Technologies for Improvement in Sintering Process

Technologies for Improvement in Sintering Process

The sintering process is used to agglomerate a mix of iron ore fines, return fines, fluxes, and coke breeze, 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 the pressure and the 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 with suitable granulometry, physical quality, chemical composition, and mechanical properties suitable for charging into the BF.

The sintering process is carried out on a sinter machine which is fed with a prepared sinter mix burden. The basic fuel for the iron ore sintering process is coke breeze, the use of which can be supported by substitutional fuels (e.g. anthracite). Sinter mix is prepared by homogenization and granulation of raw mixture (also called sinter mix) of iron ore fines, limestone, dolomite, sand and quartzite fines (flux), solid fuel (coke breeze or anthracite), and metallurgical wastes (collected dusts, sludge, and mill scale etc) in a rotating drum with 7 % to 8 % water with the  objective the obtaining of a pre-agglomerated product (sinter mix), which is then delivered as a layer over a continuously moving grate or strand of the sintering machine.

Sintering process is a metallurgical process carried out on a sintering machine. It is a thermal agglomeration process. The sintering process is an energy intensive process, in which a number of parameters have to be taken into account. The process is complex and involves various physical and chemical phenomena such as heat, mass, and momentum transfer coupled with chemical reactions. These phenomena take place simultaneously which increases considerably the complexity of process.

Sintering is basically a pre-treatment process step during iron making to produce charge material called sinter for the BF. The agglomeration in the sintering process is achieved through combustion. In this process air is sucked at the sinter strand through a bed of sinter mix. The fuel particles on the top surface layer are first ignited in a furnace and as the strand move forward, the ignited or combustion front proceeds gradually downwards through the bed until the end is reached.

During the process of sintering, heat is supplied by coke breeze in the sinter mix to increase the bed temperature to achieve partial fusion and diffusion bonding. Air flow rate and flame front speed in sintering process has been found to guide the performance of the sinter plant and these parameters mainly depends on the sinter bed permeability. The flame front speed is one of the important operating parameter. Sinter productivity strongly depends on the sinter mix permeability of a sinter mix packed bed. This is since the faster the progress of re­action during sintering, the higher is the flow rate of the gas passing the sinter mix packed bed. The gas flow rate is controlled by the sinter mix permea­bility. Flowsheet of the sintering process is shown in Fig 1.

Fig 1 Flowsheet of sintering process

The prime overall objective goal of the sintering process is to achieve a high output of uniform sinter quality at low operational costs. As the main component in the BF burden, the production of high quality sinter having proper granulometry, physical properties, chemical composition, and mechanical strength is crucial for assuring high, stable productivity of the BF with a low consumption of the reductants. BF needs sinter with high strength, 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 the BF at a low fuel rate and stable operating regime. In last few decades. several technologies have been developed which have not only resulted into vast improvements in the process of sintering but also have improved the quality of sinter. Majors of these technologies are described below.

Use of calcined lime as a replacement of limestone

In sinter mix, limestone and dolomite are added as basic 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 act as a binder in the sinter mix and improve the agglomeration of the fine particles. The fluxes improve the productivity of sinter machine and reduce the specific solid fuel consumption. Size distribution of fluxes is important for the sintering productivity. 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 needed in order to produce a strong sinter. These compounds are Fe2O3·CaO (1,205 deg C) and FeO·CaO (1,120 deg C).

Calcined lime (CaO) is an active binder since it gets hydrated into {Ca(OH)2} as a result of hydrate reaction with water. As a binder, it promotes the quasi particle property in the sinter mix. It helps in increasing the micro fines input through iron ore fines. Due to better granulation of sinter mix, it improves sinter productivity. In addition to the binding property, calcined lime also reduces the coke breeze rate due to the reduced calcination of limestone during the sintering process. The use of calcined lime also reduces the crushing and screening load of harder raw limestone and hence saves energy.

