Determination of lead and gold in crude lead—Atomic absorption spectrometry
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Determination of gold in crude lead — Atomic absorption spectrometry 1 Scope This recommended method uses atomic absorption spectrometry to determine the content of Au in crude lead. It is suitable for determining Au contents in crude lead ranging from 0.50 to 5.00 g/t. 2 Principle: After the sample is dissolved in nitric acid and tartaric acid, activated carbon is used to adsorb gold, thereby separating the impurities. In a hydrochloric acid medium, the solution is sprayed into an air-acetylene flame; a gold hollow cathode lamp is used as the light source, and the absorbance is measured at 242.8 nm using an atomic absorption spectrometer. Calculate the gold content. 3 Reagents: In analyses, unless otherwise specified, only analytically pure reagents and distilled water or water of equivalent purity are to be used. 3.1 Tartaric acid. 3.2 Activated carbon (200 mesh). Soak in an ammonium bifluoride (3.9) solution for three days, filter under vacuum, wash with water until neutral, and dry at 105°C for later use. 3.3 Hydrochloric acid (specific gravity 1.19). 3.4 Hydrochloric acid (1+19). 3.5 Nitric acid (specific gravity 1.42), reagent grade. 3.6 Nitric acid (1+3). 3.7 Nitric acid (1+19). 3.8 Aqua regia: Nitric acid (specific gravity 1.42) and hydrochloric acid (specific gravity 1.19) are mixed in a (1+3) ratio. Prepared fresh to order. 3.9 Ammonium hydrogen fluoride (2%). 3.10 Gold standard storage solution: Weigh 0.1000 g of pure gold (99.99% or higher), place it in a 100 ml beaker, add 10 ml of aqua regia (3.8), and heat on a water bath until complete dissolution; then evaporate the solution until it is almost dry. Add 10 ml of hydrochloric acid (3.3) to dissolve the residue, transfer it to a 100 ml volumetric flask, dilute with water to the mark, and mix well. This solution contains 1.00 mg of gold per 1 ml. 3.11 Gold standard solution: Transfer 10.00 ml of the gold standard storage solution (3.10) into a 1000 ml volumetric flask, add 10 ml of hydrochloric acid (3.3), dilute to the mark with water, and mix well. This solution contains 10 μg of gold per 1 ml. 4 Instruments 4.1 Atomic absorption spectrometer, equipped with an air-acetylene burner and an Au hollow cathode lamp. 4.2 The atomic absorption spectrometer used shall meet the following specifications. 4.2.1 Characteristic concentration The characteristic concentration of Au should be above 0.01 μg/mL. 4.2.2 Linearity of the working curve: The working curve should be divided into five equal segments based on concentration, and the ratio of the difference in absorbance between the highest segment and the lowest segment should be at least 0.7. 4.2.3 Minimum precision requirement: Use the standard solution at the highest concentration to measure the absorbance ten times, and calculate its average value and standard deviation. This standard deviation should not exceed 1.5% of the average absorbance value. Measure the absorbance ten times using the standard solution at the lowest concentration (not the standard solution with a concentration of zero), and calculate its standard deviation. This standard deviation should not exceed 0.5% of the average absorbance of the highest concentration standard solution. China Analysis Network 5 Operating Steps 5.1 Sampling Weigh approximately 10.00 g of sample with a particle size of less than 0.125 mm, with an accuracy of 0.0001 g. 5.2 Blank test A blank test is conducted alongside the sample. 