metalresearcher
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What is 'acid' and 'base' in metallurgy?
In several publications and websites about metal refining is spoken about 'basic' and 'acid' slags. But acid means a low pH (<7) and base a higher
pH (>7) in aqueous solution as far as I know.
Slags of over 1000°C do not have H+ or OH-, so what do these terms mean?
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Hexavalent
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http://www.nptel.iitm.ac.in/courses/113104059/lecture_pdf/Le...
"Success is going from failure to failure without loss of enthusiasm." Winston Churchill
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12AX7
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Correct, it obviously isn't pH; but conveniently, the same rules still apply, broadly speaking.
Anything from the right hand side of the periodic table is acid, left is basic. Lower oxides are basic, higher oxides are acidic.
Stronger bases tend to make lower-melting slags, while neutral oxides are either inert (thickening the slag, or adding filler to ceramics) or
"amphoteric" (e.g., calcium aluminates vs. aluminum silicates).
Examples:
Acids (glass formers): silicate, borate, phosphate, titania (slightly), chromite (slightly) (note, Cr(II) and Cr(VI) generally aren't formed at these
temperatures)
Bases: alkali, lime, magnesia (mild), ferrous oxide, zinc oxide
Neutral, or nearly neutral: alumina, chromia, ferric oxide, zirconia
Phosphate glasses can be (are?) immiscible with silicate and borate glasses. Likewise, though the halides are stable at these temperatures (or... the
fluorides anyway... most chlorides boil around here), they aren't usually miscible either. (The most likely reaction would be: 2 NaCl + SiO2 + H2O =
2 HCl + Na2O.SiO2, hence chlorides tend to be corrosive.) Fluorides tend to be soluble enough to be useful, e.g., CaF2 to thin an alumina-silica
glass.
The motivating distinction for metallurgy is, bases react with sulfur and phosphorus in molten steel, improving low-temperature brittleness and high
temperature malleability. The "basic oxygen process" is mainly notable for its use of high pressure oxygen, but "basic" gets in the name for the
required use of magnesia brick lining the reaction vessel, and the basic slag used with it. The Bessemer process could be basic as well, although I
don't know if it ever was. Needless to say, a basic slag would corrode (dissolve) an acid (alumina-silica) lining very quickly.
Iron oxide, of course, wouldn't be a good thing to use in slag, because it'll dissolve into steel (reducing the carbon content, or adding oxygen), and
its oxidation state is subject to change by contact with metal and atmosphere. A pottery glaze rich in Fe2O3 might produce a lovely brown in an
oxidizing kiln, but because FeO has such a strong effect on melting point (comparable to ZnO and CaO), a reducing atmosphere might see it run right
off the pot!
I suppose a basic slag might be useful for bronzes as well, but they aren't as sensitive to sulfur and whatnot. And at lower temperatures, the
expense of basic refractory isn't really justified, so a basic slag would just be corrosive without much value..
Tim
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blogfast25
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Alumina, chromia and zirconia are better described as amphoteric. They form both stable anions and cations. E.g. zirconium salts are
plentiful but so are zirconates, including 'substituted' ones (fluoro, e.g.).
[Edited on 21-9-2013 by blogfast25]
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12AX7
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Yes. And it depends a bit, too, since some low-melting slags might not attack alumina (let alone anything with zirconia in it), in which case the
latter would be inert filler. But however you might view them, the correlation is definitely there.
Zirconia is one of the more interesting ones in that regard, because even ZrSiO4 has such a high melting point that it hardly dissolves in, or reacts
with, anything; blocks of, IIRC, zirconia, alumina and silica fused together are used in glass furnaces. They are so corrosion-resistant, a glass
furnace might run 20 years continuously on the same brick lining. And that's soda-lime glass, i.e., a low melting slag that just happens to be
transparent!
Tim
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