A method for grading recovery of gold, silver and palladium from PCB etching waste liquid

Abstract

The application discloses a kind of gold, silver, palladium fractional recovery method in PCB etching waste liquid, belong to hazardous waste liquid resource utilization technical field.The method is first to the pretreatment and copper removal of PCB etching waste liquid, obtain gold, silver, palladium containing liquid to be handled;In the second batch and subsequent batch silver deposition processing, take the silver-containing solid obtained in the previous batch silver deposition as circulating seed, after pre-activation by acid solution containing chloride ion, add to the liquid to be handled, and add chlorinated liquid in batches, so that silver is deposited in the form of silver chloride on the circulating seed, obtain silver-containing solid and the liquid phase after separating silver-containing solid;Then separate palladium from the liquid phase, and reduce gold deposition in gold-containing liquid.The method can reduce the generation of fine silver chloride particles, reduce the entrainment of palladium and salts in silver-containing solid, improve the purity of silver products and the stability of gold, silver and palladium comprehensive recovery.

Description

Technical Field

[0001] This invention belongs to the field of hazardous waste liquid resource utilization technology, and more specifically, relates to a graded recovery method for gold, silver and palladium in PCB etching waste liquid. Background Technology

[0002] The PCB manufacturing, etching, and related wastewater resource recovery processes generate copper- and chloride-containing wastewater. This type of wastewater typically contains high concentrations of copper ions, chloride ions, and soluble salts such as sodium salts, and may also contain small amounts of precious metals such as gold, silver, and palladium. Current treatment methods primarily aim to recover copper resources, reduce wastewater volume, or achieve compliance with treatment standards. For example, copper components are recovered or pollutant content is reduced through electrolysis, precipitation, extraction, or subsequent wastewater treatment. Compared to copper ions, gold, silver, and palladium are present in lower concentrations in the wastewater and are more susceptible to interference from high salt, high chloride, and other metal ions. Therefore, achieving stable, high-purity, low-cost graded recovery is often difficult in practical applications.

[0003] In the field of wet separation of precious metals, silver can typically be preferentially separated from silver-containing liquids by utilizing its property of forming insoluble silver chloride with chloride ions. Palladium and gold can then be further recovered from the liquid phase after silver separation. This type of method is relatively straightforward and applicable to various silver-containing systems. However, in liquid phases containing palladium, gold, and high salt content, simply relying on the addition of chlorides to form silver chloride precipitate is easily affected by palladium-chloride complexes, soluble salts, and the morphology of the precipitate particles, leading to unstable silver-palladium separation results.

[0004] For example, Chinese patent publication CN103555944A discloses a method for removing silver impurities from palladium-containing materials. This method involves dissolving the palladium-containing material and adding a saturated sodium chloride solution to precipitate silver as silver chloride. The silver chloride filter cake is then washed with hydrochloric acid, and the washings and filtrate are combined for further processing. While this method effectively removes silver through silver chloride precipitation, its main approach is still to add chlorides to precipitate silver, followed by subsequent washing and post-treatment to reduce the impact of impurities in the filter cake. For complex wastewater systems with high chlorine, high salt content, and trace amounts of precious metals, the silver chloride solid obtained from direct silver precipitation may still contain mother liquor, salts, or palladium components, resulting in a significant washing burden. Therefore, there is still room for improvement in the stability of silver-palladium separation.

[0005] For example, Chinese patent publication CN114959290B discloses a method for selectively leaching and stepwise recovering precious metals gold, silver, and palladium from electronic waste. This method targets electronic waste powder for selective leaching and subsequent stepwise recovery. The document also mentions that when using sodium chloride for silver precipitation, palladium may co-precipitate with silver chloride or be adsorbed by it, making silver-palladium separation difficult. This prior art demonstrates that the problem of palladium co-precipitation or adsorption during silver chloride precipitation objectively exists in precious metal separation. However, its application mainly focuses on systems obtained from the leaching of solid electronic waste, with the process emphasizing the leaching system and subsequent stepwise recovery. For high-copper, high-chlorine, and high-salt liquid phases such as PCB etching wastewater, the stability of continuous silver precipitation separation after copper removal is still not addressed, and a more suitable treatment method is not provided.

[0006] For PCB etching wastewater, even after front-end treatment to reduce copper ion content, the subsequent liquid to be recycled may still contain high concentrations of chloride ions, sodium salts, and low levels of gold, silver, and palladium. If chloride is directly added to this type of system for silver precipitation, silver easily forms fine silver chloride particles. These particles have a large specific surface area, readily adsorbing palladium chloride complexes and carrying high-salt mother liquor or sodium chloride, resulting in high residual palladium and salts in the obtained silver-containing solid, thus affecting the purity of the silver product and palladium recovery rate. Furthermore, if the washing liquid after silver precipitation is directly combined for further treatment or simply reused, palladium components may accumulate in the subsequent treatment system, further affecting the stability of continuous batch processing.

[0007] Therefore, existing technologies still require a treatment method suitable for the graded recovery of gold, silver, and palladium in PCB etching waste liquid, in order to improve the problem of palladium and salt inclusions in silver-containing solids during the silver precipitation process after reducing copper interference, improve the stability of silver-palladium separation, and take into account the process continuity of subsequent palladium and gold recovery and washing liquid treatment. Summary of the Invention

[0008] To address the problem that direct silver chlorination during the fractional recovery of gold, silver, and palladium from PCB etching wastewater often results in palladium and salt impurities being carried into the silver-containing solids, affecting the purity of silver products, palladium recovery rate, and the stability of continuous processing, this invention provides a fractional recovery method for gold, silver, and palladium from PCB etching wastewater. This method can reduce palladium and salt entrainment in the silver-containing solids obtained from silver chlorination, thereby improving the silver-palladium separation effect and the stability of comprehensive precious metal recovery.

[0009] To solve the above problems, the present invention adopts the following technical solution.

[0010] A method for graded recovery of gold, silver, and palladium from PCB etching waste liquid includes the following steps: (1) Solid-liquid separation and acidity adjustment were performed on the PCB etching waste liquid, and copper removal treatment was carried out to obtain a solution with reduced copper ion content and containing gold, silver and palladium. (2) The liquid to be treated is subjected to silver precipitation treatment to obtain silver-containing solid and liquid phase after separating silver-containing solid; in the second batch and subsequent silver precipitation treatment, the silver-containing solid obtained from the previous batch of silver precipitation is taken as a circulating seed crystal, and the circulating seed crystal is pre-activated by an acidic solution containing chloride ions and then added to the liquid to be treated in this batch, so that the silver in the liquid to be treated in this batch is deposited on the circulating seed crystal in the form of silver chloride. (3) Separate palladium from the liquid phase after separating the silver-containing solid to obtain palladium-containing material and gold-containing liquid after palladium separation; (4) Reduce the gold-containing solution to obtain a gold-containing product.

