A compressed air crystallization system and process

Abstract

This invention discloses a compressed air crystallization system and process, belonging to the field of titanium dioxide production technology. It solves the problems of low cooling efficiency, uneven grain size, and poor energy consumption and economy in traditional crystallization processes. The invention includes the following steps: pumping concentrated titanium liquid into a crystallization pot; turning on the vacuum pump to reduce the pressure inside the crystallization pot to -0.1~-0.08MPa; introducing compressed air for auxiliary cooling; reducing the solubility of ferrous sulfate, causing crystal nucleation and growth; after reaching the final temperature, turning off the compressed air, maintaining vacuum and stirring for 8~12 minutes; and conveying the crystal slurry to a filtration system. The purpose is to effectively solve the problems of uneven liquid cooling, local supersaturation, and long nucleation time in traditional natural cooling vacuum crystallization. Compared with natural cooling that relies on latent heat of vaporization and heat dissipation from the vessel wall, this invention can significantly shorten the cooling time of titanium liquid, accelerate the cooling rate to a controllable range, and make the entire crystallization process more efficient.

Description

Technical Field

[0001] This invention belongs to the field of titanium dioxide production technology, specifically relating to a compressed air crystallization system and process. Background Technology

[0002] In the sulfuric acid process for titanium dioxide production, the crystallization process is the core step in purifying the titanium liquid. Its purpose is to cool and concentrate the titanium liquid, causing dissolved ferrous sulfate to precipitate in crystalline form, thereby reducing the iron content of the titanium liquid and preventing the impact of iron ions on the whiteness and purity of the product during subsequent hydrolysis and calcination. Current cooling crystallization operations generally employ a natural cooling vacuum crystallization process. This utilizes a vacuum system to reduce the pressure inside the crystallizer, lowering the boiling point of the titanium liquid. Water evaporation absorbs heat, achieving system cooling. The titanium liquid cools naturally from the feed temperature to the final temperature, relying on latent heat of vaporization and passive heat exchange through the vessel walls. After feeding, a vacuum pump and stirrer are started, relying on vacuum evaporation and... Natural convection cooling is slow and requires a typical cooling time of 127 minutes, accounting for 63.5% of a single batch production cycle. The natural cooling rate is only 0.2~0.3℃ / min, resulting in excessively long crystallization time, which becomes a bottleneck restricting production capacity. Based on an annual production scale of 150,000 tons, the annual loss of effective production capacity is about 8.5%. The slow cooling causes the crystal growth rate to exceed the nucleation rate, resulting in larger crystal size, fewer crystal particles, long crystallization time, and low crystallization efficiency. Furthermore, due to the excessively long time span for cooling, crystallization, and nucleation, uneven crystal size is very likely to occur. Therefore, traditional crystallization processes suffer from low cooling efficiency, uneven crystal size, and poor energy consumption and economy. Summary of the Invention

[0003] To address the aforementioned technical problems, this invention provides a compressed air crystallization process. By introducing compressed air into the gas phase space above the liquid surface and coordinating it with the vacuum pump's extraction system, a circumferential turbulent airflow is formed on the liquid surface. Simultaneously, blades within the crystallization pot push the uncrystallized titanium liquid to circulate, thereby achieving a uniform distribution of liquid temperature and concentration. This effectively solves the problems of uneven liquid cooling, localized oversaturation, and long nucleation time spans in traditional natural cooling vacuum crystallization. Compared to natural cooling that relies on latent heat of vaporization and heat dissipation from the vessel wall, this invention can significantly shorten the titanium liquid cooling time, accelerate the cooling rate to a controllable range, and make the entire crystallization process more efficient.

[0004] The technical solution adopted in this invention is as follows:

[0005] A compressed air crystallization system includes a crystallization pot, characterized in that the crystallization pot has an air inlet structure inside, a feed inlet is provided on the inner wall of the crystallization pot, and the crystallization pot is also connected to a vacuum pump through a pipe; the air outlet of the crystallization pot connected to the vacuum pump is located above the air inlet structure and at the top center of the crystallization pot, the feed inlet is located on the side wall of the crystallization pot below the air inlet structure, and the lower part of the crystallization pot has a discharge outlet.

