GB/T 29875-2013Determination of lead, arsenic, mercury in phosphate rock and concentrate (English PDF)

1 Scope

This standard specifies methods for determining the lead, arsenic and mercury contents of phosphate rock and phosphate concentrate.

It applies to the determination of lead contents above 0.001 0 %, arsenic contents above 0.000 2 % and mercury contents above 0.000 01 % in phosphate rock and phosphate concentrate.

2 Normative references

The following documents are indispensable for the application of this document. For dated references, only the edition cited applies; for undated references, the latest edition, including all amendments, applies.

The documents listed are GB/T 602 (chemical reagents - preparation of standard solutions for impurity determination), GB/T 603 (chemical reagents - preparation of preparations and products used in test methods), GB/T 6003.1 (test sieves - technical requirements and testing - Part 1: test sieves of metal wire cloth), GB/T 6682 (water for analytical laboratory use - specification and test methods) and GB/T 9723 (chemical reagents - general rules for flame atomic absorption spectrometry).

3 Test sample

The test sample passes a 125 µm test sieve complying with GB/T 6003.1, is dried at 105 °C to 110 °C for 2 h or more, and is then cooled to room temperature in a desiccator.

4.1 General

Unless otherwise stated, only reagents recognised as analytical grade or better are used in the analysis, and the water used meets the requirements of grade two water in GB/T 6682.

Where no preparation method is given for a reagent or solution used in the standard, it is prepared according to GB/T 602 and GB/T 603.

4.2 Determination of lead content - atomic absorption spectrometry

Summary of the method: the test sample is dissolved with hydrochloric acid and nitric acid; in dilute nitric acid medium and with an acetylene-air flame, the absorbance of the sample solution is measured with an atomic absorption spectrometer at a wavelength of 283.3 nm, background interference is subtracted at the same time, and the lead content of the phosphate sample is obtained by the working curve method.

Reagents and solutions: hydrochloric acid (guaranteed reagent); nitric acid (guaranteed reagent); nitric acid solution 1+1; lead standard solution 1 000 µg/mL; lead standard solution 100 µg/mL, prepared by transferring 10 mL of the 1 000 µg/mL solution to a 100 mL volumetric flask, adding 10 mL of the nitric acid solution and diluting to the mark with water, so that 1 mL of the solution contains 100 µg of lead.

Apparatus: an atomic absorption spectrometer fitted with a lead hollow cathode lamp and complying with GB/T 9723.

Analytical procedure: 1 g of the test sample is weighed to the nearest 0.000 1 g into a 150 mL beaker and a blank test is run at the same time. The sample is wetted with a little water, 15 mL of hydrochloric acid and 5 mL of nitric acid are added carefully, the beaker is covered with a watch glass and heated gently on a hot plate, evaporated almost to dryness, then removed and cooled. Ten millilitres of the nitric acid solution and a little water are added, the soluble salts are dissolved by heating, the solution is cooled to room temperature and transferred to a 100 mL volumetric flask, diluted to the mark with water and mixed; it is then filtered dry, the first small portion of filtrate is discarded and the rest is collected for use. With reference to the instrument manual the working parameters are optimised, an acetylene-air flame and the lead hollow cathode lamp are used, and at 283.3 nm, with water as the zero, the absorbances of the sample solution and of the blank solution are measured in background correction mode; the corresponding lead mass concentration is read from the working curve.

Plotting of the working curve: 0.0 mL, 1.0 mL, 3.0 mL, 5.0 mL, 7.0 mL and 10.0 mL of the 100 µg/mL lead standard solution are transferred to a set of 100 mL volumetric flasks, 10 mL of the nitric acid solution is added to each, and each is diluted to the mark with water and mixed; the lead concentrations of this series are 0.0 µg/mL, 1.0 µg/mL, 3.0 µg/mL, 5.0 µg/mL, 7.0 µg/mL and 10.0 µg/mL. Their absorbances are measured in background correction mode under the same conditions as the sample solution, the reagent blank absorbance is subtracted, and the curve is plotted with lead mass concentration on the abscissa and absorbance on the ordinate.

Calculation of the result: the lead content is expressed as the mass fraction and is obtained from the lead mass concentration read off the working curve, in micrograms per millilitre, and the mass of the test portion, in grams. The arithmetic mean of parallel determinations is taken as the final result.

Permissible difference: the relative deviation of parallel analytical results is not greater than the permissible values of Table 1, which gives 30 % for a lead content below 0.002 0 %, 20 % for a lead content from 0.002 0 % to 0.010 % and 10 % for a lead content above 0.010 %.