[0011] Preferably, the recycled seed crystal is a wet silver-containing solid obtained from the previous batch of silver immersion and washed with an acidic chlorine-containing washing solution. The wet silver-containing solid is mainly composed of silver chloride and is a solid material that has not been dried after solid-liquid separation and retains pore liquid. The water content of the wet silver-containing solid is 20%-55%.

[0012] Preferably, before using the wet silver-containing solid as a seed crystal, the wet silver-containing solid is dispersed in an acidic chlorine-containing liquid to form a silver-containing solid slurry. After stopping stirring and letting it stand for 30s-180s, the unsettled upper suspension is separated, and the settled solid is collected as a seed crystal.

[0013] Preferably, the pH of the acidic solution containing chloride ions is 0.5-1.5, the chloride ion concentration is 120g / L-220g / L, and the pre-activation time of the circulating seed crystals in the acidic solution containing chloride ions is 5min-30min.

[0014] Preferably, before silver precipitation, the solution to be treated is adjusted to a pH of 0.8-1.8, a chloride ion concentration of 100g / L-190g / L, and a redox potential of 500mV-650mV.

[0015] Preferably, the amount of the recycled seed crystal added is 5%-35% of the mass of silver in the batch of solution to be treated, based on the silver content in the recycled seed crystal.

[0016] Preferably, after adding the pre-activated circulating seed crystals, a conditioning solution containing chloride ions is added to the batch of solution to be treated in 2-5 portions, with an interval of 5-20 minutes between adjacent additions, so that silver is deposited on the circulating seed crystals in the form of silver chloride.

[0017] Preferably, the silver-containing solid obtained in step (2) is subjected to a first stage of washing and a second stage of washing in sequence; The first stage of washing uses an acidic chlorine-containing washing solution with a pH of 0.5-1.5 and a chloride ion concentration of 100g / L-180g / L; The second stage of washing uses an acidic washing solution with a pH of 1.0-3.0 and a chloride ion concentration of no more than 20 g / L.

[0018] Preferably, at least a portion of the washing liquid obtained from the first washing stage is reused for the pre-activation of the circulating seed crystals, or incorporated into the liquid phase after the separation of the silver-containing solid for subsequent palladium separation.

[0019] Preferably, a palladium concentration shunting threshold is set, wherein the palladium concentration shunting threshold is 1 mg / L-20 mg / L; When the palladium concentration in the washing solution obtained from the first washing stage is lower than the palladium concentration diversion threshold, the washing solution is reused for the pre-activation of the circulating seed crystals. When the palladium concentration in the washing liquid obtained from the first washing stage reaches or exceeds the palladium concentration diversion threshold, the washing liquid is incorporated into the liquid phase after the separation of the silver-containing solid for subsequent palladium separation.

[0020] Compared to existing technologies, this invention introduces silver-containing solids from the previous batch of silver immersion as seed crystals after copper removal from the PCB etching waste liquid. This is combined with chlorinated acidic pre-activation, phased addition of chlorine-containing conditioning solution, segmented washing, and diversion and reuse of the first-stage washing solution. This allows silver to be deposited more stably on the seed crystals in the form of silver chloride, reducing the instantaneous generation of numerous fine silver chloride particles in the liquid phase. This reduces the adsorption and entrainment of palladium components and high-salt mother liquor by the silver-containing solids. Simultaneously, by first replacing the palladium components and high-salt mother liquor in the silver-containing solids with an acidic chlorine-containing washing solution, and then using a low-chlorinated acidic washing solution to reduce salt residue, and by reusing or diverting the first-stage washing solution based on its palladium concentration, palladium is prevented from accumulating batch by batch in the silver immersion recycling system. Therefore, this invention improves the stability of silver-palladium separation, reduces palladium entrainment and salt residue in silver products, increases the overall recovery rate of gold, silver, and palladium, and reduces the amount of added chloride and washing wastewater generated. Attached Figure Description

[0021] Figure 1 This is a process flow diagram of the graded recovery method for gold, silver, and palladium in PCB etching waste liquid according to the present invention. Detailed Implementation

[0022] The specific embodiments of the present invention will be described below. The following content is used to illustrate the technical solution of the present invention and is not intended to limit the scope of protection of the present invention. Without departing from the core concept of the present invention, those skilled in the art can make adaptive adjustments to some process parameters according to the composition of the PCB etching waste liquid, the treatment scale, and product requirements.

[0023] This invention provides a method for the graded recovery of gold, silver, and palladium from PCB etching waste liquid, such as... Figure 1As shown, the basic process includes: pretreatment and copper removal of PCB etching waste liquid to obtain a solution with reduced copper ion content but still containing gold, silver and palladium; silver precipitation treatment of the solution to be treated to obtain silver-containing solid and liquid phase after separation of silver-containing solid; palladium is then separated from the liquid phase after separation of silver-containing solid, and gold-containing solution after palladium separation is reduced and precipitated.

[0024] The main improvement of this invention focuses on the silver immersion treatment process. In the second and subsequent batches of silver immersion treatment, the silver-containing solid obtained from the previous batch is used as a circulating seed crystal. After pre-activation with an acidic solution containing chloride ions, it is added to the solution to be treated in this batch, causing the silver in the solution to be treated in this batch to be deposited on the circulating seed crystal in the form of silver chloride. This method can reduce the instantaneous formation of a large number of fine silver chloride particles in the liquid phase and reduce the entrainment of palladium components and salt mother liquor by the silver-containing solid.

[0025] I. Pretreatment and Copper Removal of PCB Etching Waste Liquid Solid-liquid separation is performed on PCB etching wastewater to remove suspended solids, sludge, and insoluble impurities. Solid-liquid separation can be achieved using one or more methods, including sedimentation, filtration, pressure filtration, and centrifugation. After solid-liquid separation, the acidity of the wastewater is adjusted to suit subsequent copper removal treatment.

[0026] PCB etching waste liquid refers to copper- and chloride-containing waste liquid generated during PCB manufacturing, etching, or related waste liquid resource utilization processes. This waste liquid typically consists mainly of copper ions, chloride ions, and soluble salts such as sodium salts, and contains small amounts of precious metals such as gold, silver, and palladium. Since the copper ion content is usually significantly higher than the precious metal content, directly introducing it into subsequent silver, palladium, and gold immersion steps can easily interfere with the separation of precious metals.

[0027] The pretreated PCB etching waste liquid undergoes copper removal treatment to obtain a pretreatment solution with reduced copper ion content and containing gold, silver, and palladium. The pretreatment solution refers to the liquid obtained after copper removal treatment of the PCB etching waste liquid, where the copper ion content is lower than before the copper removal treatment, but it still contains gold, silver, palladium to be recovered, as well as soluble salts such as chloride ions and sodium salts. Complete removal of copper ions is not required in the pretreatment solution; it is sufficient that its copper ion content is reduced compared to before treatment and that it can proceed to the subsequent silver immersion treatment. The pretreatment solution obtained after copper removal treatment can directly proceed to the silver immersion treatment, or the acidity, chloride ion concentration, and redox potential can be adjusted according to the silver, palladium, gold, and chloride ion content before the silver immersion treatment.

