How to Reduce Palladium Dissolved in Nitric Acid

Palladium Recovery Principles and Methods Palladium is the most chemically active of the precious metals, and it is easy to separate palladium from base metals and other precious metals in the wet process of palladium recovery. The basic idea of wet process palladium recovery is to utilize the property that palladium can be dissolved in nitric acid to separate palladium from precious metals such as gold and platinum, which are difficult to be dissolved in nitric acid, and then to utilize the property that silver can generate silver chloride precipitate in hydrochloric acid or sodium chloride solution to separate silver from palladium-containing nitric acid solution (referred to as silver splitting). In the solution after silver separation, reagents that can precipitate palladium ions are added to achieve the purpose of separation from other base metals. The wet process yields high purity palladium products with a content of 99.99% or more. The pyroprocess is often used to recover palladium from wastes with low palladium content or to enrich palladium in pyroprocesses for the recovery of other precious metals. The palladium obtained by the pyrometallurgical process is generally crude palladium, which must be purified by a wet process to obtain high purity palladium sponge or processed directly into palladium fine chemicals. (1) Recovery of palladium from palladium-containing waste liquids In the wet process of recovering gold and silver from waste appliances, palladium easily enters the solution together with gold and silver. The existence of palladium in palladium-containing waste liquid is mainly in the form of Pd (Ⅳ) and Pd (Ⅱ) oxidized palladium, which is traditionally separated and enriched by ammonium chloropalladate precipitation and dichlorodiammine palladium complex method. The ammonium chloropalladate precipitation method utilizes the ability of Pd(IV) compounds to interact with ammonium chloride to form insoluble (NH4)2PdCl6 precipitates, thus separating the palladium in the waste solution from most base metals and some precious metals in the waste water. Since palladium generally exists as Pd(Ⅱ) in chloride solutions, an oxidizing agent such as HNO3, Cl2 or H2O2 must be added to the solution before precipitation to oxidize Pd(Ⅱ) to Pd(Ⅳ). Oxidizing agent using chlorine is the most convenient: H2PdCl4 + 2NH4Cl + Cl2 → (NH4) 2PdCl4 ↓ +2HCl operation, control the solution containing palladium 40 ~ 50g / L, room temperature into the chlorine for about 5min, and then according to the theory of the amount and to ensure that there is a solution of 10% of the NH4 Cl calculated to join the amount of solid NH4 Cl to continue to pass through chlorine until the Pd completely precipitated until. Then add solid NH4 Cl according to the theoretical amount and ensure that the solution has 10% NH4 Cl. Precipitation is filtered and washed with 10% NH4 Cl solution (saturated with chlorine gas) to obtain pure palladium salt. If further purification is required, the palladium salt can be dissolved by boiling with pure water: (NH4)2PdCl6 +H2O→(NH4)2 PdCl4 +HCl+HC1O (red solid) (black-red liquid) Repeat the above process after cooling, and the purer ammonium palladium chloride is obtained by calcination and hydrogen reduction to pure palladium sponge. The ammonium chloropalladate precipitation method can effectively remove impurities such as base metals and gold, but it is difficult to remove other precious metals, so the purity of palladium can hardly reach 99.9% when the content of precious metal impurities is too high. Dichlorodiammine complex palladium method is the use of Pd (Ⅱ) chlorine complex can be generated with ammonia soluble salt: H2PdCI4 +4NH4OH →Pd (NH3) 4 C12 +2HCl +4H2O and palladium solution of other platinum-group elements, gold and some base metal impurities in alkaline ammonia solution are formed hydroxide precipitation. The palladium-ammonia complex solution obtained by filtering off the precipitate is neutralized with hydrochloric acid to produce a dichlorodiammine-palladium(II) precipitate: Pd(NH3)4 Cl2 +2HC1→Pd(NH3)2 Cl2↓ +2NH4 Cl The precipitate is filtered and washed to obtain the pure palladium salt, which is then calcined and hydrogen-reduced to obtain pure palladium sponge. To obtain higher purity palladium, dichlorodiammine palladium(II) can be dissolved in ammonia: Pd(NH3)2 Cl2 +2NH4 0H →Pd(NH3)4 Cl2 +2H2O and then neutralized with hydrochloric acid. Repeated dissolution and precipitation