A deep decalcification and magnesium removal method for industrial sodium chloride

Abstract

This invention relates to the field of inorganic salt purification and refining technology, and discloses a method for deep decalcification and demagnesification of industrial sodium chloride. The method includes: adding a homogeneous nucleation kinetic inhibitor to industrial crude brine and mixing; adding activated seed crystal slurry to the system to obtain a suspension slurry; extracting the suspension slurry for external circulation and micro-mixing, while simultaneously injecting sodium carbonate and sodium hydroxide aqueous solutions for stepwise dosing, dynamically controlling the pH rise; and performing mechanical solid-liquid separation after aging and maturation. This invention guides the heterogeneous epitaxial growth of calcium and magnesium precipitates into dense crystals through the synergistic regulation of nucleation inhibitors and activated seed crystals. The combination of micro-mixing and stepwise dosing effectively controls the local supersaturation of the system. Without the need for traditional polymeric flocculants, it improves the sedimentation and dewatering performance of the suspension slurry, reduces the physical entrainment loss of sodium chloride salts in the solid residue cake, and simultaneously reduces the organic pollution load of the refined brine.

Description

Technical Field

[0001] This invention relates to the field of inorganic salt purification and refining technology, specifically to a method for deep decalcification and demagnesification of industrial sodium chloride. Background Technology

[0002] In the refining process of industrial sodium chloride, it is usually necessary to remove impurity ions such as calcium and magnesium from the crude brine to meet the requirements of subsequent chlor-alkali chemical or vacuum salt production processes. Currently, chemical precipitation processes such as the double alkali method are commonly used in industry, which involves adding soda ash and caustic soda to the crude brine to convert calcium and magnesium ions in the system into calcium carbonate and magnesium hydroxide precipitates, respectively. However, in the traditional reaction vessel dosing and mixing process, uneven fluid mixing near the dosing point often results in high local supersaturation, which easily leads to rapid homogeneous nucleation. The precipitates generated by this nucleation method are mostly fine colloidal particles, which are fine and have a thick hydration layer on the surface. Macroscopically, this manifests as slow slurry settling speed and extremely difficult subsequent solid-liquid separation.

[0003] To improve the settling and filtration performance of fine particles, existing purification processes typically require the addition of high-molecular-weight organic flocculants such as polyacrylamide to the reaction system. These flocculants rely on the physical entrapment and bridging effects of the long polymer chains to aggregate these colloidal microcrystals. While this approach alleviates separation difficulties to some extent, it also introduces significant process drawbacks. The introduction of high-molecular-weight flocculants increases the total organic carbon content in the refined brine. In subsequent evaporation and crystallization processes, the enriched organic matter can easily cause system foaming, scaling on heat exchange surfaces, and excessive color in the crystalline salt product. Furthermore, because the flocs formed by flocculant aggregation have a relatively loose internal structure and numerous micropores, during mechanical pressure filtration, the flocs encapsulate and trap large amounts of high-concentration sodium chloride mother liquor. This not only results in a persistently high moisture content in the solid salt filter cake but also causes a significant loss of effective sodium chloride salt with the residue, thus increasing material losses and environmental treatment costs. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a method for deep decalcification and demagnesification of industrial sodium chloride. This method solves the problems of solid-liquid separation difficulties caused by the formation of fine colloidal precipitates, the loss of effective salts with salt mud, and secondary organic pollution caused by over-reliance on polymeric flocculants during the traditional purification and decalcification of industrial sodium chloride.

[0005] To achieve the above objectives, the present invention provides the following technical solution: Firstly, the present invention provides a method for deep decalcification and demagnesification of industrial sodium chloride, employing the following technical solution: A method for deep decalcification and demagnesification of industrial sodium chloride includes the following steps: Add a homogeneous nucleation kinetic inhibitor to the industrial crude brine to be treated, and start stirring to mix. The homogeneous nucleation kinetic inhibitor is an aqueous solution of polyhydroxycarboxylate. While maintaining stirring, add activated seed slurry to the mixed system and continue mixing to obtain a suspension slurry. The activated seed slurry is a mixed suspension of calcium carbonate and magnesium hydroxide with lattice defects on the surface and a high-density negative charge layer adsorbed by characteristic. The suspended slurry is extracted and subjected to external circulation fluid dynamic micro-mixing. At the same time, sodium carbonate aqueous solution and sodium hydroxide aqueous solution are injected into the mixing node of the external pipeline for step dosing, and the pH value of the suspended slurry is dynamically controlled to rise. After completely stopping the stepwise dosing, the suspended slurry is aged and matured to obtain the final slurry. The final slurry is then subjected to mechanical solid-liquid separation to collect the refined brine containing the target sodium chloride and the solid salt mud filter cake rich in impurities such as calcium and magnesium.

[0006] By employing the above technical solution, this invention introduces polyhydroxycarboxylate as a homogeneous nucleation kinetic inhibitor in the initial stage of the reaction. This effectively suppresses the explosive spontaneous nucleation of calcium and magnesium precipitates in the liquid phase and provides sufficient precipitation driving force for the subsequent controlled heterogeneous growth process by increasing the homogeneous nucleation activation energy. Simultaneously, the introduction of activated seed slurry with specific surface properties constructs a low-energy-barrier potential pool in the system, inducing controlled epitaxial growth of calcium and magnesium ions on the seed surface. This results in precipitated particles with large particle size and high density, significantly improving the solid-liquid separation effect.

[0007] Preferably, the aqueous solution of the polyhydroxycarboxylic acid salt is a sodium gluconate aqueous solution with a mass fraction of 10% to 20%.

[0008] By adopting the above technical solution, the polyhydroxy and carboxyl functional groups in sodium gluconate molecules can undergo strong coordination adsorption with the primary crystal nucleus precursor in the liquid phase. Within a specific concentration range, the homogeneous nucleation rate can be precisely controlled to prevent the formation of colloidal precipitates that are difficult to settle.

[0009] Preferably, in the step of adding the homogeneous nucleation kinetic inhibitor, the ratio of the total molar amount of polyhydroxycarboxylate to the sum of the total molar amounts of calcium and magnesium ions in the industrial crude brine to be treated is controlled to be 0.1% to 2.0%, the stirring speed is controlled to be 50 to 200 rpm, the stirring temperature is 15 to 35°C, and the continuous stirring time is 5 to 20 minutes.