5.3 Sample preparation: Place the sample in a 400 ml beaker. Add half a sheet of filter paper, 2 g of tartaric acid (3.1), and 80 ml of nitric acid (3.6); heat to dissolve until the solution becomes clear, then remove from heat and cool to room temperature. Add 0.2 g of activated carbon (3.2), stir 3–5 times, let it stand for 20 min, and filter using slow-speed quantitative filter paper. Rinse the walls of the seafood container, the filter paper, and activated carbon 5 times with warm nitric acid (3.7). Wash 5 times with warm water, then rinse twice with 2 ml of hydrochloric acid (3.3); wash 5 times with warm hydrochloric acid (3.4), and finally wash until neutral, then drain. Wrap the gold-bearing carbon, place it in a 30 ml porcelain crucible that already contains half a sheet of filter paper, and put it in a high-temperature furnace. With the furnace door slightly open, remove the carbon at a low temperature; then burn it in an environment with air circulation at 750–800°C for 10 minutes, after which it is taken out and cooled. Add 5 ml of aqua regia (3.8), heat to dissolve, evaporate on an electric hot plate until 1–2 ml remains, then transfer to a water bath and evaporate to dryness. Add 10.00 ml of hot hydrochloric acid (3.4), remove from the heat, and cool. 5.4 Measurement On an atomic absorption spectrometer, using an air-acetylene flame, the absorbance is measured at a wavelength of 324.8 nm with water used for zeroing. The gold concentration is obtained from the working curve. 5.5 Preparation of the working curve: Take 0.00, 1.00, 2.00, 3.00, 4.00, and 5.00 ml of the Au standard solution (3.11) and place each amount in a set of 30 ml porcelain crucibles. Evaporate the contents to dryness in a water bath, then add 10.00 ml of hot hydrochloric acid (3.4) to each crucible. Remove the crucibles from the bath and allow them to cool. Measure the absorbance according to procedure 5.4. The absorbance of each solution was subtracted from the absorbance of the zero-concentration solution, and a working curve was plotted with absorbance on the vertical axis and Au concentration on the horizontal axis. 6 Calculation results: The content of Au is calculated using the following formula, expressed as a mass fraction: Au = 126 × 100 / (10 × cc × Vwm), where wAu represents the mass fraction of Au, in % ； c1—The concentration of Au in the test solution, as determined from the calibration curve, in μg/mL ； c2 — the concentration of Au in the blank test solution, as determined from the working curve, in μg/mL ； V — Volume of the test solution, mL ； m – Mass of the sample, in g. 7 Precision: The difference in analysis results between laboratories should not exceed the allowable tolerances listed in the table below: Table 1 Allowable tolerances/μg/g for Au content: 0.50–2.00: 0.30; >2.00–5.00: 0.60. 8 References: YS/T 248.8—1994, Methods for chemical analysis of crude lead. China Analysis Network
Determination of Au by activated carbon adsorption-atomic absorption spectrophotometry I. Basic principle: The sample is calcined in a muffle furnace at 650–700°C to remove elemental C and S as well as organic substances; the ore sample is dissolved in aqua regia, with gold existing in the form of HAuCl4. Activated carbon is used for adsorption, and NH4HF2 is used to complex the other impurity ions other than HAuCl4 that have been adsorbed. After ashing the activated carbon adsorbent, gold exists in its elemental form. Dissolved in aqua regia, the volume was made constant, and the absorbance was measured at 242.8 nm using an atomic absorption spectrophotometer; the Au content (g/t) was determined from the corresponding calibration curve. Detection limit of measurement: 0.1 g/t. II. Reagent preparation: 1. Aqua regia solutions: 1+1, 1+19. 2. Preparation of the gold standard: ① Gold standard stock solution a: Accurately weigh 0.1000 g of Au (which has been dried at 105°C in a constant-temperature oven for 1 hour, with a purity of 99.99%) and place it in a 250 ml beaker. Dissolve it completely by gently heating it with 20 ml of 1+1 aqua regia, then transfer the solution to a 1000 ml volumetric flask. Fill the flask to the mark with water and mix thoroughly. This solution contains 100 μg/ml of Au. ②. Gold standard storage solution b: Accurately transfer 20 ml of the 100 μg/ml Au standard storage solution a into a 100 ml volumetric flask, dilute to the mark with water, and mix thoroughly. This solution contains 20 μg/ml of Au. ③. Gold standard solution: 0 ml, 2.5 ml, 5 ml, 10 ml, 15 ml, and 20 ml of the gold standard storage solution B were respectively taken and placed in 6 100 ml volumetric flasks; 4 ml of 1:1 aqua regia was added to each, and the volume was adjusted to the marked level with water, after which the mixture was thoroughly mixed. Then this solution contains 0 μg/ml, 0.5 μg/ml, 1.0 μg/ml, 2 μg/ml, 3 μg/ml, and 4 μg/ml of Au, respectively. 