results in a pure palladium product with a purity of 99.99% or more. Pure palladium ammonia complex solution can also be directly reduced with formic acid and other reducing agents to obtain sponge metal palladium: Pd(NH3)4 Cl2 +2HCOOH-Pd↓ +2NH3 +CO2 +2NH4 Cl Reduction of the solution at room temperature to the Xu Xu added formic acid and stirring, until all the palladium in the solution is reduced, filtered and washed with purified water can be dried after the drying. Palladium sponge is obtained after drying. About 2-3mL of formic acid is needed to reduce lg of palladium. This process is simple and has a high metal recovery rate. However, the resulting palladium sponge is fine-grained and has a low bulk density, so it is easy to fly and lose when packing and transferring. In addition, impurities such as copper and nickel in the solution will also be reduced, affecting the purity of the palladium. (2) Recovery of palladium from palladium-containing solid wastes The principle of wet recovery of palladium-containing solid wastes is similar to that of palladium-containing liquid wastes. The palladium-containing solid wastes can be transferred to solution with aqua regia, nitric acid and other reagents, and then be recovered and refined by the above methods of recovering palladium from the waste liquid. Commonly used processes include concentrated nitric acid separation method, ammonium chloride separation method and direct ammonia complexation method. Among them, the ammonium chloride separation method is used more often. Palladium-containing solid waste is dissolved in aqua regia, and the mixture is oxidized with HNO3. With NH4CL analysis (NH4) 2Pdcl6, and then use 1% to 5% of the NH4 Cl solution so that (NH4) 2PdCl6 into solution and get purified, the process flow as shown in Figure 5 - 10. Recovery of palladium from palladium-containing solid wastes (a) The principle of wet assimilation of palladium-containing solid wastes is similar to the principle of assimilation of palladium-containing liquid wastes, and palladium-containing assimilated wastes are assimilated with the above mentioned methods of assimilating palladium in the wastes for assimilation and refining after the palladium has been transferred into the solution with aqua regia, nitric acid and other reagents. Commonly used processes include the concentrated nitric acid separation method, the ammonium chloride separation method and the direct ammonia complexing method, etc. Among them, the ammonium chloride separation method is more commonly used. Among them, the ammonium chloride separation method is used more often. After dissolving palladium-containing solid waste in aqua regia, the mixture is oxidized with HNO3. Analyze (NH4)2 PdCl6 with NH4Cl, and then use 1% to 5% NH4Cl solution to make (NH4)2PdCl6 solution and get purified, the process flow as shown in the figure. Palladium-containing solid waste → burning → acid cooking with hydrochloric acid → insoluble material → aqua regia dissolution, with hydrochloric acid to drive nitrate → filtration ↓ filtrate ↓ NH4Cl precipitation ↓ with the collection of other precious metals ← insoluble material ← dissolved with 1% ~ 5% NH4Cl solution ↓ (NH4)2PdCl6 solution ↓ NFl4CI precipitation ↓ palladium sponge ← H2 reduction ← calcined palladium in waste board card recycling (1) palladium in waste board card recycling process flow The waste card is crushed in a crusher, and the solid mass of the crushed catty is put into a high-temperature roasting furnace to remove most of the organic matter. After cooling, the roasted slag is ball-milled to less than 200 mesh. Put the powder in the acid reactor, add dilute nitric acid in batches, according to the speed of reaction can be heated appropriately to ensure that the reaction is carried out at a relatively fast pace and gently. After cooling, the filtrate is filtered and put into a plastic tank to wait for the assimilation of palladium and silver. During this process, the palladium, silver, copper, nickel and other base metals enter the solution in good condition: the precious metals such as gold and platinum remain in the filtrate, which is washed until colorless. The wash water is added to the filtrate. The gold and platinum are recovered from the filtrate. The main chemical reactions are as follows: 3Pd+8HNO3→3Pd(NO3)2+2NO↑+4H2O 3Ag+4HNO3→3AgNO3+NO↑+2H2O MO+2HNO3→M(NO3)2+H2O (M=Ba, Pd, Mg, etc.) 3Cu+8HNO3→3Cu(NO3)2+2NO↑+4H2O Ni +4HNC)3→Ni(NO3)2+2NO2↑+2H2O The filtrate is silver-split. Palladium can be precipitated