[0028] Copper removal can be performed using selective copper extraction. For example, the pretreated PCB etching waste liquid is contacted with an organic liquid phase for copper extraction, allowing copper ions to enter the organic liquid phase. After phase separation, the aqueous phase is obtained as the liquid to be treated. The organic liquid phase used for copper extraction can employ a commonly used copper extractant system in the art, such as an extraction system containing oxime copper extractants and solvent oil. The organic liquid phase after copper extraction can be regenerated by acid back-extraction and reused for copper removal. The purpose of this copper removal treatment is to reduce the interference of copper ions on the subsequent silver precipitation and palladium separation processes, and to provide relatively stable liquid phase conditions for the subsequent silver precipitation process.

[0029] II. First Batch of Poured Silver Treatment At the start of continuous batch processing, the first batch does not yet have the silver-containing solid obtained from the previous batch of silver immersion as a seed crystal for recycling. Therefore, the first batch can use externally added silver chloride seed crystals, pre-prepared wet silver-containing solids, or a small amount of silver chloride precipitate as the initial seed crystals; alternatively, the first batch of silver-containing solids can be obtained by conventional silver chloride immersion, and then the silver-containing solid obtained from the previous batch of silver immersion can be used as a seed crystal for recycling starting from the second batch.

[0030] The silver-containing solid obtained in the first batch, after subsequent washing, can be used as a seed crystal source for the second batch of silver immersion treatment. Thus, the cyclic seed crystal system of the present invention is formed starting from the second batch and continues to operate in subsequent batches.

[0031] Before silver precipitation, the solution to be treated can be adjusted to a pH of 0.8-1.8, a chloride ion concentration of 100 g / L-190 g / L, and a redox potential of 500 mV-650 mV. Within this range, silver is easily separated as silver chloride, while palladium and gold are more easily retained in the liquid phase, which helps reduce palladium entrainment in silver-containing solids. The redox potential can be measured using a conventional ORP electrode, and the same measurement method can be used in the same batch or consecutive batches.

[0032] III. Obtaining Cyclic Seeds From the second and subsequent batches of silver immersion treatment, the silver-containing solid obtained from the previous batch of silver immersion is used as a recycling seed crystal.

[0033] Silver-containing solids refer to solid materials obtained from the liquid phase after silver precipitation treatment, and their main component is silver chloride. These silver-containing solids may contain small amounts of mother liquor, sodium chloride, palladium chloride complexes, or other impurities, and are not limited to pure silver chloride solids.

[0034] Recycled seed crystals refer to solid materials taken from the silver-containing solids obtained in the previous batch of silver immersion treatment and used in the current batch of silver immersion treatment. Since this solid material is mainly composed of silver chloride, it can provide an adhesion basis for the continued deposition of silver chloride in this batch, thereby reducing the instantaneous formation of a large number of fine silver chloride particles in the liquid phase by silver ions in the liquid to be treated in this batch.

[0035] Preferably, the seed crystals used in the recycling process are wet silver-containing solids obtained from the previous batch of silver immersion and washed with an acidic chlorine-containing washing solution. Wet silver-containing solids refer to solid materials that have not undergone drying after solid-liquid separation and retain some pore liquid and surface adhering liquid. Since the pore liquid and surface adhering liquid still contain chloride ions, this wet state helps maintain the chlorine-containing acidic environment on the surface of the silver chloride particles, making them more suitable as seed crystals for the next batch of silver immersion.

[0036] In some embodiments, the moisture content of the wet silver-containing solid is 20%-55%. If the moisture content is too low, the chlorine-containing liquid phase on the solid surface and in the pores is insufficiently retained, and its function is similar to that of a dry seed crystal; if the moisture content is too high, the effective solid content in the circulating seed crystal decreases, and excessive mother liquor may be introduced. Preferably, the moisture content of the wet silver-containing solid is 25%-45%.

[0037] The moisture content can be calculated by weighing the wet silver-containing solid, drying the wet silver-containing solid until its mass is basically constant, weighing the dry basis mass, and then calculating the moisture content according to the ratio of the water mass to the total mass of the wet silver-containing solid.

[0038] IV. Grading treatment of recycled seed crystals To reduce the accumulation of fine silver chloride particles in continuous batch processing, a grading process can be performed before the use of recycled seed crystals.

[0039] Specifically, the wet silver-containing solid obtained in the previous batch is dispersed in an acidic chlorine-containing liquid to form a silver-containing solid slurry. After stopping stirring and letting it stand for 30s-180s, the unsettled upper suspension is separated, and the settled solid is collected as a recycled seed crystal.

[0040] The purpose of sedimentation classification is to reduce the entry of difficult-to-settle fine silver-containing particles into the circulating seed crystals. Sedimented solids are silver-containing solids that settle to the bottom of the container within a set set settling time, without being limited by absolute particle size. Difficult-to-settle fine silver chloride particles typically have a larger specific surface area, making them more prone to adsorbing palladium chloride complexes and entraining high-salt mother liquor. Through classification, the accumulation of fine particles in the circulating system can be reduced, improving subsequent solid-liquid separation performance and reducing palladium and salt entrainment in the silver-containing solids.

[0041] The separated upper suspension may still contain fine silver particles. This portion of material is not used as seed crystals for the next batch of recycling. Instead, it can be incorporated into subsequent silver product preparation steps or combined with the silver-containing solids obtained in this batch for subsequent reduction, smelting, or refining steps. This avoids the direct discharge of fine silver particles, which would result in silver loss, and also prevents them from repeatedly entering the silver immersion system as seed crystals.

[0042] The acidic chlorine-containing solution can be an acidic solution prepared from one or more of hydrochloric acid, sodium chloride, and ammonium chloride, or it can be obtained by adjusting the acidity of the washing liquid obtained from the first washing stage. The function of this acidic chlorine-containing solution is to maintain a chlorine-containing acidic environment on the surface of the silver-containing solid during dispersion, preventing significant changes in the state of the silver chloride solid. The acidic chlorine-containing solution used to disperse wet silver-containing solids can have the same or similar composition as the acidic chloride-containing solution used for pre-activating the circulating seed crystals.

[0043] V. Pre-activation of recycled seed crystals The recycled seed crystals were pre-activated by adding them to an acidic solution containing chloride ions to obtain pre-activated recycled seed crystals.

[0044] An acidic solution containing chloride ions refers to an aqueous solution containing chloride ions and exhibiting acidity. It can be prepared from one or more of hydrochloric acid, sodium chloride, and ammonium chloride, or it can be obtained by adjusting the washing solution obtained from the first washing stage. This solution is used to replace or adjust the liquid phase environment on the surface and in the pores of the circulating seed crystals, ensuring that the circulating seed crystals remain in a chloride-containing acidic state before entering the current batch of treatment solution.

