Method for recovering lithium carbonate from wastewater containing impure lithium fluoride mother liquor

Abstract

The present application belongs to the technical field of industrial wastewater treatment containing inorganic salt, and discloses a method for recovering lithium carbonate from wastewater containing impurity lithium fluoride mother liquor. The method comprises the following steps: firstly, determining the end point according to the time when the conductivity value changes to the lowest point in online monitoring, thereby accurately controlling the addition of carbonate precipitant, so that lithium ions and impurities co-precipitate to form a mixed solid phase; then, carbon dioxide is introduced into the slurry re-prepared from the solid phase, and the reaction end point is determined according to the time when the pH value change rate reaches the peak value in online monitoring, so that the lithium carbonate in the solid phase is selectively converted into soluble lithium bicarbonate into the liquid phase; finally, high-purity lithium carbonate is recovered by heating the solution rich in lithium bicarbonate. The present application establishes a solidification and temporary storage first and then selective liquefaction operation process, separates the two mutually restrictive goals of wastewater purification and resource recovery in the process time sequence, so that the thoroughness of wastewater purification is no longer at the expense of the feasibility of resource recovery.

Description

Technical Field

[0001] This invention relates to a method for recovering lithium carbonate from wastewater containing impurities in lithium fluoride mother liquor, belonging to the field of industrial wastewater treatment technology containing inorganic salts. Background Technology

[0002] Currently, the production process of lithium salts and related materials generally generates mother liquor wastewater containing lithium fluoride, lithium chloride, and various coexisting ions such as calcium and magnesium. The primary task in treating this type of wastewater is to ensure that the water discharged or reused meets strict environmental standards. The common practice is to use chemical precipitation, which involves adding alkaline precipitants such as sodium carbonate to the wastewater. This causes the calcium and magnesium hardness ions, as well as pollutants such as fluoride ions that are co-precipitated, to be converted into solid precipitates and separated from the water. This is a mature, cost-controllable, and reliable process that can ensure stable water quality compliance.

[0003] However, with the continuous increase in the value of lithium resources, the demand for recovering valuable lithium metal from the aforementioned wastewater is becoming increasingly urgent. At this point, the inherent limitations of the aforementioned technical approach, which focuses solely on pollutant removal, become apparent. Specifically, during the process of adding sodium carbonate for indiscriminate precipitation, high-value lithium ions will inevitably precipitate along with a large amount of impurity ions such as calcium and magnesium, forming a solid sludge with a complex composition and highly mixed physical form. The value of lithium is thus locked and dispersed in an impurity matrix that far exceeds its own volume, which directly reduces the economic feasibility of subsequent recovery work.

[0004] To address this issue, one direct approach is to attempt selective precipitation of the wastewater, aiming to remove impurities such as calcium and magnesium while retaining lithium ions in the liquid phase. However, the ion concentration and composition of industrial wastewater fluctuate dynamically. Achieving such precise selective separation often requires complex multi-stage pH control and expensive specialized reagents. This not only makes it difficult to guarantee process stability but also risks incomplete separation, potentially affecting the final water quality compliance, which contradicts the primary goal of wastewater treatment. Another approach is to perform secondary treatment on the already formed mixed sludge. This approach addresses the issue because the components have similar chemical properties and physical characteristics. The current technology, with its tightly integrated processes, faces challenges of high costs and lengthy procedures, regardless of whether physical sorting or complex hydrometallurgical processes are employed. This reveals an inherent contradiction between the compliance of wastewater treatment and the economic viability of recovering valuable resources within the existing technological framework. Specifically: 1. The co-precipitation operation used to ensure complete removal of pollutants is itself a direct cause of the difficulty in economically recovering valuable resources; 2. Any attempt at fine separation in the liquid phase may increase process uncertainty and pose a potential risk to the core task of ensuring stable water quality compliance. Therefore, the technical problem this invention aims to solve is to establish a new process route that first ensures the stable and complete removal of various pollutants from wastewater to meet environmental protection requirements, and then transforms the mixed precipitate, originally considered final waste, into a resource-efficient intermediate that is easily separated and purified at low cost. This would integrate the conflicting goals of environmental governance and resource recovery into a unified whole. Summary of the Invention

[0005] This invention provides a method for recovering lithium carbonate from wastewater containing lithium fluoride mother liquor with impurities. Its main purpose is to solve the problem that there is a contradiction between the co-precipitation operation adopted in the prior art to ensure the compliance of wastewater treatment and the economic requirements for the subsequent recovery of valuable lithium from mixed sludge.

[0006] To achieve the above objectives, this invention provides a method for recovering lithium carbonate from wastewater containing impurities in lithium fluoride mother liquor. This method establishes a procedure that decouples wastewater purification and resource recovery in a temporal sequence. The procedure includes the following steps:

[0007] Step a: During the continuous addition of carbonate precipitant to wastewater containing lithium fluoride mother liquor containing impurities, the conductivity value of the wastewater is continuously monitored. When the conductivity value changes from a continuous decrease to no longer decreasing or begins to rise, the addition of carbonate precipitant is stopped to form a mixed solid precipitate composed of lithium carbonate and impurity solids.

