In order to get the water to higher concentrations, a process called 'vacuum distillation' is often adopted. That is the subject of this post.
Water that contains almost all deuterium and almost no protium is often called 'heavy water'. You might remember that the 1965 movie "Heroes of Telemark" was a dramatisation of the true story of the destruction of a German heavy water plant in occupied Norway during the second world war. This was necessary because heavy water is useful in some of the techniques used to produce enriched uranium for a nuclear weapon. (As it turned out, German technology was not going in quite the right direction, but that was not known at the time.)
However, heavy water is also a source of the deuterium that is needed for the entirely peaceful and environmentally friendly fusion reactors of the near future, and it is only for this reason that I care about it enough to write this series.
In the last instalment, we reached the point where we have water with 20 to 25% deuterium atoms and the remainder still containing protium. You might remember that I explained in part 1 that some lakes around the world (which have no rivers flowing out of them) have slightly higher concentrations of deuterium than 'ordinary' water. This is because water molecules containing deuterium evaporate slightly less easily than water molecules containing protium.
Now you might be able to imagine a method of using this process on an industrial scale. If you study the physics of the evaporation of water, you find that you can choose the right conditions of temperature and pressure where the H2O evaporates preferentially and leaves the HDO and D2O behind. In fact it turns out that the best conditions for this are at a temperature slightly above the freezing point of water, and in rather a good vacuum.
In an ideal world you would like to be able to evaporate the product that you want to keep in preference to the waste product. After all, the whisky industry in Scotland makes its living by doing exactly this 'distillation'. They warm a dilute mixture of alcohol in water and the alcohol evaporates preferentially. The liquid that is distilled contains more alcohol and less water. They usually repeat this distillation a few times in order to concentrate the alcohol further, taking the product from the first distillation and putting it through the process again and again to increase the purity.
Unfortunately, in making heavy water the opposite is true. The part that evaporates first is the part that you want to 'throw away' although in practise it will still contain much more deuterium than most of the water in the world. It will not be discarded but returned to an earlier stage in the process.
The water that is left behind will be a bit more concentrated than it was before the H2O was distilled away, and the last water to evaporate will be the most concentrated in deuterium. Of course the whole process has no clear cut off points where all the H has been removed, leaving all the D behind. But an iterative approach can yield higher and higher concentrations of deuterium and, in practise, a concentration of 99% can be achieved. This might be good enough for most applications of heavy water.
As a fusion fuel, a slightly better concentration might be preferred.
Fortunately the next stage in the process helps further. It is in this stage that the heavy water is converted into deuterium gas, by a process known as electrolysis. This gas is one half of the fuel we need for fusion.
More next time: Fusion fuels 4 - Electrolysis of heavy water
Other articles in this series:
Fusion Fuels: Part 1 - The isotopes of hydrogen
Fusion Fuels: Part 2 - 'Mining' deuterium.
Fusion Fuels part 5 - Tritium