[0006] Preferably, the air intake structure includes an air hood, which is a bowl-shaped structure with an opening at the center. Multiple nozzles are evenly arranged at the bottom of the air hood, and the multiple nozzles are connected to an external air source through pipes.

[0007] Preferably, the lower part of the air inlet structure is further provided with a stirring shaft, which is parallel to the horizontal plane and has multiple blades, each blade being arranged along the length of the stirring shaft; the stirring shaft and the blades are both located below the feed inlet; a temperature monitoring module is provided below the stirring shaft and the blades, and the temperature monitoring module is installed on the side wall of the crystallization pot.

[0008] A compressed air crystallization process includes the following steps:

[0009] Step 1: Pump the concentrated titanium liquid into the crystallization pot through the feed inlet;

[0010] Step 2: Start the vacuum pump to reduce the pressure inside the crystallization pot to -0.1~-0.08 MPa;

[0011] Step 3: Compressed air is introduced into the gas phase space above the liquid surface through the nozzle to form a turbulent airflow above the liquid surface; at the same time, the crystallizing liquid is stirred by the blades, so that the liquid flows from the bottom of the crystallization pot to the liquid surface and then down to the wall of the crystallization pot to form a circulation flow; during the process of introducing compressed air, the vacuum pump is used to extract air, and the coupling control of compressed air and vacuum extraction is carried out to make ferrous sulfate crystals grow uniformly in the liquid.

[0012] Step 4: After reaching the set final temperature, stop introducing compressed air, maintain vacuum pumping and stirring for 8-12 minutes to promote the maturation and further growth of ferrous sulfate crystals;

[0013] Step 5: The crystal slurry is conveyed through the outlet to the filtration system for separation.

[0014] Preferably, in step 1, the concentration of the concentrated titanium liquid is 126~130 g / L, and the initial temperature is 50~65℃; the liquid level of the titanium liquid is controlled at 70~80% to ensure uniform liquid circulation.

[0015] Preferably, in step 3, the tips of the upper blades are 10-15 cm above the liquid surface, and there are 3-6 blades. The tips of the bottom blades are 20-50% of the diameter of the crystallizing pot from the outlet.

[0016] Preferably, in step 3, the stirring speed is 20~40 rpm, the stirring power is 7.5 kW, and the stirring speed is controlled in conjunction with the compressed air supply.

[0017] Preferably, in step 3, the compressed air inlet pressure is 0.4~0.6 MPa.

[0018] Furthermore, when the pressure inside the crystallization pot is ≥-0.08 MPa, the compressed air flow rate is automatically reduced or the pumping speed of the vacuum pump is increased; when the pressure inside the crystallization pot is ≤-0.1 MPa, the air flow rate is automatically increased or the pumping speed of the vacuum pump is reduced, thereby achieving coupled control of compressed air and vacuum pumping.

[0019] Preferably, in step 3, the compressed air is introduced in a phased control manner, with the first phase used for rapid cooling and the second phase used for controlling nucleation and protecting crystal growth.

[0020] Furthermore, based on the total air intake flow rate, it is divided into multiple nozzle inputs. In the initial rapid cooling stage, the total compressed air intake is 100~150L / min, and the stirring speed is 30~40 rpm; in the crystallization and nucleation stage, the total compressed air intake is 70~100L / min, and the stirring speed is 25~35 rpm; in the final temperature curing stage, the total compressed air intake is 50~70L / min, and the stirring speed is 20~25 rpm.

[0021] Furthermore, the initial rapid cooling stage is at a temperature of 40~65℃, the crystallization and nucleation stage is at a temperature of 38~40℃, during which the solute can be observed to gradually precipitate; the final temperature ripening stage is at a temperature of 37~38℃; the final temperature is 37℃, at which point the average particle size is 150μm, and the stirring speed is maintained at 20~25 rpm.