Optimal granulation moisture value

A study has been carried out in Japan to determine the optimal granulation moisture value of sinter raw materials, testing the effect of added moisture for six types of ores adjusted to the same particle size distributions. The optimal moisture value to achieve maximum packed bed permeability has been determined for each type of ore. From the results achieved, an optimal moisture value determination method has been adopted, based on the hypothesis that the moisture does not affect the optimal moisture for the mix materials, and a weighted average has been determined for the optimal moisture for the mix proportions of each type of ore and auxiliary materials. The application of the optimal moisture allows the added moisture value to be reduced compared to the conventional method. The packed bed permeability in the sinter plant is increased and productivity has risen by 11 tons per hour.

Intensive mixing and granulation system

The raw materials for sintering which contain iron ore fines, fluxing agents and waste materials are from different sources and have varying characteristics. They need to be blended to form a homogeneous mixture. Sinter productivity is directly related with bed permeability. In turn, permeability is related with the granule size distribution and the average granule size, which are dependent upon the moisture addition. Permeability rises to a maximum value as a function of the moisture.

Intensive mixing and granulation system enables an optimum preparation of the sinter mix by homogenizing the raw material feed and eliminates the need for blending yards. The system basically consists of high speed agitating mixer and a granulation drum. The system results in increased granulation rate, improvement in bed permeability, more equalized burn through zone and optimum burn through point control. With this system, a more homogeneous sinter mix is prepared which reduces coke breeze consumption upto 5 % and increases the sinter productivity by upto 2 %. The system facilitates use of higher percentage of ultra fines in the sinter mix.

Alongside the intensive mixing and granulation system, a preliminary granulation stage consisting of a mixer with a high stirring rate followed by a drum mixer is also used in some sinter plants. This two stage granulation system has made it possible to treat fine iron ores while increasing the flame front speed, permeability, and productivity of the process.

Selective granulation

Selective granulation process is used to allow the sintering of iron ores with high alumina content, which are otherwise difficult to sinter due to the low reactivity of alumina-bearing materials and the high viscosity of primary melts. Selective granulation consists of screening the ore and sending the larger size fraction which has lower alumina content to the conventional granulation circuit, while the smaller size fraction with higher alumina content is pelletized into 2 mm to 5 mm granules which are incorporated in the conventional granulation circuit. The smaller size fraction contains clayish ores which are high in alumina and need higher melting temperatures.

Laboratory scale study, which has been carried out using iron ores with a 0.2 % Al2O3 to 3.2 % Al2O3 content for studying the formation of the primary melt resulting from reaction of the ore fines with limestone, has shown that selective granulation achieves a granule nucleus with a  higher alumina content than conventional granulation. With this process, the fines has adhered to the nucleus, with lower alumina content, by reacting with the limestone and promoted the formation of the primary melt at a lower temperature. Fig 2 shows typical process flow of selective granulation process.

Fig 2 Typical process flow of selective granulation process

Coating granulation

It is advantageous to improve the conventional granulation process, especially when using goethite and limonite ores which normally have a higher Al2O3 content than hematite and lead to deterioration of sinter properties. In this respect, studies have shown that sintering improves when the conventional granulation stage in the drum mixer is followed by a second stage. In the first stage the mix of iron ore and return fines is placed in the drum. In the second stage, coke plus limestone plus dolomite is added to the mix resulting from the first stage and the granule obtained is formed by a nucleus composed mostly of iron ore surrounded by coke and flux.

The coating granulation process improves the flux formation reaction due to the segregation of CaO from the limestone on Fe(iron) from the iron ore. This makes sintering take place at a lower temperature, improves permeability and productivity, and decreases the formation of secondary hematite, with the consequent improvement in the RDI. The TI (tumbler index) and reducibility also improve, due to the formation of more micro-pores, which also prevent the propagation of cracks responsible for deterioration of the RDI.

The mixing time in the drum in the second stage is very important, and around 50 seconds has been established as the optimum time. A shorter time does not allow the nucleus to become well coated with coke plus flux. A longer time causes destruction of the quasi-particles, due to the inclusion of coke and flux in the granules (of the nucleus), and yields a similar quasi-particle to that obtained in single-stage conventional granulation. 

