Techniques of Hearth Protection in Blast Furnace R&D and SS Department Venkatesan J

Why hearth protection is necessary ? To increase the campaign life of furnace Relining of the hearth is not easy To avoid heat loss at hearth and to hold hot metal To avoid hearth break out 10 BFs in China have accidents due to breakout of hearth and bottom Reference : AISTech 2015 Proceedings © 2015 by AIST

BF Hearth and Bottom Carbon based grades Carbon Blocks - Calcinated anthracite baked with tar Micropore Grades - Carbon blocks with Al2O3 impregnated with tar in vacuum - Baking twice to reduce size and number of pores Super Micropore Grades - Metallic Si and Al2O3 mixed with Carbon Graphite Blocks To smooth out temperature gradients To conduct heat to the side walls Semi - Graphite Blocks Very low porosity and high thermal conductivity

BF Hearth and Bottom Ceramic Material – High Alumina brick/ Clay brick Carbon Brick Damages Flushing wear abrasion Chemical erosion Advantages High temperature resistant Anti –erosion Resistance to wear

Hearth Deterioration Penetration corrosion of carbon brick by hot metal Corrosion damage of carbon brick by hot metal Mechanical flushing & erosion of high temperature slag and iron Working Conditions of Blast Furnace hearth Chemical corrosion by alkali metal and alkali oxide Oxidative damage of oxidising substances such as H2O Damage due to differential temperature thermal stress

following the path of least resistance Hearth Deterioration Floating deadman hot metal flows through bottom of the hearth – through coke free zone increases the hearth pad temperature decreases the side wall temperature Sitting deadman larger temperature variation inside the deadman following the path of least resistance Floating deadman Sitting deadman

Tap hole outlet temperature following the path of least resistance Hearth Deterioration Erosion profile Tap hole outlet temperature following the path of least resistance

TiO2 provides protection to BF hearth lining against premature erosion Hearth Protection Possible ways to decrease wear rate of BF hearth Lowering the BF Productivity Reducing coal injection rates Grouting of the ramming mass between staves and carbon blocks Increasing the cooling rates of the wall Addition of the TiO2 containing materials TiO2 provides protection to BF hearth lining against premature erosion

Methods of Charging TiO2 into BF Added with burden materials and charge into furnace from top Ilmenite - Titanium Magnetite Ore {(Fe,Ti)3O4} TiO2 mixed with Sinter Synthetic TiO2 containing materials Characteristics Reactions initiate and proceed with increasing temperature Delayed time of action Impairment of slag quality in cohesive zone Occasional deposition in the BF shaft High input rates

Methods of Charging TiO2 into BF Fine particled TiO2 Source Injected into furnace through tuyeres Ilmenite - Titanim Magnetite Ore {(Fe,Ti)3O4} Synthetic TiO2 containing materials Fine particled TiO2 Source Characteristics Injection occurs in the immediate vicinity of the damaged worn out area TiO2 direct has interaction with gas, metal and slag phases No delayed time of action No accumulation of TiO2 containing materials in shaft Lower input rates Higher efficiency of conversion to Ti(C,N) compounds Improved slag quality with less TiO2 in slag

Protection Layers Titanium Rich Layer Iron Rich Layer Slag Rich Layer Graphite Rich Layer

Protection Layers Titanium Rich Layer Mechanism Formation of solid solution of Titanium Carbide and Titanium Nitride Most effective method Change of blast furnace operating conditions Variation of the hot metal temperature N2 partial pressure Mechanism TiO2 + 2 C -- > [Ti] + 2CO Then Ti in hot metal diffuses to low temperature regions H = 169773 Kcal/mol Once Ti in HM reaches the saturation point ---> TiC and TiN Deposit in the abnormal hearth erosion areas Regions where the hot metal circulation has a low fluid speed

Protection Layers Titanium Rich Layer TiO2 concentration in slag greater than 1.2 %, TiO2 will be reduced and precipitate as titanium carbonitride. TiO2 increases the viscosity of liquid slag TiO2 in slag should be less than 3% and Ti in Hot metal should be less than 0.3 %  Factors which affect Ti/TiO2 equilibrium Hearth temperature Slag basicity Si levels in the hot metal

Protection Layers <1150oC Iron Rich Layer Lining erosion over time promotes the heat loss through the furnace lining It reduces the hot face temperature If it below freezing point – Iron layer forms due to solidification of hot metal at hot face Embrittlement of carbon blocks occurs due to alkali & thermal stress Iron-rich layer can effectively prohibit <1150oC direct contact of hot metal with carbon blocks and also prevent the infiltration of alkalis

Protection Layers Slag Rich Layer Density of molten slag – 2.4 g/cm3 Density of hot metal – 7.0 g/cm3

Protection Layers Graphite Rich Layer Graphite precipitates in the hearth sidewall and hearth bottom areas at low temperatures Furnace temperature fluctuation leads to solidification of hot metal promotes the graphite precipitation  Factors influencing precipitation of graphite Transformation ability of carbon in hot metal to graphite Cooling rate of hot metal

Technical route for control of protection layer formation

Technical route for control of protection layer formation Increase the refractory conductivity Control of flow of cooling water Reduce the circulation intensity of hot metal Block the tuyeres above the abnormal erosion regions Charging high quality coke to increase permeability of deadman

Technical route for control of protection layer formation Increase C in hot metal Increase blast pressure Lowering the level of oxygen enrichment through low RAFT by reducing S content in HM

Technical route for control of protection layer formation Si content in hot metal Compare to C, Si has stronger tendency to combine with Fe It promotes precipitation of graphite Increase of Si in hot metal leads the rise in solubility of Ti

Technical route for control of protection layer formation S content in hot metal It forms ternary eutectic product ( C-0.17%, S- 31.7%) It inhibits the atom diffusion process Precipitation of graphite is adversely affected It increase hot metal viscosity Low S content is desirable

Technical route for control of protection layer formation Ti content in hot metal [Ti] – determines the effectiveness of hearth protection Protection layer starts when the TiC exceeds the critical value This layer decreases the hot face temperature and HM viscosity

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