Efficient metal palladium recovery method for PCB (Printed Circuit Board) copper melting process

Abstract

The invention discloses an efficient metal palladium recovery method for a PCB (Printed Circuit Board) copper melting process, which comprises the following steps of: obtaining residual liquid, namely adding pretreated activated carbon into a palladium-containing solution to obtain the residual liquid; a fusion step: adding strong base into the residual liquid, stirring, and ensuring that the pH value is greater than 14; a precipitation step; the method comprises the following steps: adding a flocculating agent into a solution rich in strong base to enrich and precipitate all small particles in the solution; a filtering step: filtering the precipitated supernatant liquid, taking out the flocculent precipitate on the lower layer, and drying the flocculent precipitate; and a recovery step: separating the dried palladium-containing solid mixture according to different metal ions to obtain separated palladium. The problems that in an existing PCB manufacturing process, metal palladium is low in recovery efficiency and not thorough, and more residues exist are solved.

Description

Technical Field

[0001] The invention relates to the field of circuit board technology, and in particular to a method for efficiently recovering metal palladium used in a PCB copper plating process. Background Art

[0002] Precious metal recovery in PCB (printed circuit board) process mainly involves extracting precious metals such as gold, silver, palladium, platinum, etc. from waste materials containing precious metals generated during the waste or production process. These metals are widely used in electronic components due to their high conductivity and corrosion resistance.

[0003] In the existing palladium recovery process, activated carbon adsorption is adopted, and then metal palladium is desorbed by other methods, so as to realize the palladium recovery in the activation cylinder in the PCB copperization process. However, the residual liquid after activated carbon adsorption still remains at the ppm level (parts per million). Palladium is a precious metal, and the Pd concentration in the activation cylinder itself is at the ppm level. Therefore, the residual liquid after activated carbon adsorption still has recovery value. However, the current recovery method using activated carbon adsorption is not thorough, which will cause some Pd (palladium) to remain, resulting in the Pd with recovery value being directly discharged as waste liquid, causing a large cost loss.

[0004] In view of this, this technical solution proposes a method for efficiently recovering metal palladium in the PCB copper process, which further enriches the residual Pd by adding strong alkali and sulfur ions, and uses flocculants to enrich and precipitate the particles for preliminary separation, and finally recovers the palladium efficiently and thoroughly. This solution has simple procedures and does not make too many improvements to the existing extraction process. The final extraction effect is high and the recovery degree is high. Summary of the invention

[0005] The technical solution of the present invention aims to solve at least one of the technical problems in the related art to a certain extent. To this end, the main purpose of the present invention is to provide a method for efficiently recovering metal palladium in the copper-plating process of PCB, aiming to solve the problems of low efficiency, incomplete recovery and high residue of metal palladium in the existing PCB manufacturing process.

[0006] To achieve the above object, the present invention provides a method for efficiently recovering metal palladium in a PCB copperization process, comprising the following steps:

[0007] Residue acquisition,

[0008] adding the pretreated activated carbon to the palladium-containing solution to obtain a residual solution;

[0009] The fusion step,

[0010] Add strong alkali into the residual solution and stir, and make sure the pH value is greater than 14;

[0011] precipitation step;

[0012] Add flocculants into a solution rich in strong alkali to enrich and precipitate all small particles in the solution;

[0013] Filtration step,

[0014] The upper clear liquid after precipitation is filtered, and the flocculent precipitate at the lower layer is taken out and dried;

[0015] Recycling steps,

[0016] The dried solid mixture containing palladium is separated according to different metal ions to obtain separated palladium.

[0017] As a further embodiment of the present invention, in the residual liquid obtaining step, the pretreatment of the activated carbon comprises the following steps:

[0018] Cleaning,

[0019] Put the activated carbon into a container, add deionized water, stir to make the activated carbon fully contact with water, wash it several times until the washing liquid is clear, and then filter it to remove the washing liquid;

[0020] Pickling,

[0021] Soak the cleaned activated carbon in dilute acid, stir to allow the acid to fully contact the activated carbon, and soak for 1-2 hours, filter to remove the acid, and rinse with deionized water until the pH value is neutral;

[0022] Alkaline washing,

[0023] Soak the acid-washed activated carbon in a dilute alkali solution, stir to allow the alkali solution to fully contact the activated carbon, and soak for 1-2 hours, filter to remove the alkali solution, and rinse with deionized water until the pH value is neutral;

[0024] dry,

[0025] Place the cleaned activated carbon in an oven and dry it at 105-120°C for 2-4 hours. After cooling to room temperature, seal and store.

