CaO in turn reacts with silica impurities in the ore to form calcium silicate:
Calcium silicate becomes part of the slang that floats on the denser molten iron at the bottom of the furnace. The largest blast furnaces, operating continuously, produce 10000 to 15000 metric tons of iron a week in response to increasing world demand for products made of steel. Figure 11.15 shows annual worldwide steel production since the beginning of the 21st century.