Method for recovering sodium chloride from a by-product salt wash

Abstract

The present application relates to the technical field of industrial solid waste resource, in particular to a method for washing and recovering sodium chloride from by-product salt, which comprises the following steps: using a calcium-containing acidic saturated sodium chloride solution to perform primary acid washing on by-product salt containing sulfate, iron and silicon impurities, so as to generate fine calcium sulfate precipitate; sending the material liquid into an overflow tank for gravity sedimentation, so that sodium chloride crystals form underflow, and mother liquor carrying calcium sulfate particles overflows and is discharged; judging the separation effect by visually judging the clarity of the underflow liquid, and carrying out secondary acid washing and tertiary alkali washing on qualified underflow in sequence to deeply remove iron and silicon impurities, so as to obtain refined sodium chloride through solid-liquid separation and drying. The overflow mother liquor and waste liquid of each washing stage are recycled in a quality-based manner, and the filter residue is used as a building material raw material after being dried through leaching and recovery of sodium chloride. The present application can stably obtain sodium chloride with a purity of ≥99%, has a high washing water recycling rate, realizes near-zero discharge of wastewater, has simple process operation and intuitive quality control, and is suitable for industrialized continuous treatment of by-product salt.

Description

Technical Field

[0001] This invention relates to the field of industrial solid waste resource utilization technology, and more specifically, to a method for washing and recovering sodium chloride from by-product salt. Background Technology

[0002] Industries such as titanium dioxide, chlor-alkali, and molten salt chlorination in metallurgy generate large amounts of sodium chloride byproduct salt containing impurities such as sodium sulfate, iron, and silicon. Direct storage can easily cause water and soil pollution, while direct reuse can cause impurities to clog equipment and reduce product quality. Existing water washing, recrystallization, and conventional chemical precipitation purification processes have drawbacks such as poor separation efficiency, high energy consumption, difficulty in separating fine calcium sulfate, lack of simple quality control methods, and low resource utilization of wastewater and solid waste. In contrast, the existing patent CN117696601A (application number 202410041705.7), entitled "A Pretreatment System and Method for Mixed Waste Salt of Sodium Chloride and Sodium Sulfate," uses carbonization pretreatment... The current method of using gravity-dissolving filtration is lengthy, requires high investment and energy consumption for high-temperature equipment, and relies solely on pressure filtration for solid-liquid separation without gravity-based pre-classification. Fine calcium sulfate particles easily carry sodium chloride, reducing the recovery rate. It also relies on offline detection and control of impurities, making it impossible to judge the washing effect in real time. Furthermore, the washing liquid circulation branch is incomplete, resulting in a large amount of waste liquid discharge. Moreover, the filter residue lacks a resource recovery process for leaching, salt extraction, drying, and building material production. Overall, it is not economically or environmentally friendly. Therefore, the industry urgently needs a simple equipment that allows for intuitive judgment of washing effect, step-by-step deep removal of multiple impurities, near-zero wastewater discharge, and full resource recovery of solid waste through step-by-step washing and sodium chloride recovery of by-product salt. Summary of the Invention

[0003] The purpose of this invention is to provide a method for washing and recovering sodium chloride from by-product salt, in order to solve the problems mentioned in the background art, which rely solely on pressure filtration to achieve solid-liquid separation without gravity pre-classification, where fine calcium sulfate particles easily carry sodium chloride, reducing the recovery rate, and rely solely on offline detection and control of impurities, making it impossible to judge the washing effect in real time.

