Protection of Blast Furnace Hearth Lining by the Addition of TiO2
Protection of Blast Furnace Hearth Lining by the Addition of TiO2
The campaign life extension of a blast furnace (BF) is of a great concern. The necessity of prolonging the campaign of the BF is well known. The campaign life improvement is to be achieved while maintaining high productivity for lowering of the unit capital cost. The refractory lining of the BF hearth is the most critical and greatly influence the BF campaign life. In fact, it is one of the most important factors which limit the BF campaign life. The wear of the hearth refractories is a serious concern for the BF operators since its influence on the BF campaign life is the maximum.
The hearth is the most severely exposed zone in the BF, because of the chemical attack, dissolution of the carbon bricks, flows of slag and hot metal (HM), and thermal stresses. The most critical region is the transition region between the furnace wall and bottom of the hearth. The campaign life of the BF is normally determined by erosion of the hearth wall refractories. In addition to the proper design of the lining, it is crucial to minimize the hearth wall erosion. Hence, methods which can lengthen the service life of the hearth and BF walls without interrupting the production are of significant economic and technical interest.
The abrasive and erosive effects on the hearth of a BF are due to various conditions namely (i) high ambient temperatures, (ii) continuous movement of the liquid smelting products, (iii) chemical activity from the products, (iv) pressure and chemical activity from the gases, and (v) entry of moisture into the BF hearth. The main reasons for the wear out of the BF hearth refractories are (i) high furnace productivity, (ii) frequency of long furnace shut downs (more than 2 days), (iii) water leakage from furnace water cooling system and (iv) quality of the charge materials
There are several measures to reduce the erosion of the BF hearth which includes (i) lowering the BF productivity, (ii) reducing coal injection rates, (iii) grouting of the ramming mass between staves and carbon blocks, (iv) temporary plugging of the tuyeres, (v) increasing the cooling rates of the wall, and (vi) addition of the TiO2 (titanium oxide) containing materials. Improvement of the lining life of the BF hearth by the addition of TiO2 containing compounds is the most widely used method. TiO2 provides protection to BF hearth lining against premature erosion.
The most frequently TiO2 containing material which is fed into the BF through the furnace top is the ilmenite ore a natural source of Ti. This ore occurs in the form of titanium magnetite (Fe,Ti)3O4 or FeTiO3 and is a mechanical mixture of ilmenite with iron minerals (magnetite and partially hematite). The typical composition of ilmenite is TiO2 – 33 %, Fe2O3 – less than 36 %, SiO2 – less than 25 %, Al2O3 – less than 8 %, MgO- less than 5 %, and moisture – 6 %. The size of the ore ranges between 10 mm to 40 mm. Another way of charging TiO2 containing material in the BF along with the burden from top is through sinter , pellet, or synthetic TiO2 containing materials.
The present technological practice for reduction of wear and repair of damaged areas in the hearth is through the input of ilmenite which generates chemically and thermally stable titanium carbonitrides Ti(C,N). These compounds accumulate primarily at the damaged points and have the effect of a so-called ‘hot repair’. Fig 1 shows deposits of Ti(C,N) in the BF hearth.
Fig 1 Deposit of titanium carbonitrides in BF hearth
The use of appropriate amount of titanium (Ti) bearing materials into the BF is found to be an effective way to protect the hearth wall. Addition of the Ti bearing materials is believed to promote the formation of a protection layer, so called ‘titanium bear’, on the refractory brick. ‘Titanium bear’ is a precipitate of carbide , nitride and carbo-nitride of Ti , which can form in the BF hearth area, if TiO2 is present in the feed. Tab 1 shows some important characteristic properties of TiN and TiC compounds.
|Tab 1 Properties of titanium carbide and titanium nitride|
|1||Colour||Gray metallic||Copper colour|
|3||Melting point||Deg C||3,157||2,950|
|4||Thermal conductivity||W/(m. K)||29||38|
|5||Hardness (Mohs scale)||9||9|
|7||Solubility in hot metal (1400 deg C)||%||Less than 0.01||Less than 0.01|
|8||Crystal type||Face centered cubic||Face centered cubic|
|10||Coefficient of expansion at 25 deg C to 100 deg C||(10)-6 1/K||7.3||7.3|
|11||Modulus of elasticity||GN/cum||320||260|
|12||Specific electrical resistance||(10)-5 W.cum||7||3|
This purpose of the addition of Ti bearing materials is based on the generation of high-temperature and high-wear resistant Ti(C,N) compounds, which shows temperature-dependent solubility in the HM. When the solubility limit is reached due to the temperature decrease, which is the case at areas of damage in the hearth because of the result of higher heat flux and loss of heat to the outside, the respective Ti(C,N) compounds are precipitated out of the HM and deposited in the more severely damaged zones of the refractories, with an intrinsic ‘hot-repair effect’.
