[Jewelry making - Article 00656] - [Orchid] Recovery of gold by bombing

[bhupesh mulik]

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    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% 

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