Thornhill (USA 1910)
applied a wet extraction method on cinnabar ores. This method consisted in
leaching cinnabar concentrates with sodium sulphide
and sodium hydroxide.
refineries roast spharelite( Zincsulphide ) to obtain a calcine
of leachable zinc oxides. In this process, where fluid bed kilns are used for
this calcination dusts are produced that contain variable amounts of mercury , selenium and other minor metals. These dusts are
removed in cottrell dust collectors and a gas washing tower.
1987, when we were working at the Zinc Refinery of Cajamarquilla( located near Lima City, capital of Peru, South
America), these flues (actually wet muds) were stocked untreated.
that time the owner of this plant was Minero Peru,
the state owned mining enterprise, owner also of a gold mine where mercury was
needed for gold amalgamation and we thought we could apply Thorhill's
method to these muds.
We succeded in recovering
almost all the mercury and selenium from these muds.
Silver( ~ 60 oz / mt
) and lead precipitate with mercury and they are separated when washing mercury
with dilute nitric acid.
In practice what is really done is leaching wet mud with 1:1 Na2S and 1:0.25
NaOH ( weighed on a dry basis ), about an hour at room
temperature and with proper agitation; then discarded aluminum cathodes sheets
are inmersed in this pulp. Reaction is exothermic and
violent, but can be controlled through varying the depth of aluminum sheets inmersion. Reaction reaches a maximum temperature of 92
degrees centigrade. Al sheets are left overnight and the day after mercury is
separated from the mother liquor.
liquor is then treated following equation "c" until pH lowers to
about 3.0. In this step H2S gas evolves and in the case that instead of using
sodium sulphide we use only sodium hydroxide H2Se
would be produced.