As you read these pages you will notice frequent references to sand molds. Lunar regolith is such a good insulator (heat conductivity 0.001 to 0.01 W/K*m) that metals and ceramics poured into sand molds will take a long time to solidify. Heat will radiate from the top of the open mold into space and cast basalt and ceramics will cool and solidify from the top down very slowly. Metals will conduct heat rapidly from bottom to top and solidify more uniformly.
There are more problems caused by extremely slow cooling in sand molds. Basalt is a mixture of minerals with different solidification points. Some will solidify first and settle out and others may just float up to the top of the liquid mixture. The basalt must cool rapidly so all these minerals are "frozen" throughout the casting of basalt. Water cooling will not be applied but the mold might have drilled passages and inert gas like helium or argon, or even CO2 pumped through it.
When ferrous metals cool slowly they anneal; that is they soften. This makes them more easily worked. After they have been worked into various shapes they can then be heated and cooled rapidly by plunging them into water (this must be done inside pressurized modules) to harden them. Then final grinding or milling operations are done to finish the part. Thus, the slow cooling of iron and steel in sand molds covered with regolith or slag is a good thing.
As for ceramics, the only answer is the use of molds made out of metal, possibly with silica linings. But this is only speculation on my part and the reality may be that sand molds are sufficient for casting ceramic blocks.
Early industrial bases on the Moon will have access to solar energy, free vacuum, low gravity that makes if easier for humans and machines to lift things, and free regolith on land that doesn't have to be licensed of leased from anyone. Magma electrolysis, also called molten silicate electrolysis, could be used to get oxygen, ceramics consisting of a mixture of spinel (MgAlO4) and silicates, and ferrosilicon. Oxygen will contain impurities of Na, K, P and S that could be separated from oxygen gas in cold traps. These trace elements have many uses. Ferrosilicon is a mixture of iron and silicon abbreviated FeSi. This alloy is not very strong, but perhaps we could make use of it for this low stress application as a ceramic block cooling mold. If we upport some lightweight titanium molds (mp 1800 C) we could cast FeSi molds for other ceramics including cast basalt. We could also upport some lightweight carbon molds. We need molds to make molds ! It might be necessary to drill cooling passages in ferrosilicon molds and use a shielded radiator to keep the molds for softening too much or even melting. If they are large and thick they will have more thermal inertia and not soften so much or melt.
If serial electrolysis is possible we could draw off iron, then FeSi, then silicon from magma electrolysis. Iron might work better than FeSi and iron has many uses of it's own. If we can obtain volcanic glass and purify it to pure silica we could line molds with welded or brazed silica bricks (mp silica 1700-1800 C.) However, silica is a good insulator. We will need to make metal molds with or without fused silica brick linings that are sort of like an ice cube tray to make large numbers of cast basalt and other ceramic blocks from furnaces.
Aluminum and magnesium can be cast in simple plaster molds, but iron cannot because it will absorb sulfur from the plaster and molten iron is too hot for plaster.. Plaster can be obtained by sulfuric acid leaching of regolith. See: Acid Leaching and Electrostatic Separation |
