IRON ORE BENEFICIATION
Beneficiation is a process which removes the gang particle like Alumina, Silica from the Iron Ore. Basically, it separates Fe2O3 or Fe3O4 from other impurities in the iron ore. In this process the Fe content is improve to maximum possible extent. The highest can be 70% i.e. purest form.
To produce the high grade pellet requires a good quality of raw material which is having chemical composition as follows:
|IRON ORE CONCENTRATE||63.5% Fe (BF grade)||3%||2%||0.005 Max||0.0008 Max|
|66.5% Fe (DR grade)||1.5%||1%|
- Normally this kind of raw material is not available readily from the mines. But we can achieve the same by beneficiating the available iron ore.
- Normally a raw material is collected from couple of mines and different pockets due to which there is a variation in the quality & chemical composition of the raw material which is not desirable for Pelletizing. Hence these ores are blended together and Beneficiated to improve its quality as near to the ideal requirement.
- The quality of pellet and the output of the pellet plant are depending upon the raw materials. This way the good Beneficiated ores will produce the best quality of pellet and maximum output can be achieved from the Pellet Plant.
Pellet Plant - Straight Grate Technology
Iron ore fines reclaimed from the blending stockpile shall be conveyed into a surge bin within the beneficiation plant building. Ore drawn from the surge bin by a belt weigh feeder is fed to a spiral screw type classifier.
Washed ore from spiral classifier is screened for +4 mm and -4 mm fractions over a scalping screen. Undersize fraction of -4 mm is pumped to sizing screens for screening off -1 mm fraction. Oversize fractions of +4 mm from the scalping screen and +1 mm from the sizing screens are ground in a primary ball mill in closed circuit with sizing screens to get 100% -1 mm solids suitable for gravity separation in spirals.
Washed sizing screen underflow fraction of -1 mm is pumped to dewatering cyclones. Underflow of dewatering cyclones is beneficiated by gravity separation through two stage spirals viz., rougher and cleaner spirals. Concentrate from spirals circuit is ground to a size consistency of 100% passing 100 mesh and ~70% passing 325 mesh in secondary ball mills in closed circuit with classifying cyclones. Ground concentrate from the classifying cyclones overflow as well as the overflow from dewatering cyclones ahead of spirals are pumped to concentrate thickener. Concentrate thickener underflow is thereafter filtered to get a product with 8% moisture max. The filter cake is conveyed to stockpile.
Tailings from the spirals circuit is pumped to a linear screen to ensure a 100% -1 mm size solids in the slurry being fed to high gradient magnetic separators to recover feebly magnetic Fe units. Concentrate from high gradient magnetic separators is diverted to secondary ball mill discharge pump box for grinding along with spiral concentrate, to desired fineness.
Tailings from high gradient magnetic separators is fed to tailings thickener.
Spiral classifier overflow is pumped to de-sliming cyclones. Overflow from these de-sliming cyclones is fed to the tailings thickener. Underflow from de-sliming cyclones is diverted to spiral tailings pump box in turn to high gradient magnetic separators to recover Fe units as much as possible.
Tailings thickener underflow is pumped to tailings settling pond.
Clear water from concentrate thickener, tailings thickener and tailings settling pond flows by gravity back into the process water sump for recirculation.
Beneficiation includes selection of effective crushing and grinding technologies, wet and dry beneficiation, and the control of moisture levels in material.
A wet beneficiation plant for producing high-grade iron ore concentrate requires screens, grinding Mills, spirals, hydrosizers / hydrocyclones, magnetic separators, Agitators, Floatation, Filters and thickeners.
The screen is a surface with multiple apertures of given dimensions. Material of mixed size presented to that surface will either pass through or be retained, according to whether the particles are smaller or larger than the governing dimensions of the aperture. The efficiency of screening is determined by the degree of perfection of separation of the material into size fractions above or below the governing dimensions of the aperture.
