Understanding Blast Furnace Ironmaking with Pulverized Coal Injection
Understanding Blast Furnace Ironmaking with Pulverized Coal Injection
Injection of pulverized coal in the blast furnace (BF) was initially driven by high oil prices but now the use of pulverized coal injection (PCI) has become a standard practice in the BF operation since it satisfies the requirement of reducing raw material costs, pollution and also satisfies the need to extend the life of ageing coke ovens. The injection of the pulverized coal into the BF results into (i) increase in the productivity of the BF, i.e. the amount of hot metal (HM) produced per day by the BF, (ii) reduce the consumption of the more expensive coking coals by replacing coke with cheaper soft coking or thermal coals, (iii) assist in maintaining furnace stability, (iv) improve the consistency of the quality of the HM and reduce its silicon (Si) content, and (v) reduce greenhouse gas emissions. In addition to these advantages, use of the PCI in the BF has proved to be a powerful tool in the hands of the furnace operator to adjust the thermal condition of the furnace much faster than what is possible by adjusting the burden charge from the top.
Pulverized coal has basically two roles in the operation of a BF. It not only provides part of the heat required for reducing the iron ore, but also some of the reducing gases. For understanding the HM production in a BF with the injection of pulverized coal, it is necessary to understand what is happening inside the BF as well as the chemical reactions and the importance of permeability within the furnace and how the raw materials can affect this parameter.
The BF is essentially a counter-current moving bed furnace with solids (iron ore, coke and flux), and later molten liquids, travelling down the shaft. Pulverized coal and oxygen (O2) enriched hot air blast is injected at the tuyere level near its base. The gases which are formed by the various reactions taking place move up the furnace shaft, reducing the iron ore as it descends.
The BF charge consist of (i) iron bearing raw material (iron ore lump, pellets, and sinter) and flux (limestone, dolomite, and quartz), and (ii) coke. These charge materials are alternatively charged at the top of the furnace. The materials after getting charged are dried and preheated by the gases leaving the furnace.
As the charge travels down the furnace, it is heated and, at a temperature of around 500 deg C, indirect reduction of the ore by the carbon monoxide (CO) and hydrogen (H2) in the ascending gases starts. The transformation of higher oxides of iron to iron oxide (FeO) starts in this zone. As the charge descends further and is heated to a temperature level of around 900 deg C to 950 deg C, direct reduction of the iron oxide by solid coke takes place. The ore is reduced by CO and H2, and the carbon dioxide (CO2) formed is immediately reduced by the coke back to CO. The net effect is the reduction of the ore by the coke. The reactivity of the coke to CO2 is an important parameter since it determines the temperature range where the transition from indirect to direct reduction takes place.
Lower down the furnace there is a region which is known as the cohesive zone. Here, slag starts to form at around 1100 deg C. Initially it is relatively viscous, and surrounds the iron oxide particles, preventing further reduction. As the temperature increases to level of around 1400 deg C to 1450 deg C, it melts and reduction continues. This region is critical in terms of burden permeability.
The next zone in the BF is known as the fluid or active coke zone. In this zone, the temperature increases to around 1500 deg C, the process of melting the iron ore and slag continue. There is substantial movement in this region and the coke feeds from it into the raceway. The raceway is the hottest part of the BF, where the temperatures can reach upto 2200 deg C. It is created when hot air blast is injected through tuyeres into the furnace. Pulverized coal is injected with the hot air blast directly into the raceway. Combustion and gasification of the coal, and coke occurs, generating both reducing gases (CO and H2) and the heat needed to melt the iron ore and slag and to drive the endothermic reactions.
The hot air blast is enriched with oxygen (O2) in order to maintain the desired flame temperature and to improve combustion efficiency. A BF has many tuyeres (the number of tuyeres depends on the hearth diameter of the furnace), each with its own raceway, arranged symmetrically around its periphery. The depth of each raceway is normally in the range of 1 metre (m) to 2 m, depending on the kinetic energy of the hot air blast.
Unburnt material leaves the raceway and moves up the furnace into the bosh and shaft regions. The molten metal and slag pass through the ‘deadman’ (stagnant coke bed) to the hearth of the furnace where they are collected and periodically removed through the taphole. The slag is then skimmed off from the liquid iron. Some furnaces have separate tapholes for the slag and iron. It can take around 6 hours (h) to 8 h for the raw materials to descend to the hearth of the furnace, although coke can remain for days, or even weeks, within the deadman. The liquid iron, termed as HM is transported to the steelmaking facilities. The quality requirements of HM for steelmaking are a consistent HM quality and a temperature which is to be as high as possible.