directly with ammonia or ammonium chloride and two fine chemicals of palladium - tetraammine palladium(II) dioxide and high purity palladium sponge - can be obtained directly in the process. From the palladium-containing waste card in the direct production of each of the fine process products of the process route shown in Figure. Waste card, pretreatment ↓ sampling analysis of palladium and impurity metal content ↓ HNO3 acid dissolution, filtration → filtrate for the recovery of platinum, rhodium, gold and other precious metals discarded ↓ filtrate plus hydrochloric acid in addition to silver, heating to drive the nitric acid, filtration → filtrate (AgCl) → recovery of silver ↓ filtrate (containing H2PdCl4) and concentrated ammonia, adjust the PH & lt; 7.5. filtration → filtrate discarded ↓ filtrate slag Pd (NH3) 4PdCl4, add concentrated ammonia to PH = 8 ~ 9, 80 ℃ → can be directly obtained tetraammine palladium dichloride (Ⅱ) ↓ Pd (NH3) 4Cl2, add hydrochloric acid acidification, PH = 1 ~ 1.5, filtration → solution discarded ↓ slag, Pd (NH3) 4Cl2 precipitation → washing. Drying to get dichlorodiammine palladium(II) products ↓ hydrazine hydrate reduction ↓ get palladium sponge products (2) leaching acid roasting slag after ball milling with nitric acid leaching. Palladium is easily dissolved in nitric acid. When dissolving, both the leaching speed and leaching rate should be considered, and attention should also be paid to economic problems. The leaching rate reaches 99% when 25% of nitric acid is used at 80℃ for 2h. After acid dissolution, the filtrate is filtered and the slag is washed. The filtrate goes on to the next step. Depending on the source of the material, the filtrate may contain platinum, rhodium and gold and other precious metals, which should be recovered with care. (3) In addition to silver, drive nitrate filtrate in the heating and stirring conditions drop acid until a small amount of liquid test no Ag + until. Settling, filtration to remove silver cyanide precipitation (further with the collection of silver), the filtrate will be heated and boiled and add a small amount of hydrochloric acid from time to time to facilitate the escape of nitrogen oxides. The solution should be transparent reddish-brown color after the nitrogen oxides are driven out. At this time, the filtrate of palladium composition for H2PdCL4, at the same time contains nitric acid soluble base metals. (4) Ammonia complexation The purpose of ammonia complexation is to remove the metal impurities in the material solution and obtain qualified dichlorotetrazolium palladium(II) [Pd(NH3)4C12] and dichlorodiammine palladium(II) [Pd(NH3)3C12] products. The solution of chloropalladite obtained after nitrate removal and filtration is heated to 80-90 °C and ammonia is added dropwise with constant stirring. The pH value of the solution was controlled to be less than 7.5, so that the palladium in the feed solution turned into flesh-red palladium tetrazolium complexed with chloropalladite Pd(NH3)?6?1 Pd C14 and precipitated. The precipitate is washed repeatedly with deionized water so that most of the base metal remains in solution and is removed by filtration. In the precipitate obtained by filtration, continue to add ammonia to PH=8~9, and continue to keep warm (80~90℃) under constant stirring for 1h, so that all the flesh-red precipitate is dissolved. At this time, the main component of the solution is tetraammine palladium(II) dichloride. Filter, take a small amount of filtrate with atomic emission spectroscopy to determine the content of impurity metal. If the content of impurity metal is lower than the specified value, the resulting light metal will be removed from the solution. If the impurity metal content is lower than the specified value, the resulting light yellow Pd(NH 3)4Cl2 filtrate is concentrated. Stop heating when a film appears on the liquid surface. Let it crystallize by natural cooling. Recrystallize the resulting light yellow product in deionized water just once to remove the free ammonia present in the crystals. The recrystallized crystals were dried in a vacuum oven (50°C). The product [Pd(NH3)4Cl2 is packed into the warehouse after passing the laboratory test. Reaction equation is as follows: 2H2PdCl4+4NH4OH→Pd(NH3)4 Pd Cl↓+4HCl+4H2O Pd(NH3)4?6?1Pd C14+4NHOH→2Pd(NH3)4C12+4H2O If the filtrate obtained from the above operation is used to determine the content of impurity metals by AES, and the result is higher than the specified value of the impurity metal content, then