Introduction to Iron ore Pellets and Pelletizing processes
Introduction to Iron ore Pellets and Pelletizing processes
Pelletizing is a process which involves mixing of very finely ground particles of iron ore fines having a size which is less than 200 mesh (0.074 mm) with additives like bentonite and then shaping them into near oval/spherical balls having size in the range of 8 mm to 16 mm in diameter by a pelletizer and hardening the balls by firing with a fuel. It is an agglomerating process of converting iron ore fines into ‘uniformed sized iron ore pellets’ which can be charged directly into a blast furnace (BF) or into a vertical furnace or rotary kiln normally used for the production of direct reduced iron (DRI). The iron ore pellets are shown in Fig 1.
Fig 1 Iron ore pellets
The typical properties of the iron ore pellets are given in Tab 1
|Tab 1 Typical properties of iron ore pellets|
|Chemical analysis(on dry basis)||Unit||Value||Tolerance|
|SiO2 + Al2O3||%||5||Maximum|
|Bulk density||t/cum||2.2||± 0.2|
|Abrasion index||%||3||Min ± 0.5 %|
|Cold crushing strength (Avg.)||kg/p||250||Minimum|
|8 mm – 16 mm||%||94||Minimum|
There are four stages involved in the production of iron ore pellets. These stages consist of (i) raw material preparation, (ii) formation of green balls or pellets, (iii) induration of the pellets, and (iv) cooling, storage and transport of pellets.
Raw material preparation – During the process for pelletization iron ore concentrate from iron ore beneficiation plant is dried and heated to around 120 deg C. The dried material is fed to the ball mill for grinding. Concentrate/ground iron ore of typical size 80 % less than 45 microns (0.045 mm) with a moisture content of around 9 % is required for the pellet production. Suitable binder (normally bentonite) is added to the concentrate which is thoroughly mixed in high intensity mixer.
Formation of green balls or pellets – Green pellets with a size range of 8 mm to 16 mm are prepared in a balling drum or in a disc pelletizer. Disc pelletizer is preferred for the production of the quality green pellets since in the disc pelletizer it is easy to control the operation with minimum of foot space. The disc pelletizer is an inclined pan having around 5 metres (m) to 7.5 m diameter. It rotates at around 6 rpm (rotations per minute) to 8 rpm. The inclination of disc is around 45 degrees and it can be adjusted in the off-line position between 45 degrees to 49 degrees.
The pre wetted mix is fed into the disc at a controlled rate. In the disc, the material is coagulated and due to the continuous rotary motion gets formed into nodules/ pellets. 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 pellets grow 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. These pellets are called green pellets as they do not have the required strength. The green pellets are then screened in a roller screen and the required size material is fed to the traveling grate of a pelletizing machine.
Surface tension of water, capillary action of water, and the gravitational force in the balling disc are the forces which act on the ore particles. Hence, they get coalesce together and form nuclei which grow in size and into ball shape. These forces responsible for the agglomeration of iron ore fines are generated in the balling disc due the rotational movement in the balling disc. When the solid particles come in contact with water, the ore surface is wetted and coated with water film. Because of the surface tension of the water film, liquid bridges are formed. As a result of the movement of particles inside the balling disc and because of the combination of the individual water droplets containing ore grains, the particles first agglomerates. The initial agglomeration of the particles causes formation of seeds. The liquid bridges in the interior of these seeds hold the particles together as if the particles are 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 also plays an important role.
Induration – During the induration, heat hardening of green pellets is carried out. Induration of green pellets consists of three main steps namely (i) drying of green pellets, (ii) firing of pellets at around 1300 deg C to sinter the iron oxide particles, and (iii) cooling of hot pellets before discharging.
During drying (temperature range of 180 deg C to 350 deg C), moisture content of 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 temperature. During pre-heating stage (temperature range of 500 deg C to 1,100 deg C), decomposition of carbonates and hydrates takes place. Gasification of solid fuels like coal or coke and conversion of iron oxides like goethite, siderite, and magnetite to higher oxide state hematite, also takes place during this stage. Commencement of solid oxide bonding and grain growth are the important steps of this stage. During firing stage (temperature range of 1250 deg C to 1340 deg C), the temperature is below the melting temperature of major oxide phase but within the reactivity range of gangue components and additives. Formation of oxides and slag bonds is decisive of this stage.
Bonding of mineral grains developed during induration of pellets is affected by the three factors consisting of (i) solid oxide bonding, (ii) re-crystallization of iron oxide, and (iii) slag bonding. Solid oxide bonding is due to the oxidation of ferrous iron oxides to ferric iron oxides which results in bonding and bridging, but only by a limited amount. Re-crystallization of iron oxides is essentially a physical process in which smaller particles consolidate into larger ones with the loss of surface energy. During the re-crystallization of iron oxides, continued growth of iron oxide crystals imparts sufficient strength. During the process, the grain growth for hematite starts at around 1100 deg C. In case of slag bonding, gangue by forming melt transport medium for ferrous or ferric oxides, facilitates grain growth and crystallization of oxide grains. It also enables the mechanism to proceed at lower temperatures than what is needed in its absence.
