Method for preparing sodium hydroxide on basis of sodium sulfate wastewater generated during lithium battery recovery

Abstract

A method for preparing sodium hydroxide on the basis of sodium sulfate wastewater generated during lithium battery recovery. The method comprises the following steps: adding a calcium oxide powder to sodium sulfate wastewater, and mixing and reacting same, so as to obtain a mixed reaction solution; filtering the mixed reaction solution, so as to obtain a reaction mother solution; subjecting the reaction mother solution to a nanofiltration operation, so as to obtain a sodium hydroxide solution and a concentrated sodium sulfate solution; subjecting the sodium hydroxide solution to a reverse osmosis operation, so as to obtain the sodium hydroxide solution; subjecting the sodium hydroxide solution, which has undergone the reverse osmosis, to an electrodialysis operation, so as to obtain the sodium hydroxide solution; and subjecting the sodium hydroxide solution, which has undergone the electrodialysis, to an evaporation and concentration operation, so as to obtain the sodium hydroxide solution.

Description

Method for preparing sodium hydroxide based on sodium sulfate wastewater generated by lithium battery recovery Technical Field

[0001] The present application relates to a method for preparing sodium hydroxide based on sodium sulfate wastewater generated by lithium battery recovery. Background Art

[0002] As of 2022, my country's production and sales of new energy vehicles have ranked first in the world for eight consecutive years. With the explosive growth of new energy vehicles, a "retirement wave" of power batteries has followed, and the battery recycling and processing industry has developed rapidly.

[0003] However, a large amount of high-COD and high-sodium sulfate wastewater is generated during the lithium battery recycling process, which is currently mainly used to produce anhydrous sodium sulfate as a by-product.

[0004] According to data released by the National Bureau of Statistics, China's proven reserves of thenardite (sodium sulfate decahydrate) reached 119.06 billion tons in 2019, ranking first in the world. Sodium sulfate produced from the mineral thenardite is high in purity and low in cost. Another source of sodium sulfate is as a byproduct in various chemical production processes, such as acid-base neutralization, sodium extraction for vanadium extraction, non-ferrous metal smelting, and lithium carbonate production. Currently, the anhydrous sodium sulfate market faces severe overcapacity. Converting sodium sulfate into bulk chemical raw materials like sodium hydroxide would significantly alleviate the sodium sulfate oversupply problem and achieve high-value utilization, thereby reducing environmental pollution and improving economic efficiency for businesses.

[0005] The current mature sodium hydroxide production process has high energy consumption. The sodium hydroxide produced by existing technologies has high impurity content, low concentration, and poor quality, which does not meet market requirements. The market urgently needs a low-energy sodium hydroxide production process. At the same time, a large amount of sodium sulfate wastewater needs to be treated. Developing a low-energy process for producing high-concentration, high-quality sodium hydroxide using sodium sulfate wastewater as raw material has significant economic significance. If it can be industrialized, it can provide an excellent technical process for the comprehensive utilization of the large amount of high-COD and high-sodium sulfate wastewater generated by the battery recycling industry.

[0006] Sodium hydroxide is prepared using sodium sulfate and calcium hydroxide, and the relevant equation is: Ca(OH)2=Ca 2+ +2(OH) - , Na2SO4+Ca(OH)2==CaSO4↓+2NaOH, CaSO4=Ca 2+ +SO4 2- , NaOH→Na++OH-, this process has always had two major difficulties:

[0007] One is the low reaction conversion rate. Since calcium hydroxide is slightly soluble and calcium sulfate is also slightly soluble, the equilibrium constant expression of the reaction is as follows: The solubility product KSP is a constant, and the OH produced by the ionization of calcium sulfate and sodium hydroxide is - With Ca 2+ All of these will further reduce the solubility of calcium hydroxide, making it difficult for Na2SO4+Ca(OH)2==CaSO4↓+2NaOH to proceed. At room temperature, the solubility product of calcium hydroxide KSP=4.7×10 -6 When the concentration of sodium hydroxide is 1.5% and the molar concentration is 0.375 mol / L, the solubility of calcium hydroxide is calculated to be 2.5×10 -4 At this time, the reaction is difficult to proceed.

[0008] Secondly, the low-concentration sodium hydroxide solution after the reaction contains a large amount of sulfate radicals, calcium ions and suspended calcium hydroxide solids, making it particularly difficult to remove impurities and separate the sodium hydroxide solution. At the same time, the low-concentration sodium hydroxide solution (concentration of 0.5% to 1.5%) is concentrated to a 32% sodium hydroxide solution (high-quality sodium hydroxide solution) using an evaporator, but the concentration cost is too high and the process has no practical value.

[0009] Summary of the Invention

[0010] Based on this, it is necessary to provide a method for preparing sodium hydroxide from sodium sulfate wastewater generated by lithium battery recovery, which can prepare sodium hydroxide solution with low energy consumption.

[0011] A method for preparing sodium hydroxide based on sodium sulfate wastewater generated by lithium battery recovery, characterized in that it comprises the following steps:

[0012] adding calcium oxide powder to sodium sulfate wastewater for a mixed reaction to obtain a mixed reaction liquid;

[0013] filtering the mixed reaction solution to obtain a reaction mother solution;

[0014] performing nanofiltration on the reaction mother liquor to obtain a sodium hydroxide solution and a sodium sulfate concentrate;

[0015] performing a reverse osmosis operation on the sodium hydroxide solution to obtain the sodium hydroxide solution;

[0016] performing electrodialysis on the sodium hydroxide solution after reverse osmosis to obtain the sodium hydroxide solution;

[0017] The sodium hydroxide solution after electrodialysis is subjected to evaporation and concentration operation to obtain the sodium hydroxide solution.