4.3.1 Determination of arsenic content - atomic fluorescence spectrometry (referee method)

Summary of the method: the test sample is dissolved with hydrochloric acid and nitric acid; under acidic conditions trivalent arsenic reacts with potassium borohydride to form arsine, which is carried by the carrier gas (argon) into a quartz atomiser where it decomposes to atomic arsenic. Under the irradiation of a special arsenic hollow cathode lamp the ground state arsenic atoms are excited to a high energy state and, on returning to the ground state, emit fluorescence of characteristic wavelength; within a certain concentration range the fluorescence intensity is proportional to the arsenic content, and the arsenic content of the phosphate sample is obtained by the working curve method.

Reagents and solutions: hydrochloric acid and nitric acid (guaranteed reagent); hydrochloric acid solutions 1+1 and 5+95; potassium hydroxide solution 5 g/L; potassium borohydride solution 10 g/L, prepared by dissolving 1 g of potassium borohydride in 100 mL of the potassium hydroxide solution and used freshly prepared; ascorbic acid solution 50 g/L, freshly prepared; thiourea solution 50 g/L; arsenic standard solution 1 000 µg/mL, from which are prepared in turn a 50 µg/mL solution (10.0 mL diluted to 200 mL), a 1 µg/mL solution (10.0 mL of the 50 µg/mL solution diluted to 500 mL) and a 100 ng/mL solution (10.0 mL of the 1 µg/mL solution diluted to 100 mL).

Apparatus: an atomic fluorescence spectrometer fitted with an arsenic hollow cathode lamp.

Analytical procedure: the test portion is weighed according to Table 2 to the nearest 0.000 1 g into a 150 mL beaker and a blank test is run at the same time. The sample is wetted with a little water, 15 mL of hydrochloric acid and 5 mL of nitric acid are added carefully, the beaker is covered with a watch glass, heated gently on a hot plate and evaporated to 1 mL to 2 mL, then removed and cooled to room temperature; a note warns that the sample solution is not to be evaporated to dryness, since the result would then be low. Ten millilitres of the 1+1 hydrochloric acid solution and a little water are added, the soluble salts are dissolved by heating, the solution is cooled to room temperature and transferred to a 100 mL volumetric flask, 10 mL of the ascorbic acid solution and 10 mL of the thiourea solution are added, the flask is diluted to the mark with water, mixed and left to stand for 30 min; the solution is then filtered dry, the first small portion of filtrate is discarded and the rest is collected. With the instrument parameters optimised, the 5+95 hydrochloric acid solution is used as the carrier and the potassium borohydride solution as the reductant, and the fluorescence values of the sample solution and of the blank solution are measured in turn.

Table 2 grades the test portion mass by arsenic content: 0.2 g to 0.5 g for an arsenic content of 0.000 2 % to 0.001 0 %, 0.1 g to 0.2 g for 0.001 0 % to 0.002 0 %, and 0.05 g to 0.1 g above 0.002 0 %.

Plotting of the working curve: 0.0 mL, 1.0 mL, 2.0 mL, 5.0 mL, 10.0 mL and 20.0 mL of the 100 ng/mL arsenic standard solution are transferred to a set of 100 mL volumetric flasks, 10 mL of the 1+1 hydrochloric acid solution, 10 mL of the ascorbic acid solution and 10 mL of the thiourea solution are added to each, and each is diluted to the mark, mixed and left for 30 min; the arsenic concentrations of the series are 0.0 ng/mL, 1.0 ng/mL, 2.0 ng/mL, 5.0 ng/mL, 10.0 ng/mL and 20.0 ng/mL. The fluorescence values are measured under the same conditions as the sample solution, the reagent blank value is subtracted, and the curve is plotted with arsenic mass concentration on the abscissa and fluorescence value on the ordinate.

Calculation and permissible difference: the arsenic content is expressed as the mass fraction and is obtained from the arsenic mass concentration read off the working curve, in nanograms per millilitre, and the mass of the test portion, in grams; the arithmetic mean of parallel determinations is the final result. Table 3 gives the permissible relative deviation as 30 % for an arsenic content below 0.000 5 %, 20 % for 0.000 5 % to 0.002 0 % and 10 % above 0.002 0 %.

4.3.2 Determination of arsenic content - Ag-DDTC spectrophotometry

Summary of the method: the test sample is dissolved with nitric acid and perchloric acid; in hydrochloric acid medium arsenic acid is reduced to arsenious acid with tin dichloride and potassium iodide, the arsenious acid is then reduced to arsine gas with metallic zinc, the gas is absorbed in a triethanolamine-chloroform solution of silver diethyldithiocarbamate to form a red colloidal silver, and the absorbance is measured at a wavelength of 530 nm; the arsenic content of the phosphate sample is obtained by the working curve method.