the solution should be controlled in a controlled manner. If the result of the filtrate obtained in the above operation is higher than the specified value of impurity metal content after determination of impurity metal content by atomic emission spectrometry, then under the condition of controlling the palladium content of the solution to be lower than 80g/L, add concentrated hydrochloric acid dropwise with stirring to the solution PH=1~1.5, at this time, a large amount of yellowish flocculent precipitate will appear in the solution. Continue stirring for 1h. After 1h of stirring, the solution was left to settle, filtered, and the precipitate was washed repeatedly with deionized water. The obtained homogeneous body is diammine palladium dioxide (Ⅱ), which is dried in vacuum (50 ℃). The dried solid is analyzed according to Pd(NH3)3C12. If it passes the test, it is packed and put into storage, and the product is obtained as palladium(II) dichloride. Since the solubility of palladium(II) dichloride in water is very small, the precipitation can be washed with water repeatedly to obtain high purity palladium(II) dichloride. Practice shows that the impurity metal content of the dichlorodiamminepalladium(II) product obtained by acidification of the intermediate product of dichlorodiamminepalladium(II) is reported to be low, and generally meets the prescribed standard. If the impurity content of the obtained dichlorodiamminepalladium(II) crystal is high, it can be purified by the following method. The following method can be adopted for purification: Dichlorodiamminepalladium(II) is dissolved in ammonia, and then acidified with hydrochloric acid to obtain dichlorodiamminepalladium(II) precipitate, and if repeated, products with a content of dichlorodiamminepalladium(II) greater than 99.9% can be obtained. (5) Preparation of palladium sponge After the above diamminedichloropalladium(II) precipitation is wetted with a small amount of deionized water, hydrazine hydrate solution is added dropwise with stirring, and the mixture is heated to 60°C. When the mixture no longer displays an obvious yellow color, the mixture is filtered. The resulting black powder is palladium sponge. The purity is generally above 99.9%. Recovery of palladium from palladium-containing solid waste (ii) Recovery of palladium from capacitors Capacitors contain a wide range of precious metals, with silver and palladium having the highest content. In the process of collecting and utilizing electronic components and waste household appliances, different types of components are usually separated and placed as far as possible during dismantling. There are many ways to recover palladium and silver from dismantled capacitors, but the following is a wet process for assimilating and refining palladium sponge from capacitors. (1) Pre-treatment and acid immersion The waste boards are crushed in a crusher, and the crushed solid blocks are roasted in a high-temperature roaster to remove most of the organic matter. After cooling, the roasted slag is ball-milled to 200 mesh or less. The powder is placed in an acid-resistant reactor, adding dilute nitric acid in batches, according to the speed of the reaction can be heated appropriately to ensure that the reaction is carried out at a faster rate and gently. After cooling, it is filtered and the filtrate is put into a plastic tank to wait for the co-collection of palladium and silver. During this process, the palladium, silver, copper, nickel and other base metals enter the solution in a good way: the precious metals such as gold and platinum remain in the filtrate, which is at least colorless in the wash water. The wash water is incorporated into the filtrate as described above. The gold and platinum are recovered again from the filtrate slag. (2) Sodium chloride silver in the filtrate add sodium chloride saturated solution, stir thoroughly, take a small amount of the upper layer of clear liquid. Dropwise addition of sodium chloride solution to test the effect of silver. After all the silver ions in the solution are converted to silver chloride precipitate, leave to settle, filter, and the filtrate is used for further extraction of palladium. The filtrate is used for further palladium extraction. The filtrate slag is mainly silver chloride. Silver chloride solid drying, with dry silver chloride quality of 60% of industrial caustic soda, 3% of industrial potassium nitrate, mixed evenly, the mixture will be placed in a graphite crucible compaction, with intermediate