[0010] By adopting the above technical solution, the inhibitor is ensured to be fully and uniformly dispersed in the crude brine, and the pretreatment of the entire nucleation environment is achieved with low energy input.

[0011] Preferably, in the step of adding the activated seed slurry, the dry weight of the activated seed slurry is controlled to be 1.0% to 5.0% of the total mass of the industrial crude brine to be treated, the stirring speed is maintained at 50 to 150 rpm, and the stirring time is 3 to 10 minutes.

[0012] By adopting the above technical solution, a moderate solid content is maintained in the system to ensure sufficient surface area during heterogeneous crystallization, so that calcium and magnesium ions in the liquid phase can quickly migrate to the seed crystal surface and reduce bulk supersaturation.

[0013] Preferably, the step of extracting the suspended slurry for external circulation hydrodynamic micro-mixing is as follows: the suspended slurry is extracted by an external circulation pump and flows through an external pipeline containing a Venturi injector or a static mixer before returning to the main reactor, and the fluid flow rate in the external pipeline is controlled to be 1.5 to 4.0 m / s.

[0014] By employing the above technical solution and utilizing the highly turbulent shear environment in the external circulation pipeline, uniform mixing of the reagents at the molecular scale on the microsecond level was achieved. This hydrodynamic micro-mixing can eliminate the phenomenon of excessively high local concentrations, ensuring stable and precise pH control during the stepwise dosing process.

[0015] Preferably, the specific operation of the stepwise dosing is as follows: First, dynamically control the dosing flow rate so that the pH value of the reaction system rises uniformly to 9.0-10.0 within 10-25 minutes, then stop the dosing and maintain external circulation for 2-8 minutes; then restart the dosing and continue to dynamically control the dosing flow rate so that the pH value of the system rises uniformly to 10.5-11.5 within 5-15 minutes, and complete the injection of the remaining sodium carbonate and sodium hydroxide aqueous solutions during this stage.

[0016] By adopting the above technical solution, the step-by-step pH control is matched with the thermodynamic stability of the precipitate. The first stage mainly promotes the controlled growth of calcium carbonate, while the second stage promotes the precipitation and co-precipitation of magnesium hydroxide by rapidly increasing alkalinity. The step-by-step control effectively avoids the entrapment and mutual interference between different types of precipitates.

[0017] Preferably, the mass fraction of the sodium carbonate aqueous solution injected simultaneously is 10% to 25%, the mass fraction of the sodium hydroxide aqueous solution is 10% to 30%, and the total amount of injected sodium carbonate and sodium hydroxide is 1.05 to 1.15 times the sum of the theoretical molar equivalents of calcium and magnesium removal.

[0018] By adopting the above technical solution, while ensuring the deep removal of calcium and magnesium ions, the excess coefficient of the reagent is controlled at a low level, thereby reducing production costs and the burden of subsequent treatment.

[0019] Preferably, the specific operation for aging and maturing the suspended slurry is as follows: maintain external circulation and low-speed stirring, and continuously age and mature the system at 15-35°C for 15-40 minutes.

[0020] By adopting the above technical solution and utilizing the Ostwald ripening principle, fine particles are further dissolved and deposited on the surface of large particles, which enhances the mechanical strength of the particles and facilitates subsequent pressure filtration or centrifugation operations.

[0021] Secondly, the present invention provides a method for preparing activated seed slurry, which adopts the following technical solution: A method for preparing activated seed slurry includes the following steps: mixing a double-alkali salt mud filter cake as the basic seed crystal, an aqueous sodium hydroxide solution as the seed crystal etching activator, and a refined sodium chloride solution, and then performing high-shear pulping.

[0022] By employing the above-mentioned technical solution, the high-shear pulping process utilizes strong mechanical shear force and cavitation effect to deagglomerate particles in the dual-alkali salt mud. Simultaneously, sodium hydroxide acts as an etching activator to chemically modify the crystal surface. This synergistic effect of physical and chemical processes transforms the originally low-activity salt mud into activated seed crystals with numerous vacancies, lattice defects, and a layer of negatively charged adsorption on their surface, greatly enhancing their attraction to metal ions in the liquid phase.

[0023] Preferably, the activated seed crystal slurry is prepared from the following raw materials by high-shear pulping in parts by weight: 100 portions of salt mud filter cake produced by the double alkali process; 20-50 parts of a sodium hydroxide aqueous solution with a mass fraction of 20%-40%; 113–316 parts of refined sodium chloride solution; The process parameters for high-shear pulping are: shearing speed of 2000-5000 rpm, pulping temperature of 30-50℃, and continuous high-shear pulping time of 10-30 minutes.

[0024] By employing the above technical solution, a slurry system with good rheological properties and stable solid content can be prepared under specific proportions and process conditions. By precisely controlling the shear speed and beating time, the density of active sites on the seed crystal surface is ensured to reach the optimal value, thereby maximizing its inducing crystallization effect in the subsequent decalcification and magnesium removal process.

[0025] This invention provides a method for deep decalcification and demagnesification of industrial sodium chloride. It has the following beneficial effects: 1. This invention alters the crystallization pathway of calcium and magnesium precipitation by combining a homogeneous nucleation kinetic inhibitor with an activated seed slurry. The homogeneous nucleation kinetic inhibitor can coordinate and adsorb with primary nucleus precursors in the liquid phase, inhibiting the homogeneous nucleation process. Simultaneously, the activated seed slurry, prepared through high-shear pulping and alkaline etching, provides heterogeneous nucleation sites with lattice defects and negatively charged layers, guiding calcium and magnesium ions to undergo heterogeneous epitaxial growth on the surface of exogenous seed crystals. This reduces the formation of fine colloidal particles, resulting in larger and more compact precipitate particles, thereby improving the settling performance of the suspension slurry and the dewatering effect during subsequent mechanical separation.

[0026] 2. This invention employs a process combining external circulation fluid dynamics micro-mixing and step-by-step dosing to achieve dynamic control of local supersaturation and pH in the reaction system. The suspended slurry is extracted for micro-mixing and then the precipitating agent is injected in stages, avoiding the problem of excessively high local concentrations that easily occur with direct dosing in the reactor, and preventing secondary nucleation caused by local supersaturation. This ensures deep removal of calcium and magnesium ions from the crude brine, and the resulting crystalline precipitate has fewer internal pores, reducing the physical entrainment of sodium chloride mother liquor in the solid cake during pressure filtration, and minimizing the loss of effective salts during production.