Return fine – mosaic embedding iron ore sintering process

In order to increase the permeability of the sintering bed for sinter ore productivity, the ‘return fine – mosaic embedding iron ore sintering’ (RF-MEBIOS) process has been developed. RF-MEBIOS process, which is a technique of return fine by-passing granulation. In this process re­turn fine as dry particle is added to granulated raw materials and then charged into the sintering machine which results into productivity increase of the sintering machine.

The productivity increase is caused by increasing the pseu­do-particle size at granulation and by decreasing the bulk density of the sinter packed bed after charging. The former is achieved by higher moisture content in the raw materials at granulation. The latter is achieved by higher friction in the sintering bed composed of a dry and wet particle compound, which has a role of decreasing bulk density. By increasing the by-pass return fine ratio and size, the sintering speed and sinter productivity is increased.

Fig 3 shows the typical material flow at the sinter plant. Transportation of return fines diverges into two routes by a di­vergence damper. One is to the existing return fine bin and the other to the new by-pass return fine bin. The return fines from the existing return fine bin and the other sinter raw materials are mixed and granulated together with water in the mixer. The return fines from the by-pass return fine bin are added after the mixer. The section given on the right side of the Fig 3 shows the positional relationship between the damper and belt conveyor. The damper position is adjusted to control the ratio of the by­-pass return fines. The damper can separate the return fines between the upper layer (by-pass return fines) and the lower layer (granulation return fines). The belt conveyor discharges course particles as the upper layer, so relatively large particles are transported to the by-pass return fine bin.

Fig 3 Typical layout of return fine transporting route at sinter plant

The permeability is increased by two factors namely (i) a low fine pseu­do-particle size (minus 0.25 mm) ratio and (ii) a low bulk density. The former is caused by granulation with high moisture content due to the ad­dition of dry return fines after the granulation when the moisture content at charging is constant.

Twin layer charging

With a uniform charging of sinter mix on the sinter strand can lead to higher temperature causing fusion of the sinter mix. This restricts downdraft air flow and the sintering process. In twin layer charging, smaller grain size charge materials with higher concentration of coke breeze is charged in the top layer. Larger grain size material (ore and sinter return) with lower coke breeze concentration is charged in the bottom layer. This ensures proper passage of heat in the lower layers, high bed permeability, and efficient use of the fuel.

Improvements in sinter mix feeding equipment

Segregated blend loading of the sinter mix results into big particles at the bottom of the sinter mix on the pellets while small particles at the top of the sinter mix on the pallets of sinter machine strand. The segregated blend loading helps in the permeability of the mix and hence helps in improving the machine productivity. There are several designs of the charging system for segregated loading. Some of them are (i) installation of an additional screen on the conventional sloping chute, (ii) intensified sifting feeder, (iii) segregated slit wire, and (iv) magnetic breaking feeder. Fig 4 shows charging system without segregated blend loading system as well as charging systems with different types of the loading systems of the sinter mix.

Fig 4 System of loading of sinter mix

Multi slit burner in Ignition Furnace

While igniting the top of the sinter mix bed on the sinter machine in the ignition furnace, flame stability of the burner is essential. Multi slit burners help produce a single wide large stable flame which eliminates no flame areas and supplies minimum heat input for ignition. This in turn results into saving of energy input in the ignition hood. It has been reported in a Japanese plant that the total heat input for ignition with multi slit burners has been reduced by around 30 % when compared with conventional burners. Outline of multi slit burner is shown in Fig 5.

Fig 5 Outline of a multi slit burner

Stand support sintering

A new sintering technique called ‘stand support sintering’, to support the sinter cake with steel stands (bars or plates) attached to pallets has been developed in Japan. This technique improves shrinkage, porosity, and reducibility rates. Due to stand support system, the productivity of sinter machine increases considerably and the machine runs more stably.

