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Factors influencing Sinter and Sintering Process

• satyendra
• July 8, 2013
• 8 Comments
• alumina, Burn through point, Flame front, Granulated mix, Grate, Iron ore, LTD, mixing drum, RDI, RI, SFCA, sinter, sinter machine, TI,

Factors influencing Sinter and Sintering Process

The sintering process is used to agglomerate a mix of iron ore fines, return fines, fluxes, and coke, with a particle size of  less than 10 mm, so that the resulting sinter, with a screened size of 5 mm to 30 mm, can withstand pressure and temperature conditions in the blast furnace (BF). The process of sintering of iron ore fines was primarily developed to convert the fines generated in mining and converting these ore fines into a product which is suitable for charging into the BF.

The BF needs high quality sinter with high strength, the lowest possible fines content, a good average size, a low RDI (reduction degradation index), high RI (reducibility index), low fines content, good average calibrated sinter size, and little variation in chemical composition in order to operate in a steady state regime. Sinter quality control, by means of adequate sintering, is important in order to operate BF at a low fuel and stable operating rate.

The sintering process is based on treating a raw mix layer (iron ores fines, return fines, and fluxes etc.) in presence of coke fines to the action of a burner placed in the surface of the layer. In this way, heating takes place from the upper to the lower sections. The raw mix layer rests over a strand system and an exhausting system allows to the whole thickness to reach the suitable temperature for the partial melting of the mix, and the subsequent agglomeration. In the Dwight-Lloyd system, the sintering grate is a continuous chain of large length and width, formed by the union of a series of pallet cars which make the sintering strand.

In an integrated steel plant, the sintering process plays an important role of furnishing raw materials to the BF. From the point of view of natural resources, the sintering process is the key technology which allows recycling of the plant waste materials (by-products or dust) produced within the steel plant other facilities. The process is complex involving various physical and chemical phenomena. The raw materials used can vary to a wide extent, from iron ore to dust recycling and fluxing agents. The natural resources of iron ores vary widely depending on the mineral composition and mining technology applied to produce the sinter feed materials.

Sintering process of iron ore fines is a metallurgical process which is carried out on a sintering machine. The strand width and length depends on the capacity of the machine and varies for each steel plant. It is basically an agglomeration process achieved through combustion. The process energy, of the order of 360 million calories to 480 million calories per ton of sinter, is supplied mainly by combustion of the coke. The flowsheet of the sintering process is shown in Fig 1.

Fig 1 Flowsheet of sintering process

The iron ore fines are natural ore of size 0 mm to 10 mm. The raw mix (also called sinter mix) is made with the weighed amount of iron ore fines, limestone, dolomite, sand or quartzite fines (flux), solid fuel (coke breeze or anthracite) and metallurgical wastes (collected dusts, sludge and mill scale etc). This sinter mix is added with water (6 % to 8 %) and the return sinter fines and then granulated or pelletized in a rotating drum before it is charged as a bed on to the moving strand of the sintering machine. The granulated mix is continuously charged together with returned sinter from the undersize of a sieving process to form a thick bed of around 300 millimeters to 500 millimeters.

Sintering is a continuous process. The sinter strand is formed by a series of pallets, each of which has side walls and a permeable grate. The granulated mix is loaded onto the permeable sinter strand grate. The pellets loaded with the granulated sinter mix, pass under the ignition hood, are subjected to downdraught suction, tipped, and then return to the loading position.

As the strand moves forward, the fuel particles on the top surface layer are first ignited in a furnace by burners of fuel gas (BF gas or mixed coke oven and BF gas). The hot gas, generated by the combustion with air, is then sucked in through the packed bed from the wind boxes equipped with blowers placed below the grate. The strand continues to move forward and the ignited or combustion front proceeds gradually downwards through the bed until the end is reached.  The combustion of coke fines or other carbonaceous materials begins at the top of the layers, and as it moves, a relative narrow band of combustion front (flame front) moves down through the bed, heating each layer successively. In the bed the granules are heated to 1,250 deg C to 1,350 deg C to achieve their softening and then partial melting.