frequency furnace or oil furnace at about 1100 ℃ for melting, to get the content of about 98% of the crude silver, and then electrolytic purification, you can get the content of more than 99.99% of the electrolytic silver. If the silver obtained by wet purification is used. Can be obtained after the wet silver chloride (without drying). Directly add concentrated ammonia. So that the silver chloride dissolved into silver ammonia solution, filtered in the filtrate really add water table hydrazine, oxalic acid, ascorbic acid and other organic reductants, in the appropriate temperature reduction to get silver powder. Generally speaking, the purity of silver powder obtained by wet treatment of silver chloride precipitation can reach more than 99.9%. (3) Palladium precipitation by xanthanate or ammonia The filtrate after silver separation generally contains a large number of base metal ions (such as Ti3+, Mg2+, Pd2+, Cu2+, Ni2+, etc.), and it is often used to precipitate palladium in the solution by the following two methods. ①Add a certain amount of industrial sulfuric acid to the solution after silver separation, so that the lead, barium and other ions in the solution are firstly intergraded into precipitate and removed, heat the filtrate to boiling, and add a small amount of hydrochloric acid in batches to drive the nitrate. After the nitrate solution directly add yellow drug solution to precipitate palladium, rapid filtration. The filter residue is palladium xanthate precipitation. Due to the palladium xanthate precipitation of the solubility product of 3 × 10-43, than a base metal and silver xanthate precipitation of the solubility product is much smaller. Therefore, the efficiency of palladium precipitation with xanthate is very high: palladium precipitation of chlorine can reach more than 99%, palladium precipitation with xanthate is a highly efficient method of extracting palladium. ②In the solution after silver separation. Directly add industrial ammonia, so that palladium ions into flesh-colored Pd (NH3) 3 C12 precipitation, after standing and filtration and filtrate with the absolute part of the base metal ions in the filtrate to separate, Pd (NH3) 3C12 precipitation with hydrochloric acid, and then ammonia precipitation, according to the needs of the repeated times precipitation and dissolution. The main reaction of palladium precipitation by xanthic acid is as follows: Pd(NO3)2+2ROCSSNa→(ROCSS)2Pd+2NaNO3 M(NO3)2+2ROCSSNa→(ROCSS)2M+2NaNO3 (M=Ba, Pd, Mg, Cu, Ni, etc.) (ROCSS)2M+Pd(NO3)2→(ROCSS)2Pd +M(NO3)2 (4) Refinement of palladium from palladium(II) xanthate or Pd(NH)3 C12 precipitate Calcine palladium(II) xanthate precipitate at 600°C for 2h to decompose palladium(II) xanthate. The crude palladium was obtained by reduction with hydrogen. Pd(NH3)3Cl2 precipitation is dissolved with a little hydrochloric acid. Add ascorbic acid and other organic reducing agents. The reduction speed is controlled to obtain coarse palladium with larger particles. The crude palladium is dissolved in a small amount of aqua regia or nitric acid. Reduction with hydrazine hydrate results in a palladium sponge with a content greater than 99.95%. Pilot study on the recovery of palladium from waste palladium-carbon catalysts Yanxia Han and Hongxia Cao (College of Chemical Engineering, Kaifeng University, Kaifeng 475004, Henan, China) Abstract The pilot process of palladium chloride recovery from waste palladium-carbon catalysts by roasting, hydrazine hydrate reduction, hydrazine hydrate dissolution, nitroxide, ammonia adjustment, hydrazine hydrate reduction, and purification was investigated, and the optimum conditions of hydrazine hydrate dissolution were identified as follows: the temperature of 80-90 ℃, reaction time of 8 h, and palladium purification of 8.0%. The optimum conditions for the dissolution of aqua regia were determined as follows: temperature 80-90 ℃, reaction time 8 h, and the mass ratio of palladium slag to aqua regia (8.7 kg of nitric acid + 37.0 kg of hydrochloric acid) 1:8. The highest palladium yield of 97% was achieved under these reaction conditions. Keywords Palladium, palladium-carbon catalyst, recycling, Pilot-scale study on recycling process of palladium chloride using disused Pd-C catalyst Han Yanxia, Cao Hongxia. (Chemical Engineering of Kaifeng University, Kaifeng, China). of Kaifeng University,Kaifeng Henan 475004) Abstract: A recycling process of palladium chloride was expatiated in the paper, in which roasting process, deoxidizing using