Chromite - Mine To Metal

Geology

Chromium is an essential alloying element for the production of all types of stainless steels and special categories of alloy steels, on account of the special properties it imparts to Steel. Chromite is the main chromium bearing mineral of economic importance and around 99% of domestic chromite consumption in India is in ferro-alloy/charge chrome industry and a very nominal amount is consumed for refractory and other industry.

Chromite (FeO.Cr2O3) is a member of the spinel group of minerals and has the general formula R+2O. R+32O3, where R+2 stands for a bivalent metal (mainly ferrous iron and magnesium) and R+3 represents trivalent chromium, aluminum, iron and titanium. The formula may be written as (Fe, Mg) O. (Cr, AI, Fe)2O3.

Chromite (FeO.Cr2O3) in its purest form contains 68% chromium oxide and 32% FeO with Cr: Fe ratio is 1.8:1. This composition of FeO.Cr2O3 cannot occur as a magmatic mineral because natural magma always contains an abundance of aluminum and magnesium. Thus, invariably, magnesium and aluminum frequently substitute for iron and chromium respectively and thereby the chrome ore analyses around 60% chromium oxide only.

 Chromite occurs as a primary mineral of ultrabasic igneous rocks and is normally associated with peridotite, pyroxenite, dunite and serpentinite.

 Chromite is the only economic source of Chromium. It has a wide range of uses in metallurgical, chemical, refractory industries. The properties of chromium that makes it most versatile and indispensable are its resistance to corrosion, oxidation, wear and galling and enhancement of hardenability.

 The Cr:Fe ratio is one of the important factors that decides the end-use of the chromite mineral. For ferro-chrome Cr:Fe should be 2.8:1; Cr2O3 48% (min) and for charge chrome Cr:Fe should be 1.6:1; Cr2O3 44% (min.).

In metallurgical industry, chromite is used for manufacturing low-carbon and high carbon ferrochrome and charge chrome which in turn are used as additives in making stainless steels and special alloy steel.

Hard lumpy chromite is used for producing high carbon ferrochrome while friable ore and briquetted fines are used for low carbon ferrochrome. Both briquetted fines and lumpy ores are used in production of charge chrome.

Occurrence in India

Major share (98.6%) of chromite resources in the country is located in state of Odisha. Other states contributing to the country’s resources of chromite are Manipur, Karnataka, Jharkhand, Maharashtra, Tamil Nadu and Andhra Pradesh.  

All these deposits are associated with ultramafic rocks. The chromite deposits of Odisha occur in number of localities associated with ultramafic complexes of Sukinda (District: Jajpur), Baula-Nausahi (District: Keonjhar) and similar occurrences of ultramafic rocks at Bhalukasoni (District: Balasore) and Ramgiri (District: Koraput).

Chromite deposits in India fall into the following types: (1) evenly scattered (or disseminated) eg. Kodapalle, A.P.; (2) Schlieren bands as bands and segregations, eg. Chaibasa, Singhbhum District, Keonjhar, Odisha; (3) Stratified (with a bedded appearance), eg. South of Kaliapani, Sukinda; (4) Sack-form (or pockety) eg. Kodapalle, A.P.; and (5) Fissure form (or vein-like) eg. Shinduvalli, Mysore, Sittampundi Salem District, Tamilnadu.

Sukinda chromite alone contributes nearly 95% of total chromite reserves of the country.

Five major seams have been identified in Sukinda ultramafic complex comprising Dunite partially or fully altered to feebly schistose talc-serpentine mass is the host rock for chromite ore. All the ore seams are mostly friable and partly lumpy, except for the Mahagiri Ore seam, which is lumpy in nature.

Chromite occurs as persistent thick bands, seams, lenses within ultramafic complex. All the seams are fairly thick and friable in nature. The chromite deposit of Sukinda belt is mainly stratiform type and can be classified into the categories like lumpy ore, granular ore, friable ore, ferruginous ore, disseminated ore and banded ore.