[0045] Preferably, the pH of the acidic solution containing chloride ions is 0.5-1.5, the chloride ion concentration is 120 g / L-220 g / L, and the pre-activation time of the circulating seed crystals in the solution is 5 min-30 min. More preferably, the pre-activation temperature is 20℃-50℃.

[0046] Pre-activation maintains a high chloride ion concentration and an acidic environment on the surface of the circulating seed crystals and in the pore liquid. This condition facilitates the deposition of silver in the current batch of solution as silver chloride on the circulating seed crystals, while reducing the adsorption or hydrolytic deposition of palladium chloride complexes on the circulating seed crystals.

[0047] The applicant discovered that if the silver-containing solid from the previous batch is directly added to the next batch of treatment solution, the composition of the mother liquor on the surface of the silver-containing solid may differ from that of the current batch of treatment solution. This can easily cause fluctuations in local acidity and chloride ion concentration, thereby affecting the stability of the silver chloride deposition process. Pre-activation with an acidic solution containing chloride ions can readjust the surface of the circulating seed crystals and the pore liquid to a chloride-containing acidic environment suitable for silver deposition, making it more stable as a basis for silver chloride deposition in subsequent silver deposition processes.

[0048] VI. Recycled Seed Silver Immersion Treatment The pre-activated circulating seed crystals are added to the current batch of solution to be treated, so that the silver in the current batch of solution to be treated is deposited on the circulating seed crystals in the form of silver chloride. After separation, a new silver-containing solid and a liquid phase after separating the silver-containing solid are obtained.

[0049] Before silver precipitation, the solution to be treated can be adjusted to a pH of 0.8-1.8, a chloride ion concentration of 100 g / L-190 g / L, and a redox potential of 500 mV-650 mV. These conditions are conducive to the formation of silver chloride solid and to keeping palladium and gold as much as possible in the liquid phase. If the pH is too high, palladium may undergo hydrolysis or be entrained; if the chloride ion concentration is too high, the risk of silver solid entraining salts increases; if the redox potential is not properly controlled, it may affect the stability of gold and palladium in the liquid phase.

[0050] The amount of recycled seed crystals added can be determined based on the mass of silver in the current batch of solution to be treated. Preferably, the amount of recycled seed crystals added is 5%-35% of the mass of silver in the current batch of solution to be treated, based on the silver content in the recycled seed crystals. The mass of silver in the current batch of solution to be treated can be calculated based on the volume of the solution to be treated and the detected silver concentration. If the amount of recycled seed crystals added is too low, the guiding effect on silver chloride deposition will be insufficient; if the amount of recycled seed crystals added is too high, the solid circulation load will increase, and it may carry in too much solid entrainment from the previous batch.

[0051] After adding the pre-activated seed crystals, a conditioning solution containing chloride ions can be added to the batch of solution to be treated in 2-5 portions, with an interval of 5-20 minutes between each addition, so that silver can be gradually deposited on the seed crystals in the form of silver chloride.

[0052] The conditioning solution containing chloride ions can be hydrochloric acid, sodium chloride solution, ammonium chloride solution, or a combination thereof. Adding it in stages is to avoid the instantaneous formation of a large number of fine silver chloride particles in the liquid phase caused by adding a large amount of chloride at once. By adding it in stages, the rate of silver chloride formation in the system is more gradual, and the newly formed silver chloride is more likely to adhere to existing circulating seed crystals for further deposition, thereby reducing the number of fine particles and decreasing palladium adsorption and entrainment in the high-salt mother liquor.

[0053] In the direct chlorination precipitation of silver, if a large amount of chloride is added at once, a large number of fine silver chloride particles will rapidly form in the liquid phase. These fine particles have a large specific surface area, easily adsorb palladium chloride complexes, and carry over chlorine- and sodium-containing mother liquor, leading to an increase in palladium and salt residues in the silver-containing solid. This invention uses the silver-containing solid obtained from the previous batch of precipitated silver as a circulating seed crystal, and pre-activates it with an acidic solution containing chloride ions before use, maintaining a chlorine-containing acidic environment on the surface of the circulating seed crystal and in the pore liquid. Then, by adding a conditioning solution containing chloride ions in stages, silver is gradually deposited on the circulating seed crystal in the form of silver chloride. This method can reduce the formation of fine silver chloride particles through instantaneous nucleation in the liquid phase, making the silver-containing solid easier to separate from the liquid and reducing the entrainment of palladium components and high-salt mother liquor.

[0054] Furthermore, the recycled seed crystals are not simply added to the system as inert fillers, but participate in the subsequent silver chloride deposition process. After the previous batch of silver-containing solids is classified, the fine silver-containing particles with poor settling properties are no longer repeatedly added to the system as recycled seed crystals; the pre-activated recycled seed crystals provide a more stable solid phase basis for the deposition of this batch of silver. After adding the conditioning solution containing chloride ions in stages, the newly generated silver chloride tends to continue to deposit on the existing solid phase, rather than forming a large number of new fine particles in the liquid phase. This change in the deposition path allows the silver deposition process to simultaneously achieve silver separation, particle morphology adjustment, and entrainment reduction effects.

[0055] VII. Segmented washing of silver-containing solids The silver-containing solid obtained in step six is ​​subjected to a first stage of washing and a second stage of washing in sequence.

[0056] The first stage of washing uses an acidic chlorine-containing washing solution with a pH of 0.5-1.5 and a chloride ion concentration of 100 g / L-180 g / L. The main function of the first stage washing is to displace the high-salt mother liquor from the pores and surface of the silver-containing solid and to return the palladium chloride complexes entrained or adsorbed in the silver-containing solid to the liquid phase. Because the first stage washing solution still has a high chloride ion concentration, it can maintain the stability of the silver chloride solid during the washing process and reduce the dispersion of fine particles or the redeposition of impurities caused by sudden changes in the washing environment.

[0057] The second washing stage uses an acidic washing solution with a pH of 1.0-3.0 and a chloride ion concentration not exceeding 20 g / L. The main purpose of the second washing stage is to reduce the amount of sodium chloride and other soluble salts entrained in the silver-containing solid, thereby improving the purity of the subsequent silver product. The second washing stage is set after the first washing stage to first displace the palladium component back to the liquid phase as much as possible under acidic conditions with a high chloride ion concentration, and then reduce salt residues by using an acidic washing solution with a lower chloride ion concentration.

[0058] The order of the first and second washing stages should not be interchanged. If an acidic washing solution with a low chloride ion concentration is used directly, the chloride-containing environment on the surface and in the pores of the silver-containing solid will change significantly. The entrained or adsorbed palladium components will not be easily and stably displaced back into the liquid phase, and the risk of dispersion of fine silver chloride particles may increase. Performing the first washing stage before the second washing stage is beneficial for reducing both palladium entrainment and salt entrainment.

