A crystallization method and special device with strictly controlled crystal size

Abstract

The present invention discloses a crystallization method and dedicated device with strictly controlled crystal particle size. After passing through a primary cooler, the solution splits into two streams, one entering the primary crystallizer and the other entering a secondary cooler, and then entering the secondary crystallizer. The supercooled solution from the secondary cooler mixes with the mother liquor from the mother liquor pump in the secondary crystallizer. The crystal slurry in the secondary crystallizer is transported from the bottom to the primary crystallizer via the secondary crystal slurry pump. In the primary crystallizer, the solution from the primary cooler mixes with the crystal slurry from the secondary crystal slurry pump, and crystals continue to grow. The crystal slurry from the primary crystallizer is transported to a thickener for thickening. The thickened crystal slurry enters a centrifuge for solid-liquid separation. The mother liquors from the thickener and centrifuge are collected in a mother liquor tank. The material at the bottom of the mother liquor tank enters the secondary crystallizer via the mother liquor pump for circulation. The present invention achieves precise control of crystal particle size and prevents the formation of scaling of fine crystals on the heat exchange surfaces of the cooler.

Description

Technical Field

[0001] The present invention relates to a crystallization method and a device. Background Art

[0002] Several conventional general-purpose cooling crystallizers, such as forced circulation and OLSO types, all utilize slurry circulation to achieve cooling. Whether using slurry circulation or fully mixed operation, both crystallizers suffer from a significant disadvantage: the collision of the crystals collected by the circulating slurry with the high-pressure impeller generates a large number of secondary crystal nuclei, reducing the average particle size of the product and producing a large number of fine crystals, which increases the CV value. Another disadvantage of slurry circulation is that it limits production capacity, as the liquid circulation flow rate and suspension density must be restricted, limiting the clearing interface of the suspension in the crystallizer below the overflow port to prevent significant crystals from being carried over into the mother liquor.

[0003] When the circulating liquid overflows from the top of the crystallizer, the solution basically does not contain crystal grains, thus avoiding the contact nucleation phenomenon between the impeller and the crystal grains. The crystal size classification effect is more favorable. However, it is often difficult to achieve precise control in industrial production, and the circulating liquid will still contain a certain number of crystal grains.

[0004] Even if the circulating liquid contains only a small amount of fine crystals, when the circulating liquid passes through the cooler, the solution temperature drops and becomes supersaturated, and nucleation is likely to occur in the solution. These crystal nuclei will adhere to the heat exchange surface of the cooler and form a large area of ​​crystal scale after continuous enrichment.

[0005] Due to the above shortcomings, the operating cycle of cooling crystallizers in actual industrial applications is relatively short, requiring frequent switching or shutdown for cleaning, and even the phenomenon of heat exchange tubes being blocked by crystals. Summary of the Invention

[0006] The object of the present invention is to provide a crystallization method and a special device with strictly controlled crystal size, which can achieve accurate control of crystal size and avoid scaling of fine crystals on the heat exchange surface of the cooler.

[0007] The technical solution of the present invention is:

[0008] A crystallization method with strictly controlled crystal size, characterized by comprising the following steps:

[0009] Step 1: After passing through the primary cooler, the solution splits into two streams. One stream enters the primary crystallizer to promote crystal growth; the other stream enters the secondary cooler and then enters the secondary crystallizer. The secondary cooler keeps the solution in a certain supercooled state to promote crystal nucleation.

[0010] Step 2: The supercooled solution from the secondary cooler is mixed with the mother liquor from the mother liquor pump in the secondary crystallizer, which not only eliminates the supersaturation of the supercooled solution but also provides space for the growth of fine crystals.

[0011] Step 3: The slurry in the secondary crystallizer is transported from the bottom to the primary crystallizer through the secondary crystallizer pump;

[0012] Step 4: In the first-stage crystallizer, the solution from the first-stage cooler is mixed with the slurry from the second-stage slurry pump, and the crystals continue to grow;

[0013] Step 5: The slurry from the primary crystallizer leaves the bottom of the primary crystallizer and enters the primary slurry pump, and is transported to the thickener for thickening. The thickened slurry enters the centrifuge for solid-liquid separation to separate the crystals from the mother liquor. The mother liquor from the thickener and centrifuge is collected in the mother liquor tank. The material at the bottom of the mother liquor tank (i.e., mother liquor) enters the secondary crystallizer through the mother liquor pump to participate in the circulation, and the upper clear liquid overflows and leaves the crystallization system.