[0027] 3. The processing method of this invention achieves the separation of calcium and magnesium solid phases based on crystallization kinetics, eliminating the need for additional addition of conventional polymeric polyacrylamide flocculants to the system. Traditional processes often rely on the trapping and bridging effects of polymeric flocculants to aggregate fine precipitates, easily leading to a high amount of residual organic matter in the effluent. This invention introduces only a small amount of polyhydroxycarboxylate aqueous solution as a nucleation inhibitor, limiting the total amount of exogenous organic matter added. This keeps the total organic carbon content in the purified brine obtained after separation at a low level, reducing the operational risks and equipment cleaning pressure caused by organic matter enrichment in subsequent evaporation and crystallization processes. Attached Figure Description

[0028] Figure 1 This is a comparison chart of the Zeta potential test results of the seed slurry samples of each group in Test Example 1 of the present invention; Figure 2 This is a comparison chart of the sedimentation kinetic curves of the suspended slurry generated in each group of reaction systems in Test Example 2 of the present invention; Figure 3 This is a comparison diagram of the particle size distribution characteristics of the co-crystallized precipitates generated in each group of the reaction in Test Example 3 of the present invention; Figure 4 This is a comparison chart of the residual calcium and magnesium ion mass concentrations in the refined brine systems obtained from each embodiment and comparative example in Test Example 4 of this invention. Figure 5This is a comparison chart showing the percentage of sodium chloride entrained and lost in the mechanically filtered solid salt mud obtained in each embodiment and the comparative example of Test Example 4 of this invention. Detailed Implementation

[0029] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0030] Preparation Examples 1-3: Preparation Example 1: This preparation example provides a method for preparing a homogeneous nucleation kinetics inhibitor solution and an activated seed slurry, including the following steps: (1) Preparation of homogeneous nucleation kinetic inhibitor solution: Weigh 10 kg of industrial grade sodium gluconate as a polyhydroxy carboxylate salt, dissolve it in 90 kg of deionized water, mix thoroughly, and prepare sodium gluconate aqueous solution as a homogeneous nucleation kinetic inhibitor for later use.

[0031] (2) Preparation of activated seed slurry: Weigh 100 kg of double alkali salt mud filter cake with a water content of 30% as the basic seed crystal and put it into the main pulping kettle with jacket temperature control and high shear disperser; inject 50 kg of sodium hydroxide aqueous solution with a mass fraction of 20% as seed crystal etching activator into the pulping kettle, and add 316 kg of refined sodium chloride solution to adjust the mud fluidity; turn on the high shear disperser, control the speed to 2000 rpm, adjust the system temperature to 30℃ by the jacket circulating water, and continuously perform strong shear pulping for 10 minutes; after pulping, the activated seed slurry with etched surface and lattice defects is obtained and placed in the storage tank for cooling and use.

[0032] Preparation Example 2: This preparation example provides a method for preparing a homogeneous nucleation kinetics inhibitor solution and an activated seed slurry, including the following steps: (1) Preparation of homogeneous nucleation kinetic inhibitor solution: Weigh 15 kg of industrial grade sodium gluconate as a polyhydroxy carboxylate, dissolve it in 85 kg of deionized water, mix thoroughly, and prepare sodium gluconate aqueous solution as a homogeneous nucleation kinetic inhibitor for later use.

[0033] (2) Preparation of activated seed slurry: Weigh 100 kg of double alkali salt mud filter cake with a water content of 30% as the basic seed crystal and put it into the main pulping kettle with jacket temperature control and high shear disperser; inject 30 kg of sodium hydroxide aqueous solution with a mass fraction of 30% as seed crystal etching activator into the pulping kettle, and add 150 kg of refined sodium chloride solution to adjust the mud fluidity; turn on the high shear disperser, control the speed to 3500 rpm, adjust the system temperature to 40℃ by the jacket circulating water, and continuously perform strong shear pulping for 20 minutes; after pulping, the activated seed slurry with etched surface and lattice defects is obtained and placed in the storage tank for cooling and use.

[0034] Preparation Example 3: This preparation example provides a method for preparing a homogeneous nucleation kinetics inhibitor solution and an activated seed slurry, including the following steps: (1) Preparation of homogeneous nucleation kinetic inhibitor solution: Weigh 20 kg of industrial grade sodium gluconate as a polyhydroxy carboxylate salt, dissolve it in 80 kg of deionized water, mix thoroughly, and prepare sodium gluconate aqueous solution as a homogeneous nucleation kinetic inhibitor for later use.

[0035] (2) Preparation of activated seed slurry: Weigh 100 kg of double alkali salt mud filter cake with a water content of 30% as the basic seed crystal and put it into the main pulping kettle with jacket temperature control and high shear disperser; inject 20 kg of sodium hydroxide aqueous solution with a mass fraction of 40% as seed crystal etching activator into the pulping kettle, and add 113 kg of refined sodium chloride solution to adjust the mud fluidity; turn on the high shear disperser, control the speed to 5000 rpm, adjust the system temperature to 50℃ by the jacket circulating water, and continuously perform strong shear pulping for 30 minutes; after pulping, the activated seed slurry with etched surface and lattice defects is obtained and placed in the storage tank for cooling and use.

[0036] Here, it is necessary to explain the principle of the product of the above preparation example and the hydrodynamic operation of the subsequent embodiments: The salt mud filter cake used in the above preparation example has calcium carbonate and magnesium hydroxide as its main chemical components. After high-shear pulping and activation treatment, the filter cake is transformed into a calcium carbonate and magnesium hydroxide mixed suspension with lattice defects and a high-density negative charge layer adsorbed on the surface. In this invention, the homogeneous nucleation kinetic inhibitor is used to coordinate and adsorb with the primary nucleus precursor in the liquid phase to increase the homogeneous nucleation activation energy and provide precipitation thrust; the activated seed slurry is used to construct a low-energy barrier potential sink in the system to provide pull force for the heterogeneous epitaxial growth of calcium and magnesium ions. In addition, in the subsequent embodiments, the operation step of extracting the suspension slurry and returning it to the reactor after flowing through an external pipeline containing a Venturi ejector or a static mixer is essentially a micro-mixing of the slurry through external circulation to enhance the mass transfer efficiency of the reaction system.

