Introduction:
Vanadium is typically found within magnetite iron ore deposits as well as other iron ores. Subsequently, during the titrimetric estimation of iron against potassium dichromate, the vanadium present in the ore is also oxidized leading to erroneous results. In this work presented herein, we have removed the vanadium present (if any) in the iron ore prior to the titration and a detailed comparison of the iron concentrations has been carried out with and without the removal of vanadium.
Experimental:
To carry out the experiment, a set of five iron ores were taken. At first, the iron ores were digested using concentrated HCl and the iron content was determined by titrating against standard potassium dichromate following the usual method. On the other hand, the vanadium content of the same set of iron ores were determined using ICP-OES and the vanadium was removed using the following method prior to titration against standard potassium dichromate. The results of the two sets are summarized in table 1.
Estimation and removal of vanadium:
The sample was fused with Na2O2 in a nickel crucible and the fused mass was extracted with water followed by filtration. The vanadium was converted to sodium vanadate and remained in the filtrate. The iron remained in the residue as Fe(OH)3. The vanadium content was estimated from the filtrate using ICP-OES.
The residue was dissolved with 1:1 HCl and R2O3 was precipitated using ammonium chloride and ammonium hydroxide. The precipitate was filtered, dissolved in 1:1 HCl, reduced by stannous chloride and titrated following the usual method against standard potassium dichromate using barium diphenyl sulphonate as indicator.
Reactions involved :
Cr2O72-+ 6Fe2+ + 14H+ → 2Cr3+ + 6Fe3+ +7H2O
6 VO2+ + Cr2O72- + 2H+ → 6 VO2+ + 2Cr3+ + H2O
Table 1: Comparison of iron content of various iron ores with and without removal of vanadium
| Experiment no | Total % Fe without removal of Vanadium |
% Vanadium | Total % Fe After removal of Vanadium |
| 1 | 61.82 | 1.82 | 60.05 |
| 2 | 61.70 | 1.67 | 59.90 |
| 3 | 61.85 | 1.88 | 59.78 |
| 4 | 62.05 | 1.76 | 60.10 |
| 5 | 61.95 | 1.87 | 60.05 |
| Mean | 61.87 | —– | 59.98 |
| Std dev | 0.12 | ——- | 0.13 |
Conclusion:
As evident from table 1, removal of vanadium prior to titration is of significant importance in estimation of total iron in vanadium containing iron ores. Potassium dichromate, being a strong oxidizing agent in acid medium, can simultaneously oxidize vanadium and iron (II) thereby leading to higher dichromate consumption and erroneous results. The variation in iron concentration is almost equal to the amount of vanadium present which justifies the necessity of removal of vanadium from the iron ores to obtain the actual iron content.