It was found that HNO3 (at least 20%) is the
best leaching agent, gold not being dissolved: two
consecutive leaching steps were carried out using the
selected leaching agent and the solid residue was
leached with aqua regia in order to verify mass
balances.
In Table 2, as an example, the results using
liquid/solid (L/S) ratio equal to 3:1, HNO3 20% in 24
h are reported. During the treatment tin precipitates
as oxide/acid metastannic. This precipitate was
separated, then gravimetric and XRF analyses were
performed, thus confirming this hypothesis. Applying
this leaching conditions, both Au and SnO2/Sn(H2O)n
can be recovered from the residue by a physicalmechanical
process with a yield of 99%.