[0059] The applicant discovered that the palladium component and the entrainment of salts in silver-containing solids are not the same issue. The palladium component is more related to adsorption on the surface of silver chloride particles, entrainment in pore liquid, or local hydrolysis deposition; while salt impurities are more likely to originate from high-salt mother liquor residue. While directly using a low-salt washing solution is beneficial for reducing soluble salts, it is not conducive to displacing the palladium component back into the liquid phase while maintaining the stability of the silver chloride solid. This invention first treats the silver-containing solid with an acidic first-stage washing solution containing a high chloride ion concentration, and then uses a second-stage washing solution with a lower chloride ion concentration to reduce salt residues, allowing the reduction of palladium entrainment and salt entrainment to be completed under more suitable liquid phase conditions.

[0060] The silver-containing solids after the first and second washing stages can proceed to subsequent silver reduction, smelting, or refining steps to obtain silver products or silver-containing intermediate materials.

[0061] 8. Reuse and diversion of the first stage washing solution The washing solution obtained from the first washing stage can be at least partially reused for the pre-activation of the recycled seed crystals, or incorporated into the liquid phase after the separation of the silver-containing solid for subsequent palladium separation.

[0062] In some embodiments, a palladium concentration splitting threshold is set, which is 1 mg / L-20 mg / L. When the palladium concentration in the washing solution obtained from the first washing stage is lower than this splitting threshold, the washing solution is reused for pre-activation of the circulating seed crystals; when the palladium concentration in the washing solution obtained from the first washing stage reaches or exceeds this splitting threshold, the washing solution is incorporated into the liquid phase after separating the silver-containing solid for subsequent palladium separation. The palladium concentration in the first washing solution can be determined using inductively coupled plasma atomic emission spectrometry, atomic absorption spectrometry, or other noble metal concentration detection methods.

[0063] The first washing solution contains displaced palladium components and chloride ions. When the palladium concentration is low, it is reused for pre-activation of circulating seed crystals, utilizing the chloride ions and acidity to maintain the surface environment of the circulating seed crystals and reducing the amount of added chloride and acid. When the palladium concentration is high, continued reuse will cause palladium to re-enter the silver immersion system, increasing the risk of palladium inclusions in silver-containing solids in subsequent batches. Therefore, this invention diverts the first washing solution according to the palladium concentration, allowing the circulation system to utilize the chloride ions while preventing palladium from accumulating batch by batch in the closed-loop silver immersion cycle.

[0064] This diversion method differs from conventional washing solution reuse. Conventional reuse typically focuses on water volume and reagent consumption, while the first-stage washing solution in this invention serves two purposes: firstly, its high chloride ion concentration and acidity can maintain the pre-activation environment for the circulating seed crystals; secondly, it may carry palladium components displaced from the silver-containing solid. If reused indiscriminately, this would cause palladium to migrate back and forth between the circulating seed crystals and the silver precipitation system. By setting a palladium concentration diversion threshold, the chloride ions and acidity of the first-stage washing solution can be utilized under low palladium conditions, while it can be introduced into the palladium separation step under high palladium conditions, thereby preventing the accumulation of palladium load in the closed-loop silver precipitation cycle.

[0065] As can be seen from the above-described process of obtaining recycled seed crystals, graded treatment, pre-activation, staged chlorination deposition, segmented washing, and the diversion and reuse of the first stage washing solution, this invention does not simply reuse silver-containing solids as seed crystals. Instead, by controlling the source of recycled seed crystals, the amount of fine particles entering, the surface liquid phase environment, the silver chloride deposition rate, and the palladium load in the washing solution, it prevents the accumulation of fine silver chloride particles during continuous batch silver precipitation, the accumulation of palladium components in the silver precipitation system, and the long-term entrainment of salt mother liquor in the silver-containing solids. The combined effect of these control methods can simultaneously improve silver separation efficiency, silver-palladium separation stability, silver-containing solid washing efficiency, and washing solution reuse stability in the same silver precipitation system.

[0066] IX. Palladium Separation and Gold Recovery The liquid phase after separating the silver-containing solid is subjected to palladium-selective separation to obtain the palladium-containing material and the gold-containing solution after palladium separation. Palladium-selective separation can be achieved through one or more of precipitation, adsorption, extraction, or ion exchange. For example, oxime-containing precipitants, sulfur-containing precipitants, mercapto resins, amine resins, or other materials capable of selectively binding with palladium can be used.

[0067] The gold-containing solution after palladium separation is subjected to reduction precipitation to obtain the gold-containing product. Reduction precipitation can be performed using sulfites, metabisulfites, ascorbic acid, oxalates, or other reducing agents suitable for reducing and separating gold.

[0068] Palladium separation and gold recovery are subsequent precious metal recovery steps after silver precipitation. The specific reagents and equipment can be selected based on the concentrations of palladium and gold in the liquid phase and product requirements. The main improvement of this invention lies in the aforementioned improvements in the stability of silver-palladium separation and the control of silver-containing solid salt inclusions through the circulating seed crystal precipitation, fractional chlorination, staged washing, and diversion and reuse of washing liquid.

[0069] By using the above method, the formation of fine silver chloride particles caused by direct silver chloride precipitation can be reduced in the high-chlorine, high-salt liquid phase of PCB etching waste liquid after copper removal, and the entrainment of palladium components and sodium chloride in silver-containing solids can be reduced. At the same time, by reusing and diverting the first stage washing liquid, the discharge of washing liquid can be reduced and the continuous accumulation of palladium in the recycling system can be avoided, thereby improving the stability of gold, silver and palladium graded recovery.

[0070] The following provides further details and implementation data.

[0071] Example 1: The PCB etching waste liquid after pretreatment and copper removal was used as the treatment solution. The concentrations of Cu, Ag, Pd, Au, and Cl- in the treatment solution were 1.18 g / L, 21.6 mg / L, 9.4 mg / L, 4.2 mg / L, 146 g / L, pH 1.31, and redox potential of 568 mV.

[0072] The wet silver-containing solid obtained from the previous batch of silver immersion and after the first washing stage was used as the seed crystal for recycling. The wet silver-containing solid was mainly composed of silver chloride and had a water content of 36.2%. The wet silver-containing solid was dispersed in an acidic chlorine-containing solution to form a silver-containing solid slurry. Stirring was stopped and the mixture was allowed to stand for 90 seconds. The unsettled upper suspension was separated, and the settled solid was collected as the seed crystal for recycling.

[0073] The recycled seed crystals were pre-activated for 15 min in an acidic solution containing chloride ions (pH 1.0, Cl- concentration 162 g / L) at 32 °C. The amount of recycled seed crystals added was 18% of the mass of silver in the solution to be treated, based on the silver content in the recycled seed crystals. The pre-activated recycled seed crystals were added to the solution to be treated, followed by the addition of a chloride-containing conditioning solution in three separate additions, with a 10-min interval between each addition, allowing silver to deposit on the recycled seed crystals as silver chloride. After silver precipitation, solid-liquid separation was performed to obtain a silver-containing solid and a liquid phase after separation of the silver-containing solid.

