Calcium is rarely found in chromite ores in amounts greater than a few tenths of a percent, and calcium analysis is rarely requested on ore samples. Calcium compounds are, however, used as slag conditioners for ferrochrome smelting. Most of the chromite-related samples received for calcium analysis at this Center have been slag samples. The data produced are used mainly to establish material balance in smelter calculations.
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 atomic absorption (AA) spectroscopy.
The method for the preparation of the sample for calcium analysis is identical to the preparation of the sample for aluminum and magnesium (previous section). Usually, the dilution prepared for aluminum is a proper concentration for calcium determination using 2- and 5-ppm calcium standards. Occasionally, the dilution prepared for the magnesium determination is used when the calcium concentration is high. The undiluted solution (step 12 of the aluminum-magnesium method) is used when the calcium concentration is low. When analyzing for calcium, a reagent blank must be carried along to correct for the calcium content of the sodium peroxide. Better precision is obtained if the sodium peroxide flux is weighed when preparing the fusions for calcium analysis.
The absorbance of the calcium solution is measured at 211.3 nm with the other AA instrument parameters set according to the manufacturer’s recommendations. When calcium determination is combined with the determination of aluminum and magnesium, samples may be run rapidly and large sample loads may be analyzed quickly without sacrificing reliability.