Aluminum and Magnesium in Chromite and Ferrochrome

Aluminum and magnesium occur as gangue components in chromite ores and ferrochrome slags. At this Center, ores from many different sites and of many varieties are examined for commercial feasibility. The beneficiation of these ores is a continuing activity that produces most of the samples that are analyzed with this method. Data from this method are used in economic feasibility studies, to decide the best beneficiation methods for different ore types, and to calculate the smelter charge.

The method of choice at this Center is fusion with a minimum of sodium peroxide, solution with hydrochloric acid, and dilution to a proper range for determination by atomic absorption (AA) spectroscopy. This technique is rapid and accurate enough to handle a considerable sample load without sacrificing reliability.

Equipment

• Zirconium crucible (approximately 30-mL capacity).
• Meker or similar gas-air mix burner.
• 250-mL beaker
• Watchglass to fit 250-mL beaker.
• Stirring rod.
• 100-mL volumetric flask.
• Assorted volumetric flasks.
• Hotplate.
• Rubber policeman.
• Assorted pipettes.
• AA spectrophotometer

Materials

• Sodium peroxide, reagent grade, granular, 20 mesh or finer.
• Hydrochloric acid, reagent grade, concentrated.
• Appropriate AA standards.

Procedure

1. Weigh 0.2 g of sample into a zirconium crucible.
2. Add 3 to 4 g of sodium peroxide, and mix thoroughly.
3. Fuse over a burner until all sample particles are dissolved. Swirl and inspect occasionally to keep unattacked particles dispersed.
4. When the fusion is complete, allow the crucible and melt to cool.
5. When they are cool, tap the crucible to dislodge the melt and place the melt in the 250-mL beaker.
6. Add 10 to 20 mL of distilled water to the beaker, and cover quickly with the watchglass.
7. Add about 5 mL of distilled water and 3 to 5 mL of concentrated hydrochloric acid to the crucible.
8. When the leaching action has subsided in the beaker, uncover and rinse the cover and sides with a minimum of distilled water.
9. Police the crucible, and pour the solution slowly into the beaker. Rinse the crucible carefully.
10. Slowly, with stirring, acidify the leach in the beaker with concentrated hydrochloric acid; then add approximately 5 mL excess.
11. Place the beaker on a hotplate at low to medium heat for 15 to 30 min to clarify the solution.
12. Remove the beaker from the hotplate, cool, and transfer the solution to a 100-mL volumetric flask. Make the flask up to the mark and mix.
13. Make a 10:1 dilution of the sample solution for determination of aluminum and a 100:1 dilution for determination of the magnesium.
14. Measure the absorbance of the aluminum at 309.3 nm and the magnesium at 285.2 nm. Set the other AA instrument parameters in accordance with the manufacturer’s recommendations.

Procedure Notes

1. Experience has shown that 0.2 g is a convenient amount for nearly all chromite samples.
2. The amount of sodium peroxide used needs to be the bare minimum necessary for reliable sample attack so that the AA instrument will not be overloaded with dissolved salts.
3. The burner should be capable of bringing the crucible bottom to red heat. Mixing is important. If particles are allowed to aggregate, fusing time may have to be extended, increasing the attack on the crucible itself and increasing the tendency of the cooled melt to stick to the crucible.
4. If the melt is to be leached quickly, the crucible may be set on any heat-resistant surface to cool. If the crucible must be set aside awhile after cooling, it should be set on a hotplate on low heat to keep the melt from absorbing atmospheric moisture. Because of its small volume, the melt is usually easier to remove from the crucible if it has solidified with the crucible tilted.
5. The small sample and melt size sometimes makes removal of the cooled melt difficult. The higher the iron content, the more vigorous the leach reaction. Police the inside of the crucible thoroughly. When finished, the solution volume should be less than 100 mL.
6. If extended heating was necessary and the crucible attacked, the solution may not clarify because of the presence of hydrolyzed zirconium. (This very rarely happens.)
7. The solution may be left to evaporate if its final volume is greater than 100 mL.
8. A 10:1 dilution will yield a final dilution factor of 1,000:1 for the aluminum. A 100:1 dilution will yield a final dilution factor of 10,000:1 for magnesium. Most samples may then be determined using 10- and 20-ppm standards for the aluminum and 2- and 4-ppm standards for the magnesium.