How is the process of converting calcium carbonate (CaCO3) into calcium oxide (CaO)?
From wikipedia article of calcium oxide


  Calcium oxide is usually made by the thermal decomposition of
  materials, such as limestone or seashells, that contain calcium
  carbonate ($\ce{CaCO3}$; mineral calcite) in a lime kiln. This is
  accomplished by heating the material to above 825 °C (1,517 °F), a
  process called calcination or lime-burning, to liberate a molecule
  of carbon dioxide ($\ce{CO2}$), leaving quicklime.
  
  $$\ce{CaCO3(s) → CaO(s) + CO2(g)}$$
  
  The quicklime is not stable and, when cooled, will spontaneously react
  with $\ce{CO2}$ from the air until, after enough time, it will be
  completely converted back to calcium carbonate unless slaked with
  water to set as lime plaster or lime mortar.