[0008] Step b: Separate the mixed solid precipitate formed in step a from the purified liquid phase;

[0009] Step c: Introduce the carbon dioxide gas carrying temperature generated in step e into the slurry reconstituted from the mixed solid precipitate separated in step b, and continuously monitor the pH value of the slurry during this process, calculate the first derivative of the pH value with respect to time, and stop introducing carbon dioxide gas when the absolute value of the first derivative reaches its peak, so as to obtain a lithium-rich solution rich in soluble lithium bicarbonate.

[0010] Step d: Separate the undissolved solid impurities from step c from the lithium-rich solution;

[0011] Step e: Heating the lithium-rich solution causes the soluble lithium bicarbonate to decompose and precipitate lithium carbonate, releasing carbon dioxide gas.

[0012] Preferably, the carbon dioxide gas introduced in step c carries the temperature of 80 to 95 degrees Celsius when it enters the slurry. This temperature causes the dissolved silicon present in the slurry to precipitate out in the form of solid silica and be separated from the solid impurities in step d.

[0013] Preferably, the execution of step c is limited to a pH value between 6.5 and 7.5, and the heating process of step e is limited to a temperature range between 80 degrees Celsius and 95 degrees Celsius.

[0014] Preferably, the operation of adding carbonate precipitant in step a is specifically to determine the stopping time by monitoring the change trend of conductivity value when the total molar ratio of carbonate to precipitable metal cations in wastewater reaches 1.1 or higher.

[0015] Preferably, step c, which involves stopping the introduction of carbon dioxide gas, further includes obtaining the turbidity value of the slurry after the absolute value of the first derivative reaches its peak value but before performing the stopping operation, and only performing the stopping operation if the obtained turbidity value is lower than a preset threshold.

[0016] Preferably, the operation of stopping the addition of carbonate precipitant in step a further includes detecting the concentration of impurity cations in the wastewater after the conductivity value reaches a point where it no longer decreases or begins to rise, but before performing the stopping operation, and only performing the operation of stopping the addition of carbonate precipitant if the detected concentration is lower than the emission standard limit.

[0017] Preferably, the calculation of the first derivative of pH value with respect to time in step c is obtained by continuously collecting pH values ​​at a fixed sampling period and dividing the difference between the pH values ​​of the next period and the previous period by the sampling period duration.

[0018] Preferably, the preparation of the slurry in step c is carried out using the purified liquid phase separated in step b as the liquid medium.

[0019] Preferably, the condition in step c for determining the peak value of the absolute value of the first derivative at the moment when the introduction of carbon dioxide gas is stopped is determined by the following relationship: ,in, This represents the pH value for the current sampling period. This is the pH value from the previous sampling period. The duration of the sampling period. The derivative threshold is the threshold at which the buffering capacity of the characterization system, determined based on historical batch data, undergoes a fundamental change.

[0020] Preferably, the lithium carbonate precipitated in step e has a total mass fraction of calcium and magnesium ions of less than 0.05%.

[0021] Compared with the prior art, the beneficial effects of the present invention are:

[0022] 1. This invention establishes a new operational sequence for wastewater treatment and resource recovery. First, by adding a carbonate precipitant, lithium ions and impurity ions such as calcium and magnesium in the wastewater are intentionally converted into a mixed solid precipitate. This precipitate serves as an intermediate carrier at this stage, allowing the purified water to be separated and meet discharge requirements. Subsequently, this solid carrier is re-pulped, and the mild acidic environment created by introducing carbon dioxide selectively converts lithium carbonate in the solid phase into soluble lithium bicarbonate, which then enters the liquid phase. The original impurities continue to exist in solid form. This process of first solidifying and temporarily storing, and then selectively liquefying, separates the two mutually restrictive operations of achieving water quality standards and purifying valuable substances in traditional processes in terms of process sequence, ensuring that the thoroughness of wastewater purification does not come at the expense of the feasibility of resource recovery.

[0023] 2. The method of this invention does not rely on a fixed reaction time or a preset pH target value. Instead, it continuously monitors the rate of change of pH in the slurry during the carbon dioxide introduction process. When the lithium carbonate, which serves as the main buffer in the slurry, is completely consumed, the rate of change of pH in the system will reach a turning point. This method determines the endpoint of leaching and stops the introduction of carbon dioxide based on the identification of this process characteristic inflection point. This approach changes the basis for endpoint control from an externally set static parameter to a dynamic characteristic that is fed back in real time by changes in the chemical state inside the reaction system. This allows the leaching operation to autonomously adapt to the fluctuations in the chemical composition of each batch of mixed solid precipitate, thereby ensuring that lithium ions are fully dissolved while avoiding secondary dissolution of impurities due to over-leaching.