[0074] The silver-containing solid underwent a first-stage washing and a second-stage washing. The first-stage washing used an acidic chlorine-containing washing solution with a pH of 1.0 and a Cl- concentration of 138 g / L; the second-stage washing used an acidic washing solution with a pH of 2.1 and a Cl- concentration of 7.5 g / L. A palladium concentration splitting threshold of 5 mg / L was set for the washing solution obtained from the first-stage washing. The portion below this threshold was recycled for pre-activation of the circulating seed crystals, while the portion reaching or exceeding this threshold was incorporated into the liquid phase after separating the silver-containing solid for subsequent palladium separation steps. After palladium separation, a palladium-containing material was obtained, and the gold-containing solution after palladium separation was then subjected to reduction precipitation of gold.

[0075] Example 2: This example uses the same processing procedure as Example 1, except that the concentrations of Cu, Ag, Pd, Au, Cl- in the solution to be treated are 0.86 g / L, 16.8 mg / L, 6.5 mg / L, 2.7 mg / L, 112 g / L, pH is 0.94, and redox potential is 522 mV.

[0076] The seed crystals used were the wet silver-containing solids obtained from the previous batch, with a water content of 31.5%. After dispersion, the wet silver-containing solids were allowed to stand for 60 seconds, and the settled solids were collected as seed crystals. The seed crystals were pre-activated for 10 minutes in an acidic solution containing chloride ions (pH 0.7, Cl- concentration 128 g / L) at 25°C. The amount of seed crystals added was 9% of the mass of silver in the solution to be treated. The chloride-containing conditioning solution was added in two portions, with a 12-minute interval between each addition.

[0077] The first stage washing solution has a pH of 0.8 and a Cl- concentration of 108 g / L; the second stage washing solution has a pH of 1.6 and a Cl- concentration of 5.2 g / L. The palladium concentration splitting threshold for the first stage washing solution is set at 3 mg / L.

[0078] Example 3: This example uses a basically the same processing procedure as Example 1, except that the concentrations of Cu, Ag, Pd, Au, Cl- in the solution to be treated are 2.06 g / L, 27.4 mg / L, 13.2 mg / L, 5.8 mg / L, 181 g / L, pH is 1.67, and redox potential is 632 mV.

[0079] The seed crystals used were the wet silver-containing solids obtained from the previous batch, with a water content of 43.8%. After dispersion, the wet silver-containing solids were allowed to stand for 150 seconds, and the settled solids were collected as seed crystals. The seed crystals were pre-activated for 25 minutes in an acidic solution containing chloride ions (pH 1.4, Cl- concentration 205 g / L) at 45°C. The amount of seed crystals added was 31% of the mass of silver in the solution to be treated. The chloride-containing conditioning solution was added in five portions, with an 8-minute interval between each addition.

[0080] The first washing solution has a pH of 1.3 and a Cl- concentration of 168 g / L; the second washing solution has a pH of 2.7 and a Cl- concentration of 16.4 g / L. The palladium concentration splitting threshold for the first washing solution is set at 10 mg / L.

[0081] Example 4: This example is used to investigate the stability of continuous batch processing. Five batches were processed continuously under similar process conditions as in Example 1. The first batch used pre-prepared wet silver chloride seed crystals as the initial seed crystals; from the second batch onwards, the wet silver-containing solid obtained from the previous batch's silver immersion and after the first stage of washing was used as the circulating seed crystals.

[0082] The concentrations of Cu, Ag, Pd, Au, and Cl- in each batch of the solution to be treated were 1.05 g / L-1.42 g / L, 19.8 mg / L-23.5 mg / L, 8.2 mg / L-10.6 mg / L, 3.8 mg / L-4.9 mg / L, 138 g / L-158 g / L, with a pH of 1.20-1.42 and a redox potential of 550 mV-590 mV. The water content of the circulating seed crystals was 33.5%-39.6%, the settling and fractionation time was 90 s, the pH of the pre-activation solution was 0.9-1.1, the Cl- concentration was 150 g / L-170 g / L, and the pre-activation time was 15 min. The conditioning solution containing chloride ions was added in three portions, with a 10-min interval between each addition.

[0083] The palladium concentration splitting threshold for the first washing solution is set to 5 mg / L. When the Pd concentration in the first washing solution is below 5 mg / L, it is recycled for pre-activation of the circulating seed crystals; when the Pd concentration reaches or exceeds 5 mg / L, it is incorporated into the liquid phase after the separation of silver-containing solids and enters the palladium separation step.

[0084] Example 5: This example is used to examine the applicability under expanded processing conditions. 1000L of the pretreated and copper-removed solution was taken, with Cu 1.64g / L, Ag 24.2mg / L, Pd 11.6mg / L, Au 5.1mg / L, Cl- 156g / L, pH 1.38, and redox potential 584mV.

[0085] The wet silver-containing solid obtained from the previous batch of silver immersion was used as the circulating seed crystal, with a water content of 38.4%. The wet silver-containing solid was dispersed in an acidic chloride-containing solution, stirring was stopped, and the mixture was allowed to stand for 120 seconds. The precipitated solid was collected as the circulating seed crystal. The circulating seed crystal was pre-activated for 18 minutes in an acidic chloride-containing solution with a pH of 1.1 and a Cl- concentration of 176 g / L at a pre-activation temperature of 35 °C. The amount of circulating seed crystal added was 22% of the silver mass in the solution to be treated. The chloride-containing conditioning solution was added in four portions, with a 10-minute interval between each addition.

[0086] The first washing solution has a pH of 1.1 and a Cl- concentration of 152 g / L; the second washing solution has a pH of 2.3 and a Cl- concentration of 9.8 g / L. The palladium concentration splitting threshold for the first washing solution is set to 5 mg / L.

[0087] Comparative Example 1: Direct one-time silver chloride precipitation The same solution as in Example 1 was used, without the addition of recycled seed crystals or pre-activation with an acidic solution containing chloride ions. Silver precipitation was performed by directly adding sodium chloride solution in a single step, followed by conventional water washing to treat the silver-containing solid.

[0088] Comparative Example 2: Using externally added dry silver chloride seed crystals The same solution with the same composition as in Example 1 was used, with dry silver chloride powder added as seed crystals in the same amount as in Example 1. The wet silver-containing solid obtained in the previous batch was not used, nor was pre-activation with an acidic solution containing chloride ions performed. The conditioning solution containing chloride ions was added in three portions, and the remaining conditions were basically the same as in Example 1.

