Sustainable Development through Beneficiation of Low Grade Chromite Ores

Prabhash Gokarna*, Amit Ranjan

a, CR Nayak

a and Y Rama Murty

b

a Tata Steel Limited, Sukinda Chromite Mines, Sukinda, Orissa, India

b Tata Steel Limited, Jamshedpur, Jharkhand, India

Abstract

Mineral conservation has been the focus of the mining industry, owing to stringently enforced

laws for the preservation depleting valuable resources and to growing self realization. Chrome

ore has a limited availability in India. 95% of the ore occurs in the Sukinda valley of Odisha. The

Cr2O3 of the ore varies from 10% to over 50%. While high and medium grade ores can be

directly used for making ferrochrome or used in refractories and pigments; low grade ore need to

be beneficiated to make it suitable to use.

Embodying the pioneering spirit of our Founder who laid the foundation stone of Industrial

Development in India by setting up the first integrated steel plant in Asia in 1907; which also led

to the discovery of Chromite ore in India at Sukinda(1949); Tata Steel Limited, established

India’s first Chrome Ore Beneficiation Plant (COB) in 1990. In small but significant steps of

debottlenecking the plant, plant expansion, and technology infusion, the concentrate production

capacity of Tata Steel’s COB Plant has increased to four times of the initial capacity.

Orienting our internal process goals and objectives to conservation of mineral led to a 60%

increase in plant yields, much of it due to the people who contributed in bringing breakthrough

technological innovations. The recovery of valuable mineral (a key performance indicator of a

beneficiation plant) has increased by 20% as a result of several projects undertaken in reducing

tailings loss and improving yield.

Tata Steel at Sukinda is aligning itself with the nation’s aim to develop and implement schemes

to utilize low and medium grade ores and conserve high grade ores. To cater to future needs, our

endeavor is to set up a tailings beneficiation plant at Sukinda that seeks to scale the heights of

excellence in the field of mineral processing and conservation of valuable mineral resources

while protecting the environment and meeting customer requirements.

Keywords: Mineral; Conservation; Chrome; Beneficiation; Yield

*Corresponding Author:

Prabhash Gokarn, Head Projects, FAMD, Tata Steel Sukinda Chromite Mines, Sukinda, Orissa

Contact No : +91-77 5200 4399, E Mail Address : prabhash@tatasteel.com,

1.0. INTRODUCTION

Chromium occurs as a chromium spinel, a complex mineral containing magnesium, iron,

aluminum and chromium in varying proportions depending upon the deposit. Chromium is

replaced by ferric iron & aluminum and iron is replaced by magnesium. It is this replacement

that improves the Cr/Fe ratio in chromite.

Chromium ore occurs exclusively in ultramafic igneous rocks. Commercial chromite deposits are

found in two forms : stratiform seams and irregular podiform/lenticular deposits. (Figure 1 –

Types of Chrome Ore Deposits)

Chrome ore deposits can also be classified as siliceous type (silica rich) and ferruginous type

(iron rich). Associated gangue minerals are talc, quartz, hematite, goethite, limonite, gabbro,

serpentine, anorthosite, dunite, and pyroxinite.

Most of the chromite reserves in the world are concentrated in Africa and Asia followed by

Europe, Australia and Brazil.

Ore Geological

deposit type Composition of Ore & Cr: Fe ratio

Principal end use Industry

High- Chromium

Podiform, Stratiform

46- 55% Cr2O3 and Cr : Fe over 2 : 1 Metallurgical

High- Iron Stratiform 40- 46% Cr2O3 and Cr : Fe of 1.5-

2.1: 1

Metallurgical

Chemical

High-Aluminium

Podiform

32-38% Cr2O3 ,

Refractory 22- 34% Al2O3 and

Cr : Fe over 2- 2.5: 1

Figure 1 - USGS Classification of Chrome Ore Deposits Source: UGCS

1.1. Present status of chrome ore

Almost 95% of chrome ore mined is consumed by metallurgical industries for making of

ferrochrome/charge chrome which is used in making of alloy – including stainless - steels. 2% of

the demand comes from the chemical industry and rest from refractory and foundry industries.

Stainless steel being the largest consumer of ferrochrome, any change in the dynamics of the

stainless steel industry impacts overall chrome ore demand significantly. Global production of

Stainless Steel is currently about 35.5 million tons in CY2012, an increase of ~5% over 2011;

leading to a ferrochrome consumption of ~9.2 million tons and a chrome ore demand of ~25

million tons. Chrome ore demand is estimated to reach ~29 million tons by 2015 at a annual

growth rate averaging ~6%. (Figure 2 – Global Stainless Steel Production, FerroChrome &

Chrome Ore Demand)

Figure 2 – Global Stainless Steel Production, Ferro Chrome & Chrome Ore Demand (mn Tons)

Source : CRU, NiCrMo & Own Estimates

2.0 CHROMITE SCENARIO

2.1 World:

Major chrome ore and concentrate producing countries are South Africa, India and Kazakhstan,

who represent 70% of the world production. South Africa and Zimbabwe hold about 90% of the

world’s proven chromite resources (Figure 3 Distribution of Chrome Reserves). Zimbabwe is

the only country to exploit both stratiform and podiform deposits while Kazakhstan has podiform

deposits in the Southern Ural Mountain region. The ores vary greatly in chromium content and in

Cr/Fe ratios. India’s output is mainly from podiform bodies on the east coast of the Orissa state..

