Metal Extraction From The Venus Planetary Surface
The geology of the Venus planetary surface and what it tells us about which metals can easily be extracted.
The surface of the planet Venus is extremely hostile, with surface temperatures of almost 500 degrees celsius and pressures of 93 bars (same pressure as 900 meters underwater on earth). Running complex mining operations on the Venus surface will be unlikely for early colonizers. As discussed in my story about Venus colonization, habitats will most likely be built as floating habitats about 50 km above the surface. At this altitude pressure and temperature is Earth-like.
The colonists dwelling there will likely want to get their hands on metals and other minerals. One can imagine sending down contraptions picking stuff straight off the Venus surface. A lot of deep digging and drilling will probably not be realistic. So the question we ask ourselves in this story is: Given these limitations, what kind of metals can we get hold of?
To answer this, we need to learn what rocks exist on the Venus surface. The types of rocks available will not be as large as on Earth because our rocks have been heavily influence by the presence of biological life (limestones), ice and water eroding the landscape in various ways.
For this reason the surface of Venus is largely made up of Basalt rocks which are formed by rapidly cooling lava on the surface. However there are not only solid rocks, wind and chemical erosion has crated plenty of sand and ash from volcanic eruptions which form dunes.
Basalt is made of the minerals:
- pyroxene. Made of (Ca, Na)(Mg, Fe, Al)(Al, Si)₂O₆. Meaning There are a lot of possible combinations of metals but they will all contain oxygen.
- plagioclase. CaAl₂Si2O₈ or NaAlSi₃O₈.
- olivine. (Mg, Fe)₂SiO₄
Which means we can theoretically get following metals:
Getting and Making Iron
Turning this into iron could be done with some different alternatives.
Either use CO from the RWGS (water gas shift) reaction. To do this requires iron-chrome or copper catalyst as well as simple steel pipe and some more.
H₂ + CO₂ → H₂O + CO (1)
We could then get Iron by doing:
Fe₂O₃ + 3CO → 2Fe + 3CO₂ (2)
Alternatively we use hydrogen directly (e.g. from electrolysis).
Fe₂O₃ + 3H₂ → 2Fe + 3H₂O (3)
The reactions are described in Robert Zubrin’s book “The Case For Mars” on page 511
Getting and Making Aluminum
Robert Zubrin discusses making Aluminum on Mars and given that Mars has some similar composition as Venus with lots of basalt I assume the process he describes is similar.
That means using the aluminum oxide called alumina (Al₂O₃). The process requires dissolving alumina in molten cryolite at 1000 °C and then use electrolyze. The process is:
Al₂O₃ + 3C → 2Al + 3CO
Zubrin describes the process as complex and extremely energy intensive. 20 kWh of electricity to produce 1 Kg of aluminum.
Thus Zubrin considers aluminum not to be a material used much on Mars. On Venus due to 4x the radiation from the sun, energy production may easier and hence making this cheaper.
However I think it is safe to assume iron will be the preferred metal, despite aluminum obviously having an advantage in an airship due to its lightness. We will instead have to use carbon fibre to accomplish that.
Silicon For Fuel, Glass and Electronics
Earth, Mars and Venus is believed to have similar crust composition and thus prevalence of Silica based minerals (Silicon dioxide SiO₂), about 40%. Quartz minerals are made of Silica. Think of this like the difference between graphite, diamond, coal, graphene etc. It is all made of carbon, but we give it different names because the atoms are arranged differently.
Glass is also made of Silica, but the molecules are not arranged the same way as in quartz.
Glass could be made from almost anything. All that is required is for melted rock to cool quickly to get the atoms arranged like in glass. If it cools more slowly it will grow crystals instead and we get e.g. quartz rocks. The main reason for focusing on quartz is that it is what give us clear transparent class. If you turn into glass a rock with metal content it will get dark like obsidian.
When you see descriptions of glass making, you often see a number of other chemical besides silica added. That is strictly not needed. You could still make glass without it, but it may be more difficult. We add compounds called fluxes to lower the melting temperature and to improve properties such as durability compounds called stabilizers are added. Chemistry Explained details this very well.
The reason for me focusing this much on glass is that with limited ability to survey the Venus surface and locate metal ore, it should be easier to obtain raw materials for glass making as silica is so abundant in the crust. Since so much production will be based on chemical processing glass is a good alternative to metal for making tanks and equipment for chemistry laboratories.
But lets look at the semi-conductor silicon itself and how we extract it.
You just mix silica with carbon and heat it in an electrical furnace.
SiO₂ + 2C ⟶ Si + 2CO
These reactions will not produce silicon pure enough for making semi-conductors. So usually the mix is made to react with hot hydrogen gas to make Silane (SiH₄). The silane is easy to separate from other gases and can thus be changed back to silicon of high purity.
Perhaps more importantly silane is a great candidate as a fuel, because it burns in carbon dioxide. The chemical reaction is:
SiH₄ + 2CO₂ ⟶ SiO₂ + 2C + 2H₂O
Which has some resemblance to how methane burn. Silane is basically methane where you replace the carbon with silicon.
CH₄ + 2O₂ ⟶ CO₂ + 2H₂O
Oxygen takes up a lot of the weight if we carry both oxygen and methane with us, as can be calculated by looking at the atomic mass of the compounds involved.
CH₄ + 2O₂ = 16 + 2*32 = 80
2O₂ / (CH₄ + 2O₂) = 2*32/80 = 0.8
A whooping 80% of the total mass of fuel and oxidizer would be made up of oxygen. Lets calculate how much less weight using silane will require:
SiH₄ / (CH₄ + 2O₂) = 32/80 = 0.4
So silane requires just 40% of the weight to carry the same amount of fuel.