In the stand support sintering method, the load of sinter cake in the upper portion of the sinter mix bed is supported by steel stands during the sintering process. The load of the sinter cake on the combustion melting zone below it makes the sinter mix bed shrink (bed compaction), and thus significantly deteriorates the permeability of the bed. The support stands installed inside the sintering pallets begin to support the load of the sinter cake above at the time when the sinter mix bed portion around the tops of the stands begins to solidify after heating and melting. The sintering process of the lower portion of the bed proceeds thereafter under a reduced load, and a permeation network develops well in the portion to improve permeability.

Waste heat recovery

Heat recovery at the sinter plant is a means for improving the efficiency of sinter making process. Hot sinter is required to be cooled. The recovered heat from the sinter cooler is used to preheat the combustion air for the burners in the ignition furnace or to generate high pressure steam which can be used for generation of electric power. In case  of high pressure steam generation, the facility configuration of the waste gas energy recovery system consists of hood, dust catcher, heat recovery boiler, circulation fan, and de-aerator.

Sintering plant consists of two measure sections namely (i) sintering section, and (ii) hot sinter cooling section. Heat recovery from both parts has been developed from sintering section exhaust gas and from cooling section cooling gas. Fig 6 shows the gas temperature distribution of both sections. As shown in the figure, there is large temperature difference depending on the position of the section. Average gas temperature in both sections is in the range of 100 deg C to 150 deg C which is too low for effective heat recovery. Heat recovery is limited to high gas temperature zone, the final part of sintering section and the initial part of the cooling section, where gas temperature of 300 deg C or higher is available. Although heat recovery zone is limited, the gas volume of sintering process is large enough for practical heat recovery which is commercially viable. Also, due to its corrosiveness, the gas temperature after heat recovery is to be kept above acid due point of the gas.

Fig 6 Waste heat recovery in sinter plant

Sintering machine exhaust gas heat recovery can be categorized to circulation type and non-circulation type (Fig 6). In circulation type, gas after heat recovery is circulated to sintering machine as cooling gas replacement, whereas in non-circulation type, the gas after heat recovery is led to gas treatment facility directly. Circulation type is adopted to improve heat recover efficiency.

Besides recovery of heat, the system helps in reduction in SOx, NOx and particulate emissions and in improving the productivity, yield and cold strength of the sinter. Energy recovery to a level of 30 % is being achieved by this method.

Emission optimized sintering system

The high volume of exhaust gases and the low concentration of elements to be cleaned has always been one of the problems of sinter plants. The fundamental objective of the emission optimized sintering (EOS) system is to reduce the volume of the gases to be cleaned (potentially achieving a reduction of upto 50 %) by placing a hood above the sinter grate which is fed with both clean air and recycled air from the wind boxes. Fig 7 shows sinter strand with EOS system.

Fig 7 Sinter strand with emission optimized sintering system

Dust emission control

Increase of production in sinter machines leads to higher dust generation which means higher particulate emissions. These emissions are dust laden and contain a wide variety of organic and heavy metal hazardous air pollutants (HAPs). By sending the waste gas to electrostatic precipitators through negatively charged pipes, the particulate matter in the waste gas stream becomes negatively charged. Routing this stream past positively charged plates then attract and collect the negatively charged particulate matter, thereby producing clean waste gas and increasing the quantity of steam recovery. Coarse dusts are removed in dry dust catchers and recycled. Use of ESP (electro static precipitator) reduces the dust level of the off-gases.

EFA process

This process is known as ‘entrained flow absorber (EFA). It was developed by Paul Wurth. The EFA process is installed at the end of the sinter plant process. It essentially consists of an entrained flow absorber and a bag-type filter. Using this equipment, from the sinter plant off-gas, dust, sulphur oxides, hydrochloric, hydrofluoric acids, dioxins and furans are captured. The absorber operates with hydrated lime (calcium hydroxide) and brown coal coke to absorb dioxins and furans. The optimum reaction conditions are reached by means of water sprayed into the reactor at high pressure and maintaining the temperature in the range of 80 deg C to 110 deg C. The injected water is evaporated and dust from the off-gas is collected in the bag-type filter. The sulphur content is lower than 50 mg per cum at STP, dust content is lower than 5 mg per cum at STP and furans/ dioxins content are lower than 0.1 nano grams per cum at STP.