[0018] The details of one or more embodiments of the present application are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the present application will become apparent from the description, drawings, and claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, without paying any creative work, they can also obtain drawings of other embodiments based on these drawings.

[0020] FIG1 is a schematic flow diagram of a method for preparing sodium hydroxide from sodium sulfate wastewater generated by lithium battery recycling in one embodiment;

[0021] FIG2 is a process flow chart of a method for preparing sodium hydroxide from sodium sulfate wastewater generated by lithium battery recycling. DETAILED DESCRIPTION

[0022] To facilitate understanding of the present application, a more comprehensive description of the present application will be provided below with reference to the accompanying drawings. The accompanying drawings illustrate preferred embodiments of the present application. However, the present application can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a more thorough and comprehensive understanding of the disclosure of the present application.

[0023] It should be noted that when an element is referred to as being "fixed to" another element, it may be directly attached to the other element or there may be an intermediate element. When an element is referred to as being "connected to" another element, it may be directly connected to the other element or there may be an intermediate element. The terms "vertical," "horizontal," "left," "right," and similar expressions used herein are for illustrative purposes only and do not represent the only implementation methods.

[0024] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which this application pertains. The terms used herein in the specification of this application are intended only to describe specific embodiments and are not intended to limit this application. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0025] Please refer to FIG1 , which shows a method for preparing sodium hydroxide based on sodium sulfate wastewater generated by lithium battery recycling according to one embodiment, including some or all of the following steps:

[0026] S100, adding calcium oxide powder to sodium sulfate wastewater for a mixed reaction to obtain a mixed reaction liquid.

[0027] In this embodiment, since calcium oxide has a low cost and does not introduce new impurities during the entire reaction process, when calcium oxide powder is added to sodium sulfate wastewater, it first reacts with water to produce calcium hydroxide, and then the calcium hydroxide and sodium sulfate undergo a replacement reaction to obtain the target product, sodium hydroxide.

[0028] S200, filtering the mixed reaction liquid to obtain a reaction mother liquid.

[0029] In the present embodiment, the solid suspended matter, i.e., calcium sulfate dihydrate, in the mixed reaction solution is removed by filtering. Since calcium hydroxide is slightly soluble and the sodium sulfate wastewater is excessive during the entire reaction process, calcium hydroxide continues to react with sodium sulfate when calcium sulfate is removed by filtration, so that the precipitate obtained by filtration is calcium sulfate dihydrate, and thus no solid suspended matter will occur in the reaction mother liquor.

[0030] However, in the conventional process, calcium hydroxide is used as a reactant. Due to the limited solubility of calcium hydroxide, the generated calcium sulfate will wrap around the undissolved calcium hydroxide. Furthermore, the present application slowly adds calcium oxide powder to the sodium sulfate wastewater, so that the calcium oxide is quickly dissolved and reacted in the sodium sulfate wastewater, thereby reducing the situation where the calcium hydroxide is wrapped by calcium sulfate. Moreover, while the calcium sulfate is being filtered, the calcium oxide can quickly dissolve and react to generate calcium hydroxide and react with sodium sulfate, so that the calcium sulfate will not wrap around the calcium hydroxide during filtration, thereby effectively improving the utilization rate of the calcium oxide and allowing the reaction to continue.

[0031] S300, performing nanofiltration on the reaction mother liquor to obtain a sodium hydroxide solution and a sodium sulfate concentrate.

[0032] In this embodiment, due to SO4 2- and Ca 2+ Cannot pass through the nanofiltration membrane, while OH - and Na + It can pass through the nanofiltration membrane and separate sodium sulfate and sodium hydroxide through nanofiltration operation, and it can also remove Ca in the sodium hydroxide solution. 2+ In this way, a low-concentration sodium hydroxide solution (concentration of 0.5% to 1.5%) can be obtained, and the problem of difficulty in removing impurities and separating the sodium hydroxide solution is overcome.

[0033] S400, performing a reverse osmosis operation on the sodium hydroxide solution to obtain the sodium hydroxide solution.

[0034] In this embodiment, the principle of reverse osmosis is that under the action of a pressure higher than the osmotic pressure of the solution, other substances cannot pass through the semipermeable membrane, thereby separating other substances from water. The pore size of the reverse osmosis membrane is very small, which can effectively remove dissolved salts, colloids, microorganisms, organic matter, etc. in the water. Therefore, the reverse osmosis operation also plays a role in removing impurities, thereby purifying the sodium hydroxide solution. At the same time, the reverse osmosis operation can also be used to concentrate a sodium hydroxide solution with a concentration of 0.5% to 1.5% to a sodium hydroxide solution with a concentration of 4% to 6%. Furthermore, the reverse osmosis operation has the advantages of low energy consumption, no pollution, and simple process operation, which can meet the concept of low-energy production of sodium hydroxide.

[0035] It should be noted that reverse osmosis can only concentrate sodium hydroxide solution to below 10%.

[0036] S500, performing electrodialysis operation on the sodium hydroxide solution after reverse osmosis to obtain the sodium hydroxide solution.