[0024] 3. By directly using the carbon dioxide gas, which carries heat and is released during the heating of the lithium bicarbonate solution in the subsequent thermal desorption purification step, as the input gas for the selective leaching step, this process coupling makes the product and heat from the later stage a functional input for the earlier stage. In this process, the carbon dioxide gas performs its chemical function of dissolving lithium carbonate, while the heat it carries increases the overall temperature of the leaching slurry. This temperature condition will cause any dissolved silicon that may be present in the wastewater to precipitate in advance in the form of solid silica. In the subsequent solid-liquid separation, the pre-precipitated solid silica will be filtered out along with other impurities, avoiding the formation of gels in the subsequent low-temperature or heating stages, which would affect the filtration efficiency or contaminate the final product. At the same time, it also realizes the recycling of materials and energy within the system. Attached Figure Description

[0025] Figure 1 This is a process flow diagram of the adaptive lithium carbonate recovery of the present invention;

[0026] Figure 2 This is a diagram of the real-time feedback control system architecture of the present invention;

[0027] Figure 3 This is a timing diagram of mass-energy synergy and cyclicity within the system of this invention. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the embodiments. It should be understood that these embodiments are only used to illustrate the present invention and are not intended to limit the scope of protection of the present invention.

[0029] This invention provides a method for recovering lithium carbonate from wastewater containing impurity lithium fluoride mother liquor. The process flow aims to separate the two process objectives of wastewater purification and resource recovery in terms of treatment sequence. The method first converts precipitable cations in the wastewater, including lithium ions and impurity cations such as calcium and magnesium, into a mixed solid phase precipitate through a co-precipitation step, and separates the purified liquid phase. Subsequently, through a selective leaching step, lithium carbonate in the aforementioned mixed solid phase is selectively converted into soluble lithium bicarbonate under specific chemical conditions. Finally, high-purity lithium carbonate product is precipitated by thermal desorption of the lithium bicarbonate-rich solution, and the process materials are recycled.

[0030] In a specific application, this method is used to treat lithium fluoride mother liquor containing impurities discharged from lithium salt production facilities. The difficulty in treating this type of wastewater lies in the coexistence of lithium ions with recoverable value, along with calcium and magnesium hardness ions and fluoride ions whose concentrations fluctuate dynamically. This creates a technical contradiction between ensuring that the discharged liquid phase consistently meets water pollutant discharge standards and economically recovering lithium resources from solid waste. To ensure the thoroughness of wastewater purification, step a of this invention, the co-precipitation step, involves continuously adding a sodium carbonate solution as a carbonate precipitant to the wastewater containing lithium fluoride mother liquor containing impurities. During this process, the conductivity of the wastewater is continuously monitored using an online conductivity meter. The working principle is that when the sodium carbonate solution is added, the highly conductive divalent calcium and magnesium ions react with carbonate ions to form precipitates and are removed from the solution, while being replaced by monovalent sodium ions with relatively low conductivity. This process causes the overall conductivity of the wastewater to show a continuous downward trend. After the impurity cations have basically reacted completely, if sodium carbonate solution is added further, it is equivalent to adding a net increase of freely moving sodium ions and carbonate ions to the system. At this point, the conductivity value will change from continuously decreasing to no longer decreasing or starting to rise. The inflection point of this conductivity curve represents the stoichiometric endpoint of the impurity cation precipitation reaction in real time. Therefore, the process controller is configured to capture the moment when the conductivity value reaches its lowest point in real time and trigger the operation to stop adding carbonate precipitant at that moment. It should be noted that, to ensure sufficient impurity removal, the endpoint judgment logic based on the conductivity change trend can be activated when the total molar ratio of carbonate to precipitable metal cations in wastewater reaches, for example, 1.1 or higher. Through this control method based on real-time process feedback, step a achieves precise addition of precipitant while forming a mixed solid phase precipitate composed of lithium carbonate and impurity solids, avoiding the increased cost and sludge volume caused by excessive addition, and contributing to the stable compliance of the purified water.

[0031] After step a is completed, step b, the first solid-liquid separation step, uses a plate and frame filter press or other solid-liquid separation equipment to separate the mixed solid precipitate formed in step a from the purified liquid phase. The separated purified liquid phase, because its impurity cation concentration is lower than the emission standard limit, can be directly discharged or reused. The compositional fluctuation of industrial wastewater places demands on the stability of process control, especially in the resource recovery stage. If not properly controlled, secondary dissolution of impurities can easily occur, thus contaminating the final product. To address this problem, step c of the method of this invention, the selective leaching step, establishes an adaptive leaching endpoint determination method. Specifically, the procedure involves first using a portion of the aforementioned purified liquid phase as a liquid medium to reconstitute the mixed solid precipitate separated in step b into a slurry with a preset solid-liquid ratio. Then, carbon dioxide gas is introduced into the slurry. Carbonic acid formed by carbon dioxide dissolving in water preferentially reacts with lithium carbonate in the slurry, selectively converting it into highly soluble lithium bicarbonate, which then enters the liquid phase. The chemical reaction formula is as follows: During this process, an online pH meter is used at a fixed sampling period. For example, the pH value of the slurry is monitored continuously for 1 second. Simultaneously, the process controller calculates the absolute value of the first derivative of the pH value with respect to time in real time by dividing the difference between the pH values ​​of the next cycle and the previous cycle by the sampling cycle duration. .

