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Waste materials from ore processing will be mostly silicate rich slag.  A few years back on this forum, we discussed the production of basalt fibres for tensile applications.  The fibres are as strong as maraging steel and weigh less than half as much.  Melting basalt is considerably less energy intensive than smelting steel, because the temperatures involved are more modest (1200°C vs 1600) and chemical reduction is not necessary.  We are melting the material and not chemically reducing it.  If we have enough energy to melt silica rich slags, then the liklihood is that we would find uses for these materials.

For high compressive strength and low thermal conductivity, I would recommend magnesium oxide.
https://www.azom.com/properties.aspx?ArticleID=54
See also: https://nvlpubs.nist.gov/nistpubs/jres/103/4/j34sli.pdf

Thermal conductivity declines as temperature increases.  As the engine will be operating in vacuum, you could also incorporate porositity to reduce thermal conductivity even further, with some loss of compressive strength.  The thrust of 0.5 tonne-force is a quite modest.  If it is distributed over 1m2, it amounts to a pressure of 5KPa .  So the material probably does not require a lot of strength.  Vibration could be a problem, as magnesium oxide is a brittle ceramic.

The older I get, the more impressed I am by simplicity.  Maybe it is a sign of mental deterioration?  :-)

This article discusses furnicular railways.
https://solar.lowtechmagazine.com/2009/ … le-trains/

These are gravity powered cable railways, that are used for transporting people up and down hills.  The simplicity of this idea has always impressed me.  These vehicles do not require any engine.  Motive power is provided by adjusting a counterweight on each vehicle, which is a tank of water.  The ascending train is connected by cable to a descending train.  By filling a water tank at the top of the hill, the descending train is made heavier than the ascending train.  As it descends and applies tension to the cable, it pulls the ascending car up the hill.

Cable driven street trains pre-date the introduction of electricity as a power source.  Before electrification, cable driven public transit systems were built in many cities across the world.  Only a few now survive, with the San Fransisco cable cars being the most famous example.  Any town with a varying gradient could build gravity powered transit networks.  The cable would run through a conduit under the road surface.  It would not need to be powered, as the energy needed would come from filling water tanks in vehicles at the top of the gradient and emptying them at the bottom.  All of the cars are attached to the cable in a loop.  So the gravitational potential energy released by a single car as it descends a gradient will help drive all the other cars around the loop simply by pulling the cable down the slope.

Nothing is free of course.  The energy needed to run the network comes from the gravitational potential energy of water at the top of a hill.  This can be recharged using a pump, to transfer water from a lower reservoir to an upper reservoir.  Provided that the two reservoirs are appropriately sized, this task can be accomplished using intermittent energy.  A mechanical windmill at the top of the hill could drive a positive displacement pump at the bottom, by pulling on a cable.

There are variations in how such a system could work.  A gravity powered cable car could work on completely flat topography, by raising and lowering weights through pits and towers.  Towers could function as raised weight hydraulic accumulators.  Discharged hydraulic fluid would drive turbines, pulling the cable.  A system equipped with multiple towers could recharge them whilst others towers are discharging.

Water has a high heat of fusion due to hydrogen bonding.  This makes a water tank useful as a thermal capacitance for a heat pump.  I don't see the point of trying to pump viscous ice slush.  It would make more sense just storing the ice in a static tank and using the outside air and ground heat to remelt it.

I finished reviewing this document last night.
https://pmc.ncbi.nlm.nih.gov/articles/PMC9417644/

The paper is specifically concerned with production of iron nanoparticles, by reducing iron oxide in hydrogen gas at various temperatures and pressures.  At high pressure, it is possible to produce consistant spherical nanoparticles reliably at temperatures <300°C.

For our purposes, reduced iron is a precursor material for production of steel in electric furnaces.  I made a few notes of points that I found interesting.

'A100bar hydrogen pressure is sufficient for complete reduction at temperatures down to 270°C with 3 hours'.

Complete reduction occurs in 3 hours at the following temperature-pressure combinations.
1. 390°C &1bar.
2. 270°C &100bar.
3. 230°C &200bar.
4. 220°C &400bar.
5. 210°C &530bar.

Reaction rate is not a linear function of pressure as I had initially assumed.  But it increases rapidly with increasing temperature.

On Mars, we would apply this method to iron rich regolith.  After the iron is reduced to small metallic particles, it will be removed from silicate contaminants by magnetic separation, after the mixture has been crushed.

