High Alumina Slag and Blast Furnace Operation
High Alumina Slag and Blast Furnace Operation
Blast furnace (BF) process of ironmaking is a process where liquid iron (hot metal) and liquid slag are produced by the reduction of iron bearing materials (sinter and / or pellet and lump ore) with coke and by fluxing of the gangue material of the feed materials. The process is the result of a series of chemical reactions which takes place in the BF. The gangue materials and coke ash melt to form slag with the fluxing materials. The separation of slag from the hot metal takes place in liquid state. Slag has a lower melting point and is lighter than the HM. In the BF it is at a higher temperature than the HM. A good quality slag is necessary for good quality HM.
BF slag contain predominantly silica (SiO2), alumina (Al2O3), lime (CaO) and magnesia (MgO) along with smaller amounts of FeO (ferrous oxide), MnO (manganese oxide), TiO2 (titanium dioxide), Na2O (sodium oxide), K2O (potassium oxide), and S (sulphur). BF slag composition has very important bearing on its physico-chemical characteristics which influences the BF performance. The physico-chemical properties of liquid slags play significant roles in determining to a great extent the degree of desulphurization, smooth operation of the BF, slag handling, coke consumption, BF productivity, and the quality of the HM.
There are four kinds of slags with distinct compositions which are produced at different regions inside the BF due to a series of reduction reactions. These four types of slags, namely primary slag, bosh slag, tuyere slag and final slag, are generated respectively in the cohesive zone, dripping zone, raceway and hearth. Good tapping is dependent mainly on the final slag which is required to have low liquidus temperature and good fluidity.
The slag fluidity in the BF affects softening-melting behaviour in the cohesive zone, permeability in the lower part of the furnace due to liquid hold-up in a dripping zone, liquid flow in the furnace hearth, and the ability of the drainage of the slag by a taphole. The slag fluidity is affected by the temperature and composition of the slag, with the latter influenced by ore gangue minerals and ash materials of coke and pulverized coal. CaO/SiO2 ratio in the slag compositions is normally adjusted to a range of 1.2 to 1.3 by an auxiliary material for improving the fluidity and desulphurization ability of the slag. The Al2O3 concentration in the slag is considered to be a factor which degrades the slag fluidity, which is semi-empirically set at the upper limit of around 18 % (lower the better) in order to avoid the accumulation of the iron and slag and the deterioration of permeability in the lower part of the furnace.
If the high Al2O3 ore is extensively used in the BF, then deviation takes place from the normal slag system to the new slag with Al2O3 content which can reach as high as 30 %. In addition, recent process changes in the BF including the increase of pulverized coal injection (PCI) also increased the concentration of Al2O3. These results in the slag system transition from silicate based to aluminate based. Slag is a complex oxides system, and its properties have a great relationship with the composition. Several studies have been carried out to determine the physico-chemical properties of BF slags. However, most of these studies were carried out over low Al2O3 BF slags with Al2O3 content normally less than 20 %.
The slag properties which affect most are viscosity, sulphide capacity, alkali capacity and liquidus temperature. These properties have great influence on the overall BF process. Low Al2O3 BF slag (with Al2O3 usually less than 15 %) normally has low viscosity, high sulphide capacity, and low liquidus temperature as well as lower slag volume as compared to high Al2O3 slag. High Al2O3 slag normally has Al2O3 content more than 15 %. High Al2O3 slag is encountered mainly because of high Al2O3/SiO2 ratio in iron ore as well as in sinter and high ash content in coke. These slags are highly viscous.
In the case of BF ironmaking, slag viscosity is a very important physical property as it influences the furnace operation in many ways. Slag viscosity is a transport property which relates to the reaction kinetics and the degree of reduction of the final slag. Slag viscosity also determines the slag– metal separation efficiency, and subsequently the metal yield and impurity removal capacity. In operation, the slag viscosity is indicative of the ease with which slag can be tapped from the furnace, and therefore relates to the energy requirement and profitability of the process.
