Effect of Alkalis on Ironmaking Process in a Blast Furnace
Effect of Alkalis on Ironmaking Process in a Blast Furnace
One of the main objectives during the operation of a blast furnace (BF) is to maximize the production of hot metal (HM) of the desired chemical composition at the minimum costs. This needs a high quality of a raw material base and a regular, smooth operation of the blast furnace. The quality of the burden materials is very important to avoid problems in the process caused by unwanted elements entering the furnace. In this area, attention is needed also to be paid to the content of unwanted elements in the input charge. These unwanted elements cause a number of technological problems in the BF. Additionally, they significantly affect the production cost of the HM. The main unwanted elements present in the charge which can be troublesome with respect to removal and the performance of the BF are the alkali compounds of the metals potassium (K) and sodium (Na).
In the BF process, the presence of alkalis is known to have negative influences on the process. Alkalis cause higher reactivity of coke, premature softening of the ore charge, decomposition of sinter, swelling of the pellets, and are involved in the formation of the deposits on the refractory lining of the BF which accelerate the wear of the lining. The negative effects of alkalis are due to the catalytic effect on coke gasification, destruction of carbon (C) structure due to the inserting of alkali, scaffold formation, and refractory attack. Alkalis are introduced with the raw materials and due to alkali circulation (Fig 1) alkalis are picked-up on the way to the higher temperature zone in the BF.
Fig1 Theoretical representation of K circulation
The presence of alkali leads to lowered production and higher coke consumption in the BF, around 4.5 % and 2.3 % respectively for each kg/tHM alkali added with the top charge of raw material. Alkali decreases the production due to the lowering of the threshold for the Boudouard reversible reaction, C (s) + CO2 (g) = 2CO (g), increased coke gas and reduced strength of the coke. Gas permeability is decreased due to the coke degradation and scaffolding on the walls can happen thus reducing the volume of the BF.
Alkali reactions in the BF
In all the alkali reactions given in this article, K can be substituted by Na. The reduction of alkali silicates by C can take place according to the reversible reactions K2SiO3 (slag) + C (s) = 2K (g) + SiO2 (slag) + CO (g), and K2SiO3 (slag) + 3C (s) = 2K (g) + Si (HM) + 3CO (g). The extent of these reactions taking place depends on the temperature and the partial pressure of CO (carbon monoxide). Alkali oxides can be reduced by either C in the coke or by the CO as per the reversible reactions K2O + C (s) = 2 K (g) + CO (g), and K2O + CO (g) = 2 K (g) + CO2 (g).
Alkalis are volatilized as elements or react with C and N2 (nitrogen) in the bosh region of the BF forming vapours of potassium cyanide (KCN) or sodium cyanide (NaCN) as per reversible reaction 2K (g) + N2 (g) = 2KCN (g). The vapours are carried up along with the furnace gas and do not dissolve in the HM or into the slag. The melting point and the boiling point for K are 63.4 deg C and 759 deg C respectively while the corresponding melting and boiling temperatures for Na are 97.7 deg C and 883 deg C respectively. The melting points of KCN and NaCN are 622 deg C and 562 deg C respectively, and the boiling points are 1625 deg C and 1530 deg C respectively. The state of these compounds is liquid in the lower shaft and gaseous in the raceway and hearth zone, depending on the temperature. The gas is carried up by the fast moving gases in the BF.
In the shaft where the oxygen (O2) potential increases (at around 1100 deg C), K and KCN are no longer stable and are oxidized by the carbon dioxide (CO2) to alkali carbonates (K2CO3, Na2CO3) according to the reactions 2K (g) + 2CO2 (g) = K2CO3 + CO (g), and 2KCN (g) + 4CO2 (g) = K2CO3 (s) + N2 (g) + 5CO (g) respectively and alkali oxides by the CO according to the reversible reaction 2KCN (g) + CO (g) = K2O + 3C + N2 (g). The generated alkali carbonates leave in form of white fines, which are distributed on the surface of the burden material or located on the brick lining.
Alkali components adsorbed into the burden material and coke form new compounds according to their relative stability. Potassium carbonate (K2CO3) and sodium carbonate (Na2CO3) are solid at temperatures below 891 deg C and below 851 deg C respectively. The alkali bearing compounds descends with the burden materials and are reduced and vaporized again reaching the high temperature zone, according to the reversible reaction K2CO3(l) + 2C = 2K (g) = 3CO (g).