[0037] In this embodiment, the principle of electrodialysis is that two solutions are separated by a membrane. When an electric current passes through the membrane and the potential difference between the two solutions is large enough, an electric current is generated. As the current flows, organic matter, metal ions, and other ions in one solution are transferred to the other solution. Therefore, the electrodialysis operation not only concentrates a 4% to 6% sodium hydroxide solution into a 10% to 16% sodium hydroxide solution, but also removes organic matter, metal ions, and other impurities from the sodium hydroxide solution, further purifying the sodium hydroxide solution.

[0038] It should be noted that, since electrodialysis operation can generally only concentrate sodium hydroxide solution to less than 20%, and electrodialysis operation has lower energy consumption than evaporation concentration, it can meet the concept of low-energy consumption preparation of sodium hydroxide.

[0039] S600, performing an evaporation and concentration operation on the sodium hydroxide solution after electrodialysis to obtain the sodium hydroxide solution.

[0040] In this embodiment, if the low-concentration sodium hydroxide solution that has not been concentrated by reverse osmosis and electrodialysis is evaporated and concentrated to a sodium hydroxide solution with a concentration of 32%, the energy consumption is not in line with the concept of low-energy preparation of sodium hydroxide solution. However, the low-energy concentration of sodium hydroxide solution is achieved through the ingenious combination of reverse osmosis, electrodialysis and evaporation concentration, which is in line with the concept of low-energy preparation of sodium hydroxide solution.

[0041] In one embodiment, before adding calcium oxide powder to sodium sulfate wastewater for mixing reaction, the method for preparing sodium hydroxide from sodium sulfate wastewater further comprises the following step: performing electrode oxidation operation on the sodium sulfate wastewater.

[0042] It should be noted that due to the high COD content in sodium sulfate wastewater generated by lithium battery recycling, the electrocatalytic oxidation process using a boron-doped diamond electrode (BDD) as the anode by adding high-COD and high-sodium sulfate wastewater to the electrode oxidation tank can effectively remove organic matter from the sodium sulfate wastewater by utilizing direct oxidation at the anode and indirect oxidation by strongly oxidizing active groups generated during the electrochemical process. This can reduce the COD value of the sodium sulfate wastewater. Furthermore, the electrode oxidation operation does not introduce new impurities, making it clean and efficient.

[0043] In one embodiment, the oxidation current density of the electrode oxidation operation is 20 mA / cm 2 ~90mA / cm 2 It is understood that by controlling the oxidation current density of the electrode oxidation at 20 mA / cm 2 ~90mA / cm 2 In this way, the organic matter in the sodium sulfate wastewater can be effectively removed, thereby reducing the COD value of the sodium sulfate wastewater.

[0044] In one embodiment, the oxidation time of the electrode oxidation operation is 0.5 h to 3 h. It is understood that by controlling the oxidation time of the electrode oxidation to be between 0.5 h and 3 h, it is possible to ensure that the organic matter in the sodium sulfate wastewater is fully oxidized, thereby greatly reducing the COD value of the sodium sulfate wastewater.

[0045] In one embodiment, after the sodium sulfate wastewater is subjected to electrode oxidation and before calcium oxide powder is added to the sodium sulfate wastewater for mixing reaction, the method for preparing sodium hydroxide from sodium sulfate wastewater further comprises the following step: pre-cooling the oxidized sodium sulfate wastewater.

[0046] It should be noted that although calcium oxide is relatively low in cost and does not introduce new impurities in this reaction process, it releases heat when dissolved in water, causing the system temperature to rise continuously. Once the wastewater temperature is too high, it will damage the membrane modules in the subsequent process, resulting in poor concentration results. Therefore, in this embodiment, the oxidized sodium sulfate wastewater is pre-cooled by a cooling system. When the pre-cooled sodium sulfate wastewater is mixed with calcium oxide powder, the heat generated when the calcium oxide dissolves in water can be neutralized. This can prevent the damage of the membrane modules in the subsequent process due to excessive temperature, thereby ensuring the normal operation of the process.

[0047] In one embodiment, the pre-cooling temperature of the pre-cooling operation is 20°C to 25°C. It is understandable that since calcium oxide releases heat when dissolved in water, the system temperature continues to rise. Once the wastewater temperature exceeds 45°C, the membrane group of the subsequent process will be damaged, resulting in poor concentration effect. Therefore, in this embodiment, by pre-cooling the oxidized sodium sulfate wastewater, the temperature of the sodium sulfate wastewater is between 20°C and 25°C, which can neutralize the heat released by the wastewater after the addition of calcium oxide powder, so that the temperature of the wastewater can be controlled between 35°C and 40°C. The wastewater temperature between 35°C and 40°C will not cause damage to the membrane group of the subsequent process, thereby ensuring the normal progress of the subsequent process.

[0048] In one embodiment, the specific steps of adding calcium oxide powder to sodium sulfate wastewater for mixing reaction are:

[0049] First, the pre-cooled sodium sulfate wastewater is added to the reactor;

[0050] Then, calcium oxide powder is added into the reaction kettle and stirred to obtain the mixed reaction liquid.

[0051] It should be noted that the pre-cooled sodium sulfate wastewater is added to the reactor, and then the calcium oxide powder is slowly added into the reactor to mix and react with the sodium sulfate wastewater. Since the sodium sulfate wastewater is pre-cooled, the heat released when the calcium oxide powder is dissolved in water can be neutralized. At the same time, the stirring operation can enable the sodium sulfate and the calcium hydroxide produced after the calcium oxide powder is dissolved in water to fully react, thereby effectively improving the reaction conversion rate of sodium sulfate.