Viscosity of the slag affects the gas permeability, heat transfer, and the reduction of SiO2, FeO etc. It is desirable to search for slag systems which can provide good fluidity even at low temperatures. There are several data reported on viscosity of BF slags. But most of these data are mainly for low Al2O3 slags with Al2O3 in the range of 10 % to15 %. Further, these viscosity data represent slags with high CaO/SiO2 ratio, very high amounts of additives like FeO, TiO2, and Fe2O3 etc. which are not so common in final BF slag.
BF slag Al2O3 content is mainly dependent on the Al2O3 content of the input materials mainly iron ore. In those cases where iron ore Al2O3 content is less than 1 % the Al2O3 content in the slag hardly goes above 10 %. But in those iron ores where Al2O3 content is 2 % and higher, raise the Al2O3 levels in BF slag to 20 % and higher. To operate a BF with such high Al2O3 slag is quite difficult and need a different type of skill from the BF operators since with the increase in the Al2O3 content of the slag, the BF operation has problems such as excess accumulation of liquid slag in the BF hearth and increase in the pressure drop at the lower part of the furnace. Hence it is important to keep the Al2O3 content of the slag in a limit which ensures good slag fluidity during the operation of BF so as to have good permeability and good drainage of slag during tapping.
Viscosity of the BF slag depends upon composition and temperature. Low viscosity not only helps to govern reaction rates by its effect on the transport of ions in the liquid slag to and from the slag / metal reacting interface, but also ensures a smooth running of the furnace. Both an increase of basic oxides and that of temperature above the liquidus temperature of the slag decrease viscosity. In the case of the CaO-MgO-SiO2-Al2O3 system, Al2O3 and SiO2 are not equivalent on molar basis in their effect although both increase the viscosity of these melts. The effect of the former on viscosity depends on the lime content of slag. This is because Al3+ ion can replace Si4+ ion in the silicate network only if associated with Ca2+ ion to preserve electrical neutrality.
BF is expected to produce HM with S content of the order of 0.05 % or less. Hence, it is always of great interest to know the desulphurizing capacity of slag or in other words, the S partition ratio between HM and slag. It has been shown that that the S partitioning between HM and slag attains equilibrium in the BF for slags with Al2O3 content very close to 15 % or less. These slags have liquidus temperatures lower than furnace temperature and also have low viscosity. These conditions are favourable for attaining equilibrium.
In order to conduct a systematic evaluation for the effects of Al2O3 concentration in the slag on the BF operation, which is focused on slag drainage for each in-furnace area and permeability in the lower part of the BF, from the view point of the slag fluidity, a high Al2O3 slag (slag Al2O3 20 %) operation experiment was carried out in the experimental BF (Fig 1) in Japan.
Fig 1 Cross-sectional view of experimental blast furnace
Some of the observations during the experiment are graphically shown in Fig 2. The results of the experiment are summarized below.
- The phenomenon of the slag drainage in the BF hearth is a fluidization phenomenon which is dominated by the viscosity. The rate of slag drainage decreases as the slag viscosity increases. Hence, for maintaining slag drainage under high Al2O3 content of the slag, for example, an increase of the MgO concentration in the slag is effective. Also, the effects of the slag crystallization temperature on the slag drainage rate are relatively small in comparison to the effects of slag viscosity.
- The pressure drop in the dripping zone increases as the Al2O3 concentration in the slag increases. Even if the CaO/SiO2 ratio increases, the pressure drop in the dripping zone increases. The pressure drop is mainly caused by the effect of wettability as a result of the slag static hold-up, and little due to the effects of dripping slag viscosity and crystallization temperature. In the high Al2O3 concentration slag, to suppress the increase of the pressure drop in the dripping zone, it is effective when there is the decrease of the hold-up by the decrease of CaO/SiO2 ratio.
- The permeable resistance in the cohesive zone is subject to Al2O3 concentration due to the permeable resistance index of the sintered ore at high temperature. The permeable resistance increases as the Al2O3 concentration in the ore increases. For example, the increase of permeable resistance can be suppressed by an increase of MgO in the ore. From the above considerations, in high Al2O3 slag operation in the BF, the effects of slag fluidity on each area in the furnace has been probed and evaluated systematically. As a result, in order to properly maintain slag drainage and permeability, it has been determined that the BF slag design which increases MgO in the slag and decreases CaO/SiO2 ratio in the slag is effective.