The major portion of alkali is drained from the BF with the slag. However, a part of alkali in the slag is reduced generating alkali vapour which ascends with the surrounding gas. The alkali vapour condenses in the upper part of the BF where a part leaves with the top gas, while the remaining condenses on the inner walls or on the feed material. Because of the volatilization and condensation of the alkali in the different thermal zones, alkali tends to cycle within the BF, leading to an accumulation and interactions with other feed materials. This can have a significant impact on the process, even when the alkali is charged in small amounts, generally less than 5 kilograms per ton of hot metal (kg/tHM). A simplified view of alkali circulation in the BF is shown in Fig 2. Studies of excavated BFs have shown that the alkali level is highest where the temperature is above 1000 deg C, which means that there is an increased alkali concentration in the lower part of the BF.
Fig 2 A simplified view of alkali circulation in the BF
Alkali cycle in the BF
Alkalis normally enter the BF with the iron material and with the coke in the form of silicates. For smooth and efficient BF operation, it is desirable to limit the amount of alkali to around 1.5 kg/tHM to 5 kg/tHM but normally it ranges from 2.5 kg/tHM to 7.5 kg/tHM in different plants. Of the two alkali substances Na and K, K is normally the main compound entering the BF. Most of the alkalis leave with the slag while some become a part of the top gas and leave the furnace with the top gas. Recirculating alkali can either be removed by the slag or by the gas. K goes to the top gas to a higher degree as it is more volatile compared to Na which goes out more with the slag.
Alkalis enter into the BF in the form of silicates (K2SiO3). Studies of the alkali cycle have shown that the silicates descend with the burden and the cycle starts with the alkali silicate being reduced by the C of the coke in the melting zone as per the equation K2SiO3 + C (s) = 2K (g) + SiO2 + CO (g). The reactions take place at around 1550 deg C as per the thermodynamic data for the reactions. Any alkali oxides which enter or are formed in the BF react further up in the BF at lower temperatures according to the equation K2O + CO (g) = 2K (g) + CO2 (g) since they are not stable. K2O can also dissolve into the primary slag.
Further the K vapours produced at the level of hearth in the BF, react with the injected pulverized coal and the N2 of the hot air blast as per the equation 2K (g) + 2C (s) + N2 (g) = 2KCN (g, l). The boiling point for KCN is 1625 deg C so as the potassium cyanide (KCN) rises away from the hot air blast from the tuyeres, it transforms into a liquid phase when the temperature drops. The time in the tuyere zone is very short due to the high gas flow so the alkali cyanides have time to move up the BF before transforming into the liquid phase. Further up in the BF, the alkali cyanides react with CO2 (carbon dioxide) to form more stable carbonates at temperatures below 1100 deg C as per the equation 2KCN (l) + 4CO2 (g) = K2CO3 + N2 (g) + 5CO (g). The carbonates either follow the top gas out as gas, or get deposited on the burden as they start to condensate below 900 deg C.
Compared to alkali silicates, alkali cyanides are unstable so any SiO2 present in the hearth part of the BF can react with the alkali cyanides to again form alkali silicates. The process of alkali silicates reducing into alkali vapour, which ascend in the BF, leave with the top gas, or react with CO2 to form carbonates, is known as the alkali cycle. There are some different views which have slightly different summarizing of the process by differing on exactly which reactions that take place. There are doubts whether carbonates actually are formed at all at the top of the BF since carbonates are not found during the excavation of BFs. However, the main process that alkali cyanides are formed and that the alkali circulates in the BF is agreed on in all the views.
Fig 3 shows circulation of alkalis in the BF. The charged material descend to the high temperature zone before alkali silicates either get decompose to alkali vapours or be absorbed by the primary slag phase in the form of K2O or Na2O. The cycle also indicates around when the alkali vapour reacts with silicates to again form silicates. The distribution of alkali vapours through the BF depends on the gas flow paths and the extent of central gas flow. Gas flows have a great effect on how heat is distributed in a BF. More central flow means more melting in the middle and less in the periphery of the BF.