In Brazil, production is concentrated in Bahia and Minas Gerais where there are mainly

stratiform deposits. (Figure 4 Global Chrome Reserve Types)

0

5

10

15

20

25

30

35

40

45

50 in

mill

ion

To

ns Growth in Demand for Chrome Ore comes mainly from

Growth in Stainless Steel Production

Stainless Steel Prod FeCr Demand Chrome Ore Demand

Figure 3 Distribution of Chrome Reserves ICDA Publication : Chromium in 2012(Aug 2013)

Stratiform Deposits( cumulates) Podiform Deposits (alpine type)

Occurs in parallel zones or seams in large layered igneous rock complexes and extensive bands with little deformation

Occurs in highly- folded ultramafic peridotites and serpentinites, small pod- shaped bodies characterized by extreme deformation

High Iron Ore High Chromium Ore; Higher Magnesium content

Great lateral extent covering large area; Uniform layers

Limited in size with small to medium reserves; Highly irregular

Prime source of Chromite for Steel & Ferroalloys industries

Prime source of Chromite for non- metallurgical use, some FeCr

Deposits found in Bushveld Igneous complex, South Africa; Great Dyke, Zimbabwe; Tsaratanana District, Madagascar; Orissa Complex, India

Typical deposits occur in Ural mountains of the CIS; Tethyan mountain chains of Albania, Iran, Greece & Turkey

Figure 4 Global Chrome Reserve Types Roskill – Chromium 2009 (2010)

2.2. India

India ranks 3rd

in chromite production and 5th

in terms of proven chrome ore reserves in the

world. Chromite deposits occur in several Indian states (Figure 5 Occurrence of Chromite Ore

in India) like Tamil Nadu, Goa, Karnataka, Maharashtra and Orissa in the form of discontinuous

South Africa, 5500, 73%

Zimbabwe, 930, 12%

Kazakastan, 320, 4%

Finland, 120, 2%

India, 70, 1%

Turkey, 20, 0%

Brazil, 17, 0%

Others, 626, 8%

Global Chrome Ore Reserves (Proven) mn Tons

bands, lenses and pockets in different host rock associations. Though small in the context of

world resources, India is endowed with appreciable quantities of good grade chrome ores.

Around 90% of the chromite resources of India are concentrated in Sukinda valley of Jajpur

district and Boula-Nuasahi belt of Orissa state. Sukinda Ultramafic Belt (SUB) and Boula-

Nausahi Igneous Complex (BNIC) of Orissa, India possesses the largest chromite deposit of the

country. The chromites of both SUB and BNIC and elsewhere in India can be categorized as

hard massive, banded, disseminated, friable, granular and lateritoid type depending on the

amount of gangue mineral and their textural arrangements , Sahoo et.al (2).

The Indian deposits are typically characterized as ferruginous and siliceous type ores. The ores

of Sukinda valley are mostly high grade, soft and friable in nature besides a small amount of hard

lumpy ore, formed in separate bands. These are mainly associated with laterite, altered ultramafic

rock, nickeliferrous limonite, goethite and talc serpentine schist.

In order to conserve these valuable ore resources strategically, several companies have

established chrome ore beneficiation plants in this region.

Figure 5 Occurrence of Chromite Ore in India Geological Survey of India – Dossier on Chromite (2004) .

3.0 BENEIFICIATION SCENARIO:

Beneficiation of Chrome Ore is well established throughout the world, with the beneficiation

process customized to the nature of ore and the specific properties desired in the output. Chrome

ores contain a variety of gangue minerals such as goethite, gibbsite, serpentines, olivine etc.,

which needs specific concentration techniques for separation and thus beneficiation of the ore.

During mining operations, large quantities of sub-grade ores ranging 10-35% Cr2O3 are also

excavated. These sub-grade ores are fed to the beneficiation plant for up-gradation to achieve

marketable grades.

Beneficiation begins with the milling of mined run of mine ore (ROM) to make the ROM

suitable for further activities to recover chrome values. The processing / beneficiation techniques

vary, and depend on the characteristics of the ore found in the region.

Generally, the process starts with milling operations (like crushing and grinding) to produce

liberated, uniform sized particles followed by enrichment of the liberated ore by a combination

of gravity separation techniques.

