Pig iron production: chemistry and physics

Primary pig iron is produced in blast furnaces by reducing iron oxides to metallic iron. The conditions in these ovens are as follows, that iron is excessively carbonated and exits the blast furnace as cast iron. This cast iron, Besides, contains a high content of various silicon impurities, manganese, phosphorus and others.

The principle of pig iron production has remained practically unchanged since ancient times.. Ancient blast furnaces were made of clay and produced several kilograms of pig iron per day.. Modern blast furnaces - the largest furnaces in the world - are capable of producing up to 6000 tons of pig iron.

Pig iron is produced by chemical reactions of iron and manganese ores with reducing agents - carbon monoxide and atomic carbon in a blast furnace. These reducing agents are formed as a result of burning fuel in the furnace. – coke, fuel oil, natural gas and pulverized coal. In addition to iron ore and fuel, other materials are used., Firstly, flux. Fluxes are needed to lower the melting point of iron ore gangue, conversion of sulfur into slag, phosphorus, ash, combustion fuel and the formation of low-melting liquid slag, which is removed from the oven.

Blast Furnace Basic Materials

1) Iron ore - a source of iron.
2) Coke - fuel and reducing agent.
3) Limestone - decomposes at high temperatures to form CaO, which acts as a flux and converts siliceous waste rocks into CaSiO slag3.
4) Air - Supports coke combustion with heat release. Removes some non-metallic impurities (silicon, arsenic) as volatile oxides. Oxidises iron oxide FeO in ore to Fe2O3, which contributes to the preservation of iron in the ore. Iron oxide FeO, basic by nature, reacts with SiO2 with the formation of a slag FeSiO3. Air makes the ore porous, which promotes uniform iron reduction.

Iron ore

The earth's crust contains about 50 % iron in the form of oxides, sulfides and other compounds - only about 200 various minerals. Rocks, from which it is technically possible and economically feasible to extract metals called ores.

Iron ores include red, brown, magnetic and spar iron ore. These ores contain many iron compounds, from which it is extracted, and waste rock, which is relatively easy to separate during processing.

Minerals in iron ores

The main ore-forming iron minerals are hematite, limonite and magnetite.

Hematite - red iron ore. Contains iron as anhydrous iron oxide Fe2O3. The iron content in red iron ore is 45-65 % with a small amount of harmful impurities.

Limonite - brown iron ore. Contains iron in the form of nFe hydrous oxides2O3×mH2O. In brown iron ore - 25-50 % gland.

Magnetite - magnetic iron ore. Contains iron mainly in the form of iron oxide Fe3O4, magnetic. Magnetites - the richest iron ore - contain 40-70 % gland.

Preparation of ore for pig iron production

For normal operation of the blast furnace, it must be loaded with lumpy material of optimal dimensions. Too large pieces of ore and other materials will not have time to properly react, and some of the material will go useless. Pieces that are too small to fit too tightly together, leaving the necessary passages for the passage of gases, making it difficult to operate the oven.

The size of the charge pieces is considered optimal 30-80 mm. Larger pieces are crushed to the optimum size.

On the other hand, when crushing materials and when mining ore, fines are formed along with large pieces, also not suitable for melting. Such materials are agglomerated and rolled to the required size..

In addition to agglomeration and rolling, ore is enriched. Beneficiation is the preliminary treatment of ore without changing the chemical composition of the main minerals and their state of aggregation.. Ore beneficiation is performed to increase the iron content in it.. At the same time, a significant part of the waste rock is removed from the ore.. Various methods are used in ore dressing: ore washing, flotation method, gravity method and magnetic enrichment.