[0022] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are:

[0023] 1. This invention introduces compressed air into the gas phase space above the liquid surface and coordinates it with the vacuum pump's extraction system to create a circumferential turbulent airflow on the liquid surface. Compressed air is continuously introduced through nozzles, forming a circumferential turbulent airflow above the liquid surface. The low-temperature air absorbs heat from the liquid upon contact with it and is then drawn away by the vacuum pump at the top of the crystallization pot, creating a continuous airflow circulation. This continuously removes heat from the liquid surface, achieving rapid and uniform cooling of the liquid surface. Simultaneously, it reduces the liquid surface temperature gradient, ensuring that the titanium liquid remains in a uniform supersaturated state throughout the crystallization process, which is beneficial for the homogeneous nucleation growth of ferrous sulfate crystals. Furthermore, the blades within the crystallization pot promote the circulation of the undried titanium liquid, thus achieving a uniform distribution of liquid temperature and concentration. This invention effectively solves the problems of uneven liquid cooling, localized supersaturation, and long nucleation time in traditional natural cooling vacuum crystallization. Compared to natural cooling that relies on latent heat of vaporization and heat dissipation from the vessel wall, this invention can significantly shorten the titanium liquid cooling time, accelerate the cooling rate to a controllable range, and make the entire crystallization process more efficient.

[0024] 2. By coupling the control of compressed air, vacuum pumping, and stirring speed, uniform nucleation growth of ferrous sulfate crystals in liquid is achieved. In traditional processes, slow cooling leads to a crystal growth rate exceeding the nucleation rate, easily resulting in large, few crystals and uneven crystal size distribution. This invention adjusts the compressed air flow rate and stirring speed in stages to provide optimal growth conditions for the crystals at different stages. In the initial rapid cooling stage, a large compressed air flow rate and high stirring speed can quickly reduce the liquid temperature and establish a uniform supersaturated state, providing conditions for nucleation. In the crystallization and nucleation stage, the air flow rate and stirring speed are moderately reduced to ensure uniform nucleation distribution while avoiding excessive liquid surface disturbance that could lead to crystal breakage. In the final temperature ripening stage, the air flow rate and stirring speed are further reduced to allow the crystals to grow stably under mild conditions, resulting in uniform particle size and a suitable particle size distribution. Through this precise control, ferrous sulfate crystals with uniform grains and sufficient particle quantity are obtained, reducing the risk of crystal breakage during crystal formation, which could lead to excessively small crystals penetrating the filter membrane.

[0025] 3. This allows the crystallization system to be promptly delivered to the filtration system after reaching the set temperature and crystal maturation state, ensuring the continuity of crystal slurry delivery and the efficiency of the filtration process. It also guarantees the stability of subsequent hydrolysis and calcination processes and reduces the adverse effects on product whiteness caused by uneven crystal size and excessive iron content in the residual liquid.

[0026] 4. By using compressed air to assist in cooling, the evaporation efficiency is improved. Compared with traditional natural cooling crystallization, the cooling time is reduced and the single-batch production cycle is shortened. This means that under the same production scale, energy input and equipment occupancy time are saved, and the capacity utilization rate is improved. Attached Figure Description

[0027] The present invention will be described by way of example and with reference to the accompanying drawings, wherein:

[0028] Figure 1 This is a schematic diagram of the connection structure of a compressed air crystallization system according to the present invention.

[0029] Figure label:

[0030] 1-Crystallization pot, 2-Vacuum pump, 3-Air hood, 4-Nozzle, 5-Inlet, 6-Stirring shaft, 7-Blade, 8-Outlet. Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0032] The following is combined with Figure 1 The present invention will be described in detail below.