MEROS process

The maximized emission reduction of sintering (MEROS) process is an innovative technology which was developed by Primetals Technologies to reduce polluting emissions from sinter plants. Through the use of specific additives, the polluting components in the gas flow are combined and separated in a connected fabric filter. The process is ‘semi-dry’ and hence 100 % effluent-free.

MEROS process is a cleaning process for the removal of dust, acidic gases, toxic metals and organic compounds in several stages. The process consists of three steps namely (i) injection of carbon-based adsorbents and desulphurization agents into the sinter off-gas stream in the counter-current direction to bind heavy metals and organic compounds, (ii) circulation of the gas stream through a conditioning reactor where the gas is moisturized and cooled to a temperature of around 100 deg C by means of an injected fine mist (accelerating the chemical reactions needed for binding and removing SO2 and other acid gas components, and (iii) the off-gas stream which leaves the conditioning reactor passes through a bag filter where the dust with the trapped pollutants is removed.

In this process, dust, acid gases, hazardous metals, and organic compounds present in the sintering gases are eliminated with high efficiency rates. In 2007, the first installation started operating at Linz (Austria), with a gas treatment capacity of 1 million N cum per hour. Dust emissions with the MEROS process are lowered to less than 5 mg per N cum. Emissions of mercury, lead, organic compounds (such as dioxins and furans (PCDD/F)), HCl, HF, and total condensable VOCs (volatile organic compounds) are lowered to less than 0.1 nano gram per N cum. One of the most outstanding characteristics of MEROS process is that it fulfils current environmental regulations and can work within the restrictions which can be set in the foreseeable future. Basic flowsheet of the MEROS process is shown in Fig 8.

Fig 8 Basic flowsheet of MEROS process

Selective waste gas recirculation system

During the sintering process the sucked air volume is normally higher than required for complete combustion of the fuel in order to allow a high velocity of the flame front. Sinter waste gas hence typically contains around 12 % to 15 % residual oxygen. It is also at a temperature which is well above the critical dew point. This is sufficient for recirculation to the sintering process after the addition of a small amount of supplementary air.

In the ‘selective waste gas recirculation system’, the off-gas from selected zones of the sinter machine is mixed with cooler off-air and is then recirculated to the sinter strand. The selective waste-gas recirculation system was developed initially to keep the off-gas volume at a constant level while increasing the sintering capacity and decreasing specific emissions. This allows investment and operating costs for gas-cleaning facilities to be held at acceptable levels.

Typical schematic diagram of selective waste gas recirculation system is given in Fig 9. In this figure, hot waste exhaust gases from the first and third sections of the sinter machine is mixed with off-air of sinter cooler and ambient air and is recirculated back to the second section of the sinter machine. A part of the waste gas is recycled back to a hood which covers a part of the sinter strand.

Fig 9 Typical schematic diagram of selective waste gas recirculation system

The advantages of the system are (i) reduced waste gas volume per unit sinter by around 50 %, (ii) reduced specific solid fuel consumption by 10 % to 15 % because of waste gas heat utilization and CO (carbon mono-oxide) post combustion, (iii) lower investment and operating costs for waste-gas cleaning plant, (iv) level of productivity and sinter quality is maintained, (v) decreased CO2 emissions, and (vi) lower specific emissions of SOx, NOx, PCDD/PCDF (dibenzo-p-dioxins/dibenzofurans), and heavy metals.

