Steve, you can use hot lye or potash solutions to dissolve silica. Alternatively, consider using ammonium bifluoride (''dry'' HF).
Story time:
Let's just say that I was dissolving some PGM sponge material that a member here had sent me. Everything was going fine but I kept seeing a recurrent precipitate that reminded me very much (it was a milky suspension) of something called Cab-O-Sil, which is basically fumed silica powder, a very, very fine powder. I removed it through microfiltration processes and this gummed up my filters and really ticked me off. What should've been a day's work became a week long endeavor.
I did a powder XRD on a sample of the material and confirmed my suspicions. I also dissolved a sample in HF and played around with it to see if I couldn't hydrolyze it back out in the filtrate but that's another story and another problem...
Anyway, long story short is that the zinc that many people here are using is contaminated with silica, SiO2. Steve's comment here in this thread helped me make the real causal link and establish that this is something that other people are experiencing rather than only the client who sent me his precipitates. It was the straw that broke the camel's back so to speak. I'm used to using pure zinc powder out of a supply house like Fisher--and that comes with complete analysis and any additives are noted. It's also murderously expensive to get it shipped (1200 mesh zinc is considered pyrophoric) so I hadn't considered possible reagent contamination, rather I thought it was residual artifacts from his other processes (which some still is!).
Now that I see others experiencing this phenomenon, I know what its cause is-- that fine silica powder decreases flammability and flowability and makes the material much easier to ship. It also makes it cheaper by adding bulk. I'm so thoroughly vexed at having my time wasted by these damn scoundrels selling inferior products to my customers and friends that I'm considering buying 100# of good material and parting it out!! Zinc is the premier material for dropping values. It washes out nicely, unlike iron and copper, it's also relatively cheap, and very fast, and also about idiot-proof.
Again, thank you Steve for confirming my suspicions and you are right about garbage in, garbage out. I realize that not everyone has access to quality, standardized chemicals, but there is something beautiful about not needing to worry about what reagents you're using.
Lou
