Characterisation and Pre-Concentration of Chromite

Classification Chromite Ores

Classification is a method of separation of fines from coarse particles and also lighter particles from heavier particles. This is performed on the basis of the velocity with which the grains fall through a fluid medium generally water or air. In view of the fact, that the velocity of particles in a fluid medium is dependent not only on the size, but also on the specific gravity and shape of the particles. The conventional classifiers, such as, hydro cyclones or mechanical classifiers, decreases the efficiency of the grinding and concentration circuits due to their imperfect separation. Several attempts have been made to improve the efficiency of classification. They include the use of screens instead of classifiers, the use of cone classifiers to process hydro cyclone under flow and two stage classification by hydro cyclones.

Improving the Efficiency of Classification

In the process of improving the efficiency of classification, a device that has been gaining popularity in recent years is the teeter-bed or hindered-bed separator such as Floatex density separator. The upward flow of elutriation water creates a fluidized “teeter-bed” of suspended particles. The small interstices within the bed create high interstitial liquid velocities that resist the penetration of the slow settling particles. As a result, small/light particles accumulate in the upper section of the separator and are eventually carried over the top of the device into a collection launder. Large/heavy particles, which settle at a rate faster than the upward current of rising water, finally pass through the fluidized bed and are discharged out through the bottom of the separator as underflow.

Indian Chromite Deposits

The major Indian chromite deposits are located at Sukinda region of Orissa state. The depletion of high grade ore resources and having a variety of gangue minerals such as goethite, serpentine, olivine and talc, etc., has lead to the utilisation of lean ore after beneficiation. Further, ample amount of plant tailing generation have gained importance from the economics, conservation and ecology point of view.

Processing Chromite Ores

Generally for processing chromite ores, different types of gravity methods are employed after crushing, grinding followed by classification. The Tata Steel Chrome Ore Beneficiation (COB) plant, designed to produce concentrate of +46% Cr2O3 with a recovery of 70% from a feed of 30- 35% average Cr2O3. Presently the COB plant produces 50 tph of tailings analysing 17% Cr2O3, which is high. This suggested for the incorporation of additional circuit comprising of hydro cyclone for pre-concentration by de-sliming and multigravity separator and wet high intensity magnetic separator for the upgradation up to the required quality. For recovering chrome values from the Karagedik Concentrate tailings a circuit comprising of wet high intensity magnetic separator and column flotation for producing a concentrate assaying 46 to 48% Cr2O3 was studied by Guney et. al.

A critical review on practice of the plant operating personnel is concerned in the grade-recovery characteristics of unit operations. But separation insight and influence of different operating and process parameters are essential to understand and control the process. The objective of the present investigation was to study the effect of the important operating variables on floatex density separator and pre-concentration for the further beneficiation process.