N2(Pd) catalyst, and deoxidizing using N2(Pd) catalyst were used. H2O, dissolving with aqua fortis, moving off nitric acid, adjusting ammonia, deoxidizing using N2H4.H2O repeatedly, refining were carried out in turn. H2O repeatedly, refining were carried out in turn. And the best dissolution condition with aqua fortis was that, in which the highest yield percentage of 97% was reached, the temperature was 80-90 ℃, the temperature was 80-90 ℃, and the temperature was 90 ℃. And the best dissolution condition with aqua fortis was that, in which the highest yield percentage of 97% was reached, temperature was 80~90 ℃, reaction time was 8 hours, and amount of aqua fortis used was 1:8 (g:g). Keywords: palladium chloride;Pd-C catalyst;recycling The hydrogenation reaction for the production of doxycycline in China's pharmaceutical industry uses a palladium-carbon catalyst. It is produced by treating powdered pharmaceutical activated carbon with palladium chloride, hydrochloric acid and reducing agent. The palladium content is in the range of 1% to 2% by mass. Upon completion of the hydrogenation reaction, the catalyst is deactivated and needs to be replaced with a new catalyst once a day [1]. Together with other product requirements, the amount of palladium catalyst used is very high[2] . Currently, domestic palladium resources are limited and production is very small, far from meeting the demand. Most of them are still imported. Therefore, the treatment of spent palladium catalysts to recover the precious metal palladium is of great significance in solving the shortage of palladium resources [3-5]. There are various methods to recover palladium from spent palladium-carbon catalysts, such as aqua regia, oxidative roasting, hydrochloric acid leaching, caustic soda leaching, and incinerator system [6-8]. In this paper, we optimize the process and conduct a pilot study on the recovery of palladium from the waste palladium-carbon catalyst produced in the production of doxycycline. 1 Process flow of recovering palladium chloride from waste palladium-carbon catalyst The process flow of recovering palladium from waste palladium-carbon catalyst is as follows: waste palladium-carbon catalyst → roasting → hydrazine hydrate reduction → aqua regia dissolution → nitroxide driving → ammonia adjustment → hydrazine hydrate reduction → palladium sponge refining. 1.1 Roasting The deactivated palladium-carbon catalyst is firstly ground into 100 mesh fine powder. Soak it in 90 ℃ hot water for 1 h. Filter and dry it to remove the impurities. The catalyst was then roasted in a muffle furnace at 550~600 ℃ for 2 h to remove the organic impurities. 1.2 Hydrazine hydrate reduction Weigh 7.5 kg of palladium charcoal after cultivation and soaking in appropriate amount of water, add 300 g of sodium hydroxide, and then raise the temperature, the temperature rose to 80 ℃, while stirring and slowly add 7.5 L hydrazine hydrate. After holding for 3 h and cooling naturally, when the temperature is reduced to about 30 ℃, the upper layer of clear liquid was sucked out, and then add appropriate amount of purified water to wash the palladium slag, repeat the above operation for 4~5 times, and wash the palladium slag to near neutral. 1.3 Dissolving in aqua regia Transfer the palladium slag to the nitrification kettle and add the prepared aqua regia dropwise. Increase the temperature to about 80 ℃ and time the reaction for 3 h. Preparation of aqua regia: (1) Ratio 1, nitric acid is reagent nitric acid, 8.7 kg nitric acid + 37.0 kg hydrochloric acid; (2) Ratio 2, nitric acid is fuming nitric acid, 6.3 kg nitric acid + 39.0 kg hydrochloric acid. The recovery of palladium mainly depends on the operating conditions of aqua regia dissolution, so the appropriate reaction temperature, reaction time and the amount of aqua regia were determined experimentally. 