Silica in Alumino Silicate Refractory Materials
Scope/Purpose:
This SOPs covers a method for determination of silica in alumino silicate refractory materials.
Principle:
The sample is decomposed by fusion mixture and extracted with acids. Silica is determined by dehydration and baking followed by hydrofluorization. The residual silica is determined photometrically and combined.
Apparatus/Equipment:
- Platinum crucibles
- Muffle Furnace (950- 1000 °C)
- Balance
- UV-Vis Spectrometer
- porcelain dish
- Water bath
- Hot oven
- Desiccator
Reagent
- Fusion mixture – Mix equal amount of sodium carbonate and potassium carbonate.
- Potassium iodide – 1 percent (m/v).
- Dilute hydrochloric acid – 1:1, 1:3 and 1:20 (v/v).
- Dilute sulphuric acid – 1:1 (v/v).
- Methyl alcohol
- Concentrated hydrochloric acid (R.D. = 1.16)
- Hydrofluoric acid – 40 percent,
- Potassium bisulphate – Solid.
- EDTA solution (0.05 M) – Dissolve 18.611 g of EDTA (disodium salt) in 1 liter of water.
- Phenolphthalein indicator solution – 0.1 percent (m/v) in ethanol.
- Dilute ammonia solution – 1:1 (v/v).
- Ammonium molybdate solution – Dissolve 10 g of ammonium molybdate [ (NH4)6 /10,024.4Hs0] in 100 ml of water, filter if necessary.
- Oxalic acid solution -10% (m/v).
- Ascorbic acid solution – 10% (m/v) stored in an amber color bottle (stable up to 5 days).
- Standard silica solution
- a) Fuse 0.20 g of pure silica with 5.0 g of fusion mixture in a platinum crucible. Cool and dissolve the melt in 300 ml of water containing 20 g of sodium hydroxide in a polythene beaker. Warm the solution on a water bath and dilute to 500 ml in a volumetric flask. Store in a polythene bottle.
- b) Prepare immediately before use, a standard working solution containing 0.04 mg/ml of silica (SiO2) by diluting the above stock solution.
Procedure:
- Weight 1 gm approx. of finely ground test sample platinum crucible.
- Add 6gm of fusion mixture followed by 2-3 drops of potassium iodide solution and mix well.
- Heat the platinum crucible over a gas burner. Increase the temperature and keep it for 20 mins. Then cool it and observe that a transparent glass is obtained.
- Transfer into a porcelain dish. Add Dil. HCL 1:1 then add 2 ml Dil. Sulphuric acid.
- Add 10 ml of methanol and placed the dish on a Water bath. To remove boron trioxide by treating 2-3 times with 10 ml of methanol and evaporate the solution to dryness until the smell of HCL is no longer detected.
- Finally, place the dish in a hot oven, dry it at 110°C. Cool the dish then add 10 ml of conc. HCL and 40 ml of hot water, stir it.
Determination of major silica by Gravimetric method-
- Cover the dish and allow the mixture to digest on a water bath for 10 mins. Use Whatman paper no. 42 for filtration then transfer the mixture to filter paper along with dil. HCL 1:20. Wash the precipitate 5 times with hot dil. HCL 1:20 then wash again with hot water.
- Reserve the filtrate (Solution A).
- Transfer the precipitated filtrate paper to platinum crucible which is ignited and weighted.
- Ignite at low temperate until the precipitate is free from carbonaceous matter. Then heat in muffle furnace at 950- 1000 °C to constant mass. Cool the crucible in desiccator and weight it.
- Moisten the content with water. Add 5 drops of Dil. Sulphuric acid and 10 ml of hydrofluoric acid (HF). Evaporate to dryness on hot plate.
NOTE: – Do not use Glass equipment for HF.
- Heat the crucible and residue cautiously first over gas flame and finally for 2 min at 950 – 1000 °C in the furnace. Cool in desiccator and weight to constant mass.
Preparation of main solution
- Remaining residue (HF). Add 1 gm of potassium bisulphate in platinum crucible. Head the crucible over a gas burner for 5 mins. Dissolve the melt in water containing a few drops of concentrated hydrochloric acid and add the solution to the main filtrate (solution A). Cool and dilute to 500 ml in a volumetric flask.
Determination of Residual Silica by Photometric method-
Preparation of calibration solution
Prepare calibration solution from standard solution (1ml= 40µg SiO2)
Vol. from Std. Sol. Conc. (in 100ml) Conc. In sample
Blank
1ml 0.4 µg 40mg/l
2ml 0.8 µg 80mg/l
3ml 1.2 µg 120mg/l
4ml 1.6 µg 160mg/l
6ml 2.4 µg 240mg/l
- Take 0 ml ,1ml,2ml,3ml,4ml and 6ml of standard solution in six 100ml volumetric flask and take 10 ml of main solution in 100ml of volumetric flask.
- Add 5ml EDTA solution followed by a drop of phenolphthalein indicator and then add dil. Ammonium solution until the color changes to red.
- Add 5ml of ammonium molybdate sol. And allow to stand for 10 mins.
- Add 10ml of oxalic acid followed by 5ml of ascorbic acid and make up100 ml with D/W.
- Allow the solution to stand for half an hour until proper color is reached. Measure the absorbance at 810nm against a blank solution.
Silica (in g) = Abs. × final vol. × GCF / vol. of sample× Slope×1000
Calculations:
Total silica (as SiO2), % – {(M1 – M2) + M3 / M} × 100
Where,
M1, = mass in g of the crucible and the impure silica before hydrofluoric acid treatment,
M2, = mass in g of the crucible and residue after hydrofluoric acid treatment,
M3 = mass in g of residual silica present in 500 ml of the main sample solution A, and
M = mass in g of the sample taken.
Reference Standard: IS 12107: 1987 part -2