[0026] activation,

[0027] Heat the dried activated carbon at 300-500°C, introduce inert gas, and heat for 1-2 hours. After cooling, seal and store;

[0028] Screening tests,

[0029] The pretreated activated carbon was obtained after passing through a standard sieve and testing the specific surface area, porosity and adsorption capacity.

[0030] As a further embodiment of the present invention, in the dissolution step, the strong base added into the residual liquid for stirring is sodium hydroxide and potassium hydroxide.

[0031] As a further solution of the present invention, in the residual liquid obtaining step, activated carbon is added to the palladium-containing solution and then heated at 40-60°C.

[0032] As a further embodiment of the present invention, in the precipitation step, the flocculant added to the solution rich in strong alkali is polyaluminium chloride.

[0033] As a further solution of the present invention, in the recovery step, different metal ions are separated by ion exchange, and the steps include:

[0034] The solids are mixed to form a palladium-containing solution, which is passed through a column filled with an ion exchange resin;

[0035] When the palladium ions are adsorbed by the resin and other metal ions pass through the column, the palladium is eluted from the resin with an eluent and collected.

[0036] As a further solution of the present invention, in the dissolving step, stirring is continued for 10-30 minutes when the strong base is added, and after the pH value is greater than 14, stirring is continued for 20-60 minutes.

[0037] The beneficial effects of the present invention are as follows:

[0038] The present invention proposes a highly efficient method for recovering metal palladium in the PCB copper process. Without changing the existing mainstream method of recovering Pd by activated carbon, the residual liquid after activated carbon recovery can be further enriched by adding strong alkali / sulfide ions, and then the particles can be enriched and precipitated by flocculants for preliminary separation. After separation, a special Pd extraction process is performed to separate Pd from other impurity metal ions. The residual Pd after activated carbon adsorption can be achieved and the Pd residue can be ensured to be far below the ppm level, and can reach 10 ―10 The method adds a flocculation stage on the existing basis to achieve the enrichment of Pd precipitation / suspended particles, and simultaneously reduces the amount of metal impurities in the activation tank, which can reduce the difficulty of treating the activation tank solution and avoid Pd waste to the greatest extent. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] In order to more clearly illustrate the embodiments of the technical solutions of the present invention or the technical solutions of the present invention in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the technical solutions of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the structures shown in these drawings without paying creative work.

[0040] Figure 1 It is a schematic block diagram of the steps of the present invention. DETAILED DESCRIPTION

[0041] as follows:

[0042] Please refer to the attached Figure 1 ,

[0043] The implementation steps mainly include the following:

[0044] Residual liquid acquisition, adding pretreated activated carbon to the palladium-containing solution to obtain residual liquid; dissolution step, adding strong base into the residual liquid and stirring, and ensuring that the pH value is greater than 14; precipitation step: adding flocculant into the solution rich in strong base to enrich and precipitate all small particles in the solution; filtering step, filtering the upper clear liquid after precipitation, and taking out the flocculent precipitate of the lower layer, and drying it; recovery step, separating the dried palladium-containing solid mixture according to different metal ions to obtain separated palladium.

[0045] In PCB manufacturing, the activation cylinder of the electroless palladium plating process may contain tens to hundreds of ppm of palladium ions. By adding activated carbon to the activation cylinder solution, palladium ions are adsorbed onto the activated carbon, and the palladium ion concentration in the residual liquid is reduced to less than 1ppm. The activated carbon adsorbed with palladium can be further processed to recover palladium, and the residual liquid can be safely discharged or further processed. By enriching the palladium in the activation cylinder onto the activated carbon, palladium can be effectively recovered and waste can be reduced. The specific steps include activated carbon pretreatment, adsorption, separation, residual liquid treatment and palladium recovery, among which key parameters such as pH value, temperature and time need to be optimized according to actual conditions to improve adsorption efficiency.