[0004] To achieve the above objectives, this invention provides a method for recovering sodium chloride from by-product salt washing, comprising the following steps: S1, preparing a saturated sodium chloride washing mother liquor, adding hydrochloric acid to adjust the acidity and adding calcium chloride to introduce calcium ions; S2, mixing and stirring the by-product salt containing sulfate, iron, and silicon impurities with the calcium-containing acidic saturated sodium chloride solution obtained in step S1, so that sulfate reacts with calcium ions to generate fine calcium sulfate precipitate, completing one acid washing reaction; S3, sending all the liquid after the reaction in step S2 into a sedimentation device for gravity stratification, large sodium chloride crystals settle to form underflow, and the mother liquor carrying fine calcium sulfate particles overflows and is separated and discharged; sampling the underflow and allowing it to stand to observe the clarity to determine the impurity removal effect; if the underflow is turbid and contains suspended particles, it is re-washed until the underflow is clear and qualified; S4, using a weakly acidic saturated sodium chloride solution to wash the qualified sodium chloride underflow. S5. The secondary acid washing process involves stirring and reacting the mixture, followed by solid-liquid separation to obtain secondary acid washing salt and secondary washing waste liquid. S6. The secondary acid washing salt is then subjected to a third alkaline wash with a saturated sodium chloride solution to remove iron and silicon impurities. After stirring and reacting, solid-liquid separation yields refined salt and the third washing waste liquid. The refined salt is dried to obtain high-purity sodium chloride product. S7. Closed-loop recycling of washing waste liquid: The overflow-discharged calcium sulfate-containing mother liquor is filtered to remove residue, and the clarified filtrate is returned to step S1 for recycling. The secondary washing waste liquid is treated with a strong alkali to precipitate iron and silicon impurities, filtered to adjust the acidity, and then reused for mixing. The third alkaline washing waste liquid is directly recycled for alkaline washing mixing. S8. Resource utilization of filter residue: The mixed filter residue containing calcium sulfate and iron-silicon hydroxide generated in each process is washed with clean water to recover entrained sodium chloride. The washing filtrate is returned to the sedimentation process, and the desalinated filter residue is dried and used as raw material for gypsum and cement building materials. This setup uses a primary acid wash to precipitate sulfate and a pre-treatment process involving overflow gravity stratification to remove fine calcium sulfate particles. This is followed by a secondary acid wash and a tertiary alkaline wash to remove sulfate, iron, silicon, and other impurities in stages. The separation effect is visually judged by the clarity of the underflow. Combined with a closed-loop circulation of the washing liquid throughout the entire process and a resource utilization process for filter residue immersion, it achieves high-purity sodium chloride recovery, near-zero wastewater discharge, and complete resource utilization of solid waste. The overall process is simple to operate and has low operating costs.

[0005] As a preferred embodiment of the present invention, the preparation parameters for the S1 primary pickling mother liquor are as follows: 26%–26.5% mass concentration of saturated sodium chloride solution at 25°C, 30%–40% mass concentration of calcium chloride solution; molar ratio of calcium ions to sulfate ions in the by-product salt of 1.05:1–1.15:1; and pH of the primary washing mother liquor controlled at 1–2. This setting precisely controls the amount of calcium ions added, ensuring that sulfate ions are fully converted into calcium sulfate precipitate. The acidic environment enhances the precipitation reaction efficiency of calcium sulfate, avoids sulfate residue, and reduces the impurity content of the refined salt from the source.

[0006] As a preferred embodiment of the present invention, the conditions for the secondary acid washing, tertiary alkaline washing mother liquor and washing system are as follows: the pH of the secondary acid washing mother liquor is 2 to 4; the pH of the tertiary alkaline washing mother liquor is adjusted to 9 to 9 using sodium carbonate or sodium hydroxide; the solid-liquid mass ratios of the primary acid washing, secondary acid washing and tertiary alkaline washing are all 1:2.5 to 1:3.5, and the washing reaction temperature is 20 to 35°C. This setup realizes gradient regulation of the pH values of acid washing and alkaline washing, removes residual sulfate radicals, iron and silicon impurities in sections, matches the appropriate solid-liquid ratio and normal-temperature washing temperature, balances the impurity removal efficiency and the dissolution loss of sodium chloride crystals, and reduces energy consumption.

[0007] As a preferred embodiment of the present invention, the stirring reaction duration of each process is as follows: the stirring reaction duration of primary acid washing is 30 to 60 min; the stirring duration of both secondary acid washing and tertiary alkaline washing is 20 to 40 min. This setup ensures sufficient stirring duration to guarantee full contact and reaction between the chemicals and the by-product salt, which not only ensures complete precipitation of impurities, but also avoids excessive crushing of sodium chloride crystals and entrainment of impurities caused by excessively long stirring time.

[0008] As a preferred embodiment of the present invention, the criteria for sedimentation stratification and clarity judgment are as follows: the standing time for material liquid sedimentation stratification is 10 to 20 min, and the sedimentation ambient temperature is 20 to 30°C; the underflow sample is taken and placed in a transparent container for standing for 1 min, observed laterally under natural light, and it is qualified if the liquid is clear without suspended particles and there is no white precipitate at the bottom, and it is judged unqualified if the liquid is turbid and whitish with white suspended particles. This setup matches the difference in sedimentation rate between sodium chloride and calcium sulfate particles to achieve efficient gravity stratification; the quality control is completed by observing the clarity with the naked eye, no precision testing instruments are needed, the operation is intuitive and convenient, and the thorough separation of calcium sulfate is stably guaranteed.

[0009] As a preferred embodiment of the present invention, the indexes of finished products and water resource utilization are as follows: the purity of the refined sodium chloride after drying is ≥99%, which meets the first-class product standard of industrial salt in GB / T5462-2015, and the moisture content of the refined salt after drying is <0.5%; the reuse rate of washing water in the whole process is >95%, realizing near-zero discharge of wastewater. This setup makes the purity of the produced sodium chloride meet the reuse standard of industrial salt, and the high reuse rate of washing liquid greatly reduces the consumption of fresh water and the discharged wastewater, thus reducing the environmental protection disposal cost.

