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

Scope:

ASTM E1805-21 is used to determine the gold content of copper concentrates ranging 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, during which the sample is inquartated. 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. Subsequently, after fusion, the sample is poured into the fusion assay mold. Once cooled, the lead button is separated from the slag by tapping it with a steel hammer. Next, the cupellation process is conducted by placing the cupel into the furnace and heating it to 900 ºC for 30 minutes, after which 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 the temperature is lowered to 820 °C. When the process of cupellation is near the end, oily-looking drops of PbO can be seen collecting 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 revolves around a shifting axis. When the last trace of PbO has been removed, this play of colors disappears, and the Dore bead becomes duller than a normal metallic luster. At this point, the cupel is removed from the furnace and kept for cooling. Afterward, the cooled cupel is taken, and the Dore bead containing the precious material is removed with tongs. Next, the Dore bead is brushed to eliminate any adhering cupel material. Following this, parting is performed using HNO3 and NH4OH. Finally, 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 is a flux blend of borax glass, flour, lead oxide, potassium carbonate, and silica. The fire assay flux mixture is a 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

Read more: ASTM E1568 Determination of Gold in Activated Carbon by Fire Assay Gravimetry

Conclusion:

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