I. e., what if we just burn trash completely or dissolve it in acid, get the resulting paste and extract pure chemical elements out of it by applying some physics and/or chemistry to it until no more can be extracted? Just curious.


To be perfectly clear: my main concern is whether there are any means to extract any useful element from this bulk of burned/dissolved/blender-ed trash. For example, we burn with Thermite the whole contents of some waste disposal site, until it's only multicolored ash there. Or pour the HSbF6 over it. From there, can we get the pure carbon out of the resulting mass? Pure aluminium? Pure iron? Gases, maybe, like oxygen?


Sure, this absolutely can be done. In particular, you cannot destroy elements using conventional means like burning, so all the initial elements are still in the mass. The question is whether or not this is a good way to extract what you want. There may be better chemical techniques to do the extraction that are short of burning. The branch of chemistry which deals with the optimal conversion of elements and chemicals to other useful substances is chemical engineering, and figuring out which process is "best" (e.g. fastest, cheapest, highest yield) is crucial for industry.

There is a very funny story related to this idea on the Periodic Table of Videos. A scientist was trusted with the entire UK supply of plutonium (10mg, a very small amount) during World War II. He accidentally spilled it on to his lab bench. He then took a saw, sawed a large circle out of bench, burned that wood, and recovered nearly all the plutonium! So yes, this works. Here's the video, and this story begins around 15:30: Plutonium Video

EDIT: As to your update, there are lots of methods available, but it depends greatly on the starting conditions and materials. For example, burning is a great way to extract carbon: burning hydrocarbons or carbohydrates burns off the hydrogen or water (respectively) and leaves mostly pure carbon.

But extracting metals might take more work. Often, metals exist as ionic compounds. For example, silver nitrate, $\ce{AgNO3}$, contains siver $\ce{(Ag)}$ in the +1 state. While it might be possible to dissociate the silver with high enough temperature, it is far easier to extract metallic silver by reducing silver in the presence of a more reactive metal. "More reactive" simply means a metal that will more easily give up an electron to the less reactive sliver, changing the silver ion $\ce{Ag+}$ to metallic $\ce{Ag}$. Silver nitrate is highly soluble in water, and if you simply drop in a piece of metallic copper $\ce{(Cu)}$ metallic silver will slowly come out of solution and plate the copper with silver, while copper dissolves and creates copper nitrate. The equation is:

$\ce{2AgNO3 + Cu->2Ag + Cu(NO3)2}$

but the crucial part is the ionic equation:

$\ce{2Ag+ + Cu->2Ag + Cu^2+}$

which shows that the silver ion is reduced to silver, and the copper metal is oxidized to copper ion.

Again, Periodic Videos has the footage of this exact reaction: Silver Video

Now I picked this example precisely because of this footage, but it's not the best example because silver is so low in reactivity that it exists metallically in nature (i.e. you can find lumps of silver if you are lucky!) But many metals are so reactive that it takes a huge amount of energy to extract them. Examples are sodium or potassium, which are so reactive they explode in the presence of water (go search YouTube and have a field day!). To extract them as metals commercially usually requires electrolysis, which uses electricity to force electron transfer which would not otherwise occur spontaneously.

And on and on with other methods. There is no universal extraction method, at least not yet, and this is why we have chemcial engineers to figure out better and better procedures to use.

  • $\begingroup$ I've updated my question with more elaborate explanation of my idea. If there really are such methods, why they don't implement them everywhere? $\endgroup$ – hijarian Mar 26 '13 at 13:30
  • $\begingroup$ Can you, please, extend your consideration about the universal extraction method? What do you think are possible choices? Radiation with proper signal? Changing structure of the substance and then doing something? $\endgroup$ – Léo Léopold Hertz 준영 Dec 7 '14 at 19:28

Sounds like you're referring to plasma waste conversion. If you heat matter to 4-figure temperatures all molecular bonds are broken and you have a gaseous mass of dissociated atoms. It is theoretically possible to extract the constituent elements from the plasma.

If you want to preserve a particular element then the cooling of the plasma has to be controlled so that the element does not come into contact with others with which it naturally reacts until it is below the threshold temperature for those reactions.

Fun bonuses: Not only can you recover specific elements from homogeneous waste streams, but plasma waste reactors can also run with an energy surplus created by the "burning" of energy-rich inputs. They can also be designed to produce synthetic gas from organic feedstocks.

  • $\begingroup$ Amazing! So, it seems that such technology exists at all! Exactly what I was curious about the most. I think it's very unprobable, though, that this method will become widespread. I suspect energy requirements for manipulations with positions of just the elements we're interested in inside the plasma stream would be enormous. $\endgroup$ – hijarian Jul 28 '14 at 16:21
  • $\begingroup$ My original thought was about liquefying the waste, not torching it, however. :) Less stuff blowing up, which is safer. $\endgroup$ – hijarian Jul 28 '14 at 16:25
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    $\begingroup$ The problem with liquefying, blending, or anything short of molecular disintegration of "waste" (i.e., heterogeneous compounds of indeterminate composition) is you can't decompose everything to elements. Plasma waste conversion is just one step beyond incineration, which is already widely used on both municipal waste and weapon stockpiles, including chemical weapons! There's no problem with things "blowing up" when that's the objective. The thing that isn't widespread is elemental extraction from general waste plasma, probably because general waste doesn't contain enough valuable elements. $\endgroup$ – feetwet Jul 28 '14 at 21:58

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