1.3.1 The effect of reaction temperature on the recovery of palladium Under the conditions of reaction time of 8 h and the mass ratio of palladium slag and aqua regia (ratio 1) of 1:8, the change of palladium recovery with reaction temperature is shown in Fig. 1. From Fig. 1, it can be seen that, when the reaction temperature is lower than 60 ℃, palladium can not be dissolved by aqua regia due to the slow reaction speed and the palladium recovery is only about 86%. When the reaction temperature is 80-90 ℃, the palladium recovery rate can be increased to about 97%. Therefore, the appropriate reaction temperature should be 80-90 ℃. Figure 1 Effect of reaction temperature on palladium recovery 1.3.2 Effect of reaction time on palladium recovery Under the conditions of reaction temperature of 85 ℃ and the mass ratio of palladium slag to aqua regia (ratio 1) of 1:8, the change of palladium recovery with the reaction time is shown in Fig. 2. Fig. 2 Effect of reaction time on palladium recovery From Fig. 2, it can be seen that the palladium recovery increases with the increase of reaction time. After 8 h, the reaction is basically complete, and prolonging the reaction time will not increase the palladium recovery rate. Therefore, the appropriate reaction time should be 8 h. 1.3.3 The effect of the amount of aqua regia on palladium recovery Under the conditions of reaction temperature of 85 ℃ and reaction time of 8 h, the effect of the amount of aqua regia (ratio 1) on the recovery of palladium is shown in Fig. 3. Fig. 3 The effect of the amount of aqua regia on palladium recovery According to the stoichiometric equation, the theoretical mass ratio of palladium slag to aqua regia is 1:6. However, from Fig. 3, it can be seen that at this time, palladium recovery is only about 84%. However, as can be seen from Fig. 3, the palladium recovery rate is only about 84% at this time. This is because the reaction speed is too slow in the late stage of the reaction due to the low amount of aqua regia, and the palladium cannot be leached out completely. When the amount of aqua regia is too high and the mass ratio of palladium slag to aqua regia is 1:8, the reaction is complete and the palladium recovery reaches a high level. Through the study of the reaction temperature, reaction time and the amount of aqua regia, the optimal conditions for the dissolution of aqua regia were found to be: the temperature was 80-90 ℃, the reaction time was 8 h, and the mass ratio of palladium slag to aqua regia was 1:8, and the highest palladium recovery was achieved at 97% under the reaction conditions. 1.4 Catching nitrate After the palladium slag is dissolved in aqua regia, 3 L of concentrated hydrochloric acid is added to catch nitrate each time, and the process is repeated for 4-5 times, with the end point that no more reddish-brown gas is produced after the addition of concentrated hydrochloric acid. End of the nitrate, add 10 kg of purified water to drive hydrochloric acid, then add 50 kg of purified water, filtration, filter cake with about 1% (v/v) of hydrochloric acid washed twice and stored, the filtrate is transferred to the ammonia adjustment kettle. 1.5 Adjustment of ammonia Slowly add ammonia to adjust pH=8.7~8.8, after 10 min, re-measure the pH is unchanged. Increase the temperature to 80 ℃, hold for 30 min and then filter while hot, the filter cake is washed with 10 L of purified water and then stored separately, the filtrate is adjusted with concentrated hydrochloric acid pH = 1.0 ~ 1.5 (during the adjustment of acid, open the jacket cold water to cool down, and control the temperature of filtration does not exceed 30 ℃). Stir for 10 min and check the pH, then stir for another 30 min, then filter. 1.6 Hydrazine hydrate reduction: Mix the yellow filter cake with 30 L of purified water and pump it into the reduction kettle, slowly add 6 L of hydrazine hydrate (control the speed of dropping to avoid flushing the material), the amount of hydrazine hydrate is subject to the material in the kettle turns black and the supernatant becomes clear, then stir for 30 minutes, then it can be filtered and palladium sponge can be obtained. 1.7 Palladium sponge refining Put the filtered palladium sponge into the refined palladium nitrification kettle, add aqua regia (ratio 2), raise the temperature to 80 ℃, after 1 h, use a small amount of concentrated hydrochloric acid to drive the nitrate, 2 L each time, about 4 to 5 times, add 5 kg of purified water to drive the hydrochloric acid, add water, and then discharged. The final amount of water is suitable to put down the material and clean the kettle and pipeline as little as possible. 2 CONCLUSION Palladium was recovered from waste palladium-carbon catalysts by this pilot process. Through the study of reaction temperature, reaction time and the amount of aqua regia, the optimal conditions for the dissolution of aqua regia were found to be as follows: the temperature was 80-90 ℃, the reaction time was 8 h, and the mass ratio of palladium slag to aqua regia (8.7 kg of nitric acid + 37.0 kg of hydrochloric acid) was 1:8. The highest yield was obtained under these conditions, and the palladium yield reached 97%.