[0033] Example 1

[0034] A compressed air crystallization system, see attached figure. Figure 1 The crystallization pot 1 is characterized in that an air inlet structure is provided inside the crystallization pot 1, a feed inlet 5 is provided on the inner wall of the crystallization pot 1, and the crystallization pot 1 is also connected to a vacuum pump 2 through a pipe; the air outlet of the crystallization pot 1 connected to the vacuum pump 2 is located above the air inlet structure and at the center of the top of the crystallization pot 1, the feed inlet 5 is located on the side wall of the crystallization pot 1 below the air inlet structure, and a discharge outlet 8 is provided at the lower part of the crystallization pot 1.

[0035] After feeding, the liquid level is always kept below the air inlet structure.

[0036] In this embodiment, the air intake structure includes an air hood 3, which is a bowl-shaped structure with an opening at the center. Multiple nozzles 4 are evenly arranged at the bottom of the air hood 3, and the multiple nozzles 4 are connected to an external air source through pipes.

[0037] In this embodiment, a stirring shaft 6 is also provided at the lower part of the air inlet structure. The stirring shaft 6 is parallel to the horizontal plane and has multiple blades 7. Each blade 7 is arranged along the length direction of the stirring shaft 6. The stirring shaft 6 and the blades 7 are both located below the feed inlet 5. A temperature monitoring module is provided below the stirring shaft 6 and the blades 7. The temperature monitoring module is installed on the side wall of the crystallization pot 1.

[0038] Example 2

[0039] A compressed air crystallization process includes the following steps:

[0040] Step 1: Pump the concentrated titanium liquid into the crystallization pot 1 through the feed port 5;

[0041] Step 2: Start vacuum pump 2 to reduce the pressure inside crystallization pot 1 to -0.09 MPa;

[0042] Step 3: Compressed air is introduced into the gas phase space above the liquid surface through nozzle 4 to form a turbulent airflow above the liquid surface; at the same time, the crystallizing liquid is stirred by blade 7, so that the liquid flows from the bottom of the crystallization pot 1 to the liquid surface and then sinks down the wall of the crystallization pot 1 to form a circulation flow; during the process of introducing compressed air, vacuum pump 2 is used to extract air, and the coupling control of compressed air and vacuum extraction is performed to make ferrous sulfate crystals grow uniformly in the liquid.

[0043] Step 4: After reaching the set final temperature, stop the introduction of compressed air, maintain vacuum pumping and stirring for 12 minutes to promote the maturation and further growth of ferrous sulfate crystals;

[0044] Step 5: The crystal slurry is conveyed to the filtration system through the discharge port 8 for separation.

[0045] In this embodiment, in step 1, the concentration of the concentrated titanium liquid is 128 g / L and the initial temperature is 60°C; the liquid level of the titanium liquid is controlled at 75% to ensure uniform liquid circulation.

[0046] In step 3 of this embodiment, the tip of the upper blade 7 is 12 cm above the liquid surface, and there are 6 blades 7. The tip of the bottom blade 7 is 35% of the diameter of the crystallization pot 1 from the outlet. The width of the blade 7 is 0.2 times the diameter of the crystallization pot 1.

[0047] In this embodiment, in step 3, the stirring speed is 35 rpm, the stirring power is 7.5 kW, and the liquid volume is maintained at 30 m³. 3 The stirring power to liquid volume ratio is 0.25 kW / m³. 3 It is also controlled in conjunction with the compressed air intake.

[0048] In this embodiment, in step 3, the compressed air supply pressure is 0.4~0.6 MPa.

[0049] The vacuum is maintained at approximately -0.09 MPa, with fluctuations not exceeding 0.005 MPa. The compressed air flow rate or the pumping rate of vacuum pump 2 is automatically adjusted to achieve coupled control of compressed air and vacuum pumping.

[0050] In this embodiment, in step 3, the compressed air is introduced in stages, with the first stage used for rapid cooling and the second stage used for controlling nucleation and protecting crystal growth.

[0051] In this embodiment, based on the total air intake flow rate, it is divided into multiple nozzles 4 inputs. In the initial rapid cooling stage, the total compressed air intake is 130 L / min, and the stirring speed is 35 rpm; in the crystallization and nucleation stage, the total compressed air intake is 90 L / min, and the stirring speed is 30 rpm; in the final temperature curing stage, the total compressed air intake is 60 L / min, and the stirring speed is 22 rpm.