[0046] When the residual liquid is obtained, a strong alkaline substance (such as sodium hydroxide NaOH or potassium hydroxide KOH) is added to the residual liquid, and the solution is evenly mixed by stirring, and the pH value of the solution is adjusted to greater than 14 to create a strong alkaline environment. The purpose of this step is to optimize the recovery efficiency of palladium or meet specific process requirements. In palladium recovery or PCB process, the residual liquid usually contains palladium ions (Pd 2+) and other metal ions (such as copper, nickel, iron, etc.). In order to further separate and recover palladium, it is necessary to adjust the pH value of the solution to a strong alkaline condition (pH>14) to achieve the following goals: promote the dissolution or complexation of palladium ions; make other metal ions form hydroxide precipitation and thus separate from palladium; improve the adsorption efficiency of palladium (such as using activated carbon or ion exchange resin). The specific steps can be, (1) Add strong base, choose strong base: Commonly used strong bases include sodium hydroxide (NaOH) and potassium hydroxide (KOH), which can quickly increase the pH value of the solution. Addition method: Slowly add the strong base solid or concentrated solution to the residual liquid, while stirring to avoid local over-alkalinity. (2) Stirring, purpose: to evenly disperse the strong base in the solution and ensure that the pH value is evenly distributed. Use a magnetic stirrer, mechanical stirrer or manual stirring tool to continue stirring until the solution is completely mixed. (3) Adjust the pH value, goal: adjust the pH value of the solution to greater than 14, that is, strong alkaline conditions. Monitoring method: Use pH test paper or pH meter to monitor the pH value of the solution in real time. Add strong base gradually until the pH reaches the target range.

[0047] The purpose of ensuring that the pH is greater than 14 is to:

[0048] (1) Promote specific chemical reactions, dissolution and separation of palladium: In the palladium (Pd) recovery process, a strong alkaline environment (pH ≥ 14) can promote the dissolution or complexation of palladium ions, making it easier to separate from other metal ions. For example, palladium can form a stable complex with ammonia under strong alkaline conditions (such as [Pd(NH3)4] 2+ ), which is convenient for subsequent separation and recovery. Selective precipitation of metal ions: Some metal ions (such as copper and nickel) will form hydroxide precipitates under strong alkaline conditions, while palladium ions remain in the solution, thus achieving selective separation.

[0049] (2) Improve adsorption efficiency, activated carbon adsorption: Under strong alkaline conditions, the functional groups on the surface of activated carbon (such as hydroxyl and carboxyl) are more likely to adsorb or complex with palladium ions, thereby improving the adsorption efficiency of palladium. Ion exchange resin: Some ion exchange resins have a higher selective adsorption capacity for palladium ions under strong alkaline conditions.

[0050] (3) Inhibit interference from impurities and reduce interference from other metal ions: Under strong alkaline conditions, many metal ions (such as iron, aluminum, and zinc) will form hydroxide precipitates, thereby reducing their interference with the palladium recovery process. Prevent the hydrolysis of palladium: Under strong alkaline conditions, palladium ions are not easily hydrolyzed to form insoluble precipitates, thereby maintaining their stability in the solution.

[0051] (4) Meet process requirements, electroplating process: In some electroplating processes, a strong alkaline environment is necessary to ensure the uniformity and adhesion of the coating. Chemical palladium plating: In the process of chemical palladium plating, strong alkaline conditions can promote the reduction and deposition of palladium ions.

[0052] The role of strong alkaline conditions is to promote the dissolution or complexation of palladium. Under strong alkaline conditions, palladium ions can form stable complexes with ammonia or other complexing agents (such as [Pd(NH3)4] 2+ ), thereby keeping it in solution for subsequent separation and recovery. Precipitate other metal ions. Many metal ions (such as copper, nickel, iron, zinc, etc.) will form hydroxide precipitates (such as Cu(OH)2, Ni(OH)2, Fe(OH)3) under strong alkaline conditions, thereby separating from palladium ions. For example: Cu2++2OH-→Cu(OH)2↓Cu2++2OH-→Cu(OH)2↓Ni2++2OH-→Ni(OH)2↓Ni2++2OH-→Ni(OH)2↓. Improve adsorption efficiency. Under strong alkaline conditions, the adsorption efficiency of activated carbon or ion exchange resin for palladium ions is significantly improved, thereby more effectively enriching palladium. Suppress impurity interference. Strong alkaline conditions can reduce the interference of other metal ions and ensure the selectivity and efficiency of the palladium recovery process.

