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I have multiple questions linking into a potential recycling method; I'm curious if any of these ideas are possible, or if I'm having a pipe dream:

  • Can any gas be turned into plasma?
  • Can multiple types of gases together be turned into plasma at one time?
  • Can plasmas be turned into another state of matter?
  • In a plasma state, do like molecules attract?
  • Do plasmas of different elements have different densities?

I was thinking about a recycling system where trash is burned and the resulting toxic gases are captured and ionized into plasma. In the plasma state, they are piped to another storage area where they are sorted into like elements based off density.

If it is possible to change the state of matter for such waste products, is it then possible to take the resultant distilled elements and change them back into useable solids/liquids/gases?

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Can any gas be turned into plasma?

Yes, in principle, any element can form a plasma. For instance, the sun's outer atmosphere, called the corona, contains iron in multiple charge states as a plasma constituent.

Can multiple types of gases together be turned into plasma at one time?

Yes, again the solar corona is a perfect example of this. It is comprised of hydrogen, helium (both singly and doubly ionized states), carbon, nitrogen, oxygen, iron, etc. The latter heavy ions are found in multiple ionization states.

Can plasmas be turned into another state of matter?

Yes, if you reduce the energy of the system, the matter will eventually undergo recombination to form neutral particles without further external work done on the system. From that point, the normal phase transitions expected from thermodynamics are expected.

In a plasma state, do like molecules attract?

No, not in general. In a plasma there is a principle called quasi-neutrality whereby the charged particle fields act to cancel each other out over distances larger than the Debye length. Thus, two fully ionized iron nuclei are not likely to attract each other but rather repel each other due to their large electrostatic potentials.

Do plasmas of different elements have different densities?

I am not entirely sure what you are asking but I think the answer is no and yes. The number density of a plasma is not necessarily determined by the constituent species but rather the boundary conditions of the system. The center of massive stars can be considered plasmas of heavy elements and are much denser than corresponding outer layers of lighter elements during the latter part of their lifetimes. Early in their life their cores are often more heavily populated by light elements like hydrogen and helium with extremely high number densities.

The mass density, however, will differ from one mass species to another for the same number density, of course.

In the plasma state, they are piped to another storage area where they are sorted into like elements based off density.

They would not be sorted by density like oil and water but rather more likely by mass-to-charge ratio using magnets or centrifuges.

If it is possible to change the state of matter for such waste products, is it then possible to take the resultant distilled elements and change them back into useable solids/liquids/gases?

If energy were not an issue, then yes, in principle, this would be a possible procedure for neutralizing and eliminating toxic waste. The problem is that many toxic waste products are composed of complex compounds which would first be dissociated prior to any ionization, which alone would require tremendous amounts of energy for any industrial-scale application. To then ionize the dissociated parts would require, in most cases, even more energy.

The typical energies here are hundreds to thousands of kJ/mol for dissociation and ionization. Industrial-scale applications would involve millions to billions of moles of toxic materials, thus billions to trillions of Joules just to get to the ionization stage (this is probably a weak lower bound). These estimates all ignore containment and efficiency of the dissociation and ionization processes (the latter two of which are almost certainly not 100%). My quick, hand-wavy guess would be that such a factory would consume power in the range of megawatts, i.e., the amount produced by a typical nuclear power plant.

In summary, unless the price of energy drops precipitously and the need for such a system becomes paramount, I doubt this is currently feasible.

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  • $\begingroup$ Thank you for the excellent breakdown, I really appreciate it! $\endgroup$ – Samuel Knight Johnson Dec 20 '18 at 1:53
  • $\begingroup$ @SamuelKnightJohnson (Fyi, once somebody posts an answer you like, it's customary to mark their answer as accepted - there should be a checkmark on the left you can click, I think.) $\endgroup$ – Erhannis Jul 9 at 20:45
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Any gas can in principle be turned into plasma, by decomposing all of the molecules and separating the electrons from the nuclei. The temperature at which this happens is generally very, very high, and differs based on the particular gas involved. Multiple gases can be turned into a plasma; just raise the temperature above the highest threshold for any individual gas. Plasmas can also be created using very high electric fields (again, the electric field threshold is different for every material).

Plasmas can be turned back into gases by allowing them to cool down or turning off the electric field, at which point the electrons and nuclei recombine into atoms and the atoms recombine into molecules (assuming that the original gas was not a noble gas, in which case the gas remains as free atoms, and assuming that the density is high enough that interactions between atoms are common. Interstellar gas clouds often contain free atoms or "incomplete" molecules because they are below this density threshold). Further state changes can be achieved by manipulating the temperature and pressure of the material; the specific points at which these state changes occur can be found by consulting a phase diagram.

There are no molecules in a fully-ionized plasma.

The density of a plasma is dependent on several things: the total number of nuclei and electrons, the mass of the nuclei involved, and either the volume of the container (if the volume is fixed) or the temperature and pressure of the plasma (if the volume is variable).

The process you are describing is similar to the basic operating principle of a mass spectrometer. They are often used in analytical chemistry for determining the composition of a product.

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