Grinding mill with spherical ball charge are used in mineral processing plant to grind coarse concentrates and middling. Final concentrates and tailing are separated after grinding mill and fed with material that has already been ground and treated in one or more processing steps. In the grinding mill relatively small sized balls are used. They can be operated in both open and closed circuit. The purpose of grinding and regrinding is to reduce the ore to a size small enough to liberate and recover the valuable minerals. Closed circuit grinding minimizes over grinding of very friable ore normally found in the ore bodies of our region. The more the recirculation load the less is the over grinding of particles.
For this application, 100% load is maintained. The grinding process achieves two goals, 1st the particles are crushed to achieve the liberation size for upgrading and 2nd the particles must be fine enough such that after combining them with the deslimed minus 0.150 mm fraction, the product will have the blaine needed for pelletizing.
A Spiral Concentrator is a flowing film separation device. General operation is a continuous gravitational laminar flow down on an inclined surface. The mechanism of separation involves primary and secondary flow patterns. The primary flow is essentially the slurry flowing down the spiral trough under the force of gravity. The secondary flow pattern is radial across the trough. Here the upper-most fluid layers comprising higher density particles move away from the centre while the lower-most concentrate layers of higher density particles move towards the centre. Spirals require addition of water at various points down the spiral to assist washing of the concentrate, i.e. transporting away the light gangue from the concentrate band. The amount of wash waster and its distribution down the spiral trough can be adjusted to meet the operating requirements. Point control minimizes the total water requirements by efficiently directing water into the flowing pulp at the most effective angle.
Hydrocyclones / Classifiers
Conventionally, the modelling and simulation of classifiers and cyclones are based on 50% separation size commonly known as d50 size. However, the design and construction of mechanical classifiers are primarily based on limiting / elimination size. The metallurgical performance depends on the key hydro-cyclones variables such as pressure, feed density, size of vortex and apex. Depending on the ore characteristics, operating parameters of hydro-cyclone can be altered to improve performance. The Hydrosizers (classifiers) allow splitting of the low grade ore into two parts, at roughly 50% plus 0.150 mm and 50% minus 0.150 mm respectively. The desired split size of 0.150 mm is chosen based on the fact that particles below that size are normally small enough for the pelletizing process.
Magnetic separator uses composite, permanent magnet materials such as Nd-Fe by applying a new magnetic force concentration technique, a suitable product can be obtained. It is simple in construction, uses no extra power to generate the magnetic field and has high processing capacity per unit weight. Some tests on a tungsten ore have shown that , compared to a double-disk magnetic separator, it has four times higher capacity per unit weight and produces a tungsten concentrate with 3.79% higher grade and 5.27% higher recovery. This new device would be suitable for concentrating weakly magnetic minerals. For fine particles down to around 38 microns, spirals give more satisfactory results. Smaller particles than 38 microns must be upgraded by HGMS or possibly centrifuges. The centrifuge unit until recently had a limited capacity of only 45 dmt/hr and they were operating in batch discharging mode. Now the capacity of centrifuge unit is over 100 dmt/hr and they have continuous discharge. Like HGMS, a centrifuge works much better in a narrow size consist. The final tail is dewatered in a high rate thickener before deposition in the tailing pond.
The concentrate from the two stages of magnetic and gravity separation is thickened to about 60% - 70% solids. Thickener underflow and hydro-separator underflow are pumped into the agitated slurry storage tanks before filtration.
Flotation columns are widely used in mineral processing industry to effect separation between disperse phases based on differences in their affinities for air bubbles. However the effective design and scale-up of flotation columns remain a difficult problem. Their design has been based on the use of analogy with chemical kinetics. Use of this procedure is contrasted with mass transfer.
Thickener / Filters
Thickening / filtering removes most of the liquid from both slurried concentrates and mill tailings. Thickened tailings are discharged to a tailings pond and the liquid is usually recycled to a holding pond for reuse in the mill. Chemical flocculants such as aluminium sulfate, lime, iron, calcium salts, and starches, may be added to increase the efficiency of the thickening process. A distributor is required to feed each filter from the concentrate thickener. The most efficient filter i.e. pressure filter is now replacing older technology filters. At a Blaine of 1800 cm2/g on specular hematite ore pressure filter/ disc filter produces close to 100 dmt / hr at 8.5% moisture. On earthy hematite limonite ore, to control the filter cake moisture at 10% H20 or below, the capacity would be in the range of 80 dmt / hr more likely.