Fig 3 Circulation of alkalis in the BF
Formation of ammonia and hydrogen cyanide in the BF
Ammonia (NH3) is believed to be formed in the BF. Basic reactions behind the NH3 and hydrogen cyanide (HCN) formation in the BF are 2KCN + 3H2O = K2CO3 + 2NH3 + C, and NH3 + CO = HCN + H2O. The ratio between these reactions depend on several parameters such as (i) the amount of moisture available, (ii) the amount of available KCN in the top, and (ii) the temperature during the reactions. The temperature threshold for NH3 is around 600 deg C and the NH3 formation continuously decreases exponentially until 500 deg C and afterwards it is not detected. Further NH3 which is formed is oxidized by either Fe2O3 or CO2 and the amount of NH3 formed decreases. The oxidizing of NH3 by Fe2O3 or MnO2 depends on temperature. At lower temperature MnO2 is a stronger oxidant and at higher temperature Fe2O3 is the stronger oxidant.
As HCN and NH3 can be found in the top gas, the oxidization kinetics for NH3 is not fast enough to remove it completely. The higher water found in the top gas means there is more formation of NH3 which can take place according to the above reaction. The formation of NH3 in the BF is complex as several parameters as given below affect its formation.
Top gas temperature – The top gas temperature depends on the ratio between endothermic / exothermic reactions in the BF. The temperature affects the moisture content. Lower temperature can also lead to increased solubility of NH3 in water and HCN is miscible in water, so presence of water can decrease its presence in the top gas.
Flame temperature – The flame temperature has a minor effect on the amount of alkali vapour produced and the total alkali load. A high temperature is needed to reduce the alkali silicates in to alkali gas which starts the alkali circulation. Lowered flame temperature hence leads to more alkali leaving the BF through the slag.
Basicity – A lower basicity leads to higher alkali uptake in the slag, hence there is lower circulating alkali in the BF and less NH3 is produced.
Moisture content – Less moisture introduced with the charge or through other ways in the BF gives lesser water for the NH3 forming reaction to happen.
Effect of alkali and removal
The concentration of alkali in the raw material has increased in the recent years due to the decreased availability of high quality coking coal. Alkali is detrimental to the BF operation since it causes an increased reactivity of coke due to the catalytic effect on the solution loss reaction (Boudouard reaction).
One of the major negative effects of the alkali is that it catalyzes the Boudouard reversible reaction, lowering the temperature for the reaction from 900 deg C to 950 deg C down to around 750 deg C to 850 deg C and increasing the coke reactivity depending on the coke quality. It also affects the coke structure negatively. The lowered threshold for the Boudouard reaction means that more C is getting consumed in the BF in a strongly endothermic reaction. Thus, increasing of the coke addition is needed into the BF to keep a stable operation with 2 kg to 10 kg of coke per kg alkali or with 6 kg to 11 kg of coke depending on sources used.
Alkali which has entered the coke structure decreases the coke strength in the lower part of the BF due to the increased reactivity of coke and thus increased degradation. The apparent reaction rate of the coke at the tuyere level has shown to be ten times of the feed coke reaction rate, which is related to the total amount of presence of K. K is known to increase the reactivity of coke with CO2 and can result in coke fracture because of swelling of the coke grains by insertion of K. An increased in reactivity can be advantageous since it reduces the BF operating temperature, the temperature in the thermal reserve zone and thus lowering the temperature for reduction of iron oxide. However, the purpose of coke is also to act as burden support within the furnace, and hence, the coke fracture needs to be minimized.
Studies on the penetration of K vapour into coke have confirmed that the structure of coke minerals can be broken because of the volume expansion of alkalized minerals especially kalsilite. The surface area between the minerals and the C matrix increases because of the breakage of the minerals in the coke. This facilitates the interaction of mineral matter with C and BF gases accelerating the coke gasification. An expansion in coke can also occur when alkali vapours penetrate into the crystal layers of coke to form interlayer compounds. The coke strength and support of the burden during the BF operation is critical as a collapse of the burden reduces gas and liquid permeability, which reduces the efficiency both regarding HM production and increased CO2 emissions.
