Interstitial Alloys
The Bronze Age began to fade about 3000 years ago with the discovery that iron oxide could be reduced to iron metal in wood or charcoal fires by limiting the air supplied to the fire. As in copper smelting, the reducing agent for iron oxide is carbon monoxide:
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Iron quickly replaced bronze as the metallic material of choice for fabricating tools and weapons both because iron ore is much more abundant in Earth's crust than the ores of copper and tin, and because tools and weapons made of iron and iron-containing alloys are much stronger than those made of bronze.
Today, the reduction of iron ore is done in blast furnaces, enormous reaction vessels (some more than 50 m tall) that operate continuously at about 1600 degree (Figure 11.14). Iron ore hot carbon (coke), and limestone are added to the top of the vessel and molten iron and solid by-products (called slag) are harvested from the bottom (figure 11.4a). Blast furnaces get their name from blasts of air preheated to about 1000 degree that are injected through nozzles near the bottom and that suspend the reactants until the iron reduction process is complete. It may take as long as eight hours for the reactants to fall to the bottom of a blast furnace. On their way down, O2 in the hot are partially oxidizes coke (carbon) to carbon monoxide, and the CO reduces the iron in iron ore as described in Equation 11.4. Limestone (CaCO3) decomposes to calcium oxide (CaO) or lime:
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