Gravity concentration techniques have been adopted in several mines worldwide. This

classification technique takes the advantage of the specific gravity difference between chromite

and gangue minerals. The sequence of operation from mining to beneficiation is shown

schematically below (Figure 6 Sequence of Operations from Mining to Beneficiation).

Figure 6 Sequence of Operations from Mining to Beneficiation

3.1 Beneficiation of Chrome Ore

Ore existing in the Sukinda valley is primarily friable ferruginous and lumpy siliceous. High

grade ore is directly used for FerroChrome making whereas low to medium grade friable ore is

beneficiated to make it a usable grade chromite in the form of chrome concentrate. These

concentrates are used by Metallurgical, Chemical and Refractory industries.

The typical beneficiation plan has a two process size preparation and beneficiation circuit.

(Figure 7 Chrome Ore Beneficiation Plant) The feed ore is reduced from ROM size of about

220mm to the libration size which is <1mm in case of the ore existing in the Sukinda valley. A

combination of a Double Toggle Jaw Crusher, a Cone crusher and a Grinding Mill, coupled with

a High Frequency Screen is used to reduce size from 220mm to <1mm.

Figure 7 Chrome Ore Beneficiation Plant

The beneficiation circuit is based on the gravity concentration method and uses differences in

specific gravity between Chromites and Gangue minerals at different particle size ranges for

classification. Typically the equipments for beneficiation are Hydro cyclones, Flotex Gravity

Separators, Spirals and Shaking Tables.

4.0 IMPROVEMENT JOURNEY IN BENEFICIATION OF CHROME ORE FOR BETTER

MINERAL CONSERVATION :

For the sustainability of chromite resources in the face of continuously rising demand for chrome

ore (Figure 2 Growth in Demand for Chrome Ore), beneficiation of lean/sub-grade ores as

well as tailings is imperative.

This challenge has given rise to the need for developing economic and efficient classification

processes for the recovery of chromite values from lean & sub-grade ores and from tailings.

Most gravity separation processes suffer large chromite losses in the tailing. Thus, any attempt

towards decreasing tailing losses would not only help in preserving the limited chromite reserves

but also make the economics of the operations more attractive.

Apart from the liberation phenomena, in many of the operating plants considerable quantity of

values are lost in the tailings due to inconsistent feed quality or the inappropriate selection of

operating and design parameters.

Indian chrome ore beneficiation plants also suffer from these ills. Hence our research efforts

have been directed towards improving feed quality and optimizing operating & design

parameters. This has helped us in decreasing the loss of Cr2O3 in tailings and also enabled us to

upgrade low grade chromites which are not being processed earlier due to a lack of suitable

technology.

The critical issues related to the chromite processing industries have been categorized as:

Tackling Variations in Input Ore

Daily Management for Improved Process Control

Improving recovery and reducing tailing losses

Optimizing particle size

Improving recovery of ultrafine chrome particles

Beneficiation of low and sub-grade chromite ore (10–35%Cr2O3).

Reprocessing of stockpiled tailings containing valuables.

Process Control Systems

4.1 Tackling Variations in Input Ore : The challenges faced by Chrome Ore Beneficiation

circuits are derived mainly from the large variation in the input feed ROM(Run of Mines). Any

process plant is adjusted to optimize its output for a consistent input feed. The results would vary

if the input feed available to the plant is inconsistent and that largely depends on the

characteristic of the ore deposit.

In large steel plants, variations in input feed are minimized by building large buffers and

facilities for blending. This is usually not possible to implement at the mining site due to space

and other operational constraints.

The problem of inconsistent feed has been countered by a) creating specific optimum operating

process parameter schedules for major known variations in the ROM characteristic, b) blending

in a small scale to achieve some consistency in feed and c) operating the plant at the optimum

schedule for the feed quality.

4.2 Daily Management for Improved Process Control : Improved process control helps getting

the optimum performance out of an equipment. This can only be achieved through adoption of

daily managenment practices. Daily management practices involves establishing performance

parameters for each critical process and controlling the process by repeated PDCA(Perform-Do-

Check-Act) and SDCA(Standardize-Do-Check-Act) cycles. An example of optimizing bed level

set point with respect to tailing losses is shown in Figure 8 (Figure 8 Optimizing Bed Level Set

Point with respect to Tailing Loss)

Figure 8 Optimizing Bed Level Set Point with respect to Tailing Loss

4.3 Improving recovery and reducing tailing losses : The graph in Figure 8 shows the

correlation between Bed Level Set Point vs Tailings Loss of chromite in a Flotex Density

Seperator. It can be seen that as the bed level increases, the loss of chromite through tailings also

increases.