[0052] In one embodiment, the stirring operation is performed at a stirring speed of 30 rpm to 60 rpm. It is understood that stirring the mixed reaction liquid by a stirring paddle and controlling the stirring speed of the stirring paddle between 30 rpm and 60 rpm can not only accelerate the dissolution of the calcium oxide powder, but also accelerate the reaction of sodium sulfate and calcium hydroxide, thereby effectively improving the reaction conversion rate of sodium sulfate.

[0053] In one embodiment, the reaction time of the mixing reaction is 20 min to 90 min. It is understandable that due to the - and Ca 2+ It will affect the progress of the replacement reaction. Therefore, by controlling the single mixing reaction time between 20 minutes and 90 minutes, it is possible to ensure the full reaction of the single mixing reaction, so that sodium sulfate can be converted into sodium hydroxide to a greater extent.

[0054] In one embodiment, the mass concentration of the sodium sulfate wastewater is 6% to 10%. It can be understood that the mass concentration of the sodium sulfate wastewater is between 6% and 10%, that is, by controlling the excess of sodium sulfate, calcium hydroxide can fully react, and at the same time, SO4 in the excess sodium sulfate 2- It will inhibit the ionization of calcium sulfate, causing calcium sulfate to ionize in water. 2+ The amount of sodium sulfate in water decreases, thereby reducing the impact on the solubility of calcium hydroxide, thereby effectively improving the conversion rate of calcium hydroxide and further improving the reaction conversion rate of sodium sulfate.

[0055] In one embodiment, the specific steps of filtering the mixed reaction liquid are:

[0056] First, the mixed reaction liquid is subjected to a sedimentation and filtration operation in an inclined plate sedimentation tank to obtain a precipitate and a supernatant;

[0057] Then, the precipitate is filtered through a filter press to obtain calcium sulfate dihydrate;

[0058] Finally, the supernatant is subjected to a slag removal filtration operation through a candle filter to obtain the reaction mother liquor.

[0059] It should be noted that the mixed reaction liquid is separated into a precipitate and a supernatant by passing through an inclined plate sedimentation tank. The precipitate is further filtered through a filter press to obtain calcium sulfate dihydrate, and the supernatant is further deslagging through a candle filter to obtain a reaction mother liquor. In this way, fixed suspended matter can be removed, overcoming the problem that solid suspended matter in the sodium hydroxide solution is difficult to remove. At the same time, calcium sulfate dihydrate by-product can be generated, thereby effectively improving the utilization rate of sodium sulfate wastewater.

[0060] In one embodiment, the specific operation steps of performing nanofiltration operation on the reaction mother liquor are as follows: the reaction mother liquor is transported to the nanofiltration membrane group by a high-pressure pump for nanofiltration operation to obtain the sodium hydroxide solution and the sodium sulfate concentrate with a concentration of 0.5% to 1.5%. It should be noted that there are negatively charged groups in the nanofiltration membrane, and the charged groups form electrostatic interactions with the electrolyte ions. Through the electrostatic interaction, they hinder the penetration of multivalent ions and allow monovalent ions to pass through, thereby achieving low concentrations of NaOH and SO4 through nanofiltration operation. 2- , Ca 2+ The separation of Na2SO4 and the concentration of Na2SO4 were achieved at the same time. And the OH in the wastewater was removed by nanofiltration membrane separation. - So that the remaining Na2SO4 (sodium sulfate concentrate) in the wastewater can be fed back into the reactor to undergo the Na2SO4+Ca(OH)2=CaSO4↓+2NaOH reaction again, thereby completely converting the sodium sulfate into sodium hydroxide.

[0061] In one embodiment, the number of nanofiltration membrane groups is at least one. It is understood that at least one nanofiltration membrane group can ensure the normal operation of the nanofiltration operation. Depending on the condition of the reaction mother liquor, at least one or more nanofiltration membrane groups are selected for concentration. Multiple nanofiltration membrane groups can ensure a good concentration effect while also achieving a good purification effect. That is, the multivalent ions in the reaction mother liquor face multiple layers of obstacles, thereby further purifying the sodium hydroxide solution.

[0062] In one embodiment, each nanofiltration membrane group is provided with at least one nanofiltration membrane layer. It is understood that since each nanofiltration membrane group is provided with one or n nanofiltration membrane layers, at least one or more nanofiltration membrane layers are selected for concentration according to the conditions of the reaction mother liquor. The multi-layer nanofiltration membrane combination can improve the ion barrier performance of a single nanofiltration membrane group, so that the single nanofiltration membrane group has better concentration and purification effects, thereby further purifying the sodium hydroxide solution.

[0063] In one embodiment, the specific operating steps of performing reverse osmosis operation on the sodium hydroxide solution are: transporting the sodium hydroxide solution with a concentration of 0.5% to 1.5% to the reverse osmosis membrane group through the high-pressure pump for nanofiltration operation to obtain the sodium hydroxide solution with a concentration of 4% to 6%.

[0064] It should be noted that although the reverse osmosis membrane group can only concentrate the sodium hydroxide solution to less than 10%, compared with the energy consumption of concentrating and evaporating low-concentration sodium hydroxide solution to 4% to 6%, reverse osmosis operation has significant advantages. In addition, the principle of reverse osmosis is to separate substances from water under the action of a pressure higher than the solution's osmotic pressure, based on the fact that other substances cannot pass through the reverse osmosis membrane group. The reverse osmosis membrane has a very small pore size and can effectively remove dissolved salts, colloids, microorganisms, organic matter, etc. from the water. In addition, by controlling the pore size of the reverse osmosis membrane, the permeability of sodium hydroxide can be adjusted, thus achieving the concentration of the sodium hydroxide solution and achieving a good impurity removal effect, so that the sodium hydroxide solution can be further purified.

