Title of Invention

"A PROCESS FOR RECOVERY OF PALLADIUM AS PALLADIUM CHLORIDE FROM SPENT SILICA"

Abstract

The present invention provides a process for the recovery of palladium as palladium chloride from spent silica. This invention relates to two alternative routes for the recovery of Pd from spent silica. In the first route, spent silica is thermally treated in air to partially burn the organic moiety of the complex followed by palladium extraction by acid digestion and purification by judiciously adjusting the pH of filtrate obtained. Recovered silica retains its structure and can be re-used. Alternatively, palladium-phthalocyanine complexes from silica are extracted by treating it with aqueous HF. Silica dissolves as H2SiF6, which is a value-added product and palladium complex is filtered off. Subsequently, Pd is recovered as PdCl2 in aqueous phase by acid digestion of insoluble phthalocyanine complexes and judiciously adjusting the pH of filtrate obtained.

Full Text

A PROCESS FOR RECOVERY OF PALLADIUM AS PALLADIUM CHLORIDE FROM SPENT SILICA Field of the invention The present invention relates to the recovery of precious metals from spent catalysts or adsorbent or inorganic waste. More particularly, the present invention relates to a process for the recovery of palladium from silica used as an adsorbent in column chromatography or as a catalyst support in many catalysts. Background of the invention Precious transition metal ions and their coordination complexes find industrial applications as supported catalysts and performance chemicals in fine chemicals industries. For example metals like Ag, Au, Pd, Pt and Rh are used in a variety of industrial applications as catalysts for oxidation, hydrogenation and dehydrogenation reactions. Various polymeric materials, modified silica, zeolite or /and various clay materials are used as support for these metals. Similarly, coordination metal complexes of Pd, Pt, Ru and Rh are used as commercial catalysts in homogeneous conditions for hydroformylation and hydrogenation reactions. However, owing to difficulties in separation of metal complexes from product mixtures, there is growing need to developed heterogenized catalysts where coordination metal complexes are supported on polymeric or inorganic solid support like silica, carbon, zeolite, and alumina. Commercially, it is important to recover the precious metals from the support to the maximum extent possible once the catalyst is deactivated. In certain situations, the catalysts are degraded during the reaction cycle or subsequent work-up and the reaction effluent may comprise of various remnants, e.g., complexes of various valence states for the metal ions. In many cases, the presence of excessive contaminants reduces the feasibility of recycling of the catalyst. However, the problem related to degradation of catalysts over the repeated cycles, contaminants arise from side reaction and leaching of the effective catalyst for the solid support are common in some cases of heterogeneous catalysis. In view of the environmentally stipulated restrictions on disposal of metal containing waste, effective recovery of the residual precious metals from the reaction effluent is of paramount importance for a process to be environmentally acceptable and economically viable. Furthermore, during the recovery of precious metals from ore or scrap including spent catalysts, the use of solvent extraction to separate the precious metals from one another and from base metals that may also be present is becoming more widespread. The hydrometallurgical processes employed for the separation and recovery of the platinum group metals, (e.g., platinum, palladium and rhodium), typically involve dissolving the metal ions by some type of oxidative acidic chloride leach, typically with aqua regia or hydrochloric acid/Ch followed by solvent extraction. The application of coordination complexes of precious metals in fine chemicals industries also requires prior purification of these complexes using silica gel as a stationary phase. For example Pd-phthalocyanine complexes namely Palladium phthalocyanine complex, Bromo phthalocyanine complex and Irgaphor green, which find applications as pigment material for compact disc coating, are purified using silica gel. Significant quantity of these complexes (up to 0.5 wt% Pd) is adsorbed in the pores of silica gel, and is difficult to dislodge from the pores of silica (spent silica) by conventional elution with a solvent. Many technologies, at times specific to support and metal, are used for recovery of precious metals from spent catalysts, effluent solutions. Reference is made to British Patent No. A-2127001, 1984 wherein