Journal of Scientific & Industrial Research Vol. 65, January 2006, pp. 65-67
Comparison of different extraction methods to determine gold in geological samples P V Sunder Raju* National Geophysical Research Institute, Hyderabad 500 007 Received 03 May 2005; revised 25 August 2005; accepted 14 October 2005 Different petro-chemically varied lithological samples were collected from the Chitradurga schist belt, Karnataka, to identify auriferous zones. The efficiencies of gold estimation at very low levels by different extraction methods were compared. The extraction methods were fire-assay (FA), cyanide leach, and methyl isobutyl ketone (MIBK) solvent extraction. The rock samples include banded iron formations (silicate, oxides, carbonate and sulphide facies), ferruginous cherts, massive milky white quartz veins and highly sheared fractured translucent dark grey to bluish grey quartz veins. After separation and pre-concentration of gold, MIBK solvent extraction procedures have been studied using flame atomic absorption spectrophotometer (F-AAS). Keywords: Chitradurga, F-AAS, MIBK IPC Code: C22B11/00
Introduction In nature, gold occurs as native or refractory type. Refractory gold ores are not free milling and cannot be leached without some form of physical, chemical or pyro metallurgical pretreatment. Refractory gold is found in association with sulphides (pyrite, arsenopyrite and chalcopyrite), whereas native gold is associated with tellurides, bismuthides, silver, antimony and copper. In general, fire-assay (FA), cyanide leach, open aquaregia and microwave (closed) are some of the important extraction methods. The present study is an attempt to find any difference in the Au content of the splits of large sample after crushing, to understand the efficiency of the analytical methods based on aquaregia leach, NaCN leach and methyl isobutyl ketone (MIBK) extraction, and compare these methods with the classical and widely accepted lead fire assay method. Experimental Methods Sampling and Powdering
Samples were collected from geologically favorable hosts for gold mineralization from Chitradurga and Gadag schist belts. Bulk samples (10-15 kg) were collected and powdered (200-250 mesh). Samples were coned, quartered and ______________ *Tel: 040-23434700; Fax: 040-23434651 E-mail:
[email protected]
systematically sampled to give a weight of 20 g. Prior to chemical dissolution, the samples1 were mixed with ammonium nitrate and subjected to open air roasting at 600-650° C in a porcelain crucible. Instrumentation
SpectrAA 220 (Varian, Australia) flame atomic absorption spectrophotometer (F-AAS) with deuterium background correction was utilized. A model Z 300 bench top centrifuge (Hermle Labor Technik, Germany) was employed for centrifuging sample solutions in MIBK extraction. Gold speck obtained by cyanidation process was weighed using a model UMT-2 microbalance (Metler, Switzerland). The instrumental parameters are as follows: lamp type, hollow cathode lamp; flame type, air acetylene; background correction, deuterium; wavelength, 242.8 nm; slit width, 1.0 nm; lamp current, 4 mA; measurement time, 1.5 sec; airflow, 3.5 l/min; and acetylene flow, 1.5 l/min. Anaytical Methods Lead Fire Assay
Finely powdered sample (80-90%
