Sustainable Development through Beneficiation of Low grade Chromite Ores
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Sustainable Development through Beneficiation of Low grade Chromite Ores

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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. Tata Steel ...

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. Tata Steel Limited, established India’s first Chrome Ore Beneficiation Plant (COB) in 1990. 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.

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Sustainable Development through Beneficiation of Low grade Chromite Ores Sustainable Development through Beneficiation of Low grade Chromite Ores Document Transcript

  • Sustainable Development through Beneficiation of Low Grade Chromite Ores Prabhash Gokarna *, Amit Ranjana , CR Nayaka and Y Rama Murtyb 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 , Refractory22- 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 inmillionTons 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. References: 1. Li, X, McKee, D,J, Horberry.T, Powell, M,S, (2011). The control room operator: The forgotten element in mineral process control, Minerals Engineering Vol.24, pp 894–902. 2. Rama Murthy. Y, Tripathy.S.K., Raghu Kumar. C (2011). Chrome ore beneficiation challenges & opportunities - A Review, International Journal Minerals Engineering. Minerals Engineering 24 (2011) 375–380. 3. Rama Murthy, Y.,, Sunil K Tripathy, Veerendra Singh, Vilas D Tathavadkar and A Ranjan (2012). Studies on reduction in chromite losses in tailing in an operating plant. XXVI International Mineral Processing Congress (IMPC) 2012 Proceedings / New Delhi, India / 24 - 28 September 2012.Pp- 04377-04385.
  • 4. Sahoo. R.K., Mohanty J.K., Das S.K., Paul A.K. Chromites of India Their Textural and Mineralogical Characteristics. IMMT Bhubaneswar. 5. Sunil Kumar Tripathy, Rama Murthy,Y., Veerendra Singh 2013. Characterization and separation studies of Indian Chromite beneficiation plant tailing. International Journal of Mineral Processing 122 (2013) 47–53. 6. Amit Ranjan, LS Divakera, CR Nayak, Y Rama Murty, 2013, Conservation of Mineral Resource in Chrome Ore Beneficiation Plant, AQN Congress 2013 Proceedings (paper 184).