Iron Ore Pellet Project
The integral development of a Iron Ore Pellet Plant project entails some high risks due to the multiple variables involved. VT CORP ensures fulfillment of user requirements, meeting the schedule, qualities and costs.
Our solutions include:
- Techno-Economical Feasibility Report (TEFR)
- Engineering services
- Supply of Equipment
- Project Management, Supervision and Inspection services
- Start-Up / Commissioning
STEP 1 – CONCEPTUAL DESIGNING
Concept design is used to identify project technical and economic feasibility and to set out guidelines for basic and detailed engineering development. This is based on a previous study (feasibility study) and on the definition of project requirements.
The main concepts analysed and studied in this phase are:
- Products and production capacity
- Standards and regulations
- Manufacturing process description and user requirements
- General installation description.
- Plot plan, block diagrams, room distribution, personnel and materials flow diagrams, classified area drawings, basic process diagrams
- Requirement estimates for auxiliary services
- Preliminary equipment list
- Economic investment estimate, ± 30%
In order for this phase to be successful, VT CORP has to fully assimilate the customer's requirements, which means that the involvement of the latter is essential right from the very beginning of the project. A joint customer/VT CORP work team is therefore established with experts from both parties.
STEP 2 - BASIC ENGINEERING
The basic engineering will definitively include the user's requirements, basic specifications, the execution timetable and the economic evaluation.
The following jobs are defined during this phase:
- Detailed review of the concept engineering and user requirements
- Data sheets for all rooms (critical and non-critical)
- Heat load and air flow calculations for each room
- Basic water and HVAC P&ID
- Services points of use distribution
- Room layout review, including service areas
- Consumptions list
- Equipment list
Basic engineering is developed in two phase: the first comprises data acquisition and user requirement preparation and, in the second, the development of all other previously described jobs. The approval of this engineering forms a solid base for detailed engineering development.
STEP 3 - DETAILED ENGINEERING
VT CORP develops the detailed engineering within the project execution scope, using of the very latest calculation, drawing and 3D design tools, which guarantee maximum quality results from start right through to qualification.
The range of activities in this phase is:
- Detailed review of basic engineering
- Technical specifications for materials and equipment
- Functional specifications
- Dimensioning of conduit, piping and electrical installations
- Lists of equipment, instrumentation, accessories and materials
- Detailed installation drawings: piping and conduit layout, isometrics, architecture details, single-wire electrical drawings etc
CORE EQUIPMENT SUPPLY
We are having a team of experienced technical experts in the field of Iron Ore Beneficiation and Iron Ore Pelletisation for technology and process. We offer turnkey solutions from concept to commissioning for Iron Ore Pellet Plant.
We are having a tie-up with world renowned technology provider M/s. Uralmash, Russia. Uralmash has supplied more than 50 successful Indurating machine installations worldwide. We can supply Indurating machine with international standards at best price in least delivery period.
VT CORP is a diversified business group and catering to various industries. We have three manufacturing facilities located at Hyderabad, Nagpur and Mumbai. Our mineral process division is located at Bangalore.
Project management in our fields of operation consists of the application of knowledge, abilities, tools and techniques to the current project activities in order to satisfy customer requirements and thus accomplish the final goal with efficacy and efficiency.
The project manager is responsible for identifying requirements and establishing clear possible goals, leading and coordinating a highly qualified work team in each of the disciplines.
VT CORP establishes a Project Quality Plan that guarantees success in terms of time, scope, cost and quality. The project manager will be responsible for executing the PQP with regards to the following:
- Project structure
- Project equipment management and coordination
- Project supervision and coordination
- Cost and time control
- Purchase support
- Detailed project review
- Project management means added value in our wide range of activities and services.
In Project commissioning, VT CORP is assuring that all systems and components of Iron Ore Pellet Plant is designed, installed, tested, operated, and maintained according to the operational requirements of the client.
In practice, the commissioning process comprises the integrated application of a set of engineering techniques and procedures to check inspect and test every operational component of the project, from individual functions, such as instruments and equipment, up to complex amalgamations such as modules, subsystems and systems.