Two common approaches for the addition of TiO2 in the BF are (i) preventive approach, and (ii) remedial approach. In the remedial approach, TiO2 is charged on a regular basis to build and maintain a protective layer of Ti(C,N) precipitate on the BF hearth. In the remedial approach, relatively large quantities of TiO2 are charged when hearth temperatures increase beyond critical levels. These large quantities of additions are maintained until hearth temperatures stabilize at an acceptable level. Tab 2 gives the typical parameters during TiO2 additions in the BF during these two approaches.
|Tab 1 Typical parameters during TiO2 additions in BF|
|Sl. No.||Subject||Unit||Preventive measure||Remedial measure|
|1||Charge TiO2 units||Kg/tHM||3-5||5-20|
|2||Ti concentration in HM||%||0.05 – 0.1||1.0 – 1.5|
|3||TiO2 concentration in slag||%||1.0 – 1.5||1.5 – 3.0|
Mechanism of chemical reactions of TiO2
Ilmenite is a natural ore which consists of iron titanates (Fe,Ti)3O4 or FeTiO3. It first needs to be broken down in the BF into FeO and TiO2 by means of the supply of energy (coke consumption 3 kg/t to 10 kg/t of ilmenite) before the generation of Ti (C,N) compounds can occur.
Three basic technical mechanisms in case of additions of TiO2 containing compound in BF are (i) thermodynamic calculations indicate that TiO2 is in equilibrium with Ti(C,N) in the slag at the tuyere level when the slag TiO2 concentration is around 1.2 %, (ii) at concentrations more than 1.2 %, TiO2 is reduced and and precipitated as Ti(C,N), (iii) due to the increase in the viscosity of liquid slag and the maximum TiO2 level in the slag and the maximum Ti concentration in the HM are to be controlled with the respective upper limits of TiO2 level in slag being 3 % and the maximum Ti concentration in the HM being 0.3 %, and (iv) higher Ti / TiO2 partition is favoured by higher Si levels in the HM and higher basicity of the slag.
The process of Ti(C,N) deposit is an interface reaction. It is necessary for the Ti levels to rise through the slag / HM metal-interface to achieve an effective reaction of the Ti sources. It is hence advantageous to produce finely dispersed droplets of Ti with high quantity and high specific area as soon as possible. Fine dispersed droplets of Ti have been proved to be particularly favourable for the formation of high quantity of Ti(C,N). The large Ti(C,N) crystals on the C blocks in the hearth can be attributed to the infiltration-induced concentration on the surface of the refractory material. This accumulation results in accelerated crystal growth and thus in stabilization of the depositions.
Ti containing materials charged into the BF are reduced only by direct reduction as shown in the equation TiO2 + 2 C = Ti + 2 CO ; H = 169773 Kcal/mol. Formation of carbonitrides is controlled by diffusion process and hence needs more time. Ti after reduction from TiO2 precipitates into HM and reacts with carbon and nitrogen to form Ti(C,N) which forms a protective layer on the hearth. Successful formation of the protection layer at the eroded regions of the hearth lining depends largely on the flow and the heat transfer of the HM and hence the furnace operating conditions. Further, the amount of the TiO2 containing material needed is to be enough to form the protection layer but at the same time is to be minmized as excess amount causes adverse effect on the post processing of the HM and the slag. The mechanism of the formation of Ti(C,N) wear developed is as described below.