[0037] Furthermore, the units such as g / L and mg / L described in the embodiments of this invention are standard parameters used to characterize the absolute mass concentration of solute or free ions in an aqueous system (Note: 1 kg = 1000 g, 1 gram = 1000 mg). When calculating the dosage in the embodiments, the absolute mass calculated based on the above concentration parameters and the volume of crude brine is matched with the total kilograms of the reagent in the preparation example, together constituting the process implementation basis of this invention. Examples 1-3: Example 1: This example provides a method for deep decalcification and demagnesification of industrial sodium chloride, including the following steps: (1) 1000 liters of industrial crude brine to be treated (test indicators: calcium ion content 850 mg / L, magnesium ion content 220 mg / L, sodium chloride mass concentration 300 g / L) was pumped into the main reactor, and the homogeneous nucleation kinetic inhibitor solution prepared in Preparation Example 1 was added. The ratio of the total molar amount of sodium gluconate to the sum of the total molar amounts of calcium and magnesium ions in the crude brine was controlled to be 0.1%. The stirring paddle of the main reactor was started, and the stirring speed was controlled to be 50 rpm. The mixture was stirred continuously for 5 minutes at 15°C.

[0038] (2) While maintaining the stirring state, add the activated seed slurry prepared in Example 1 into the main reactor, control the dry basis mass of the activated seed slurry to be 1.0% of the total mass of the crude brine to be treated, maintain the stirring speed at 50 rpm, and continue stirring for 3 minutes.

[0039] (3) Turn on the external circulation pump connected to the bottom of the main reactor, extract the suspended slurry and return it to the reactor after flowing through the external pipeline containing the Venturi injector. Control the fluid flow rate in the external circulation pipeline to 1.5 m / s. According to the total amount of reagent required for calcium and magnesium removal, which is 1.05 times the theoretical molar equivalent, inject 10% sodium carbonate aqueous solution and 10% sodium hydroxide aqueous solution into the mixing node of the external pipeline through the metering pump for the first step of dosing. Dynamically control the dosing flow rate so that the pH value of the system in the reactor rises to 9.0 at a uniform rate within 10 minutes. Stop the dosing and maintain the external circulation for 2 minutes. Restart the metering pump for the second step of dosing. Continue to dynamically control the dosing flow rate so that the pH value of the system rises to 10.5 at a uniform rate within 5 minutes. At the same time, complete the injection of the remaining theoretical amount of reagent.

[0040] (4) Completely stop adding chemicals, keep the external circulation pump running and the main reactor stirred at low speed, and let the system age and mature continuously at 15°C for 15 minutes. After aging, turn off the external circulation pump and pump the final slurry in the reactor into a plate and frame filter press for mechanical solid-liquid separation, and collect the refined brine and solid salt mud filter cake respectively. Example 1

[0041] This embodiment provides a method for deep decalcification and demagnesification of industrial sodium chloride, including the following steps: (1) 1000 liters of industrial crude brine to be treated (test indicators: calcium ion content 850 mg / L, magnesium ion content 220 mg / L, sodium chloride mass concentration 300 g / L) was pumped into the main reactor, and the homogeneous nucleation kinetic inhibitor solution prepared in Preparation Example 2 was added. The ratio of the total molar amount of sodium gluconate to the sum of the total molar amounts of calcium and magnesium ions in the crude brine was controlled to be 1.0%. The stirring paddle of the main reactor was started, and the stirring speed was controlled to be 120 rpm. The mixture was stirred continuously for 12 minutes at 25°C.

[0042] (2) While maintaining the stirring state, add the activated seed slurry prepared in Example 2 into the main reactor, control the dry basis mass of the activated seed slurry to be 3.0% of the total mass of the crude brine to be treated, maintain the stirring speed at 100 rpm, and continue stirring for 6 minutes.

[0043] (3) Turn on the external circulation pump connected to the bottom of the main reactor, extract the suspended slurry and return it to the reactor after flowing through the external pipeline containing the static mixer. Control the fluid flow rate in the external circulation pipeline to 2.5 m / s. According to the total required amount of 1.10 times the theoretical molar equivalent of calcium and magnesium removal, inject 15% sodium carbonate aqueous solution and 20% sodium hydroxide aqueous solution into the mixing node of the external pipeline through the metering pump for the first step of dosing. Dynamically control the dosing flow rate so that the pH value of the system in the reactor rises to 9.5 at a uniform rate within 15 minutes. Stop the dosing and maintain the external circulation for 5 minutes. Restart the metering pump for the second step of dosing. Continue to dynamically control the dosing flow rate so that the pH value of the system rises to 11.0 at a uniform rate within 10 minutes. At the same time, complete the injection of the remaining theoretical amount of dosing.

[0044] (4) Completely stop adding medicine, keep the external circulation pump on and the main reactor stirred at low speed, and let the system age and mature continuously at 25°C for 25 minutes; after aging, turn off the external circulation pump and pump the final slurry in the reactor into a centrifuge for mechanical solid-liquid separation, and collect the refined brine and solid salt mud filter cake respectively. Example 2

[0045] This embodiment provides a method for deep decalcification and demagnesification of industrial sodium chloride, including the following steps: (1) 1000 liters of industrial crude brine to be treated (test indicators: calcium ion content 850 mg / L, magnesium ion content 220 mg / L, sodium chloride mass concentration 300 g / L) was pumped into the main reactor, and the homogeneous nucleation kinetic inhibitor solution prepared in Preparation Example 3 was added. The ratio of the total molar amount of sodium gluconate to the sum of the total molar amounts of calcium and magnesium ions in the crude brine was controlled to be 2.0%. The stirring paddle of the main reactor was started, and the stirring speed was controlled to be 200 rpm. The mixture was stirred continuously for 20 minutes at 35°C.

[0046] (2) While maintaining the stirring state, add the activated seed slurry prepared in Example 3 into the main reactor, control the dry basis mass of the activated seed slurry to be 5.0% of the total mass of the crude brine to be treated, maintain the stirring speed at 150 rpm, and continue stirring for 10 minutes.