[0032] The mechanism of this method is that when a large amount of lithium carbonate, which acts as the main buffer in the slurry, is consumed, the pH value of the system decreases slowly. The pH value remained at a low level, but when the lithium carbonate was almost completely consumed, the buffering capacity of the system decreased significantly. At this point, even a small amount of carbon dioxide introduced subsequently could cause a rapid drop in pH, thus... A peak is formed on the real-time curve; this peak represents the moment when the selective dissolution of lithium carbonate is basically complete, and is the reaction endpoint to avoid the dissolution of impurities such as calcium carbonate and magnesium carbonate due to excessive leaching; therefore, the process controller is set to determine that the reaction has reached its endpoint and immediately stop introducing carbon dioxide gas when the absolute value of the first derivative reaches the peak value, in order to obtain a lithium-rich solution rich in soluble lithium bicarbonate; the condition for determining the peak moment can be determined by the following relationship: ,in, This represents the pH value for the current sampling period. This is the pH value from the previous sampling period. The duration of the sampling period. A derivative threshold; derivative threshold This can be determined through calibration procedures: analyzing historical batch data or experimental data, and comparing them. By analyzing the curves and the concentration data of calcium and magnesium ions in the liquid phase at different times, we can identify and determine a method that ensures both lithium ion dissolution rate and keeps impurity dissolution below acceptable levels. The value, and set it as a threshold. For example, in one implementation, the threshold It was determined to be 0.05. With this adaptive endpoint control, step c can achieve selective dissolution of lithium within a pH range, for example, 6.5 to 7.5.

[0033] To further improve the operational reliability of the system, the method of the present invention can also integrate a decision arbitration mechanism; specifically, in step c, when the process controller bases on... After the peak condition is determined to have been reached, but before executing the physical operation to stop introducing carbon dioxide, the system performs a decision arbitration step. This step includes acquiring the turbidity value of the slurry at this moment. The principle is that the selective leaching process converts solid lithium carbonate into a clear solution, and at the optimal endpoint, the turbidity of the liquid phase should be at a low level. Therefore, the system only confirms the validity of the decision and executes the operation to stop introducing carbon dioxide if the acquired turbidity value is below a preset threshold, such as 5 NTU. If the turbidity value is higher than this threshold, it indicates that there may be a problem such as an abnormal pH meter signal. In this case, the system will reject the stop operation command and trigger an alarm, thereby providing safety redundancy for the control system. After separating the undissolved solid impurities, mainly calcium carbonate and magnesium carbonate, from the lithium-rich solution in step c through step d, the second solid-liquid separation step, the system proceeds to step e, the thermal desorption purification step. This step heats the lithium-rich solution to, for example, 80°C. Up to 95 Within a certain temperature range, lithium bicarbonate undergoes a decomposition reaction, reprecipitating as high-purity lithium carbonate solid and releasing carbon dioxide gas. The chemical reaction formula is as follows: The final precipitated lithium carbonate, after washing and drying, has a total mass fraction of calcium and magnesium ions of less than 0.05%. Furthermore, the method of this invention constructs an internal material and energy circulation path to address the risks posed by dissolved silicon in the wastewater. This circulation path uses the heat-carrying carbon dioxide gas released during heating in step e as the input gas for selective leaching in step c, without cooling. During this process, the temperature of the heat-carrying carbon dioxide gas entering the slurry can be maintained at 80°C. Up to 95 The heat it carries raises the overall temperature of the leaching slurry. Based on the characteristic that the solubility of dissolved silica in neutral or weakly acidic aqueous solutions decreases with increasing temperature, this heating operation causes the dissolved silica present in the slurry to precipitate out as solid silica. In this way, the precipitated solid silica can be separated from other solid impurities in subsequent step d, thereby avoiding the problem of it forming a gel in subsequent stages, which would affect filtration efficiency or contaminate the product. This improves the adaptability of the method to raw materials while realizing the recycling of materials and energy.

[0034] Example 1: In a continuously operating battery material production facility, the quality of its discharged lithium fluoride mother liquor wastewater containing impurities fluctuates irregularly, particularly the calcium ion hardness, which varies between batches. To meet wastewater discharge standards, the facility's original treatment method involved adding far more carbonate precipitant than theoretically required to each batch of wastewater. While this method ensured water quality compliance, it also resulted in excessively high chemical costs and solid sludge disposal fees. Attempts to recover valuable lithium from this complex sludge were not feasible due to economic reasons, creating a conflict between environmental compliance requirements and the economic requirements of resource recovery. To address this situation... A batch of 10 cubic meters of wastewater was taken for treatment. Step a was initiated, and while sodium carbonate solution was pumped into the wastewater, the online conductivity meter showed that the initial conductivity reading began to decline steadily. When the system detected that the conductivity value had reached its lowest point and showed a rebound trend, the addition of sodium carbonate solution was stopped. At this time, the purified liquid phase separated was tested, and its impurity cation concentration was lower than the emission standard limit. This precipitation endpoint control based on real-time response of wastewater replaced the original fixed excessive addition method, so that the consumption of precipitant matched the actual impurity content of the batch of wastewater. Under the premise of achieving the water quality purification target, the cost of chemical addition and the amount of sludge generated were reduced.

