jgourlay
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Seperation of metals: how does this work?
Gents, could you all sketch out for me the general dimensions of the snake pit when it comes to separation of metals from metals? For example, I was
reading a little about Germanium and the source said it was a zinc impurity present at levels between 2% and 4%.
And I'm thinking, "how in the hell do you get that out of there?"
So....how in the hell DO you get things like that out of there. Ditto for traces amounts of silver in lead/copper and on and on.
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blogfast25
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That's such a broad question that only a very broad answer can be given.
In essence the separation of metals is done much in the same way other elements are separated form each other: by relying on differences in their
chemical properties, often supplemented by differences in their physical properties.
As regards difference in chemical properties possibly the most often used method relies on differences in solubility. The solvents par excellence for
metals are the acids, primarily the mineral ones (hydrogen halides, nitric acid, sulphuric acid and other oxo acids and some others). Off the top off
my head Germanium would in concentrated H2SO4 oxidise to GeO2 but not dissolve. Zn however is soluble in conc. H2SO4.
The difference in solubility of some metal compounds in water is another example: Ni and Cu can be separated because the difference in solubility of
their sulphides (despite both being classed as 'water insoluble').
Another method based on solubility is based on amphoterism: some metal oxides are soluble in strong acids as well as strong alkali (these are known as
amphoteric). Since as Al is amphoteric but Fe is not, by treating raw Al ore (Bauxite: crude alumina) with strong alkali the alumina can be separated
from the contaminating Fe2O3.
Solubility is also affected by complexation: Zn2+ and Cu2+ (e.g.) form strong, stable complexes with ammonia, which allows the separation from
elements that cannot thus be complexed. Another complexing agent is cyanide (CN-), good for complexing a whole range of metal elements.
General differences in solubility in various organic solvents is also often exploited: acetone dissolves FeCl3, useful to get rid of Fe contamination
in some cases.
The use of differences in physical properties is also important: differences in density can be used in flotation separation, difference in boiling
points can be used to separate by distillation (distilled calcium, distilled magnesium).
All in all the separation of the metals should be seen simply as the separation of chemical elements.
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jgourlay
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Blogfast, thanks. That's exactly the degree of depth and detail I was looking for: a good overview.
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JohnWW
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I presume that you want to extract the germanium from raw, as-smelted but unpurified, zinc, as germanium is rarer and more valuable than zinc.
According to http://en.wikipedia.org/wiki/Germanium , germanium is obtained primarily from the mineral sphalerite, ((Zn,Fe)S), which is the chief ore of zinc,
though it is also recovered from silver, lead, and copper ores. It was discovered comparatively late (by the famous German chemist Winkler in the
1880s) because very few minerals contain it in high concentration. However, there is a rare mineral, germanite, Cu26Fe4Ge4S32, that has a high
concentration of it, found in polymetallic sulfide hydrothermal deposits, the original and major source of it being the Tsumeb mine in Namibia,
formerly known as German South-West Africa until 1918, but it also contains gallium, zinc, molybdenum, arsenic, and vanadium as impurities; see http://en.wikipedia.org/wiki/Germanite .
Germanium could be separated from unpurified zinc through the fact that is insoluble in dilute acids and alkalis (although dissolves slowly in
concentrated sulfuric acid and reacts violently with molten alkalis to produce germanates ([GeO3]2− ). Separation from a strongly alkaline
solution of zincate and germanate, by varying the pH, seems also to be a possibility. However, impure as-smelted Zn, obtained from sulfide ores, is
liable to also contain Cd and Ga.
[Edited on 22-6-10 by JohnWW]
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12AX7
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AFAIK, the primary method for purifying zinc is distillation. Boil it off (this occurs at a fairly pedestrian 907C, red-orange hot) and condense.
Lighter impurities (Mg, Cd, etc.) come off sooner, heavier impurities remain (Al, Cu, Fe, Ag, etc.).
Electroplating is also used, probably more common with Pb and Cu and such than Zn. The impurities either dissolve (e.g., SeO3), precipitate (Al,
Sn?), or slough off (Ag, Au, etc.), while the majority material (e.g., Cu) plates across happily. What impurities remain (anode sludge and dissolved
materials) are processed for values.
Tim
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not_important
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Electrorefining is increasingly used with zinc, for the recovery of germanium, indium, and other rare metals. believe it can be more energy
efficient than distillation, although higher in capital costs.
Adding to 12AX7's comment, electrorefining is basically electroplating. The idea is to use near the minimum voltage needed to get the target metal to
deplate from the crude metal of the anode, and replate on the cathode. Ideally more electropositive metals will not plate out from the electrolyte,
and less electropositive metals will not dissolve from the anode; in practice this isn't always so, and additional means are needed, A contaminating
metal may form an insoluble compound with a reagent in the bath and precipitate out, or a complexing agent may bind it so tightly that it doesn't
plate.
Electrorefining tends to give 3 product sets, the purified metal, the stuff that stays in solution, and the stuff that doesn't dissolve. The last two
are generally mixtures, with trace elements now in high enough concentration to allow their separation. The anode mud from refining copper would be
smelted and cast into anodes of high silver content, the anode mud from refining those would be worked up for gold and platinum metals.
Solvent extraction is also used. Indium is extracted from acidic chloride solutions into organic solvents as a chloride complex; the trick is to
adjust parameters to get as much In as possible without extracting excessive amounts of Fe.
Distillation is used for other metals than zinc, germanium as GeCl4 is an example, and V(+5) as its lower alkoxides has been tried.
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jgourlay
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Not_important: the problem with your explanation is that it's sufficiently detailed to make me want to try it!
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blogfast25
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@ jgourlay:
You're the home schooler, right? Seems to me you've caught the chem bug a bit... 
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not_important
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jgourlay - it would be easy to do a demonstration type set-up, noting that there's some real tuning needed to get things right.
Gunmetal AKA red brass is a type of bronze, in this case an alloy of copper, tin, zinc, and often lead. G bronze/Copper Alloy No. C90300 is 88%
copper, 8% tin, and 4% zinc; U.S. Government bronze specification G is composed of 88% copper, 10% tin, and 2% zinc. You should be able to tune
conditions so as to plate fairly pure copper, with zinc remaining in solution and tin possibly remaining as anode mud.
It would be a bit tricky, electroplating style electrorefining is typically used where the main metal is in quite high concentration, 95% or better.
In the case of copper it produces high purity copper with good electrical and physical qualities, and allows the recovery of the precious metal
content - metals present in only fractional percentages.
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ScienceSquirrel
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Mood: Dogs are pets but cats are little furry humans with four feet and self determination! 
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Zinc is one of the purest metals available over the counter to the home chemist, at least 99% for zinc sheet used for roofing.
The source zinc ore may contain traces of silver, germanium, etc but after distillation or electro winning they are profitable by products and the
zinc sold to the customer is really pure zinc!
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