Preparation of ammonium chloroplatinate by a precipitation stripping of Pt(IV)-loaded alamine 336 or TBP
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INTRODUCTION
IN the laboratory of co-author S. Sanuki, a direct-reduction process for Pd(II) or the Pt(IV)-loaded organic phase was investigated using a stripping solution containing NaHPO2.[1] The result was that Pd(II)-loaded TBP and Alamine 336 were reduced to a metal state and recovered as fine particulates. In contrast, Pt(IV)-loaded TBP or Alamine 336 are very difficult to reduce. The recovery of Pt(IV) chlorocomplexes from an organic phase by an ordinary stripping method is not easy.[2] Crystallization of (NH4)2PtCl6 has been known since ancient times to be an efficient and effective method of purifying Pt(IV).[3] It is considered possibile to recover (NH4)2PtCl6 from Pt(IV) loaded in an organic phase by using NH3 or NH4Cl as a stripping agent. Products obtained as (NH4)2PtCl6 can be readily decomposed by gaseous reduction at an elevated temperature, producing sponge-like Pt metal.[4] Since the ammonium salt of Pd(II) chlorocomplexes is soluble, it may be possible to separate Pt(IV) from Pd(II) by precipitation stripping of insoluble (NH4)2PtCl6. Using a stripping solution containing various agents, precipitation stripping of (NH4)2PtCl6 from Pt(IV)-loaded Alamine 336 or TBP was examined. Furthermore, the possibility of separating Pt(IV) as (NH4)2PtCl6 from Pt(IV) and Pd(II)-loaded Alamine 336 was investigated. II. PRELIMINARY EXAMINATION OF THE PRECIPITATION STRIPPING OF PT-LOADED ALAMINE 336
A. Precipitation Stripping of Pt(IV)-Loaded Alamine 336 The extraction of Pt(IV) chlorocomplexes from an HCl solution by Alamine 336 can be expressed by Eqs. [1] and [2]. (R3N)o 1 HCl 5 (R3NH+?Cl2)o 2(R3NH+?Cl2)o 1 PtCl22 6 2 5 ((R3NH+)2?PtCl22 6 )o 1 2Cl
METALLURGICAL AND MATERIALS TRANSACTIONS B
[2]
As is shown by Eq. [2], the extraction of Pt(IV) involves an anion exchange between Cl2 and PtCl22 6 , and, thus, amines in the organic phase form two ion-pair species, (R3NH+?CL2)o and ((R3NH+)2?PtCl622)o. In contrast, the stripping of Pt(IV) from Pt(IV)-loaded Alamine 336 can follow one of three routes: hydrolysis, anion exchange, or reduction.[5] For reduction stripping, it is necessary to use a stripping solution, whose reducing power is strong, and the solubility of the reduced product is low. Therefore, precipitation stripping reactions available for the formation of (NH4)2PtCl6 include only hydrolysis and anion exchange. Application of an alkaline stripping solution makes the hydrolysis reaction possible. The reaction using NH3 or NaOH can be expressed by Eqs. [3] and [4], respectively. ((R3NH+)2?PtCl22 6 )o 1 2NH4OH 5 2(R3N)o 1 (NH4)2PtCl6 1 2H2O
SUMIKO SANUKI, Associate Professor, is with the Department of Material Systems Engineering and Life Science, Faculty of Engineering, Toyama University, Toyama City, Japan 930-8555. YUKAKO MATSUMOTO, formerly Graduate Student, Department of Materials Science and Engineering, Faculty of Engineering, Toyama University, is Research Engineer, K.K. Kuwayama Kikinzoku, Uozu City, Japan 937-0853. HIROSHI MAJIMA, is Professor Emeritus, Kyoto University, Kyoto, Japan 6068501,
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