Several chemical reactions and phase transformations take place within the bed, part of the materials melt when the local temperature reaches the melting temperature (burn through point) and as it moves, the re-solidification phenomenon and phase transformations occur with considerable changes on phases composition and thermo physical properties. In these series of reactions a semi-molten material is produced which, in subsequent cooling, crystallizes into several mineral phases of different chemical and morphological compositions consisting mainly hematite, magnetite, ferrites and gangue composed mostly of calcium silicates.  Fig 2 shows the thermal cycle of the materials in the sinter stand.

Fig 2 Thermal cycle of the materials in the sinter stand

Strand speed and sucked air flow are controlled to ensure that ‘burn through’ (the point at which the burning fuel layer reaches the base of the strand) occurs just prior to the sinter being discharged. The temperature of the sintering process is mainly controlled by the combustion of the fuel particles.

During the process of sintering, there are different zones on the sinter machine strand. These zones from the charging side are wet zone, preheating and drying zone, combustion zone and product zone. In the combustion zone liquid phase is formed between iron ore, flux and other elements and the unreacted iron ore particles are bonded together by the liquid. The final product ‘sinter’ is a porous solid material having certain strength.

The partial melting and diffusion within the materials causes the particle to agglomerate forming a continuous porous sinter cake. After the sintering cake is tipped off the pallets, the sinter is hot screened, and the fine fraction (return fines, less than 5 mm) is recycled to be mixed with the raw materials while the coarse fraction is cooled and sent to the BF hoppers. The wind boxes below the strand are connected to an exhaust fan through a gas scrubbing system. In general, the hot gas produced during sintering can also be re-circulated for better thermal efficiency.

The strand can vary from small to large machines with the area and bed height compatible with the auxiliary equipments used for suction of the outlet gas. The area of the strand and the suction power together with the bed permeability determines the maximum speed and hence, the productivity of the process. However, depending on the selected operational parameters and raw materials, quality of the sinter produced can vary widely and can strongly affect the subsequent BF process operation.

There are several factors which affect the process of sintering, productivity of the sinter machine, and the quality of the sinter. Major amongst them are described below.

Chemical composition – The chemical and structural composition are very important in sinter, and it is good for them to be stable so that both primary and final slags possess adequate characteristics in terms of softening and melting temperatures, liquid temperature and viscosity for the stable operation of the BF. It is important to have a high iron content, low gangue content, and basicity of the order of 1.6 to 2.1. Sinter reducibility, and sinter quality in general, improves with a higher level of hematite than magnetite, and its structure improves with a higher level of primary or residual hematite and ferrites than secondary or precipitated hematite.

Sinter structure – Because of the diversity of the mineralogical components which comprise the raw mix, as well as the heterogeneity of the mix, the sinter structure is complex since it is being formed mainly by grains of iron oxide and calcium ferrites bonded by a gangue matrix. The ferrites, whose amount increases with the basicity index, are easily reduced. By increasing the mechanical toughness of the sinter to certain levels, they are considered to be very useful components. The ferrites are SFCA type and are formed by a solid-liquid reaction between hematite and the Fe2O3·CaO melt, with the subsequent assimilation of SiO2 and Al2O3 in the melt. The gangue is composed of calcium, iron and magnesium silicates which are difficult to reduce, and come to form part of the slag in the BF.

The structure and composition of sinter includes the presence of primary hematite (non-assimilated or residual), secondary hematite (precipitated), primary magnetite (non-assimilated or residual), secondary magnetite (precipitated) and ferrites as major phases, along with a smaller amount of gangue. There is sufficient porosity to favour the reducibility of the sinter, including micro-pores in many cases. The optimum structure for reducibility is formed by a nucleus of primary hematite surrounded by a lattice of acicular ferrites.

Effect of iron ore fines quality – In the sintering process, the main raw material used is iron ore fines, Quality of the iron ore fines influences the process of sintering to a great extent. An increase in the mean size of the iron ore fines promotes the productivity of the sintering machine, saves the specific fuel consumption but reduces the sinter strength. Dense low alumina iron ores gives a better sinter strength and lower specific fuel consumption. Very high level of micro-fines in the ore decreases the granulation efficiency and hence, decreases the bed permeability and affects the productivity of sintering adversely. The iron ore porosity has effect on the sinter porosity and its physical and metallurgical properties. Iron ores with high loss on ignition affects the sintering process in a negative way by reducing the productivity, increasing the specific fuel consumption and reducing the sinter strength.