precious metals are recovered rapidly and efficiently from cyanide containing leach solution by loading onto an activated carbon fibre body. The main limitation of the process is that cyanide ligand should have high affinity and should form stable complexes with the metal ions to be leached out. Reference is made to CS-B-251467 (1988); Chemical Abstract 109 (24): 213964 wherein palladium is recovered from acidified wastewater by sorption on activated carbon, pre-treated with an alkali metal salt of Ethylenediaminetetraacetic acid (EDTA). However, this is useful for homogenous aqueous catalyst waste and is not directly applicable to non-aqueous systems. Reference is made to Japanese Patent JP 54 9597 (1978) wherein the regeneration and recovery of precious metal like palladium involves a burning step. It is disclosed in that patent, that the spent residue obtained from a catalysed reaction performed in homogeneous conditions is subjected to heating to burn out the organic fragment and then by subsequent dissolution of the palladium species in acid to the corresponding palladium salt. However, this invention has a limited scope in the sense that it is only concerned with degradation of metal complex having lower aliphatic mono-carboxylic acids as the organic moiety. Reference is made to US Patent 4,331,634 (1982) wherein strongly acidic solution of sulphuric, hydrochloric, perchloric or nitric acid is used as stripping solvent for the extraction of the palladium from the organic acidic solution containing oxime as extracting reagent. The organic phase may also contain an anionic phase transfer material or catalyst to aid the extraction process. In this patent, the oximes employed for the extraction of palladium are hydroxyoxime and derivatives, which are expensive considering the industrial application. Reference is also made to US Patent 4,578,250 (1986) wherein aqueous ammonia is used as the stripping solution and along with oxime as an extracting solvent similar to those of the U.S. Patent 4,331,634 (1982) but here oxime used is ether oxime, in which the hydroxy group of the oximes are converted to ether groups. In this case also, the oximes employed for the extraction of palladium are expensive and not user friendly considering the industrial application. Furthermore, it is reported that after recycling certain oxime containing solvents over a period of time (e.g., ten extractions of palladium followed each time by stripping with 6M hydrochloric acid solution), the rate of palladium extraction deteriorates considerably. Reference is made to Japanese Patent JP Patent 61-238, 927 (1996) wherein palladium is recovered by extraction using an aldoxime. The main limitation of these processes is the use of expensive organic ligands and organic solvents, which are difficult to recover. Y. Baba, K. Inoue, K. Yoshizuka and T Furusawa, (Industrial Engineering Chemistry-Reearch, Volume 29, (1990), page 2111) describe the use of non-cheating oximes such as dodecanal oxime, decanal oxime, octanal oxime and hexanol oxime for the extraction of palladium metal. This report also has drawback in the sense that the oximes employed for the extraction are expensive considering the industrial application Ion exchange resins can also be used to recover precious metal ions, but it is difficult to achieve 100% metal recovery in this method and all precious metals are not also not present in the ionic state or too much of contaminants is present in the effluents to be treated. According to above prior art, most of the metal recovery methods are applicable to the homogeneous reaction mixtures and employs chelating agents for extracting metal ions. Process known in the prior art for recovery of metals fromsupported catalysts also makes use of mineral acids and invariably destroyed the support structure. Objects of the invention The main object of the invention is to provide a process for the recovery of precious metals from spent catalysts or adsorbent or inorganic waste, which obviates the drawbacks as detailed above. Another objective of the invention is to provide a process for the recovery of precious metals or their complexes supported on solid support as in adsorbent/catalysts, specially palladium from spent silica wherein palladium is present as a mixture of palladium phthalocyanine complexes adsorbed on silica gel. Yet another objective of the invention is to provide a process for enabling 100% recovery of palladium with more then 99% purity. Yet another objective of the invention is to recover phthalocyanine from silica along with Pd metal. Yet another objective of the invention to provide a process wherein silica is recovered as a value added