Blast furnace design

The blast furnace is a shaft type furnace. A typical blast furnace has a bottom diameter 6-8 m and height 20-36 m. The largest blast furnace, japanese, has a diameter 14,9 m. The blast furnace profile and its temperature zones are shown in the figure. 1.

schema-domnyPicture 1 - Blast furnace profile. Input and output materials.
Basic chemical reactions

Blast furnace smelting

Blast furnace smelting consists in separate charging into the upper part of the furnace (grate) fluxed sinter and coke. They are placed in the oven in layers. The charge is heated by the heat of combustion of coke in hot air, which is blown into the bottom of the blast furnace. The charge gradually goes down. As a result of the physicochemical interaction of the charge components and the rising gases in the lower part of the furnace - the hearth - two immiscible liquid layers are formed - cast iron on the hearth of the hearth and slag - above the cast iron.

Liquid iron is released every 2-3 o'clock, in large ovens - every hour. Slag from the furnace is tapped out together with cast iron. They are separated using special closures..

The blast furnace usually runs continuously for several years. – to 10 years.

Physicochemical processes in a blast furnace

The following processes take place simultaneously in the blast furnace:

1) combustion of fuel carbon and formation of reducing agents;
2) decomposition of charge components;
3) reduction of oxides;
4) iron carburization and iron formation;
5) slag formation.

Fuel combustion and formation of reducing agents

The combustion of fuel carbon occurs in the lower part of the furnace when air interacts at a temperature 1000-1300 ºS with coke:

FROM + ABOUT2 = SO2 .

The resulting carbon dioxide rises to the hot coke and reacts with it to form reducing agent CO:

SO2 + C = 2CO.

Reducing agent CO in the presence of iron decomposes by reaction with the formation of atomic soot reductant C:

2СО = С + SO2.

Reduction of iron oxides

The main task of the blast furnace process is the reduction of iron from its oxides.. The main roles in the reduction of iron are played by carbon monoxide and atomic black carbon., which are formed as a result of the blast furnace process.

The reduction reaction zones and their temperatures in the blast furnace are shown in the figure 2.

vosstanovlenie-zhelezaPicture 2 - Scheme for the reduction of iron oxides
in the production of pig iron in a blast furnace

The reduction of iron oxides proceeds in the following sequence:

Fe2O3 → Fe3O4 → FeO → Fe

The main recovery reactions are the following:

Fe2O3 + 3C = 2Fe + CO
3Fe2O3 + CO = 2Fe3O4 + CO2
Fe3O4 + CO = 3FeO + CO2
Ugly + CO = Fe +CO2

Hydrogen is also involved in the reduction of iron., which is formed from water, which is contained in the charge.

Carburizing iron

Iron carburization occurs due to the interaction of solid spongy iron with carbon:

3Fe + 2CO = Fe3C + CO2.

Iron-carbon alloy has a melting point below, than pure iron. As a result, droplets of liquid iron are formed., that flow to the bottom of the forge (bream) through a layer of hot coke, saturated with carbon.

Blast furnace slag formation

The main reactions of slag formation are as follows:

CaCO3 → CaO + CO2
High + SiO2 = CaSiO3

Adverse reactions of impurity reduction

As a result of side reactions, the reduction of impurity elements - manganese, silicon and phosphorus:

MnO2 + 2C = Mn + 2CO
SiO2 + 2C = And + 2CO
That3(PO4)2 + 3SiO2 = 3CaSiO3 + P2O5
P2O5 + 5C + 2P + 5CO

In this way, in the blast furnace we get a kind of contaminated iron, that is, cast iron, which contains large amounts of free carbon, as well as impurity elements – manganese, silicon and phosphorus.

Blast furnace pig iron

Typical chemical composition of blast furnace primary pig iron:
Iron (Fe) = 93,5-95,0%
Silicon (And) = 0,30-0,90%
Sulfur (S) = 0,025-0,050%
Manganese (Mn) = 0,55-0,75%
Phosphorus (P) = 0,03-0,09%
Titanium (You) = 0,02-0,06%
Carbon (C) = 4,1-4,4%

Blast furnace primary pig iron is smelted. Steel smelting process, roughly speaking, consists in reducing the carbon content in iron and cleaning it from excessive manganese content, silicon, phosphorus and other impurities.