Analysis of Slag content in Ferro Alloys Part II

ANALYSIS OF SLAG CONTENT IN FERRO ALLOYS

USING

FOURIER TRANSFORMED INFRARED SPECTROMETRY

PART II – HIGH CARBON FERROCHROME

Ferrochrome is an alloy of chromium and iron containing between 50% and 70% chromium. It is the major chromium source for use as the main raw material for stainless steel production.  Submerged-arc furnaces are most commonly utilized to smelt chromite ores by using suitable carbonaceous reductants such as coke, bituminous, char, etc. During the smelting process slag is created and metallic ferrochrome coalesces into droplets, which being the heavier phase, separate out of the slag by settling through the slag to the bottom of the furnace. The basic slag system for FeCr production is MgO-Al2O3-SiO2-Cr2O3 in different proportions but also smaller amounts of CaO, chromium and iron oxides, with significant quantities of chromium in the form of PAC and entrained alloy.

Typical bulk composition of slags in the production of high carbon ferrochrome is in the following ranges:

SiO2 – 15 – 30%, typically 23%

Al2O3 – 19 – 33%, typically 25%

MgO – 13 – 25%, typically 20%

CaO – 1 – 5%, typically 3%

Fe – 1 – 12% (may be as FeO or as entrained ferrochrome alloy), typically 8%

Cr – 1 – 18% (may be as Cr2O3 or entrained ferrochrome alloy), typically 12%

In high carbon ferrochrome slag, the Chromium oxide present as both divalent and trivalent state of chromium based on the FeCr process condition. In pure alloy the metals, Fe and Cr present in metallic bonding. But in slag they are present as metal – oxygen and Si – O bonding. These have a characteristic absorption in the IR region.

Following this principle, this method tried to identify and quantify the slag content (present as contamination) in high carbon ferrochrome alloy using a Perkin Elmer FTIR (spectrum 100 series) with HATR. Pure HCFC slag was made 200 mesh size and oven dried. This was scanned and found to have absorptions at 985 and 894 cm^-1 with a max. absorption at 894 cm^-1, whereas pure alloy does not have any characteristic absorption in this range.

Five different concentrations of slag sample were prepared by using IR grade KBr and absorptiograms were taken. A standard curve was drawn with 100%, 50%, 25%, 6%, 3% & 1% slag content in KBr with a correlation (R² value) of 0.9978.

Another set of absorptiongrams were generated using pure high carbon ferrochrome alloy as diluent. A six point calibration curve was drawn with 100%, 50%, 12%, 6%, 3% and 1% slag in HCFC alloy. This shows a correlation (R² value) of 0.9986.
Another calibration curve was drawn with 1%, 3% & 12% slag concentration in HCFC alloy which shows a correlation (R² value) of 1.0000.

Quantification has been done for a number of samples with known value of slag content with this technique w.r.t. the above drawn standard curve. This shows a good agreement with the estimated value of slag content.

So this method could be utilized for estimation of slag content in high carbon ferrochrome alloy which may be present as contamination instead of classical analysis using gravimetric separation.

Contributed by: Mrs. Nandita Das under guidance of Prof. Barun Gupta