ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT
Refining of Platinum and Rhodium y an Ion Exchange Process CHARLES K. BUTLER Research, Developmenf, and Engineering l o b o r a f o r i e s , Owens-Corning Fiberglos C o r p . , Newark, Ohio
P
LATINUM-rhodium alloys are frequently used as materials of construction of devices for the manufacture of fibrous glass. As such they are subject to corrosive action of molten glass, combustion atmospheres, high temperature, and base metal contaminants and must frequently be recovered and purified. Alloys of platinum and rhodium have been chemically refined in the past by oxidation of the metals to chloroplatinic and chlororhodic acids and precipitation of the respective aminechloride salts. A partial separation occurs and it then becomes necessary t o complete the separation if the separated materials are to be used in the preparation of a definite alloy of platinum and rhodium. It would obviously shorten the work and time required if the base metals could be separated without disturbing the ratio of platinum t o rhodium. The proposed method, making use of ion exchange resins, removes the base metals by cation exchange but allows the platinum and rhodium to pass through as anionic chloride complexes. MacKevin and Crummett ( 2 ) have demonstrated that chloroplatinate, (PtClo)--, and chlororhodate, (RhCls)---, anionic complexes do not appreciably dissociate and are not adsorbed on strong-acid cation exchange resins, The conditions for quantitative adsorption of simple cations on cation exchange resins are well hnown and widely reported in the literature (3). Thus, it was believed that a process could be derived based on these principles. A satisfactory process, described graphically in Figure 1, involves the dissolution of the contaminated alloy, conversion of the precious metals to anionic chloro- complexes, separation of the base metal cations fiom the precious metal anions by cation exchange, and reduction of the mixture of platinum and rhodium salts simultaneously to the metals with hydrazine. The refined metals are obtained in their original ratio. Apparatus and materials
Synthesis of Contaminated Alloys. A study of experience in refining platinum-rhodium alloys in the laboratory has revealed the following contaminants t o be most prevalent: iron, copper, nickel, lead, chromium, zinc, cobalt, manganese, aluminum, and the alkaline earth metals. The total impurities seldom exceed 1% by weight. Before reworking, the metal must be totally refined t o a minimum of 99.90% precious metals. In order t o avoid a progressive build-up of impurities in the entire precious metal stock, a quantitative separation of the contaminating metals from the precious metals is desirable. If an analytical separation of alloys composed of 96% precious metals and 4% base metals could be achieved, the purification requirements for a new process would be more than met. Stock solutions of the precious metals and the base metals were prepared by dissolution in mineral acids. The contaminated alloys were synthesized by combining predetermined volumes
April 1956
of the two solutions prior to the first evaporation in the chlorideconversion procedure. STOCKCONTAMINATING SOLUTION.Pure metals were used t o prepare the solution for contaminating the precious metals. Equal weights of copper, nickel, and manganese we1 e dissolved in dilute nitric acid. The same quantities each of iron, chromium, lead, zinc, cobalt, and aluminum were dissolved in dilute hydrochloric acid. The two solutions were combined and the concentration of weight of metal per unit volume was calculated. Alkaline earth metals were eliminated from the stock contaminating solution, as it was believed that they would adsorb quantitatively on Dowex 50. STOCKPRECIOUS METALSOLCTION. The stock precious metal solution was made by dissolving 96-gram samples of purified platinum-rhodium alloy with aqua regia. The composite base metalprecious metal solution was made by adding a volume of stock contaminating solution, which was equivalent to 4 grams of reduced base metals, t o each 96-gram sample of precious metal solution. The efficiency of adsorption of base metal cations from the composite solution was studied as a function of influent base metal concentration. For this experiment, solutions containing 3 and 12% base metals were used. §election of Ion Exchange Resin. Originally, the intent was to adsorb the precious metals as anions on an anion exchange resin, leaving the contaminating metals as cations in the effluent wastes. Chloroplatinate and chlororhodate anions adsorb quantitatively on either Amberlite-IR4B or Dowex 2. An attempt was made to develop an elution technique that would result in a fairly concentrated platinum