration heat exchanger to clean the residual low-concentration HF and HCl mixed acid.

[0017] Further, in step 1), the temperature of the aging acid is 20℃-60℃, the mass concentration of the aging acid is ≥42%, the circulation time of the aging acid is 12h-24h, and the feed amount of the aging acid is 80%-100% of the designed feed amount of the waste acid raw material.

[0018] Further, in step 1), the temperature of the process water is 80℃-90℃, and the circulation time of the process water is 8h-12h.

[0019] Further, in step 1), the aging acid and the process water are circulated to clean the pipes inside the waste acid concentration heat exchanger 2-3 times.

[0020] Further, in step 2), the total mass concentration of the mixed acid of low-concentration HF and HCl is 3%-5%, the mass ratio of low-concentration HF to HCl is 2:1-4:1, the temperature of the mixed acid of low-concentration HF and HCl is 80℃-90℃, and the soaking time is 12h-24h.

[0021] Further, in step 2), the waste acid concentrate heat exchanger external pipe is soaked 2-3 times with the mixed acid of low concentration HF and HCl.

[0022] Furthermore, fresh steam is used as a heat source to heat the aging acid, the process water, and the mixed acid of low concentration HF and HCl.

[0023] Furthermore, the cleaning cycle for the internal pipes of the waste acid concentration heat exchanger is 10-12 days, and the cleaning cycle for the external pipes of the waste acid concentration heat exchanger is 160-190 days.

[0024] Furthermore, the waste acid generated in the sulfuric acid process for titanium dioxide production is concentrated and then subjected to a aging reaction in an aging acid storage tank to obtain the aging acid.

[0025] Furthermore, the aging acid includes ferrous sulfate.

[0026] Compared with existing technologies, the beneficial technical effects of this invention are as follows: This invention uses aging acid obtained by aging reaction after concentration of 20% waste acid and process water as raw materials for cleaning the graphite inner pipes of the heat exchanger in the waste acid concentration system. The 20% waste acid is a byproduct of titanium dioxide production from Panzhihua titanium resources. Simultaneously, a mixed acid of low-concentration HF and HCl is used as raw material for cleaning the graphite outer tubes of the heat exchanger in the waste acid concentration system. Fresh steam is used as a heat source to heat the aforementioned aging acid and process water. This invention provides a simple, effective, and economical method for cleaning scale buildup in the waste acid concentration heat exchanger of sulfuric acid titanium dioxide production using the high-calcium, silicon-rich Panzhihua titanium resources. This significantly reduces the labor intensity and cost of cleaning the heat exchanger, providing important technical support for ultimately achieving efficient and stable operation of the waste acid concentration system, clean production of sulfuric acid titanium dioxide in my country, and the promotion and application of Panzhihua titanium resources. Attached Figure Description

[0027] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0028] Figure 1 This is a schematic flowchart of the cleaning method for the waste acid concentration heat exchanger of the sulfuric acid process for titanium dioxide production according to the present invention.

[0029] Figure 2 This is a flow chart of the waste acid concentration process of the present invention. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be further described in detail below with reference to specific examples and the accompanying drawings.

[0031] This invention primarily addresses the technical challenges of long cleaning cycles, high cleaning costs, and high labor intensity in the scaling process of waste acid concentrators for titanium dioxide production. To solve these challenges, the overall concept of the technical solution adopted in this invention is as follows: Figure 1 As shown, the scale buildup in the graphite inner pipes of the waste acid concentration heat exchanger is flushed sequentially with hydrated acid and process water, and the scale buildup in the graphite outer pipes of the waste acid concentration heat exchanger is soaked in a mixture of low-concentration HF and HCl.

[0032] The main theoretical basis of the method of this invention is as follows:

[0033] ① The large amount of ferrous sulfate solid particles in the aging acid obtained after the waste acid is concentrated and then aged will have a strong scouring effect on the graphite inner pipes of the heat exchanger in the waste acid concentration system, so that the scale attached to the graphite inner pipes of the heat exchanger will be slowly washed off.

