Molten salt electrolysis
Passing current through a molten chloride or oxide to win a metal too reactive to smelt — the only route to magnesium, sodium, lithium and calcium.
Some metals cannot be smelted. Carbon reduces iron oxide readily and copper oxide easily, but it will not take oxygen or chlorine away from magnesium, sodium, lithium or calcium — those bonds are stronger than the ones carbon can form. For as long as reduction meant a furnace and charcoal, these elements could not be isolated at all.
Electrolysis supplies the missing agent. Current forced through a molten salt strips the metal out at the cathode and releases chlorine or oxygen at the anode, and the strength of the bond stops being a barrier and becomes a bill. This is why the reactive metals were all isolated within a few decades of the electric battery, and why their price still tracks the cost of electricity more than the cost of ore.
Processing
The salt must be molten, because a solid conducts by electrons rather than ions and nothing is transported. That means running at several hundred degrees, in a lined cell, and it is where most of the energy goes.
Water cannot be used as the solvent: these metals are reactive enough that they would reduce the water instead, producing hydrogen and leaving the metal as a hydroxide. That constraint — the reason the process is molten rather than aqueous — is the same one that makes the metals valuable.
Environmental impact
Electrolysis is only as clean as its electricity, and these cells consume a great deal of it. A magnesium or aluminium plant is generally sited where power is cheap rather than where ore is, which is why smelters cluster around hydroelectric capacity.
The anode product matters too: chloride cells release chlorine, which is captured and sold rather than vented, and carbon anodes in oxide cells are consumed and emit carbon dioxide directly, independent of how the electricity was generated.
How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.
How this connects
Where a connection has been confirmed by an outside reference, that reference is named beside it.
produces
- Magnesium — element · from molten magnesium chloride, including the chloride returned by the Kroll process
- Sodium — element · from molten sodium chloride, with chlorine released at the anode
- Lithium — element · from molten lithium chloride, after concentration from brine or spodumene
- Calcium — element · from molten calcium chloride; there is no smelting route
- Potassium — element · difficult even electrolytically, because the metal dissolves in its own molten salt
- Mischmetal — alloy · reduced from the mixed chlorides without separating the lanthanides first, which is the entire economic point of it
- Fluorine — element · electrolysis of potassium bifluoride, which is molten and conducts — there is no chemical oxidant strong enough to displace fluorine from a compound, so electricity is the only route and always has been
was succeeded by
- Hall–Héroult process — process · the Hall–Héroult process is molten salt electrolysis specialised to alumina dissolved in cryolite
takes as input
- Lepidolite — mineral · after concentration, as one of the two routes to lithium chloride
succeeded
- Brine evaporation — process · the concentrated salt still has to be reduced; evaporation delivers a compound, and electrolysis delivers the metal
Sources
- Material WorldOur own writing