Metallic Iron in Ferrochrome Slags

It would truly be unusual to find a sample of mineral chromite with metallic iron in it. However, (his Center has handled many samples of prereduced smelter charges with several percent of metallic iron and ferrochrome slags with measurable quantities of metallic iron. Therefore, this method finds its use in the later stages of mineral chromite processing.

Results from this method have been used to calculate total reduction in smelter runs, the efficiency of the partition of metaliics and slag materials, and the final smelter charge.

The method is rapid and reliable and yields high-quality results on a routine basis. Reproducibility has typically been well within 1 relative percent.

Materials

• Mercuric chloride, reagent grade, crystals.
• Hydrochloric acid, reagent grade, concentrated.
• Phosphoric acid, reagent grade, concentrated.
• Sodium diphenylamine sulfonate solution.
• 0.1N potassium dichromate solution, standardized.

Procedure

1. Weigh the sample into a 100-mL volumetric flask
2. Using nonmetallic tools, add 8 to 10 g of mercuric chloride
3. Immediately add 40 to 50 mL of distilled water, and swirl the flask vigorously to mix the contents.
4. Bring the solution to a boil, and boil gently for 1 min.
5. Remove the flask from the heat, and immediately screw the cap on snugly.
6. Cool the solution to room temperature or below.
7. When the solution is cool, dilute to the mark with distilled water, reseal the flask, and mix the contents of the flask thoroughly.
8. Filter into a dry beaker using a dry funnel and a dry, medium-speed qualitative paper (such as S&S 597).
9. Pipette 50 mL of filtrate into a 250-mL Erlenmeyer flask.
10. Add 5 to 10 mL of concentrated hydrochloric acid, 5 to 10 mL of concentrated phosphoric acid, and three to five drops of sodium diphenylamine sulfonate indicator. Titrate with 0.1N potassium dichromate solution to a purple endpoint.

Titration equation:

3Fe+² + Cr+6 → 3Fe+³ + Cr+³

Calculation:

mL titer x N Cr +6 x eq wt Fe x 2/sample wt x 1,000 x 100 = pct metallic Fe.

1 mL 0.1000N K2Cr2O7 = 5.585 mg Fe.

Procedure Notes

1. A maximum sample size of about 2 g is used to minimize the error caused by solid material in the volumetric flask.
2. Metallic spatulas show definite signs of attack after contact with mercuric chloride; therefore, all handling should be done with glass, plastics, or porcelain utensils, and thorough caution should be used. The final solution after boiling should be saturated with mercuric chloride with a few excess crystals in evidence.
3. The mercuric chloride begins attack upon contact with any metal. The attacked metal then becomes much more subject to air oxidation, so delays should be minimized until the solution is boiled and tightly stoppered.
   Chromium may be taken into solution also, but it does not interfere except by imparting a green color to the solution.
4. Shake the flask until any materials caked on the bot-tom are thoroughly dispersed. At this point, the flask may be left for up to 24 h before continuing.
5. In summary, take a “dry aliquot” of 50 mL into a 250-mL Erlenmeyer flask. Fine materials sometimes slime through the filter paper. They may be ignored unless the filtrate becomes too murky to see the endpoint.
   10. The hydrochloric acid provides the proper acid medium for the titration. The phosphoric acid complexes with ferric iron. The endpoint sharply turns to purple. The titrations should be carried out promptly.