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Energy Use: Metal Casting - Magnesium

The total energy required to produce one ton of magnesium is around 35 to 40 MWh for both the electrolytic and thermal processes. Other thermal process steps are comparable in energy use. Where they differ is in use of raw materials, energy for the production of lime, and the conversion process. (There is a theoretical minimum of 7 MWh/ton of metal).

The other thermal processes are comparable in energy use. Where they differ is in the use of raw materials, the energy consumed in the production of lime, and the conversion process.

Electrolysis:
Additional thermal energy (per ton of metal produced) is required in the electrolytic process for various process steps which may include the following:

  • Lime: if manufactured by metal producer at 10 gigajoules per ton (3 MWh equivalent in power) to convert natural and magnesium chloride source.
  • Production of hydrochloric acid: including the "syngas" step to produce hydrogen (to react with liberated chlorine). Energy (gas) is presumed to be about the same as lime (5-10 gigajoules per ton).
  • Magnesium chloride preparation: about 16 gigajoules (5 MWh equivalent in power).
  • Acid recovery: about 15 gigajoules per ton (5 MWh/ton).

Notes:

  • 1 MWh is 3.6 gigajoules. At around one-third electric conversion efficiency, 1 MWh of power will require 10 gigajoules of gas. (1 gigajoule equals 947,817 Btu which equals 945 scf). Therefore, 10 gigajoules results in about 9,450 cf of natural gas.
  • Typically, two-thirds of the chlorine produced in electrolysis is used in preparation steps. So for each ton of magnesium produced, about one ton of chlorine is available for sale. If the chlorine is sold to a "petchem" project or for use in domestic water treatment, it provides a theoretical maximum credit of around 60 gigajoules per ton (20 MWh per ton of magnesium). However, this depends on the efficiency of chlorine use (with up to two-thirds consumed in the process and disposed as waste); so the credit reduces to as low as 6 MWh.

Thermal Reduction
This process uses power. No breakdown is available, but total energy consumption is approximately the same as electrolysis (i.e., around 35 MWh per ton of magnesium metal produced). There is no credit for chlorine as a user of magnetite or dolomite. This includes thermal energy for preparation of dolomite (or mixture of magnesia and lime) at 15 gigajoules (4 MWh equivalent).