Sinter mix preparation – The sintering performance depends on the efficiency of the mixing of the components of the sinter mix and granulation of the sinter mix in the mixing drum. This activity when carried out in two stages in two mixing drums instead of being carried out in a single satge in a single drum gives better performance with respect to permeability of the sinter bed and hence results in improvement in the sintering productivity.

Sintering mechanism – The sintering mechanism consists of two different phenomena namely physical phenomenon of heat transfer from the top layer due to the bottom layer due to the action of the sucked air passing through the bed and chemical phenomenon of fuel combustion generating heat and a chemical reactions front. These two phenomena are independent and need to have the same propagation speed to ensure maximum flame temperature. The equilibrium between these two phenomena is imporatnt for the high performance of the sintering process, low specific fuel consumption, and high quality of sinter.

Sinter bed permeability – The sintering bed needs a good permeability for the air to be sucked through the sinter mix. A good permeability of the sinter mix ensures a high strand productivity and a proper efficiency of the solid fuel burnt.

Moisture of the sinter mix – It plays an important role in the granulation process and hence affects the sinter productivity. Moisture forms liquid bridges between particles for beginning the granulation process. Capillary forces are responsible to decrease the distance beteen the particles and increase the granules resistance. Moisture content is to be optimum (normally it varies between 6 % to 8 %) since the excess of moisture has a harmful effect on the granules and on the sinter bed permeability and stability. Effect of moisture on the granulation process is shown in Fig 3.

Fig 3 Effect of moisture on the granulation process

System of loading of sinter mix – The segregated blend loading system for loading the sinter mix on the sinter machine strand helps in the permeability of the mix and hence in improving the machine productivity. Fig 4 shows charging systems without  and with segregated blend loading systems.

Fig 4 Systems of loading of sinter mix

Ignition system – The time and temperature of ignition is essential for proper progress of the flame front and a proper sinter quality. Excess of ignition causes decrease in bed permeability and the metallurgical properties of the sinter. This increases the generation of return fines and hence the productivity. The use of multi slit burners improves the ignition efficiency and reduces the ignition energy by around 30 %.

Type of solid fuels – Solid fuels of very low reactivity or very high reactivity promote difference between the front of combustion and heat transfer. A suitable size distribution of solid fuel is necessary to optimize the sintering process.

Automatic process control – It improves the sinter machine productivity. This technology results into savings of 2 % to 5 % in the energy consumption.

Installation of emissions optimized sintering – It reduces solid fuel consumption in the range of 6 kg per ton of sinter to 12 kg per ton of sinter.

FeO in sinter – The FeO content is an important control parameter in the sinter plant. When the chemical composition of the ore mix is fixed, FeO can provide an indication of sintering conditions, in particular the coke rate. A 2 % increase in the FeO content in sinter has been found to lower (improve) the RDI by 8 points. However, a higher FeO content negatively affects reducibility. It is important to find an optimum FeO level in the sinter in order to improve the RDI without altering other sinter properties.

SiO2 in sinter – A higher percentage of silica in the sinter mix counters the effect of high alumina in th ore. Silica combines with FeO and CaO to form compounds with a low melting point which favour the formation of the primary melt consisting of FeO·SiO2 (1,180 deg C), 2FeO·SiO2 (1,205 deg C), and FeO·SiO2·CaO (1,223 deg C). Increasing the silica content and the basicity of the adherent fines causes the primary melt formation temperature to drop, which is favourable for the subsequent assimilation reaction at the liquid-solid interface between the fines and the nucleus particles. Normally, the desirable alumina / silica ratio in the sinter mix is 0.5 or below.

Al2O3 in sinter – Alumina plays an important role in reshaping and coalescing process during the sintering by changing the physio-chemical properties of the primary melt. This leads to a unique sinter pore structure. It has been seen that the pore area increases drastically and the pore shape becomes more irregular as alumina increases from 1.6 % to 2.4 %.

The most harmful effect of alumina is to worsen the sinter RDI, which increases as the alumina content increases. Industrial experience with the BF shows that within a 10 % to 10.5 % CaO content range an increase of 0.1% in the alumina content increases the RDI by 2 points. The strength and quality of sinter deteriorate as the alumina content increases. Alumina promotes the formation of SFCA (silico ferrite of calcium and aluminum), which is beneficial for sinter strength, but the strength of the ore components is lower, since a high alumina content in their lattice has been reported to be the main cause of the observed lower strength. Alumina increases the viscosity of the primary melt which forms during the sintering process, leading to a weaker sinter structure with more inter-connected irregular pores.