[0035] This method transforms a multi-component separation process in the liquid phase into a single-component extraction process in the solid phase through the co-precipitation operation in step a, namely, the selective leaching in step c. After the mixed solid precipitate separated in the previous step is re-pulped, carbon dioxide gas carrying temperature, recovered from the subsequent step e, is introduced into the slurry. During this process, the process controller continuously monitors the pH value and calculates the absolute value of its first derivative. Because the precise control of the preceding step a results in a dynamic change in the ratio of lithium to impurities in each batch of the generated mixed solid precipitate, a leaching control method with a fixed duration or a fixed target pH value cannot simultaneously guarantee lithium recovery and product purity. Furthermore, based on... The adaptive endpoint determination mechanism does not rely on the initial composition of the solid material, but rather determines the reaction endpoint by identifying the moment when the system's buffering capacity decreases. This mechanism creates a functional connection between the precise precipitation in step a and the adaptive leaching in step c. That is, although the output of the former fluctuates, the control logic of the latter can autonomously adapt to these fluctuations, thereby stably obtaining a high-quality lithium-rich solution under changing operating conditions. When the controller detects... Reaching the preset threshold At the peak moment, the system stops introducing carbon dioxide and performs a second solid-liquid separation on the slurry. The separated lithium-rich solution is heated to 90°C in step e, and lithium carbonate solid is precipitated. Analysis shows that its total calcium and magnesium content is less than 0.05%, and the volume of solid impurities that were originally considered waste is also reduced due to the precise control in step a. The entire treatment process transforms the original single environmental protection task of wastewater treatment into a closed-loop process that takes into account both environmental benefits and resource value recovery.

[0036] Example 2: This example aims to verify the effectiveness and feasibility of the adaptive endpoint control based on real-time process feedback used in the method of this invention when treating lithium fluoride mother liquor wastewater with fluctuating composition and containing impurities, through experimental data. The experiment was conducted in a 100L reactor equipped with an adjustable speed stirrer and a jacket temperature control system. The system integrates an online monitoring and control unit with the following functional specifications: a conductivity meter with a measurement range of 0-20 mS / cm and a resolution of 0.01 mS / cm, a pH electrode with a measurement range of 0-14 and a resolution of 0.01, and a process controller capable of executing preset logic to control the start and stop of the reagent pump and gas flow meter based on sensor signals. For comparison, the experiment set up a sample group of this invention and a control sample group. The initial wastewater used was artificially prepared lithium fluoride mother liquor containing impurities under simulated high hardness conditions. The concentrations of the bond components were analyzed and kept consistent. During the co-precipitation process in step a, the control group used a fixed excess addition method, i.e., after calculating the theoretically required molar amount of precipitant based on the total hardness of the initial wastewater, 1.5 times the theoretical amount of 25% w / w sodium carbonate solution was added, consuming a total of 7.5 L. The present invention group used a control method based on the lowest conductivity point to determine the endpoint, continuously pumping the same sodium carbonate solution until the process controller detected the lowest conductivity reading and stopped pumping, consuming a total of 5.2 L. During the selective leaching process in step c, the generated mixed solid precipitates were re-pulped with the same solid-liquid ratio. The control group used a fixed-duration operation, i.e., carbon dioxide gas was introduced at a constant flow rate for 60 minutes. The present invention group used a control method based on the pH change rate to determine the endpoint, introducing carbon dioxide at the same flow rate until the process controller detected the lowest conductivity reading. The value reaches the calibrated threshold. When to stop the flow, the pH data sampling period here. The period was set to 1 second. This period was determined after balancing the real-time performance of data acquisition with the data processing load of the system. Under the conditions of aqueous reaction kinetics, it can capture rapid changes near the reaction endpoint while avoiding interference from signal noise introduced by high-frequency sampling on derivative calculation.

[0037] After completing all treatment steps, the key performance indicators of the two sample groups were tested. The results showed that the sodium carbonate dosage of the sample group of the present invention was significantly lower than that of the control sample group. This indicates that the control method based on the lowest conductivity point in step a ensures that the dosage of precipitant matches the actual impurity content of the batch of wastewater, avoiding chemical waste caused by fixed excessive dosage. Simultaneously, the purified liquid phase obtained by the sample group of the present invention contains less residual... The concentration was 8.1 mg / L, lower than the 18.5 mg / L of the control group, indicating that this control method has an advantage in the thoroughness of the precipitation reaction; in addition, the final lithium recovery rate of the sample group of this invention was 98.6%, higher than the 91.3% of the control group, and the final product purity was 99.96%, with a total Ca / Mg content of 0.041%, while the product purity of the control group was 99.72%, and the total Ca / Mg content was 0.082%; the difference between the data indicates that in step c, based on The adaptive control based on peak value determination, compared to fixed-duration operation, can terminate the reaction when lithium carbonate is fully dissolved but impurity carbonates have not yet dissolved in large quantities. This improves the recovery rate of valuable resources while suppressing the contamination of the final product by impurities. Experimental data shows that, compared with the fixed-parameter control method, the adaptive control method based on real-time process feedback adopted in this invention can reduce chemical consumption, improve wastewater purification effect, and achieve higher resource recovery rate and product purity when treating lithium fluoride mother liquor wastewater containing impurities.