The induration treatment causes certain chemical reactions to occur which change the specific metallurgical properties of the pellets. These reactions can include the oxidation of magnetite and dehydration of earthy hematite. For BF grade, fluxed pellets are produced with additions of limestone, dolomite, silica, etc. to the balling feed. These additions react with the gangue in the iron ore to enhance the performance of the pellets in certain downstream processing steps.
Pellet cooling and handling – The pellets are cooled and screened after the induration. The over sized pellets are crushed and are sent along with the undersized to the stock house bins where they are reprocessed. Cooled pellets are sent to the storage for their transport to the downstream plants for further processing.
There are several iron ore pelletizing processes/technologies which are available for the production of the pellets. Some of these are (i) shaft furnace process, (ii) straight travelling grate process, (iii) grate kiln process, (iv) cement bonded processes (Grangcold process, MIS Grangcold Process, and char process etc.), and (v) hydro-thermal processes, (COBO process, MTU process, and INDESCO process etc.). However, at present, only the straight travelling grate (STG) process and the grate kiln (GK) process are more popular processes.
Straight travelling grate process
The process was developed by former Lurgi Metallurgie and accounts for global major installed capacities. In this process, a double deck roller screen ensures right size of green pellets (8 mm to 16 mm in size) is evenly distributed across the width of the travelling grate. The grate carries the green pellets on a bed having a height in the range of 300 mm to 550 mm through a furnace having several zones. These zones are with updrafting, downdraft drying, preheating, firing, after firing and heating zones. A flowsheet of the process is given at Fig 2.
Fig 2 Flowsheet of straight travelling grate process
Grate kiln process
The grate kiln process was developed by former Allis Chalmer and the first plant on this technology was constructed in 1960. In the grate kiln process (Fig 3) the traveling grate is used to dry and preheat the pellets. Material moves on straight travelling grate till it attains the temperature in the range of 800 deg C to 1000 deg C. After that, the material is transferred to refractory lined rotary kiln for induration where the temperature is further raised in the range of 1250 deg C to 1300 deg C. At 800 deg C, the FeO of the magnetite iron ore gets converted into Fe2O3 in an exothermic reaction. The liberated heat hardens the green balls which is helpful to withstand the tumbling impact due to the rotation of the rotary kiln. A circular cooler is used for cooling of the fired pellets. Flowsheet of grate kiln process is given at Fig 3.
Fig 3 Flowsheet of grate kiln process
A comparison of the two processes is given in the Tab 2.
|Tab 2 Comparison between straight travelling grate process and grate kiln process|
|Sl. No.||Straight travelling grate process||Grate kiln process|
|1||Drying , preheating, induration, and cooling cycle is carried out in a single unit||Drying , preheating, induration, and cooling cycle is carried out in different units|
|2||Green pellets remain undisturbed during the process||Entire process takes place in three equipments namely travelling grate, rotary kiln and circular cooler hence pellets transfer takes place.|
|3||Grate cars moves at the same speed in the drying, induration and cooling zones. Any disturbance in one zone affects the other zones||Independent control of the three zones hence the process has better operational flexibility|
|4||Fines generation is negligible since there is no transfer of materials||Since material transfer takes place at several places hence there is higher generation of fines|
|5||There is no strength requirement of intermediate product||Before transfer to the kiln, the green pellets are to be sufficiently hardened|
|6||Process availability is higher||Process availability is lower|
|7||Higher specific energy consumption||Lower specific energy consumption|
|8||Lesser maintenance||Higher maintenance|
|9||Lower dust generation||Higher dust generation|
|10||Higher investment cost||Lower investment cost|
|11||Suited both for hematite and magnetite ores||Process is more suited for magnetite ores..|
A comparison of temperature distribution during the two processes is shown in Fig. 4
Fig 4 Comparison of temperature distribution during the two processes
Advantages of pellets
The various advantages of iron ore pellets are given below.
- Iron ore pellet is a kind of agglomerated fines which has better tumbling index when compared with the iron ore and it can be used as a substitute for the iron ore lumps both in the BF and for DRI production.
- Pellets have good reducibility since they have high porosity (25 % to 30 %). Normally pellets are reduced considerably faster than sinter as well as iron ore lumps. High porosity also helps in better metallization in DRI production.
- Pellets have a uniform size generally in a range of 8 mm -16 mm.
- Pellets have spherical shape and open pores which give them good bed permeability.
- Pellets have low angle of repose which is a drawback since it creates uneven binder distribution.
- The chemical analysis is uniform since it gets controlled during the beneficiation process. Fe content varies in the range of 63 % to 68 % depending on the Fe content of ore fines. Absence of LOI (loss on ignition) is another advantage of the pellets.
- Pellets have high and uniform mechanical strength and can be transported to long distances without generation of fines. Further, it has got resistance to disintegration. High mechanical and uniform strength of pellets is even under thermal stress in reducing atmosphere.