The reduction will take place in batches.  Finely milled iron rich regolith would be loaded into baskets.  The baskets will be preheated to ~300°C using solar heat in an oven.  Next, a crane will lift the basket into a pressure vessel, which will be jacketed in a water loop and also maintained at a 300°C.  The pressure vessel will then be sealed.  Solar heat will then be used to boil water, generating steam that drives a piston, forcing hydrogen through a one-way valve into the vessel at ~100bar.  The compression will heat the hydrogen to ~270°C and force it into the iron oxide dust with the basket inside of the pressure vessel.  The water jacket around the vessel will continue heating the vessel and its contents as the hydrogen reacts with the iron oxide producing reduced iron.

After a 3 hour cook time, the vessel is vented to the Martian atmosphere, opened and the basket is removed.  The basket is then allowed to cool and is then emptied onto a conveyor.  The mixed iron and silicate mass is then milled, allowing crude iron powder to be seperated from the silicate slag using an electromagnet.
******

Additional: By my calculations, we need about 10MJ of electrical energy to make sufficient hydrogen to produce 1kg of reduced iron.  The remainder of the energy needs can be met entirely by solar heat in the 270-300°C range.  I havn't calculated how much solar heat will be needed yet.  But I think the liklihood is that we would be looking to build our iron works in a region where there is both a source of natural hydrogen or methane and iron rich ore or regolith.  Iron powder will be converted to steel in an electric furnace close to the base.  So it would be advantageous to build the plant such that it can be packed up and assembled at a suitable site.

This link provides data on the average composition of Martian soils.
https://ntrs.nasa.gov/api/citations/201 … 005414.pdf

The alkali and alkali earth oxides will not react with hydrogen.  Not sure about the manganese and chromium oxides.  The crude iron that is produced will need to be melted and treated using the basic oxygen process, which will burn off sulphur and phosphorus impurities.  Carbon can be added to reduce melting point.  High carbon cast irons can melt at temperature as low as 1200°C.  There are plenty of direct uses for grey iron.  For high quality steels melting temperature will be 1400 - 1600°C.  The higher end is for low alloy steels and mild steel.  Low alloy steels will dominate demand on Mars.  But cast irons will have significant uses, especially in structures and machine housings.

TH, I couldn't say at present.  The bins will need to be handled by crane, but this is unlikely to be a limiting factor for batch size.  The size of the batch will be limited by the volume of the pressure vessel.  If PVs are made from alloy steel, the wall thickness will increase as enclosed volume increases.  Beyond a certain point, it becomes very difficult to weld.  We could use concrete pressure vessels with steel stress carrying tendons.  It is possible to build much larger vessels in this way.  Pre-stressed cast iron vessels can take higher pressure, as crush strength of iron is much greater than that of concrete.  The higher the pressure, the more rapidly the iron oxide will reduce into pure iron.

In chemistry, reaction rate is proportional to reactant concentration, which at constant temperature scales linearly with pressure.  Double the pressure and reaction rate doubles.  Reaction rate will be governed by the Arhenius equation.  This states that reaction rate increases exponentially with temperature: R = e^kT.

This reference discusses CO2 dissociation in ceria tubes at temperatures of 1600°C.
https://www.sciencedirect.com/science/a … 4721016119

This might be tough to achieve with concentrated solar heat.  The concentration factor must be extremely high.

The raised weight hydraulic accumulator is an old energy storage technology that could see use in the future.
https://en.m.wikipedia.org/wiki/Hydraul … sed_weight

It is a form of gravity energy storage in which a raised mass is used to pressurise hydraulic fluid in a cylinder.  The amount of energy stored is m x g x h, where m is mass in kg, g = 9.81 and h is the height that the weight is raised.  A 100 tonne weight raised to a height of 10m will store some 9.81MJ, or 2.73kWh.

The downside of this technology is a relatively low energy density.  This will make it expensive to install.  The advantages are simplicity, use of simple materials like steel, concrete and stone, a potentially high discharge rate and extreme longevity.  Raised weight accumulators installed in the 19th century were still in use in the UK until the 1970s.  They were retired because electrically driven systems replaced the entire hydraulic network due to commercial availability of electrical systems.

