What is flux?

A flux is a substance added during smelting to bring down the softening point of the gangue materials, to reduce the viscosity of the slag and to decreased the activity of stable or unstable components in the slag phase.

● The molten oxide product of the smelting is knowns as slag which formed by the combination of gangue and the flux. The gangue arises in a blast furnace from the ore and the ash of the coke.

Limestone and Dolomite :

● In iron making coke, silica and alumina are the main constituents as a gangue. Therefore limestone and dolomite are required as a fluxes. Some of the continental ore are self fluxing when mixed with siliceous ores and there by no additional flux is required.

● The amount and type of flux required in a blast furnace is determined according to the composition of the burden vise versa, the softening point obtained from the relevant diagram.  Once the temperature of the slag is fixed the oxide equilibrium diagram gives all the possible composition range which will have this as it's softening point.

Evaluation of a Flux :

● Lime or magnesia content of the flux gives a fallaceous impression about the liquid or value of the flux. The value of a flux is expressed in terms of available base since it may itslef contain oxide impurities similar to the gangue which it aims to flux away.

● The available base value depends upon the basicity of the slag under operating condition. In a normal blast furnace the basicity ( CaO +MgO ) /SiO2 is usually in the range of 1.0 - 1.3.
The available base is

                 % (CaO + MgO) - % (SiO2).B
Where B is the basicity
This will be available for fluxing of the gangue of the ore and ash.

● The lower is the softening point of the slag the lower is it's viscosity at a given temperature and hence the flux is needed in such a way that slag of the lowest possible softening point formed. In blast furnace slag is also retained of much sulphur content. A basicity of 1.0 to 1.3 can achieved in blast furnace slag.

● Here limestone and dolomite should contain minimum of phosphorus, sulphur, silica and alumina. The lower are these impurities the higher is it's value as a flux. CaCO3 content should not be less than 95% and SiO2 should be less then 5% and MgO and Al2O3 each below 3% depending upon SiO2 content.

● Nearly 0.5 t of limestone is required for making a tonne of pig iron. The beneficial product will be size in which from it isn't suitable as a charge directly in the furnace, bug if is a better feed for making fluxed sinter.

Metallurgical Coal :

● Natural coal is too dense/ fragile to be as a fuel in modern blast furnace. Therefore certain coals when heated out of contact with air result in a carbonaceous, nearly
1.volatile matter free
Metallurgical coal - cooking coal
Cooking Coal
2. Strong and porous mass
3. High fuel efficiency
Is called coke, which is just the right type of fuel need to run blast furnaces.

● The coal which can be converted in to coke is either called metallurgical coal or coking coal and it is the raw material required for ironmaking. Coke cost nearly 20% of overall cost of iron and steel production. Quality of coke is at centre off all consideration of ironmaking operation. If the quality of the iron ore reduces 1% then the coke consumption reduces nearly 0.8 to 1.2%.

● The quality of coke used for iron making has a fundamental effect on the operation and output of the furnace. The coal should be uniformly low in ash and low in deleterious impurities like sulphur and phosphorus and high in fixed carbon. But the coke should contain below 2% volatile matter and 10% ash with 85% fixed carbon. The phosphorus content may vary from 0.018 to 0.04% and sulphur from 0.6 to 1.5%. Coke is the chief source of sulphur in the blast furnace.

● A reduction of 0.1% sulphur in coke can reduce coke rate by 2% and limestone consumption by 1.5 to 2.0% with a proportional increase in productivity of the furnace.

● Finally it has led to develop of more economical blends of metallurgical coals for their end use in iron making. And important of limestone and dolomite as flux which place in producing good quality sinters.

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