[0065] Furthermore, by controlling the membrane pore size of the reverse osmosis membrane group to gradually decrease and all of them can pass through sodium hydroxide, impurities are continuously removed from the sodium hydroxide solution when passing through the multi-layer reverse osmosis membrane, and water in the sodium hydroxide solution can also be removed, so that the sodium hydroxide solution is effectively concentrated and further purified.

[0066] In one embodiment, the number of reverse osmosis membrane groups is at least one. It is understood that at least one reverse osmosis membrane group can ensure the normal operation of the reverse osmosis operation. According to the impurity content of the sodium hydroxide solution, at least one or more reverse osmosis membrane groups are selected for concentration. Multiple reverse osmosis membrane groups can ensure a good concentration effect while playing a good impurity removal and purification role. That is, the impurities in the sodium hydroxide solution face multiple obstacles, thereby further purifying the sodium hydroxide solution.

[0067] In one embodiment, each reverse osmosis membrane assembly includes at least one reverse osmosis membrane layer. It is understood that since each reverse osmosis membrane assembly includes one or n reverse osmosis membrane layers, at least one or more reverse osmosis membrane layers are selected for concentration based on the impurity content of the sodium hydroxide solution. The multi-layered reverse osmosis membrane assembly can improve the barrier performance of a single reverse osmosis membrane assembly, enabling the single reverse osmosis membrane assembly to achieve better concentration and impurity removal and purification effects, thereby further purifying the sodium hydroxide solution.

[0068] In one embodiment, the specific operation steps of performing an electrodialysis operation on the sodium hydroxide solution after reverse osmosis are as follows: performing an electrodialysis operation on the sodium hydroxide solution with a concentration of 4% to 6% through an electrodialysis system to obtain the sodium hydroxide solution with a concentration of 10% to 16%. It should be noted that since electrodialysis can concentrate the sodium hydroxide solution to below 20%, the electrodialysis system can not only concentrate the sodium hydroxide solution, but also remove organic matter and other impurity ions in the sodium hydroxide solution through permeation, thereby further purifying the sodium hydroxide.

[0069] In one embodiment, the operating pressure of the electrodialysis system is 0.5 kg / cm 2 ~3.0kg / cm 2 It is understood that by controlling the operating pressure of the electrodialysis system at 0.5 kg / cm 2 ~3.0kg / cm 2 In this way, the sodium hydroxide solution can be better concentrated and further purified.

[0070] Furthermore, the operating voltage of the electrodialysis system is 100 V to 250 V, and the operating current of the electrodialysis system is 1 A to 3 A. It is understood that by controlling the operating voltage of the electrodialysis system to be between 100 V and 250 V and the operating current to be between 1 A and 3 A, the sodium hydroxide solution can be better concentrated and further purified.

[0071] In one embodiment, the specific operation steps of evaporating and concentrating the sodium hydroxide solution after electrodialysis are: evaporating and concentrating the sodium hydroxide solution with a concentration of 10% to 16% by an MVR evaporator to obtain the sodium hydroxide solution with a concentration of 32%.

[0072] It should be noted that, since neither reverse osmosis nor electrodialysis can concentrate the sodium hydroxide solution to 32%, the energy consumption of concentrating and evaporating a sodium hydroxide solution with a concentration of 10% to 16% through an MVR evaporator is significantly lower than the energy consumption of concentrating and evaporating a low-concentration sodium hydroxide solution through an MVR evaporator. That is, the combination of reverse osmosis, electrodialysis and MVR evaporator achieves low-energy preparation of high-quality sodium hydroxide solution, and can also effectively treat sodium sulfate wastewater, which is in line with the environmental protection concept of green recycling.

[0073] Furthermore, the first-effect evaporation of the MVR evaporator adopts a falling film evaporator, which can achieve low-rate evaporation and concentration, and the second-effect evaporation adopts a forced reverse circulation evaporator, which can achieve high-rate evaporation and concentration. The combination of the first-effect evaporation and the second-effect evaporation can not only save energy consumption, but also meet the production requirements of different working conditions and different concentration ratios, so that the versatility of the MVR evaporator is effectively improved, thereby enabling the preparation method of the present application to adapt to the production requirements of different working conditions and different concentration ratios.

[0074] To better illustrate that the method for preparing sodium hydroxide of the present application has a lower concentration energy consumption cost, the method for preparing sodium hydroxide of the present application and the traditional method for preparing sodium hydroxide are described below using a specific case:

[0075] For the traditional preparation method of sodium hydroxide, 1% sodium hydroxide solution is directly concentrated to 32% using an MVR evaporator. At this time, the comprehensive cost of one ton of water is 130 yuan. If 32 tons of 1% sodium hydroxide solution is evaporated and concentrated into 1 ton of 32% sodium hydroxide solution, the entire concentration process consumes 31 tons of water, and the evaporation cost is 4030 yuan.