Hydraulic accumulators are charged by filling them with hydraulic fluid under pressure.  This system is compatible with a hydraulic power transmission system.  This is useful in applications where mechanical power is needed in static machinery.  Hydraulics allow power transmission without the use of electricity.  The systems needed to produce pressurised hydraulic fluid are mechanical pumps.  These could be simple positive displacement piston pumps, driven by direct mechanical wind turbines.  When the wind level is low, the pumps will operate more slowly.  The end use is a turbine that converts the mechanical energy of a moving fluid into rotational energy for a machine.

A modern hydraulic power network could use a combination of wind and solar energy to provide a more continuous supply of mechanical power, which can be buffered by the hydraulic accumulators.  If solar PV is used, the PV network could produce pressurised hydraulic fluid using piston pumps driven by DC motors directly coupled to the panels themselves.  This removes the need for inverters, as the speed of the DC pumps will vary as the power output from the panels varies.  It also eliminates the need for electrical power transmission, as pumps can be installed at regular intervals along a line of photovoltaic panels.

Accumulators are most suitable for relatively short term energy storage.  For periods were there is little or no sun or wind, such a system would need either a backup power supply like a DG or would need to rely on curtailment.  One way of achieving this would be to oversize the system and have some functions on floating switches.  For example, heat pumps could be activated when power levels were high, with hot water stored in insulated tanks.  Grinding and polishing operations can be switched on and off, provided that total output is sufficient over a long period.  With careful organisation, industrial processes could work on an intermittent power supply.

This might be a system that we could put into use on Mars as well.  Due to the lower gravity, accumulators would only store 2/5 of the energy that they would on Earth.  But the system is simple in ways that would make it easier to make, with limited available materials.  Hydraulically powered machines are simpler than electrically powered systems and static hydraulic networks and machinery can be produced almost entirely from low alloy steels.

I like the solar pond idea.  Assuming that the top of the pond achieves a maximum temperature of 10°C, the required pressure to prevent boiling is 1228Pa.  That is effectively double atmospheric pressure (610Pa) at Mars datum.  In the northern lowlands, atmospheric pressure is greater.  So the structure covering the pond needs to be able to withstand a maximum differential pressure of 618Pa, or 13.1lb/ft2.  That is low enough for a tempered glass paned, metal frame greenhouse structure to serve as cover.  This has the advantage of being significantly more abrasion resistant than a polymer membrane.  It is also insensitive to UV damage.

The pond is divided into two distinct layers.  A dense brine layer sits at the bottom.  A lighter brackish layer floats on top.  The denser brine can be warmer, as it cannot rise by convection through the lighter brackish.  A small ion exchange filter could be used to maintain the salinity gradient within the pond.  A thin layer of polyethylene could provide a physical barrier between the two layers.  This has the additional advantage that a convective boundary layer will form on both sides of it, providing additional insulation to the warm water underneath.

A solar pond would work better on Mars, as the lower gravity would tend to weaken thermal convection.  The lower the gravity, the less efficient convection is at removing heat.

In biomining, we are replacing direct acid leaching with bacteria that excrete acids to do the same thing.  It is an interesting approach.  There are both direct and indirect methods.  The direct method involves injecting the bacterìa into the ground in a solution containing their food source.  The fluid is then withdrawn, ions are seperated by ion exchange membranes and the solution is reinjected along with fresh energy source (i.e acetate).  In the indirect method, the bacteria are grown in a vat.  They metabolise sugar or acetate producing an organic acid, which is then seperated and injected into the ground.  I don't have a strong opinion of which is best.  The indirect method allows bacteria to grow under more controlled conditions.

Many low grade uranium ores are mined using insitu acid leaching.  So the method does come with operational experience.  It will recover a lot of other metals along the way.  The only problem I can see with doing this on Mars is the cold.  The fluid must be heated prior to injection.  Freezing point suppression with salts won't work, because brine becomes progressively more viscous at temperatures below zero.  It may technically still be liquid, but its viscosity will be comparable to treacle.

These are interesting ideas Void.  We definitely need some creative approaches to steel production on Mars.  If we have to reduce iron using electrolytically derived carbon monoxide, then each kg of molten iron requires 15.51kWh of electricity.
https://www.sciencedirect.com/science/a … 6522003460

That is about the same energy intensity as aluminium on Earth.  It isn't an attractive option unless electricity is cheap.  Which it won't be for while to come.  Using bulk sources of biomass like blue green algae is an attractive option.  It will be easy enough to dry the material by exposing it to Martian atmospheric pressure.  Human waste and refuse is something that could be used as well.