GB/T 3654.3-2019Ferroniobium—Determination of silicon content—Gravimetric method (English PDF)

7.1 Sample Amount

Weigh-take 0.50 g of sample, accurate to 0.0001 g.

7.2 Times of Determination

In terms of the same sample, at least conduct independent determination twice.

7.3 Blank Test

Conduct blank test along with the sample.

7.4 Determination

Along the inner wall of the crucible, add 4 drops of sulfuric acid (4.6) to wet the dry residue. Add 4 mL ~ 5 mL of hydrofluoric acid (4.4). Place it on the electric heating plate, then, heat it up, evaporate it, till it emits sulfuric acid smoke. Slightly cool it down, then, along the inner wall of the crucible, add 4 mL of hydrofluoric acid (4.4). Continue to heat it up, evaporate it, till it emits all the sulfuric acid smoke. Place the crucible in 1,050 °C high-temperature furnace (5.4), burn for 20 min. Then, take it out to slightly cool it down. Place it in a dryer, cool it down to room temperature, then, weigh it. Repeatedly burn it, till it reaches a constant mass.

7.4.1 Sample decomposition

Place the sample (7.1) in the nickel crucible (5.2); add 2 g of sodium hydroxide (4.1), 4 g of sodium peroxide (4.2), mix them up. On an electric furnace, heat it up, bake it, till it reaches a dry and hard state. Put on the crucible cover; place it into the high-temperature furnace (5.4). From room temperature, slowly heat it up to around 700 °C.

At 700 °C, continue melting for 15 min ~ 20 min; take it out to cool it down. Use water to wash the bottom of the crucible; place it into a 500 mL beaker, which contains 150 mL of sulfuric acid (4.7) and is covered with a watch glass, to leach the molten block.

Wash the crucible and the cover; stir it, till there is no precipitation of hydroxide in the solution.

7.4.2 First dehydration of silicic acid

Place a beaker on an electric heating plate or a low-temperature electric furnace.

Conduct cryo-concentration, till it emits sulfuric acid smoke. Then, at a high temperature, heat it up to make the sulfuric acid smoke reflux in the beaker for 15 min ~ 20 min. Then, take it down to slightly cool it down. Slowly add 10 mL of hydrochloric acid (4.3), the, cool it down; add 150 mL of hot water and 5 mL of hydrogen peroxide (4.5), then, evenly stir it. Heat it up, till it nearly reaches boiling; dissolve the precipitated niobium pentoxide hydrate and soluble salts. If the solution is not pellucid, 1 mL ~ 2 mL of hydrogen peroxide (4.5) may be added to dissolve it. Use medium-speed quantitative filter paper, which adds a little amount of paper pulp, to filter it. Use glass rod, which has a rubber head, to wipe the watch glass, the glass road and the inner wall of the beaker. Use hot sulfuric acid washing solution (4.8) to wash it. Then, rinse and precipitate it for 7 times ~ 9 times; transfer the precipitation, along with the filter paper, into the platinum crucible (5.3) for reservation.

7.4.3 Secondary dehydration of silicic acid

Add 5 mL of sulfuric acid (4.6) into the filtrate; place it on the electric heating plate, heat it up, till it emits sulfuric acid smoke for 5 min ~ 10 min. Then, take it down to slightly cool it down. Proceed in accordance with the subsequent procedures in 7.4.2.

7.4.4 Burning precipitation

Place the precipitation, along with the filter paper, into the platinum crucible (7.4.2;

7.4.3). On an electric heating plate, at a low temperature, dry it. At 500 °C ~ 600 °C, in the high-temperature furnace (5.4), ash it. Then, continue to raise the temperature to 1,050 °C; burn for 40 min. Take it out to slightly cool it down. Then, place it in a dryer to cool it down to room temperature, then, weigh it. Repeatedly burn it, till it reaches a constant mass.

8.1 Result Calculation

In accordance with Formula (1), calculate the mass fraction of silicon in the sample, expressed in (%):

Where, wSi---mass fraction of silicon, expressed in (%); m1---the mass of precipitation and platinum crucible before hydrofluoric acid treatment, expressed in (g); m2---the mass of residue and platinum crucible after hydrofluoric acid treatment, expressed in (g); m3---the mass of precipitation and crucible in the blank test before hydrofluoric acid treatment, expressed in (g); m4---the mass of precipitation and crucible in the blank test after hydrofluoric acid treatment, expressed in (g); […]