JPMS Conference Issue Materials 2011 Aqib Ali Khan, Yaseen Iqbal, Phase and Microstructure …. 7 PHASE AND MICROSTRUCTURE OF CHROMITE: A DIFFERENT APPROACH Aqib Ali Khan * , Yaseen Iqbal Materials Research Laboratory, Institute of Physics & Electronics, University of Peshawar *email: aqibkhan@upesh.edu.pk ABSTRACT Chromite ores are basically classified on basis of Cr 2 O 3 content and Cr to Fe ratio in a sample. The higher content of Cr increases the value of the ore but this is greatly affected by the presence of impurities in form of silicates, oxides or hydroxides. The present study was performed to analyze the impurities of chromite ore collected from Prangghar (Mohmand agency), Pakistan. From x-ray diffractormeter analysis a single phase (magnesiochromite-ferroan, ICDD card # 09-0353) was identified. SEM/EDX analysis along with metallurgical microscopy images were in partial agreement with XRD, as some impurity phases were also identified. In spite of these impurities, mostly silicates, the ore was best suited for metallurgical industry due to its high Cr content. 1. INTRODUCTION Normally chromite occurs in nature with an (AB 2 O 4 ) cubic spinel structure (Fd3m) 1-3 . Here A and B are metal ions of 2+ and 3+ valance state and occupy tetrahedral and octahedral sites respectively. Figure 1 shows the AB 2 O 4 spinel structure 4-5 . Natural chromite being a cubic spinel compound has a formula unit (Mg,Fe 2+ )O(Cr,Al,Fe 3+ ) 2 O 3 3 . In chromite the tetrahedra of oxygen atoms surround divalent metal ions and the octahedra of oxygen surround chromium ions 6 . At the A and B sites of naturally occurring chromite, the substitution of Fe +2 by Mg and Cr by Al and even by Fe +3 in lower proportion occurs depending upon the type of minerals which are usually found with the chromite, thus describing the presence of serpentines, chlorites, olivine and other minerals 7 . For industrial use chromite minerals are classified into different grades by the total amount of Cr 2 O 3 present and Cr to Fe ratio. The general grade wise classification of chromite ores is as follow: i) metallurgical grade with Cr 2 O 3 content above 46–48% and Cr/Fe ratio above 3; and 25% alumina and magnesia together ii) chemical grade with Cr 2 O 3 content above 44% and Cr/Fe ratio above 1.5% iii) refractory grade with Cr 2 O 3 between 30 and 40% and Cr/Fe ratio between 2 and 2.5 and above 63% of Cr 2 O 3 and Al 2 O 3 together. The properties of the chromite mineral for refractory grade are improved by the content of magnesia 8-9. Figure 1. An AB 2 O 4 spinel structure 1 . A higher proportion of chromium increases the heat resistance of the mineral, but this can be modified by the type and relative amount of the impurities. For example, the presence