The hot gas leaving the top of the furnace is known as top BF gas. It is cooled, cleaned, and is utilized in steel plant as a fuel gas with its major portion is used for hot blast stoves heating. Fig 1 shows the cross-section of a BF indicating different zones along with their temperatures.
Fig 1 BF cross-section showing different zones along with their temperatures
Chemical reactions in the BF
The BF can be considered as a counter-current heat and mass exchange furnace since the heat is transferred from the ascending gas to the BF burden and O2 from the descending burden to the gas. The counter-current nature of the reactions makes the overall process a very efficient process. The chemical reactions taking place within the BF are of complex nature. The major reactions taking place within the furnace are described here.
The principal chemical reaction is the reduction of the iron ore charge to metallic iron. This simply means the removal of O2 from the iron oxides of the ore burden by a series of chemical reactions which are termed as gas reduction or indirect reduction. These reactions are as given below.
3Fe2O3 + CO = 2Fe3O4 + CO2 (starts at around 500 deg C)
3Fe2O3 + H2 = 2Fe3O4 + H2O
Fe3O4 + CO = 3FeO + CO2 (takes place in the temperature range of 600 deg C to 900 deg C)
Fe3O4 + H2 = 3FeO + H2O
FeO + CO = Fe + CO2 (occurs in the temperature range of 900 deg C to 1100 deg C)
FeO + H2 = Fe + H2O
The above reactions are exothermic and generate heat. At the same time as the iron oxides are going through these reactions, they are also beginning to soften and melt. At the high temperatures near the fluid zone, carbon (C) of coke reduces FeO to produce iron (Fe) and CO. This reaction, termed as ‘direct reduction’, is highly endothermic, and the heat that drives it is provided by the specific heat contained in the hot raceway gases.
FeO + C = Fe + CO
Combustion and gasification of coal, and coke generate the reducing gases (CO and H2) which flow up the furnace. As coal and coke enter the raceway, they are ignited by the hot air blast and immediately combust as per the following reaction to produce CO2 and heat.
C + O2 = CO2
Since the reaction takes place in the presence of excess C at a high temperature, the CO2 is reduced by the following ‘Boudouard or solution loss reaction’ to CO which is an endothermic reaction.
CO2 + C = 2CO
In addition, water vapour (H2O) produced during combustion is reduced as per the following reaction which is again an endothermic reaction.
H2O + C = CO + H2
Injection of H2 bearing coal augments indirect reduction. H2 is a more effective reducing gas than C (direct reduction). The H2 regeneration reaction (H2O + C = CO + H2) is less endothermic and proceeds faster than CO regeneration (Boudouard reaction). Higher H2 content in the BF upward moving gases promotes higher rates of iron ore reduction, and hence increases the productivity of the BF. However, a higher H2 concentration can also lead to higher amounts of coke fines in the furnace shaft.
The limestone and dolomite descends in the furnace and remains a solid whilst it goes through the following calcination reactions.
CaCO3 = CaO + CO2
(Ca,Mg)CO3 = (Ca,Mg)O + CO2
These reactions are endothermic and begin at around 870 deg C. The CaO (calcium oxide) and MgO (magnesium oxide) help the removal of sulphur (S) and acidic impurities from the ore burden to form the liquid slag. It can also help remove S released from the coke and coal.
The stable operation of a BF depends on the uniform distribution of the upward moving gas flow and the unhindered flow of HM and slag to the BF hearth. Hence, maintenance of the permeability in the furnace is important for the stable furnace operation, and therefore the BF productivity. The majority of the technical issues associated with increasing rates of coal injection are a response to permeability requirements. Some of the issues essentially associated with the high PCI rates are shown in Fig 2.
Fig 2 Technical issues associated with high PCI rate in a BF
Permeability within the furnace is influenced by the properties of the ore burden, coke, and coal. Fines generated from these materials can accumulate, blocking both gas and liquid flows. Unburnt char from coal and coke fines, for example, can accumulate in the bird’s nest, a relatively compact zone between the raceway and deadman, and around the bottom of the cohesive zone. This can result in gas flow fluctuations and unstable operation. Peripheral gas flow can take place leading to increased heat load on the furnace walls, particularly in the lower part of the furnace. This can shorten the life of the furnace refractory lining, thus accelerating the need for an early capital repair.
