Method for the determination of gold by activated carbon adsorption iodometry

First, the method summary

The sample is decomposed with aqua regia, and Au is enriched in activated carbon in a rare aqua medium. After activated carbon ashing, it is re-dissolved with aqua regia, HCI is evaporated, and a small amount of Fe, Cu, etc. is masked by NH4HF2 and EDTA in acetic acid medium. Element, KI is added to reduce Au3+ to Au+, and free I2 is precipitated, and starch is used as an indicator, and titrated with sodium thiosulfate standard solution. This method is applicable to the determination of ω(Au)/10-6>0.5 in general ore.

Second, reagent preparation

Activated carbon - pulp: The activated carbon is first treated. The activated carbon with a particle size of 0.074 mm was immersed in a 20 g/L NH4HF2 solution for 3 days, filtered, and washed 7 to 8 times with HCl (2+98) and hot water. The treated activated carbon and the pulp were mixed at a mass ratio of 1+2 when dry.

Gold standard solution: Weigh 0. 0500g pure gold (99.99% or more) in 100mL porcelain crucible, add l0mL aqua regia, cover the surface dish, add and dissolve 8~10 drops 250g/L immediately after heating and dissolving in 60~70°C water bath The NaCl solution was heated to dryness on a boiling water bath, taken off, and 1 mL of HCI was added, and evaporation was continued on a boiling water bath. Remove a small amount of water, slightly heat the salt to dissolve, remove it to room temperature, transfer it to a 500mL volumetric flask containing 5mL HCI, dilute to the mark with water, and mix. This solution contained 100 μg/mL Au.

Sodium thiosulfate standard solution: weighed 25.2 g of sodium thiosulfate (Na2S2O3 · 5H2O) dissolved in freshly boiled and cooled distilled water, added 0.1 g of Na2CO3, diluted with water to 1 L (solution pH 7.2 ~ 7.5), 1mL of this solution is equivalent to 10mg Au.

Take 30mL and 100mL of the above solution in a l0L lower mouth bottle, add 1g of anhydrous Na2CO3 and 10mL of chloroform, dilute to 10L with cooled fresh distilled water, shake up, and set a week, then calibrate: take 30μg or 100μg Au in 50mL In the porcelain crucible, add 3~5 drops of 250g/L NaCl solution, add 2~3mL aqua regia, dilute in water bath, add 3~4 drops of HCI, evaporate and repeat twice. Then using two standard solution of sodium thiosulfate the amount of iodine by the titration. The mass of the Au solution was determined to be equivalent to the mass of Au (1 mL, respectively, equivalent to 30 μg and 100 μg), μg/mL.

Third, the analysis procedures

Weigh 10-30g (accurate to 0.01g) in a porcelain ark, burn in a high temperature furnace at 600 ° C for 40min, remove and let cool, transfer the sample into a 400mL beaker, wet with a small amount of water, add 100mL aqua regia (1+1), heated to boil for 30~60min, shake in the middle several times, remove the heat and add l0mL l0g/L animal glue solution, stir, dilute to 100~120mL with warm water, and inject the solution into the package. In a Buchner funnel with an activated carbon-pulp adsorption column, the beaker was washed twice with hot HCl (2+98) and the residue was 7 to 8 times. Remove the Buchner funnel, wash the activated carbon with hot 20g / L NH4HF2 solution 3~4 times, then wash it with hot HCI (2+98) 3~4 times, wash it with warm water 3~4 times, drain it. Take out The activated carbon-pulp adsorption cake is placed in a 50 mL porcelain crucible, placed on an electric furnace pan for low temperature drying, and heated to carbonization, and then transferred to a high temperature furnace at 650 to 700 ° C for burning to the presence of no carbon particles, and taken out for cooling. Add 5 drops of 250g/L NaCl solution to the porcelain crucible, add 2-3mL aqua regia along the crucible wall, steam dry on the boiling water bath, add 3~5 drops of HCI, continue to evaporate, repeat twice, and finally steam. No sour taste. Remove the mash, add 3~5mL of acetic acid (7+93), stir to dissolve all soluble salts, and cool to room temperature. Add 0.1 g of NH4HF2 and mix well. Immediately after adding a few drops of 25 g/L EDTA solution, 0.5 g of KI was added, and the solution was titrated to a pale yellow color with a sodium thiosulfate standard solution, and 3 to 5 drops of a 10 g/L starch solution were added, and the titration was continued until the blue color disappeared.