Commissioning activities, in the broader sense, are applicable to all phases of the project, from the basic and detailed design, procurement, construction and assembly, until the final handover of the unit to the owner, including sometimes an assisted operation phase.
Technology: Pellet Plant
TECHNOLOGY: STRAIGHT GRATE TECHNOLOGYVT CORP offers a proven straight travelling grate technology. It can handle all types of pellet feed whether 100% hematite, 100% magnetite or a blend of both. Entire process comprising :
- After-firing and
are carried out on a single straight travelling grate furnace.
Advantages of this process are its
- No rotary equipment, and
- Highest possible fuel efficiency
Straight Grate Indurating Machines
The high reliability and long life indices are achieved through the use:
- A system of pallets state diagnostics and equipment protection in emergency situations;
- Parts of pallets made of heat-resistant steels and alloys;
- Grate bars with the heads fitted with projections and recesses to prevent skewing of grates;
- Symmetrical under-grate bar frame of the pallet which can be turned over repeatedly and thus repaired quickly.
- Pallet carrier mounted on antifriction bearings;
- Discharge device with load hold-down decreasing spilling of pellets when discharging;
- Efficient device with automatic grippers to replace pallets which excludes manual labour and reduces downtime.
Efficient system of gas-air flows promotes the maximum use of heat of process gases thanks to their recirculation over the process zones;
- Two-layer charge of green pellets with subsequent drying by layers improves the quality of pellets;
- Transfer of the high-temperature heat carrier directly from the cooling zones to the firing ones without using blowing means reduces consumption of electric power;
- Application of injection burners permits using of hot air-oxidizer which reduces fuel consumption;
- Tight seals reduce consumption of electric power;
- Gas leakage through seals of wind boxes is used for aspiration of blowing-down through seals of the pressure chambers, thus the electric power consumption for aspiration is reduces;
Economical and efficient electric drives allow all the requirements of the process to be realized. The level of their diagnostics minimizes the number and time of shutdowns.
Green Pelletizing Section has Pelletizing discs.
These discs are fed with mixed material at a controlled rate from the day bins. Green pellets discharging from the discs are conveyed over a common conveyor to a double deck roller screen via oscillating and wide belt conveyor system.
Dry material does not pelletize and presence of moisture is essential to roll the powder into balls.
- Surface tension of water in contact with the particle plays a dominant role in binding the particle together.
- Rolling of moist material lead to the formation of balls of very high density which otherwise is attainable by compacting under high pressure.
- The ease with which material can be rolled into balls is almost directly proportional to the surface area of particles i.e. it’s fineness
Kinetics of balling
Growth of pellets in a disc occurs in two stages
- Formation of nuclei of seeds.
- Growth of nuclei to form pellets
- Water saturated particles form the nuclei or seeds which grow by repeated collision with other nuclei or wet particles to become pellets.
- Major forces holding the particles together – surface tension of water.
- As the nuclei grows: the centrifugal force overcomes the friction and these are ejected from the nucleation zone and enter the pellet growth region.
- During the process of growth, excess water is expelled from the voids of the grown pellet by compaction and rolling on the surface of the disc.
Ball growth region
- Growth by assimilation is possible when balling proceeds without the addition of fresh feed material.
- Growth by layering is possible when balling proceeds with the addition of fresh feed material.
- The rolling action breaks some of the granules particularly the smaller ones, and the material coalesces with those which grow.
- Since smaller granules are weaker, they are the first victim and growth of bigger balls take place at their expense
- Growth continues till such size when the torques tending to separate the contacting balls exceeds the bond strength of coalescence.
- More predominant in drum pelletizers.
Growth by layering
- Growth of the seeds is said to be taking place by layering when the balls pick up material while rolling on a layer of fresh feed.
- The amount of material picked up by the balls is directly proportional to its exposed surface.
- Predominant in disc pelletizers.
Induration of green balls means their thermal treatment in the following stages:
- Drying of green balls.
- Heating of dried pellets & oxidation of magnetic pellets up to induration temperature.
- Firing at induration temperature.