[0089] Comparative Example 3: Wet cyclic seeding was used, but sedimentation and fractionation were not performed. Using the same composition of the solution to be treated as in Example 1, the wet silver-containing solid obtained from the previous batch was used as a seed crystal for recycling, but no sedimentation and classification treatment was performed; all the wet silver-containing solid was directly used for silver precipitation in this batch. The pre-activation, chlorination in stages, and washing conditions were the same as in Example 1.

[0090] Comparative Example 4: Recycled seed crystals were not pre-activated. Using the same composition of the treatment solution as in Example 1, the wet silver-containing solid obtained from the previous batch was taken and, after sedimentation and classification, was used as a circulating seed crystal, but without pre-activation in an acidic solution containing chloride ions, it was directly added to the treatment solution. The remaining silver precipitation, washing, and diversion conditions were the same as in Example 1.

[0091] Comparative Example 5: A conditioning solution containing chloride ions was added at once. The same solution with the same composition as in Example 1 was used, with the wet silver-containing solid obtained from the previous batch as the seed crystal for recycling. Sedimentation classification and pre-activation were performed. However, the conditioning solution containing chloride ions was added all at once, not in multiple additions. All other conditions were the same as in Example 1.

[0092] Comparative Example 6: No first wash performed The same solution with the same composition as in Example 1 was used as the treatment solution, and the wet silver-containing solid obtained from the previous batch was used as the seed crystal for recycling. Sedimentation classification, pre-activation, and silver precipitation by chlorination in stages were performed. The silver-containing solid obtained by silver precipitation was not washed with the first stage acidic chlorine-containing washing solution, but was directly washed with the second stage acidic washing solution.

[0093] Comparative Example 7: The first stage of washing solution was completely reused and not diverted according to palladium concentration. The continuous batch processing conditions were similar to those in Example 4, except that the palladium concentration of the first washing solution was not detected, nor was it diverted according to the palladium concentration, but was entirely reused for pre-activation of the circulating seed crystals.

[0094]

[0095]

[0096]

[0097] Among them, the silver precipitation rate is the proportion of the amount of silver reduced in the liquid phase before and after silver precipitation to the total amount of silver before silver precipitation; the palladium entrainment rate is the proportion of the amount of palladium detected in the silver-containing solid to the total amount of palladium entering the silver precipitation treatment; the NaCl residue is the mass percentage of NaCl in the dry basis of the silver-containing solid after washing; the filtration time is the time required to treat an equal amount of silver precipitation slurry under the same filtration area and the same vacuum degree; the data of Example 4 and Comparative Example 7 are the average values ​​of the results of 5 consecutive batches of treatment.

[0098]

[0099] The reduction in the amount of added chloride and the reduction in the amount of washing wastewater were calculated based on the direct one-time chlorination silver precipitation process of Comparative Example 1. The data of Example 4 and Comparative Example 7 are the average values ​​of the results of five consecutive batches of treatment.

[0100]

[0101] As shown in Tables 1 and 2, the solutions to be treated in each embodiment differ in terms of residual copper, precious metal content, chloride ion concentration, pH, and redox potential, but all can achieve fractional recovery of gold, silver, and palladium using the method of this invention. Based on the process parameters of each embodiment, it can be seen that the method of this invention is not limited to a single raw material composition or a single operating condition, and has good applicability in high-chlorine, high-salt liquid phase systems of PCB etching wastewater after copper removal.

[0102] As shown in Table 3, in Comparative Example 1, which used direct one-time silver chlorination precipitation, the palladium entrainment rate and NaCl residue in the silver-containing solid were both high, the filtration time was significantly prolonged, and the purity of the resulting silver product was low. Compared with this method, the palladium entrainment rate, NaCl residue, and filtration time of the silver-containing solid in Examples 1 to 5 were significantly reduced, and the purity of the silver product increased to 96.5%-97.4%. These results indicate that using the previous batch of silver-containing solid as a circulating seed crystal, and pre-activating it with a chlorinated acidic solution before participating in this batch of silver precipitation, helps reduce the palladium adsorption and high-salt mother liquor entrainment problems caused by the large-scale generation of fine silver chloride particles during direct silver chlorination precipitation.

[0103] Comparative Example 2 used externally added dry silver chloride seed crystals for silver deposition. Its palladium entrainment rate, NaCl residue, and filtration time were lower than Comparative Example 1, but still significantly higher than Example 1. This result indicates that simply adding externally added dry silver chloride seed crystals can only improve the formation state of the silver particles to a certain extent. The wet silver-containing solid obtained in the previous batch, due to the retention of certain pore liquid and surface adhering liquid, is more suitable as a deposition base for continuous batches of silver deposition after pre-activation with a chlorinated acid solution, thus making the silver deposition process more stable.

[0104] Comparative Example 3 used the wet silver-containing solid obtained from the previous batch as the recycled seed crystal, but without sedimentation fractionation. The results showed that the palladium entrainment rate, NaCl residue, and filtration time in the silver-containing solid were all higher than in Example 1. This result indicates that if all the silver-containing solid obtained from the previous batch is directly reused in the next batch of silver precipitation, fine silver particles that are difficult to settle will re-enter the system with the recycled seed crystal, increasing the risk of palladium adsorption and entrainment by the high-salt mother liquor. Reducing the entry of fine particles into the recycled seed crystal through sedimentation fractionation helps maintain the stability of the continuous batch silver precipitation process.

[0105] Comparative Example 4 used wet circulating seed crystals and underwent sedimentation classification, but did not undergo chlorinated acid pre-activation. Compared with Example 1, Comparative Example 4 showed increased palladium entrainment and NaCl residue. This result indicates that the pre-activation step can adjust the liquid phase environment on the surface and in the pores of the circulating seed crystals, maintaining it in a chlorinated acid state suitable for silver chloride deposition, thereby reducing palladium adsorption and impurity entrainment caused by local liquid phase environment fluctuations after the circulating seed crystals enter the batch of treatment solution.

[0106] Comparative Example 5 used a single-stage addition of chlorine-containing conditioning solution for silver precipitation. Its silver precipitation rate was similar to that of Example 1, but the palladium entrainment rate, NaCl residue, and filtration time were significantly increased. This result indicates that similar silver precipitation rates do not necessarily mean identical purification effects for the precipitated silver solid. Single-stage addition of the chlorine-containing conditioning solution easily leads to the rapid formation of a large number of fine silver chloride particles in the liquid phase, while staged addition allows for a relatively gradual silver chloride formation rate, making silver more likely to deposit on existing recycled seed crystals, thereby reducing the formation of fine particles, improving filtration performance, and reducing entrainment.

[0107] Comparative Example 6 did not undergo the first stage of acidic chlorine-containing washing; instead, it directly used the second stage of acidic washing solution. In this comparative example, the residual NaCl content was reduced, but the palladium entrainment rate was still significantly higher than in Example 1. This result indicates that palladium entrainment in silver-containing solids is not a problem of the same nature as salt entrainment. The first stage washing solution, under acidic conditions and a high chloride ion concentration, can displace the high-salt mother liquor from the pores and surface of the silver-containing solid, and return some of the entrained or adsorbed palladium components to the liquid phase; the second stage washing is mainly used to reduce soluble salt residue. The two stages of washing, used in sequence, can simultaneously reduce palladium entrainment and salt residue.