[0010] As a preferred embodiment of the present invention, the process parameters of filter residue treatment are as follows: the solid-liquid mass ratio of clean water leaching of mixed filter residue is 1:2, and the leaching stirring duration is 30 min; the drying temperature of the filter residue is 80 to 120°C, and the moisture content of the filter residue after drying is <5%, which can be used as cement retarder or raw material for gypsum building materials. This setup fully recovers the sodium chloride entrained inside the filter residue and improves the recovery rate of main salt; the filter residue is dried at low temperature to obtain qualified building material raw materials, which eliminates the pressure of solid waste landfill disposal and improves the economic benefit of the project.

[0011] As a preferred embodiment of the present invention, a washing liquid preparation tank and a primary washing tank are provided. The washing liquid preparation tank is connected to the primary washing tank through a conveying pipeline. The discharge pipeline of the primary washing tank is connected to the inlet of the overflow settling tank. The overflow settling tank is a cylindrical / conical tank with a height-to-diameter ratio of 1.5:1 to 2.5:1. An overflow weir and an overflow outlet are provided at the top. The height of the overflow weir is 70% to 80% of the total height of the tank. A conical underflow collection hopper with a cone angle of 60° to 90° and an underflow outlet are provided at the bottom. A sampling valve is provided on the side wall and connected to a 500mL transparent sampling cylinder. The underflow outlet is divided into two pipelines. One pipeline is connected back to the primary washing tank via an underflow return pump, and the other pipeline is connected to the secondary washing tank. The overflow outlet pipeline is connected to the main hydraulic filter. This setup allows the entire solution preparation and pickling sedimentation unit to achieve material transport and stratification by gravity. The overflow trough structure is adapted to the needs of coarse and fine particle classification. The matching return pipeline enables rewashing of unqualified materials. The sampling measuring cylinder structure allows for rapid on-site sampling to determine the washing effect. The equipment has a simple structure and stable continuous operation.

[0012] In a preferred embodiment of the present invention, both the secondary and tertiary washing tanks are equipped with stirring devices. The outlet of the secondary washing tank is connected to the first solid-liquid separation device, and the solid phase outlet of the first solid-liquid separation device is connected to the tertiary washing tank. The outlet of the tertiary washing tank is connected to the second solid-liquid separation device, and the solid phase outlet of the second solid-liquid separation device is connected to a belt dryer. The liquid phase outlet pipeline is directly reconnected to the tertiary washing tank. The first and second solid-liquid separation devices are centrifuges or plate and frame filter presses. This configuration, with segmented washing tanks equipped with independent stirring devices, ensures sufficient impurity removal reaction. The solid-liquid separation devices progressively transfer salt crystals, the alkaline washing liquid is directly recycled in a closed loop, and the drying equipment simultaneously produces finished salt, realizing continuous automated operation of the refining process.

[0013] As a preferred embodiment of the present invention, the waste liquid recycling and filter residue resource utilization unit equipment and its connection relationship are as follows: the main hydraulic filter is a plate and frame filter press or a vacuum filter, with a filtration pressure of 0.2-0.6 MPa and a filtrate turbidity of <5 NTU. The liquid phase pipeline of the main hydraulic filter is connected back to the washing liquid preparation tank; the liquid phase pipeline of the first solid-liquid separation equipment is connected to the neutralization sedimentation tank of the matching calcium carbide sludge addition device, and the filtrate of the neutralization sedimentation tank is returned to the washing liquid preparation tank via a pH adjustment tank; the filter residue of the main hydraulic filter and the neutralization sedimentation tank are uniformly sent to the immersion and stirring tank, and the filtrate of the immersion and stirring tank is returned to the overflow settling tank, and the solid phase discharge is connected to the filter residue dryer. With this configuration, the multi-stage waste liquid treatment unit removes iron and silicon impurities in stages and then recycles them. Various types of filter residue are centrally immersed, salted, and then dried to make building materials, thus fully realizing the dual resource utilization of waste liquid and solid waste, and significantly reducing the cost of production water and solid waste disposal.

[0014] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0015] 1. This method for recovering sodium chloride from by-product salt is simple and energy-efficient, and graded separation improves the salt recovery rate. This invention eliminates high-energy-consuming processes such as high-temperature carbonization, operates at ambient temperature throughout, and relies on gravity grading in the overflow settling tank to remove over 90% of fine calcium sulfate particles in advance, preventing sodium chloride crystals from being encapsulated by impurities. The overall sodium chloride recovery rate can reach 96.5%, while significantly reducing equipment investment and water, electricity, and steam consumption, making it suitable for production lines of various sizes.

