GB/T 3884.2-2012Methods for chemical analysis of copper concentrates - Part 2: Determination of gold and silver contents - Flame atomic absorption spectrometric method and fire assay method (English PDF)

4.1 Principle

The sample is dissolved in nitric acid, hydrofluoric acid, perchloric acid, hydrochloric acid. In a dilute hydrochloric acid medium, the absorbance of silver is measured using a flame atomic absorption spectrometer, at a wavelength of 328.1 nm with an air- acetylene flame. The silver content is calculated using the standard curve method.

4.2 Reagents

Unless otherwise specified, only reagents confirmed to be of analytical grade and distilled or deionized water or water of equivalent purity shall be used in the analysis.

4.2.1 Nitric acid (rho = 1.42 g/mL).

4.2.2 Hydrochloric acid (rho = 1.19 g/mL).

4.2.3 Hydrofluoric acid (rho = 1.42 g/mL).

4.2.4 Perchloric acid (rho = 1.67 g/mL).

4.2.5 Silver standard stock solution. Weigh 0.1000 g of silver (wAg >= 99.99%) into a

200 mL beaker; add 20 mL of nitric acid (1 + 1); heat until completely dissolved; cool to room temperature; transfer to a 200 mL amber volumetric flask; dilute to the mark with chloride-free water; mix well. 1 mL of this solution contains 500 µg of silver.

4.2.6 Silver standard solution. Transfer 10.00 mL of the silver standard stock solution

(4.2.5) to a 100 mL volumetric flask; dilute to the mark with hydrochloric acid (1 + 3);

mix well. 1 mL of this solution contains 50 µg of silver.

4.3 Instruments

Flame atomic absorption spectrometer with a silver hollow cathode lamp.

Under optimal instrument operating conditions, any instrument meeting the following specifications may be used.

- Characteristic concentration. In a solution consistent with the matrix of the sample solution, the characteristic concentration of silver shall not exceed 0.018 µg/mL;

- Precision. When measuring absorbance 10 times with the highest concentration standard solution, the standard deviation shall not exceed 1.0% of the average absorbance. When measuring absorbance 10 times with the lowest concentration standard solution (not the "zero" standard solution), the standard deviation shall not exceed 0.5% of the average absorbance of the highest concentration standard solution;

- Linearity of the working curve. Dividing the working curve into 5 equal segments according to concentration, the ratio -- of the absorbance difference in the highest segment to the absorbance difference in the lowest segment -- shall not be less than

0.85.

4.4 Samples

4.4.1 The sample particle size shall not exceed 100 µm.

4.4.2 The sample shall be dried at 100 °C ~ 105 °C for 1 hour, then placed in a desiccator and cooled to room temperature.

4.5 Test procedure

4.5.1 Sample

Weigh 0.5 g of sample (4.4), accurate to 0.0001 g.

4.5.2 Parallel test

Perform two parallel tests and take the average value.

4.5.3 Blank test

Perform a blank test along with the sample.

4.5.4 Determination

4.5.4.1 Place the sample (4.5.1) in a 250 mL PTFE beaker; add a small amount of water to moisten; add 15 mL of nitric acid (4.2.1); heat on a hot plate at low temperature to dissolve; evaporate to a volume of about 10 mL; remove and let cool slightly. Add 5 mL of hydrofluoric acid (4.2.3); continue heating to dissolve while shaking occasionally;

dissolve to a small volume; remove and let cool slightly. Add 5 mL of perchloric acid (4.2.4); cover with a PTFE watch glass; heat to dissolve until there is no black residue at the bottom of the beaker; remove and let cool slightly. Rinse the watch glass and beaker walls with water; add 5 mL of hydrochloric acid (4.2.2); continue heating until the perchloric acid emits white fumes. Evaporate to near dryness; then remove and cool.

4.5.4.2 Add hydrochloric acid (4.2.2) according to Table 1; rinse the watch glass and beaker walls with water; heat to dissolve the salts; remove and cool to room temperature.

Transfer the solution to a volumetric flask of the appropriate volume (the volume of the volumetric flask is the measurement volume in Table 1); dilute with water to the mark;

mix well; let it stand (or filter dry).

Table 1 -- Constant volume and volume for adding hydrochloric acid

4.5.4.3 Using an air-acetylene flame, at the analytical wavelength of silver (328.1 nm), zero the instrument with water and measure the absorbance of the blank solution and the sample solution. Determine the corresponding silver mass concentration from the working curve.

4.5.5 Plotting the working curve

4.5.5.1 Pipette 0 mL, 0.20 mL, 1.00 mL, 2.00 mL, 3.00 mL, 4.00 mL, 5.00 mL of silver standard solution (4.2.6) into a set of 100 mL volumetric flasks (when the mass fraction of silver in the sample is 10.0 g/t ~ 20.0 g/t, add an equal amount of copper and iron matrix solution to the sample solution). Add 10.0 mL of hydrochloric acid (4.2.2); dilute to the mark with water; mix well.

4.5.5.2 Under the same conditions as the sample solution, at the analytical wavelength

of silver (328.1 nm), zero the instrument with water and measure the absorbance of the silver standard solutions. Subtract the absorbance of the "zero" concentration solution from the series of standard solutions. Plot the working curve with the mass concentration of silver as the abscissa and the corresponding absorbance as the ordinate.

4.6 Test data processing

The silver content is expressed as a mass fraction wAg, in g/t, which is calculated according to formula (1).

Where.

rho1 - Mass concentration of silver in the test solution obtained from the working curve, in micrograms per milliliter (µg/mL);

rho0 - Mass concentration of silver in the blank solution obtained from the working curve, in micrograms per milliliter (µg/mL);

V - Total volume of the test solution, in milliliters (mL);

m0 - Mass of the sample, in grams (g).