[0014] The solution passing through the primary cooler is divided into two streams, one entering the primary crystallizer and the other entering the secondary cooler. The flow ratio of the two solutions is interlocked with the particle size in the secondary crystallizer. When the particle size in the secondary crystallizer deviates from the set upper limit of the crystal particle size, the flow entering the secondary cooler is reduced and the flow to the primary crystallizer is increased; conversely, when the particle size in the secondary crystallizer deviates from the set lower limit of the crystal particle size, the flow entering the secondary cooler is increased and the flow to the primary crystallizer is reduced.

[0015] The temperature difference between the discharge temperature of the secondary cooler and the cooling medium is controlled between 2~4℃. The solution outlet temperature and the low-temperature water temperature of the secondary cooler shell side are interlocked. By adjusting the low-temperature water flow rate, the temperature difference between the secondary cooler outlet temperature and the cooling medium is kept within the set range.

[0016] The operating temperature of the secondary crystallizer is set according to the super solubility curve of the specific solution. The temperature inside the secondary crystallizer is interlocked with the flow rate of the mother liquor pump. When a lower temperature is required, the flow rate of the mother liquor pump is reduced; conversely, when a higher temperature is required, the flow rate of the mother liquor pump is increased.

[0017] A special device for a crystallization method with strictly controlled crystal particle size is characterized by comprising: a primary cooler, the outlet of the primary cooler being divided into two pipelines connected to a secondary cooler and a primary crystallizer respectively, the lower slurry outlet of the secondary cooler being connected to the primary crystallizer via a secondary slurry pump, the lower slurry outlet of the primary crystallizer being connected to a thickener via a primary slurry pump, the upper mother liquor outlet of the thickener being connected to a mother liquor tank via a pipeline, the lower slurry outlet of the thickener being connected to an inlet of a centrifuge, the mother liquor outlet of the centrifuge being connected to the mother liquor tank via a pipeline, the centrifuge being provided with a crystal outlet, the upper part of the mother liquor tank being provided with a mother liquor overflow outlet, and the lower mother liquor outlet of the mother liquor tank being connected to the secondary crystallizer via a mother liquor pump.

[0018] The present invention achieves precise control of crystal particle size, and the mother liquor is cooled without circulating through the cooler, thereby avoiding the occurrence of scaling of fine grains on the heat exchange surface of the cooler. The nucleation and growth of the crystals are carried out in separate spaces, avoiding the back-mixing of different crystal particles in the slurry. Combined with detection instruments and automatic control systems, the long-term stable operation of cooling crystallization can be ensured. The present invention is suitable for systems in which the solubility of salt in the solution changes significantly or moderately with temperature, such as KCl, NaNO3, CuSO4*5H2O, Na2SO4*10H2O, FeSO4*7H2O, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The present invention will be further described below with reference to the accompanying drawings and examples.

[0020] Figure 1 It is a schematic diagram of a special device for the crystallization method of the present invention with strictly controlled crystal particle size.

[0021] In the figure: ① primary cooler, ② secondary cooler, ③ primary crystallizer, ④ secondary crystallizer, ⑤ primary slurry pump, ⑥ secondary slurry pump, ⑦ mother liquor pump, ⑧ mother liquor tank, ⑨ thickener, ⑩ centrifuge. DETAILED DESCRIPTION

[0022] Example 1:

[0023] A total flow rate of 3 t / h of ferrous sulfate solution with a temperature of 42°C and a mass concentration of 25% enters the primary cooler, where the temperature is reduced to 25-27°C. 2.4 t / h enters the primary crystallizer, and the remaining 0.6 t / h enters the secondary cooler for further cooling, with the final cooling temperature reaching 8°C. Both the primary and secondary crystallizers are equipped with double-paddle agitators operating at a stirring speed of 35 rpm. The solids content of the slurry in the primary crystallizer is 23%, and the average crystal particle size is 1.2 mm. The slurry from the primary crystallizer enters the primary slurry pump at the bottom and is transported to the thickener. The thickener is equipped with an agitator to gently stir the slurry, thickening it to a solids content of 45%. The material at the bottom of the thickener flows by gravity into an HR400 two-stage piston pusher centrifuge, producing 660 kg / h of ferrous sulfate heptahydrate (FeSO4*7H2O) with a moisture content of ≤5% at the centrifuge outlet.

