STATUS OF IO PELLET INDUSTRY IN INDIA:
Pelletisation of agglomerated fines has taken a great stride recently in the KarnatakaGoa region. Thus Jindal-Vijaynagar steel plant is setting up a slime disposal facility in the Bellary district in Karnataka with an investment of 11.7 crores. VSP-NMDC has recently submitted a report on the pelletisation and slime beneficiation at Donnamali, Karnataka. Ministry of steel has proposed a feasibility report on the pelletisation plant in Goa. Janaki Corporation, a private organization is working on a feasibility report on the installation and pelletisation of 0.6 MTPY pellet plant at Bellary and Sidimongla village near Bellary and Tungabhadra Mineral Pvt. Ltd. proposes 1.2 MTPY iron ore pelletisation plant in Karnataka region. The status of the various commissioned pellet plants and the plants under commissioning/ Engineering in India are given in Annexure-I.
Pelletisation of iron ore fines is practiced by the major iron ore producing countries. The status of Global pellet capacity and production as well as export of iron ore pellets is given in Annexure- II.
TECHNO-ECONOMIC POLICY OPTIONS:
Before coming to process technology, let us understand the mechanization of pellet formation.
(I) MECHANISM OF PELLET FORMATIONS:
Ball Formation – Surface tension of water & gravitational force creates pressure on Particles, so they coalesce together & form nuclei which grow in size into ball.
Theory of ball formationThe forces responsible for the agglomeration of ore fines are surface tension and capillary action of water and gravitational forces of particles due their rotation in balling unit. When the solid particles are come in contact with water, the ore surface is wetted and coated with water film. Due to the surface tension of water film, liquid bridges are formed. As result of the movement of particles inside the balling unit and of the combination of the individual water droplets containing ore grains, first agglomerates, called seeds, are formed. The liquid bridges in the interior of these seeds hold them together as in a network. With the further supply of water, the agglomerates condense and become denser. Capillary forces of liquid bridges are more active in this stage of green ball formation. The optimum of this ball formation phase is attended when all the ports inside the balls are filled with liquid. When the solid particles are fully coated with water, the surface tension of water droplets becomes fully active dominating the capillary forces. Besides this effect, the rolling movement of grains and movement or shifting of particles relative to each other plays an important role.
Pelletizing in discsGreen pellets with a size range of 8-16 mm are prepared in balling drum or discs. Discs are preferred to produce quality green pellets as these are easy to control operation with minimum foot space. The disc is an inclined pan of around 5 to 7.5 meters diameter rotating at around 6 to 8 rpm. The inclination of disc is around 45 o and it can be adjusted in the off-line between 45 o to 49 o . The pre wetted mix is fed into the disc at a controlled rate. Ore fines are lifted upwards until the friction is overcome by gravity and the material rolls down to the bottom of the disc. This rolling action first forms small granules called seeds. Growth occurs in the subsequent revolutions of the disc by the addition of more fresh feeds and by collision between small pellets. As the pellet grows in size, they migrate to the periphery and to the top of the bed in the discs, until they overflow the rim.
Pellet growth is controlled by the small amount of water sprayed in the disc and the adjustment in the disc rotational speed.
Induration (Heat Hardening):
Pellet induration consists of three main steps: 1. Drying of green pellets. 2. Firing of pellets at around 1300 o C to sinter the iron oxide particles. 3. Cooling of hot pellets before discharging.
During drying (180 o -350 o C), moisture content in the green pellet is evaporated.
Surface and interstitial moisture evaporates at lower temperatures where as chemically combined water (as goethite or limonite) or any hydrate or hydroxide combinations lose their water at slightly higher temp. During pre-heating (500- 1100 o C), decomposition of carbonates, hydrates takes place. Gasification of solid fuels like coal or coke and conversion of iron oxides like goethite, siderite to higher oxide state, hematite, also takes place. Commencement of solid oxide bonding and grain growth are the important steps in this stage. During firing stage (1200- 1300 o C), the temp. is below the melting temp. of major oxide phase but within the reactivity range of gangue components and additives. Formation of oxides and slag bonds is decisive in this stage.
Bonding of mineral grains developed during induration of pellets is affected by the following factors:
- Solid oxide bonding: Oxidation of ferrous iron oxides to ferric iron oxides results in bonding and bridging, but only to limited amount.
- Recrystallization of iron oxides: Essentially a physical process in which smaller particles consolidate into larger ones with the loss of surface energy. Continued growth of iron oxide crystals imparts sufficient strength. Grain growth for hematite starts at around 1100 o C.
- Slag bonding: Gangue by forming melt transport medium for ferrous or ferric oxides facilitate grain growth and crystallization of oxide grains. It also enables the mechanism to proceed at lower temperatures than would be required in its absence.
(II) PROCESS TECHNOLOGY:
There are several iron ore pelletizing processes/technologies available in the world. Some of them are Shaft Furnace Process, Straight Travelling Grate Process, Grate Kiln Process, Cement Bonded Process (Grangcold Process, MIS Grangcold Process, Char process etc. and Hydrothermal Processes, (COBO Process, MTU Process, INDESCO Process) etc. However, currently, Straight Travelling Grate Process and Grate Kiln Process are more popular processes.