The efficiency of the BF process depends on the rate of removal of O2 from the ore burden. The more the gas removes O2 from the ore burden, the more efficient is the process. Therefore, intimate contact between the gas and the ore burden is important. For the optimization of this contact, the permeability of the ore layer is to be as high as possible. The ratio of the gas flowing through the ore burden and the amount of O2 to be removed from the burden is also to be in balance.
The permeability of an ore layer is largely determined by the amount of fines (less than 5 mm) within it. The majority of the fines are normally generated by sinter, if it is present in the charged burden, or from lump ore. There are two sources of fines, those that (i) form part of the iron ore charge, and (ii) generated by degradation of the ore burden materials during their transport and charging, and within the furnace shaft.
Thus it is important to screen the burden materials to remove the fines before they are charged into the BF. The preferred sizes for different materials are typically 5 mm to 30 mm for sinter, 8 mm to 16 mm for pellets and 10 mm to 30 mm for the iron ore lump ore. Most of the BFs operating these days at high PCI rates use a large proportion of prepared ore burden consisting of over 80 % sinter and/or pellets.
In the case where fines are generated by degradation of the ore burden materials during their transport and charging, and within the furnace shaft, it is important to control the degradation characteristics of the burden materials. There are standard tests for determining the resistance of the iron burden materials to physical degradation by impact and abrasion, and for measuring disintegration during reduction at low temperatures.
Ore burden with a high reducibility is preferred. There are standard methods available for the determination of the reducibility of the materials constituting the ore burden. It is rather unfortunate that improving reducibility can increase the degradation and disintegration of the ore materials. Lower silica (SiO2) and calcium oxide (CaO) contents, and higher alkali contents increase reducibility but also increase disintegration.
The permeability for gas flow gets reduced as soon as the burden material starts softening and melting. Hence, it is essential that the burden materials start melting at relatively high temperatures so that they do not slow down the gas flow while they are still high up in the shaft. A fast transition from the solid to liquid state is also desired. Melting properties are determined by the slag composition. Melting of pellets and lump ore typically starts in the temperature range of 1000 deg C to 1100 deg C, whereas the basic sinter starts melting at higher temperatures.
Further, the quality of the burden materials is to be consistent for ensuring stable operation of the BF. Also, it is to be distributed into the BF in such a way as to achieve smooth operation with high productivity.
Coke has three main functions in a BF. These are (i) chemical, (ii) thermal, and (iii) physical. Due to its chemical function, it plays the role of a reducing agent. Its combustion provides gases to reduce the iron oxides, and other oxides such as silica (SiO2). It also supplies C for carburization of the HM. Because of its thermal function, coke combusts in the raceway which provides a source of heat to melt the iron and slag, and also to drive the endothermic processes. Due to its physical function, coke provides support for the iron burden on a permeable matrix, through which the gases and liquid iron and slag can flow.
Coal contributes to the first two functions but not to the third ‘physical’ function. Here, the coke has to ensure permeability for the furnace gas in the region above the cohesive zone, within the cohesive zone, and for gas and molten products in the bosh and hearth regions. Role of coke is particularly very important in the cohesive zone where the softening and melting of the iron ore can form impermeable layers, separated by permeable coke layers or windows. Furthermore, in this zone coke forms a strong grid which supports part of the weight of the overlying burden. Due to the physical function of the coke, there is a limit to the amount of coal which can be injected.
A high as well as consistent quality of coke is required to decrease the fines generation which can lead to poor permeability, unstable operation of the BF, with resultant lower productivity. The rate at which the coke degrades and generates fines as it descends through the furnace is mainly controlled by the Boudouard reaction, thermal stress, mechanical stress and alkali accumulation, depending on its position within the furnace and also on the operational conditions. Thus the following principal coke properties are of interest.
- Cold strength (within the BF) – Cold strength determines the resistance to breakage and abrasion during handling. Shattering and abrasion mechanisms dominate fines generation in the upper part or shaft of the furnace, and these mechanisms are often related to the coke’s cold strength. Standard tests for assessing the mechanical degradation (cold strength) of coke are available.
- Hot strength – Due to the hot strength of the coke, there is the retention of structural integrity in the coke lumps when reacted with CO2 at high temperatures. The reaction of coke with CO2 (Boudouard reaction) in the raceway promotes its degradation and the production of fines. In addition, degradation caused by impact with the high speed hot air blast can occur. Coke with lower hot strength can result in distorted raceway and cohesive zones, and accumulation of coke fines in the deadman leading to permeability problems. Therefore, the strength and stability of the coke structure after its reaction with CO2 at high temperature is an important parameter. Two indices are used to provide an indication of the potential behaviour of coke at high temperatures. These are (i) the Coke Reactivity Index (CRI) and (ii) Coke Strength after Reaction (CSR). These are determined using standard tests.