[0038] Example 3: This example combines Figures 1 to 3 A method for recovering lithium carbonate from wastewater containing impurities in lithium fluoride mother liquor is described, such as... Figure 1As shown, the process flow disclosed in this invention begins by introducing lithium fluoride mother liquor wastewater containing impurities into the co-precipitation unit in step a. In this unit, the precipitation endpoint is adaptively determined based on the moment when the conductivity reaches its lowest point, using real-time feedback provided by online conductivity monitoring. This solidifies lithium ions and impurities into a mixed solid precipitate. This precipitate then enters the solid-liquid separation unit in step b, where a purified liquid phase that meets discharge or reuse standards is separated from the mixed solid phase that enters subsequent steps. In the selective leaching unit in step c, lithium carbonate in the mixed solid phase is selectively converted into soluble lithium bicarbonate using the material and energy cycle from step e. This process is also based on real-time feedback, using online pH change rate monitoring. The leaching endpoint is determined by the moment the peak value is reached. The leached slurry then enters the solid-liquid separation unit II in step d, where solid impurities such as calcium carbonate and magnesium carbonate are separated from a lithium-rich solution containing soluble lithium bicarbonate. This lithium-rich solution then enters the thermal desorption purification unit in step e, where high-purity lithium carbonate is precipitated by heating. During this process, the released substances and energy are recycled to step c.

[0039] like Figure 2 As shown, the stable operation of the process is centrally managed by a process controller. This controller is embedded with a conductivity endpoint judgment algorithm, a pH change rate endpoint judgment algorithm, a turbidity decision arbitration logic, and a historical data and threshold library as core components. At the coprecipitation reactor node, the process controller receives conductivity data from the online conductivity meter and sends start / stop control commands to the carbonate precipitant injection pump through its algorithm logic. After passing through solid-liquid separation unit I, the mixed solid phase enters the selective leaching reactor node. The online pH meter and online turbidity meter in this node send pH and turbidity signals to the process controller. The controller issues flow control commands to the carbon dioxide gas flow meter according to its pH change rate endpoint judgment algorithm and turbidity decision arbitration logic. Finally, the lithium-rich solution separated by solid-liquid separation unit II is sent to the thermal desorption purification reactor node for processing.

[0040] like Figure 3 As shown, the internal material and energy cycle of the system manifests as a coordinated temporal logic. After the cycle system is started, the purified liquid phase produced by the first separation system is used as the pulping liquid medium for the selective leaching system. Simultaneously, the thermal desorption system heats the lithium-rich solution to 90°C, releasing heat-carrying energy. Gas, which transfers heat energy to The circulating system, supplied at temperatures between 80°C and 95°C and received by the selective leaching system, introduces heat energy that promotes the precipitation of soluble silicon, thereby forming solid silica. Under these temperature-controlled conditions, the system selectively dissolves lithium carbonate. In a continuously circulating closed loop, the thermal desorption system continuously releases... The solution is continuously supplied to the selective leaching system via a circulation system to maintain the reaction temperature, while the liquid phase recycling system replenishes the pulping water, ultimately achieving zero-emission recycling.

[0041] Example 4: In this example, the derivative threshold used to determine the reaction endpoint in the selective leaching step c. The determination provides a reproducible calibration procedure for setting a derivative threshold for the process controller in step c when treating a lithium fluoride mother liquor containing impurities with fluctuating component characteristics during the commissioning or process adjustment phase of a wastewater treatment system. This is a prerequisite for ensuring lithium recovery rate and product purity; if the threshold... Improper setting of the derivative threshold can lead to several problems. If the threshold is too low, the reaction may terminate prematurely before the lithium carbonate is fully dissolved, resulting in a decrease in recovery. Conversely, if the threshold is too high, carbon dioxide may not be stopped promptly after the lithium carbonate has dissolved, causing secondary dissolution of impurities such as calcium carbonate and magnesium carbonate, thus contaminating the product. Therefore, it is crucial to determine the appropriate derivative threshold for a specific raw material system. The following calibration procedure must be performed in a 5L laboratory-scale reactor that simulates the temperature and stirring conditions of the production environment and is equipped with a pH electrode of the same functional specifications as the production equipment. The data sampling period is... The reaction time was set to 1 second, and the apparatus was equipped with a mass flow meter for controlling the flow rate of carbon dioxide gas and a sampling device with a 0.22-micron filter head for extracting liquid phase samples. First, a mixed solid-phase precipitate sample obtained under stable production conditions through steps a and b was taken and, according to the solid-liquid ratio set in the production process, re-slurryed with the purified liquid phase in the reactor. Stirring and temperature control were initiated, and then carbon dioxide gas was introduced into the slurry at a constant flow rate. Real-time pH data was recorded simultaneously. Every 2 minutes from the start of carbon dioxide introduction, a filtered liquid phase sample was extracted using the sampling device until the reaction had proceeded for 90 minutes, ensuring that all reaction stages were covered by data. After the experiment, all extracted liquid phase samples were analyzed using inductively coupled plasma atomic emission spectrometry (ICP-AES) to obtain the concentration data of calcium and magnesium ions. Simultaneously, based on the recorded pH data, the overall process parameters were calculated. A curve that changes over time.

