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
The entire process (Analysis of Gold & Silver) consists of three steps:
- 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.
- 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.
- Quantification of metalling gold and silver by either by gravimetry or by ICP-OES or AAS.
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:
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%
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
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
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
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%
- Fire assay is globally accepted technique for sample concentration for gold & silver analysis.
- 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.