Iron is the second major element in chromite. In smelting operations where ferrochrome is the product, a specific range in the chromium-to-iron ratio is desired. Where pure metallic chromium is the product, the iron concentration is not as important. In both cases, it is important to have a good value for the chromium-to-iron ratio. A typical chromite or chromite concentrate can range from less than 10 pct iron to over 20 pct iron.

To cope with this variability and with the difficulty in attacking chromite samples, the method of choice at this Center is titration after alkaline fusion. This method is also used on ferrochrome slags, even though they may occasionally contain less than 1 pct iron.

While this method is not as fast as the total chromium method, rapid and reliable results are easily obtained.

Equipment

• Zirconium crucible (approximately 30-mL capacity).
• Meker or similar gas-air mix burner.
• 400-mL beaker and cover.
• Glass stirring rod.
• 500-mL Erlenmeyer flask.
• Hotplate.
• Funnel.
• Filter paper.
• Burette.

Materials

• Sodium peroxide, reagent grade, granular, 20 mesh or finer.
• Hydrochloric acid, reagent grade, concentrated.
• Aqueous ammonia, reagent grade, concentrated.
• Hydrochloric acid, reagent grade, diluted 1:1 with distilled water.
• Stannous chloride, reagent grade, 10-pct solution in 20- pct hydrochloric acid.
• Saturated mercuric chloride solution.
• Phosphoric acid, reagent grade, concentrated.
• Sodium diphenylamine sulfonate solution.
• Potassium dichromate, reagent grade, 0.1N solution in distilled water.

Procedure

1. Weigh the sample into a zirconium crucible.
2. Add 4 to 12 g of sodium peroxide, and stir until the peroxide and sample are thoroughly mixed.
3. Fuse over a gas and air burner until all sample par-ticles 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 gently to free the solidified melt from the bottom of the crucible.
6. Place the solidified melt and the emptied crucible in a 400-mL beaker, add 150 to 200 mL of cold distilled water, and cover quickly.
7. When leaching activity has subsided, remove and rinse the cover and wash down the beaker sides.
8. Slowly, with stirring, add concentrated hydrochloric acid until all the precipitated iron dissolves.
9. Remove and thoroughly rinse the crucible, and police it if necessary.
10. Slowly, with stirring, add concentrated aqueous ammonia until a permanent iron precipitate is obtained; then add 20 to 25 mL excess.
11. Bring the solution to a boil, and boil for a few minutes to destroy any remaining peroxide and produce a more easily filterable precipitate.
12. Remove the beaker from the hotplate, and rinse the sides.
13. Filter the solution through a fast-speed, qualitative paper (such as S&S 595). Wash the beaker once and wash the precipitate three to five times with distilled water, and discard the filtrate and washes.
14. Wash the precipitate back into its original beaker with distilled water.
15. Wash the filter paper free of iron with alternating rinses of hot 1:1 hydrochloric acid and distilled water. Make sure any precipitate particles stuck on the beaker sides are redissolved.
16. Add 20 to 25 mL of concentrated hydrochloric acid, stir until all the precipitate dissolves, and transfer the solution to a 500-mL Erlenmeyer flask.
17. Set the flask on a hotplate and bring it to a boil. Reduce the iron with dropwise additions of 10-pct stannous chloride solution with agitation until the solution is colorless, or Light green if Cr+3 is present. Add three to five drops excess, and bring to a boil again.
18. Cool the solution to room temperature or below.
19. When the solution is cool, add 10 mL of a saturated mercuric chloride solution, and mix. Immediately add 7 to 10 mL of concentrated phosphoric acid and three to five drops of sodium diphenylamine sulfonate indicator, and titrate with standard potassium dichromate solution to the first permanent purple.

Titration equation:

3Fe+² + Cr+6 → 3Fe+³ + Cr+³

Calculation:

mL titer x N K2Cr2O7 x eq wt Fe/sample wt x 1,000 x 100 = pct total Fe.

1 mL 0.1000N Cr+6 = 5.585 mg Fe.

Procedure Notes

1. Sample size is usually approximated to yield a 10- to 30-mL titration. However, a minimum of about 0.2 g is used when adequate sample is available. A maximum of about 1 g is used to keep the melt volume to a manageable level.
2. An approximate ratio of 20 parts of peroxide to 1 part of sample is used. At high sample weights, resulting melt volume must be considered, however. The flux and sample must be thoroughly mixed, or the sample particles may aggregate and stubbornly resist attack.
3. The burner should be capable of bringing the entire crucible bottom to red heat. Careful agitation must be done to keep the particles from aggregating. Extended fusion time increases the attack on the crucible itself and adds a significant concentration of zirconium to the analyte solution. It also tends to make the cooled melt stick to the crucible.
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 at low heat to keep it from absorbing atmospheric water.
5. If gently tapping does not free the melt, try moderately hard tapping. If the melt still sticks, lay the crucible on its side on the bottom of the beaker.
6. The leaching reaction can range from very slow to very vigorous. A high iron concentration speeds the reaction. A high aluminum concentration tends to slow the reaction. A slow leach may become a very fast leach when acid is added. Once the leach is finished, the addition of acid is generally accompanied by mild effervescence unless there is a considerable amount of carbonate present to evolve carbon dioxide.
7. An ammonia precipitation is done to free the iron of the large amount of sodium in the solution and to produce larger precipitate particles. A high sodium concentration has been found to depress the indicator change in the titration. It has also been found that with a sodium hydroxide precipitation, a very fine-grained iron precipitate is produced. The coarser precipitate produced by the ammonia is easily filtered. A small excess of sodium peroxide added to the ammonia precipitation after step 10 will allow the precipitate to be freed of nearly all of the chromium present. Sodium peroxide is added 1 or 2 g at a time until a moderate effervescence is produced by gentle stirring and the beaker contents take on a deeper brown color. Chromium, usually present as hydrated chromic oxide (Cr2O3) is oxidized to Cr+6 by the sodium peroxide and remains in the filtrate. This operation can be used to measure iron and chromium on the same sample but usually needs to be repeated to remove all visible traces of chromium from the iron solution. If chromium is to be determined, the filtrate is acidified with 1:1 sulfuric acid and titrated as for total chromium. Results are slightly low, and the method is not recommended unless insufficient sample is available for a separate determination.
8. Bubbles from peroxide decomposition cause mechanical problems in the filtration, often slowing it unnecessarily.
9. It has been found convenient, after rinsing the beaker, to wash the sides once with hot 1:1 hydrochloric acid from a wash bottle.
10. A glass wash bottle with insulation for holding is used to contain the 1:1 hydrochloric acid for washing.
11. The Erlenmeyer flask is a matter of preference; the titration may be done just as well in the beaker. If a beaker is used, it should be covered during cooling.
12. The stannous chloride reduces the Fe+3 to the Fe+2 form necessary for the titration. Interfering elements that are reducible by stannous chloride are rare in chromite mineral samples and ferrochrome slags. The excess stannous chloride protects the Fe+2 from air oxidation while cooling.
13. Saturated mercuric chloride solution is added in excess to destroy the excess stannous ion; it does not otherwise participate in the titration reaction. Phosphoric acid is added as a complexing agent for Fe+3. If much titanium or zirconium is present in the solution, a gellike precipitate may form when the phosphoric acid is added. This interferes with mixing efficiency and requires a slower and more careful titration. It does not interfere chemically. Hydrofluoric acid may be substituted for the phosphoric acid or added, in small amounts, in addition to the phosphoric acid to avoid this condition. The indicator is at first colorless, turns green as the endpoint is approached (very hard to see when much Cr+3 is also present), and sharply turns to purple at the endpoint.