Small coke particles of different size decrease the voids in the coke bed in the wet zone and the surface area of coke increases. Breakage of coke also promotes flooding, choking of the hearth, and increased burning of tuyeres and slag notch, all of which limit furnace output. The tendency for the ascending gas to be transported near the walls increases when the permeability is lowered, which decreases the utilization of the gas and thus increases the coke rate.
Another effect of alkali is the increased chance of scaffold formation in the shaft as alkali condenses on the lining and can bind the fine material to it. Scaffolds are a build-up of solid material on the furnace wall and project towards the furnace centre. Scaffolds can occur at any place from the middle to upper part of the shaft. The effect of scaffold formation is that the burden descend is slowed down or interrupted, and in extreme cases leading to hanging, slipping, and uncontrolled charging. The scaffolds also reduce the working volume of the BF. The gas velocity is increased in the unaffected parts in the shaft, which result in an increased top gas temperature and decreased utilization of the CO. When the scaffolds break away from the furnace wall it often results in a chilled hearth. The scaffolds can be removed by temporary increasing the furnace temperature or is removed when a gas flow of high temperature reaches the area.
The burden material can also be glued together by condensed liquid cyanides and carbonates which reduces the bed permeability. Moreover, alkali can attack the refractory material, especially C based refractories normally used in the lower part of the furnace. Alkali compounds penetrate the C blocks which are consumed forming alkali vapour. This results in a shorter life of the refractory lining and a need for a more frequently relining.
Removal of alkali is mainly done with the slag and is best performed at lower basicity values. Of the alkali removed over 90 % is removed through the slag. Here the basicity is considered as CaO/SiO2. Results of several studies have shown that the lower slag basicity increases the amount of alkali in the slag. However, problem with too low basicity is that a higher level of sulphur (S) remains in the HM, as the S can be counteracted by CaO present in BF slag and CaO is lower when the basicity is lower. The lower limit of the basicity for keeping the HM quality under control differs from plant to plant. The limit is dependent on the BF parameters and the quality of the raw material used. A basicity value just above or around 1 can be seen as the limit if alkali is to be removed and the HM quality is to be maintained.
For impeding the gasification of alkali silicates, the partial pressure of CO is needed to be kept high. The high temperature for the reaction at 1550 deg C means that a lower flame temperature also can be used to slow down the reduction and gasification and thus lower the alkali circulation. Removal of alkali need decreased re-circulation of alkali containing materials to the BF as alkali gets otherwise just reintroduced to the BF.
Lowering of the catalyzing effect of alkali on coke gasification can be done by coke ash additions which can bind the existing alkali in more stable forms. It has been tried with certain mineral addition before. As alkali diffuse through the coke, a coating of the minerals addition can stabilize the alkali at the surface of the coke stopping it from degrading the inner parts of the coke.
Control of alkali load
For controlling of the alkali load in the BF, the alkali input is to be kept low as far as possible. If there is a variation in the ingoing material the ore mix is to be blended well to minimize the risk of high alkali sections in the BF. Another precaution is to avoid recirculation of flue dust with high levels of alkali. A way to minimize the risk of scaffold formation is to increase the strength of the ferrous burden and the coke to avoid the formation of fines.
The BF can be operated with a central gas flow. Because of it, the top gas temperature in the centre is increased to such level that part of the alkali leaves the furnace as vapour with the top gas. However, higher top gas temperatures results in increased heat losses.
The recirculated alkali can be decreased and instead leave the BF via the slag by operating the BF with an acid slag. The lower the slag basicity the more easily the basic K and Na are absorbed into the slag since lime (CaO) and magnesia (MgO) occupies the same sites in the silica network as alkali oxides. A decreased basicity can be achieved by decreasing the basicity in the ferrous burden and flux additions. However, a lower basicity also results in increased S (sulphur) content in the HM as S binds to Ca in the slag.
Another possibility to decrease the alkali load is to increase the slag volume, which decreases the activity of alkali oxides in the slag and hence increase the absorption of alkali and the output through slag, but an increased slag volume also increases the coke rate.
A lower temperature in the BF results in decreased reduction and vaporization of alkali. Operating the BF with lower flame temperature results in a decreased alkali load but also decreased productivity and higher coke rate. The alkali load can also be decreased by operating the BF with high partial pressure of CO since it opposes alkali gasification reaction. This can be achieved by O2 enrichment of the blast or high top pressure.