[0052] In this embodiment, the initial rapid cooling stage is at a temperature of 40~60℃, the crystallization and nucleation stage is at a temperature of 38~40℃, and the gradual precipitation of solute can be observed; the final temperature ripening stage is at a temperature of 37~38℃; the final temperature is 37℃, which is the condition for reaching the final temperature. At this time, the average particle size is 150μm, and the stirring speed is maintained at 20~25 rpm.

[0053] Example 3

[0054] Unlike Example 2, in this example, with the liquid volume remaining constant, the total compressed air flow rate is 120 L / min and the stirring speed is 32 rpm during the initial rapid cooling stage; the total compressed air flow rate is 85 L / min and the stirring speed is 28 rpm during the crystallization and nucleation stage; and the total compressed air flow rate is 50 L / min and the stirring speed is 20 rpm during the final temperature ripening stage.

[0055] Comparative Example 1

[0056] With the liquid volume unchanged, only vacuum pump 2 and stirring blade 7 are turned on, without introducing compressed air, and the pressure inside the pot is maintained at -0.09 MPa; the stirring speed is 30 rpm throughout the process. After the temperature drops to 37℃, stirring and vacuum pump 2 are maintained for 10 minutes before discharging.

[0057] Comparative Example 2

[0058] Unlike Example 2, in this comparative example, with the liquid volume unchanged, the compressed air flow rate was kept constant at 120 L / min throughout the entire process, the stirring speed was 30 rpm throughout the entire process, the vacuum pump 2 pumping speed was not adjusted, and only normal fixed operation was maintained. The actual pressure fluctuation range inside the pot was between -0.075 MPa and -0.095 MPa.

[0059] Comparative Example 3

[0060] With the liquid volume remaining constant, the compressed air is controlled in stages as in Example 2, and the vacuum is controlled in conjunction with Example 2. However, the stirring speed is fixed at 40 rpm throughout the entire process, and the stirring rate is not reduced in the final temperature stage.

[0061] Comparative Example 4

[0062] With the liquid volume remaining constant, the compressed air flow rate is kept constant at 100 L / min throughout the process, and is linked to the vacuum pump 2 for control.

[0063] The crystallization methods according to Examples 1-2 and Comparative Examples 1-4 are shown in Table 1 below:

[0064] Table 1: Comparison of evaluation index data obtained from the crystallization methods of Examples 2-3 and Comparative Examples 1-4

[0065]

[0066] Examples 2 and 3 significantly shortened the total crystallization time, especially Example 2, which was shortened from 127 min to 110 min, a reduction of 17 min. Traditional natural vacuum crystallization, due to slow cooling, causes crystal nucleation to occur dispersed over a longer period of time, resulting in a large difference in growth time between crystals that nucleate sequentially, leading to a larger average particle size, a higher particle size distribution coefficient, and a significantly longer filtration time. In contrast, this invention uses compressed air assistance and the coupled control of compressed air flow rate, vacuum rate, and stirring rate to make the nucleation and growth process more concentrated and uniform, thereby obtaining crystals with a more suitable particle size and more uniform distribution.

[0067] Although Comparative Example 2 also introduced compressed air, the pressure fluctuation inside the pot was large and the mass transfer of liquid surface evaporation was unstable because the compressed air and vacuum pumping were not coupled and controlled. Therefore, it was not as good as Example 2 in terms of crystallization time, particle size distribution and filtration performance.

[0068] Comparative Example 3 is similar to Example 2 in terms of the initial cooling rate, but due to the high-speed stirring throughout the process, the crystals are subjected to greater shearing and collision during the later stage of crystal maturation, resulting in a higher proportion of fine crystals and a worse particle size distribution.

