ANALYSIS OF GOLD & SILVER BY USING FIRE ASSAY TECHNIQUE FOLLOWED BY ICP-OES

Introduction:

Fire assaying is the process of concentration of precious metals from non-ferrous ores and estimated either gravimetric or ICP-OES instrument.

FIRE ASSAY FURNACE

 

ICP-OES

 

The entire process (Analysis of Gold & Silver) consists of three steps:

1. Separation of elemental gold & silver from ores by formation of lead alloy along with formation of slag for rest of the materials present in ore.
2. In second step, the lead alloy is heated in stream of oxygen at 950 0C in a cupel using Cupellation furnace when most of the lead is removed as PbO leaving gold and silver in metallic state.
3. Quantification of metalling gold and silver by either by gravimetry or by ICP-OES or AAS.

 

Description:

Formation of lead alloy button of gold and silver is done by mixing adequate quantity of ore with a synthetic flux mixture at 1100 0C in PIIT furnace when gold and silver is converted to lead alloy and rest of the ore is separated as slag. The synthetic flux mixture contains Lead oxide, Flour, Fluorspar, Silica, Sodium carbonate & Borax.         Second stage is Cupellation of lead button which contains lead with Gold and Silver. Lead is converted to lead oxide, most of which is removed by stream of air blowing through the furnace and rest of lead oxide was absorbed by calcium phosphate crucible.

 

After cupellation, pure gold and silver is treated with nitic acid, when silver dissolves but not gold. The solution containing silver nitrate is filtered off and estimated using ICP-OES instrument for silver content. Gold is left as residue, dissolved in aqua regia and estimated for gold content by ICP-OES.   RESULTS:

1. Rock Sample:

   Sl no	Sample No.	Declared Result	Obtained Results
   01	S4-internal std	Au-38 ppm	Au-34 ppm
   02	S2 -internal Std	Ag -0.215%	Ag- 0.208%

2.  Lead Ore:Analysis was carried out by fire assay followed by ICP-OES, following finding was observed.

   Sl no	Sample No	Declared Result	Obtained Result
   01	PT sample	Ag-33 PPM	Ag-31 PPM

3.  Zine ore: Analysis was carried out by fire assay followed by ICP-OES, following finding was observed

   Sl no	Sample No	Declared Result	Obtained Results
   01	OREAS 351	Ag-125 PPM	Ag -118 PPM

4. Copper ore: Analysis was carried out by fire assay followed by ICP-OES, following finding was observed

   Sl no	Sample No	Declared Result	Obtained Results
   01	CCU 1-D copper 0re	Au-14 PPM	Au-12.5 PPM
   02	CCU 1-D copper 0re	Au-14 PPM	Au-13 PPM

5. Recovery of Pure Metal

   Sl no	Sample No	Declared Result	Obtained Results
   01	AuBr3 (100%) pure	Au -100%	Au - 93.80%
   02	AuBr3 (100%) pure	Au -100%	Au - 91.86%
   03	Ag2SO4	Ag-100%	Ag- 93.30%
   04	Ag2SO4	Ag-100%	Ag- 94.48%

  Conclusion:

1. Fire assay is globally accepted technique for sample concentration for gold & silver analysis.
2. Since after cupellation only gold & silver are separated, there is no possibility of any interference during analysis. Therefore, results achieved are accurate and repeatable.

  PRESENTED BY: Sukhenjit Datta, Arnab Pattanayak and Priya Das under the guidance of Prof B.K. Gupta.