[0076] For the preparation method of sodium hydroxide of the present application, 32 tons of 1% sodium hydroxide solution are converted into 6 tons of 5.3% sodium hydroxide by reverse osmosis, reducing 26 tons of water and processing costs of 52 yuan. 6 tons of water are concentrated to 2 tons of 16% sodium hydroxide solution by electrodialysis, reducing 4 tons of water and processing costs of 120 yuan. Then, an MVR evaporator is used for evaporation, and 2 tons of 16% sodium hydroxide solution are converted into 1 ton of 32% sodium hydroxide finished product, reducing one ton of water and processing costs of 130 yuan. Therefore, the concentration cost of preparing 1 ton of 32% sodium hydroxide solution from 32 tons of 1% sodium hydroxide solution using the preparation method of the present application is 302 yuan, while the traditional method for preparing sodium hydroxide directly requires 4030 yuan for concentration and evaporation through an MVR evaporator. It can be seen that the concentration cost of the preparation method of sodium hydroxide of the present application is reduced by 3728 yuan, so that sodium hydroxide solution can be prepared with low energy consumption.

[0077] Examples are listed below, but it should be noted that the following examples do not exhaust all possible situations, and the materials used in the following examples can be obtained from commercial channels unless otherwise specified.

[0078] Example 1

[0079] 1. Pump 32 tons of 8% sodium sulfate wastewater into the electrode oxidation tank, and use the boron-doped diamond electrode (BDD) as the anode for electrocatalytic oxidation. The oxidation current density is 100mA / cm 2 , the electrochemical oxidation time is 1 hour.

[0080] 2. Pre-cool the sodium sulfate wastewater after electrode oxidation to 22°C and then add it to the reactor. Turn on the reactor agitator, then add 280 kg of calcium oxide powder into 32 tons of 8% sodium sulfate wastewater at a flow rate of 3 kg per minute. Calcium oxide releases heat when it comes into contact with water, and the reaction proceeds at 38°C for 40 minutes.

[0081] 3. The liquid after the reaction is pumped into the inclined plate sedimentation tank through a high-pressure pump. After passing through the inclined plate sedimentation tank, the sedimentation tank residue is filtered through a filter press to obtain calcium sulfate dihydrate. The clear liquid is pumped into a candle filter through a high-pressure pump to filter and remove the residue.

[0082] 4. After deslagging, the reaction mother liquor is sent to 3 groups of nanofiltration membrane groups (each group of nanofiltration membrane groups contains 5 layers of nanofiltration membranes) through a high-pressure pump to remove sodium sulfate and calcium ions. The sodium sulfate concentrate is reused, and the obtained 1% sodium hydroxide is sent to 3 groups of reverse osmosis membrane groups (each group of reverse osmosis membrane groups contains 6 layers of reverse osmosis membranes) through a high-pressure pump to be concentrated to 5%.

[0083] 5. Then 7 tons of 5% concentrated liquid is sent to electrodialysis through the feeding pump for further concentration to 15%. The operating pressure of the electrodialysis system is 3.5 kg / cm 2 , operating voltage 300V, current 3.8A.

[0084] 6. Then, 2.5 tons of 15% concentrate was concentrated to 32% by a two-effect MVR evaporation device combining falling film evaporation and forced circulation to obtain 1.2 tons of 32% high-quality sodium hydroxide. The test data of 32% sodium hydroxide are shown in Table 1.

[0085] Table 1

[0086] Example 2

[0087] 1. Pump 32 tons of 7% sodium sulfate wastewater into the electrode oxidation tank, and use the boron-doped diamond electrode (BDD) as the anode for electrocatalytic oxidation. The oxidation current density is 90mA / cm 2 , the electrochemical oxidation time is 1.5 hours.

[0088] 2. Pre-cool the sodium sulfate wastewater after electrode oxidation to 25°C and then add it to the reactor. Turn on the reactor agitator, then add 240 kg of calcium oxide powder into 32 tons of 7% sodium sulfate wastewater at a flow rate of 3 kg per minute. Calcium oxide releases heat when it comes into contact with water, and the reaction proceeds at 40°C for 30 minutes.

[0089] 3. The liquid after the reaction is pumped into the inclined plate sedimentation tank through a high-pressure pump. After passing through the inclined plate sedimentation tank, the sedimentation tank residue is filtered through a filter press to obtain calcium sulfate dihydrate. The clear liquid is pumped into a candle filter through a high-pressure pump to filter and remove the residue.

[0090] 4. After deslagging, the reaction mother liquor is sent to two groups of nanofiltration membrane groups (each group of nanofiltration membrane groups contains three layers of nanofiltration membranes) through a high-pressure pump to remove sodium sulfate and calcium ions. The sodium sulfate concentrate is reused, and the obtained 1% sodium hydroxide is sent to two groups of reverse osmosis membrane groups (each group of reverse osmosis membrane groups contains five layers of reverse osmosis membranes) through a high-pressure pump to be concentrated to 5%.

[0091] 5. Then 6.4 tons of 5% concentrated liquid is sent to electrodialysis through a feed pump for further concentration to 15%. The operating pressure of the electrodialysis system is 3.0 kg / cm 2 , operating voltage 250V, current 3A.

[0092] 6. 2.13 tons of 15% concentrate was concentrated to 32% by a two-effect MVR evaporation device combining falling film evaporation and forced circulation to obtain 1 ton of 32% high-quality sodium hydroxide. The test data of 32% sodium hydroxide are shown in Table 2.

[0093] Table 2

[0094] Comparative Example 1

[0095] 1. Pump 32 tons of 8% sodium sulfate wastewater into the electrode oxidation tank, and use the boron-doped diamond electrode (BDD) as the anode for electrocatalytic oxidation. The oxidation current density is 100mA / cm 2 , the electrochemical oxidation time is 1 hour.

