Gravimetric Silica in Mineral Chromite and Ferrochrome Slags

Silica is present in chromite ores as a basic silicate. In characterization and beneficiation studies performed on chromites at this Center, the chromium-silica ratio determines the purity of the gangue material being analyzed. Accurate characterization of the original ore material and the further feasibility of smelting to ferrochrome are identified by using this ratio.

The method of analysis for silica at this Center is dehydration of the silicic acid with sulfuric and perchloric acids. One dehydration is accurate enough for characterization of the ore.

Equipment

• Zirconium crucible (approximately 30-mL capacity).
• Platinum crucible (approximately 20-mL capacity).
• Meker or similar gas-air mix burner.
• 400-mL beaker and cover.
• Boiling or bump cup for 400-mL beaker.
• Glass stirring rod.
• Hotplate.
• Rubber policeman. Funnel.
• Filter paper and pulp.
• Fume cabinet or hood suitable for the fuming of perchloric acid.
• Muffle furnace.

Materials

• Sodium peroxide, reagent grade, granular, 20 mesh or finer.
• Hydrochloric acid, reagent grade, concentrated.
• Sulfuric acid, reagent grade, diluted 1:1 with distilled water.
• Perchloric acid, reagent grade, concentrated.
• Hydrofluoric acid, reagent grade, 48 to 51 pct.
• Hydrochloric acid, reagent grade, diluted 20:1 with distilled water.

Procedure

1. Weigh 0.5 g of the sample into a zirconium crucible.
2. Add 4 to 12 g of sodium peroxide, and stir until the sample is thoroughly mixed.
3. Fuse the crucible contents using the gas and air burner until all particles are dissolved completely.
4. When the fusion is complete, allow the crucible and its contents to cool.
5. Gently tap the melt into a 400-mL beaker, and add 50 to 75 mL of distilled water.
6. Carefully add to the crucible 10 to 15 mL of distilled water and 5 mL of concentrated hydrochloric acid. When the chemical action ceases, slowly pour the contents of the crucible into the 400-mL beaker containing the main melt.
7. Police the crucible, being careful to remove any adhering silica particles from the sides.
8. Acidify the solution in the 400-mL beaker with concentrated hydrochloric acid.
9. To the beaker add 30 mL of 1:1 sulfuric acid and 40 mL of concentrated perchloric acid.
10. Place the beaker and its contents on a hotplate in a perchloric acid hood, and slowly evaporate the solution until dense white fumes of perchloric acid appear.
11. Put a cover glass over the beaker, and reflux at a high-heat setting on the hotplate for 15 to 20 min. A boiling or bump cup made of aluminum or similar material helps avoid splattering.
12. When the beaker is cool, add 150 mL of distilled water to the mass in the beaker and stir. Slightly heat, if necessary, to dissolve any insoluble material other than the silica.
13. Filter the solution through a medium-speed paper (such as S&S 589 white ribbon) plus a little paper pulp, and police the 400-mL beaker to remove any silica that adheres to the sides.
14. Wash the silica caught in the filter paper, alternately, five times with distilled water and five times with the 5-pct hydrochloric acid wash solution (heat slightly).
15. After allowing the filter paper and its contents to drain properly, place them in a clean platinum crucible.
16. Starting at a low temperature in a muffle furnace, ignite the crucible and contents to 1,200° C for approximately 1 h.
17. Cool in a desiccator, and weigh the platinum crucible containing the ignited silica and its associated impurities.
18. Add 3 to 5 mL of hydrofluoric acid and one to two drops of 1:1 sulfuric acid to the platinum crucible.
19. Slowly volatilize the silica as silicon tetrafluoride on a hotplate at a low heat.
20. As soon as the hydrofluoric acid and silica have evaporated and fumes of sulfuric acid appear, increase the hot-plate temperature to a high setting.
21. When drying is complete, ignite the platinum crucible and impurities to a red heat over a burner.
22. Weight the platinum crucible and impurities when cool.

Calculation:

(wt Pt crucible + SiO2 + impurities) – (wt Pt crucible + impurities)/sample wt x 100 = pct total SiO2.