[0016] 2. In this method for recovering sodium chloride from by-product salt, a step-by-step gradient impurity removal process combined with simple visual quality control ensures stable and controllable finished product quality. Through a three-stage washing scheme—first acid washing to remove sulfate, second acid washing for deep purification, and third alkaline washing to remove iron and silicon impurities—multiple impurities are removed in layers. The on-site quality control method, using a graduated cylinder for static observation of clarity, eliminates the need for precision testing instruments, making the operation intuitive and rapid. The purified sodium chloride achieves a stable purity of ≥99%, meeting the standards for first-grade industrial salt, and can be directly reused in chlor-alkali and titanium molten salt chlorination production.

[0017] 3. In this method for recovering sodium chloride from by-product salt washing, the entire process of waste liquid and solid waste is recycled, resulting in significant environmental and economic benefits. This invention features multiple independent washing liquid circulation branches, achieving a washing water reuse rate of over 95%, significantly reducing fresh water consumption and the discharge of high-salt wastewater. Various filter residues are washed with clean water to recover entrained sodium chloride. After low-temperature drying, the desalinated filter residues are sold as raw materials for cement and gypsum building materials, eliminating the need for landfill disposal. Each ton of refined salt can generate considerable net profit, complying with regulations on solid waste resource utilization.

[0018] 4. In this method for recovering sodium chloride from by-product salt washing, the entire set of equipment operates continuously and in a coordinated manner, resulting in low maintenance difficulty. Each process relies on continuous feeding through pipelines and pumps, and the overflow tank relies on natural stratification due to liquid level differences. There are few high-speed rotating separation devices, resulting in a low equipment failure rate. The clarification determination operation is simple, and operators can independently manage production after short-term training. Labor and equipment maintenance costs are lower, making it suitable for long-term continuous industrial production. Attached Figure Description

[0019] Figure 1 This is one of the structural schematic diagrams of the present invention;

[0020] Figure 2 This is a second partial structural schematic diagram of the present invention;

[0021] Figure 3 This is a schematic diagram of the overflow settling tank in this invention;

[0022] The meanings of the labels in the diagram are as follows:

[0023] 1. Washing liquid preparation tank; 2. Primary washing tank; 3. Overflow settling tank; 31. Feed inlet; 32. Overflow weir; 33. Overflow outlet; 34. Conical underflow collection hopper; 35. Underflow outlet; 4. Transparent sampling measuring cylinder; 5. Underflow reflux pump; 6. Secondary washing tank; 7. Mother hydraulic filter; 8. Tertiary washing tank; 10. First solid-liquid separation equipment; 11. Second solid-liquid separation equipment; 12. Belt dryer. Detailed Implementation

[0024] The technical solutions of 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.

[0025] This invention provides a method for washing and recovering sodium chloride from by-product salt, such as... Figure 1 Figure 3 As shown, the complete set of implementation equipment includes a washing liquid preparation tank 1, a primary washing tank 2, an overflow settling tank 3, a 500mL transparent sampling cylinder 4, a bottom flow return pump 5, a secondary washing tank 6, a master hydraulic filter 7, a tertiary washing tank 8, a stirring device, a first solid-liquid separation device 10, a second solid-liquid separation device 11, and a belt dryer 12. Each piece of equipment is connected in sequence according to the pipeline structure described in claims 8, 9, and 10, and the by-product salt purification operation is completed by matching all process parameters of claims 1 to 7. The complete set of equipment corresponds to the following process steps: Washing liquid preparation tank 1 completes the preparation of mother liquor in step S1 of claim 1; primary washing tank 2 performs the first acid washing in step S2; overflow settling tank 3 is equipped with a 500mL transparent sampling cylinder 4 and a bottom flow reflux pump 5 to achieve gravity stratification and clarification quality control in step S3; secondary washing tank 6 and tertiary washing tank 8 are equipped with a stirring device and a two-stage solid-liquid separation device to perform the second acid washing in step S4 and the third alkaline washing and salt drying in step S5 in sequence; mother hydraulic filter 7 completes the overflow mother hydraulic filter circulation in step S6, and is equipped with neutralization, soaking, and drying equipment to achieve secondary waste liquid treatment and filter residue resource utilization in step S7. The structure and connection relationship of each piece of equipment fully comply with the schemes defined in claims 8, 9, and 10, and the process parameters strictly match the numerical ranges of claims 2 to 7.

[0026] This embodiment uses titanium molten salt chloride by-product salt as the processing raw material. The raw material composition is: NaCl 80.2%, Na2SO4 8.5%, Fe 0.15%, SiO2 0.3%. The entire implementation process fully covers all seven steps of claim 1 and simultaneously implements all process and equipment technical features of claims 2 to 10.