Sinter reducibility is determined by the chemical and mineralogical composition and by the pore structure. Due to the complexity of the effects of alumina on each of these factors, consideration of how alumina affects reducibility has produced contradictory results. In a study carried out in a sinter pot loaded with 65 kg of ore mixes with different alumina contents, an increase in the alumina content from 2 % to 5.5% increased the sinter RI from 58 % to 64 %.

Effect of flux – In sinter mix, limestone or lime and dolomite or calcined dolomite are added as basic fluxes while sand or quartzite fines are used as acidic fluxes. The fluxing oxides in the sinter are required to modify the BF slag chemistry in such a way so as to have the desired characteristics. Basic fluxes added to the sinter mix in the form of lime and calcined dolomite also act as a binder in the sinter mix and improve the fine particles agglomeration. They improve the productivity of sinter machine and reduce the specific solid fuel consuption. Size distribution of fluxes is important for the sintering productivity.

CaO in sinter – CaO combines with the iron oxides to form compounds with a low melting point which favours the formation of the primary melt, a minimum level of which is required in order to produce a strong sinter. These compounds are Fe2O3·CaO (1,205 deg C) and FeO·CaO (1,120 deg C). The properties of the melt formed during sintering determine the structure of the bonding phases originated in the sinter. The melt properties in the moments prior to solidification depend to a large extent on the chemical composition of the fines layer adhered to the granules and the assimilation of nucleus particles. 

MgO in sinter – MgO provides for an optimum BF slag condition in terms of both good flowability and desulphurization. It can be added to the BF as raw flux in the form of dolomite or dunite, or as sinter. The addition of MgO to the raw mix improves the RDI, since MgO stabilizes magnetite and thus decreases the hematite content, giving rise to less stress in the sinter during the hematite to magnetite reduction in the BF stack.

It has been determined that replacing CaO with MgO in the form of dolomite for basicity levels of 1.6 to 1.9 leads to a slight reduction in sinter strength, reducibility and productivity. In a study carried out in a sinter pot with 65 kg of raw mix, the MgO content of four produced sinters was increased from 1.4 % to 2.6 % by the addition of dolomite to the mix. The iron ore used was having a low MgO content (0.01 %) and a high Al2O3 content (3 %). It was seen that raising the MgO level in the sinter, from 1.4 % to 2.6 %, increased the FeO content and decreased the productivity and the RI, RDI and TI (tumbler index) indices.

Granulometric distribution – Adequate size distribution (low dispersion, high average particle size) allow for higher sinter bed permeability and hence higher sintering process productivity. Higher mean size of the sinter feeds normally allow for higher permeability of the sinter bed.

After being tipped from the pallets in the sintering machine, the sinter is hot screened. Its granulometric distribution is an important process parameter. The 10 mm to 30 mm fraction is sent directly to the BF hoppers, the larger fraction is crushed to obtain smaller sized fractions, and the less than 5 mm fraction (return fines) is recycled to the sinter plant hoppers.

For the good operation of the process, it is important to keep a balance (B) between the generation and recycling of return fines (RF). For good operation, B = RF generated / RF returned and B is to be in the range of 0.95 to 1.05.

The sinter is screened and each of the resulting fractions is weighed (more than 40 mm, 20 mm to 40 mm, 10 mm to 20 mm, 5 mm to 10 mm, and less than 5 mm. The combined weight of all the fractions comprises the total cake weight. The useful sinter is the total cake minus the return fines generated (less than 5 mm fraction). The average grain size is calculated as a function of the kg of sinter corresponding to each fraction, and can vary over a broad interval between 25 mm and 45 mm.

Sinter porosity – Sinter porosity is an important parameter which affects the sinter properties considerably, in particular its reduction behaviour. Porosity (P) is calculated by determining the real density (Dr) and apparent density (Da) of sinter before and after being subjected to the reducibility test. It is given by the equation P = (Dr- Da) / Dr. Sinter experiences a strong increase in porosity after undergoing the reducibility test.