- Chemical composition- With regards to the chemical composition, its ash, S (which affects the S in the HM) and alkali contents are important. Alkalis (and other basic oxides such as iron oxides) increase the coke’s reactivity towards CO2 due to their catalytic effect, and lower its abrasion resistance. Thus the coke is more susceptible to degradation. The ash in coke has normally an adverse effect on the performance of the BF.
- Mean size and size distribution – The undersize material is required to be screened out before charging to avoid possible permeability problems. The size distribution impacts directly on furnace permeability, both in the area of the shaft and the lower parts of the furnace. The average mean size of charged coke is typically in the range 20 mm to 50 mm. Under stable BF operation, the majority of the coke fines are consumed within the furnace by the Boudouard reaction, HM carburization and reaction with the slag, with only a small amount leaving with the top BF gas.
Coke rates of below 300 kilograms per ton of HM (kg/tHM) have become state-of-the-art practice in the modern BFs with PCI. The lowest values for coke rate which are being achieved are around 240 kg/tHM. The use of nut coke is becoming common, the amount depending on local conditions. Nut coke increases the overall C yield of the BF. It protects coarse coke from excessive size degradation as it is preferentially gasified in the shaft.
Emissions and environmental concerns
Globally the iron and steel industry accounts for the highest share of CO2 emissions (around 27 %) from the manufacturing sector. This is because of its energy intensive production, its dependence on coal as the main energy source and the large volume of steel production. Around 60 % of the global steel production is through the BF route and ironmaking by BFs are one of the major sources of emissions within a steel plant, since most of the energy consumption is related to the BF process at around 2.4 giga calories per ton (Gcal/t) to 3.1 Gcal/t of crude steel, including the hot blast stoves. Since CO2 is associated with climatic change, its reduction is an important concern. Described below are the amount, composition and calorific value (CV) of the top BF gas, air emissions, and CO2 emissions and their reduction, as well as briefly the liquid and solid wastes.
The hot dirty BF gas leaves the top of the furnace, under pressure, and passes through a gas cleaning plant (GCP) where the particulates (principally unburnt char, soot and coke fines) and water are removed, and the BF gas is cooled. The amount of dust required to be removed increases with increasing coal injection rates. Modern GCPs are multiple-step units where the coarse particles are first removed by gravity separation (dust catchers or cyclones), followed by fines removal by wet scrubbers or wet electrostatic precipitators to reach a dust content below 10 milli grams per cubic metres (mg/cum). In some of the BFs dry removal of fines using air bag filters is also practiced. The modern GCPs even allow the extracted dust to be sorted into different types for its effective re-use.
The top BF gas contains around H2 – 4 %, CO – 25 %, and CO2 – 20 %, with the remainder being mostly nitrogen (N2). It has a CV of around 810 kilo calories per cubic meters (kcal/cum), that is around 35 % to 40 % of the energy content of the coal and coke is extracted from the BF in the BF gas. The cleaned BF gas is used in various places in the steel plant including in the heating of blast air in the hot blast stoves. The CV of the BF gas influences its use in the downstream processes.
Modern BFs are normally operated at high pressure to increase furnace productivity. In many of the BFs, a top-pressure recovery turbine is used to generate electricity from the pressure remaining in the top gas. The power output of top-pressure recovery turbine can cover around 30 % of the electricity requirement for all equipment of the BF, including the blowers for air blast. The amount, composition and CV of the BF gas are influenced by the properties of the coal as well as the operating conditions. For example, HV coals typically have a higher H2 content and lower CV than LV ones, and therefore can generate the BF gas with a higher H2 content and lower CV. Injecting HV coals typically increases the amount of dust in the BF gas compared to LV coals. The amount of fine dust in the BF gas is usually higher when a very high VM (volatile matter) coal is injected. The increased carbonaceous material in the fine dust is identified as soot, originating from the incomplete combustion of coal VM.
Only small information is available with regards to the changes in air emissions when coal is injected into a BF. Injecting coal does not cause an increase in the S content of the BF gas when coals with the S content of around 0.8 % are injected in the BF. A study for the life cycle inventory for BFs has shown that both SO2 and NO2 emissions actually decreases by around 22 % and 16 %, respectively, when the PCI rate increases from 16 kg/tHM to 116 kg/tHM.
