page 12
Basic oxygen furnace
The basic oxygen sequence
Diagram sequence of Basic Oxygen Furnace steel making with roll over changes
Photographs of sequence of Basic Oxygen Furnace steel making with roll over changes
There are three main stages in the operation of the furnace. You can see a diagram and photograph of each step by rolling over the highlighted word.
1. Charging the furnace
The BOS converter is charged first with scrap. This is used as a coolant. It helps to control the very high temperatures produced by the violently exothermic reactions in the furnace. After the scrap, three or four times as much hot metal (up to 300 tonnes) is poured into the furnace from a ladle.
2. Blowing
After charging, the furnace is blown by blasting oxygen through a lance that is lowered into the molten metal. The furnace needs no heating because the oxygen combines very exothermically with the impurity elements, carbon, silicon, manganese and phosphorus. Carbon is oxidised to carbon monoxide and much of the carbon in the metal escapes as this gas. The other impurity elements also form oxides. These are acidic and are separated from the metal by adding basic calcium oxide (lime) to the furnace. This combines with the oxides and removes silicon, manganese and phosphorus in a slag.
3. Tapping the furnace
After the blow has continued for about 20 minutes, the metal is sampled. Its composition at this stage is shown in Bar Chart 3. The BOS process is now complete and the furnace can be tapped. Steel is run out of the taphole into a ladle, separating it from the lighter slag, which is later emptied as waste.
Graph of steel composition
Secondary steel making
Low levels of S and P in the metal are acceptable to the customer but the levels of C, Si and Mn suit the customer's requirements for tinplate. This will involve changes to theprimary steel. The next step in the process is called secondary steelmaking. At this stage, adjustments are made to the composition of the steel to bring it still closer to the customer’s specification. You can find out about this on the next page.
Question 12

Describe the changes that have taken place in the composition of the metal after blowing the furnace.