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A town in a mountain valley has access to geothermal energy due to the volcanic activity of the mountain range, somewhat like Yellowstone. This has already given them access to hot springs that they can bathe in (though most are too dangerous, either being too hot or containing noxious chemicals). They use pipe to distribute it to heat their homes.

With this rich source of heat, the residents have been wondering if it's possible to use the heat to smelt ores of various metals (copper, tin, zinc, iron etc.) through geothermal heat alone.

Can they?

Some clarifications:

  • The technology is late Victorian but the knowledge is modern (long story)

  • Due to this, they know they could use electricity using a Stirling engine to smelt metals, but they lack any graphite deposits to use as electrodes due to the being over a volcano.

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    $\begingroup$ What metals do they have access to? There's a reason I ask, also, how advanced is their chemistry, is wood available? $\endgroup$ Feb 20 at 8:10
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    $\begingroup$ Are you asking "Can the heat from geothermal sources be used directly to smelt ores?" Or are you asking "Can the energy from the difference between the heat provided by geothermal sources and local ambient temperatures be converted or concentrated to allow smelting ores?" $\endgroup$
    – Makyen
    Feb 20 at 18:01
  • $\begingroup$ I was asking for the latter as the former could easily be done through something like a Stirling engine for electricity production. Course I have my answer now. $\endgroup$
    – Seraphim
    Feb 20 at 19:19
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    $\begingroup$ Why do they need graphite deposits? You can make graphite out of practically any carbon source with a modicum of effort... $\endgroup$
    – Perkins
    Feb 22 at 17:42
  • $\begingroup$ Directly, how could geothermal energy ever come close to 'smelting power'? Do these guys have some kind of heat exchanger/extractor that might later boost the geothermal energy? $\endgroup$ Feb 22 at 21:17

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I can't argue with @L.Dutch's answer and I upvoted it

But let's add to the list of reasons why this can't work in a context.

Exposed lava is a dangerous creature. It exists because of geological instabilities that let (if you'll excuse the drama) the demons of the deep out of their cages. Generally, the faster it moves, the hotter it starts and the hotter it remains.

When that instability occurs under an aquifer, what you get are hot springs. The cooling effect of the water lowers the volatility of those raging demons in a way that happens to be mighty pleasant useful to everything from humans to Macaques.

But that doesn't help you. You need much more heat than liquid water can deliver. You need access to the lava. Except that exposed lava, even when predictable, is a mighty dangerous demon.

Kilauea Volcano has been erupting almost continuously since 1983. Although it is noted for its approachable lava flows, the volcano undergoes many changes that can be unpredictable and life threatening. Without knowledge of the landscape and the processes that form it, visitors can find themselves in dangerous, even deadly, situations. Understanding volcanic hazards and taking the right equipment are keys to safely exploring volcanic landscapes. (Source)

Hawaii activates National Guard as Mauna Loa’s unpredictable lava flow creeps within 2 miles of critical highway (Source)

You can't control it

And with Victorian-era technology, you can't control it. Heck, with circa 2024 technology we still can't control it. Those lovely buildings in the Star Wars movies that straddle lava flows for whatever energy-gathering or manufacturing purposes? They've never existed in human history.

"Armed with shovels and pickaxes and protecting themselves against heat with wet sheep-skins", according to one academic account, the Catanians opened an artificial breach, cutting off the lava's path.

The residents of nearby Paterno were not pleased. They believed the diversion pointed the lava directly at their own community and decided to stop Catania's attempts. The breach sealed up and the lava continued flowing toward Catania, destroying a large portion of the town.

It was the modern start to what Dr Shannon Nawotniak, a professor of geology at Idaho State University, calls a "spectacularly poor success rate" of stopping lava. (Source)

Recommendation

Dump the requirement. The idea is cool. I'd hate for it to not be used because it couldn't factually exist historically or today.

Or, if you insist on using a solution, you're required to shift to geothermal electricity production. Time to import that graphite — and a Victorian-era technology with modern knowledge without trade is unbelievable.