[0034] ② After cleaning the waste acid concentration system with aging acid, hot water is then used to clean the graphite inner tube of the waste acid concentration heat exchanger. Soluble salts in the scale have high solubility in water, so the soluble salt crystals wrapped in the scale will be dissolved. At the same time, this prevents the soluble salts from decreasing in solubility due to the lower temperature and eventually depositing in the waste acid concentration heat exchanger pipes, thus preventing blockage of the graphite inner tubes of the heat exchanger. The two methods are used alternately to clean the graphite inner tubes of the waste acid concentration system heat exchanger until they are clean.

[0035] ③ Use a mixture of low-concentration HF and HCl to soak and wash the scale on the graphite outer tube of the waste acid concentration heat exchanger, so that the scale on the graphite outer tube of the waste acid concentration heat exchanger is dissolved, and finally rinsed with process water to clean it.

[0036] The cleaning method for the waste acid concentration heat exchanger of the sulfuric acid process for titanium dioxide production of the present invention specifically includes the following steps:

[0037] 1) Clean the pipes inside the waste acid concentrate heat exchanger.

[0038] Introduce hydrated acid into the pipes of the waste acid concentration heat exchanger and circulate it to clean the pipes for a predetermined time, then discharge the hydrated acid.

[0039] Process water is introduced into the pipes inside the waste acid concentration heat exchanger and circulated to clean the pipes inside the waste acid concentration heat exchanger for a predetermined time, and then the process water is discharged.

[0040] Repeat the above steps until the pipes inside the waste acid concentrate heat exchanger are thoroughly cleaned.

[0041] The specific process is as follows: After the waste acid concentration system has been running continuously and stably for 10-12 days, the feeding is stopped, and the concentrated acid in the waste acid concentration system is pumped into the aging acid storage tank through the drain valve on the circulation pipeline (see...). Figure 2 After undergoing a aging reaction, aging acid is obtained, and then the aging acid in the aging acid storage tank is added to a waste acid concentration system. In a preferred embodiment, the aging acid is not separated by plate and frame pressing, and therefore includes ferrous sulfate.

[0042] To achieve a significant turbulent scouring effect within the waste acid concentration heat exchanger pipes, 80%-100% of the designed feed rate of 20% waste acid raw material is introduced into the pipes of the waste acid concentration heat exchanger to clean the pipes. Simultaneously, fresh steam is introduced to heat the hydrated acid. In a preferred embodiment, the temperature of the hydrated acid is 20℃-60℃, the mass concentration of the hydrated acid is ≥42%, and the circulation time for hydrated acid cleaning is 12h-24h. After the circulation time, the hydrated acid is pumped into the hydrated acid storage tank through the drain valve on the circulation pipe. Then, process water is pumped in from the 20% waste acid inlet, and at the same time, the fresh steam valve is opened to maintain the temperature of the process water at 80℃-90℃. The circulation time of the process water is 8h-12h. After the circulation time, the drain valve on the circulation pipe is opened to drain the water into a trench. The hydrated acid and process water circulate to clean the pipes 2-3 within the waste acid concentration heat exchanger until the graphite pipes of the heat exchanger are completely clean. In a preferred embodiment, the pipes inside the waste acid concentration heat exchanger are cleaned every 10-12 days.

[0043] 2) Clean the external pipes of the waste acid concentrate heat exchanger.

[0044] A low-concentration mixture of HF and HCl is introduced into the external pipe of the waste acid concentration heat exchanger to soak the external pipe for a predetermined time, and then the mixture is discharged.

[0045] Repeat the above steps until the external pipes of the waste acid concentrate heat exchanger are thoroughly cleaned.

[0046] Process water is introduced into the external pipes of the waste acid concentration heat exchanger to clean the residual low-concentration mixed acid of HF and HCl.

[0047] The specific process is as follows: Open the top cover of the heat exchanger, then add a mixture of low-concentration HF acid and HCl with a total mass concentration of 3%~5%. Introduce fresh steam to maintain the temperature of the mixture at 80~90℃, and soak for 12~24 hours. The mass ratio of low-concentration HF to HCl is 2:1-4:1. After soaking, open the drain valve at the bottom of the heat exchanger to drain the mixture. Once drained, close the drain valve and add the same mixture back to the top of the heat exchanger. Repeat the soaking operation 2-3 times until the graphite outer pipes of the heat exchanger are clean. Finally, rinse 2-3 times with process water at 20-80℃ to drain any remaining low-concentration HF acid and HCl mixture from the graphite outer pipes. Those skilled in the art should understand that the heat exchanger shell is lined with an acid-resistant protective layer. In a preferred embodiment, the external pipes of the waste acid concentration heat exchanger are cleaned every 160-190 days.