Modelling of sintering process

During the process of sintering, several chemical reactions and phase transformations take place, not only due to the heat front changes, but also due to the modifications of local gas composition and initial melting temperatures of the mixture of raw materials. When local temperature and composition of the solids is reached, mostly the phase transformations are driven by heat supply and diffusion which take place within the particles bed with the mechanism of liquid formation playing the major role. The materials partially melt down when the local temperature reaches the melting temperature and as it moves, the contact with cold gas promotes the re-solidification and thus, the particle agglomeration forms a continuous porous sinter cake. The final sinter cake properties are strongly dependent upon the thermal cycle, initial chemical composition of the raw materials, and the thermo-physical properties developed during sintering. The mathematical model of the sintering process simulates the phenomena taking place within the sinter machine in the industrial production of sinter to the blast furnace.

The method for modelling the sintering process of an industrial strand machine is based on multi-phase, and multi-component transport equations of momentum, mass, and energy for gas, solid and liquid phases taking into account the local phenomena of porous sinter formation (Fig 10). The model considers the phases interacting simultaneously and the chemical species of each phase is calculated based on the chemical species conservation equations. The accurate descriptions of rate exchange for momentum, energy, and chemical reactions are essential to the whole accuracy of the model.

Fig 10 Multi-phase transfer of momentum, mass, and energy considered for sintering model

The chemical species are individually taken into account by solving the transport equation of each chemical species of the gas and solid phases. The solid phase accounts for the mixture of iron ore sinter feed, fine sinter (returned fine sinter), coke breeze (or other solid fuel), scales (fines from steel plant), and fluxes. The liquid phase is composed of melted and formed components in the liquid phase. The re-solidified phase comprises the liquids re-solidified and phases formed during the re-solidification process and strongly depend on the local liquid composition and heat exchange. The final sinter cake is formed by a mixture of these materials and its quality depends upon the final compositions and volume fractions of each of these materials and their distribution within the mosaic sinter structure.

In the sinter process model, it is assumed that the liquid phase formed moves together with the remaining solid phase due to the viscosity and considering that the liquid is formed attached on the surface of the unmelted particles, thus, equations for momentum transfer and enthalpy of the solids account for this mixture of viscous liquid and solid materials. In the model, the temperature-composition dependent thermo-physical properties are assumed to follow the mixture rule to take into account the individual phase properties pondered by their phase volume fractions.

Automation and control system

With the ultimate aim of stabilizing the sintering process, increasing the productivity, and lowering of the production costs, automation and control system is needed in the sinter plant to assure optimum and stable operation throughout the sintering process. For this purpose, several efforts have been made to understand in-bed phenomena and steer the process towards optimum operation. The main control techniques in sintering are charge density control to achieve uniform sintering across the width of the strand and pallet speed control to maintain optimum productivity and sinter quality.

The automation and control system for sinter plant is a three-level hierarchical system which uses the distributed control system (DCS), centralized process computer system (PCS), and central computer systems (CCS) of the steel plant. DCS performs functions such as measuring wind velocity distribution and gas temperature distribution along the sinter strand, and also ‘direct digital control’ (DDC). PCS performs functions such as process control to optimize sinter plant operation, and information services to operators. CCS performs functions such as planning, managing, and data analysis of production and operation based on the general-purpose data base. Fig 11 shows the automation and control system for sinter plant.

Fig 11 Automation and control system for sinter plant

The closed-loop sinter expert system is designed so that operator has to take ‘as few actions as possible, as many as necessary’ with the target to enable an optimized sinter operation needing minimal operator interactions. The expert system, which is designed as a rule based decision system , counteracts process fluctuations caused by changes in the raw mix composition and quality, human factors or process conditions. The sooner the system responds to an abnormal or changing process situation, the smoother the overall sinter operation is. The accurate timing of control activities and anticipation of disturbances are of utmost importance to avoid critical process conditions and to maintain a high production rate at low costs.

With the automation and control system, optimum process control conditions are achieved since perfect alignment of process parameters takes place. Integrated level 2 automation system at the sinter plant, the standard deviation of quality parameters can be decreased by around 5 % to 10 %. This system also helps in reduction of coke breeze consumption can be reduced by around 3 % and productivity can be increased by around 3 % to 5 %.

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