Procedure Notes

1. A sample size suitable for most chromite ores is 0.5 g. If considerable (over 40 pct) silica is present, reduce the sample size to 0.25 g.
2. Increase the sodium peroxide as the sample size increases, at an approximate ratio of 20 parts of peroxide to 1 part of sample. Again, however, consideration must be given to melt volume at high sample weights. Thorough mixing cannot be overstressed. Sample particles in the melt can aggregate and stubbornly resist attack.
3. The burner should be capable of bringing the crucible to red heat on the bottom. Mixing remains important. If particles are allowed to aggregate, fusing time may have to be extended, which increases the attack on the crucible itself, shortening its useful life, increasing the tendency of the cooled melt to stick to the crucible, and adding a significant concentration of zirconium to the analyte solution.
4. If the melt is to be leached quickly, the crucible can be set out on any heat-resistant material to cool. If the melt has to be set aside for a while, it should be set on a hotplate on low heat to keep it from absorbing atmospheric water (which makes the melt stick to the crucible).
5. If gentle tapping does not free the melt, try moderately hard tapping. If the melt still sticks, lay the crucible on its side in the bottom of the beaker.
6. The leaching reaction can range from very slow to very vigorous. A high aluminum concentration tends to slow the reaction. A slow leach reaction may become a very fast leach reaction when sulfuric acid is added, so care must be exercised. Once the leach reaction has finished, the addition of sulfuric acid is generally accompanied by moderate effervescence unless a significant amount of carbonate is present.
7. Using glassware will introduce negligible silica into the analysis, although one must be careful not to unintentionally add silica by accidentally chipping a stirring rod, etc. Carefully police all glassware and crucibles to remove particles of silica. Nonignited silica is quite sticky and adheres readily to most surfaces.
8. The purpose of using sulfuric and perchloric acids is to dehydrate the soluble silicic acid SiO2•H2O to insoluble silica (SiO2). These two acids are excellent for this purpose.
9. A special perchloric acid hood is needed for evaporation since most commercial hoods are not designed for this purpose and may react explosively to perchloric acid fumes.
10. When the solution reaches a volume of approximately 100 to 125 mL, salts start to precipitate. As this happens, occasional stirring keeps the solution well mixed and prevents bumping. A bump or boiling cup is a good preventative measure against splattering.
11. An acid volume of 50 to 75 mL is satisfactory for re- fluxing to begin. Do not fume off all of the perchloric acid. If this happens, insoluble chromium salts will appear, which will not redissolve upon adding water. Should this occur, add more perchloric acid and refume. Refluxing for 15 to 20 mm removes almost all of the water from the silicic acid and also oxidizes the iron and chromium in solution to an orange color.
12. After adding distilled water to the cooled and refluxed silicic acid, do not let the solution stand for long, as some of the colloidal silica will redissolve. It may be necessary to heat the beaker and its contents slightly or to add several milliliters of concentrated hydrochloric acid to redissolve any material that is difficult to dissolve.
13. Wash the filter paper carefully to remove all traces of perchloric acid. If this is not done, the remaining perchlorates will ignite explosively in the furnace.
14. The ignition to 1,200° C effectively removes all traces of water from the silica.
15. Ignited silica (SiO2) is light and fluffy. Avoid air drafts and spillage when weighing and igniting. The weighed silica is always impure, and with chromites it may have a light greenish color. Impurities include aluminum, chromium, iron, titanium, vanadium, and zirconium, as well as beryllium, calcium, and strontium, if present in the initial material. The weight of impurities should be extremely small in comparison to the weight of silica obtained if the analysis is carefully done.
16. Adding hydrofluoric acid to the weighed silica produces silicon tetrafluoride (SiF4), which is a gas. This step must be done in a hood as fumes of hydrofluoric acid and silicon tetrafluoride are extremely toxic. Sulfuric acid also must be added to the platinum crucible containing the ignited silica and impurities. When the platinum crucible is reignited with its residue of iron, aluminum, etc., everything remaining will be reignited to an oxide form, as originally done at 1,200° C rather than to a fluoride form. As an example, ferric sulfate [Fe2(SO4)3] as an impurity in the platinum crucible is converted to ferric oxide (Fe2O3) and sulfur trioxide (SO3) upon reignition.
17. Dirty platinum crucibles may be cleaned by fusing sodium bisulfate in them until the sides and bottom are free of stuck particles.