In the ultramafic complex of Nausahi area the chromite bodies occurring in form of discontinuous bands and lenses, are confined to the altered dunite / peridotite. The chromite seams in the Baula-Nausahi area are thin, small, mostly hard and lumpy. The chromite deposit is mainly straitiform type with categories like lumpy and granular ore, friable ore, ferruginous ore, banded ore, disseminated ore and magnetic ore. The magnetic chromite ore is found in Baula area where both chromite and magnetite are so intimately mixed that poses problem for their separation.

Mineral Processing of Chromite Ore

India comes under high grade group with countries having average grade of 39% Cr2O3. Chromite ore is mainly used for ferro-alloy production (Fero-chrome/charged chrome) that is input material of chromium for steel making. High-grade lumpy ore/briquetted fines assaying in the range of over 44 to 48% Cr2O3 having minimum Cr: Fe ratio of 1.6:1 & 2.8:1, is needed for charge chrome and ferro-chrome production respectively.

In order to meet such stipulation of ferro-alloys, the practice adopted in India is to consume medium to high grade chrome ores (+44% Cr2O3) and is achieved by resorting to selective mining coupled with scrubbing cum hand sorting of high grade lumpy ore. For friable fine ores, selective mining and blending practice is on vogue.

The low to medium grade r.o.m. ore (below 40% Cr2O3) either in-situ deposit or generated on account of selective mining and hand sorting is however resorted to crude beneficiation technique with prime objective is to meet the physical as well as chemical stipulations necessitated for ferro-alloy industry.

Some of the rudimentary chrome ore processing techniques practiced for friable ore is discussed herewith. These process routes involved:

• Manual screening cum washing of r.o.m. low to medium-grade ore by spray of water in two mm size screen followed by vigorous manual churning of screen under-size in a settling tank.
• The coarse chromite concentrates were settled nearby whereas fine ferruginous/siliceous/aluminous materials were washed away as slimes.
• The oversize stacked and or sold to nearby available market for beneficiation. Crushing & screening to all -1-mm size and subjecting to either:

1. Sluice in a narrow gentle sloping drain. Coarse concentrates settle in the slope nearby rejecting slimes of ferruginous/siliceous/aluminous material, which are carried away to a farther distance by action of water.
2. Treatment in hydro-cyclone followed by jet-sizers i.e. hindered settling classifiers.
3. Treatment in Hydro-cyclone to reject overflow slimes followed by tabling of hydro-cyclone underflow (sand).
4. Scrubbing cum screening of ROM ore to -1 mm size, followed by crushing and/or grinding of oversize to all -1-mm size and classification (spiral classifier/ Hydro-cyclone). The underflow constitutes the final concentrate while the overflow is rejects.
5. Crushing and grinding of r.o.m. ore followed by de-sliming (Hydro-cyclone), hydro-sizing of cyclone underflow into two size fractions followed by gravity concentration by spiral and /or table.
6. Scrubbing of r.o.m. ore followed by screening, crushing and grinding of oversize to all minus 1 mm size, followed by classification (spiral classifier / Hydro-cyclone) to plus & minus 100 ?m size, followed by gravity concentration employing spiral & table.

• The first three process routes of beneficiation (practiced by large number of minor players), have caused grave resource waste with a low rate of recovery, which is averaged between 40 to 50 % Cr2O3 as it concentrated chromite grains limited to over 75 ?m (200 mesh) size only.
• In the subsequent process routes, chromite recovery is marginally better but then again it is limited to 50 ?m size only with a loss mostly at size below 50 ?m and is inevitable due mainly to lack of properly developed process flow sheet and control in its process parameters. The maximum chromite recoveries in all these later process COB plants are ranging between 60 to 70% Cr2O3 only.

The compromise in optimum/best possible chromite recovery by the lessees is primarily due to surplus ore reserves, readily available flourishing and unrestricted (no quantity ceiling) export market and ease of value addition, technology of which is supplied by a few engineering consultant and equipment manufacturer/supplier. Primary aim of these consultants and equipment manufacturers was sale of their machinery/equipment for easy recovery and rather than optimum recovery in the interest of conservation. Thus, development of proper flow sheet and scientific material accounting was seldom in their agenda.

Disclaimer: The views expressed are personal and after interactions with Industry Experts

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