[0069] Comparative Example 4 shows that even with proper vacuum and stirring control, if the compressed air is kept constant throughout the process, it is impossible to simultaneously achieve rapid cooling in the front stage and crystal ripening in the back stage. Example 2 uses staged airflow control, which improves the cooling efficiency in the front stage and avoids excessive disturbance in the back stage, thus achieving the best overall effect.

[0070] It should be noted that:

[0071] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A compressed air crystallization system comprising a crystallization kettle (1), characterized in that, The crystallization pot (1) is provided with an air inlet structure inside. The crystallization pot (1) is provided with a feed inlet (5) on its inner wall. The crystallization pot (1) is also connected to a vacuum pump (2) through a pipe. The air outlet of the crystallization pot (1) connected to the vacuum pump (2) is located above the air inlet structure and at the top center of the crystallization pot (1). The feed inlet (5) is located on the side wall of the crystallization pot (1) below the air inlet structure. The crystallization pot (1) is provided with a discharge outlet (8) at its lower part.

2. A compressed air crystallization system as defined in claim 1, wherein, The air intake structure includes an air hood (3), which is a bowl-shaped structure with an opening at the center. Multiple nozzles (4) are evenly arranged at the bottom of the air hood (3), and the multiple nozzles (4) are connected to an external air source through pipes.

3. A compressed air crystallization system as defined in claim 1, wherein, The lower part of the air inlet structure is also provided with a stirring shaft (6), which is parallel to the horizontal plane. Multiple blades (7) are provided on the stirring shaft (6), and each blade (7) is arranged along the length direction of the stirring shaft (6). The stirring shaft (6) and the blades (7) are both located below the feed inlet (5). A temperature monitoring module is provided below the stirring shaft (6) and the blades (7), and the temperature monitoring module is installed on the side wall of the crystallization pot (1).

4. A compressed air crystallization process characterized by, Includes the following steps: Step 1: Pump the concentrated titanium liquid into the crystallization pot (1) through the feed port (5); Step 2: Start the vacuum pump (2) to reduce the pressure inside the crystallization pot (1) to -0.1~-0.08 MPa; Step 3: Compressed air is introduced into the gas phase space above the liquid surface through the nozzle (4) to form a turbulent airflow above the liquid surface; at the same time, the crystallizing liquid is stirred by the blade (7) so that the liquid flows down from the bottom of the crystallizing pot (1) to the liquid surface and down the wall of the crystallizing pot (1) to form a circulating flow; during the process of introducing compressed air, the vacuum pump (2) is used to pump air and the coupling control of compressed air and vacuum pumping is carried out so that ferrous sulfate crystals grow uniformly in the liquid. Step 4: After reaching the set final temperature, stop introducing compressed air, maintain vacuum pumping and stirring for 8-12 minutes to promote the maturation and further growth of ferrous sulfate crystals; Step 5: The crystal slurry is conveyed to the filtration system through the discharge port (8) for separation.

5. A compressed air crystallization process according to claim 4, wherein, In step 1, the concentration of the concentrated titanium liquid is 126~130 g / L, and the initial temperature is 50~65℃; the liquid level of the titanium liquid is controlled at 70~80% to ensure uniform liquid circulation.

6. The compressed air crystallization process according to claim 4, characterized in that, In step 3, the tips of the upper blades (7) are 10-15 cm above the liquid surface, and there are 3-6 blades (7). The tips of the bottom blades (7) are 20-50% of the diameter of the outlet (8) of the crystallizing pot (1).

7. The compressed air crystallization process according to claim 4, characterized in that, In step 3, the stirring speed is 20~40 rpm, the stirring power is 7.5 kW, and it is controlled in conjunction with the compressed air supply.

8. The compressed air crystallization process according to claim 4, characterized in that, In step 3, the compressed air is introduced at a pressure of 0.4~0.6 MPa.

9. The compressed air crystallization process according to claim 4, characterized in that, In step 3, the compressed air is introduced in stages, with the first stage used for rapid cooling and the second stage used to control nucleation and protect crystal growth.

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