and rhodium effluent. A solution of ammonium chloride and ammonia water was used as an eluent; in accordance with the observations of MacNevin and Crummett ( d ) , the platinum eluted inefficiently as a complex amine cation. Rhodium could not be detected in the effluent. As the platinum and rhodium complexes could not be quantitatively eluted, efforts were directed toward separation of the base metals from the precious metals by cation exchange. For this work, Dowex 50, a nuclear-sulfonic, strong-acid, cation exchange resin was used. Carboxylic-type resins were not tried because the p H of the influents were by necessity t o be in the order of unity. The Dowex 50 resin was procured from the Dow Chemical Co., Midland, Mich. Construction of Column System. The concentration of platinum and rhodium in solutions t o be processed was arbitrarily set a t 25 grams per liter. The value of these solutions is approximately $80 per liter. Consequently, it was necessary to design a column system that was shatterproof, chemically inert, and transparent. A 52.5-inch section of Lucite tubing, 1.5 inches in outside diameter, with walls '/a inch thick, was used for the main column. Base plates for the top and bottom were machined from '/z-inch Plexiglas sheet. The bottom plate was permanently sealed t o the column with a methyl methacrylate-acetone
INDUSTRIAL AND ENGINEERING CHEMISTRY
71 1
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT paste and the top was sealed with stopcock grease. The column was fitted with influent feed and internal pressure release through the top plate, effluent outlet and backwash inlet through the bottom plate, and an overflow through the side of the column 2 inches from the top. The overflow was also a backwash outlet. All five fittings were made of 2-inch lengths of Lucite tubing 1/4 inch in outside diameter, with walls I/IP, inch thick. The column was tied into the various sources with transparent plastic tubing. The resin was retained in the column with graded, crushed borosilicate glass. Experimental procedure removes base metal contaminant by cation exchange
Synthesis of Contaminated Alloy Solution. The precious metal solution, having been deliberately contaminated with 4 &.eight 70 of base metals, is heated to 62' C. A sufficient quantity of solid sodium chloride is added stoichiometrically t o the chloroplatinic and chlororhodic acids to result in a minimum of interference with the adsorption of base metal cations. The solution is evaporated to dryness and 3 ml. of concentrated hydrochloric acid per gram of platinum-rhodium alloy are added. The solution is evaporated t o dryness again. The hydrochloric acid treatment is repeated twice, with evaporations to destroy the nitric acid. The final evaporation is continued until hydrogen chloride fumes are no longer detectable. The salts are diluted to a concentration of 25 grams per liter and then are heated until all have dissolved.
+ CONTAUIXATED METAL
-+NaCl
REGEKERANT
Figure
1.
PASS THROUGH DOWEX 50
WAST REGENERANT 4
WASTE
Schematic diagram of process
Purification by Cation Exchange. The solution is pumped into the column containing a 30-inch head of 50- to 100-mesh sodium-form Dowex 50x8. A hydrostatic head of 4 feet gives a flow rate of 35 ml. per minute per square inch of column cross section. Analysis of Effluent. A 400-ml. volume of effluent, equivalent to 10 grams of precious metals, is taken for colorimetric determination of chromium, iron, and manganese. The base metals are first isolated by precipitation as hydrated oxides. This is accomplished by adding 10 grams of solid sodium nitrite to develop the nitrite complexes of platinum and rhodium, which are stable in alkaline eolutions. The base metals are precipitated a t pH 9.6 a t 80" C. and the solution is filtered while hot. The base metals are determined colorimetrically: chromium by the di-
712
phenylcarbazide method, iron by the thiocyanate method, and manganese by the periodate method ( 4 ) . Reduction of Precious Metals. The remainder of the effluent ia dispensed in increments of 2.5 liters per 4-liter beaker. The purified solutions are heated to 65" C. and stirred mechanically, a8 40 ml. of 85% hydrazine hydrate are slowly added. Sodium hydroxide is then added until the solution is distinctly alkaline. If the metal blacks are not coagulated, the addition of several milliliters of hydrazine hydrate will complete the coagulation. Two 5-ml. aliquots are examined qualitat,ively for platinum and rhodium. To one of the aliquots concentrated hydrochloric acid is added until distinctly acid, followed by 1 ml. of 10% pot,assium iodide. A red color indicat,es that platinum is present (4). The other aliquot is treated with sodium bisulfite and acidified with