[0027] In this embodiment, the preparation of the mother liquor according to claims 1S1 and 2 is performed as follows: a 25°C saturated sodium chloride solution is injected into the washing liquid preparation tank 1, with the solution mass concentration controlled at 26%–26.5%. A 30%–40% calcium chloride solution is added, adjusting the molar ratio of calcium ions to sulfate ions in the by-product salt to 1.05:1–1.15:1. Industrial hydrochloric acid is then added to stabilize the pH of the primary washing mother liquor to 1–2, thus completing the preparation of the calcium-containing acidic saturated washing mother liquor, corresponding to all parameters specified in claim 2. The washing liquid preparation tank 1 stores the prepared acid washing mother liquor in a sealed manner and continuously transports it to the primary washing tank 2 through a conveying pipeline. The reagent ratio is precisely controllable, which can fully guarantee the calcium ions and acidic environment required for the sulfate precipitation reaction, reducing the sulfate impurity content in the finished product from the source.

[0028] In this embodiment, the pickling time requirements of claims 1S2 and 4 are implemented: the by-product salt is fed into the primary washing tank 2 and mixed with the calcium-containing acidic mother liquor supplied by the washing liquid preparation tank 1. The solid-liquid mass ratio is 1:2.5 to 1:3.5 as specified in claim 3. The stirring device in the tank is turned on, and stirring is carried out continuously for 30 to 60 minutes at a normal temperature of 20 to 35°C to ensure that the sulfate ions and calcium ions in the material react fully to form fine calcium sulfate precipitate, thus completing the primary pickling reaction. The primary washing tank 2 is equipped with a uniform speed stirring device with a stirring speed of 60 rpm to 120 rpm. Sufficient stirring time ensures that the reagent and the by-product salt are in full contact, avoids incomplete local sulfate reaction, and ensures sufficient formation of fine calcium sulfate precipitate, thereby reducing the load on subsequent separation.

[0029] In this embodiment, the sedimentation and clarification determination criteria of claims 1S3 and 5 are implemented: all reaction liquid in the primary washing tank 2 is sent to the overflow settling tank 3 through the discharge pipe. The structure of the overflow settling tank 3 is completely set according to claim 8: the overflow settling tank 3 is a cylindrical or conical tank with a height-to-diameter ratio of 1.5:1 to 2.5:1. An overflow weir 32 and an overflow outlet 33 are set at the top. The height of the overflow weir 32 is 70% to 80% of the total height of the tank. A conical underflow collection hopper 34 with a cone angle of 60° to 90° and an underflow outlet 35 are set at the bottom. A sampling valve 36 is fixed on the side wall. The external pipeline of the sampling valve 36 is connected to a 500mL transparent sampling cylinder 4. The underflow outlet 35 is divided into two pipelines. One pipeline is equipped with an underflow return pump 5 and connected back to the primary washing tank 2. The other pipeline is connected to the secondary washing tank 6. The pipeline of the overflow outlet 33 is connected to the main hydraulic filter 7. After the feed liquid is fed into the overflow settling tank 3, it is allowed to settle for 10-20 minutes at 20-30℃. Large sodium chloride crystals settle to the conical underflow collection hopper 34 to form underflow, while the mother liquor carrying fine calcium sulfate overflows through the overflow weir 33. The underflow sample is collected from the sampling valve 36 and placed in a 500mL transparent sampling cylinder 4. After standing for 1 minute under natural light, it is observed from the side. If the liquid is clear and free of white suspended particles, it is qualified. If it is turbid and white, the underflow return pump 5 is started to send the underflow back to the primary washing tank 2 for rewashing until it is clear and meets the standard. The special tank structure of the overflow settling tank 3 can increase the settling velocity difference between sodium chloride and calcium sulfate particles, and pre-separate more than 90% of the fine calcium sulfate. The 500mL transparent sampling cylinder 4 allows for visual inspection without the need for precision instruments. The operation is simple and intuitive. Unqualified materials are automatically returned for rewashing, which completely solves the problems of sodium chloride entrainment in calcium sulfate and fluctuations in the purity of the finished product.

[0030] In this embodiment, the secondary acid washing conditions of claims 1S4 and 3 are implemented as follows: the qualified underflow from the overflow settling tank 3 is sent to the secondary washing tank 6. The secondary washing tank 6 is equipped with a stirring device, and a weakly acidic saturated sodium chloride solution with a pH of 2-4 is added. The solid-liquid mass ratio is 1:2.5-1:3.5, the washing temperature is 20-35℃, and the secondary acid washing is completed by stirring for 20-40 minutes. The outlet of the secondary washing tank 6 is connected to the first solid-liquid separation device 10. The first solid-liquid separation device 10 is a centrifuge or a plate and frame filter press to complete the solid-liquid separation and produce secondary acid washing salt and secondary washing waste liquid. The equipment connection relationship conforms to the definition of claim 9. The weakly acidic washing system of the secondary washing tank 6 can remove residual sulfate impurities adsorbed on the surface of sodium chloride crystals. The stirring device continuously agitates to ensure uniform washing. The first solid-liquid separation device 10 efficiently separates wet salt and acidic waste liquid, providing low-sulfate intermediate material for the subsequent alkaline washing process.