product that can be reused. Summary of the invention Accordingly, the present invention provides a process for the recovery of palladium as palladium chloride from spent silica, comprising reacting the spent silica with an oxidising agent or hydrofluoric acid at a temperature in the range of 350-450°C for 3-5 hours to dissolve silica to form H2SiF6 and palladium phthalocyanine complexes recovered by filtration followed by extraction of palladium by reacting with 2 to 6 molar aqueous hydrochloric acid at a temperature in the range of 70-90°C under constant stirring for 4-6 hours followed by further recovery and purification of palladium metal from filtrate by precipitation of the metal salt at pH in the range of 5 to 10.5 and recovering phthalocyanine ligands by solvent extraction using organic solvent selected from chloroform and dichloromethane and dried at a temperature in the range of 80 to 110°C to obtain palladium chloride . In one embodiment of the invention, the silica is recovered by cooling the oxide or H2SiF6 by cooling to ambient temperature and filtration and washing and then drying at a temperature in the range of 80 to 110°C. In another embodiment of the invention, the palladium metal salt is recovered from the filtrate by precipitation of the metal salt at different pH in the range of 5 to 10.5 and then purified. In another embodiment of the invention, the spent silica is treated with hydrofluoric acid to dissolve silica to form H2SiF6 and palladium phthalocyanine complexes recovered by filtration followed by extraction of palladium by digestion with aqueous hydrochloric acid followed by further recovery and purification of palladium metal from filtrate by precipitation of the metal salt at different pH in the range of 5 to 10.5 and recovering phthalocyanine ligands by solvent extraction. In a further embodiment of the invention, the aq. HC1 solution used is a 2 molar solution. In another embodiment of the invention, the pH is maintained in the range of 6 to 9.5. In another embodiment of the invention the acid used to digest palladium is selected from the group consisting of hydrochloric acid, sulphuric acid and nitric acid. In another embodiment of the invention, the silica is at least partially oxidised by heat treatment in air at a temperature in the range of 350-450°C for 3-5 hours. In yet another embodiment of the invention, the palladium metal is recovered as a salt by digesting it from the spent silica using a mineral acid selected from the group consisting of hydrochloric acid, sulphuric acid and nitric acid in the range 2- 6 molar aqueous solution at a temperature in the range of 70-90°C under constant stirring for 4-6 hours. In another embodiment of the invention, the recovered silica is washed and then dried in an in oven at a temperature in the range of from 80 to 110°C. In another embodiment of the invention, the spent silica starting material is treated with hydrofluoric acid to dissolve silica to form H2SiF6 and palladium recovered as palladium phthalocyanine complexes by filtration followed by extraction of palladium by digestion with 2 molar aqueous hydrochloric acid and further recovery and purification of palladium metal from filtrate by precipitation of the metal salt at different pH in the range of 6 to 10.5 and phthalocyanine ligands recovered by solvent extraction. In another embodiment of the invention, spent silica-containing palladium in the range 0.1 to 0.5 wt% is calcined in air in the temperature range 350 to 450°C for 4-6 hours. In another embodiment of the invention calcined silica is refluxed with 2-6 molar aqueous hydrochloric acid solution for 4-6 hours to leach out palladium. In yet another embodiment of the invention silica is filtered and the pH of the filtrate is maintained in the range 9.5 to 10.5 to precipitate palladium as palladium chloride. In a further embodiment of the invention silica is dissolved in 40% hydrofluoric acid under stirring as FfcSiFe and palladium phthalocyanine complex, which is insoluble in hydrofluoric acid is recovered by filtration. In yet another embodiment of the invention suspended Pd-complex is recovered by solvent extraction process using organic solvent selected from chloroform and dichloromethane. In a further embodiment of the invention extracted palladium phthalocyanine complex is de-metalled using aqueous hydrochloric acid under reflux conditions and then phthalocyanine ligands recovered by solvent extraction using an organic solvent selected from chloroform and dichloromethane and palladium recovered as palladium chloride from aqueous phase by precipitation. In another embodiment of the invention, phthalocyanine fragments remaining in organic