hydrochloric acid. Two milliliters of 10% st,annous chloride are added to the boiling solution. The presence of a rose-red color indicatee rhodium ( 4 ) . Sequestration and Melting of Blacks. The supernatant liquid from the reduction is decanted from the precious metal blacks and washed twice by decantation with distilled water. The blacks are then heated for 1 hour at 85' C. in a slightly acidic solution of sodium ethylenediamine tetraacetate. They are then washed twice by decantation and the sequestration is repeated in a solution made slightly alkaline Tvith sodium hydroxide. The purpose of the dual-sequester treatment is to complex and remove any residual cations that might be entrained in the metal blacks. The blacks are then filtered in a 5-inch Buchner funnel under a vacuum and washed Kith water until the water is chlcride-free. These washings remove reaction by-products that form slags and objectionable vapors when melted. A final washing with methanol removes the excess water. The blacks are air dried, placed in Coors No. 5 crucibles, and sintered a t 1000" C. for 4 hours in a neutral or reducing atmosphere. The sintered masses ape melted by high frequency induction in a zircon crucible and poured in a graphite mold a t 3280" F. Best conditions for separation were determined by experiment
The sodium-form resin is rarely employed for separation of cations from mildly acidic solutions. The acid-form resin, which has received a great deal more attention ( 3 ) , was tried for the separation of the nine base metal contaminants from chloroplatinic and chlororhodic acids. The hydrochloric acid that wm retained by the mixed acids a t 62' C. in the chloride conversion step was sufficiently high to prevent efficient adsorption of the base metal cations on hydrogen-form Dowex 50. Evaporation a t higher temperatures caused oxides of platinum and rhodium to form. A final evaporation a t 62' C. with water eliminated the excess hydrochloric acid, but caused hydrolysis of the precious metal complexes. Although a vacuum technique could possibly have been developed to eliminate the excess hydrochloric acid, it waa much easier to convert to the sodium salts of the chloro-complex acids. These salts are highly soluble and, when evaporated, do not retain hydrochloric acid as do the mixed acids. The resin was converted to the sodium form in order to preserve equilibrium between the sodium in the resin and the sodium in the precious metal salts in solution. The quantity of sodium chloride added following dissolution and prior to the first evaporation is dependent upon the concentration of precious metals. This quantity is based on the stoichiometric equivalent required to form disodium chloroplatinate and trisodium chlororhodate. The efficiency of base metal cation adsorption vas dependent upon the level of conversion of the mixed acids to the salts. -4t the 100 mole % level of conversion, 90% of the base metal cations are taken up by the resin. Decreasing the sodium chloride to 70 mole % increasea the adsorption efficiency to 98.5%. Decreasing the sodium
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 48,No. 4
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT chloride further increases the exchange efficiency slightly, but results in increased desorption of the sodium ions from the resimn, requiring more frequent regeneration. Also, the precious metal complexes are a p t t o hydrolyze during evaporation with lesser amounts of sodium chloride. The satisfactory level of conversion was found t o be 70 mole %. Of several reductants considered for recovering the platinum and rhodium, hydrazine was by far the better one. One milliliter of 85% hydrazine hydrate reduces approximately 2 grams of platinum or rhodium. Reduction is best accomplished a t 65” C. The hydrazine should be added before alkalizing, in order t o avoid formation of colloidal precious metal blacks. The reduction is instantaneous and quantitative with the addition of the sodium hydroxide. The sequestration treatments increase the separation efficiency from 98.5% by ion exchange to 99.5%. The efficiency of adsorption as a function of inffuent base metal concentration was studied. For this series of experiments, solutions containing 3, 4, and 12 weight base metals were used. The efficiency was independent of the influent concentration and varied between 98 and 99.5%. The column was regenerated with a saturated solution of sodium chloride and the effluent was examined qualitatively for platinum and rhodium. Platinum was absent, but rhodium was usually present as a trace. Regeneration was continued until the effluent was free of iron by the thiocyanate test. If regeneration is continued with 1.OM hydrochloric acid, more iron will desorb from the