[0048] The following examples illustrate the cleaning method of the waste acid concentration heat exchanger for sulfuric acid titanium dioxide production according to the present invention.

[0049] Comparative Example

[0050] Comparative Example 1: After the new waste acid concentration system has been running continuously and stably for 12 days, all the scaled heat exchangers in the new system will be replaced with new ones. Then, the system will be fed with 20% acid at a rate of 12m³ / h. 3 Waste acid was concentrated at a rate of 23.0% per hour, with a fresh saturated steam pressure of 0.40 MPa, a fresh saturated steam temperature of 151.3℃, and a fresh saturated steam flow rate of 5.4 t / h. Then, a aging reaction was carried out. After the operation stabilized, the concentration of the aged acid was measured.

[0051] Comparative Example 2: After the new waste acid concentration system operated continuously and stably for 12 days, the scaled heat exchanger of the new waste acid concentration system was not cleaned. Then, the feed rate was 12m³ / h of 20% acid. 3 Waste acid was concentrated at a rate of 23.0% per hour, with a fresh saturated steam pressure of 0.40 MPa, a fresh saturated steam temperature of 151.3℃, and a fresh saturated steam flow rate of 5.4 t / h. Then, a aging reaction was carried out. After the operation stabilized, the concentration of the aged acid was measured.

[0052] Example 1

[0053] Example 1-1

[0054] After the new waste acid concentration system operated continuously and stably for 12 days, the pipes inside the waste acid concentration heat exchanger, which had accumulated scale, were cleaned twice in rotation using hydrated acid and process water. The flow rates of both the hydrated acid and process water were 17 m³ / s. 3 / h (design feed rate of raw materials 17m) 3 / h, which is 100% of the designed raw material feed rate), the aging acid cleaning time is 24 hours per cycle, the process water cleaning time is 12 hours per cycle, the aging acid temperature is 50℃, the aging acid mass concentration is ≥42%, and the process water temperature is 80℃. After cleaning, the acid feed rate is 12m³ / h. 3 Waste acid was concentrated at a rate of 23.0% per hour, with a fresh saturated steam pressure of 0.40 MPa, a fresh saturated steam temperature of 151.3℃, and a fresh saturated steam flow rate of 5.4 t / h. Then, a aging reaction was carried out. After the operation stabilized, the concentration of the aged acid was measured.

[0055] Examples 1-2

[0056] After the new waste acid concentration system has been running continuously and stably for 12 days, the internal pipes of the waste acid concentration heat exchanger are cleaned according to the cleaning parameters in Example 1-1. Thereafter, the internal pipes of the waste acid concentration heat exchanger are cleaned every 12 days according to the cleaning parameters in Example 1-1. Then, after the new waste acid concentration system has been running for six months, the external pipes of the waste acid concentration heat exchanger are not cleaned. Instead, after the new waste acid concentration system has been running continuously and stably for another 12 days, the internal pipes of the waste acid concentration heat exchanger, which have accumulated scale, are cleaned twice in alternating cycles using hydrated acid and process water. The flow rates of both the hydrated acid and process water are 17 m³ / s. 3 / h (design feed rate of raw materials 17 m³) 3 / h, which is 100% of the designed raw material feed rate), the aging acid cleaning time is 24 hours per cycle, the process water cleaning time is 12 hours per cycle, the aging acid temperature is 50℃, the aging acid mass concentration is ≥42%, and the process water temperature is 80℃. After cleaning, the acid feed rate is 12m³ / h. 3 Waste acid was concentrated at a rate of 23.0% per hour, with a fresh saturated steam pressure of 0.40 MPa, a fresh saturated steam temperature of 151.3℃, and a fresh saturated steam flow rate of 5.4 t / h. Then, a aging reaction was carried out. After the operation stabilized, the concentration of the aged acid was measured.