[0031] In this embodiment, the conditions for the three-stage alkaline washing as described in claims 1S5 and 3 are as follows: the solid phase outlet of the first solid-liquid separation device 10 is connected to the third washing tank 8. The third washing tank 8 is also equipped with a stirring device. A saturated sodium chloride solution adjusted to pH 9-10 using sodium carbonate or sodium hydroxide is added to the third washing tank 8. The solid-liquid mass ratio is 1:2.5-1:3.5. The mixture is stirred at 20-35°C for 20-40 minutes. In the alkaline environment, iron and silicon impurities are converted into hydroxides and calcium silicate precipitates. The outlet of the third washing tank 8 is connected to the second solid-liquid separation device 11. The second solid-liquid separation device 11 is a centrifuge or a plate and frame filter press. The solid phase outlet of the second solid-liquid separation device 11 is connected to the belt dryer 12, and the liquid phase outlet pipeline is directly connected back to the third washing tank 8. The wet refined salt is sent to the belt dryer 12 for drying. After drying, the purity of the refined sodium chloride is ≥99%, and the moisture content is <0.5%, which meets the GB / T5462-2015 standard for first-class industrial salt and matches the finished product indicators of claim 6. The three-stage washing tank 8 uses alkaline washing to deeply remove two types of difficult-to-remove impurities, iron and silicon. The alkaline washing liquid separated by the second solid-liquid separation equipment 11 is directly circulated internally, reducing the need for additional alkaline reagents. The belt dryer uses low-temperature drying to prevent salt crystal agglomeration, and the produced refined salt can be directly reused in chlor-alkali and titanium molten salt chlorination production units.

[0032] In this embodiment, the waste liquid recycling equipment parameters of claims 1S6 and 10 are implemented as follows: the overflow settling tank 3 overflows and discharges the calcium sulfate-containing mother liquor, which is then transported to the mother hydraulic filter 7. The mother hydraulic filter 7 is a plate and frame filter press or a vacuum filter, with a filtration pressure controlled at 0.2-0.6 MPa. The turbidity of the filtrate after filtration is <5 NTU. The liquid phase pipeline of the mother hydraulic filter 7 is connected back to the washing liquid preparation tank 1, and the clarified filtrate is directly reused in the S1 liquid preparation process. The secondary washing waste liquid produced by the first solid-liquid separation equipment 10 is separately set up for neutralization and acid adjustment. After being adjusted to pH 11-12 by calcium carbide mud to precipitate iron and silicon, it is filtered and then adjusted to pH <2 by hydrochloric acid before being sent back to the washing liquid preparation tank 1 for circulation. The tertiary washing waste liquid produced by the second solid-liquid separation equipment 11 is directly returned to the tertiary washing tank 8. The entire process has a washing water reuse rate of >95%, achieving near-zero wastewater discharge, which fully meets the water resource indicators of claim 6. The mother hydraulic filter 7 has a limited filtration pressure that can completely trap calcium sulfate filter residue. The low-turbidity filtrate can be reused without deep treatment. The three-way washing waste liquid is recycled separately, which greatly reduces the use of fresh water and lowers the environmental treatment cost of high-salt wastewater.

[0033] In this embodiment, the filter residue treatment parameters of claims 1S7 and 7 are implemented as follows: The mixed filter residue of calcium sulfate, ferric hydroxide, and calcium silicate produced by the master hydraulic filter 7 and the neutralization process is uniformly sent to the immersion washing equipment. The solid-liquid mass ratio of the mixed filter residue to clean water is 1:2. The filter residue is stirred and immersed for 30 minutes to recover sodium chloride entrained in the filter residue. The immersion filtrate is returned to the front end of the overflow settling tank 3. The desalinated filter residue is sent to the drying equipment, and the drying temperature is controlled at 80-120℃. After drying, the moisture content of the filter residue is <5%. The finished filter residue is used as a cement retarder or as a raw material for gypsum building materials. Immersion washing of the filter residue with clean water can recover sodium chloride remaining in the pores of the filter cake, improving the overall salt recovery rate. Low-temperature drying does not damage the calcium sulfate crystal form, and all solid waste is recycled and sold externally, eliminating the cost of solid waste landfill disposal.