[0047] (3) Turn on the external circulation pump connected to the bottom of the main reactor, extract the suspended slurry and return it to the reactor after flowing through the external pipeline containing the Venturi injector. Control the fluid flow rate in the external circulation pipeline to 4.0 m / s. According to the total required amount of 1.15 times the theoretical molar equivalent of calcium and magnesium removal, inject 25% sodium carbonate aqueous solution and 30% sodium hydroxide aqueous solution into the mixing node of the external pipeline through the metering pump for the first step of dosing. Dynamically control the dosing flow rate so that the pH value of the system in the reactor rises to 10.0 at a uniform rate within 25 minutes. Stop the dosing and maintain the external circulation for 8 minutes. Restart the metering pump for the second step of dosing. Continue to dynamically control the dosing flow rate so that the pH value of the system rises to 11.5 at a uniform rate within 15 minutes. At the same time, complete the injection of the remaining theoretical amount of dosing.

[0048] (4) Completely stop adding chemicals, keep the external circulation pump running and the main reactor stirred at low speed, and let the system age and mature continuously at 35°C for 40 minutes. After aging, turn off the external circulation pump and pump the final slurry in the reactor into a plate and frame filter press for mechanical solid-liquid separation, and collect the refined brine and solid salt mud filter cake respectively.

[0049] Comparative Examples 1-4: Comparative Example 1: Compared with Example 2, the difference is that: in step (1), no homogeneous nucleation kinetic inhibitor solution is added, and the crude brine is directly subjected to subsequent operations, while the rest are the same.

[0050] Comparative Example 2: Compared with Example 2, the difference is that the seed crystals added in step (2) were not subjected to the high-concentration sodium hydroxide and high-shear activation pulping treatment as in Example 2. Instead, 100 parts by mass of the basic seed crystals were directly mixed with water to prepare ordinary slurry and added to the main reactor. All other aspects are the same.

[0051] Comparative Example 3: Compared with Example 2, the difference is that in step (3), the external circulation pump is not turned on and the stepwise addition of chemicals is not carried out. Instead, the same amount of sodium carbonate aqueous solution and sodium hydroxide aqueous solution are poured directly into the top of the liquid surface of the main reactor at one time, so that the macroscopic pH in the reactor rises to 11.0 at one time. The rest are the same.

[0052] Comparative Example 4: This comparative example uses the traditional mainstream dual-alkali process. The difference from Example 2 is that only 1000 liters of industrial crude brine with the same test indicators are taken, and an excess of sodium hydroxide and sodium carbonate solution is added directly under normal stirring until the pH reaches 11.0. Then, 3 ppm of polyacrylamide (PAM) polymeric flocculant is added, and after stirring and aging for 25 minutes, solid-liquid separation is carried out directly.

[0053] Test Examples 1-4: Test Example 1: Determination of Seed Interface Charge Characteristics Based on Zeta Potential 50 ml of the ordinary basic seed slurry from Comparative Example 2, and 50 ml of the activated seed slurry from Preparation Example 1, Preparation Example 2, and Preparation Example 3 were taken as test samples.

[0054] Considering that excessively high ionic strength of in-situ industrial slurry can interfere with electrophoretic light scattering signals and cause electrode polarization, a sodium chloride aqueous solution with a concentration of 0.05 mol / L was prepared as a background dispersion.

[0055] Take 0.5 ml of each of the four test samples and add it to 49.5 ml of background dispersion. Stir magnetically at 150 rpm for 5 minutes to disperse it evenly and prepare the test suspension.

[0056] The suspension to be tested is injected into the folded capillary sample cell of the Zeta potential analyzer, and after removing air bubbles, it is placed in the test chamber.

[0057] An electrophoretic conversion equation suitable for aqueous dispersion systems was used to convert the measured particle electrophoretic mobility into the final Zeta potential value. Each group of samples underwent three independent scan tests, and the Zeta potential values ​​were recorded.

[0058] Table 1. Results of Zeta potential test on the surface of seed crystal samples Ordinary basic seed slurry -2.14 -1.58 -2.87 -2.20 Preparation Example 1: Activated Seed Slurry -16.42 -18.25 -15.91 -16.86 Preparation Example 2: Activated Seed Slurry -25.63 -24.11 -27.38 -25.71 Preparation Example 3: Activated Seed Slurry -32.45 -30.82 -33.16 -32.14 Based on the data in Table 1 and Figure 1 The potential change trend shown indicates that the average Zeta potential of the ordinary basic seed crystal surface without the combined treatment of high alkali and mechanical force is -2.20 mV, which is near the isoelectric point close to electrical neutrality, indicating a low surface charge density. After the treatments in Preparation Examples 1 to 3, as shown... Figure 1As shown in the bar chart, the Zeta potential on the seed crystal surface shifts negatively, and the absolute value of the negative potential increases from 16.86 mV to 32.14 mV with increasing sodium hydroxide concentration and shear speed during the processing. In the high-concentration sodium hydroxide system, excess hydroxide ions overcome the mass transfer resistance at the solid-liquid interface under shear force, undergoing specific adsorption on the exposed crystal surface of the calcium carbonate and magnesium hydroxide blend seed crystals. The shear rate disrupts the original flat crystal surface, exposing lattice defects such as steps and twisting sites, increasing the number of active sites anchoring hydroxide ions per unit specific surface area, and constructing a high-density negative charge layer on the seed crystal surface.

[0059] Combination Figure 1 The data reflects the interface modification results, showing that the input of activated seed crystals with high-density negative charges into the main reactor altered the crystallization and precipitation pathways of calcium and magnesium ions in the system. Due to electrostatic attraction, the negatively charged activated seed crystals constitute a low-energy-barrier potential sink in the liquid phase system. Under the premise that the polyhydroxycarboxylate inhibits the homogeneous nucleation activation energy of free calcium and magnesium ions, the local supersaturation induced by the added precipitant is not released through nucleation within the liquid phase itself. Instead, calcium and magnesium ions in the solution are adsorbed onto the defect sites of the activated seed crystals, undergoing heterogeneous epitaxial growth. This process eliminates the conditions for the formation of primary colloidal microcrystals, resulting in subsequent precipitates that are dense granular, providing a prerequisite for improving macroscopic solid-liquid separation efficiency.