[0057] Examples 1-3

[0058] After the new waste acid concentration system had been running continuously and stably for 12 days, the internal pipes of the waste acid concentration heat exchanger were cleaned according to the cleaning parameters in Examples 1-2. Thereafter, the internal pipes of the waste acid concentration heat exchanger were cleaned every 12 days according to the cleaning parameters in Examples 1-2. Then, after the waste acid concentration system had been running for six months, the external pipes of the waste acid concentration heat exchanger were cleaned twice using a mixed acid with a mass ratio of HF:HCl = 3:1 (mass ratio), a total mass concentration of 3%, and a temperature of 80°C, along with process water at a temperature of 80°C. The soaking time for the mixed acid was 12 hours. After cleaning, when the waste acid concentration system had been running continuously and stably for another 12 days, the scaled internal pipes of the waste acid concentration heat exchanger in the new waste acid concentration system were cleaned twice in alternating cycles using aging acid and process water. The flow rates of both the aging acid and process water were 17 m³ / s. 3 / h (design feed rate of raw materials 17 m³) 3 / h, which is 100% of the designed raw material feed rate), the aging acid cleaning time is 24h per cycle, the process water cleaning time is 12h per cycle, the aging acid temperature is 50℃, the aging acid mass concentration is ≥42%, and the process water temperature is 80℃. After cleaning, according to the 20 acid feed rate of 12m 3 Waste acid was concentrated at a rate of 23.0% per hour, with a fresh saturated steam pressure of 0.40 MPa, a fresh saturated steam temperature of 151.3℃, and a fresh saturated steam flow rate of 5.4 t / h. Then, a aging reaction was carried out. After the operation stabilized, the concentration of the aged acid was measured.

[0059] Table 1 Comparison of results from Example 1 with Comparative Examples 1 and 2

[0060]

[0061] A comparison of the results of Example 1 and Comparative Examples 1 and 2 above shows that:

[0062] Comparative Example 1 replaced the waste acid concentration heat exchanger directly after one cycle of operation. Although the concentration effect was guaranteed, the cycle and cost of sending it out for chemical cleaning were quite high, making it impossible for sulfuric acid process titanium dioxide enterprises to bear the time and cost.

[0063] Comparative Example 2 did not clean the waste acid concentration heat exchanger. Although this saved on cleaning cycles and costs, the severe scaling in the pipes of the waste acid concentration heat exchanger caused the concentration of the aging acid obtained after the waste acid concentration and aging reaction to decrease to 45.2% during the next operation. This was a decrease of 10.7% from the initial aging acid concentration of 55.9%, which was a significant drop. Furthermore, the pipes inside the waste acid concentration heat exchanger would become completely blocked after the third round of operation, causing the waste acid concentration system to malfunction.

[0064] In Example 1-1, after the new waste acid concentration system had been running continuously and stably for 12 days, the concentration of aging acid was basically restored compared to that in Comparative Example 1 due to the cleaning of the pipes inside the waste acid concentration heat exchanger. This reflects that the waste acid concentration effect was also basically restored. Moreover, the cost and cleaning cycle of a single cleaning were much less than the direct replacement method in Comparative Example 1. Sulfuric acid process titanium dioxide production enterprises can fully accept such time and cost.

[0065] Although the internal pipes of the waste acid concentration heat exchanger in Examples 1-2 were cleaned, the external pipes of the waste acid concentration heat exchanger were not cleaned after six months of operation of the new waste acid concentration system. Although the scaling rate of the external pipes of the waste acid concentration heat exchanger is relatively slow, after six months of operation, the thickness of the scale layer gradually increases, which will significantly affect the heat transfer effect of the waste acid concentration heat exchanger. This can be confirmed by the decrease in the outlet temperature of the secondary heat exchanger, which decreased from 82℃ in Comparative Example 1 to 75℃ in this example, and the concentration of hydrated acid decreased from 55.9% in Comparative Example 1 to 52.4% in this example. Therefore, after a certain period of operation, it is necessary to clean the external pipes of the waste acid concentration heat exchanger, which uses secondary steam as a heat source.