Metallic iron is needed as the catalyst for the conversion of TiO2 to Ti(C,N). By addition of TiO2 into the BF, it gets dissolved in the slag phase and reduced to metallic Ti by silicon or carbon at the phase-interface of HM and slag as per the equations (i) TiO2 + C = Ti + CO2 and / or (ii) TiO2 + Si = Ti + SiO2. This formed Ti then dissolves (because of its high solubility) in the HM immediately. The Ti which is in the enriched HM, gets transported with the HM flow to the damaged zones of the hearth. The dissolved metallic Ti reacts with the C and N dissolved in the HM to form Ti(C,N) compounds as per the equation xTi + yC, zN = TiN, TiC and Ti(C,N). The Ti(C,N) compounds precipitate at the locations with lower temperatures (high heat flux areas) when Ti(C,N) solubility is in HM is low. Fig 2 shows visualization of the mechanism of formation Ti(C,N) protection layer in the BF hearth. And a piece of protection layer of Ti(C,N) on the hearth taken from a BF after it has been stopped for relining.
Fig 2 Protection of BF hearth by TiO2
Factors affecting the [Ti] / [TiO2] equilibrium in the BF operation include (i) hearth temperature, (ii) slag basicity, and (iii) silicon levels in the HM. Typical relationships between temperature and TiO2 load at different of Ti/TiO2 equilibrium and HM silicon are shown in Fig 3. Ti(C,N), which have a melting point of 2959 deg C, precipitate on the hearth bottom and walls. The precipitated build up over time protects the inner face of the hearth lining and helps extend BF campaign life. This has been proven from the large deposits of Ti(C,N) found in the salamanders of the blown out BFs.
Fig 3 Relationship between hearth temperature and titanium load at different Ti / TiO2 equilibrium and HM silicon levels
Method of charging TiO2 in the BF
The TiO2 containing compounds can be added with burden materials into the BF from the top or can be injected into the BF through several tuyeres. In the case of TiO2 containing materials charged along with the burden from top, distribution occurs throughout the whole length of the shaft, and as a consequence there exists a delay in the reaction. As a result, the quantities charged are higher than actually needed, compromising the quality of the slag and occasional deposits in the shaft (inactive burden). Normally, Ti is uniformly distributed throughout the cross-section of the BF. However, Ti is needed only in the wall zones of the hearth. Hence, higher input quantities are necessary and this has a negative effect on qualities of the HM and the slag. The increase of the Ti content in the HM from the increase in charging TiO2 bearing materials, results in more TiO2 contained in the slag and this can be a limiting factor in the use of slag as an additive in cement production.
TiO2 bearing materials when they are injected into the BF, are in the form of fine particle of TiO2 synthetic materials. These fine particle of TiO2 synthetic materials are injected into the BF through the tuyere in the vicinity of the BF hearth. The local injection of fine-particle TiO2 sources through the tuyeres directly in the vicinity of the hearth zone is a more effective method of importing TiO2 into the BF. This technique offers a whole series of advantages such as (i) injection occurs in the immediate vicinity of the endangered areas of the refractories which means that best possible results can be achieved systematically by low input quantities, (ii) the delay period is shorter before the reparative action occurs, even in case of ‘hot spots’ in the furnace wall, (iii) there is no accumulation of TiO2-containing materials in the BF shaft, (iv) the TiO2-containing materials are conveyed to the reaction site directly at tuyere level and in the hearth, where they are able to influence directly the interactions of the gas, metal and slag phases, irrespective of the reactions occurring in the shaft and in the cohesive zone, (v) lower input rates and higher efficiency of conversion to Ti(C, N) compounds result in improved slag quality, because of the lower TiO2 contents in the slag, and hence no deterioration in the quality of the BF granulated slag .
The industrial use of the synthetic source of TiO2 indicates a considerable reduction of temperature upon systematic injection into critical BF hearth zones. Precision injection of the materials permits a rapid repair of the damaged areas in case of occurrence of a ‘hot spot’. However, an injection system is necessary for the use of synthetic products. This system consists of a storage-bin, a pressure lock, a feed vessel, a rotary feeder with an ejection-nozzle, and correspondingly dimensioned conveying lines for simultaneous delivery to upto 4 tuyeres. The delivery rate is to be around 10 kg/minute to 60 kg/minute. The most appropriate tuyeres can be selected and supplied, depending on the requirements and needs. The automation concept enables entirely automated operation possible, with the exception of filling of the storage-bin.