Iron and Steel

USAGE OF LIME IN IRON AND STEEL INDUSTRY

Quicklime - both high calcium and dolomitic - is used most commonly in the iron and steel industry to purify the steel in the electric arc furnace (EAF) and in blast furnaces. Lime is particularly effective in treating phosphorus, sulfur and silica and, to a lesser extent, manganese. Lime also has important uses in the secondary refining of steel and in the manufacture of steel products.

Electric Arc Furnaces

In electric arc furnaces, scrap iron and steel, pig iron, iron ore and enriched iron ore are placed in an oven and heated by the use of heat from electric current. A lime flux is added with a mixture of quicklime or slaked lime and dolomitic lime. The total amount of flux is used around 50 Kilograms per tonne of steel (up to 50% can be dolomitic lime).

The benefits of lime flux: The lime flux purifies impurities and forms a slag that can be separated from the steel and is removed from the oven as a liquid. It also reduces refractory wear and gassing and can provide foam slag for long arc operation. lime is used unless special product is required by special furnace injection applications.
 

Limestone Use in Blast Furnaces

Iron is generally produced by burning iron ore (Fe2O3, Fe3O4) in the blast furnace (more than 100 m) with coke and air in reducing environment.

Fe2O3  +  3 CO  ®  2 Fe  +  3 CO2 ­

Meanwhile, the combustion gases (CO, CO2, N2, ..) which are formed cannot find suitable ways to exit the fine and frequently grained surfaces of the iron ores (0-8 mm) and therefore the furnace is in danger of being blocked. In order to prevent this problem, the ore must be agglomerated before being fed into the furnace, thereby increasing the hardness and gas permeability, which is called agglomeration.
 

In order to agglomerate the iron ore, pulverized coal and lime products are placed in it and burned at high temperatures (1200 - 1400 ° C). According to applied process, it is called sintering, briquetting or pelletizing. As a result of this process ore in 0-8 mm is transformed into hard particles in 2-2.5 cm size.

Lime products have important contribution to agglomeration process. Milled limestone (CaCO3), which is added to 10-12% ore with coal dust, reacts with the silicon and aluminum in the ore and calcined at approx 1200 - 1400 ° C and forms the calcium silicate - aluminum ferrite system.

This compound binds the fine particles of iron ore and imparts a certain hardness.

Another important role of limestone in the mixture is to keep the molar ratio of CaO: SiO2 at certain intervals (1.5-1.7 which is required for slag formation in high furnace

Some producers add 1 to 2% or more grinded quicklime (CaO) to the ore.

Quicklime

1. It is hydrated in mixing and ensures adherence of small grains (0.5-2 mm) ores and increases the permeability of the beds.

2. The heat released during hydration (270 kcal / kg CaO) provides energy economy by reducing the sintering temperature from 1400 ° C to 1000 ° C.

3. It increases sintering capacity and mechanical strength of sinter pellets.

The role of dolomite in the mixture is to adjust the slag composition that will occur in the blast furnace and to protect the furnace refractories.

Analysis of a typical sintered iron ore is as follows:

Slaked lime, which is used in pellet and briquette production up to 5% ratio, allows the grains of grains to stick and form.

Pig Iron production (blast furnace process)

The liquid iron is obtained by burning the ore given to the blast furnace after curing.

Fe2O3  +  3 CO  ®   2 Fe  +  3 CO2 ­
FeO       +     CO  ®      Fe  +      CO2 ­

Limestone is used to remove impurities (such as sulfur, silicon, aluminum, manganese) from ore or coke during the reduction of iron ore to iron in blast furnaces; The impurities in the ore generally have high melting temperatures (E.Ex Al2O3: 1730°C; lime: 2570°C.) and are therefore very difficult to separate at furnace temperatures. However, as a chance, the lime compounds of these impurities can be removed from iron by melting at temperatures as low as 700 - 1400 ° C.

The limestone reacts effectively with impurities by turning into lime at high temperatures in the kiln and removes them as a low-density slag (approx. 250 - 400 kg slag / t-iron).

CaCO3  ®   CaO + CO2­
CaO + FeS  ®  CaS (curuf) +  FeO
CaO + SiO2  ® CaSiO3 (curuf)

Generally slag components

CaO - Al2O3 - SiO2 - MgO - S - Na2O
(%90)  like this.

The fluidity of the slag should be high and the melting point should be low. For this, it is preferable to have a basic structure of slag. The limestone given to the blast furnace increases the fluidity of the slag compound (low viscosity) and It forms compounds at low temperatures with impurities.

When the lime dosage given according to type and amount of impurities in the ore, is choosen to be ensure the degree of basicity (CaO / SiO2) as more than1.1, a slag is obtained from basic structure with good properties (viscosity viscosity optimal: 1 - 5)

The amount of limestone given to the furnace depends on the purity of the ore, but usually it is between 160 and 200 kg per ton of iron. Blast furnace slag is used in Portland cement.production.
 

Secondary treatment

Where purer steel is required, secondary refining is usually required. Many secondary refining processes use lime to perform important functions, such as adjusting the temperature or chemistry of the steel, removing additional impurities, and preventing impurities from being sucked back. In addition, it can be used in combination with other materials such as florspar or alumina to form synthetic slag that is used as the lime to remove the quicklime, sulfur and phosphorus.

Steel Products

The extinguished lime (whether dry or juicy) has a number of applications in the manufacture of steel products. Widely used in wire drawing from steel rods or used to prevent sticking on cast iron molds

Lime is used to neutralize the acid-based waste pickling solution in which the iron salts are also precipitated. Lime bath is usually used to neutralize the pickling acid remaining in steel products. In addition, the steel product may be coated with lime to protect temporary corrosion. Slaked lime is used to neutralize the acid in the coke by-products.