- Cooling of indurated pellets
- UPDRAUGHT DRYING
- DOWNDRAUGHT DRYING
- PRE HEATING
- AFTER FIRING
- COOLING ZONE
|Travelling-Grate Induration Machine|
|1||Production of qualified fired pellets, MTPA||≥0.3||≥0.45||≥0.6||≥1.2|
|3||Total area of process zones, m2||52||72||108||189|
|4||Length of process zones, m||26||36||36||63|
|5||Width of process zones, m||2||2||3||3|
|6||No. of wind boxes||13||18||12||21|
|7||Area per wind box, m2||4||4||9||9|
|8||Pallet strand speed, m/min||0.25-1.25||0.25-1.25||0.1-1.5||0.3÷2.0|
|9||Hearth layer height, mm||80|
|10||Side layer height (per side), mm||80||80||80||80|
|11||Total pellet bed including hearth layer, mm||450||450||450||450|
|12||Emergency layer height, mm||300||300||300||300|
|13||No. of burners|
VT CORP offers Straight Grate Technology with the technology partner M/s.URALMASH, Russia for Iron Ore Pellet Plant with the capacities 0.3, 0.45, 0.6, 1.2, 2.0 MTPA and so on.
URALMASH Machine-building Corporation (MK URALMASH) is a leader of the Russian heavy machine-building sector.
Within the Corporation, Uralmash Engineering Company was created with its own competitive manufacturing capabilities, an extensive know-how background in the field of technology and design of individually tailored machinery. Manufacturing sites of MK URALMASH are located in Ekaterinburg (Uralmashzavod) and Orsk (ORMETO-YUMZ).
Uralmash Machine-building Corporation
Designs and supplies complete equipment for all the branches of heavy industry. Provides modernization and revamping of previously supplied equipment. Supplies replacement equipment and spare parts. Designs drive and automation systems. Our scorecard lists reference installations, both implemented as the “turn-key” solutions and based on modernization of the existing systems and process. MK URALMASH offers complete supply of technology and equipment, coordinates activities of designers, process engineers and manufacturers and other sub suppliers involved in the projects.
Integration and further development of the manufacturing capacities is a guarantee of reliable supplies of a wide spectrum of equipment in Russia and abroad.
Uralmash Expertise in Pellet Plant Technology
Uralmash plant has a wide-ranging experience in production of induration machines used at pelletizing plants. Since 1964 about fifty production complexes with induration machines of 108m2, 306m2 and 520m2 effective areas have been designed and put into commission. Along with these, component parts and accessory equipment intended for various methods have been manufactured and put into operation. Uralmash plant induration machines are used for production of pellets from iron-ore and nickel concentrates, chromitites and phosphorites, as well as for cement-clinker production and waste-free processing of bituminous shale and in other induration-related processes.
Uralmash Experience in India:
Starting from 1950-ies Uralmash plant has supplied every large-scale metallurgical plant in India with its equipment. The partnership has started from the deliveries of Uralmash plant equipment to Bhilai Metallurgical plant. A number of Uralmash machines have been operated at the plant. These are: "1150" blooming mill with annual production rate of 1 million tons of ingots, continuous billet-production mill (which production rate is 430 thousand tons of billets per year), rail-and-beam mill with annual production rate of 700 thousand tons of work-pieces (in the mid of 1990-ies Uralmash plant modernized the mill).
In 1970-ies Uralmash plant has been involved into construction of one of the largest metallurgical plants located in Bocaro. Thus, Uralmash plant has manufactured "2000" continuous four-housing mill and "200" one-housing mill of manufacturing capacity - 575 thousand tons of mill products per year.
In the mid 1980-ies Uralmash plant has delivered sintering machine of 312m2 work area to Visakhapatnam Metallurgical plant.
In the late 1990-ies Rourkela Metallurgical plant has been equipped with Uralmash plant sintering machine which work area was 192m2.
Uralmash Machine-Building Corporation is one of the leaders in the Russian market of metallurgical, mining, oil- and gas-producing industrial equipment, as well as the equipment for construction-materials industry and power industry. The development strategy of Uralmash Machine-Building Corporation is to create a state-of-the-art machine-building corporation, which will be able to fully satisfy the customers' needs for equipment.