[0042] Calculated The curves were compared with the total calcium and magnesium ion concentration curves obtained from the analysis, plotted on the same time coordinate system. It can be observed that... The curve reaches its peak at a specific time point, while the total calcium and magnesium ion concentration curve remains at a baseline level in the initial stage, and then begins to show an upward trend at another time point; the purpose of calibration is to determine The temporal relationship between the time point when the curve reaches its peak and the time point when the total calcium and magnesium ion concentration begins to rise; observed in a specific calibration experiment. The reaction reached its peak value of 0.052 at 46 minutes. The total calcium and magnesium ion concentrations remained below the baseline of 0.5 mg / L until 50 minutes, and then began to rise after 50 minutes. This data indicates that... The peak value, taken as the reaction endpoint, provides a time window within which lithium carbonate has essentially dissolved while secondary dissolution of impurities has not yet occurred. Based on the results of this calibration experiment, to allow for a control margin in industrial production, 95% of the measured peak value is determined as the final derivative threshold. In this case, the derivative threshold is... Set to 0.049 By implementing this procedure, a control parameter can be transformed into a traceable engineering setpoint with clear physical meaning and experimental data support.

[0043] Example 5: To ensure that the control logic based on the lowest conductivity point to determine the endpoint can withstand interference from sensor signal noise in the industrial application of step a, a field debugging procedure for control parameters must be executed before the initial deployment of the system or before treating wastewater with changed characteristics. This procedure is implemented by performing a trial run on a batch of wastewater. During this process, the process controller is configured to collect conductivity data at high frequency and store it in a short-term time buffer while continuously adding carbonate precipitant. Its built-in endpoint determination algorithm determines the endpoint by identifying inflection points that meet preset trend conditions. The determination logic is set as follows: when the controller continuously detects at least 5 data points with a monotonically decreasing conductivity reading in the buffer, it begins to search for the lowest value of subsequent readings. Only when at least 3 data points with a monotonically increasing reading are continuously detected after the lowest value appears, does the controller finally confirm the lowest point as an effective reaction endpoint and terminate the addition of precipitant. This procedure is used to reduce the impact of false inflection points caused by local turbulence or instantaneous signal fluctuations on endpoint determination.

[0044] To set an effective safety threshold for the turbidity monitoring-based decision arbitration mechanism in step c, it is necessary to perform the derivative threshold in Example 3. During the calibration procedure, an online turbidity meter with a measurement range of 0-100 NTU and a resolution of 0.1 NTU is simultaneously activated, and the turbidity change curve throughout the selective leaching process is recorded; by comparing this turbidity curve with... Correlation analysis between the curve and the total calcium and magnesium ion concentration curve in the liquid phase can determine the concentration of calcium and magnesium ions in the liquid phase. The baseline turbidity value corresponding to the optimal reaction endpoint when the peak value is reached; in a specific calibration test, when When the peak value is reached, the measured liquid reference turbidity stabilizes at 3.5 NTU. The safety threshold is determined by adding a fixed redundancy above this reference turbidity value. If the redundancy is set to 2.0 NTU, the turbidity safety threshold under this condition is determined to be 5.5 NTU. This value is written into the safety monitoring module of the process controller as the basis for judging whether the pH measurement system is abnormal.

[0045] Example 6: To determine the key process parameters of the method of the present invention and configure its response logic under specific boundary conditions before industrial deployment, an offline process window optimization and verification procedure needs to be executed. The primary goal of this procedure is to determine an objective function that can synergistically optimize the silicon removal effect and operating energy consumption for the temperature-carrying carbon dioxide gas introduced in step c. To this end, a set of gradient experiments needs to be conducted in multiple parallel laboratory-scale reactors containing mixed solid precipitates from the same batch that have been re-slurried after treatment in steps a and b. During the selective leaching step c, carbon dioxide gas with precisely controlled temperature is introduced into each reactor, with temperature setpoints of 70°C and 70°C respectively. 75 80 85 90 95 and 100 In each group of experiments, the result was achieved by After the reaction endpoint was determined by peak value, the resulting lithium-rich solution was sampled and analyzed to detect the concentration of residual dissolved silicon and to record the unit energy consumption of each group. Experimental data showed that when the gas temperature was below 80°C... At that time, the concentration of residual dissolved silicon in the solution is relatively high, while when the gas temperature is higher than 95°C... At that time, the concentration of residual dissolved silicon did not decrease significantly further, but the unit energy consumption of the system increased significantly. Based on this, 80 Up to 95 The optimal solution interval for the objective function is determined, i.e., the process temperature window.

