From our Ore to Your Door - Episode 13: Calcination (Multiple Hearth Furnaces)

The process of converting Magnesium Carbonate (MgCO3) into Magnesium Oxide (MgO) is undertaken using QMAG’s three multiple hearth furnaces.
The calcination process happens when the ore is heated up to approximately 1000ºC, allowing the chemical reaction to take place. The multiple hearth furnaces use natural gas to heat designated hearths inside the furnace to the required calcination temperatures.

An offgas system removes small particles and any combustion gases from the furnace and filters out the dust particles. The dust particles are then captured in the baghouse and introduced back into the furnace for calcination using blowpots.

The calcined material (MgO) exiting the furnace needs to be cooled to a temperature that the downstream process can handle, so it is passed through a rotary cooler.

What is a multiple hearth furnace?
The multiple hearth furnace is a large cylindrical vessel, containing 17 hearths stacked on top of each other. The furnace is lined with heat resistant bricks, which separate the furnace into the 17 hearths. Hearths 7 to 14 are fired hearths when the furnace is in operation to meet a designated temperature profile within the furnace. Each of the fired hearths has four burners, and a mixture of natural gas and pre- heated combustion air enters the furnace through these burners.

A shaft runs from the top to the bottom of the furnace. This shaft has arms attached to it on each hearth, with ploughs attached to the arms that “rake” the material across the floor of the hearth to drop into the hearth below. This is how the magnesite moves through the furnace, alternating between inner and outer hearths as it flows from the top to the bottom of the furnace, exiting the furnace as magnesium oxide.