There is a lot of waste products in the layer of slag that is formed when the process of obtaining molten iron is carried out. The table below shows the usual composition of slag showing the different waste products created:
At 500°C and 850°C
Iron Oxide combines with the carbon monoxide formed when coked reacts with the hot air to create iron oxide and carbon dioxide removing some of the oxygen in the iron oxide. This is known as indirect reduction
At Higher Temperature 1600°C to 1900°C
Direct reduction occurs where the iron oxide is directly reduced by coke to form Iron and Carbon monoxide.
Fe2O3 + 3CO → 3CO2 + 2Fe this is the overall equation at its basic level of the reaction occurring in the blast furnace.
Manufacture Of Steel By The BOS Process
Steel is made from the impure iron removed from a blast furnace. Though the iron has ahd some impurities removed from it, there are still many other elements dissolved in it such as carbon, silicon, manganese, phosphorous and sulphur.
Metal with a composition as shown in the table below is very brittle.
To make this metal into steel most of the phosphorus and sulphur needs to be removed and the carbon content lowered. Other elements also need to be added. The process used to achieve all this is called the Basic Oxygen Steelmaking Process (BOS). It is a batch proves in which 300 tonnes of high quality steel are made in about 40 minutes. The step by step stages involved are outlined below:
Removing Unwanted Elements
300 tonnes of molten iron form the blast furnace are poured into a device called a ladle.
Removing Sulphur
Sulphur is removed first and this happens separate from the main steelmaking process. To do this several hundred kilograms of magnesium powder is injected into the ladle by a tube called a lance. By a violent exothermic reaction the sulphur reacts with the magnesium forming Magnesium Sulphide on the surface which is raked off.
Mg + S → MgS
Other elements
Carbon, Phosphorus and the other impurities are removed by direct oxidation [oxidation is where substances are oxidised so gain oxygen but loose electrons]
C + ½ O2 → CO
Si + O2 → SiO2
Mn + ½ O2 → MnO
4P + 5O2 → P4O10
Fe + ½ O2 → FeO This is not a desired reaction but occris anyway.
The ladle then takes the desulphrised iron to a steelmaking vessel which already contain some scrap steel. (see diagrams below)
The converter itself then turns into a vertical position and a water cooled lance (tube) slowly inches its way down close to the surface of the iron. A very quick blast of oxygen under pressure forces its way into the vessel and creates a seething foam of molten metal and gas which is blasted up the walls of the converter. Over the next 20minutes or so most of the impurities such as carbon, silicon, manganese and phosphors, and some iron are oxidised.
The oxides created of Phosphorus and silicon are acidic meaning they will react with bases to form salts. [this occurs because the acid donates a proton to the base resulting in a reaction forming a new substance which is called a salt] After the oxygen has been blown into the iron a mixture of calcium oxide and magnesium oxide is added to the converter. These basic oxides react with the acidic oxides to form molten slag which floats to the surface, the oxides of manganese and iron also collect in the slag. [a basic oxide is one that would react with water to produce a base], [an acidic oxide is one that would react with water to produce an acid]
Different Types Of Steel – Composition and Uses
Steel is formed from the refining and removal of impurities of iron, because of this the amount of certain impurities in the steel can determine the properties and so the uses of the steel. For this reason different compositions of steel can be made to be used differently.
Mild Steel
Mild steel is a form of steel that has a very low carbon content in fact it is composed of around 0.25% carbon only. Also mild steel composes of 0.4-0.7% Manganese and 0.1-0.5% Silicon. The low carbon content makes mild steel easy to use when heated it can be welded and so this is one of the uses of it. Mild steel is also used in the construction of car bodies. Mild steel is also used in school workshops and for engineering purposes.
Stainless Steel
There are different types of stainless steel but all follow a certain type of composition. They contain a maximum of 0.15% carbon, 2% manganese, 1% silicon, 0.045% phosphorus, 0.03% sulphur, 8- 14% nickel and 12-20% Chromium. The key to stainless steel being water resistant and rust resistant is the chromium in its composition. The chromium reacts with the oxygen in the atmosphere creating a thin layer of chromium contain oxygen that preventing oxygen getting to the surface of the steel and so causing rust. The result of this stainless property is that stainless steel can be used to make medical instruments that need to be sterilised Stainless steel is used wherever metal may need to come in contact with air or water. Kitchen sinks are made of stainless steel, window frames; banisters for stairs are made of stainless steel, fridge doors and in many other areas of farming and industry. Stainless steel has a wide variety of uses.
Alloy Steel
Alloy steels have specific compositions depending on their use. They contain carbon as well as other elements like vanadium and molybdenum. There is still large amounts of Manganese, silicon and copper in the alloy. Alloy steels are used in axles, vehicle gears, rollers and carving knives.