Value talent cultivation and encourage creative innovation
Effective Team Work
Pursue organizational solidity and press on effective team-work
Ensure first and best of product quality and service
What is Pelletization?
The process of pelletization enables converting Iron Ore Fines into “Uniformed Sized Iron Ore Pellets” that can be charged into the blast furnaces or for Production of Direct Reduced Iron (DRI).
Pellets are uniform size, with purity of 63%- 65% contributing to faster reduction and high metallization rates.
Pellets with their high, uniform mechanical strength and high abrasive strength increase production of sponge iron by 25% to 30% with same amount of fuel.
Advantages of Iron Ore Pellets Over Sized Iron Ore:
- Iron ore pellets are superior to iron ore lumps in the subsequent iron making processes such as Blast furnace, DR furnace etc.
- Iron ore pellets with a nominal 64- 67% Fe, uniform porosity and better size help in faster reduction and higher metallization rates than the conventional iron ore lumps.
- The inherent higher mechanical strength and abrasion resistance of pellets enhance the production rate of sponge iron by approx. 20% under identical operating conditions.
- Iron ore pellets are of spherical shape with a size range of 9-16 mm.
- Iron ore pellets are not vulnerable to degradation during transportation due to their high abrasion resistance.
Pelletizing Facility Description
The Iron Ore Pelletisation facility can be broadly grouped into four sections as under.
- Iron Ore Pellet feed and additives storage, proportioning and mixing section
- Green balling section
- Indurating Furnace section
- Iron Ore Pellet product stacking and reclaiming section
Pellet feed is basically the iron ore concentrate with a minimum of 64% Fe, ground to 100% minus 100 mesh, with a Blaine index of ~1800.
Additives are limestone and / or dolomite, bentonite and coal / coke breeze, all ground to minus 200 mesh.
While limestone and / or dolomite are added to control the basicity as needed in the downstream iron making processes, bentonite is used as a binder and coal / coke breeze is a solid fuel supplement.
Requirement of pellet feed and additives is as under:-
|Quantity required per tonne of pellets||1033 kgs|
|Size consist and Blaine number||100% passing 100 mesh with ?1800 Blaine|
|Fe %||? 63% Fe|
|Gangue (SiO2 + Al2O3)||3% to 5%|
|Loss on ignition||? 3.5%|
|Quantity required per tonne of pellets||15 – 20 kgs|
|Size consist||100% passing 200 mesh|
|CaO||50% ± 1%|
|SiO2||3% ± 0.5%|
|Loss on ignition||~ 42%|
|Quantity required per tonne of pellets||6 - 10 kgs|
|Size consist||100% passing 200 mesh|
|Coal / Coke|
|Quantity required per tonne of pellets||15 kgs max.|
|Size consist||100% passing 200 mesh|
|Calorific value||~ 6500 kcals / kg|
|Volatile matter (coal)||10% max.|
|Ash fusion point||13300 C|
Iron Ore in the form of ground filter cake and ground additives, are conveyed to their respective day bins. The stored materials are transferred to mixer in the required proportion for homogenization ahead of balling.
Iron ore mixed with the additives is fed to balling discs. Green pellets discharging from the balling discs are conveyed to a double deck roller screen.
On-size green pellets are fed to the straight travelling type indurating machine for heat hardening.In this process, the wet pellets are dried, preheated, indurated and cooled on a continuously moving grate without intermediate transfers. The process air introduced for pellet cooling is circulated from the cooling zone of the grate in a multi-pass manner to the other process zones to obtain maximum possible thermal efficiency. Only the relatively cool and moisture laden gases are discharged to process gas cleaners and then back into atmosphere.
Fired pellets, after separation for hearth layer recycle, are discharged onto belt conveyors for storage in the product stockpile.
Auxiliary facility required for normal operation of the facility will be provided within the process plant battery limits, a brief description of which is as follows.
Pellet feed and additives storage, proportioning and mixing sectionGround iron ore concentrate and additives are conveyed into individual storage bins. The raw materials drawn from the respective bins at the desired rate are fed to the mixer for homogenization.
