GB/T 6730.9-2016Iron ores - Determination of silicon content - The silicomolybdic blue spectrophotometric method reduced by ammonium ferrous sulfate (English PDF)

4.1

Mixed flux, take 3 portions of anhydrous sodium carbonate and 1 portion of boric acid, evenly grind and mix them.

4.2 Sulfuric acid

rho = 1.89 g/mL.

4.3 Sulfuric acid

5 + 95.

4.4

Oxalic acid solution, 50 g/L.

4.5

Ammonium ferrous sulfate solution, 30 g/L.

Weigh-take 3 g of ferrous ammonium sulfate [(NH4 )2 Fe(SO4 )2 - 6H2 O], add 1 mL of sulfuric acid (1 + 1), use water to dilute to 100 mL; dissolve and filter for use. It shall remain valid for

1 week.

4.6

Ammonium molybdate solution, 50 g/L. Store it in a plastic bottle.

4.7

Silicon standard solution.

4.7.1

Silicon stock solution, 200.0 µg/mL.

Weigh-take 0.2140 g of silicon dioxide (above 99.9%) that has been burned to a constant mass at 1,000 °C, place it in a platinum crucible containing 4 g of mixed flux (4.1), carefully and mix it well, then, use 1 g of mixed flux (4.1) to cover it. Cover the platinum cover, melt and decompose it at 900 °C ~ 950 °C for 30 min, take it out, slowly turn it to make the molten liquid evenly distributed on the inner wall of the crucible and cool it down. Put the crucible and the cover into a polytetrafluoroethylene beaker filled with water, at a low temperature, heat and leach it, then, wash the crucible and the cover. Cool it to room temperature, transfer the leaching solution into a 500 mL volumetric flask; use water to dilute to the mark, mix it well, then, immediately transfer it to a plastic bottle for storage.

7.2 Amount of Specimen

Weigh-take 0.20 g of specimen, accurate to 0.0001 g.

7.3 Blank Test and Verification Test

7.3.1 Blank Test

Weigh-take the same amount of high-purity ferric oxide (iron content greater than 99.98%) as the specimen; along with the specimen, conduct a blank test.

7.3.2 Verification test

Along with the specimen, analyze the same type of reference sample.

7.4 Determination

7.4.1 Specimen decomposition

Place the specimen in a platinum crucible containing 4 g of mixed flux (4.1), carefully and mix it well, then, use 1 g of mixed flux (4.1) to cover it. Cover the platinum cover, melt and decompose it at 900 °C ~ 950 °C for 15 min ~ 30 min, take out the crucible, slowly turn it to make the molten liquid evenly distributed on the inner wall of the crucible and cool it down.

Put the cooled crucible in a beaker filled with 200 mL of sulfuric acid (4.3), at a low temperature, heat and leach it, then, wash the platinum crucible and the cover, and cool to room temperature.

Transfer the leaching solution into a 250 mL volumetric flask, use water to dilute to the mark, mix it well and reserve it for later use. If there is precipitation, adopt the dry filtration method to filter it.

7.4.2 Color development

Respectively take two portions of 5.00 mL (when the silicon content is less than 1%, respectively take 10.00 mL) of the solution (7.4.1) in 100 mL volumetric flasks. One is used as a color developing solution, and the other is used as a reference solution.

a) Color developing solution

Take one portion of the test solution, add 5 mL of ammonium molybdate solution (4.6), add 30 mL of water to mix it well, and let it stand for 15 min (when the room temperature is lower than 15 °C, let it stand for 40 min or heat it in a boiling water bath for 30 s, then, immediately cool it). Add 10 mL of oxalic acid solution (4.4) and mix it well; within 30 s after the solution becomes clear, add 10 mL of ammonium ferrous sulfate solution (4.5), use water to dilute to the mark and mix it well.

b) Reference solution

Take the other portion of the test solution, add 10 mL of oxalic acid solution (4.4), 5 mL of ammonium molybdate solution (4.6) and 10 mL of ammonium ferrous sulfate solution (4.5); use water to dilute to the mark and mix it well.

7.4.3 Absorbance measurement

At a wavelength of 760 nm of the spectrophotometer, use an appropriate cuvette to measure the absorbance, subtract the absorbance of the blank test solution, and find out the content of silicon in the test solution on the calibration curve. For specimens with a silicon content lower than

0.5%, the absorbance may be measured at a wavelength of 810 nm.

7.4.4 Preparation of calibration curve

7.4.4.1

When the silicon content is 0.1% ~ 1.0%

Respectively take 0 mL, 1.00 mL, 3.00 mL, 5.00 mL, 7.00 mL and 9.00 mL of silicon standard solution (4.7.3) into 100 mL volumetric flasks. Add 10 mL of the blank test solution (7.3.1) and

5 mL of ammonium molybdate solution (4.6), use water to dilute to 40 mL and mix it well. Let it stand for 15 min (when the room temperature is lower than 15 °C, let it stand for 40 min or heat it in a boiling water bath for 30 s, then, immediately cool it). Add 10 mL of oxalic acid solution (4.4) and mix it well; within 30 s after the solution becomes clear, add 10 mL of ammonium ferrous sulfate solution (4.5), use water to dilute to the mark and mix it well. Take the color developing solution without silicon as a reference, at a wavelength of 760 nm of the spectrophotometer, select an appropriate cuvette, measure the absorbance and draw a calibration curve.

7.4.4.2

When the silicon content is 1.0% ~ 5.0%

Respectively take 0 mL, 1.00 mL, 2.00 mL, 3.00 mL, 4.00 mL and 5.00 mL of silicon standard solution (4.7.2) into 100 mL volumetric flasks. Add 5 mL of the blank test solution (7.3.1) and

5 mL of ammonium molybdate solution (4.6), use water to dilute to 40 mL and mix it well. Let it stand for 15 min (when the room temperature is lower than 15 °C, let it stand for 40 min or heat it in a boiling water bath for 30 s, then, immediately cool it). Add 10 mL of oxalic acid solution (4.4) and mix it well; within 30 s after the solution becomes clear, add 10 mL of ammonium ferrous sulfate solution (4.5), use water to dilute to the mark and mix it well. Take the color developing solution without silicon as a reference, at a wavelength of 760 nm of the spectrophotometer, select an appropriate cuvette, measure the absorbance and draw a calibration curve.

8.1 Calculation of Silicon Content

In accordance with Formula (1), calculate the silicon content (mass fraction) wSi in the specimen:

8.2.3 Inter-laboratory precision

The inter-laboratory precision is used to evaluate the consistency between the final results reported by two laboratories. After the two laboratories report the results in accordance with the same steps specified in 8.2.2, in accordance with Formula (6), calculate the average valueµ12 of the final results:

Where,

µ1 ---the final results reported by Laboratory 1;

µ2 ---the final results reported by Laboratory 2;

µ12 ---the average value of the final results.

If µ1 -µ2 <= P (see 8.2.1), the final results are consistent.

8.2.4 Acceptance of analytical values

The acceptance of analytical values is verified using certified reference samples. The steps are the same as described above. After confirming the precision, the final laboratory results are compared with the standard value Ac . For example:

a) µc - Ac <= C, there is no significant difference between the measured value and the standard value.

b) µc - Ac > C, there is a significant difference between the measured value and the standard value.

Where,

µc ---the measured value of the reference sample;

Ac ---the standard value of the reference sample;

C---this value depends on the type of reference sample used.

For the reference sample determined by inter-laboratory, in accordance with Formula (7), calculate C:

Where, V (Ac ) is the variance of the standard value Ac .