The chlorine (Cl) content limitation for coal (typically to below 0.05 %) is due to the corrosive properties of the generated chlorine compounds, in particular, hydrochloric (HCl) acid. Chlorine, formed in the raceway when coal is injected, reacts with the gaseous alkalis (from the coal or coke ash) to form alkali chlorides (NaCl and KCl). Some HCl acid and minor amounts of other Cl compounds are also generated. Part of generated HCl acid is removed by the limestone in the furnace. The alkali chlorides (also generated from the iron ore) can circulate within the shaft causing sinter disintegration and thus, increased fines content and deterioration in furnace permeability. The Cl compounds can also corrode the refractory lining and the pipelines in the GCP of the BF. They are removed in the wash water in the scrubber.
Thermodynamics and metallurgy of the BF process concentrate the trace elements originating from coal, coke and iron ore into different output streams. The high volatile elements, such as cadmium (Cd) and mercury (Hg), are absorbed on the fine dust and leave in the BF top gas. They are removed in the GCP (around 75 % Cd and 90 % Hg). Cd and Hg do not go to HM or slag. The less volatile elements, such as zinc (Zn) and copper (Cu), partition between the liquid metal and slag. The majority of the Zn from all the input sources gets dissolved into the HM because of the high pressure in the BF, with around 70 % leaving in the HM and slag. Lead (Pb) has a lower evaporation temperature than Zn, and can accumulate in the BF, lowering the productivity. Most of the lead (Pb) comes from the iron ore (sinter, lump ore and pellets), followed by the coke. The majority leaves the BF absorbed on the fine dust particles from the burden materials and coke, and is removed in the GCP in the scrubber (over 80 %). Its transfer into HM is considered to be of minor importance. Only about 1 % each of Cd and Pb, and 5 % of the Hg are emitted in the gaseous metallic state.
CO2 emissions from BFs are affected by a number of factors. Smaller BFs tend to emit more CO2/tHM than the large BFs because of their lower efficiency. A larger BF is normally more efficient since the heat losses are lower and it is usually more economical to install energy efficient equipment. The energy loss for an efficient BF is less than 10 % of the total energy input. Moreover, the quality of the raw materials influences energy consumption and hence CO2 emissions. For example, lower ash coals produce lower amounts of slag than higher ash coals, and hence a better thermal efficiency is achieved since less energy is needed to melt the ash. For each percentage increase in the ash content of the injected coal, around 1.5 kg/tHM of extra coke is needed which increases the C input and thus, CO2 emissions.
Coke quality affects the quantity of the reducing agent (coke, and coal) which is needed in the BF and therefore, the CO2 emissions. A 1 % increase in coke ash raises the slag rate by 10 kg/tHM to 12 kg/tHM, and the energy demand for every 10 kg/tHM of slag is around 15 Mcal/tHM. Coke quality depends on the quality of the coal used in its production and the process of coking.
The qualities of iron ores differ in their chemical composition and iron content, which affects the energy needed for the reduction reaction to produce iron, and to melt the iron ore. The chemical composition of the gangue affects the amount of limestone or lime which is to be added to achieve the needed basicity of the slag. In total these factors can make a difference of around 240 Mcal/t to 480 Mcal/t difference in the energy needs for a BF. However, the quality of iron ore is deteriorating due to the depletion of high quality deposits. Therefore, the energy needs for ironmaking is increasing due to this factor.
PCI reduces overall CO2 emissions of a steel plant compared to all-coke operations. This is mainly because PCI reduces the need for coke and hence energy consumption and CO2 emissions from the coke oven batteries. The energy saved is on average around 840 Mcal/t coke replaced. PCI can also lower energy consumption within the BF.
A life cycle assessment (LCA) study evaluates the environmental performance of products and materials from mining of the raw materials through to end-of-life and waste disposal. The initial phases of a LCA involve performing a life cycle inventory, which quantifies the material, energy and emissions associated with a particular system. The iron and steel industry has complex flows of energy and materials, both inside and outside the steel plants. Many of the products can be sold ‘over the fence’ and some can be dispatched to long distances. Therefore, the full production energy use and CO2 emissions may be considerably higher or lower than the site footprint suggests. For instance, buying coke and/or electricity reduces CO2 emissions at the site but increase the emissions elsewhere. LCA results are dependent on where the system boundaries are set.