[0066] Examples 1-3 not only cleaned the internal pipes of the waste acid concentration heat exchanger every 12 days, but also cleaned the external pipes after the waste acid concentration system had been running for six months. This cleaning scheme allowed the heat transfer effect of the waste acid concentration heat exchanger to be restored after cleaning. As can be seen from the data of Examples 1-3, the mass concentration of the hydrated acid (55.9%), the outlet temperature of the first-stage heat exchanger (109°C), and the outlet temperature of the second-stage heat exchanger (82°C) in Examples 1-3 all returned to the level of Comparative Example 1. At the same time, the cost and cleaning cycle of each cleaning of the internal and external pipes of the waste acid concentration heat exchanger remained very low.

[0067] In summary, the cleaning methods described in Examples 1-3 help maintain the stable and efficient operation of waste acid concentration, and the operating costs are relatively low.

[0068] Example 2

[0069] Example 2-1

[0070] The cleaning scheme in Example 2-1 is generally similar to that in Example 1-1, with the only difference being the parameters for cleaning the pipes inside the waste acid concentration heat exchanger. In Example 2-1, the flow rates of both the aging acid and process water are 13.6 m³ / s. 3 / h (design feed rate of raw materials 17m) 3 / h, which is 80% of the designed feed rate of raw materials), the cleaning time of each aging acid is 12h, the cleaning time of each process water is 8h, the temperature of aging acid is 20℃, the mass concentration of aging acid is ≥42%, the temperature of process water is 90℃, and the aging acid and process water are used alternately to clean the pipes inside the waste acid concentration heat exchanger 3 times.

[0071] Example 2-2

[0072] The cleaning scheme in Example 2-2 is generally similar to that in Example 1-2, with the only difference being the parameters for cleaning the pipes inside the waste acid concentration heat exchanger. In Example 2-2, the flow rates of both the aging acid and process water are 13.6 m³ / s. 3 / h (design feed rate of raw materials 17m) 3 / h, which is 80% of the designed feed rate of raw materials), the cleaning time of each aging acid is 12h, the cleaning time of each process water is 8h, the temperature of aging acid is 20℃, the mass concentration of aging acid is ≥42%, the temperature of process water is 90℃, and the aging acid and process water are used alternately to clean the pipes inside the waste acid concentration heat exchanger 3 times.

[0073] Example 2-3

[0074] The cleaning schemes in Examples 2-3 are generally similar to those in Examples 1-3, with the only difference being:

[0075] ① The parameters for cleaning the pipes inside the waste acid concentration heat exchanger differ. In Examples 2-3, the flow rates of both the aging acid and process water are 13.6 m³ / s. 3 / h (design feed rate of raw materials 17m) 3 / h, which is 80% of the designed feed rate of raw materials), the cleaning time of each aging acid is 12h, the cleaning time of each process water is 8h, the temperature of aging acid is 20℃, the mass concentration of aging acid is ≥42%, the temperature of process water is 90℃, and the aging acid and process water are used alternately to clean the pipes inside the waste acid concentration heat exchanger 3 times.

[0076] ② The parameters for cleaning the external pipes of the waste acid concentration heat exchanger are different. In Examples 2-3, HF:HCl = 2:1 (mass ratio), the total mass concentration of the mixed acid of HF and HCl is 5%, the temperature is 90℃, and the external pipes of the waste acid concentration heat exchanger are cleaned 3 times with process water at a temperature of 20℃. The soaking time of the mixed acid of HF and HCl is 24 hours.

[0077] Table 2 Comparison of results from Example 2 with Comparative Examples 1 and 2

[0078]

[0079] A comparison of the results of Example 2 and Comparative Examples 1 and 2 shows that:

[0080] Comparative Example 1 replaced the waste acid concentration heat exchanger directly after one cycle of operation. Although the concentration effect was guaranteed, the cycle and cost of sending it out for chemical cleaning were quite high, making it impossible for sulfuric acid process titanium dioxide enterprises to bear the time and cost.

[0081] Comparative Example 2 did not clean the waste acid concentration heat exchanger. Although this saved on cleaning cycles and costs, the severe scaling in the pipes of the waste acid concentration heat exchanger caused the concentration of the aging acid obtained after the waste acid concentration and aging reaction to decrease to 45.1% during the next operation. This was a decrease of 11.0% from the initial aging acid concentration of 56.1%, which was a significant drop. Furthermore, the pipes in the waste acid concentration heat exchanger would become completely blocked after the third round of operation, causing the waste acid concentration system to malfunction.