In the straight grate system, a continuous parade of grate cars moves at the same speed though the drying, induration and cooling zones. Any change in one section effects the residence time in another.
In the Grate-Kiln System, independent speed control of the grate, kiln and cooler are available to the operator. This provides process flexibility to adjust to changes in concentrate feed Many Indian entrepreneurs have been looking at foreign, particularly, the Chinese suppliers for setting up pelletization plants. However, in China and most foreign countries, where pellets are used in blast furnaces, the feed material for making pellets is magnetite ore concentrate, which is much easier to pelletize. Furthermore, out of the two commonly used pelletization processes, i.e. Straight grate and Grate-kiln, but suitable more for low LOI magnetite ores. India having both hematite and magnetite types of ores, which contain varying LOIs and alumina levels, pose a serious challenge to the Indian researchers and entrepreneurs to chose the right process and use it economically and efficiently. This is also pertinent because of typical nature of Indian iron ores containing high chemically combined water, high alumina and a very fine concentrate arising out of a high degree of grinding required during beneficiation. The pellet manufacturing technology, followed in North America, China and other advanced countries, largely apply to magnetite ores. The adaptability of these processes for the typical Indian hematite/ goethite ores needs to be examined in detail. Already some of the plants set up recently based on Chinese technology are struggling.
Main technology supplier in Indian pellet industry is Outocompo, based on which about 26 million tonnes annual capacity has been set up/being set up in the country. All these plants are of international capacity of 3-5 million tonnes per annual. However to revolutionize pellet manufacturing in India , there is need to develop indigenous technology based on our raw materials suitable to smaller capacities (60,000-150,000 TPA).This call for promoting R & D efforts in the country. It is a matter of satisfaction that some initiatives have been taken by the Ministry of Steel who are providing financial assistance for three such projects. More such efforts are required to be made involving private sector players.
(i) The country’s iron ore requirement is going to increase substantially (almost 340 Mt/yr by 2020) in line with domestic steel production requirement outlined in National Steel Policy. Limited reserves of high grade iron ore pose a great challenge in long term sustainability of Indian iron & steel industry. In order to ensure optimum use of existing iron ore resources with special emphasis on conservation of high grade ores, there is a pressing need to utilize existing low grade iron ores including slimes and dump fines which are stockpiled in millions of tonnes in different mine heads. The solution lies in beneficiation which will not only help in utilization of hitherto wasted low grade ores/ slimes by upgrading its Fe value, but also mitigate environmental hazard, arising out of large stockpiling of slimes/rejects.
(ii) Newer and modern beneficiation techniques are required for up- gradation of low grade iron ores with high yield. Because of varying mineralogical characteristics of ore bodies, specific beneficiation technology solution need to be developed by R&D organizations for recovery of micro-fines for each of the deposit through extensive test work and development of flow sheet.
(iii) Present estimate of proved reserves had been arrived at long back and have become outdated. Therefore, a need is felt for taking up fresh exploratory investigations by GSI & IBM to ascertain proved reserves as per International Convention. Further, the present classification of ore reserves need to be modified by reclassifying the reserves into four grades viz. high grade (Fe +62 %), low grade (Fe +52%), poor ores (Fe +45%) and very poor ores (Fe below 45%) and for each grade, proved reserves need to be separately estimated.
(iv) The Government needs to provide all sorts of encouragement by way of policy support, incentives, so as to facilitate entrepreneurial initiative towards upgradation of low grade iron ores including slimes in tailing ponds by beneficiation and subsequent utilization of concentrate for sintering/pellet making.
(v) Beneficiation of low grade ores, mostly at micro-fines level, provides concentrate which can be used in iron making in the form of pellets. Therefore, pelletisation technology will have a predominant role in supplying the prepared burden for iron making. Adoption of suitable pelletisation technology of varying capacity is encouraged to meet specific needs with respect to availability and type of iron ore fines/concentrate in the country.
(vi) Major efforts are going on to develop new cost effective hot metal/steel production technologies based on utilization of iron ore fines/slimes and non coking coal (because of limited global reserves of coking coal).Some of these are-Hismelt technology. Finnex technology, Romelt technology etc. There is need to encourage adoption of some of these technologies taking in account of Indian raw materials. This will not only lead maximization of fines but will conserve our natural resources.
(vii) Prospective entrepreneurs are apprehensive about the continuous availability of iron ore fines from same sources on long term basis because of sensitivity of the beneficiation technology to different iron ores. Therefore, there is need to develop some mechanism to provide iron ore linkages to them.
(viii) In view of increasing demand of high quality iron ore and for environmental protection against dumped fines and slimes in the tailing pond, an improved beneficiation technique for iron ores , dumped fines and slimes, use of blue dust, through sintering and pelletizing need to be encouraged.
(ix) While allocating /renewing iron ore mines, preference should be given to those who undertake to produce pellets/sinters.
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