The World Steel Association (WSA) has used a LCA approach to quantify resources use, energy and environmental emissions associated with the production of fourteen steel industry products from the extraction of raw materials through to the steel plant’s gate (‘cradle-to- gate’). The life cycle inventory included both the BF/basic oxygen furnace and electric arc furnace routes. A life cycle inventory for BFs has shown around 6.5 % CO2 reduction when the PCI rate is increased from 16 kg/tHM to 116 kg/tHM. There is a limit, though, on the amount of coal that can be injected. A study has indicated that the maximum rate coal which can be injected is around 250 kg/tHM. However, according to the WSA, an increase of coal injection above 180 kg/tHM does not reduce the coke amount, and the additional coal is just gasified and produces more of BF gas.
In one of the study carried out by the LCA methodology, it is estimated that the CO2 reduction effects of PCI is 0.07 kg CO2 at an injection rate of 0.1 kg/kg of HM.
Various LCA studies show that injecting H2-containing reducing agents, such as coal, can lower CO2 emissions (compared to all-coke operation since the H2 content of coke is only around 0.5 %). Further CO2 reductions can be achieved by lowering the C input (coke and coal). Measures to accomplish this can be divided into the following two groups.
- Those which promote higher efficiency BF operation. These include higher blast temperatures, improved shaft efficiency, and a lower thermal reserve zone temperature. However, these measures also reduce the supply of top BF gas to downstream processes. Minimization of top BF gas production reduces CO2 emissions but may not be possible at plants where utilization of the BF gas in downstream processes is important.
- Measures which promote energy savings in the ironmaking process, such as the reduction of BF heat loss, charging of metallic iron, lower slag rate, and operating with a lower sinter ratio or pre-reduced sinter.
Furthermore, recycling the decarbonized BF gas to the BF lowers CO2 emissions. This technology, commonly termed top gas recycling (TGR), first removes the CO2 by a commercial process such as ‘Selexol’, before reheating and injecting the BF gas into the furnace shaft and/or through the tuyeres. It requires operating the furnace with a pure O2 blast to avoid N2 accumulation due to recycling. The captured CO2 can be stored underground. The BF gas, which principally consists of CO and H2, reduces the C consumption and increases the BF productivity. ULCOS (Ultra-Low CO2 Steelmaking), a consortium of 48 European companies and organizations, is pioneering this technology. Pilot-scale testing of the technology over a six week period resulted in upto a 76 % reduction in CO2 emissions, provided the captured CO2 is stored.
Waste water and by-products
Steel production is a water intensive process, consuming around 180 cum to 200 cum of water per ton of steel. BF consumes around 14 cum to 17.5 cum of water per ton of HM, the majority of which is used for cooling purposes (to cool the BF walls and tuyeres), and to quench the slag. Water is further utilized in the BF GCP. Waste water generated from these processes is treated before it is recycled with over 90 % of the water is recycled. The amount and composition of the waste water partly depends on the quality of the BF raw materials. For example, high salt raw materials can require significantly higher volumes of wash water in the top BF gas scrubbers. Water treatment process can remove Cd and other heavy metals in the waste water before it is recycled or discharged.
Integrated iron and steel production results in around 450 kg to 500 kg of residues and by-products per ton of crude steel produced. Of this, more than 375 kg/t is slag and around 60–65 kg/t is dust and sludge from flue gas cleaning and scale. Around 86 % of all residues and by-products can be recycled internally and externally, after treatment. The coarse dust removed from the BF top gas by dry separation can be recycled internally. The sludge containing the finer particles from the BF GCP is typically landfilled. In some plant it is also recycled through sinter plant.
Different forms of slag are produced depending on the method used to cool the liquid slag. These include air cooled slag, expanded or foamed slag, pelletized slag, and granulated slag. The majority of the slag can be sold, with only a small amount being used for landfilling (less than 10 %). Thus BF slag is considered to be a by-product rather than a waste. The slag can be utilized in cement production, road construction, as a building material, and for special purposes. The possible uses depend on the properties and form of the slag.
The composition of the slag depends on the quality of the BF raw materials. It is formed from the gangue material in the iron ore, and the ash from the coke and coal. It consists principally of silicates and alumino-silicates of Ca (calcium) and Mg (magnesium), together with other compounds of S, Fe (iron), Mn (manganese), and other trace elements.
The amount of slag generated increases with rising injection rates and increasing ash and S content of the reductants. S in the slag originates mainly from the coal. However, the S is effectively captured within the slag. It is only any S present on the surface which is potentially leachable. The trace elements also probably are captured within the slag.