[0082] In Example 2-1, after the new waste acid concentration system had been running continuously and stably for 12 days, the concentration of aging acid was basically restored compared to that in Comparative Example 1 due to the cleaning of the pipes inside the waste acid concentration heat exchanger. This reflects that the waste acid concentration effect was also basically restored. Moreover, the cost and cleaning cycle of a single cleaning were much less than the direct replacement method in Comparative Example 1. Sulfuric acid process titanium dioxide production enterprises can fully accept such time and cost.

[0083] Although the internal pipes of the waste acid concentration heat exchanger in Example 2-2 were cleaned, the external pipes were not cleaned after six months of operation of the new waste acid concentration system. While the scaling rate of the external pipes was slow, the scale layer gradually thickened after six months of operation, significantly impacting the heat transfer efficiency of the waste acid concentration heat exchanger. This is evidenced by the decrease in the outlet temperature of the secondary heat exchanger, from 83°C in Comparative Example 1 to 76°C in this example, and the concentration of hydrated acid from 56.1% in Comparative Example 1 to 52.6% in this example. Therefore, after a certain period of operation, the external pipes of the waste acid concentration heat exchanger, which uses secondary steam as a heat source, need to be cleaned.

[0084] Examples 2-3 not only cleaned the internal pipes of the waste acid concentration heat exchanger every 12 days, but also cleaned the external pipes after the waste acid concentration system had been running for six months. This cleaning scheme allowed the heat transfer effect of the waste acid concentration heat exchanger to be restored after cleaning. As can be seen from the data of Examples 2-3, the mass concentration of the hydrated acid (56.1%), the outlet temperature of the first-stage heat exchanger (110℃), and the outlet temperature of the second-stage heat exchanger (83℃) in Examples 2-3 all returned to the level of Comparative Example 1. At the same time, the cost and cleaning cycle of each cleaning of the internal and external pipes of the waste acid concentration heat exchanger remained very low.

[0085] In summary, the cleaning methods in Examples 2-3 help maintain the stable operation of waste acid concentration with high efficiency and relatively low operating costs.

[0086] Example 3

[0087] Example 3-1

[0088] The cleaning scheme in Example 3-1 is generally similar to that in Example 1-1, with the only difference being the parameters for cleaning the pipes inside the waste acid concentration heat exchanger. In Example 3-1, the flow rates of both the aging acid and process water are 15.3 m³ / s. 3 / h (design feed rate of raw materials 17m) 3 / h, which is 90% of the designed feed rate of raw materials), the cleaning time of each aging acid is 20h, the cleaning time of each process water is 10h, the temperature of the aging acid is 60℃, the mass concentration of the aging acid is ≥42%, the temperature of the process water is 85℃, and the aging acid and process water are used alternately to clean the pipes inside the waste acid concentration heat exchanger 3 times.

[0089] Example 3-2

[0090] The cleaning scheme in Example 3-2 is generally similar to that in Example 1-2, with the only difference being the parameters for cleaning the pipes inside the waste acid concentration heat exchanger. In Example 3-2, the flow rates of both the aging acid and process water are 15.3 m³ / s. 3 / h (design feed rate of raw materials 17m) 3 / h, which is 90% of the designed feed rate of raw materials), the cleaning time of each aging acid is 20h, the cleaning time of each process water is 10h, the temperature of the aging acid is 60℃, the mass concentration of the aging acid is ≥42%, the temperature of the process water is 85℃, and the aging acid and process water are used alternately to clean the pipes inside the waste acid concentration heat exchanger 3 times.

[0091] Example 3-3

[0092] The cleaning scheme in Example 3-3 is generally similar to that in Example 1-3, with the only difference being:

[0093] ① The parameters for cleaning the pipes inside the waste acid concentration heat exchanger differ. In Examples 3-3, the flow rates of both the aging acid and process water are 15.3 m³ / s. 3 / h (design feed rate of raw materials 17m) 3 / h, which is 90% of the designed feed rate of raw materials), the cleaning time of each aging acid is 20h, the cleaning time of each process water is 10h, the temperature of the aging acid is 60℃, the mass concentration of the aging acid is ≥42%, the temperature of the process water is 85℃, and the aging acid and process water are used alternately to clean the pipes inside the waste acid concentration heat exchanger 3 times.