[0096] 2. Add the sodium sulfate wastewater after electrode oxidation into the reactor, turn on the reactor agitator, and then add 280 kg of calcium oxide powder into 32 tons of 8% sodium sulfate wastewater at a flow rate of 3 kg per minute. Calcium oxide releases heat when it comes into contact with water, and the reaction proceeds at a temperature of 45°C for 40 minutes.

[0097] 3. The liquid after the reaction is pumped into the inclined plate sedimentation tank through a high-pressure pump. After passing through the inclined plate sedimentation tank, the sedimentation tank residue is filtered through a filter press to obtain calcium sulfate dihydrate. The clear liquid is pumped into a candle filter through a high-pressure pump to filter and remove the residue.

[0098] 4. After deslagging, the reaction mother liquor is sent to 3 groups of nanofiltration membrane groups (each group of nanofiltration membrane groups contains 5 layers of nanofiltration membranes) through a high-pressure pump to remove sodium sulfate and calcium ions. The sodium sulfate concentrate is reused, and the obtained 1% sodium hydroxide is sent to 3 groups of reverse osmosis membrane groups (each group of reverse osmosis membrane groups contains 6 layers of reverse osmosis membranes) through a high-pressure pump to be concentrated to 5%.

[0099] 5. Then 7 tons of 5% concentrated liquid is sent to electrodialysis through the feeding pump for further concentration to 15%. The operating pressure of the electrodialysis system is 3.5 kg / cm 2 , operating voltage 300V, current 3.8A.

[0100] 6. 2.5 tons of 15% concentrate was concentrated to 32% by a two-effect MVR evaporation device combining falling film evaporation and forced circulation to obtain 1.2 tons of 32% high-quality sodium hydroxide. The test data of 32% sodium hydroxide are shown in Table 3.

[0101] Table 3

[0102] It can be seen from Table 1 and Table 2 that high-quality sodium hydroxide solution can be prepared by the sodium hydroxide preparation method of the present application.

[0103] As can be seen from Table 1 and Table 3, by comparing Example 1 and Comparative Example 1, the difference between Example 1 and Comparative Example 1 lies in the pre-cooling operation. By comparing the data of Example 1 and Comparative Example 1, it is found that the impurity data of Comparative Example 1 are all higher than those of Example 1. It can be seen that the separation effect in the membrane separation process is poor. Since the temperature of the wastewater exceeds 40°C, the separation effect of the reaction mother liquor when passing through the reverse osmosis membrane group and the electrodialysis membrane group is poor, which in turn causes the sodium hydroxide solution after evaporation and concentration to fail to meet the high quality requirements. This also shows that the temperature of the wastewater exceeding 40°C will damage the reverse osmosis membrane group and the electrodialysis membrane group, resulting in poor separation and concentration effects of the reverse osmosis operation and the electrodialysis operation. Therefore, the sodium sulfate wastewater needs to be pre-cooled before mixing with the calcium oxide powder, so that a high-quality sodium hydroxide solution can be prepared.

[0104] Compared with the prior art, this application has at least the following advantages:

[0105] 1. By adding calcium oxide powder and mixing with sodium sulfate wastewater, calcium oxide will form Ca(OH)2 after dissolving in water. - It will affect the reaction, so the solid suspended matter (calcium sulfate dihydrate) in the mixed reaction liquid can be removed by filtration, and then the low concentration of NaOH and SO4 can be achieved by nanofiltration. 2- , Ca 2+ At the same time, the OH in the mixed reaction liquid - In the reaction mother liquor, the sodium sulfate concentrate can be recycled as sodium sulfate wastewater, which can overcome the difficulty of the reaction being unable to proceed continuously and effectively improve the reaction conversion rate, thereby completely converting the sodium sulfate into sodium hydroxide.

[0106] 2. Since the energy consumption of the reverse osmosis operation and the electrodialysis operation is lower than that of the evaporation concentration operation, the sodium hydroxide solution is concentrated in sequence through the reverse osmosis operation, the electrodialysis operation and the evaporation concentration operation to obtain a sodium hydroxide solution with a concentration of 32%. In this way, the energy consumption of concentrating the sodium hydroxide solution can be reduced, thereby realizing low-energy-consumption concentrated preparation of the sodium hydroxide solution.

[0107] The technical features of the above-mentioned embodiments can be combined arbitrarily. In order to make the description concise, not all possible combinations of the technical features in the above-mentioned embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0108] The above-described embodiments merely represent several implementation methods of the present application. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that a person skilled in the art could make various modifications and improvements without departing from the spirit of the present application, all of which fall within the scope of protection of the present application. Therefore, the scope of protection of the present patent application shall be determined by the appended claims.

Claims

1. A method for preparing sodium hydroxide from sodium sulfate wastewater generated by lithium battery recovery, characterized in that: The following steps are involved: Adding calcium oxide powder to sodium sulfate wastewater for mixed reaction to obtain a mixed reaction liquid; filtering the mixed reaction solution to obtain a reaction mother solution; Performing nanofiltration on the reaction mother liquor to obtain a sodium hydroxide solution and a sodium sulfate concentrate; Performing a reverse osmosis operation on the sodium hydroxide solution to obtain the sodium hydroxide solution; Performing electrodialysis on the sodium hydroxide solution after reverse osmosis to obtain the sodium hydroxide solution; The sodium hydroxide solution after electrodialysis is subjected to evaporation and concentration operation to obtain the sodium hydroxide solution.