phase are recovered by distillation. In yet another embodiment of the invention chloroform and dichloromethane used are also recovered by distillation. Detailed description of the invention Silica is commercially used for purification of palladium phthalocyanine complexes as stationary phase. During this process up to 0.5% of palladium is trapped in the pores of silica and is not recovered by conventional techniques like solvent extraction. However owing to high cost of palladium, it is imperative to recover trapped palladium from silica. In the process invented, spent silica is thermally treated in air to partially burn the organic moiety of the palladium phthalocyanine complex followed by palladium extraction by acid digestion and purification by judiciously adjusting the pH of filtrate obtained. Recovered silica retains its structure and can be re-used. Analysis of Pd in the silica and after its recovery was done by ICP - Inductively Coupled Plasma Emission Spectrometer (ICP-spectrometer) and Spectrophotomertically as [PdI4]2" complex. Palladium is estimated in acidic aqueous solution as brown- red complex, [PdLj]2". In an acid medium (HC1, fySC^) containing excess of iodide, palladium forms a brown- red complex, [PdL»]2' The molar absorptivity of the complex is 1.02x10"* at Xmax = 410 nm (a=0.096). A calibration curve is obtained by preparing a palladium iodide complex solution by dissolving known amounts of palladium chloride and other reagent, like concentrated hydrochloric acid, ascorbic acid and potassium iodide. The detailed procedure is as follows: Accurately measured volume/weight of the sample solution/solid containing (w/v) not more than 1 mg of Pd was taken in a 100ml volumetric flask. To this was added 10 ml of 6N hydrochloric acid, 20 ml of 20% (w/v) potassium iodide solution, and 4ml of 1% (w/v) ascorbic acid solution. Volume was made up to 100ml with water in a 100ml volumetric flask, and absorbance at 410nm against water was measured Wt of Palladium in the diluted solution can be obtained as: Wt of Pd per 100 ml = (A/e). (Mol. Wt of Palladium) where A = Absorbance of the 100 ml solution at 410 nm and e = Extinction coefficient, 1.02e4 mor'cm"1 for [PdU]2 species at 410 nm Palladium is also estimated spectrophotomertically using inductively coupled plasma emission spectrometer (ICP). This is most accurate method of estimation of metals ions at PPM/ppb levels. A calibration curve was obtained by dissolving known amount of palladium chloride in hydrochloric acid and recoding its intensity on ICP spectrometer for palladium at 340.458 nm, using Perkin Elmer's Inductively Coupled Plasma Emission Spectrometer. a) Thermal treatment of spent silica at an appropriate temperature such that organic moiety is only partially oxidized without the formation of metal oxide. Scientifically speaking, during thermal treatment, palladium phthalocyanine complex, which is trapped in the pores of silica probably, are desorbed and organically rich phthalocyanine moiety is partially oxidised to volatile compounds. Partially oxidized palladium complex gets deposited on the external surface of silica and can easily react with mineral acid used for digestion. The major inventive steps involved in the present invention is selectively partially oxidizing palladium phthalocyanine complex so that it does not form palladium oxide which is difficult to react with mineral acid under normal processing conditions. b) Recovery of pure palladium with 99-100% purity and 95-100% recovery as palladium chloride by judicially adjusting the pH of the aqueous solution and thereby removing the impurities namely iron, zinc, nickel and silica c) Recovery of pure palladium without resorting to the use expensive and hazardous organic solvent and complexing ligands. d) Recovery of pure palladium without destroying structure of the inorganic support namely silica in the present case. The following examples are given by way of illustration and therefore should not be constructed to limit the scope of the present invention. Example 1 SOOg of spent silica, wherein palladium is present as a mixture of palladium phthalocyanine complexes adsorbed on silica gel, was taken in a china dish and kept at 400°C in a muffle furnace in air atmosphere for three hour and than allowed to cool to ambient temperature. 