Recovery of gold after aqua regia process.
Importance - during the aqua regia process some refiners do not know
that AGCL maskans the gold alloy metal which has AG content below
12%. during the refining some AU metal alloy may be maskened by AGCL
which we can not see visually because the colour of AGCL
(gray)matches the silica from jewellery dust.
so their is possibility of remaining some AU ,depends on the
caratage.if any factory,making only 9k or 10k white gold jewellery
then there is lessw recovery of gold from refining waste compare
with 14k or 18k working.
so our aim to reduce the losses by increasing the recovery from dust
& avoid the mistakes of vhemical reaction happens during the
refining. after the refining the waste material by aqua regia, these
waste cotains acidic medium.before going to cynide leaching, we have
to remove the acid because it creates a strong exothermic reaction &
evoives hcn gas which is deadly poisonous.
the refined waste from aqua regia process must be ignited again in
muffle furnace & make a fine powder by grinding.
this fine powder taken into alarge plastic bucket.if the diameter of
bucket is widen then it is very effective for process.add perfect
quantity of water into the dust to make a slurry of dust .now add
NAOH to maintain the ph in between 8-10.means slurry should be basic
in nature. now its time to add nacn in proper proportion depends on
quantity of au present into the dust. cynide gets complexed with the
AG & AU.but the rate of leaching is very slow ,so it requirs to
fastenes the reaction so we use an xoidising agent instead of
atmospheric oxygen in the leaching of
AU.example-permagnates,persulphates,peroxide,dichromates,
ferrocynides ,H2O2,CAO2 etc…
i always prefer H2O2 or CAO2.H2O2 is most suitable oxidising agent
for replacing atm.oxygen because only water & oxygen (no toxic
products)are fromed in decomposition of H2O2.
CAO2 is also more suitable ,H2O2 starts reactedquickly. refiners
should be aware of one point that while adding H2O2 in proper
proportion add sloely into the cn-leaching dust.while addded H2O2
always slowly stirr the leaching solution,otherwice recovery will
affects.strirring plays important role .always strirr slowly because
the reaction is bumping & exothermic.keep this solution for max. 24
hrs. after 24 hrs., filter the solution ,wash the residue fully &
collect the filter solution in large plastic bucket & now add more
NAOH& zinc in proper proportion as per the gold & silver content.
keep this solution about 4hrs., now decand or filter the solution,
the residue must be washed fully it should not contain CN because CN
is volatile. while burning the residue , gold may evaporated wth CN.
after burning the residue ,remaining ashes you can melt or treat
burn material to nitric acid to dissolve large amount of ag.when ag
dissolves in nitric acid we can see brownish coloured gold powder in
solution.fiter the AGSO4 solution,treat the residue by aqua regia
process to get pure gold.
note - according to my experience, i recovered approx. 500gm of
silver & 25gm of gold in 8kg of refined dust through this method.
Limitations-
dust v/s water -0.8% to 1.15%
NAOH v/s water-0.025%to 0.030%
NACN v/s dust-0.045% to 0.078%
H2O2 v/s dust-0.030% to 0.06%