The sample and flux mixture consisting of sodium carbonate,
borax, nickel (as metal or as oxide or carbonate) and sulfur is
fused at 1000 °C for 1.5 to 2 h. The NiS formed during the
fusion collects the gold and PGE. The crushed NiS button, after
separation from matrix, is treated with hot concentrated HCl and
the insoluble precious metal sulfides along with the dissolved
precious metals are then collected on tellurium. The tellurium
co-precipitation step effectively collects all the PGE sulfides
avoiding the H2S passing step at this stage. The black precipitate
is filtered off and analysed either directly by NAA or dissolved
in aqua regia or HCl + hydrogen peroxide and analysed by ICP
or AAS.