First I would like to thank Steven Cooke for his response last evening to my below query...
"Could someone suggest an inexpensive substitute for alcohol to be added to water in a float/sink separation tank to lower the density? Thank you."
I think I incorrectly assumed lowering the density was our solution. What we are trying to do is float/sink separate polypropylene from rubber. Currently @ 98% of the rubber is sinking with just water and a surfactant...so not a density issue. Think there are tiny bubbles or air pockets in the remaining pieces that keeps them afloat. Adding the alcohol immediately causes the remaining rubber to sink...might it be affecting the surface tension or something else? Regardless what might be a substitute for the very flammable alcohol we don't want to use? Thanks again.
You're welcome. It sounds like you have gotten into a common situation. You have assumed a possible solution and asked a direct question (good), but it is NOT the solution because you didn't really ask the correct question. We do normally go for relatively shorter, factual replies here, and relative densities or other properties or equation explanations are some of those.
However, it sounds like you are really in need of a more customized process improvement consultation than a simple factual answer based on your perception of the system. What IS the desired outcome of the process? For example, why isn't 98% yield good enough? Secondly, yes, that still does have to do with density, if not in quite the way you were first thinking of it. You can still refer to the lists I linked earlier to see what fluid options you may have.
If the problem is adhered or entrapped air bubbles in a small fraction of the plastic pieces, then the use of an ultrasonic wave (relatively inexpensive in the long run, and very effective) in the bath would eliminate them and allow the normal density of the plastic to help it to settle out.
Beyond that (or including that) I would recommend that you look for a consultant to help you improve the overall process, as there may be many other things that you aren't thinking of and that I can't imagine without looking at the physical process. Even your concern for flammability may be based on erroneous preconceptions (or practice) in how you operate the process.
Sounds like you need a highly efficient wetting agent. Surfactants can do this somewhat but you already have that. Alcohol as you have determined lowers the surface tension of water considerably. Lowering the pH with sodium hydroxide will lower the surface tension as will trisodium phosphate (TSP). Depending on the fate of your water TSP might pose a disposal concern. TSP has been removed from most detergent formulations for waste water considerations.
You might look into the Surfynol line like 465 or 485. These are also highly efficient wetting agents that can dramatically lower the surface tension of water.
Best of luck,
In addition to the two good answers you have already received I would like to add that you are practicing something very close to a well developed industrial process called ore flotation.Froth flotation - Wikipedia In it, different minerals are separated by mixing the ore with a surfactant solution and agitating it to make foam. Depending on the surface properties of the various ore constituents, surfactant will adsorb differently to different mineral surfaces. The alteration of the surface due to the surfactant is usually to make a hydrophillic surfaces into a hydrophobic ones making it more attractive to air bubbles. The air bubbles then float the hydrophobic surface minerals (whether they are naturally hydrophobic or have become so due to surfactant adsorption) and those minerals that are hydrophilic sink. The foam is then collected and the floating minerals separated from the sunk ones.
I would definitely second Drew's suggestion that you might want to try different surfactants although for a different reason. Beyond the change in the surface tension of the overall solution you will also be getting adsorption of the surfactant to the plastic and rubber particles. You want to encourage the polypropylene to float as much as possible and the rubber to sink. In this case I'm thinking you want more surfactant adsorption on the polypropylene than on the rubber. (You don't mention what surfactant you're using now so I don't know what to suggest.) One of the things that might be diagnostic is measuring the zeta potential of the two kinds of particles (Zeta potential - Wikipedia) - if one is more electronegative than the other it would be more susceptible to the adsorption of cationic surfactants, and more electopositive ones susceptible to adsorption of anionic. You can also do zeta potential measurements of particles taken from your surfactant solution to see if one of them has a bigger difference in zeta potential (pre versus post treatment) that then affects their behavior in the separation process. Drew's suggestions are nonionic so I'm not sure if they would improve the situation or not (as regards adsorption on the particles - he's right about the effect of decreasing the overall surface tension).
Of course Steven's suggestion of using a sonicator to completely eliminate any bubbles that might be floating the last bits of rubber might be just the easiest way to go. There is still the possibility that a different surfactant than what you're using now might also enhance even that especially if you used one that was a poor foamer. (And there again Drew might be taking you in the right direction - nonionics are generally much poorer foamer than anionics.)
Good luck on your enterprise.