[0060] Test Example 2: Determination of Homogeneous Nucleation Induction Period Based on Online Turbidity Industrial crude brine without added activated crystals was used as the basic test solution, divided into a blank control group and three test groups with different proportions of inhibitors. The homogeneous nucleation kinetic inhibitor solutions prepared in Preparation Example 1, Preparation Example 2, and Preparation Example 3 were added to the test groups according to the steps outlined, controlling the ratio of the total molar amount of sodium gluconate to the sum of the total molar amounts of calcium and magnesium ions in the crude brine to be 0.1%, 1.0%, and 2.0%, respectively.

[0061] Each group of basic test solutions was placed in a crystallization reactor with a jacket and temperature control. The constant temperature circulating water was turned on to stabilize the system temperature at 25°C. Mechanical stirring was started, and the speed was set to a constant 150 rpm.

[0062] Immerse the probe of the online turbidity meter below the liquid surface of the crystallization reactor, set the data acquisition frequency to once per second, and record the initial system turbidity baseline.

[0063] A mixed precipitant is added dropwise to the crystallization reactor at a constant rate using a micro-flow pump. The mixed precipitant is prepared by mixing a 15% sodium carbonate aqueous solution and a 20% sodium hydroxide aqueous solution in a 1:1 volume ratio.

[0064] Continuously record the changes in turbidity of the system over time. Define the time interval from the start of precipitant addition to the period when the turbidity value of the system suddenly jumps and continues to rise as the homogeneous nucleation induction period. Perform the operation independently three times for each test solution and record the data.

[0065] Table 2. Results of homogeneous nucleation induction period tests under different polyhydroxycarboxylate addition ratios Blank control group 23 19 25 22.3 Inhibitor addition ratio 0.1% group 142 135 158 145.0 Inhibitor addition ratio 1.0% group 418 432 405 418.3 Inhibitor addition ratio 2.0% group 824 789 841 818.0 Based on the data in Table 2 and Figure 2 The induction period trend is shown. In the blank control group, after the addition of the precipitant, the turbidity of the system jumped rapidly within a short time, and the average homogeneous nucleation induction period was 22.3 seconds. Free calcium and magnesium ions overcome the nucleation energy barrier in the alkaline and carbonate environments, undergoing homogeneous nucleation and generating a large number of colloidal microcrystals. After introducing sodium gluconate as a homogeneous nucleation kinetic inhibitor, as... Figure 2 As shown in the bar chart, the homogeneous nucleation induction period of the system increases with the increase of the inhibitor ratio. When the dosage ratios are 0.1%, 1.0%, and 2.0%, the average induction period is extended to 145.0 seconds, 418.3 seconds, and 818.0 seconds, respectively.

[0066] Combination Figure 2 The data reflects a time lag effect, where the hydroxyl and carboxyl groups in the sodium gluconate molecule undergo coordination adsorption with the growth surfaces of primary crystal nuclei or primary microcrystals in the supersaturated state of the crude brine system. This coordination adsorption blocks the diffusion of solute molecules to the primary crystal nucleus surface, alters the specific surface free energy of the solid-liquid interface, and increases the activation energy required for macroscopic homogeneous nucleation. The polyhydroxycarboxylate in the liquid phase inhibits the spontaneous precipitation of free metal ions, delaying the onset of homogeneous nucleation. The extended induction period at the external circulation micromixing node provides a time window for calcium and magnesium ions in the liquid phase to migrate to the surface of activated seed crystals with high-density negative charges and undergo heterogeneous epitaxial growth, kinetically confirming the effectiveness of the push-pull competition mechanism.

[0067] Test Example 3: Macroscopic Sedimentation Dynamics of Suspension and Dewatering Performance of Salt Mud 1000 ml of each of the suspended slurries that had completed the sedimentation and aging process but had not yet entered the pressure filtration separation stage in Examples 1 to 3 and Comparative Examples 1 to 4 were measured as test samples.

[0068] Transfer each group of test samples to a 1000 ml standard graduated cylinder and place it on a vibration-proof table. Start the timer and record the vertical distance of the descent of the clear liquid layer and the suspended slurry interface within the first 5 minutes, and calculate the initial free settling rate.

[0069] After the sedimentation test, the slurry in the measuring cylinder was transferred to a Buchner funnel with qualitative filter paper, and the vacuum system was connected. The negative pressure of the filtration was controlled to be constant at 0.08 MPa, and filtration was carried out continuously for 15 minutes. The retained solid salt mud filter cake was then collected.

[0070] Weigh the initial mass of each group of wet salt mud filter cakes and bake them in a constant temperature forced-air drying oven at 105℃ for 4 hours. Remove them and cool them to room temperature in a desiccator, then weigh them again. Repeat this baking and weighing process until the difference between two consecutive weighings is less than 0.01 grams, reaching a constant weight. Calculate the moisture content percentage of the salt mud filter cakes based on the difference between wet and dry weights.

[0071] Each group of samples underwent three independent sedimentation and dehydration tests, and the discrete test values ​​were recorded.

[0072] Table 3. Results of test on settling velocity of suspended slurry and moisture content of salt mud filter cake Example 1 7.14 7.68 6.89 7.24 38.15 36.42 39.01 37.86 Example 2 11.23 12.51 11.87 11.87 26.83 28.14 25.67 26.88 Example 3 14.86 13.52 14.41 14.26 21.05 22.38 20.91 21.45 Comparative Example 1 1.95 2.14 1.68 1.92 64.21 62.35 65.74 64.10 Comparative Example 2 2.41 2.87 2.22 2.50 59.38 57.62 61.15 59.38 Comparative Example 3 1.56 1.32 1.71 1.53 69.42 67.81 71.05 69.43 Comparative Example 4 5.82 6.31 5.95 6.03 73.18 71.55 74.62 73.12 Based on the data in Table 3 and Figure 3 The bidirectional performance comparison trend shows that the average initial settling rate of Examples 1 to 3 ranges from 7.24 cm / min to 14.26 cm / min, and the average filter cake moisture content ranges from 37.86% to 21.45%. Figure 3 The inverse correspondence between the dark gray column on the left and the light gray column on the right shows that the increase in sedimentation rate and the decrease in water content in the system exhibit a positive synergistic optimization with the increase of activation process parameters and inhibitor dosage. The process in this example blocks the formation of primary colloidal microcrystals through the synergistic effect of the homogeneous nucleation activation energy inhibition mechanism of polyhydroxycarboxylate and the low energy barrier potential sink mechanism of high negative potential seed crystals. Calcium and magnesium ions undergo directional crystallization at seed defect sites, resulting in co-crystallized precipitates with strong rigidity and dense structure, exhibiting superior macroscopic sedimentation kinetics, weak interparticle water holding capacity, and high mechanical dehydration efficiency.