[0024] The mother liquor from the centrifuge and the overflow from the thickener are collected in a mother liquor tank. The mother liquor tank is not agitated. The solution containing a small amount of crystals flows from the bottom of the tank into the mother liquor pump and is then transported to the secondary crystallizer. In the secondary crystallizer, low-temperature solution from the secondary cooler enters from the top, while solution from the mother liquor pump enters from the bottom. The two streams mix within the secondary crystallizer. The solution from the mother liquor pump helps eliminate supersaturation from the secondary crystallizer and promotes the growth of fine crystals in the mother liquor. The material temperature in the secondary crystallizer is 10°C, and the average crystal size in the secondary crystallizer is 0.7-0.8 mm. The low-temperature solution from the secondary crystallizer forms crystal nuclei when it exceeds the super solubility curve (at the above temperature, the solubility of ferrous sulfate is 20.8 and the super solubility is 23.5).

[0025] The shell side of the primary and secondary coolers uses low-temperature water (5°C). The primary cooler reduces the feed temperature to 25°C-27°C, and the secondary cooler cools it to 8°C. The feed passes through the secondary cooler in a single pass, without any mother liquor circulating through the secondary cooler. This solves the problem of scaling and scarring of fine crystals on heat exchange surfaces during the mother liquor circulation cooling process in conventional freeze-crystallization processes. The secondary crystallizer primarily functions for nucleation, while the primary crystallizer primarily functions for crystal growth. This design offers significant advantages, reducing crystal backmixing while maintaining a highly controlled nucleation count and crystal size throughout the entire process.

[0026] Comparative Example 1: After leaving the primary cooler, the 3t / h ferrous sulfate solution all enters the secondary cooler;

[0027] Comparative Example 2: The ferrous sulfate solution at the outlet of the mother liquor pump is cooled in a secondary cooler and then enters a secondary crystallizer.

[0028] Example 1 Comparative Example 1 Comparative Example 2 Average particle size 1.2mm 0.45mm 0.8mm Particle size range 0.9~1.4mm 0.3~0.7mm 0.4~1.0mm CV value 38% 61% 72% Operation cycle ≥55 days 14 days 3 days Example

[0029] A sodium sulfate solution with a temperature of 40°C and a mass concentration of 20% is flowed at a total flow rate of 5 t / h. It enters the primary cooler, where the temperature is reduced to 12°C. 4.5 t / h enters the primary crystallizer, and the remaining 0.5 t / h enters the secondary cooler for further cooling, with the final cooling temperature reaching 8°C. Both the primary and secondary crystallizers are equipped with double-paddle agitators operating at a stirring speed of 35 rpm. The solids content of the slurry in the primary crystallizer is 28%, and the average crystal particle size is 0.9 mm. The slurry from the primary crystallizer enters the primary slurry pump at the bottom and is transported to the thickener. The thickener is equipped with an internal agitator to gently stir the slurry, thickening it to a solids content of 45%. The material at the bottom of the thickener flows by gravity into an HR400 two-stage piston pusher centrifuge, producing 2800 kg / h of sodium sulfate decahydrate (Na2SO4*10H2O) with a moisture content of ≤5%.

[0030] The mother liquor from the centrifuge and the overflow from the thickener are combined and flow into the mother liquor tank. The mother liquor tank is not agitated. The solution containing a small amount of crystals flows from the bottom of the tank into the mother liquor pump and is then transported to the secondary crystallizer. In the secondary crystallizer, low-temperature solution from the secondary cooler enters from the top, while solution from the mother liquor pump enters from the bottom. The two streams mix within the secondary crystallizer. The solution from the mother liquor pump helps eliminate supersaturation from the secondary crystallizer and promotes the growth of fine crystals in the mother liquor. The material temperature in the secondary crystallizer is 10°C, and the average crystal size in the secondary crystallizer is 0.4-0.5 mm. The low-temperature solution from the secondary crystallizer forms crystal nuclei when it exceeds the super solubility curve (at the above temperature, the solubility of sodium sulfate is 9 and the super solubility is 12.5).

[0031] The shell side of the primary and secondary coolers uses low-temperature water (5°C). The primary cooler reduces the feed temperature to 12°C, and the secondary cooler to 8°C. The feed passes through the secondary cooler in a single pass, without any mother liquor circulating through the secondary cooler. This solves the problem of scaling and scarring of fine crystals on heat exchange surfaces during the mother liquor circulation cooling process in conventional freeze-crystallization processes. The secondary crystallizer primarily functions for nucleation, while the primary crystallizer primarily functions for crystal growth. This design offers significant advantages, reducing crystal backmixing while maintaining a highly controlled nucleation count and crystal size throughout the entire process.