Reagents and solutions: nitric acid, hydrochloric acid and perchloric acid (guaranteed reagent); hydrochloric acid solution 1+1; arsenic-free metallic zinc granules; potassium iodide solution 300 g/L; citric acid solution 500 g/L; tin dichloride solution 400 g/L, prepared by dissolving 40 g of tin dichloride with slight warming in 50 mL of hydrochloric acid, diluting to 100 mL with water and storing in a brown reagent bottle; silver diethyldithiocarbamate (Ag-DDTC) solution, prepared by dissolving 0.3 g of powdered Ag-DDTC in 100 mL of chloroform, adding 3 mL of triethanolamine, mixing and filtering through degreased cotton before use, the solution keeping for one week; lead acetate cotton, prepared by dissolving 11.8 g of lead acetate in 100 mL of water, adding a few drops of glacial acetic acid, immersing degreased cotton in the solution, removing it after 2 h, squeezing out the solution and drying it; arsenic standard solutions of 1 000 µg/mL, 50 µg/mL (10.0 mL diluted to 200 mL) and 2.5 µg/mL (10.0 mL of the 50 µg/mL solution diluted to 200 mL).

Apparatus: a spectrophotometer, and the arsine generation and absorption assembly shown in Figure 1. The figure is keyed as follows: 1, hydrogen generator, a 100 mL ground-mouth conical flask; 2, the ground-glass stopper connecting the gas delivery tube, carrying a hydrogen sulfide absorption bottle of 20 mm diameter filled with lead acetate cotton; 3, the rubber tube connecting the two glass delivery tubes; 4, the gas delivery tube, with an internal diameter at the outlet below 1 mm; 5, the arsine absorption tube, a 10 mL stoppered colorimetric tube.

Analytical procedure: the test portion is weighed according to Table 4 to the nearest 0.000 1 g into a 150 mL beaker and a blank test is run at the same time. The sample is wetted with a little water, 5 mL of nitric acid and 5 mL of perchloric acid are added and mixed, the beaker is covered with a watch glass and heated; once the sample has dissolved it is evaporated until white fumes appear, without going to dryness, then removed, allowed to cool slightly, the beaker wall is rinsed once with water, the solution is heated again to white fumes, removed and cooled, 20 mL of water is added and heating is continued until the salts dissolve; after cooling the solution is transferred to a 100 mL ground-mouth conical flask and mixed. A note warns that the sample solution is not to be evaporated to dryness and that it may be removed as soon as dense white fumes appear. Five millilitres of the citric acid solution and 10 mL of the 1+1 hydrochloric acid solution are added and water is added to a volume of about 45 mL. Three millilitres of the potassium iodide solution and 2 mL of the tin dichloride solution are added, the flask is shaken and left for 15 min, 5 g of zinc granules are added, the stopper is fitted quickly and the gas delivery tube is at once inserted into the absorption tube containing 10.0 mL of the arsine absorbing solution; absorption lasts 40 min. The delivery tube is then withdrawn and rinsed with a little chloroform into the absorption tube, which is made up to 10.0 mL with chloroform, stoppered and mixed. A first note requires the arsine absorption tube and the gas delivery tube to be washed clean and dried after each use; a second note requires every joint of the generation and absorption assembly to be tightly connected and checked before use, so that arsine cannot escape. Using a 1 cm cell, the absorbance of the sample solution is measured on the spectrophotometer at 530 nm against the blank solution, and the corresponding mass of arsenic is read from the working curve.

Table 4 grades the test portion mass by arsenic content: 1.0 g for an arsenic content of 0.000 2 % to 0.002 5 %, 0.2 g to 0.5 g for 0.002 5 % to 0.010 %, and 0.1 g above 0.010 %.

Plotting of the working curve: 0.0 mL, 2.0 mL, 4.0 mL, 6.0 mL, 8.0 mL and 10.0 mL of the 2.5 µg/mL arsenic standard solution are transferred to a set of arsine generation flasks, 5 mL of the citric acid solution and 10 mL of the 1+1 hydrochloric acid solution are added to each and water is added to about 45 mL; the procedure then follows the two steps used for the sample, with the reagent blank solution as reference. The curve is plotted with arsenic concentration on the abscissa and absorbance on the ordinate.

