Protection of Blast Furnace hearth lining by the addition of TiO2


Protection of Blast Furnace hearth lining by the addition of TiO2

The refractories of the blast furnace (BF) hearth is the most critical material in the iron making process by the blast furnace since the BF campaign life greatly depends on its condition. The most critical region is the transition region between the furnace wall and bottom of the hearth. The hearth refractory wear is a serious concern for many of the BF operators. The abrasive and erosive effect on hearth zone of a blast furnace are due to various conditions namely high ambient temperatures, continuous movement of the liquid smelting products, chemical activity from the products, pressure and chemical activity from the gases and 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 (> 2 days), (iii) water leakage from furnace water cooling system and (iv) quality of charge materials. The wearing of the hearth refractories affects the campaign life of a blast furnace.

Blast furnace operators use the following methods to decrease the wear rate of the BF hearth refractories.

  • Lowering the BF productivity
  • Reducing coal injection rates
  • Grouting of the ramming mass between staves and carbon blocks
  • Temporary plugging of the tuyeres
  • Increasing the cooling rates of the wall
  • Addition of the TiO2 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.

Process of BF hearth protection

The addition of TiO2 containing compounds results in the precipitation of Ti (C,N) (titanium carbonitrides) onto the bottom and the walls of the BF hearth (Fig 1 ). The accumulated precipitation helps to protect the hearth from erosion.

TiO2 protection of BF hearth

Fig 1 Deposit of titanium carbonitrides in BF hearth

 Factors which affect Ti/TiO2 equilibrium in the blast furnace operation include hearth temperature, slag basicity, and Si levels in the hot metal. Typical relationship between temperature and TiO2 load at different of Ti/TiO2 equilibrium and HM silicon is shown at Fig 2.

Relationship between TiO2 and Temperature

Fig 2 Relationship between hearth temperature and titanium load at different Ti/TiO2 equilibrium and HM silicon levels

Titanium carbonitrides, which have a melting point of more than 3000 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 titanium carbonitrides found in the salamanders of the blown out blast furnaces. Typical parameters attained during TiO2 additions in the blast furnace are shown in Tab 1.

Tab 1 Typical parameters  during TiO2 additions in BF

Subject

Unit

Preventive measure

Remedial measure

Charge TiO2 units

Kg/tHM

3-5

5 -20

Ti concentration in HM

%

0.05 – 0.1

1.0 – 1.5

TiO2 concentration in slag

%

1.0 – 1.5

1.5 – 3.0

 

Three basic technical guidelines in case of additions of TiO2 containing compound in BF are as follows.

  • Thermodynamic calculations indicate that TiO2 is in euilibrium with titanium carbonitride in the slag at the tuyere level when the slag TiO2 concentration is around 1.2 %. At concentrations greater than 1.2 %, TiO2 will be reduced and and precipitate as titanium carbonitride.
  • Due to the increase in the viscosity of liquid slag, The maximum TiO2 level in the slag and the maximum Ti concentration in the hot metal is to be controlled. The respective upper limits are 3 % and 0.3 %.
  • Higher Ti/TiO2 partition is favoured by higher silicon levels in hot metal and higher basicity of the slag.

Titanium containing materials charged into the furnace are reduced only by direct reduction as shown in the following 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 titanium carbonitrides which forms a protective layer on the hearth. Successful formation of the protection layer at the eroded regions of the hearth lining depends greatly on the flow and the heat transfer of the hot metal and hence the furnace operating conditions. Further the amount of the TiO2 containing material is required to be enough to form the protection layer but should be minmized as excess amount causes adverse effect on the post processing of the hot metal and the slag.

Method of charging TiO2 in the blast furnace

The TiO2 containing compounds can be added with burden materials into the furnace from the top or can be injected into the blast furnace through several tuyeres. The most frequently TiO2 containing material which is fed into the BF via the furnace top is ilmenite which occurs in form of titanium magnetite ore {(Fe,Ti)3O4}. These ores are mechanical mixture of ilmenite with iron minerals (magnetite and partially hematite). Another way of charging TiO2 containing material in the blast furnace along with the burden from top is through sinter or synthetic TiO2 containing materials. In this case of TiO2 containing materials charged along with the burden from top, reactions initiate and proceed  with the increasing temperatures as the burden material descends in the blast furnace. Further distribution and movement takes place throughout the entire BF shaft. The characteristics of this method of the TiO2 additions are as follows.

  • Delayed time of action
  • High necessary input quantity
  • Impairment of slag quality. High TiO2 content in the slag has negative effect on BF slag cement setting performance
  • Occasional deposition in the BF shaft

Sources of TiO2 when they are injected into the BF, are in the form of fine particled TiO2 source. These fine particled TiO2 source is injected into the furnace through tuyere in the vicinity of the BF hearth and has the following characteristics.

  • The injection occurs in the immediate vicinity of the damaged worn out area of the hearth. Hence TiO2 containing materials are able to influence directly the interactions of the gas, metal and slag phases irrespective of the reactions occuring in the shaft and in the cohesive zone.
  • There is no delayed time of action.
  • There is no accumulation of TiO2 containing materials in the furnace shaft.
  • The quantity requirement of TiO2 containing compounds is lesser.
  • Lower input rates and higher efficiency of conversion to titanium carbo-nitrides compounds results into lesser TiO2 going into slag thus results into improved slag quality.