Raw Materials II: Iron Ore and Agglomerates
9.4 THE SINTERMAKING PROCESS .1 Bedding and Blending
In sintermaking, various materials in fine form, and in given proportions have to be processed.
Therefore, one of the steps preceding sintermaking is bedding and blending so that uniformity in the feed to the sintering machine(s) can be ensured, which, in turn would guarantee that the sinter produced is of consistent chemistry.
For this purpose, the individual raw materials comprising iron ore fines, fluxes (limestone, pyroxenite, dolomite, etc.) and coke breeze are first crushed (fluxes are ground in a hammer mill, and coke breeze is ground in a roll crusher or a rod mill) and screened to the size required—
typically, iron ore between 100 mesh and 10 mm, fluxes below 3.15 mm and coke breeze below 6 mm. These materials are then individually stored in bins. The materials are discharged from these bins in the prescribed amounts onto conveyor belts. At the last stage, a shuttle-type conveyor belt takes all the materials together to the bedding and blending yard. The discharge from the conveyor is heaped in the shape of triangular piles of the mixed materials on the ground. From each of these piles, the mix is then scooped by a reclaimer, which in its turn, helps to make long, horizontal layers.
Approximately 300 individual layers make up one bed and at least two such beds are provided in any sintering plant—one that is still being laid horizontally, and the other from which the mixed materials are in the process of being sent for sintering. Immediately before sintering, flux is added and minor adjustments are often made to the coke breeze amount in the sinter mix, depending on the requirements of sinter chemistry and the prevailing thermal conditions. More often than not, solid wastes from the plant that contain some amount of iron and/or flux constituents are added to the sinter mix. Sintering provides an easy way for recycling these ‘waste’ materials.
If the fluctuations in sinter chemistry are such that the standard deviation in CaO content is 1.8–2.0% and that in the Fe content is about 0.70–0.75% without any bedding and blending, these fluctuations can be minimised to 0.6% for CaO and 0.3% for Fe by adopting bedding and blending. Thus, the need for bedding and blending operations prior to sintering is obvious.
9.4.2 Granulation
The green mix after thorough mixing is transferred to a mixing drum. In this drum, water is sprinkled and the drum is rotated to encourage ball/granule formation. This is essential for maintaining the permeability in the sinter bed, particularly if deep-bed sintering (bed depth 600 mm and more) is resorted to.
Since it is widely accepted that deep-bed sintering gives superior quality sinter, the mechanism of granule formation has received considerable attention. The growth of the granules has been described in terms of two limiting cases, or postulates. In the k-postulate, the granule size is assumed to be proportional to the seed size. Hence,
y = kx (9.1)
where y and x are the granule size and seed size, respectively, and k is a constant of proportionality. Recently, it has been shown that k can be calculated from the relationship
er er
where W is the moisture content of the granulating charge, g is a minor correction for any entrapped air bubbles in the layers and for the curvature of the liquid–air interface at the granule surface, e is the void fraction of the layer, and rl and rs are average densities of liquid and solid, respectively.
In the second theory, the so-called t-postulate, the layer thickness t is assumed to be fixed and independent of the seed size. Hence
y = x + 2t (9.3)
At present, no theoretical analysis exists for predicting the layer thickness, based on which the merits/demerits of these two postulates can be assessed.
9.4.3 Sintering
The technique of sintering, whereby the fine materials charged are partially fused at a high temperature to produce clustered lumps was developed in the 1890s, in the non-ferrous industry as a batch process. Continuous sintering of copper ore was undertaken between 1903 and 1906 by Dwight and Lloyd in Mexico, wherein, a moving-bed of fine ore particles and other additives, supported on a metallic chain type strand, was agglomerated by exposing the bed to high temperatures. The heat required was supplied primarily from external sources. The same idea was later adopted for iron ore sintering. Since then, Dwight–Lloyd technology has been used all over the world to produce iron ore sinter for blast furnaces. Sinter is typically 5 mm to 40 mm in size, made from feed ore fines of below 10 mm in size.
The layout of a Dwight–Lloyd iron ore sintering machine is shown in Figure 9.1. The sinter mix is fed at one end of an endless belt (or strand) that consists of a large number of hollow individual steel boxes (called pallets), which keep moving continuously like a chain at the desired rate, determined by the vertical sintering speed, from the feed end to the discharge end of the strand. Once the mix is fed into a pallet, heating is begun (referred to as ignition) by using external burners located in the ignition hood at the feed end of the machine. Individual wind
Machine fines
Figure 9.1 Schematic arrangement of a sintering machine in a sinter plant.
boxes located under the strand provide the suction as each pallet moves from the feed to the discharge end. Sintering is normally completed before the strand reaches the last two wind boxes at the discharge end. Suction of air through these two wind boxes cools the sinter on the strand.
9.4.4 Feed Preparation and Product Handling
The starting materials for making sinter are fine raw ore (less than 8–10 mm), coke breeze (coke that is between 3–6 mm in size), fine limestone/dolomite (less than 3 mm) and sand. An alternative to sand plus limestone/dolomite is dunite, i.e. magnesium silicate, which supplies both CaO and MgO along with silica, thereby helping to balance the sinter chemistry, without adding the three materials separately. Return sinter below 5 mm (normally referred to as return fines) and steel plant waste materials that contain some iron units, are also added. After mixing all the input materials, water is sprinkled for granule/nuclei formation as explained earlier (Section 9.4.2). These nuclei grow as mixing of the flux, carbon and iron ore fines is continued, but since the time available for granule formation is relatively short (3–4 minutes), the growth of the nuclei into large globules (more than 10 mm) is restricted. The granulated material is transferred to the sintering strand to form a bed (typically 400–600 mm deep). Burners, fired with blast furnace/coke oven gas or oil, are used to supply heat initially. Air is sucked at a fixed suction (usually 1.1–1.6 kPa) from under the strand so that the flame front after ignition travels through the entire bed of the sinter mix. The advance of the flame front is also supported by the combustion of coke breeze present in the bed.
The iron ore particles get agglomerated owing to partial melting at the surface and slag formation. The sinter thus produced is crushed and screened before charging into blast furnaces.
For this purpose, the sinter cake from the machine is cooled and then broken to a suitable size (typically 6–40 mm) in a crusher, followed by screening to remove the below 5 mm size. Part of the original sinter (usually 30–40%) becomes return fines (generated both at the sinter plant and in the blast furnaces prior to charging).