[0053] During the sedimentation step, flocculants work through two main mechanisms:

[0054] Charge neutralization: suspended particles usually carry negative charges, which repel each other and make it difficult for them to aggregate. The cations in the flocculant can neutralize the negative charges on the surface of the particles, reduce the repulsive force, and make the particles easier to aggregate. Bridging effect: the long-chain molecules of the flocculant can adsorb multiple particles at the same time, forming a "bridge" to connect the particles together to form larger flocs. In this scheme, metal Pd and Ni are both X-group metals and have similar properties. The solubility product of nickel hydroxide is about 2*10 ―15 By adjusting the pH to 14, the Ni ion concentration can reach 2*10 ―13 , which can ensure that the Pd concentration remaining in the solution is minimized, and then all recoverable Pd can be separated by precipitation enrichment, and then refined by dissolution.

[0055] After the filtering step, recovery begins, and the purpose of separating the solid mixture containing palladium after drying according to different metal ions is that palladium coexists with other metal ions (such as copper, nickel, iron, zinc, etc.) at this time. In order to further efficiently recover palladium, it is necessary to separate it from other metal ions. The selection of separation method depends on the type and concentration of metal ions in the solution and the existence form of palladium. For example, chemical precipitation, solvent extraction, ion exchange, electrochemical method, etc. By methods such as chemical precipitation, solvent extraction, ion exchange, electrochemistry or selective adsorption, palladium ions are selectively separated and recovered from a solution containing multiple metal ions. This process is of great significance in PCB technology and precious metal recovery, and can efficiently recover palladium and reduce resource waste.

[0056] A preferred embodiment of the present invention: in the residual liquid acquisition step, the pretreatment of the activated carbon includes the following steps: washing, placing the activated carbon in a container, adding deionized water, stirring to allow the activated carbon to fully contact the water, washing multiple times until the washing liquid is clear, and filtering to remove the washing liquid; acid washing, soaking the washed activated carbon in dilute acid, stirring to allow the acid liquid to fully contact the activated carbon, and soaking for 1-2 hours, filtering to remove the acid liquid, and rinsing with deionized water until the pH value is neutral; alkali washing, soaking the acid-washed activated carbon in a dilute alkali solution After stirring, the alkali solution is fully contacted with the activated carbon, and the activated carbon is soaked for 1-2 hours. The alkali solution is removed after filtering, and the activated carbon is rinsed with deionized water until the pH value is neutral; drying, the cleaned activated carbon is placed in an oven, dried at 105-120°C for 2-4 hours, and sealed and stored after cooling to room temperature; activation, the dried activated carbon is heated at 300-500°C, and an inert gas is introduced for 1-2 hours. After cooling, the activated carbon is sealed and stored; screening and testing, the pretreated activated carbon is obtained after passing through a standard sieve and testing the specific surface area, porosity and adsorption capacity.

[0057] The activated carbon pretreatment steps usually include washing, acid washing, alkaline washing, drying, and optional activation, particle size screening and quality testing. The purpose of these steps is to remove impurities, improve adsorption performance, and ensure that the activated carbon can efficiently adsorb the target substance (such as palladium obtained in this scheme) in subsequent applications. The pretreated activated carbon can be better used to enrich palladium from the activation cylinder and obtain residual liquid that meets environmental requirements.

[0058] A preferred embodiment of the present invention: in the dissolving step, the strong base added into the residual liquid for stirring is sodium hydroxide and potassium hydroxide.

[0059] A preferred embodiment of the present invention: in the residual liquid obtaining step, activated carbon is added to the palladium-containing solution and then heated to 40-60°C.

[0060] After adding activated carbon to the palladium-containing solution, it is heated at 40-60°C, mainly to optimize the adsorption efficiency of activated carbon for palladium ions and accelerate the adsorption process. Heating will increase the activity of palladium ions in the solution, making it easier to adsorb or complex with functional groups (such as hydroxyl and carboxyl) on the surface of activated carbon. Heating can also open the microporous structure of activated carbon and increase its specific surface area, thereby increasing the adsorption capacity. According to kinetic theory, increased temperature will increase the speed of molecular movement, thereby accelerating the adsorption process. Under conditions of 40-60°C, the adsorption reaction rate is significantly increased, and adsorption equilibrium can usually be reached in a shorter time. Palladium ions are more likely to form stable complexes with functional groups on the surface of activated carbon under heating conditions. Other metal ions (such as copper, nickel, iron, etc.) may form hydroxide precipitates under heating conditions, thereby reducing competition for palladium adsorption.