[0094] ② The parameters for cleaning the external pipes of the waste acid concentration heat exchanger are different. In Example 3-3, HF:HCl = 4:1 (mass ratio), the total mass concentration of the mixed acid of HF and HCl is 4%, the temperature is 85℃, and the external pipes of the waste acid concentration heat exchanger are cleaned 3 times with process water at 60℃. The soaking time of the mixed acid of HF and HCl is 20h.

[0095] Table 3 Comparison of results from Example 3 with Comparative Examples 1 and 2

[0096]

[0097] A comparison of the results of Example 3 and Comparative Examples 1 and 2 shows that:

[0098] Comparative Example 1 replaced the waste acid concentration heat exchanger directly after one cycle of operation. Although the concentration effect was guaranteed, the cycle and cost of sending it out for chemical cleaning were quite high, making it impossible for sulfuric acid process titanium dioxide enterprises to bear the time and cost.

[0099] Comparative Example 2 did not clean the waste acid concentration heat exchanger. Although this saved on cleaning cycles and costs, the severe scaling in the pipes of the waste acid concentration heat exchanger caused the concentration of the aging acid obtained after the waste acid concentration and aging reaction to decrease to 45.0% during subsequent runs. This was a decrease of 10.8% from the initial aging acid concentration of 55.8%, which was a significant drop. Furthermore, the pipes inside the waste acid concentration heat exchanger would become completely blocked after the third run, rendering the waste acid concentration system inoperable.

[0100] In Example 3-1, after the new waste acid concentration system had been running continuously and stably for 12 days, the concentration of aging acid was basically restored compared to that in Comparative Example 1 due to the cleaning of the pipes inside the waste acid concentration heat exchanger. This reflects that the waste acid concentration effect was also basically restored. Moreover, the cost and cleaning cycle of a single cleaning were much less than the direct replacement method in Comparative Example 1. Sulfuric acid process titanium dioxide production enterprises can fully accept such time and cost.

[0101] Although the internal pipes of the waste acid concentration heat exchanger in Example 3-2 were cleaned, the external pipes were not cleaned after six months of operation of the new waste acid concentration system. Although the scaling rate of the external pipes was slow, the scale layer gradually increased in thickness after six months of operation, significantly impacting the heat transfer efficiency of the waste acid concentration heat exchanger. This is evidenced by the decrease in the outlet temperature of the secondary heat exchanger, from 81°C in Comparative Example 1 to 74°C in this example, and the concentration of hydrated acid decreased from 55.8% in Comparative Example 1 to 52.3% in this example. Therefore, after a certain period of operation, the external pipes of the waste acid concentration heat exchanger, which uses secondary steam as a heat source, need to be cleaned.

[0102] Example 3-3 not only cleaned the internal pipes of the waste acid concentration heat exchanger every 12 days, but also cleaned the external pipes after the waste acid concentration system had been running for six months. This cleaning scheme allowed the heat transfer effect of the waste acid concentration heat exchanger to be restored after cleaning. As can be seen from the data of Example 3-3, the mass concentration of the hydrated acid (55.8%), the outlet temperature of the first-stage heat exchanger (108°C), and the outlet temperature of the second-stage heat exchanger (81°C) in Example 3-3 all returned to the level of Comparative Example 1. At the same time, the cost and cleaning cycle of each cleaning of the internal and external pipes of the waste acid concentration heat exchanger remained very low.

[0103] In summary, the cleaning method in Examples 3-3 helps maintain the stable operation of waste acid concentration with high efficiency and relatively low operating costs.

[0104] The technical solution of this invention is expected to enable rapid, economical, and effective cleaning of waste acid concentration heat exchangers in sulfuric acid process titanium dioxide production, significantly reducing the labor intensity of workers and improving the efficiency of waste acid concentration. Furthermore, the application of this invention requires no modification to on-site equipment and pipelines, making it easy to implement. Currently, waste acid concentration processes and equipment both domestically and internationally are largely similar, employing heating and vacuum evaporation concentration techniques, and using graphite heat exchangers. Therefore, this invention can be widely promoted and applied at waste acid concentration sites in sulfuric acid process titanium dioxide enterprises both domestically and internationally.