Thus efforts are now made for setting process parameters to the level where the tailing loss is

minimised which enhances recovery, while optimizing product quality. The process parameters

are then monitored at regular intervals for any abnormality in the process. Corrective and

preventive actions are taken in case deviations are noticed.

4.4 Optimizing Particle Size : Gravity concentration is based on the principal of differences in

specific gravity between mineral and gangue particles.

However, this difference in the specific gravity diminishes when the particle size becomes very

small which often leads to loss of mineral values in tailings in a gravity separation processes. It is

therefore imperative to optimize the size distribution in the mineral processing of Chrome Ore.

Example of such optimization is shown in Figure 9 (Figure 9. Optimization of Grinding Mill).

Figure 9. Optimization of Grinding Mill

From the graph it is evident that at higher RPM of a grinding mill the mean particle size of the

output shifts towards the coarser side, resulting in lower generation of ultra fines and thereby

reducing loss of chrome value through tailings.

As gravity concentration processes are not very effective for very fine particles (less than

75micron), other concentration processes are recommended, which need to be customized with

respect to specifics of the application.

4.5 Improving Recovery of Ultrafine Chrome Particles : The application of enhanced gravity

concentrators (MGS) and floatation columns have found wide acceptance at various

beneficiation plant flow sheets in Turkey for the recovery of fine and ultrafine chromite from

Turkish ores. For the beneficiation of Indian chrome ore these unit operations have yet to be

established.

4.6 Beneficiation of low and sub-grade chromite ore (10–35%Cr2O3) and re-processing of

stockpiled tailings containing valuables : Detailed study of chrome ore beneficiation processes

reveals that opportunities exist for recovering chromite values from the low/sub-grade ores and

tailings using conventional beneficiation processes.

Detailed characterization studies on chromite tailings from the beneficiation plant of Sukinda,

India by Tripathy et.al 2013 revealed that a chromite concentrate of marketable grades can be

produced from the tailing analyzing up to17% Cr2O3 and Cr/Fe ratio of 0.49. IBM having

defined the thresh-hold limit for Chromite at 10% makes the processing of low & sub-grade

chrome ores and tailings which contain > 10% Cr2O3 essential Setting up of processing plants for

re-processing stored tailings are being studied by various chrome majors in India.

4.7 Process control systems have been developed over the years in most mineral processing

plants. Such process control systems are generally developed from the huge data base generated

from collecting various operating parameters over time. In practice, the performance of a system

often deteriorates with time but such situations are rarely discussed in the literature (Li et. al,

2011). Unsteady operation of beneficiation units in the process circuits often arises from

numerous changes in the input to the plant such as feed properties to the circuit, overall flow

rate, mineral composition, amount of solids in the feed, the size distribution of the feed etc.

The databank collected over a period of time under various operating conditions would help

overcome future problems relating to either variations in the feed characteristics or specific

demands in the output by metallurgical industries.

A critical aspect in collecting such a data bank hinges in documenting relatively small changes/

improvements in actual practicing plant in terms of grade, recovery etc., which in turn affect the

economics.

The above concept was adopted in one of the operating chromite plants at Sukinda. Practical

problems encountered during the plant trials were well documented (described in the article by

Rama Murthy et.al.2012). The results of this trial campaign were subsequently used to optimize

the process parameters with changes in input and thus reduce chromite losses in tailings from

this operating plant significantly by making small changes.

An outline of the various initiatives taken up in different sections of the operating plant is

presented in the figure10 (Figure 10 Overview of Plant Initiatives) below :

Figure 10 Overview of Plant Initiatives

5.0 CONCLUSIONS:

Conservation of valuable and limited chromite resources while satisfying the continuously

increasing demand for chrome ore by Metallurgical, Chemical and Refractory industries makes

the beneficiation of lean & sub-grade ores essential. From the mineral conservation point of view

it is necessary to maximize utilization of lean ores to preserve high grade chromite resources. It

has also become essential to reduce loss of chromite values in tailings and recover values from

previously generated tailings.

Chrome Ore Beneficiation is particularly challenging given variations in input ROM and the

inefficiency of traditional gravity concentration systems in dealing with fine/ultrafine particles.

This challenge has given rise to the need for developing and optimizing processes/process flow

sheets for economic and efficient recovery of chromite values. Process optimization through

initiatives at the shop floor is important to improve plant recovery. Technological developments

are underway to further help in improving Chrome Ore Beneficiation processes.

.

6.0ACKNOWLEDGEMENTS

Authors are thankful to the management at Tata Steel for giving us an opportunity to publish this

work. We are grateful to Chrome Ore Beneficiation Plant members for their involvement in the

project. The support and services provided by the staff at R&D and SS Division is also duly

acknowledged. The presentation is an amalgamation of authors’ own views and thoughts. Tata

Steel does not necessarily subscribe to the views and thoughts expressed in this paper and should

not be held responsible for the same.

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