[0061] In the actual implementation process, attention should be paid to temperature control: the heating temperature should be controlled between 40-60℃ to avoid excessive temperature causing damage to the activated carbon structure or evaporation of the solution. Uniform heating: use a constant temperature water bath or heating stirrer to ensure that the solution is heated evenly. Safety protection: during the heating process, care should be taken to prevent the solution from splashing or the equipment from overheating.

[0062] A preferred embodiment of the present invention is that in the precipitation step, the flocculant added into the solution rich in strong alkali is polyaluminium chloride.

[0063] Since polyaluminium chloride has the characteristics of high efficiency, rapid sedimentation, strong adaptability, low dosage and low corrosiveness, it is an ideal material for precipitation enrichment. In this scheme, the enrichment of palladium is mainly achieved through charge neutralization and bridging. Specifically, charge neutralization: the multi-nuclear complex in polyaluminium chloride carries a high positive charge, which can neutralize the negative charge on the surface of suspended particles, reduce the repulsive force between particles, and promote particle aggregation. Bridging effect: the long-chain molecules of polyaluminium chloride can adsorb multiple particles at the same time to form a "bridge" to connect the particles (palladium) together to form larger flocs.

[0064] A preferred embodiment of the present invention is that in the recovery step, different metal ions are separated by ion exchange, and the steps include:

[0065] The solids are mixed to form a palladium-containing solution, which is passed through a column filled with ion exchange resin; when the palladium ions are adsorbed by the resin and other metal ions pass through the column, the palladium is eluted from the resin with an eluent and collected.

[0066] This scheme preferably adopts ion exchange method to collect palladium.

[0067] A preferred embodiment of the present invention is as follows: in the dissolving step, stirring is continued for 10-30 minutes when the strong base is added, and stirring is continued for 20-60 minutes after the pH value is greater than 14.

[0068] The purpose of segmented stirring is to ensure that the strong base is fully mixed with the solution, reacts completely, and optimizes the recovery of palladium (Pd). Specifically, the purpose of continuous stirring for 10-30 minutes when adding the strong base is as follows:

[0069] Ensure that the strong base is evenly dispersed to avoid excessive concentration of the strong base in the solution, which will lead to uneven reaction or local over-alkalinity. Stir to quickly disperse the strong base (such as sodium hydroxide NaOH or potassium hydroxide KOH) throughout the solution. Time: 10-30 minutes of stirring time is sufficient to evenly distribute the strong base.

[0070] Promote rapid pH adjustment and quickly adjust the pH of the solution to the target range (>14). Stirring can accelerate the reaction of strong bases with acidic components in the solution (such as H + ions) reaction, rapidly raising the pH. Time: 10-30 minutes of stirring time is usually sufficient to stabilize the pH at >14.

[0071] Prevent local over-alkali and avoid excessive strong alkali in local areas, which will cause certain metal ions (such as aluminum and zinc) to form insoluble precipitation and interfere with subsequent reactions. Continuous stirring makes the strong alkali evenly dispersed to reduce the risk of local over-alkali.

[0072] The purpose of continuing stirring for 20-60 minutes after the pH value is greater than 14 is to

[0073] Ensure that the reaction is complete and that the palladium ions in the solution react fully with other metal ions to achieve the best separation effect. Under strong alkaline conditions, palladium ions will form stable complexes with ammonia or other complexing agents (such as [Pd(NH3)4] 2+ ), while other metal ions (such as copper, nickel, and iron) will form hydroxide precipitates. Time: 20-60 minutes of stirring time can ensure that the reaction reaches equilibrium.

[0074] Promote the dissolution or complexation of palladium. Under strong alkaline conditions, palladium ions are more easily dissolved or complexed, which is convenient for subsequent recovery. Stirring can accelerate the reaction rate of palladium ions with complexing agents (such as ammonia water). Time: 20-60 minutes of stirring time can ensure that palladium ions are fully dissolved or complexed.

