Extraction Chromatographic Separation of Gold Using Bis(2...

Indian Journal of ChemistryVol. 25A. March 1986. pp. )()().)01

Extraction ChromatographicSeparation of Gold Using

Bis(2-ethylhexyl)phosphoric Acidas an Extractant

LALIT C T EUSEBIUSt & S M KHOPKAR*

Department of Chemistry, Indian Institute of Technology,Powai, Bombay 400076

Received 18 February 1985; revised 7 November 1985;accepted 20 January 1986

Bis(2-ethylhexyl)phosphoric acid (HDEHP), a liquid cationexchanger coated on silica gel from benzene quantitatively extractsgold from 7-8 M HCI and is stripped off with 2 M HCI. Gold insynthetic mixtures containing other metal ions such as Cu(I1), Ni(ll),Mn(I1), Pb(IV), Pt(IV) and Pd(I1) in the ratios actually present invarious gold alloys has been successfully separated employingHDEHP.

Bis(2-ethylhexyl)phosphoric acid (HDEHP), a liquidcation exchanger is one of the common extractantsemployed for the separation of metal ions such as rareearths, fission products and transuranium metal ions!etc. Several methods are available/ -12 for theextraction chromatographic separation of gold, butthe use of HDEHP as the extractant for gold 13 -I b islacking. Presently we have explored extractionchromatographic behaviour of gold using HDEHP asan extractant in various concentrations of hy-drochloric acid. The optimum conditions for itsextraction have been evaluated and its separation from20 metal ions have been accomplished.

ECIL model GS 866C spectrophotometer providedwith matched corex glass cuvetts of 10 mm optical pathlength was used for measuring absorbance. ECILmodel 5651 pH-meter having glass-SCE electrodeassembly was used for pH measurements. Thechromatographic column (int diam =8 mm and length= 150 mm) was used for chromatographic separation.

Stock solution of gold was prepared by dissolvingabout 0.45 g of gold chloride (Johnson & Mathey) in0.1 M hydrochloric acid and making the volume to 100ml with water. It was standardized gravimetrically byhydroquinonel7 and was found to contain 400 Jlg/mlof gold. Dilute solution (40 Jlg/ml of gold) wasprepared by appropriate dilution.

Bis(2-ethylhexyl)orthophosphoric acid (HDEHP)(Schuchardt, Munchen, Germany) was used as such.

tPresent address: Department of Chemistry. Ewing ChristianCollege. University of Allahabad. Allahabad 211003.

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Purified silica gel (60-120 mesh. BDH) was renderedhydrophobic by procedure described earlier I II. It wasthen coated with H D EH P taken in benzene. The excessbenzene was removed by centrifugation at 2500 rpmusing a Remi model T 8C centrifuge. This gel was usedfor carrying out column extraction studies as well asfor reverse phase extraction chromatographicseparation work.

General procedureA slurry of silica gel coated with HDEHP was

transferred to a chromatographic column and theheight of the bed was maintained at 8 cm. It waswashed with 2-8 M hydrochloric acid (10-15 ml) as thecase may be. An aliquot of solution containing 40 Jiggold/ml was rendered acidic (8 M) by addinghydrochloric acid. It was passed through thepretreated column at a flow rate of 1 ml/min. Afterextraction, gold was stripped with 2 M hydrochloricacid. Twenty fractions (each of 2 ml) were collected andgold from each fraction was determined spectrophoto-metrically I 9 as the gold-rhodamine complex inbenzene at 565 nm.

It was found that gold is quantitatively extractedfrom 7-8 M hydrochloric acid. The gold thus extractedhad a breakthrough volume above 60 ml and 90 ml at7 M and 8 M hydrochloric acid, respectively.Therefore at 7-8 M hydrochloric acid, the extractionbehaviour of other metal ions were studied. It wasnoted that except iron and uranium all other metal ionswere not retained by the column under the conditionsemployed for the extraction of gold and they were allquantitatively recovered from within the initialfractions (1-8) of 2 ml each. Gold which was retained inthe column was finally stripped with 2 M hydrochloricacid. Thus from 7-8 M hydrochloric acid gold wasseparated from other metal ions except iron anduranium.

It was observed that the optimum conditions forbatch extraction of gold were not same as those forextraction chromatography 10. This is the case withmany metal ions studied 11.1 II. The ions such as copper(0.01 M), nickel (not extracted), manganese (PH 4.0),lead (0.01 M), platinum and palladium (not extracted),silver (0.1 M), cadmium (0.03 M) and antimony (0.1 M)were extracted at different acidities of hydrochloricacid hence they passed through the column in 7 Mhydrochloric acid medium facilitating separation ofgold from binary mixtures.

It was possible to separate gold from binarymixtures containing copper and gold or tellurium and

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