[0034] The complete installation process of the entire set of equipment follows the connection relationships of claims 8, 9, and 10: First, lay out the conveying pipeline between the washing liquid preparation tank 1 and the primary washing tank 2, then connect the discharge pipeline of the primary washing tank 2 to the inlet 31 of the overflow settling tank 3, and install the overflow weir 32, overflow outlet 33, conical underflow collection hopper 34, underflow outlet 35, sampling valve 36, 500mL transparent sampling cylinder 4, and underflow return pump 5 in sequence; the overflow outlet 33 is connected to the main hydraulic filter 7, and the qualified underflow pipeline is connected to the secondary washing tank 6; the secondary washing tank 6 and the tertiary washing tank 8 are equipped with stirring devices, and the first solid-liquid separation device 10, the tertiary washing tank 8, and the second solid-liquid separation device 11 are connected in series in sequence, and the second solid-liquid separation device 11 is connected to the belt dryer 12 and the tertiary washing tank 8 respectively; the liquid phase pipeline of the main hydraulic filter 7 is reconnected to the washing liquid preparation tank 1, completing the entire pipeline layout. All equipment structures, connection methods, and dimensional parameters are strictly implemented in accordance with claims 8 to 10.

[0035] The complete process flow is as follows: First, prepare the primary pickling mother liquor in washing solution preparation tank 1 according to the parameters in claim 2; second, send it to primary washing tank 2 and complete the primary pickling at room temperature for 30-60 minutes according to claims 3 and 4; third, send all the liquid to overflow settling tank 3 and complete gravity stratification and visual quality control of clarity for 10-20 minutes according to claim 5, with unqualified underflow being returned for rewashing; fourth, send the qualified underflow to secondary washing tank 6 and perform weak acid washing at pH 2-4 for 20-40 minutes, followed by solid-liquid separation using the first solid-liquid separation equipment. Step 10: Separation; Step 5: Secondary acid washing salt is sent to the third washing tank 8, and alkaline washing at pH 9-10 for 20-40 minutes. After separation by the second solid-liquid separation device 11, the wet salt is sent to the belt dryer 12 to produce refined salt with a purity ≥99%; Step 6: Various washing waste liquids are circulated in three paths, and the mother hydraulic filter 7 is pressure filtered at 0.2-0.6 MPa to produce low-turbidity circulating filtrate; Step 7: All process filter residues are soaked, dried and recycled according to the parameters of claim 7. All process conditions and equipment structures in the entire process do not deviate from the technical features described in claims 1 to 10.

[0036] The actual processing results of this embodiment are as follows: the purity of refined sodium chloride is 99.25%, the residual sodium sulfate is 0.22%, and the iron and silicon dioxide content is far below the industry limit; the washing water reuse rate is 96.8%, the purity of dry-basis calcium sulfate in the filter residue is 82.3%, the processing cost per ton of refined salt is 162 yuan, and the net profit is 368 yuan. This verifies that all technical solutions of the claims of this invention can be stably implemented and have both environmental and economic benefits. The entire equipment operation process is now complete.

[0037] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely preferred examples and are not intended to limit the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for recovering sodium chloride by-product salt through washing, characterized in that: Includes the following steps: S1. Prepare a saturated sodium chloride washing mother liquor, add hydrochloric acid to adjust the acidity of the mother liquor, and add calcium chloride to introduce calcium ions; S2. Mix and stir the by-product salt containing sulfate, iron, and silicon impurities with the calcium-containing acidic saturated sodium chloride solution obtained in step S1 to allow the sulfate to react with calcium ions to form fine calcium sulfate precipitate, thus completing one acid washing reaction. S3. Send all the liquid after the reaction in step S2 into the sedimentation equipment for gravity stratification. Large sodium chloride crystals settle to form the underflow, while the mother liquor containing fine calcium sulfate particles overflows and is separated and discharged. Take a sample of the underflow and let it stand to observe the clarity to determine the impurity removal effect. If the underflow is turbid and contains suspended particles, it is refluxing and rewashing until the underflow is clear and qualified. S4. The clarified and qualified sodium chloride underflow is subjected to secondary acid washing with a weakly acidic saturated sodium chloride solution. After stirring and reacting, solid-liquid separation is carried out to obtain secondary acid washing salt and secondary washing waste liquid. S5. The secondary acid-washed salt is washed three times with an alkaline saturated sodium chloride solution to remove iron and silicon impurities from the system. After stirring and reacting, the solid and liquid are separated to obtain refined salt and waste liquid from the third washing. The refined salt is dried to obtain high-purity sodium chloride product. S6. Closed-loop circulation of washing waste liquid: After the calcium sulfate-containing mother liquid discharged from the overflow separation is filtered to remove residue, the clarified filtrate is returned to step S1 for liquid preparation and recycling; the secondary washing waste liquid is adjusted with strong alkali to precipitate iron and silicon impurities, filtered to restore acidity and then reused for liquid preparation. The waste liquid from the three alkaline washing processes is directly recycled for alkaline washing solution preparation; S7. Resource utilization of filter residue: The calcium sulfate and iron-containing silicon hydroxide mixed filter residue generated in each process are washed with clean water to recover the sodium chloride entrained. The washing filtrate is returned to the sedimentation process. The desalinated filter residue is dried and used as raw material for gypsum and cement building materials.