[0105] The application effect of the technical solution of this invention is expected to be very significant. After application, the efficiency of waste acid concentration will be improved and the hoisting cost will be reduced. It is estimated that the waste acid recycling rate will increase from the original 60% to 75%, and the limestone consumption per ton of titanium dioxide required for waste acid neutralization will be significantly reduced, with an average reduction of 1.16 tons. Therefore, the annual economic benefit = difference in limestone consumption per ton × average limestone price per ton × annual titanium dioxide production + hoisting cost = 1.16 × 123.21 × 13.7 + 200 = 2158.05 (ten thousand yuan). Therefore, this application has excellent economic benefits.

[0106] It should be noted that the components or steps in the above embodiments can be interchanged, substituted, added, or deleted. Therefore, the combinations formed by these reasonable permutations and transformations should also fall within the protection scope of this invention, and the protection scope of this invention should not be limited to the above embodiments.

[0107] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention (including the claims) is limited to these examples. Within the framework of the invention, technical features of the above embodiments or different embodiments can be combined, and many other variations of the different aspects of the invention as described above exist, which are not provided in the details for the sake of brevity. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the invention should be included within the protection scope of the invention.

Claims

1. A cleaning method for a waste acid concentration heat exchanger in the sulfuric acid process for titanium dioxide production, characterized in that, Includes the following steps: 1) Clean the pipes inside the waste acid concentrate heat exchanger. A quenching acid is introduced into the pipes of the waste acid concentration heat exchanger to circulate and clean the pipes for a predetermined time. Then the quenching acid is discharged. The waste acid generated in the sulfuric acid process for titanium dioxide is concentrated and then subjected to a quenching reaction in a quenching acid storage tank to obtain the quenching acid. The temperature of the quenching acid is 20℃-60℃, the mass concentration of the quenching acid is ≥42%, and the quenching acid includes ferrous sulfate. Process water is introduced into the pipes inside the waste acid concentration heat exchanger and circulated to clean the pipes inside the waste acid concentration heat exchanger for a predetermined time, and then the process water is discharged. Repeat the above steps until the pipes inside the waste acid concentration heat exchanger are thoroughly cleaned. 2) Clean the external pipes of the waste acid concentrate heat exchanger. A low-concentration mixture of HF and HCl is introduced into the external pipe of the waste acid concentration heat exchanger to soak the external pipe for a predetermined time, and then the mixture is discharged. Repeat the above steps until the external pipes of the waste acid concentration heat exchanger are thoroughly cleaned. Process water is introduced into the external pipe of the waste acid concentration heat exchanger to clean the residual low-concentration HF and HCl mixed acid.

2. The method according to claim 1, characterized in that, In step 1), the circulation time of the aging acid is 12h-24h, and the feed amount of the aging acid is 80%-100% of the designed feed amount of the waste acid raw material.

3. The method according to claim 2, characterized in that, In step 1), the temperature of the process water is 80℃-90℃, and the circulation time of the process water is 8h-12h.

4. The method according to claim 3, characterized in that, In step 1), the aging acid and the process water are circulated to clean the pipes inside the waste acid concentration heat exchanger 2-3 times.

5. The method according to claim 1, characterized in that, In step 2), the total mass concentration of the mixed acid of low-concentration HF and HCl is 3%-5%, the mass ratio of low-concentration HF to HCl is 2:1-4:1, the temperature of the mixed acid of low-concentration HF and HCl is 80℃-90℃, and the soaking time is 12h-24h.

6. The method according to claim 5, characterized in that, In step 2), the waste acid concentrate heat exchanger external pipes are soaked 2-3 times with a mixed acid of low concentration HF and HCl.

7. The method according to claim 1, characterized in that, Fresh steam is used as a heat source to heat the aging acid, the process water, and the mixed acid of low concentration HF and HCl.

8. The method according to claim 1, characterized in that, The time interval between two consecutive cleanings of the internal pipes of the waste acid concentrate heat exchanger is 10-12 days, and the time interval between two consecutive cleanings of the external pipes of the waste acid concentrate heat exchanger is 160-190 days.

Images