[0108] As shown in Table 5, in Example 4, during five consecutive batches of treatment, the palladium entrainment rate in the silver-containing solid remained at a low level and did not show a continuous upward trend, depending on the palladium concentration in the first washing solution being reused or diverted. In contrast, in Comparative Example 7, all of the first washing solution was reused for circulating seed crystal pre-activation. As the number of treatment batches increased, the palladium concentration in the first washing solution gradually increased, and the palladium entrainment rate in the silver-containing solid also increased accordingly. This result indicates that the diversion control of the first washing solution is not simply for reducing wastewater discharge, but rather can prevent the palladium component from repeatedly migrating between circulating seed crystal pre-activation and silver precipitation treatment, thereby inhibiting the accumulation of palladium in the closed-loop silver precipitation cycle.

[0109] As shown in Table 4, in Examples 1 to 5, the overall recovery rates of gold, silver, and palladium remained between 94.5% and 96.3%, the purity of the silver product was consistently above 96%, and the amount of added chloride and washing wastewater were also reduced. These results indicate that the present invention, while improving the purification effect of silver-containing solids, can maintain a high overall recovery level of precious metals and reduce the generation of added chloride and washing wastewater.

[0110] The data above demonstrates that the effectiveness of this invention does not stem from a single silver chloride precipitation step, but rather from a combination of factors: recycling the previous batch of silver-containing solids, sedimentation and classification, pre-activation with chlorinated acid, phased addition of chlorine-containing conditioning solution, two-stage washing, and the diversion and reuse of the first stage washing solution. This combination ensures that the formation mode of silver chloride particles, the surface state of the circulating seed crystals, the removal of silver-containing solid entrainment, and the control of palladium load in the washing solution work together during the silver precipitation process. This allows for simultaneous reduction of palladium entrainment, NaCl residue, and filtration burden in continuous batch processing, while preventing the accumulation of palladium components in the silver precipitation recycling system batch by batch. Therefore, compared to direct silver chloride precipitation, conventional external seed crystal precipitation, and simple washing solution reuse, this invention offers more stable silver-palladium separation and better process continuity.

Claims

1. A method for graded recovery of gold, silver, and palladium from PCB etching wastewater, characterized in that, Includes the following steps: (1) Solid-liquid separation and acidity adjustment were performed on the PCB etching waste liquid, and copper removal treatment was carried out to obtain a solution with reduced copper ion content and containing gold, silver and palladium. (2) The liquid to be treated is subjected to silver precipitation treatment to obtain silver-containing solid and liquid phase after separating silver-containing solid; in the second batch and subsequent silver precipitation treatment, the silver-containing solid obtained from the previous batch of silver precipitation is taken as a circulating seed crystal, and the circulating seed crystal is pre-activated by an acidic solution containing chloride ions and then added to the liquid to be treated in this batch, so that the silver in the liquid to be treated in this batch is deposited on the circulating seed crystal in the form of silver chloride. (3) Separate palladium from the liquid phase after separating the silver-containing solid to obtain palladium-containing material and gold-containing liquid after palladium separation; (4) Reduce the gold-containing solution to obtain a gold-containing product.

2. The method for graded recovery of gold, silver, and palladium from PCB etching waste liquid according to claim 1, characterized in that, The recycled seed crystal is a wet silver-containing solid obtained from the previous batch of silver immersion and washed with an acidic chlorine-containing washing solution. The wet silver-containing solid is mainly composed of silver chloride and is a solid material that has not been dried after solid-liquid separation and retains pore liquid. The water content of the wet silver-containing solid is 20%-55%.

3. The method for graded recovery of gold, silver, and palladium from PCB etching waste liquid according to claim 2, characterized in that, Before using the wet silver-containing solid as a seed crystal, the wet silver-containing solid is dispersed in an acidic chlorine-containing liquid to form a silver-containing solid slurry. After stopping stirring and letting it stand for 30s-180s, the unsettled upper suspension is separated, and the settled solid is collected as a seed crystal.

4. The method for graded recovery of gold, silver, and palladium from PCB etching waste liquid according to claim 1, characterized in that, The pH of the acidic solution containing chloride ions is 0.5-1.5, the chloride ion concentration is 120g / L-220g / L, and the pre-activation time of the circulating seed crystals in the acidic solution containing chloride ions is 5min-30min.

5. The method for graded recovery of gold, silver, and palladium from PCB etching waste liquid according to claim 1, characterized in that, Before silver precipitation, the solution to be treated is adjusted to a pH of 0.8-1.8, a chloride ion concentration of 100g / L-190g / L, and a redox potential of 500mV-650mV.

6. The method for graded recovery of gold, silver, and palladium from PCB etching waste liquid according to claim 1, characterized in that, The amount of the recycled seed crystal added is 5%-35% of the mass of silver in the batch of solution to be treated, based on the silver content in the recycled seed crystal.

7. The method for graded recovery of gold, silver, and palladium from PCB etching waste liquid according to claim 1, characterized in that, After adding the pre-activated seed crystals, add the conditioning solution containing chloride ions to the batch of solution to be treated in 2-5 portions, with an interval of 5-20 minutes between each two additions, so that silver is deposited on the seed crystals in the form of silver chloride.

8. The method for graded recovery of gold, silver, and palladium from PCB etching waste liquid according to claim 1, characterized in that, The silver-containing solid obtained in step (2) is subjected to a first stage of washing and a second stage of washing in sequence; The first stage of washing uses an acidic chlorine-containing washing solution with a pH of 0.5-1.5 and a chloride ion concentration of 100g / L-180g / L; The second stage of washing uses an acidic washing solution with a pH of 1.0-3.0 and a chloride ion concentration of no more than 20 g / L.

9. A method for graded recovery of gold, silver, and palladium from PCB etching waste liquid according to claim 8, characterized in that, At least a portion of the washing solution obtained from the first washing stage is reused for the pre-activation of the recycled seed crystals, or incorporated into the liquid phase after the separation of the silver-containing solid for subsequent palladium separation.

10. A method for graded recovery of gold, silver, and palladium from PCB etching waste liquid according to claim 9, characterized in that, A palladium concentration shunting threshold is set, wherein the palladium concentration shunting threshold is 1 mg / L-20 mg / L; When the palladium concentration in the washing solution obtained from the first washing stage is lower than the palladium concentration diversion threshold, the washing solution is reused for the pre-activation of the circulating seed crystals. When the palladium concentration in the washing liquid obtained from the first washing stage reaches or exceeds the palladium concentration diversion threshold, the washing liquid is incorporated into the liquid phase after the separation of the silver-containing solid for subsequent palladium separation.

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