2. The method for preparing sodium hydroxide from sodium sulfate wastewater generated by lithium battery recovery according to claim 1, characterized in that: Before adding calcium oxide powder to sodium sulfate wastewater for mixed reaction, the following steps are also included: The sodium sulfate wastewater is subjected to electrode oxidation operation.

3. The method for preparing sodium hydroxide from sodium sulfate wastewater generated by lithium battery recovery according to claim 2, characterized in that: The oxidation current density of the electrode oxidation operation is 20 mA / cm 2 ~90mA / cm 2 .

4. The method for preparing sodium hydroxide from sodium sulfate wastewater generated by lithium battery recovery according to claim 2, characterized in that: The oxidation time of the electrode oxidation is 0.5h to 3h.

5. The method for preparing sodium hydroxide from sodium sulfate wastewater generated by lithium battery recovery according to claim 2, characterized in that: After the sodium sulfate wastewater is subjected to electrode oxidation operation and before calcium oxide powder is added to the sodium sulfate wastewater for mixed reaction, the following steps are also included: The oxidized sodium sulfate wastewater is precooled.

6. The method for preparing sodium hydroxide from sodium sulfate wastewater generated by lithium battery recovery according to claim 5, characterized in that: The precooling temperature of the precooling operation is 20°C to 25°C.

7. The method for preparing sodium hydroxide from sodium sulfate wastewater generated by lithium battery recovery according to claim 6, characterized in that: The specific steps of adding calcium oxide powder to sodium sulfate wastewater for mixed reaction are: Adding the pre-cooled sodium sulfate wastewater into the reactor; Calcium oxide powder is added into the reaction kettle and stirred to obtain the mixed reaction liquid.

8. The method for preparing sodium hydroxide from sodium sulfate wastewater generated by lithium battery recovery according to claim 7, characterized in that: The stirring speed of the stirring operation is 30 rpm to 60 rpm.

9. The method for preparing sodium hydroxide from sodium sulfate wastewater generated by lithium battery recovery according to claim 1, characterized in that: The reaction time of the mixing reaction is 20 min to 90 min.

10. The method for preparing sodium hydroxide from sodium sulfate wastewater generated by lithium battery recovery according to claim 1, characterized in that: The mass concentration of the sodium sulfate wastewater is 6% to 10%.

11. The method for preparing sodium hydroxide from sodium sulfate wastewater generated by lithium battery recovery according to claim 1, characterized in that: The specific operation steps of filtering the mixed reaction liquid are as follows: Performing a sedimentation and filtration operation on the mixed reaction liquid through an inclined plate sedimentation tank to obtain a precipitate and a supernatant; Performing a filter press operation on the precipitate through a filter press to obtain calcium sulfate dihydrate; The supernatant is subjected to a slag removal and filtration operation through a candle filter to obtain the reaction mother solution.

12. The method for preparing sodium hydroxide from sodium sulfate wastewater generated by lithium battery recovery according to claim 1, characterized in that: The specific operation steps of performing nanofiltration operation on the reaction mother liquor are: The reaction mother liquor is transported to a nanofiltration membrane group by a high-pressure pump for nanofiltration operation to obtain the sodium hydroxide solution and the sodium sulfate concentrate with a concentration of 0.5% to 1.5%.

13. The method for preparing sodium hydroxide from sodium sulfate wastewater generated by lithium battery recovery according to claim 12, characterized in that: The number of the nanofiltration membrane groups is at least one group.

14. The method for preparing sodium hydroxide from sodium sulfate wastewater generated by lithium battery recovery according to claim 13, characterized in that: Each of the nanofiltration membrane groups is provided with at least one layer of nanofiltration membrane.

15. The method for preparing sodium hydroxide from sodium sulfate wastewater generated by lithium battery recovery according to claim 12, characterized in that: The specific operation steps of performing reverse osmosis operation on the sodium hydroxide solution are: The sodium hydroxide solution with a concentration of 0.5% to 1.5% is transported to the reverse osmosis membrane group by the high-pressure pump for nanofiltration operation to obtain the sodium hydroxide solution with a concentration of 4% to 6%.

16. The method for preparing sodium hydroxide from sodium sulfate wastewater generated by lithium battery recovery according to claim 15, characterized in that: The number of the reverse osmosis membrane groups is at least one.

17. The method for preparing sodium hydroxide from sodium sulfate wastewater generated by lithium battery recovery according to claim 16, characterized in that: Each of the reverse osmosis membrane groups is provided with at least one layer of reverse osmosis membrane.

18. The method for preparing sodium hydroxide from sodium sulfate wastewater generated by lithium battery recovery according to claim 15, characterized in that: The specific operation steps of performing electrodialysis operation on the sodium hydroxide solution after reverse osmosis are: The sodium hydroxide solution with a concentration of 4% to 6% is subjected to electrodialysis operation through an electrodialysis system to obtain the sodium hydroxide solution with a concentration of 10% to 16%.

19. The method for preparing sodium hydroxide from sodium sulfate wastewater generated by lithium battery recovery according to claim 18, characterized in that: The operating pressure of the electrodialysis system is 0.5 kg / cm 2 ~3.0kg / cm 2 .

20. The method for preparing sodium hydroxide from sodium sulfate wastewater generated by lithium battery recovery according to claim 18, characterized in that: The specific operation steps of evaporating and concentrating the sodium hydroxide solution after electrodialysis are as follows: The sodium hydroxide solution with a concentration of 10% to 16% is evaporated and concentrated by an MVR evaporator to obtain the sodium hydroxide solution with a concentration of 32%.

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