[0073] Combination Figure 3The drastic deterioration of the comparative data shows that Comparative Example 1 lacked a homogeneous nucleation kinetic inhibitor, allowing free metal ions to undergo homogeneous nucleation over a low activation energy barrier, generating a large number of submicron-sized colloidal particles. This resulted in a sharp drop in the slurry settling rate to 1.92 cm / min and an increase in the filter cake moisture content to 64.10%. Comparative Example 2 used ordinary seed crystals that had not undergone strong shear etching and specific adsorption activation, thus losing their charge attraction effect. Free ions did not undergo heterogeneous epitaxial growth, and the system precipitate had high water retention. Comparative Example 3 did not use an external circulation micro-mixing device, and the local supersaturation at the dropping point in the main reactor became uncontrolled, disrupting the push-pull balance of crystallization kinetics. The generated colloidal substances coated the outer layer of the seed crystals, leading to extreme deterioration of solid-liquid separation performance. Comparative Example 4 employed the traditional dual-alkali method with the addition of an organic flocculant. The primary microcrystals were aggregated into flocs through the bridging effect of the polymer chains, increasing the settling rate to 6.03 cm / min. However, the flocs formed by polyacrylamide had a loose internal structure, encapsulating a large amount of capillary water and free water, resulting in a filter cake moisture content as high as 73.12%. The comparison of apparent physical properties macroscopically confirmed the technical differences in separation mechanism between the pure crystallization control pathway of this invention and the traditional flocculation and sedimentation pathway.

[0074] Test Example 4: Determination of Purity of Refined Brine and Sodium Chloride Loss The refined brine mother liquor and solid dry salt mud obtained after mechanical solid-liquid separation in Examples 1 to 3 and Comparative Examples 1 to 4 were extracted as test samples.

[0075] The concentrations of residual calcium and magnesium ions in purified brine mother liquor were determined by disodium ethylenediaminetetraacetate complexometric titration. During the titration, a calcium ion endpoint was determined using a calcein-thymolphthalein mixed indicator, and the total calcium and magnesium endpoint was determined using Chrome Black T indicator. Low-concentration samples were cross-calibrated using inductively coupled plasma atomic emission spectrometry (ICP-AES), and the concentrations of residual calcium and magnesium ions were calculated and recorded.

[0076] A portion of the refined brine mother liquor was extracted and injected into a total organic carbon analyzer. The total organic carbon mass concentration in the liquid phase system was determined by high-temperature catalytic oxidation. The test results were recorded to assess the degree of mother liquor contamination caused by the introduction of exogenous organic matter.

[0077] Accurately weigh 10.00 g of each group of solid-phase dry salt mud samples dried to constant weight, put them into a conical flask containing 500 ml of deionized water, and extract them by continuous shaking at 200 rpm for 120 minutes in a constant temperature shaker.

[0078] The extracted suspension was vacuum filtered through a 0.22-micron microporous membrane to retain insoluble calcium carbonate and magnesium hydroxide precipitates, and the filtrate containing dissolved sodium chloride was collected.

[0079] The collected filtrate was titrated with a standard silver nitrate solution, using potassium chromate aqueous solution as an indicator. The titration endpoint was defined as the appearance of a brick-red precipitate that did not fade within half a minute. The mass percentage of sodium chloride entrained in the dried salt mud was calculated based on the volume of silver nitrate consumed. Each group of dried salt mud samples underwent three independent extraction and titration tests.

[0080] Table 4. Purity index of refined brine and test results of sodium chloride entrainment loss from dry salt mud. Example 1 2.34 0.91 8.12 2.51 2.18 2.76 2.48 Example 2 1.15 0.38 11.45 1.64 1.93 1.72 1.76 Example 3 0.82 0.21 18.26 1.35 1.28 1.51 1.38 Comparative Example 1 19.63 14.22 1.84 16.24 15.31 17.08 16.21 Comparative Example 2 13.81 8.75 10.93 11.56 12.82 11.19 11.86 Comparative Example 3 26.54 21.36 12.08 20.15 18.74 21.32 20.07 Comparative Example 4 9.42 6.18 94.37 23.46 24.11 22.85 23.47 Based on the data in Table 4 and Figure 4 , Figure 5 The comparative trends shown indicate that the residual calcium ion concentration in the purified mother liquor of Examples 1 to 3 ranged from 0.82 mg / L to 2.34 mg / L, and the residual magnesium ion concentration ranged from 0.21 mg / L to 0.91 mg / L; Figure 4 As shown, the total residual hardness of the system is at an extremely low level. Meanwhile, Figure 5 The results clearly show that the proportion of sodium chloride entrained in the dry salt mud of the examples was stably controlled within the range of 1.38% to 2.48%. The process of the examples introduced a homogeneous nucleation kinetic inhibitor, which hindered the spontaneous precipitation of free metal ions in the liquid phase, instead causing heterogeneous epitaxial crystallization on the surface of negatively charged activated seed crystals. This control process allowed the precipitation reaction to break through the concentration limit of the metastable region, achieving deep removal of calcium and magnesium ions, and the generated co-crystals eliminated capillary microporous structures. During mechanical pressure filtration, the high-concentration sodium chloride mother liquor trapped between solid particles was physically squeezed out, cutting off the path of effective salt loss with the solid waste residue. The introduction of polyhydroxycarboxylate in the examples slightly increased the total organic carbon of the mother liquor to 8.12 mg / L to 18.26 mg / L, still within the safe threshold that can be tolerated in subsequent evaporation and crystallization processes.