column. However, because the free acid content of the influent precious metal solutions never exceeds 0.1M hydrochloric acid, it was believed that the residual iron on the resin would not be detrimental t o the process. This belief has been subFigure 2. Ion stantiated during several months of exchange sepaoperation. ration equipment The hydrolytic separation of base metal cations and subsequent colori metric determinations were used until the exchange efficiency was raised above 99%. Spectrographic analysis replaced wet analysis and the Harvey principle of total emission was used in calculating the compositions. An attempt was made to relate the leakage of chromium, iron, and manganese while the other six cations are quantitatively taken up. Samuelson (3) has cited work t h a t tends t o explain the incomplete adsorption of iron. Ferric chloride, in very dilute solutions, hydrolyzes t o a sol that is not taken up. The extent of adsorption diminishes with decreasing concentration and also on aging of the solutions. Gustavson ( 1 ) has provided evidence that chromic chloride in solution exists in both cationic and anionic forms. No explanation for the manganese was given, except that it was adsorbed least efficiently of several cations under investigation. Consideration was given t o determination of the resin capacity, the objective being t o relate in some way the concentrations of the influent solutions and the volume processed before break-
-
through would occur. However, this was abandoned for several reasons. First, iron is always present as a contaminant and is the first t o break through the resin. The thiocyanate test is expedient and reliable for determining breakthrough. Secondly, the cationic component from one batch t o another varies in composition and concentration, so t h a t it would be virtually impossible t o relate the varied influent compositions with regeneration requirements. Finally, every batch of metal would require wet analysis, thus increasing the cost of the process tremendously. Equipment has been modifled for adaptation to commercial process
The experimental equipment has been modified to handle large volumes of metal for reclamation (Figure 2). Three 52.5-inch Lucite columns are suspended vertically in an angle iron framework. Two are 2 inches in inside diameter and the other is 1.5 inches. Beneath the columns are two 5-gallon borosilicate glass bottles on trunnions. One of the bottles contains the influent solution t o be processed and the other the purified solution. The influent is pumped t o a reservoir above the columns with a variable-speed Sigma motor pump. The reservoir, fabricated of Plexiglas sheeting, has an overflow that returns the solution t o the influent bottle when the depth of the influent exceeds 4 inches in the reservoir. The solution falls freely into the columns, under its own hydrostatic head, through hard-rubber stopcocks and clear plastic tubing. The flow rate is 25 ml. per minute per square inch of column cross section. Overflows situated 3 inches from the top of each column return the excess solution to the influent source. Backwashing is accomplished by forcing tap water through hard-rubber fittings built into the side of the column about 1 inch above the bottom of the column. Summary
Alloys of platinum, rhodium, and platinum-rhodium may be refined by extraction of simple cations from aqueous solutions of the precious metal complex chloride salts with Dowex 50 in the sodium form. The exchange removal varies with the freeacid concentration in the influent and is about 98.5% efficient. The quantitative reduction of the precious metals with hydrazine results in a powdered mass of the same relative composition as the original alloy. Dual treatment of the metal blacks iirst with acid and then with alkaline ethylenediamine tetraacetate increases the efficiency of separation t o 99.5%. This process is much cheaper, simpler, and less time-consuming than classical methods of separation. Acknowledgment
The author wishes t o express his appreciation t o W. M. MacNevin of The Ohio State University for his review of this paper and t o L. P. Biefeld of the Owens-Corning Fiberglas Corp. for his criticisms. literature cited
(1) Gustavson, K.H., J. SOC.Leather Trades Chemists 35, 160 (1951). (2) MacNevin, W. M., Crummett, W. B., Anal. Chem. 25, 1628 (19.53). ,\_._.
(3) Samuelson, O., “Ion Exchangers in Analytical Chemistry,” Wiley, New York, 1950. (4) Snell, F. D., Snell, C . T., “Colorimetric Methods of Analysis,” 3rd ed., vol. 11, p. 520, Van Nostrand, New York, 1949. ACCEPTED January 23, 1956. RECEIVED for review March 14, 1955.
E N D OF ENGINEERING, DESIGN, AND PROCESS D E V E L O P M E N T SECTION
April 1956
INDUSTRIAL AND ENGINEERING CHEMISTRY
713