In the study carried out with hematite and goethite ores, the changes caused to the initial pore structure during reduction tests at 550 deg C and 950 deg C were analyzed. It was seen that the pore diameter needs to be larger than 0.01 micrometer for the reducing gas to have sufficient access to the pores to reduce the sinter satisfactorily. When the micro-pores coalesced to pores of a size of more than 1 micrometer to 5 micrometers, the specific surface area of the sinter decreased and so did its reduction.

Study has shown that eliminating the coalescence of micro-pores and increasing the number of small pores makes it possible to increase the surface area of the sinter and achieve a substantial improvement in its reducibility. Ferrites stabilize the micro-pores and lead to a rise in porosity, hence achieving higher reducibility. The ferrite decomposition reaction for producing magnetite and silicates can be achieved at high temperature in a reducing atmosphere, and is the most important reaction to decrease sinter porosity. Besides the increase in sinter porosity after being subjected to the reducibility test, there is also an increase in volume originated during the transformation of hexagonal hematite into cubic magnetite. The increase in volume which takes place due to this transformation is 25 %.

The crystal structure of magnetite (Fe3O4) is of the spinel type, with a = 8.38 angstroms. It has a close-packed cubic lattice of oxygen ions with the smaller Fe2+ and Fe3+ ions distributed in the interstices. Hematite (alpha Fe2O3) is of rhombohedral corundum type (a = 5.42 angstroms and x = 55 degrees 14 minutes). The oxygen ions are arranged in a close-packed hexagonal lattice and two thirds of the octahedral interstices are occupied by Fe3+ ions. The oxide has a small oxygen deficit, probably because of oxygen vacancies, but possibly also due to iron ions in additional interstitial positions.

Reducibility index – Reducibility is an important characteristic of sinter. It measures the ability to transfer oxygen during reduction in the BF stack, giving an idea of fuel consumption needs in the furnace. The porosity and structure of the sinter and the mineral phases are intimately related with the sinter reducibility. A heterogeneous structure is more reducible than a homogeneous structure. It is also possible to predict reducibility behaviour from the concentration of each phase present. The reducibility of mineral phases in decreasing order is Fe2O3 greater than CaO·2Fe2O3, is greater than CaO·Fe2O3, is greater than 2 CaO·Fe2O3, and is greater than Fe3O4.

Hematite and magnetite are rapidly reduced to wustite (FeO), but the rates differ for subsequent reduction to metallic iron. From hematite, wustite is quickly and homogenously reduced, although some wustite is surrounded by metal. From magnetite, the reduction is a topochemical reaction (a chemical reaction which occurs at the boundary of solid phases), following the sequence Fe3O4 to FeO to Fe, and almost all the wustite grains are surrounded by metallic iron, which delays the subsequent reaction.

The reducibility of SFCA can be related with its morphology, porosity and whether or not it is coated with glass. Acicular ferrite (less than 10 micrometers) formed at low temperature (less than 1,300 deg C) is more reducible, while columnar ferrite (greater than 10 micrometers) formed at high temperature (greater than 1,300 deg C, possibly coated with glass) is less reducible. Primary hematite is more reducible than secondary hematite because of its intrinsic porosity. Various studies for the determination of the relationship between porosity, reducibility and the TI has shown that the higher porosity leads to greater reducibility, and the sinter with the largest surface area (open pores) present a more fragile structure and lower TI.

Studies carried out on the behaviour of chlorine and alkalis in the BF and their effect on sinter properties during reduction have shown that despite some differences the effects of chlorine, which combines to form KCl and NaCl, and alkalines on sinter, are on the whole quite similar. Sinter reduction tests at upto 1,100 deg C show that the presence of alkalis favours the reduction of hematite to magnetite, due to the catalytic action of the alkali. The presence of chlorine compounds is unfavourable, as they are deposited on the sinter surface and inhibits its reduction. The presence of alkalis leads to an increase in the sinter stress, due to an increase in the reduction of hematite to magnetite, and cracks form which increase abrasion. By inhibiting the reduction reaction, chlorine compounds assure less abrasion upto 700 deg C. At higher temperatures, the reduction reaction increases, with the corresponding rise in abrasion.

Reduction degradation index – The RDI is a very important parameter which serves to predict the degradation behaviour of the sinter in the lower part of the BF stack. Sinter degradation during reduction at low temperature is normally determined by the RDI static test, which is carried out at 550 deg C. Low values of RDI is desirable.