The calculation result is expressed to one decimal place.

4.7 Precision

4.7.1 Repeatability

Under repeatability conditions, the absolute difference between two independent test results within the average range given in Table 2 does not exceed the repeatability limit (r); the number of cases exceeding the repeatability limit (r) does not exceed 5%. The repeatability limit (r) is obtained using linear interpolation or extrapolation according to Table 2.

Table 2 -- Repeatability limits (r) for Method 1

4.7.2 Reproducibility

The absolute difference between two independent test results, which are obtained under reproducibility conditions, within the average range given in Table 3, shall not exceed the reproducibility limit (R). The cases exceeding the reproducibility limit (R) shall not exceed 5%. The reproducibility limit (R) is obtained using linear interpolation or extrapolation based on the data in Table 3.

Table 3 -- Reproducibility limits (R) of Method 1

5.1 Principle

The sample is prepared and melted at high temperature. The molten lead traps the precious metals in the sample to form lead clasps. Other substances in the sample react with the flux to form fusible slag. The lead clasps are blown away to obtain gold-silver granules. Impurities adhering to the surface of the granules are removed. Gold is separated by nitric acid; the silver content is determined by titration, or the gold content is determined by gravimetric method or flame atomic absorption spectrometry.

5.2 Reagents

Unless otherwise specified, only reagents confirmed to be of analytical grade and distilled water or deionized water or water of equivalent purity are used in the analysis.

5.2.1 Anhydrous sodium carbonate. Industrial grade, powder.

5.2.2 Lead oxide. Industrial grade, powder (wAu < 0.01 g/t; wAg < 0.5 g/t).

5.2.3 Borax. Industrial grade, powder.

5.2.4 Silica. Industrial grade, powder.

5.2.5 Starch. Industrial grade, powder.

5.2.6 Potassium nitrate. Analytical grade.

5.2.7 Sodium chloride. Industrial grade, powder.

5.2.8 Silver (wAg >= 99.99%).

5.2.9 Nitric acid (rho = 1.42 g/mL). Chloride ion free.

5.2.10 Nitric acid (1 + 1). Chloride ion free.

5.2.11 Nitric acid (1 + 7). Chloride ion free.

5.2.12 Mixed acid. 3 parts by volume of hydrochloric acid (rho = 1.19 g/mL) + 1 part by volume of nitric acid (5.2.9), prepare fresh before use.

5.2.13 Ferric ammonium sulfate solution. Take 1 part by volume of saturated ferric

ammonium sulfate solution and add 3 parts by volume of nitric acid (1 + 3); mix well.

5.2.14 Potassium thiocyanate standard titration solution. Prepare and standardize

according to the following steps.

a) Preparation. Weigh 0.5 g of potassium thiocyanate; place it in a 100 mL beaker;

dissolve it in water; transfer it to a 1000 mL volumetric flask; dilute to the mark with water; mix well. Filter after standing for one week; prepare for later use.

b) Standardization. Weigh three portions of silver (10.00 mg ~ 15.00 mg, 5.2.8);

place them separately in porcelain crucibles (5.3.7). Add 10 mL ~ 15 mL of nitric acid (5.2.10); gently heat to dissolve; evaporate to 1 mL ~ 2 mL. Add a small amount of water and 0.5 mL of ferric ammonium sulfate solution (5.2.13); titrate until a light pink color is obtained, which is the endpoint.

Calculate the actual concentration of the potassium thiocyanate standard titration solution using formula (2).

Where.

c - Actual concentration of the potassium thiocyanate standard titration solution, in moles (mol/L);

m1 - Mass of silver, in milligrams (mg);

107.868 - Molar mass of silver, in grams per mole (g/mol), [MAg = 107.868];

V1 - Volume of potassium thiocyanate standard titration solution consumed in titrating silver, in milliliters (mL);

V0 - Volume of potassium thiocyanate standard titration solution consumed in titrating the blank solution, in milliliters (mL).

If the range of the three calibration results is not greater than 4 × 10^-^5 mol/L, take the average of the three calibration results; otherwise, recalibrate.

5.2.15 Gold standard stock solution. Weigh 0.1000 g of gold (wAu >= 99.99%) into a 100 mL beaker; add 10 mL of mixed acid (5.2.12); cover with a watch glass; evaporate to near dryness in a water bath; remove and cool slightly; add 20 mL of hydrochloric acid (1 + 1); rinse the beaker and watch glass with a small amount of water; transfer the solution to a 100 mL volumetric flask; dilute to the mark with water; mix well. 1 mL of this solution contains 1 mg of gold.

5.2.16 Gold standard solution. Transfer 10.00 mL of the gold standard stock solution

(5.2.15) into a 100 mL volumetric flask; add 20 mL of hydrochloric acid (1 + 1); dilute to the mark with water; mix well. 1 mL of this solution contains 100 µg of gold.

5.3 Instruments and equipment

5.3.1 Microbalance. Sensitivity 0.0001 mg.

5.3.2 Assay furnace. Maximum heating temperature 1200 °C.

5.3.3 Refractory clay crucible. Volume approximately 350 mL.

5.3.4 Ash dishes.

5.3.5 Specimen pulverizer.

5.3.6 Porcelain crucible. Volume 30 mL.

5.3.7 Porcelain crucible. Volume 50 mL or 100 mL.

5.3.8 Atomic absorption spectrometer, equipped with a gold hollow cathode lamp.

Under optimal instrument operating conditions, any instrument meeting the following specifications may be used.

- Characteristic concentration. In a solution consistent with the matrix of the measurement solution, the characteristic concentration of gold shall not exceed

0.08 µg/mL;

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