[0046] Another objective of this procedure is to verify and configure a strategy for handling wastewater with low calcium ion concentration and high fluoride ion concentration. To simulate this boundary condition, the molar ratio of calcium ions to fluoride ions in the wastewater to be treated is pre-set to be less than 0.6. Under this condition, if step a is executed directly, the fluoride ion concentration in the subsequently separated purified liquid phase will be higher than the emission standard limit. To address this situation, the method of this invention is further configured to include a pre-treatment water quality verification and adjustment step. Before the co-precipitation operation in step a, the initial concentrations of fluoride ions and calcium ions in the wastewater to be treated are detected by an ion-selective electrode. When the process controller determines that the molar ratio is less than 0.6, a calculated amount of calcium chloride solution will be automatically added first to adjust the ratio to above 0.6, and then the carbonate precipitant addition process in step a will be started. After this adjustment, the fluoride ion concentration in the purified liquid phase can stably meet the emission standard. The execution of this procedure enables the entire process to achieve stable removal of fluoride ions for wastewater with different characteristics, improving the universality of the method.

[0047] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the present invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the present invention.

[0048] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for recovering lithium carbonate from wastewater containing an impure lithium fluoride mother liquor, characterized by, The method comprises the following steps: Step a, continuously monitoring the conductivity value of the wastewater containing impurity lithium fluoride mother liquor during the continuous addition of carbonate precipitant, and stopping the addition of carbonate precipitant when the conductivity value changes from continuous decrease to no longer decrease or begins to rise, to form a mixed solid phase precipitate composed of lithium carbonate and impurity solids; Step b, separating the mixed solid phase precipitate formed in step a from the purified liquid phase; Step c, introducing the temperature-carrying carbon dioxide gas generated in step e into the slurry re-prepared from the mixed solid phase precipitate separated in step b, and continuously monitoring the pH value of the slurry in the process, calculating the first derivative of the pH value with respect to time, and stopping the introduction of carbon dioxide gas when the absolute value of the first derivative reaches a peak, to obtain a lithium-rich solution rich in soluble lithium bicarbonate; Step d, separating the insoluble solid phase impurities in step c from the lithium-rich solution; Step e, heating the lithium-rich solution to precipitate lithium carbonate and release carbon dioxide gas; And, the temperature-carrying carbon dioxide gas introduced in step c has a temperature of 80-95°C when it enters the slurry, which causes the dissolved silicon present in the slurry to precipitate in the form of solid silicon dioxide and be separated together with the solid phase impurities in step d; The operation of stopping the introduction of carbon dioxide gas in step c also includes obtaining the turbidity value of the slurry after the absolute value of the first derivative reaches a peak, but before the stopping operation is performed, and only when the obtained turbidity value is below a preset threshold, the operation of stopping the introduction of carbon dioxide gas is performed; The condition for determining the time when the introduction of carbon dioxide gas is stopped in step c, at which the absolute value of the first derivative reaches a peak, is determined by the following relationship: wherein, is the pH value of the current sampling period, is the pH value of the previous sampling period, is the length of the sampling period, is a derivative threshold value representing a fundamental change in the buffering capacity of the system, determined from historical batch data.

2. The method for recovering lithium carbonate from wastewater containing impure lithium fluoride mother liquor according to claim 1, characterized in that, The execution of step c is limited to a pH value of 6.5-7.5, and the heating process of step e is limited to a temperature range of 80-95°C.

3. The method for recovering lithium carbonate from wastewater containing impure lithium fluoride mother liquor according to claim 1, characterized in that, The operation of adding carbonate precipitant in step a is specifically to determine the stopping time by monitoring the conductivity value trend when the total molar ratio of carbonate to precipitable metal cations in the wastewater reaches 1.1 or more.

4. The method for recovering lithium carbonate from wastewater containing impure lithium fluoride mother liquor according to claim 1, characterized in that, The operation of stopping the addition of carbonate precipitant in step a also includes detecting the concentration of impurity cations in the wastewater after the conductivity value reaches a point where it no longer decreases or begins to rise, but before the stopping operation is performed, and only when the detected concentration is below the emission standard limit, the operation of stopping the addition of carbonate precipitant is performed.

5. The method for recovering lithium carbonate from wastewater containing impure lithium fluoride mother liquor according to claim 1, characterized in that, The calculation of the first derivative of the pH value with respect to time in step c is obtained by continuously collecting the pH value at a fixed sampling period, and using the difference between the pH values of the next period and the previous period divided by the length of the sampling period.

6. The method for recovering lithium carbonate from wastewater containing impure lithium fluoride mother liquor according to claim 1, characterized in that, The preparation of the slurry in step c is carried out using the purified liquid phase separated in step b as the liquid medium.

7. The method for recovering lithium carbonate from wastewater containing impure lithium fluoride mother liquor according to claim 1, characterized in that, The precipitated lithium carbonate in step e has a total mass fraction of calcium ions and magnesium ions of less than 0.05%.

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