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    $\begingroup$ Thanks for the answer. I also found that Graphite does occur in volcanicly active places in the from of hydrothermal graphite (financialpost.com/commodities/mining/ceo-interviews/…). That and I think I found a way for the inhabitants to but there sewege to good use... link.springer.com/article/10.1007/s42250-019-00098-3 $\endgroup$
    – Seraphim
    Feb 20 at 19:19
  • $\begingroup$ Just pump the water from the hottest spring around, mix it with sewege and put it into a very thick pressure cooker. $\endgroup$
    – Seraphim
    Feb 20 at 19:26
  • $\begingroup$ In case it's not obvious, lava is molten rock, with the weight of rock because it's rock, several times higher than water, and it's also very viscous, because it's rock and rock doesn't flow very well, and it also happens to melt most things you might try to put in its path to control it, which is a problem for you, and if it doesn't melt them, they'll solidify it, creating a clog, which is also a problem for you. $\endgroup$
    – user253751
    Feb 21 at 2:36
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It's going to be difficult, for various reasons:

  • while it's true that lava has a temperature in the range of 800 to 1200 C, geothermal heat is not necessarily in that range, it can be lower.
  • to smelt ores you don't need just heat, you also need a reducing environment, to turn the oxides of the ore into native metal. Burning carbon does that, heat alone doesn't.
  • hot rocks or lava might add unwanted impurities to whatever you are producing. You mention thermal waters being loaded with chemicals.
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    $\begingroup$ The basic idea is that smelting ores is a chemical reaction. Heat helps, but still they need both reactants. $\endgroup$
    – AlexP
    Feb 20 at 9:39
  • $\begingroup$ the other problem you have is keeping the lava out of the ore, there jut is not enough heat to get the ore molten without emersion. As soon as you can interact with lava it is losing heat. $\endgroup$
    – John
    Feb 20 at 22:04
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No. Geothermal heat doesn't get hot enough.

The hottest water found in Yellowstone is deep-level brine at about 200 °C. The hottest subsea vents are about 400 °C. Very few ores can be smelted at such low temperatures. Tin, for example, requires heating to 1200 °C, while zinc requires 700 °C at the lowest.

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The first thing that comes to mind when reading your question is Iceland, known for abundant sources of geothermal energy and also having a functional manufacturing industry including smelting facilities (aluminum in particular).

I agree with @JBH's recommendation to dump the science-based tag, so making that same assumption, I think you can reasonably extrapolate from Iceland's energy economy.

As mentioned in most other answers, the actual heat generated is inadequate for smelting, both temperature-wise and because there are additional chemical reactants needed. However, if access to this geothermal heat is so abundant you can reasonably power pretty much their entire society off of it with plenty of excess, it becomes more practical to then transfer some of that energy back into process of smelting.

You mentioned Victorian-era technology as well. While Iceland didn't use the geothermal energy for much more than hot baths for centuries (only breaking away from fossil fuels for power generation in the 1970s), simply having your characters' society adapt known energy conversion techniques earlier will solve this problem. For example, going from operating a mill by having a river or other flowing body of water turn a wheel does not need much of a stretch for someone in the community to come up with a way to do the same but with steam. This can in turn raise a weight (containing the energy through its potential that can then be released at a later time), or directly piped through another system to concentrate the heat if needed.

In short, either drop the science-based or bump the technology level, and read up on Iceland!

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Heat may melt metals. Water is not hot enough, so you have to use the lava directly. Lava moves unpredictably, so you cannot build a factory for this purpose.

People may approach the edge of a slow lava flow, and place a long-handle pan so it is heated but not melted. In such a pan, they may melt stuff with a lower melting point than the pan itself. Might be useful for cheap small scale metal casting.

Ores may be placed directly on lava, if the final product will float to the top and be recoverable later when that lava cools. Others have pointed out that you may need more than just heat for such a process though.

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Perhaps the volcano has numerous fumaroles, producing unusually hot gasses. Although smelting of most metals is out of reach of even high temperature fumaroles, there are many other processes which they could be useful for:

  • Fumaroles often deposit hydrothermal minerals around the vent. So you could mine some (but probably not all) of: Gold, silver, copper, tin, zinc, and lead.
  • If you have silver and copper they often come together. Extracting them from the ore probably requires higher temperatures than the fumaroles, but yields a mixture of the metals. You can separate the silver from the copper by liquation with lead, which you may also have deposits of, and that is possible at fumarole temperatures. There may also be other materials that could be refined by liquation, I'm not sure.
  • Fumaroles produce hot steam, which could be used for steam treatment of irons and steels.
  • Even if fumarole gasses are not hot enough to smelt many ores, they may still be ducted into the furnace and used to pre-heat the furnace and it's contents, reducing fuel requirements.
  • Cowper regenerators used on blast furnaces were an early Victorian invention. They are required to get good quality steel in large quantities. If you delay their invention a bit, then you could use a brick counterflow heat exchanger running on fumarole gasses instead, and have the only working blast furnace for a thousand miles, or if delaying inventions doesn't fit your world, then you'd have a slightly cheaper to run blast furnace.
  • The hot gasses could be used to run a steam engine without fuel. That opens up all sorts of options for processing metals or making metal products. You could run a steam hammer, or a forging press. You could also run a lineshaft system with drills, mills and lathes making finished products.

So I think a bustling hub of smelters and foundries and metalwork factories could easily pop up around the town, even if they can't use the heat to directly smelt materials. And there is nothing more Victorian than steam engines, lineshafts, and factories.

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Sure.

A Stirling engine is one efficient way to use a heat gradient. But any heat gradient can turn into motion, and any motion can power dynamos. So you have a source of electricity regardless of whether that source is efficient.

You may want something like a tall tunnel with slow turbines to capture an updraft from the springs, then use that to pump water up to a high reservoir (gravity storage makes up for bad battery tech). Then you run a large dynamo off that reservoir every month or two to generate a giant pile of power and smelt a large batch at once.

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  • $\begingroup$ It gets away from the idea of using the geothermal power, but the described mountain valley setting could also be a location where wind power is particularly plentiful, without any need for that big updraft tunnel. That reservoir would be a big project for a single town to tackle all on their own, though. $\endgroup$ Feb 21 at 15:33
  • $\begingroup$ @ChristopherJamesHuff sure, usually. Here though the asker introduced conversion to electricity as part of the question, and specifically wanted it powered by the heat from the hot springs. If you're going to windpower instead, you may as well go with charcoal instead, which is, uh, a lot less Rube Goldburg-ey than either. $\endgroup$
    – fectin
    Feb 22 at 1:17
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Due to this, they know they could use electricity using a Stirling engine to smelt metals, but they lack any graphite deposits to use as electrodes due to the being over a volcano.

Steam power might be more likely for scalability/power density reasons, but that aside: copper, tin, and zinc can easily be melted or smelted with resistance heating. This is much more difficult with iron, but it can be melted with induction heating, which should be achievable with the output of an Alexanderson alternator (one of the first machines for producing high power AC at RF frequencies, invented in 1904).

Also, synthetic graphite via the Acheson process was invented in the late 1800s, so they could plausibly produce it on their own at a similar level of technology.

The real question I see is whether a "town in a mountain valley" can possibly support the population and industry required for all this.

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    $\begingroup$ Melting is a physical process, and is done by heat alone. Smelting metal from ore is a chemical process, and heat alone is not enough. Even if you use ludicrous amounts of heat and melt iron oxide, what you get is molten iron oxide, not iron; to get iron, you need to react the oxide with something, in usual practice with carbon, to reduce it to metallic iron. $\endgroup$
    – AlexP
    Feb 20 at 22:43
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    $\begingroup$ @AlexP So? The problem was to use geothermal power as a heat source. The usual reducing agent, carbon (and carbon monoxide when it's partially oxidized), will work just as well when the heat is produced electrically. $\endgroup$ Feb 20 at 22:49
  • $\begingroup$ What would be the minimum population be? $\endgroup$
    – Seraphim
    Feb 21 at 3:07
  • $\begingroup$ @Seraphim That depends a lot on the available technology and how much they need to do on their own instead of acquiring the materials by trade. You've got people who specialize in geothermal systems, electrical power production, electrically powered smelting and melting furnaces, graphite production, production of furnace components from that graphite, mining, refining, and processing of the various raw materials including metals, ceramics, glasses, charcoal/coal/coke, etc...and we haven't even gotten to the people who actually use the end products. $\endgroup$ Feb 21 at 4:06
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Great news everyone! So heat can be used to make electricity, electricity can be used to make hydrogen. Hydrogen can be used to make steel. https://worldsteel.org/wp-content/uploads/Fact-sheet-Hydrogen-H2-based-ironmaking.pdf

Its very, very inefficient though. Like multiple conversions of energy in-efficient.

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