Secondary hematite, also known as skeletal rhombohedral hematite, is the main cause of a poor value of the sinter RDI. This is based on the frequent observation of cracks around the narrow neck regions of such hematite. On the other hand, it has been suggested that the cracks which form due to the volumetric change which accompanies the transformation of the crystalline phase from hematite to magnetite are responsible for the reduction degradation of the sinter.

Studies have shown that secondary hematite is the most harmful sinter component for RDI. Secondary hematite normally contains dissolved impurities like Al2O3, TiO2 and MnO which increase the stress in magnetite by distorting the lattice. This magnetite is formed during hematite reduction at 550 deg C in the BF. It has been found in the studies that the sinter structure depends on the maximum temperature reached in the bed, and that secondary hematite is present at higher temperatures. Secondary hematite forms as a result of recrystallization during the sintering of primary hematite. At lower temperatures, a greater proportion of primary hematite (residual hematite) remains in the sinter composition. It has been observed that Al2O3 tends to be concentrated in the secondary hematite phase when the primary hematite to secondary hematite transformation takes place. It has also been seen that an increase in the Al2O3 and TiO2 concentration in sinter is harmful for the RDI.

In further studies, it was determined that the presence of a solid dissolution of Al2O3 and TiO2 in hematite originates a 4 % volume expansion during the reduction of hematite to magnetite at 550 deg C, and causes distortion of the crystalline lattice of these phases and an increase in the magnitude of lattice stresses in the magnetite formed. The presence of cracks in the sinter structure after reduction at 550 deg C are more frequent in the regions with a higher secondary hematite content, and harmful for the RDI, as has been noted.

The production rate and RDI have been studied in a sinter plant using neural networks. The model considered 55 parameters and analyzed a group of 695 RDI values recorded over a 3-year period. It was found that the production rate and the RDI depended on the same variables. A strong relationship was seen between the RDI and the outdoor ambient temperature at the plant. The RDI was also strongly dependent on the Ti content in the sinter, even when this was only very small. No relationship with alumina was found due to its low content (0.5 %) and scarce variation in the tested period. The model found the coke ratio in the sinter mix to be the most important control variable with regard to the RDI.

To improve the operation of BF, one Japanese plant lowered the SiO2 content in the sinter from 4.8 % to 4.2 %, taking into account pulverized coal injection rates of around 170 kg per ton of HM. This led to an improvement in furnace permeability and reducibility, but worsened the RDI. A relation was thus found between bed permeability and the RDI. With the combined actions of lowering the silica content in the sinter and improved melt temperature control, the plant has managed to lower the silica content in hot metal from 0.3 % to 0.2 %. It has been reported that a 6 % improvement in sinter RDI lowers the BF coke rate by 14 kg per ton of hot metal and increase the BF productivity by 3 %.

Low temperature degradation index – The degradation of sinter is determined by the RDI and the ‘low temperature degradation’ (LTD) index. Sinter degradation during reduction at low temperature is determined by the dynamic LTD test, which is carried out at 600 deg C. Degradation is originated, to a certain extent, in the transformation which takes place during the reduction of hematite to magnetite, accompanied by an increase in volume, giving rise to the presence of structural stresses in the sinter. The degradation of sinter in the BF occurs during reduction in the low temperature zone, and has a harmful effect on the burden strength in the furnace, with the resulting loss of permeability to reducing gases and an increase in coke consumption.

In the studies involving the addition of magnetite fines in a raw mix for sintering, a coke saving of 0.43 % was seen for each 1 % increase in magnetite in the raw mix, due to the fact that when hematite ore is replaced by magnetite fines, the bed temperature increases as a result of the exothermic oxidation reaction of magnetite to hematite. An increase of 5.1 % was also seen in the LTD index for each 1 % increase in hematite in the raw mix during the oxidation of magnetite, which is transformed into gamma Fe2O3 with the same cubic spinel lattice structure as magnetite. The TI and RI indices do not undergo any noticeable change when hematite is replaced by magnetite.

Tumbler index – The cold strength of sinter is determined by the tumbler test, and depends on the strength of each individual ore component, the strength of the bonding matrix components, and the ore composition. This test determines the size reduction due to impact and abrasion of the sinter during its handling, transportation, and in the BF process. Studies of the fracture strength of several mineral phases have allowed the following order to be established, primary (or residual) hematite is greater than secondary hematite, is greater than magnetite, and is greater than ferrites. Cold mechanical strength is directly related with the tendency for fines to form during transportation and handling between the sinter machine and the BF throat.

