blogfast25
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Filtration hell…
I’ve been working with this silicate based mineral (Beryl), which involves fusing it with KOH, then leaching out the KOH (or at least most of it),
followed by a sulphuric acid treatment of the alkali fused Beryl. This leaches out the Al and Be as sulphates but also converts the silica to that
highly gelatinous ‘SiO2.nH2O’. The hydrated silica and Be/Al bearing liquor then needs separating. One handbook on Be processing mentions
centrifuging and I’m beginning to see why.
This slurry of gelatinous (and highly voluminous!) hydrated silica is a nightmare to filter: it clogs up the pores of normal filter paper extremely
quickly. Hot filtering and even vacuum assisted (granted that my vacuum isn’t very high) didn’t really work: the filtration simply comes to a full
stop after a while.
I then tried to change the consistency of the hydrate by boiling the crap out of it, reasoning that ‘SiO2.nH2O’ might be less gelatinous if n is
smaller. It didn’t make a dent in it!
The only thing that seemed to work was to flip the pH from <<7 to about 14: the consistency of the slurry became much thinner. Unfortunately
this isn’t really an option for me…
Anyone have any experience dealing with strongly hydrated silica gels?
[Edited on 16-2-2013 by blogfast25]
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bahamuth
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Vacuum filter it through a bed of clean sand (or diatomaceous earth if it stands up to the acidity, I have no idea if it does), and at least one can
disturb the bed a little when it clogs to get it flowing again.
Or (as I guess you don't have access to a centrifuge) fill it a bottle, weigh it, make another dummy bottle the same weight, tape both bottles to the
spokes on a bike rim, with the bike on the side or inverted and pedal away with your hands I actually did this with 50ml tubes to separate a very fine grained amine HCl salt which took forever to settle and it worked like a
charm.
Or you could make a home made centrifuge by various means, as you probably only need like 10-50X rcf to get it to settle fast.
Any sufficiently advanced technology is indistinguishable from magic.
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smaerd
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Though I have no experience with your particular situation, I have delt with awful filtrations. One thing I found to help was using cotton balls in a
buchner funnel(vacuum on). Filtering small amounts to approximate dryness. Replacing the cotton balls then going at it again until it clogs. Over and
over and over and over. Diatomaceous earth sounds like the winner here, basically the same idea but a lot more manageable, again I have no idea how it
may react with your mixture.
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S.C. Wack
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First thought is just a kitchen funnel with glass wool and marbles. Who knows.
But hey...maybe after sitting around for a couple weeks with shaking every so often, it might change...the compact white powdery kind, not Obama's...I
do this for some three-quarters-ass chromatography, after neutralizing silicate with a lot of shaking.
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zed
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Try reacting boiling solutions from the get-go. Larger particles of SiO2 may form initially. This may be similar to the old Ag+Cl problem.
Damned hard to filter the ppt. of a "cold" reaction. Too damned fine. Once ppt. has formed, coagulation by boiling, works poorly.
Similarly, the reaction of Ca with Phosphate. When you react cold solutions you get un-filterable colloidal crap. If you react boiling solutions,
because of the increased solubility...crystals form more slowly, grow larger, and fall right out of solution.
Easy to try. More practical than centrifuging.
[Edited on 16-2-2013 by zed]
[Edited on 16-2-2013 by zed]
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blogfast25
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Thanks all, so far.
Zed: the silica was in fact formed from very hot solution, close to boiling. At first when I poured the 95 % H2SO4 on the cold alkali fused, still wet
Beryl, much heat of neutralisation heat formed but no gelatinous SiO2. At that point I must have had actual H2SiO3 (A.F. Holleman describes how it can
exist) in solution. On dilution with cold water the hot solution then dropped the gelatinous SiO2. I have a feeling that avoiding using concentrated
H2SO4 might offer solace: by adding the acid to a thinner slurry [more water] of the fused Beryl and by adding it more slowly.
[Edited on 17-2-2013 by blogfast25]
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AndersHoveland
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What about trying to dissolve the beryl ore with hydrofluoric acid, or some combination thereof ?
All the silicon should be removed as SiF4 fumes, and then you could neutralize the remaining fluorides by adding a solution of calcium chloride
(forming a much safer insoluble CaF2 precipitate).
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blogfast25
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Quote: Originally posted by AndersHoveland  | What about trying to dissolve the beryl ore with hydrofluoric acid, or some combination thereof ?
All the silicon should be removed as SiF4 fumes, and then you could neutralize the remaining fluorides by adding a solution of calcium chloride
(forming a much safer insoluble CaF2 precipitate). |
You trying to get me killed? 
If the choice is between a 20" moderate heat fusion with KOH to render the ore acid soluble on the one hand, and messing with at least 43 % HF on the
other hand, then that choice is made easily, as far as I'm concerned.
To be fair, methods involving fluoride fusion, in particular with the more 'innocuous' [cough!] Na3FeF6, have been deployed to digest Beryl in the
past. Today the prevailing process appears to be the Degussa process, which renders the ore acid soluble by means of complete vitrification by
quenching the molten ore with cold water.
[Edited on 17-2-2013 by blogfast25]
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blogfast25
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The filtration problem was solved when I read something about a counter-current process to leach the acid solubles from out of the gelatinous hydrated
silica.
Now I will simply decant off the supernatant liquor first, carefully so as not to disturb the gel too much. This liquor will first be strained using a
muslin cloth or plastic tea strainer to avoid gel hitting the filter. The rest of the acid soluble will be leached out by adding hot 5 % to the
remaining jelly and stirring vigorously, then decanting off the supernatant liquor again as above. A bit tedious but it should do the trick…
[Edited on 17-2-2013 by blogfast25]
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smaerd
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Please report back success or failure 
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Fantasma4500
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you could try pressing it with some metal filter of some sort and then decant off the liquid (this is used in some coffee machines, hot water in,
coffee stuff in and then you trap the particles on the bottom with some metal thing you press down and you can pour coffee without any particles in
but i guess if centrifuging it is used industrially it might not be that easy
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blogfast25
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| Quote: Originally posted by Antiswat | you could try pressing it with some metal filter of some sort and then decant off the liquid (this is used in some coffee machines, hot water in,
coffee stuff in and then you trap the particles on the bottom with some metal thing you press down and you can pour coffee without any particles in
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It's unlikely to work. Unlike in your coffee analogy, the water molecules in the gel are somewhat bound to the silica. That makes forcing them out
much harder than in the case of coffee dross.
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