Fig 2 Slag properties influencing BF operations
Effect of high alumina on slag
High Al2O3 content in BF slag has many adverse effects. The increase of Al2O3 in the iron ore not only affects the strength of the sinter, but also its characteristics at high-temperatures in the cohesive zone. The Al2O3 concentration in the slag is considered to be a factor which degrades the slag fluidity and increases the liquidus temperature. The effects of high Al2O3 in the slag are as follows.
- High Al2O3 slag has got high viscosity for constant basicity (CaO/SiO2). However with an increase of basic oxides and that of temperature above the liquidus temperature of slag, the viscosity of high Al2O3 slag decreases to some extent.
- The viscosity of liquid slag is dependent basically on its chemical composition and on its temperature. Slag viscosity is an important process variable of the BF process. It is the transport property of the slag which relates to the reaction kinetics and the degree of reduction of the final slag. Low viscosity helps to govern the reaction rates by its effect on the transport of ions in the liquid slag to and from the slag / metal interface. It also determines the slag metal separation efficiency, the metal yield, and impurity removal capacity. It also ensures a smooth running of the furnace.
- In BF operation, the slag drainage phenomenon in the BF hearth is a fluidization phenomenon dominated by viscosity. The slag drainage rate decreases as the slag viscosity increases.
- High Al2O3 slag has greater tendency towards silicon (Si) reduction and there is tendency towards increase of HM Si level. This can be either due to the rise in the equilibrium concentration of Si or not attaining the equilibrium levels at all.
- The S content of the HM tends to increase with the increase in the Al2O3 content of the slag. Hence the high Al2O3 slag contributes to less efficient desulphurization. It is observed that not only is the equilibrium distribution of S between metal and slag is affected adversely but the rate of attaining such a distribution is also markedly slower. Hence there is slower pick up of S by the high Al2O3 slag since the S equilibrium is not being attained within the BF.
- The pressure drop in the dripping zone increases as the Al2O3 concentration in the slag increases. Even if the CaO/SiO2 ratio increases the pressure-drop in the dripping zone increases. The pressure drop is mainly caused by the effect of wettability as a result of the slag static hold up, little due to the effects of dripping slag viscosity and crystalline temperature. The permeable resistance in the cohesive zone increases as the Al2O3 content of the slag increases.
Counter measures for diluting the effect of high alumina
The deteriorating effect of high Al2O3 in the slag is offset by increasing its MgO content. The Al2O3 concentration in the slag is semi-empirically set in many countries at the upper limit of around 16 % in order to avoid the accumulation of the iron and slag and the deterioration of permeability in the lower part of the BF.
The increase of permeable resistance in the cohesive zone can be suppressed by increase of MgO in the burden. Physical effects of increased MgO content in the slag are just the opposite to those of Al2O3. MgO helps in the maintenance of good slag drainage from the hearth during tapping. As the level of MgO increases in the high Al2O3 slag, the S content of HM improves for a given range of Si. This is probably due to the higher fluidity of the high MgO slag. High MgO slag is advantageous for the control of both S and Si. Decrease of slag basicity is also helpful. To offset the deteriorating effect of high Al2O3 slag operation of the BF, the measures which are important are described below.
Since increase in slag MgO improves the hearth drainage rate at high Al2O3 slag operation, MgO in the slag is to be kept at a level which is more than the minimum level. Higher MgO level in the slag also improves the permeability in the cohesive zone of the BF.
To suppress the increase in the pressure drop in the dripping zone, it is important to decrease the slag hold up by the decrease of CaO2/SiO2 ratio. Permeability of the dripping zone is improved by decreasing the slag CaO/SiO2 ratio in the slag to around 1 %.
One other method for reducing the effect of high Al2O3 in slag is to dilute the level of Al2O3 in the slag to lower concentrations by addition of extra slag forming materials in the BF burden. However this results into higher slag volume and involves higher flux and coke rates and lower productivity of the BF. This method can be used for control only as an occasional remedy.