Fourth, the calculation of the analysis results

The content of Au in the sample was calculated by the following formula.

ω(Au)/10-6=(fT×VT)/(ms)

Where fT - 1mL Na2S2O3 standard solution is equivalent to the mass of Au, μg / mL;

VT - the volume of the Na2S2O3 standard solution consumed by the titration, mL;

Ms - the mass of the sample, g.

Five, matters needing attention

(1) The particle size of the sample should be less than 0.074mm to ensure the uniformity of the sample. (2) Depending on the nature of the sample, it can be dissolved by the following methods: 1) Sulfide ore with high sulfur content. First dissolve with reverse aqua regia (1+1), add l0mL each time, after the end of the intense reaction, add 100-200mL aqua regia (1+1); 2) low sulfur containing mineral sample. Was added 100 ~ 120mL aqua regia (1 + 1); 3) iron oxide ore. Add 50~60mL HCl, boil, add 50~60mL aqua regia (1+1); 4) Mineral sample containing carbon and organic matter. Burn off charcoal at 600~700°C, then treat according to oxidized ore sample. Or remove carbon by oxidation with HNO3-KClO3. (3) The amount of activated carbon should be determined according to the content of gold, generally the amount of Au is less than 5mg, the amount of activated carbon is 0. 5~0. 8g; the amount of Au is 5~30mg, the dosage is about 1g; the amount of Au is 30~60mg, The amount of activated carbon is 1~1. 5g. (4) Adsorption column installation: The adsorption column is tightly packed in the circular hole of the suction filter cylinder, the pulp is added into the adsorption column, the vacuum pump is started, the pumping is compacted, and the pulp is about 4 ~5mm thick, add a little pulp to dry, add activated carbon-pulp mixture (absolutely prevent activated carbon leakage), covered with a layer of pulp. Pack the Buchner funnel and filter it with filter paper. (5) Activated carbon - When the pulp is carbonized, do not catch fire to avoid loss of Au. (6) EDTA has a reducing effect on gold trichloride. When EDTA is added, it should be added to KI immediately and titrated rapidly.

Determination of gold by activated carbon enrichment flame method

First, the method

After the sample is burned at 650 ° C, the aqua regia is decomposed, and the animal glue is condensed with silicic acid, the activated carbon is used to dynamically adsorb Au, and then ashed and extracted to prepare a HCl (1+11) solution. The test solution is taken into the air-acetylene flame, and the absorbance of Au is measured by the AAS method. The method is applicable to the determination of ω(Au)/10-6=0.05-100 in Yanyoushi and minerals.

Second, equipment and reagent preparation

Activated carbon adsorption column: insert a glass adsorption column with an inner diameter of 35mm and a height of 90mm into the suction filter hole. Place a porous plastic plate with an outer diameter of 34mm in the column, pour it into the pulp for filtration, and filter the dried pulp layer to a thickness of about 3~4mm. Then add a mixture of activated carbon and pulp (1+2), filter it, and dry it to a thickness of 5~7mm (depending on the content of Au, make the activated carbon 0.5~1g), wash the column wall with water, add another layer Thin pulp. Install the Buchner funnel, pad two filter papers on the funnel and add a little pulp for use.

Activated carbon and its treatment: The particle size of activated carbon should be 0.075mm. If the purity of the activated carbon used is not enough, it can be treated by the following two methods: (1) Place the activated carbon in a plastic beaker, add hot 40g/L NH4HF2, and immerse the activated carbon in the solution. Leave it for more than 48h, then filter it, wash it with HCl (1+24) for 8~10 times, then wash it with hot water for 8 times, place it to dry, and (2) place the activated carbon in the beaker and add HCl (1+5). Immerse the activated carbon, heat and boil for 1 h, then soak for several hours, filter, wash with water, and repeat the treatment several times until the filtration does not show the yellow color of Fe3+.