[0032] Comparative Example 3: After leaving the primary cooler, the 5t / h sodium sulfate solution all enters the secondary cooler;

[0033] Comparative Example 4: The sodium sulfate solution at the outlet of the mother liquor pump is cooled in a secondary cooler and then enters a secondary crystallizer;

[0034] Example 2 Comparative Example 3 Comparative Example 4 Average particle size 0.9mm 0.5mm 0.6mm Particle size range 0.7~1.1mm 0.3~0.7mm 0.4~0.9mm CV value 53% 61% 64% Operation cycle ≥35 days 16 days 8 days

[0035] This shows the excellent technical effects of the present invention.

Claims

1. A crystallization method with strictly controlled crystal size, characterized by: The following steps are involved: Step 1: After passing through the primary cooler, the solution is divided into two streams. One stream enters the primary crystallizer to promote crystal growth; the other stream enters the secondary cooler and then enters the secondary crystallizer. The secondary cooler puts the solution in a certain supercooled state to promote crystal nucleation; Step 2: The supercooled solution from the secondary cooler is mixed with the mother liquor from the mother liquor pump in the secondary crystallizer, which not only eliminates the supersaturation of the supercooled solution but also provides space for the growth of fine crystals. Step 3: The slurry in the secondary crystallizer is transported from the bottom to the primary crystallizer through the secondary crystallizer pump; Step 4: In the first-stage crystallizer, the solution from the first-stage cooler is mixed with the slurry from the second-stage slurry pump, and the crystals continue to grow; Step 5: The slurry from the primary crystallizer leaves the bottom of the primary crystallizer and enters the primary crystal slurry pump, and is transported to the thickener for thickening. The thickened slurry enters the centrifuge for solid-liquid separation to separate the crystals from the mother liquor. The mother liquor from the thickener and centrifuge is collected in the mother liquor tank. The material at the bottom of the mother liquor tank enters the secondary crystallizer through the mother liquor pump to participate in the circulation. The upper clear liquid overflows and leaves the crystallization system. The flow ratio of the two solutions is interlocked with the particle size in the secondary crystallizer.

2. A crystallization method with strictly controlled crystal size according to claim 1, characterized in that: The solution after the primary cooler is divided into two streams, one entering the primary crystallizer and the other entering the secondary cooler. When the particle size in the secondary crystallizer deviates from the set upper limit of the crystal size, the flow rate entering the secondary cooler is reduced and the flow rate to the primary crystallizer is increased. On the contrary, when the particle size in the secondary crystallizer deviates from the set crystal particle size lower limit, the flow rate entering the secondary cooler is increased and the flow rate to the primary crystallizer is reduced.

3. The crystallization method according to claim 1, wherein: The temperature difference between the discharge temperature of the secondary cooler and the cooling medium is controlled between 2~4℃. The solution outlet temperature and the low-temperature water temperature of the secondary cooler shell side are interlocked. By adjusting the low-temperature water flow rate, the temperature difference between the secondary cooler outlet temperature and the cooling medium is kept within the set range.

4. The crystallization method according to claim 1, wherein: The operating temperature of the secondary crystallizer is set according to the super solubility curve of the specific solution. The temperature inside the secondary crystallizer is interlocked with the flow rate of the mother liquor pump. When a lower temperature is required, the flow rate of the mother liquor pump is reduced; conversely, when a higher temperature is required, the flow rate of the mother liquor pump is increased.

5. A dedicated device for the crystallization method with strictly controlled crystal size according to claim 1, characterized in that: It includes a primary cooler, the outlet of the primary cooler is divided into two pipelines respectively connected to the secondary cooler and the primary crystallizer, the lower slurry outlet of the secondary cooler is connected to the primary crystallizer through a secondary slurry pump, the lower slurry outlet of the primary crystallizer is connected to the thickener through a primary slurry pump, the upper mother liquor outlet of the thickener is connected to the mother liquor tank through a pipeline, the lower slurry outlet of the thickener is connected to the centrifuge inlet, the mother liquor outlet of the centrifuge is connected to the mother liquor tank through a pipeline, the centrifuge is provided with a crystal outlet, the upper part of the mother liquor tank is provided with a mother liquor overflow outlet, and the lower mother liquor outlet of the mother liquor tank is connected to the secondary crystallizer through a mother liquor pump.

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