[0075] Accelerate the precipitation of other metal ions, so that other metal ions (such as copper, nickel, iron) fully form hydroxide precipitation, thereby separating from palladium ions. Stirring can speed up the precipitation reaction rate and make the precipitate evenly distributed. Time: 20-60 minutes of stirring time can ensure that the precipitation reaction is complete.

[0076] Improve the adsorption efficiency (such as using the activated carbon involved in this scheme). Under strong alkaline conditions, the adsorption efficiency of activated carbon for palladium ions is significantly improved. Stirring can accelerate the diffusion rate of palladium ions to the surface of activated carbon and improve the adsorption efficiency. Time: 20-60 minutes of stirring time can ensure that the adsorption reaches equilibrium.

[0077] The above description is only a preferred embodiment of the technical solution of the present invention, and does not limit the patent scope of the technical solution of the present invention. All equivalent structural changes made by using the contents of the technical solution description and drawings of the present invention under the conception of the technical solution of the present invention, or directly / indirectly applied in other related technical fields are included in the patent protection scope of the technical solution of the present invention.

Claims

1. A method for efficiently recovering metal palladium in a PCB copper coating process, characterized in that: The following steps are involved: S1, residual liquid acquisition, adding the pretreated activated carbon to the palladium-containing solution to obtain a residual solution; S2, the dissolution step, Add strong alkali into the residual solution and stir, and make sure the pH value is greater than 14; S3, precipitation step; Add flocculants into a solution rich in strong alkali to enrich and precipitate all small particles in the solution; S4, filtering step, The upper clear liquid after precipitation is filtered, and the flocculent precipitate at the lower layer is taken out and dried; S5, recovery step, The dried solid mixture containing palladium is separated according to different metal ions to obtain separated palladium.

2. The method for efficiently recovering metal palladium in a PCB copper coating process according to claim 1, characterized in that: In the residual liquid obtaining step, the pretreatment of the activated carbon includes the following steps: A1. Cleaning, Put the activated carbon into a container, add deionized water, stir to make the activated carbon fully contact with water, wash it several times until the washing liquid is clear, and then filter it to remove the washing liquid; A2, pickling, Soak the cleaned activated carbon in dilute acid, stir to allow the acid to fully contact the activated carbon, and soak for 1-2 hours, filter to remove the acid, and rinse with deionized water until the pH value is neutral; A3, alkali washing, Soak the acid-washed activated carbon in a dilute alkali solution, stir to allow the alkali solution to fully contact the activated carbon, and soak for 1-2 hours, filter to remove the alkali solution, and rinse with deionized water until the pH value is neutral; A4, dry, Place the cleaned activated carbon in an oven and dry it at 105-120°C for 2-4 hours. After cooling to room temperature, seal and store. A5. Activation, Heat the dried activated carbon at 300-500°C, introduce inert gas, and heat for 1-2 hours. After cooling, seal and store; A6. Screening test, The pretreated activated carbon was obtained after passing through a standard sieve and testing the specific surface area, porosity and adsorption capacity.

3. The method for efficiently recovering metal palladium in the PCB copperization process according to claim 1, characterized in that: In the dissolving step, the strong alkali added into the residual liquid and stirred is sodium hydroxide and potassium hydroxide.

4. The method for efficiently recovering metal palladium in a PCB copper coating process according to claim 1, characterized in that: In the residual liquid obtaining step, activated carbon is added to the palladium-containing solution and then heated at 40-60°C.

5. The method for efficiently recovering metal palladium in a PCB copper coating process according to claim 1, characterized in that: In the precipitation step, the flocculant added into the solution rich in strong alkali is polyaluminium chloride.

6. The method for efficiently recovering metal palladium in a PCB copper coating process according to claim 1, characterized in that: In the recovery step, different metal ions are separated by ion exchange, and the steps include: B1, mixing the solids to form a palladium-containing solution, and passing it through a column filled with ion exchange resin; B2. When the palladium ions are adsorbed by the resin and other metal ions pass through the column, the palladium is eluted from the resin with an eluent and collected.

7. The method for efficiently recovering metal palladium in a PCB copper coating process according to claim 1, characterized in that: In the dissolving step, stirring is continued for 10-30 minutes when the strong base is added, and stirring is continued for 20-60 minutes after the pH value is greater than 14.

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