The sinter strength depends to a large extent on the properties of the matrix formed by vitreous glass, silicates, olivines and ferrites. Vitreous glass presents a high degree of stress. The allotropic transformation, which starts at 697 deg C, from beta-2CaO·SiO2 to gamma-2CaO·SiO2 is accompanied by a change in volume which causes the sinter strength to decrease. Ferrites have been identified as a strong bonding material which improves sinter strength.

The TI of sinter is dependent on critical faults in the sinter and their propagation through sinter particles. Faults are unavoidable since the different minerals and phases precipitate out of the melt at different times during the cooling cycle, and changes in volume almost always accompany the transformation of a liquid into a solid.

Studies have been carried out to improve the strength of the sinter with high iron (58.8 %) and low silica (4.38 %) content. By adding of serpentine and burnt lime into the ore mix to be sintered and with a deeper sinter bed, there is a significant increase in the amount of magnesium ferrite and SFCA, which is associated with improvement in the sinter strength.

Sinter plant productivity

In a sinter plant, the requirement is the achieving of high productivity. This is done by assuring good bed permeability, and for this it is essential to optimize the granulation process. Moreover, for high sinter productivity, it is necessary to maximize the sinter output. A variety of factors can influence output, such as (i) horizontal and vertical uniformity in the sinter bed,(ii) sinter bonding strength, (iii) crushing of product sinter, and (iv) selection of return fines screen opening.

Non-uniform sintering normally results in part of the bed being more friable and can lead to high fines production. Where there is a lack of vertical uniformity, it is frequently necessary to increase the coke content in the top part of the bed. This is possible by segregation of the feed using devices such as an intensified sifting feeder or a slit bar chute (Fig 4). Horizontal uniformity is improved by using multi-segment gates on the roll feeder outlet. The problem is mainly serious near the pallet walls where the air flow is highest. This can be reduced by compacting the top of the bed close to the side walls or installing a dead bar grate near the wall.

In some sinter plants, higher production has been achieved by increasing the bed depth, normally together with a reduction in the strand speed. For this type of operation, high permeability is necessary and some improvements to granulation can be essential, such as the addition of (more) lime.

In Japan, in one of the sinter plant, air is enriched with oxygen. Oxygen is injected below the hood which covers a large part of the strand, after the ignition hood. This improves coke consumption, with the result of operating with a narrower heating zone and a higher flame front speed. It is possible to improve production by 1 ton per hour with the use of a flow of 500 N cum of oxygen.

Several sinter plants produce sinter with a 1.5 % to 3 % MgO content by adding dolomite, serpentine or olivine in the feed. Higher productivity is achieved with olivine and serpentine than with dolomite, a fact which can be attributed to the harmful effect of dolomite on sinter strength, and thus on output.

In one of the study, it has been seen that the productivity is the main challenge being faced by the users of pisolitic ore. It has been widely observed that incorporating pisolitic ores in blends causes sinter plant productivity to drop. The reason for this is a reduction in bed permeability caused by excessive melt formation. To improve productivity, water addition during granulation can be increased in order to compensate the fact that porous pisolite ore particles absorb a significant part of the added granulation water and thus reduce the amount of free water available on their surfaces for inter-particle adhesion, leading to deterioration in granulation efficiency.

Study has been carried out in a laboratory pot grate, varying the MgO content in the raw material from 1.40 % to 2.60 %. Dolomite and dunite are used as fluxes to add magnesia. Increasing the MgO content in the sinter mix means a higher temperature is needed for melt formation, and the highly fluxed composition with MgO acts as a refractory phase, raising heat consumption and reducing productivity. On the other hand, it has been found that increasing the MgO content improves the RDI, due to the drop in hematite and ferrite phases and the increase in the magnetite phase, which presents lower degradation.

In a study, it has been see that increasing the MgO content (range 1.75 % to 3.25 %) caused the plant productivity to decrease. The TI increased, but it was considered that for MgO contents of more than 4 % the TI decreases due to the formation of a vitreous matrix which shows a high degree of stress and a low formation of bonding phases. In contrast with this study, it was seen that an increase in MgO also increases the RDI. This variation can be due to differences between experimental conditions and the actual plant data.