Gold standard storage solution: Weigh 1.0000g high-purity Au in a beaker, add 20mL aqua regia, dissolve at low temperature, transfer to a 1000mL volumetric flask, add 80mL aqua regia, dilute to the mark with water, and mix. This solution contains 1000μg/mL Au.

Gold standard solution: Pipette 25.0 mL of Au standard stock solution in a 500 mL volumetric flask. Add 40 mL of aqua regia, dilute with water to the mark and mix. This solution contains 50.0 μg / mL Au.

Third, the instrument and working conditions

WFX-1B atomic absorption spectrometer. Gold hollow cathode lamp; lamp current 3mA; wavelength 242.8mm; spectral passband 0.4nm; burner height 6mm; air flow 7.0L/min; acetylene flow 1.8L/min.

Fourth, the analysis steps

Weigh 10.00~30.00g sample in porcelain crucible and raise it from low temperature to 650 °C for 2h (mixing once in the middle). Take out the cooling, transfer to a beaker, wet with water, add 80mL aqua regia, decompose at low temperature and steam to a volume of about 25mL, remove, rinse the table and cup wall with water, add 10mL 10g / L animal glue solution, stir for a few minutes Add 100 mL of water and mix well. Pour the sample solution into the Buchner funnel on the prepared activated carbon adsorption column, filter it by suction, wash the beaker and residue with hot HCl (1+19) for 4~5 times, remove the Buchner funnel, and use 50g/L NH4F. The hot solution is washed 7~8 times, then washed 7-7 times with hot HCl (1+19) and 7~8 times with hot water. Stop the suction filtration, move the activated carbon and pulp into the porcelain crucible, rub the residual activated carbon with a small piece of filter paper, combine it in the crucible, place the crucible on the electric furnace, and after the content of the crucible is dried and carbonized, transfer it to the muffle furnace at 700 °C. The mixture was burned until it was completely ashed and kept for 10 minutes. Add 5mL aqua regia to the wolfberry root, dissolve it at low temperature and dip it to 2~3mL, remove it into a 25mL volumetric flask, dilute with water to the mark, and mix. Adjust the AAS instrument to the specified working conditions, and take the test solution into the air-acetylene flame to determine the absorbance of Au.

Drawing of the working curve: separately transfer 0, 25, 50, 75, 100, 150, 200, 250μgAu standard solution into a set of 25mL volumetric flask, add 2mL aqua regia, dilute to the mark with water, and mix. It is measured in the same batch as the sample solution. Taking the mass concentration of Au as the abscissa and the corresponding absorbance as the ordinate, the working curve is drawn.

V. Calculation of analysis results

The content of Au in the sample was calculated by the following formula.

Where ωB is the mass fraction of the measured element (component), where B is the measured element (component)

ρB2——the mass concentration of the measured element in the test solution is found on the working curve, μg/mL;

ρB1——the mass concentration of the measured element in the blank test solution is found on the working curve, μg/mL;

Vs - the total volume of the sample solution, mL;

Ms—weigh the mass of the sample, g.

Six, matters needing attention

(1) The sample should be fully burned to remove organic matter, charcoal, sulfur, etc., otherwise the analysis result will be low, and (2) the ashing temperature after enrichment with activated carbon should not be too high to prevent the loss of Au. But the ashing must be complete, otherwise the result is low. (3) The volume of the test solution can be increased or decreased depending on the content of Au. (4) When analyzing a sample containing Sb and As high, it is first decomposed with HCl-SnCl2, separated by filtration, and the residue is fired at 700 ° C to remove coexisting interfering elements such as Sb and As. Then use aqua regia decomposition, foam adsorption, thiourea release. The release liquid was taken into an air-acetylene flame, and the absorbance of Au was measured by the AAS method. This method is applicable to the determination of w(As)/10-6=0.1~30 in rock minerals with high Sb and As.

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