Physical behaviour of material during reduction at High temperatures

The behaviour of material from the time are charged on the stock line until their descent in to the bottom of the stock is of direct interest in assesiass the blast furnace operation. During this period the material may disintegrate by any one or more of the of the below reason :

1. Decrepitation
2. Low temperature breakdown under reducing condition.
3. Failure under overlying load at high temperature.
4. Swelling at high temperature under reducing condition.
5. Premature softening of materials.

● The permeability of the stack may in consequence decrease and affect the furnace operation adversely. Tests have, therefore been developed under as closely simulated conditions as is otherwise feasible, to measure the tendency of burden material to degradation.

Decrepitation :

When iron bearing materials are suddenly exposed to the exhaust gas temperature at the stock level on charging, breakdown may occur due to thermal shock. This is known as decrepitation.

● It is measured by dropping a known weight of material in a previously heated to a temperature level of 400 to 600℃ under normal atmosphere, inert atmosphere or under mildly reducing conditions. After the charge attains the temperature it is removed cooled and sieved to measure the breakdown.

Low Temperature Breakdown Test :

● It has been observed in the experimental blast furnace that the iron bearing materials do disintegrate at low temperature under mildly reducing conditions, that is in the upper part in the stack affecting the furnace permeability and consequently the output adversely. It is believed that deposition of carbon in the region of the stack is also a contributory factor with sinters the breakdown has been associated with the presence of micro cracks.

Reduction Degradation Indeix Test :

● All the methods are followed the temperature of sample is raised to 900℃ and mixed gas of N2:CO :: 70:30 at the rate of 15 lit/min is passes for 3hrs. The cooled sample is weighed to find out the percentage of weight loss against 500gms.

Reducibility index = {% wt. Loss / (Fe(t) - FeO) × 48/112 + FeO × 16/56 } × 100

Coke Reactivity Test (CRI) | Coke Strength after Reaction (CSR)

● The dry coke is placed in the reacting vessel made of inconnel-600 grade steel. The vessel is pushed in an electric muffle furnace kept at 1100℃ temperature. Pure N2 is passed @ 5lit/min till the sample temperature stabilise at 1100℃. Then pure CO gas is passed for 2hrs. @ 5lit/min. Subsequently sample temperature is brought down to room temperature by passing N2 gas.

● Cold reacted coke is weighted and the % wt loss against initial wt. is reported as CRI (coke reactivity index). Then the reacted coke is rotated in an I/drum at 20rpm for 30min. Tumbled coke is screened on 10mm round screen. The percentage of +10mm is reported as CSR ( coke strength reaction)

Hot Compression Strength :

● The compression strength of the burden materials falls rapidly with increasing temperature. A cold compression strength strength of 150 - 300 kg for certain pellets fall to as low as 25kg at 1000℃. It means that practically half way down the furnace stack the pallets loose practically much of their strength.

Swelling :

● Some materials or peticupart the pellets show a maxima in their volume change with increasing degree of reduction. This is due to swelling of the pellets. If it is excessive it can give rise to serious trouble in the furnace operation. Swelling can be measured for individual particles but bulk swelling data are practical use. In the CNRM or the BISRA tests for the swelling four samples of 58 -62gms each are used.

Temperature : 1000℃
Reducing gas : 4% CO and 60% N2
Gas flow         : 1000 lit/min
Pre heating.   : Under nitrogen atmosphere.

Softening of Materials in the furnace :

● During its descent in the furnace stack the burden comes to a temperature level where it soften. The charge becomes sticky and bed permiabilper is consequently decreased resulting in a considerable pressure drop in this zone. The wider is the sticky zone greater is the pressure drop in the furnace and more the furnace operation gets affected adversely.

● The earlier the softening commence wider is the softening zone and consequently wider is the sticky zone in the furnace. The softening behaviour also varies with the degree of reduction. Therefore it necessary to measure the softening temperature of the burden material under reducing condition.

● Softening temperature vary with the nature of the iron bearing material its content, initial state of oxidation, basicity ratio etc. In particular alkali content could have a marked effect on the softening temperatures.

References :

1. Heat treatment principle and techniques  by : T.V. Rajan,  C.P. Sharma,  Ashok Sharma.
2. Physical Metallurgy Principle and Practice by : Raghavan V.,
3. Lectures of IITs & BPUT (ODISHA).

4. Iron making by : Dr. R. H Tupkary & V. R Tupkary

Author :
Subir Kumar Sahu.

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