ASTM E1805-21 Standard Test Method for Determination of Gold in Copper Concentrates by Fire Assay Gravimetry

ASTM E1805-21 Standard Test Method for Determination of Gold in Copper Concentrates by Fire Assay Gravimetry

The ASTM E1850-21 test method determines gold in copper concentrates in the content range from 0.2 μg/g to 17 μg/g. In this method the test sample of copper concentrate is fluxed and fused in a clay crucible. The precious metals are reduced, collected in a lead button, and then cupelled to remove the lead. The remaining Dore bead is parted with nitric acid to remove the silver and other impurities from the gold. The gold is then annealed, cleaned, and weighed on a microbalance.

    Scope:

    ASTM E1805-21 is used for the determination of gold in copper concentrates in the content range from 0.2 μg/g to 17 μg/g.

    Test Procedure 

    ASTM E1805-21 test is carried out in eight steps. Initially, the clay fire assay crucible is prepared for the test in which inquartation of the sample is done. Then for the next step, fusion is done in three stages: primary, secondary, and tertiary. The crucible is heated from 600 ºC to 1100 ºC in these stages. Then after fusion, the sample is poured into the fusion assay mold. Then after cooling, the lead button is separated from the slag by tapping with a steel hammer. Then the cupellation process is carried out where the cupel is placed into the furnace and heated up to 900 ºC for 30 min., then the separated lead button is added to each cupel. The lead button will melt into a bright lead puddle that will soon form a dark crust with the formation of surface PbO. The furnace temperature is kept above 888 °C, the melting point of PbO. This step is accomplished within 1 min to 2 min. 

    After 5 min, the draft to the furnace is opened, and temperature is brought down to 820 °C. When the process of cupellation is near the end, oily looking drops of PbO can be seen to collect on the surface of the remaining Dore bead. These cause a rainbow color band to move swiftly over the surface of the Dore bead giving the illusion that the bead is revolving on a shifting axis. When the last trace of PbO has been removed, this play of colours disappears and the Dore bead becomes duller than a normal metallic lustre. At this point, the cupel is removed from the furnace and kept for cooling. Then from the cooled cupel Dore bead containing the precious material is removed with tongs. Then the Dore bead is brushed to remove any adhering cupel material. Then parting is done with HNO3 and NH4OH. Then the remaining gold is weighed. Remaining slag is combined with the cupels from the duplicate fusions for each test sample, the total is weighed and recorded.

    Test Specimen:

    Cupel correction are flux blend of borax glass, flour, lead oxide, potassium carbonate, and silica. Fire assay flux mixture is blend of Borax glass, lead oxide, potassium carbonate, potassium nitrate, silica, and sodium carbonate.

    Data:

    The correction value is corrected as follows

    Where:

    A = total weight of cupel and slag, g,
    B = weight of the first correction sample weight, g,
    C = weight of the second correction sample weight, g,
    D = weight of correction samples’ gold bead, mg, and
    F = correction value, mg.

    Calculate the corrected gold concentration in the original sample as follows

    Where:

    E = weight of test sample gold, mg,
    F = correction value, mg,
    G = original test sample weight,
    H = gold content, μg/g

    Conclusion:

    Hence, we can perform ASTM E1805-21 to determine the Gold in Copper Concentrates by Fire Assay Gravimetry

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