2. The method for washing and recovering sodium chloride from by-product salt according to claim 1, characterized in that: S1 primary pickling mother liquor preparation parameters: 25℃ saturated sodium chloride solution mass concentration 26%~26.5%, calcium chloride solution mass concentration 30%~40%; molar ratio of calcium ions to sulfate ions in by-product salts 1.05:1~1.15:1; pH of primary washing mother liquor controlled at 1~2.

3. The method for washing and recovering sodium chloride from by-product salt according to claim 2, characterized in that: The conditions for the mother liquor and washing system for the secondary acid washing and the tertiary alkaline washing are as follows: the pH of the mother liquor for the secondary acid washing is 2-4; the pH of the mother liquor for the tertiary alkaline washing is adjusted to 9-10 using sodium carbonate or sodium hydroxide; the solid-liquid mass ratio for the primary acid washing, secondary acid washing, and tertiary alkaline washing is 1:2.5-1:3.5, and the washing reaction temperature is 20-35℃.

4. The method for washing and recovering sodium chloride from by-product salt according to claim 1, characterized in that: The stirring and reaction time for each process is as follows: the stirring and reaction time for the first acid wash is 30-60 minutes; the stirring time for the second acid wash and the third alkaline wash is 20-40 minutes.

5. The method for washing and recovering sodium chloride from by-product salt according to claim 1, characterized in that: Sedimentation stratification and clarification judgment criteria: The sedimentation stratification time of the liquid is 10-20 min, and the sedimentation environment temperature is 20-30℃; take the underflow sample, put it into a transparent container and let it stand for 1 min, observe it from the side under natural light. If the liquid is clear and there are no suspended particles and no white precipitate at the bottom, it is qualified. If the liquid is turbid and white and there are white suspended particles, it is unqualified.

6. The method for washing and recovering sodium chloride from by-product salt according to claim 1, characterized in that: Finished product and water resource utilization indicators: The purity of refined sodium chloride after drying is ≥99%, meeting the first-class standard of GB / T54622015 industrial salt, and the moisture content of refined salt after drying is <0.5%; the whole process washing water reuse rate is >95%, achieving near-zero wastewater discharge.

7. The method for washing and recovering sodium chloride from by-product salt according to claim 1, characterized in that: Filter residue treatment process parameters: the solid-liquid mass ratio of the mixed filter residue is 1:2 when soaked in clean water, and the soaking and stirring time is 30 minutes; the filter residue drying temperature is 80-120℃, and the moisture content of the dried filter residue is <5%, which can be used as a cement retarder or gypsum building material raw material.

8. The method for washing and recovering sodium chloride from by-product salt according to claim 1, characterized in that: Set up a washing liquid preparation tank (1) and a primary washing tank (2). The washing liquid preparation tank (1) is connected to the primary washing tank (2) through a conveying pipeline. The discharge pipeline of the primary washing tank (2) is connected to the inlet (31) of the overflow settling tank (3). The overflow settling tank (3) is a cylindrical / conical tank with a height-to-diameter ratio of 1.5:1 to 2.5:

1. An overflow weir (32) and an overflow outlet (33) are set at the top. The height of the overflow weir is 70% to 80% of the total height of the tank. A conical bottom flow collection hopper (34) with a cone angle of 60° to 90° and a bottom flow outlet (35) are set at the bottom. A sampling valve (36) is set on the side wall and connected to a 500mL transparent sampling measuring cylinder (4). The bottom flow outlet (35) is divided into two pipelines. One pipeline is connected back to the primary washing tank (2) through the bottom flow return pump (5), and the other pipeline is connected to the secondary washing tank (6). The pipeline of the overflow outlet (33) is connected to the main hydraulic filter (7).

9. The method for washing and recovering sodium chloride from by-product salt according to claim 8, characterized in that: Both the secondary washing tank (6) and the tertiary washing tank (8) are equipped with stirring devices. The outlet of the secondary washing tank (6) is connected to the first solid-liquid separation device (10), and the solid phase outlet of the first solid-liquid separation device (10) is connected to the tertiary washing tank (8). The outlet of the tertiary washing tank (8) is connected to the second solid-liquid separation device (11), and the solid phase outlet of the second solid-liquid separation device (11) is connected to the belt dryer (12). The liquid phase outlet pipeline is directly connected back to the tertiary washing tank (8). The first solid-liquid separation device (10) and the second solid-liquid separation device (11) are centrifuges or plate and frame filter presses.

10. The method for washing and recovering sodium chloride from by-product salt according to claim 8, characterized in that: Waste liquid recycling, filter residue resource utilization unit equipment and connection relationship: The main hydraulic filter (7) is a plate and frame filter press or vacuum filter, with a filtration pressure of 0.2 to 0.6 MPa and a filtrate turbidity of <5 NTU. The liquid phase pipeline of the main hydraulic filter (7) is connected back to the washing liquid preparation tank (1).

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