[0081] Combination Figure 4 and Figure 5 The degradation trend of the comparative examples shows that, in Comparative Example 1, no kinetic inhibitors were added, resulting in homogeneous nucleation and the formation of colloidal suspensions. Unreacted calcium and magnesium ions were trapped inside the colloidal flocs, leading to an increase in the residual calcium and magnesium concentration in the mother liquor. Furthermore, the amorphous colloids had strong water-holding capacity, causing sodium chloride loss to rise to 16.21%. Comparative Example 2 used unactivated seed crystals, which lost their surface electrostatic adsorption capacity and could not guide ion directional growth, resulting in incomplete crystallization and material loss. Comparative Example 3 did not employ a fluid dynamics micro-mixing device; localized supersaturation at the dosing point disrupted the crystallization push-pull balance and induced secondary nucleation, such as… Figure 4 The residual calcium ion concentration in the mother liquor spiked to 26.54 mg / L, while the disordered stacking of microcrystals led to... Figure 5Sodium chloride accounted for 20.07% of the total feed. Comparative Example 4 used a traditional dual-alkali method combined with polyacrylamide polymeric flocculant, relying on the bridging effect of long-chain macromolecules to encapsulate primary microcrystals. The introduction of polyacrylamide increased the total organic carbon in the mother liquor to 94.37 mg / L, causing severe organic pollution in the refined brine; moreover, the loose network flocs formed encapsulated a large amount of liquid mother liquor, such as... Figure 5 As shown, sodium chloride loss due to entrainment in the dry salt mud reached as high as 23.47%. The results of the apparent chemical index tests fully confirm that the pure crystallization control method of this invention has excellent engineering applicability in reducing material loss and controlling organic pollution in the system.

Claims

1. A method for deep decalcification and demagnesification of industrial sodium chloride, characterized in that, Includes the following steps: A homogeneous nucleation kinetic inhibitor is added to the industrial crude brine to be treated, and stirring is started to mix it. The homogeneous nucleation kinetic inhibitor is an aqueous solution of polyhydroxycarboxylate. While maintaining stirring, add activated seed slurry to the mixed system and continue mixing to obtain a suspension slurry. The activated seed slurry is a calcium carbonate and magnesium hydroxide mixed suspension with lattice defects on the surface and a high-density negative charge layer adsorbed by characteristic. The suspended slurry is extracted and subjected to external circulation hydrodynamic micro-mixing. At the same time, sodium carbonate aqueous solution and sodium hydroxide aqueous solution are injected into the mixing node of the external pipeline for step-by-step dosing, and the pH value of the suspended slurry is dynamically controlled to rise. After the stepwise dosing is completely stopped, the suspended slurry is aged and matured to obtain the final slurry. The final slurry is then subjected to mechanical solid-liquid separation to collect the refined brine containing the target sodium chloride and the solid salt mud filter cake rich in impurities such as calcium and magnesium.

2. The method for deep decalcification and demagnesification of industrial sodium chloride according to claim 1, characterized in that, The method for preparing the activated seed slurry includes the following steps: mixing the double-alkali salt mud filter cake as the basic seed crystal, the sodium hydroxide aqueous solution as the seed crystal etching activator, and the refined sodium chloride solution, and then performing high-shear pulping.

3. The method for deep decalcification and demagnesification of industrial sodium chloride according to claim 2, characterized in that, The activated seed slurry is prepared from the following raw materials by high-shear pulping in parts by weight: 100 portions of the double-alkali process salt mud filter cake; 20-50 parts of the sodium hydroxide aqueous solution with a mass fraction of 20%-40%; The refined sodium chloride solution is 113-316 parts; The process parameters for high-shear pulping are: shearing speed of 2000-5000 rpm, pulping temperature of 30-50℃, and continuous high-shear pulping time of 10-30 minutes.

4. The method for deep decalcification and demagnesification of industrial sodium chloride according to claim 1, characterized in that, The aqueous solution of the polyhydroxycarboxylic acid salt is a sodium gluconate aqueous solution with a mass fraction of 10% to 20%.

5. The method for deep decalcification and demagnesification of industrial sodium chloride according to claim 1, characterized in that, In the step of adding the homogeneous nucleation kinetic inhibitor, the ratio of the total molar amount of polyhydroxycarboxylate in the homogeneous nucleation kinetic inhibitor to the sum of the total molar amounts of calcium and magnesium ions in the crude industrial brine to be treated is controlled to be 0.1% to 2.0%, the stirring speed is controlled to be 50 to 200 rpm, the stirring temperature is 15 to 35°C, and the continuous stirring time is 5 to 20 minutes.

6. The method for deep decalcification and demagnesification of industrial sodium chloride according to claim 1, characterized in that, In the step of adding the activated seed slurry, the dry basis mass of the activated seed slurry is controlled to be 1.0% to 5.0% of the total mass of the industrial crude brine to be treated, the stirring speed is maintained at 50 to 150 rpm, and the stirring time is 3 to 10 minutes.

7. The method for deep decalcification and demagnesification of industrial sodium chloride according to claim 1, characterized in that, The specific steps for extracting the suspended slurry and performing external circulation hydrodynamic micro-mixing are as follows: The suspended slurry is pumped out by an external circulation pump and flows through an external pipeline containing a Venturi injector or a static mixer before returning to the main reactor. The fluid velocity in the external pipeline is controlled to be 1.5 to 4.0 m / s.

8. The method for deep decalcification and demagnesification of industrial sodium chloride according to claim 1, characterized in that, The specific operation of the stepwise dosing is as follows: First, dynamically control the dosing flow rate so that the pH value of the reaction system rises uniformly to 9.0-10.0 within 10-25 minutes. Then, stop the dosing and maintain external circulation for 2-8 minutes. The dosing was then restarted and the dosing flow rate was continuously dynamically controlled to allow the pH of the system to rise uniformly to 10.5–11.5 within 5–15 minutes, and the remaining sodium carbonate and sodium hydroxide aqueous solutions were completely injected during this stage.

9. The method for deep decalcification and demagnesification of industrial sodium chloride according to claim 1, characterized in that, The mass fraction of the sodium carbonate aqueous solution injected simultaneously is 10%–25%, and the mass fraction of the sodium hydroxide aqueous solution is 10%–30%. The total amount of injected sodium carbonate and sodium hydroxide is 1.05–1.15 times the sum of the theoretical molar equivalents of calcium and magnesium removal.

10. A method for deep decalcification and demagnesification of industrial sodium chloride according to claim 1, characterized in that, The specific operation for aging and maturing the suspended slurry is as follows: maintain external circulation and low-speed stirring, and continuously age and mature the system at 15-35°C for 15-40 minutes.

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