1000 ml 2M aqueous hydrochloric acid solution was taken in a 5 litre round bottom flask equipped with heating mantle and thermocouple to heat the content of the round.bottom flask at constant temperature. To this was added above treated silica and the whole content was refluxed at 90°C for five hours. It was filtered with Buckner funnel after cooling to ambient temperature followed by washing with 0.5 M aqueous hydrochloric acid was dried at 100°C in an oven. The pH of the filtrate, which contained palladium, was raised to 6 and precipitate formed filtered to remove impurity especially of silica, iron and other metals. The precipitates were collected and dried. The pH of the filtrate obtained at pH 6 was raised to 9.3 and precipitate collected and dried. The precipitates obtained at pH 6 and 9.3 were analysed for palladium content. 0.745 g, of the precipitate was obtained at pH 6, which contains no palladium. 0.228g of precipitate collected at pH 9.3 contains 98.67% palladium determine spectrophotomertically. The total palladium content adsorbed on the silica, estimably separately using known procedure of repeatedly treating with aqua regia and hydrochloric acid finally estimating Pd by ICP spectrometry) was 0.23 g showing 99- 100% recovery by above process. Example 2 SOOg of spent silica was taken in a china dish and kept at 700°C in a muffle furnace in air atmosphere for three hour and than allowed to cool to ambient temperature. 1000 ml 2M aqueous HC1 solution was taken in a 5 litre round bottom flask equipped with heating mantle and therr ^le to heat the content of the round bottom flask at constant temperature. To this v, oa added above treated silica and the whole content was rcfluxed at 90°C for five hours. It was filtered with Buckner funnel after cooling to ambient temperature followed by washing with 0.5M aqueous hydrochloric acid solution. Silica was dried at 100°C in an oven. The pH of the filtrate, which contained palladium, was raised to 6 and precipitate formed filtered to remove impurity especially of silica, iron and other metals. The precipitates were collected and dried. The pH of the filtrate obtained at pH 6 was raised to 9.3 and precipitate collected and dried. The precipitate obtained at pH 6 and 9.3 were analysed for palladium content. 0.74Sg, of the precipitate was obtained at pH 6 contains nil palladium, 0.226g of precipitate collected at pH 9.3 also contains nil palladium and 0.25g of the precipitate collected at pH 11 also contains nil palladium as determined spectrophotomertically. Example 3 The above experiment was repeated except that the silica was calcined at 300°C There was nil recovery of palladium from precipitates obtained after adjusting pH a different value of 6,9, and 11 of the filtrate obtained after acid digestion. Example 4 500g silica waste was calcined at 400°C in a muffle furnace in air atmosphere for three hour. After cooling the sample to ambient temperature it was digested with 1000 ml 2M aqueous hydrochloric acid solution. Filtered and washed the content to remove silica and finally filtrated was evaporated it to dryness on water bath / hot plate to get solid mass. It gave 2.07 g inorganic solid, which contained 122.5 mg of palladium chloride. Example 5 SOOg silica waste was calcined at 400°C in a muffle furnace in air atmosphere for three hours and after that it was digested with 1000 ml aqua regia (750 ml concentrated hydrochloric and 250 ml concentrated nitric acid). The content was filtered and pH of the filtrated was adjusted to 6 and filtered again. It gave 208 mg brown coloured solid with nil PdCb content. The pH of the filtered was raised to 9.5 and again filtered. It gave 740 mg solid, again with nil palladium in it. Example 6 lOOg silica wastes as such was treated with 100 ml aqua regia (75 ml c ^centrated hydrochloric acid and 25 H concentrated nitric acid) and mixtir^ kept for 3-4 hour, then decanted and the filtrate was analysed for Pd spectrophotomertically. It gave no test for palladium. Example 7 770 g of spent silica (crude deep green in colour) was added to 3500g HF (40%) and stirred by magnetic stirrer. SiOa dissolves in HF solution as F^SiFe and palladium phthalocyanine complex, being insoluble in aqueous HF media remain suspended. Palladium phthalocyanine complex (1.950g) was recovered from aqueous HF media by gravity filtrations and washed with water. Finally it was extracted with chloroform. Chloroform was removed on rotaevaporater. It gave 1.95 g of green colour palladium phthalocyanine complex. Example 8 lOOg of spent silica (crude deep green in colour) was added to 500g HF (40%) and stirred by magnetic,stirrer. SiO2 dissolves in HF solution as H2SiF6and palladium phthalocyanine complex, being insoluble hi aqueous HF media remain suspended. Palladium phthalocyanine complex (200 mg) was recovered from aqueous HF media by gravity filtrations and washed with water, and dried. Palladium phthalocyanine complex so recovered was refluxed with 100 ml 2M hydrochloric acid for 2 hour, than it was filtered to remove phthalocyanine. Palladium was recovered as PdCb (39 mg) by evaporation of aqueous acidic solution. Example 9 500g of spent silica was taken in a china dish and kept at 400°C in a muffle furnace in air atmosphere for three hour and than allowed to cool to ambient temperature. 