3. Filter pulp: Soak qualitative filter paper in water and crush it. 4. Activated carbon filter pulp: Take out some of the crushed filter pulp and slowly add activated carbon powder that has been boiled in water; continue adding until the entire filter pulp turns black. At this time, it is necessary to ensure that there is more than 0.2g of activated carbon per 15ml of filter pulp. 5. NH4HF2 and HCl solutions: both are 5% aqueous solutions. 6. Gelatin solution (20g/l): Weigh 20g of gelatin and dissolve it in 1000ml of hot water. III. Basic steps: 1. Weigh 5.0–20.0 g of sample on an electronic balance with a precision of 0.1 g, place it in a 50 ml porcelain crucible, and bake it in a muffle furnace at 650–750°C for 2 hours to ensure complete reaction of the elemental C and S as well as the organic substances present in the sample. 2. Remove the crucible, cool it, then transfer the sample from the crucible into a 250 ml beaker. Add a small amount of NH4HF2 and 100 ml of 1:1 aqua regia, and place the mixture in an electric furnace to heat it for 1 hour in order to ensure complete reaction of the Au in the sample. 3. Remove the beaker and add 50 ml of water to it to reduce acidity ； Add 2–3 ml of a 20 g/ml gelatin solution to precipitate the organic substances and silicates in the sample. 4. Install the adsorption column following this procedure: Fix the adsorption device → plastic filter plate → a layer of filter paper → turn on the air compressor → a cup of pulp → rinse to flatten → a cup of activated carbon pulp → rinse to flatten → a cup of pulp → rinse to flatten → a layer of filter paper → Büchner funnel → a layer of filter paper → a cup of pulp → a layer of filter paper. 5. Transfer all of the sample solution (including the sediment at the bottom of the beaker) to a Büchner funnel for vacuum filtration (make sure to clean the beaker thoroughly). 6. Wash the beaker and Büchner funnel 5 times each with a 5% aqua regia solution. 7. Remove the Büchner funnel, and add 5% NH4HF2, 5% HCl, and distilled water to the adsorption column respectively; rinse with each reagent 5 times. 8. Remove the adsorbed mass from the crucible, place the crucible in a muffle furnace and ash it at 750°C for 2 hours, with the furnace door slightly open. Once the activated carbon and filter paper have completely ignited and there is no open flame, remove the crucible and allow it to cool. 9. Add 5 ml of aqua regia to the crucible and place it on an electric furnace to apply gentle heat in order to dissolve elemental Au. When the solution in the crucible is 2 ml, remove the crucible and allow it to cool. 10. Bring the solution in the crucible to a constant volume in a colorimetric tube, dilute it with water to the calibration mark, and let it stand for 30 minutes. 11. Turn on the atomic absorption spectrophotometer, install the Au hollow cathode lamp, and preheat it for 10–30 minutes. 12. On AAS, using an Air-C2H2 flame, its absorbance is measured at 242.8 nm, and its purity is determined by comparing it with a calibration curve. IV. Points to note: 1. For samples with high levels of carbonates and sulfites, the addition of aqua regia for leaching must be done slowly; otherwise, the large amount of gas generated will cause the solution to splash, resulting in a lower gold content. 2. The acidity range for gold adsorption by activated carbon is quite wide. Gold can be quantitatively adsorbed in an HCl medium with a concentration of 1–6 mol/l or in aqua regia with a volume fraction of 10–40%. 3. Adding filter paper pulp to the activated carbon facilitates the adsorption of gold; meanwhile, cracks are less likely to occur during filtration, the filtration speed is fast, and the average adsorption rate is over 99.9%. 4. The relationship between the thickness of the activated carbon pulp filled in the adsorption column, the amount of activated carbon, and the amount of gold is as follows: Gold adsorption amount, Amount of activated carbon, Thickness of activated carbon pulp