There are four additives used in the pelletizing blend viz., bentonite as binder, limestone and / or dolomite as flux, coal or coke as solid fuel.
Limestone is normally used to tailor the chemistry of pellets and to produce the desired metallurgical and physical properties in the fired pellets. Limestone levels of 0.5% to about 2% of pelletizing mix are common for production of DR-grade pellets. BF-grade pellets may have levels of carbonate flux addition in the green balls exceeding 10% in some cases. Pellet chemistry is customer driven, and the pellet plant is designed to provide the entire range of limestone addition rates necessary to meet the need of individual customer. If the pellet chemistry calls for magnesium content, dolomite could be substituted for the limestone and processed in the limestone handling equipment.
Fixed carbon in the form of coal / coke breeze is simply a solid fuel added to green balls to improve furnace productivity and overall fuel economy. Coal / coke is generally a less expensive source of heat than oil or gas. When added to green balls at proper levels, coke brings the inherent fuel value of a hematite green ball equal to that of a magnetite pelletizing feed. This contained fuel value, when burnt properly in the pelletizing cycle, results in lowering consumption of the more expensive burner fuel and at the same time accelerate the pelletizing process. Both proximate analysis and CV of coal / coke breeze dictate the quantity that can be added to mixer.
Bentonite is used as a binder in production of BF-grade pellets or DR-grade pellets, depending upon the silica requirement of the pellets produced. Bentonite adds up to approximately 0.3 % by weight to the silica content (SiO2 content) of fired pellets. Bentonite is generally less expensive compared to the organic binder.
Green Balling Section:
Mixed material from the mixer is conveyed to surge bins in the balling section and fed by weigh-feeders to the pelletizing discs. Green pellets discharging from the discs are conveyed to a double deck roller screen ahead of indurating machine. The oversize and undersize green pellets are re-circulated and on-size green pellets fed to the straight travelling grate type indurating furnace.
The indurating furnace is fed continuously from the double deck roller screen feeder which lays down the green balls across the full width of the machine on top of a protective hearth layer. Induration of green pellets takes place on the travelling grate, having a number of wind boxes. The total bed height of hearth layer plus the green balls is constant up to ~ 500 mm. Speed of travelling grate is variable in the range of ~ 0.5 to ~ 1.5 meter / min. and is controlled to maintain a constant bed height.
In the updraft drying zone, gas flow removes water from lower half of pellet bed and at the same time heats this layer to a temperature where condensation will not form in the lower layers of green pellets, when gas flow reverses for downdraft drying.
The updraft drying off-gas is discarded. The heat for updraft drying is supplied by the second cooling zone.
Heat for downdraft drying air is supplied from firing zone wind boxes. Air for remainder of heating cycle comes from first cooling zone via direct recuperation duct. Provision is made to temper the initial stages of preheating with cooler air to allow better control of heat-up rate of pellets to prevent thermal shock and to avoid damage from rapidly evolving steam or gases resulting from crystalline water in the ore, calcinations of the flux and combustion of coal. Gas from the downdraft wind boxes is discarded until off-gas reaches sufficient temperature to be useful for downdraft drying.
The gas flow scheme is designed to recuperate a significant amount of heat. High level heat is recuperated directly from first cooling zone, while low level heat is recuperated from second cooling zone for drying via fans at temperatures which avoid the use of exotic materials of construction.
Process parameters may be reset to optimize the induration pattern as campaigns of different types of pellets are run. At the end of firing zone most of the bed will have reached the end temperature of about 1300°C to 1350°C. The lowest layers of the bed reach the end temperature by drawing the hottest recuperation gases through the bed in the after firing zone. No burners are required in this zone. Cooling is updraft to quickly lower the temperature of grate components and to recuperate the heat in pellets at the highest possible temperature. Heat for induration is supplied by burners firing heavy furnace oil.
A single fuel burner design system for heavy fuel oil is envisaged in this proposal. Burner system for combustion of low CV gaseous fuel = 2000 kcal/Nm3 is outside the scope of this design. Hence, Heavy oil burners will have to be removed from burner ports and gaseous fuel burners installed in the event of switching over to gaseous fuel in future. However, a dual burner system can be offered, if required.