1000 ml 2M aq. HC1 solution was taken in a 5 litre round bottom flask equipped with heating mantle and thermocouple to heat the content of the round bottom flask at constant temperature. To this was added above treated silica and the whole content was refluxed at 90°C for five hours. It was filtered with Buckner funnel after cooling to ambient temperature followed by washing with 0.5M aqueous HC1 solution. Silica was dried at 100°C in an oven. Recovered silica following heat treatment and acid digestion showed no difference in XRD, IR and surface area with respect to original silica. Example 10 10 kg calcined spend silica (calcination was done in indirect fixed calciner, which can calcine the material at 400 to 700 °C in a continuous process at 2 - 10 kg per hour rate depending upon the residence time was placed in a 50-liter capacity round bottom flask equipped with heating and stirring facilities. To this was added 3.331iter of concentrated hydrochloric acid and 16.5 litre of de-ionised water. The content of flask was refluxed at 90°C for 5 hour. After it cools down, it was filter off with Buckner funnel to remove silica and the filtrate was collected in a container. The silica was washed with de-ionised water and the washing was mixed with the filtrate. The pH of the filtrate containing Palladium was adjusted to 6.0 with sodium hydroxide and it was filtered to remove non-palladium metal impurities. Again the pH of this filtrate is adjusted to 9.3- 9.5 with sodium hydroxide solution, wherein palladium metal is precipitated as palladium chloride. This was filtered off and washed with de-ionised water till it is chloride free. The precipitate after washing are dried in oven night and analysed for palladium content. If required the sample can be further purified by re-dissolving the impure Pd containing sample in slightly warm - 1M HCI solution and then adjusting its pH to 6.3 and filtering off the impurity (if any) and then finally precipitating the pure PdCb at pH 9.3. By the above procedure, it gave 12.5g-palladium chloride of 97-99% purity. The main advantages of the present invention include: a) Thermal treatment of spent silica at reasonably lower temperature of less than 450°C b) Recovery of palladium with 99-100% purity and 95-100% recovery as palladium chloride. c) Recovery of palladium without resorting to the use expensive and hazardous organic solvent and complexing ligands. d) Recovery of pure palladium without destroying structure of the inorganic support silica. e) Possibility of recovering expensive phthalocyanine ligands as well as silica as a value added product. We claim: 1. A process for the recovery of palladium as palladium chloride from spent silica, comprising reacting the spent silica with an oxidising agent or hydrofluoric acid at a temperature in the range of 350-450°C for 3-5 hours to dissolve silica to form H2SiF6 and palladium phthalocyanine complexes recovered by filtration followed by extraction of palladium by reacting with 2 to 6 molar aqueous hydrochloric acid at a temperature in the range of 70-90°C under constant stirring for 4-6 hours followed by further recovery and purification of palladium metal from filtrate by precipitation of the metal salt at pH in the range of 5 to 10.5 and recovering phthalocyanine ligands by solvent extraction using organic solvent selected from chloroform and dichloromethane and dried at a temperature in the range of 80 to 110°C to obtain palladium chloride . 2. A process as claimed in claim 1, wherein the acid used to digest palladium is selected from the group consisting of hydrochloric acid, sulphuric acid and nitric acid. 3. A process as claimed in claim 1, wherein the spent silica-containing palladium in the range 0.1 to 0.5 wt% is calcined in air in temperature range 350 to 450°C for 4-6 hours. 4. A process for the recovery of palladium as palladium chloride from spent silica substantially as herein describe with reference to examples accompanying this specification.

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2304-DELNP-2004-Abstract-(20-07-2010).pdf

2304-delnp-2004-abstract.pdf

2304-DELNP-2004-Claims-(20-07-2010).pdf

2304-delnp-2004-claims.pdf

2304-delnp-2004-Correspondence Others-(09-05-2012).pdf

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2304-delnp-2004-Form-1-(09-05-2012).pdf

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2304-DELNP-2004-Petition 137-(20-07-2010).pdf