Calculation and permissible difference: the arsenic content is expressed as the mass fraction and is obtained from the mass of arsenic read off the working curve, in micrograms, and the mass of the test portion, in grams; the arithmetic mean of parallel determinations is the final result. Table 5 gives the permissible relative deviation as 50 % for an arsenic content below 0.001 0 %, 20 % for 0.001 0 % to 0.002 0 % and 10 % above 0.002 0 %.

4.4.1 Determination of mercury content - atomic fluorescence spectrometry (referee method)

Summary of the method: the test sample is dissolved with hydrochloric acid and nitric acid; under acidic conditions the mercury in the sample is reduced by potassium borohydride to atomic mercury, which is carried by the carrier gas (argon) into a quartz atomiser. Under the irradiation of a special mercury hollow cathode lamp the ground state mercury atoms are excited to a high energy state and, on returning to the ground state, emit fluorescence of characteristic wavelength; within a certain concentration range the fluorescence intensity is proportional to the mercury content, and the mercury content of the phosphate sample is obtained by the working curve method.

Reagents and solutions: hydrochloric acid and nitric acid (guaranteed reagent); nitric acid solutions 1+1 and 5+95; potassium hydroxide solution 5 g/L; potassium borohydride solution 1 g/L, prepared by dissolving 0.1 g of potassium borohydride in 100 mL of the potassium hydroxide solution and used freshly prepared; mercury standard solution 1 000 µg/mL, from which are prepared in turn a 50 µg/mL solution (10.0 mL diluted to 200 mL), a 1 µg/mL solution (10.0 mL of the 50 µg/mL solution diluted to 500 mL) and a 100 ng/mL solution (10.0 mL of the 1 µg/mL solution diluted to 100 mL).

Apparatus: an atomic fluorescence spectrometer fitted with a mercury hollow cathode lamp.

Analytical procedure: about 1 g of the test sample is weighed to the nearest 0.000 1 g into a 150 mL beaker and a blank test is run at the same time. The sample is wetted with a little water, 15 mL of hydrochloric acid and 5 mL of nitric acid are added carefully, the beaker is covered with a watch glass, heated gently on a hot plate and evaporated to 1 mL to 2 mL, then removed and cooled. Ten millilitres of the 1+1 nitric acid solution and a little water are added, the soluble salts are dissolved by heating, the solution is cooled to room temperature and transferred to a 100 mL volumetric flask, diluted to the mark with water, mixed and filtered dry; the first small portion of filtrate is discarded and the rest is collected. A first note warns that the sample solution is not to be evaporated to dryness, since the result would then be low; a second note allows the test portion to be reduced, or the solution to be diluted, when the mercury content of the sample is high. With the instrument parameters optimised, the 5+95 nitric acid solution is used as the carrier and the potassium borohydride solution as the reductant, and the fluorescence values of the sample solution and of the blank solution are measured in turn.

Plotting of the working curve: 0.0 mL, 1.0 mL, 2.0 mL, 3.0 mL, 4.0 mL and 5.0 mL of the 100 ng/mL mercury standard solution are transferred to a set of 100 mL volumetric flasks, 10 mL of the 1+1 nitric acid solution is added to each, and each is diluted to the mark and mixed; the mercury concentrations of the series are 0.0 ng/mL, 1.0 ng/mL, 2.0 ng/mL, 3.0 ng/mL, 4.0 ng/mL and 5.0 ng/mL. The fluorescence values are measured under the same conditions as the sample solution, the reagent blank value is subtracted, and the curve is plotted with mercury mass concentration on the abscissa and fluorescence value on the ordinate.

Calculation and permissible difference: the mercury content is expressed as the mass fraction and is obtained from the mercury mass concentration read off the working curve, in nanograms per millilitre, and the mass of the test portion, in grams; the arithmetic mean of parallel determinations is the final result. Table 6 gives the permissible relative deviation as 100 % for a mercury content below 0.000 1 %, 50 % for 0.000 1 % to 0.001 0 % and 20 % above 0.001 0 %.

4.4.2 Determination of mercury content - hydride generation atomic absorption spectrometry

Summary of the method: the test sample is dissolved with hydrochloric acid and nitric acid; under acidic conditions the mercury in the sample is reduced by potassium borohydride to atomic mercury, and the mercury vapour is carried by the carrier gas (argon) into an atomic absorption spectrometer. Mercury atomic vapour absorbs strongly the ultraviolet light of wavelength 253.7 nm and, within a certain concentration range, the absorbance is proportional to the concentration of the mercury vapour; the mercury content of the phosphate sample is obtained by the working curve method.

Reagents and solutions: the first entries of the list are hydrochloric acid (guaranteed reagent), nitric acid (guaranteed reagent) and nitric acid solution 1+1. The remainder of the list falls beyond the pages consulted for this record.