Faster IPA Recycling For Your Resin Print Workflow

If you’ve printed with photopolymer resins, you know that you need alcohol. Lots of alcohol. It makes sense that people would like to reuse the alcohol both to be environmentally responsible and to save a little money. The problem is that the alcohol eventually becomes so dirty that you have to do something. Given time, the polymer residue will settle to the bottom and you can easily pour off most of the clean liquid. You can also use filters with some success. But [Makers Mashup] had a different idea. Borrowing inspiration from water treatment plants, he found a chemical that will hasten the settling process. You can see a video of his process below.

The experimentation started with fish tank clarifier, which is — apparently — mostly alum. Alum’s been used to treat wastewater for a long time. Even the ancient Romans used it for that purpose in the first century. Alum causes coagulation and flocculation so that particles in the water wind up sinking to the bottom.

It isn’t as simple as just adding alum to the waste liquid. To encourage particles to coagulate, real waste treatment plants agitate the water and that is required here, too. For best results, the video says to mix a solution of alum and distilled water and then stir the alcohol, resin, and solution together, rapidly at first and then more slowly.

After a 45 minute stir, you’ll need to let it all settle for a few hours, but you can see large clumps forming almost immediately. In the video, he uses a magnetic stirring rig, but he also points out that if you have a wash station, it can do the same job. In fact, if you don’t have a wash station, this might be the justification you need to buy one!

One important note: the tests were done with pure aluminum sulfate (the right name for alum). If you buy alum at the store, it is likely to have potassium or other additives and that might change the results or even the safety of this procedure.

If you want to build a magnetic stirring machine, we’ve seen several homebrew units. If you wonder if you are diluting your alcohol with this procedure, you could test the proof of the resulting liquid.

28 thoughts on “Faster IPA Recycling For Your Resin Print Workflow

    1. “I’m unreasonably sad that this has nothing to do with hoppy beers…”

      But would you _want_ to recycle those beers quickly?
      (ewwww!)
      Yes, I know, you don’t really buy a beer/coffee, you rent it.

  1. “One important note: the tests were done with pure aluminum sulfate (the right name for alum).”

    I’d say that ‘aluminum sulfate’ is the right name for aluminum sulfate, not necessarily the right name for alum. ‘Alum’ doesn’t really refer to a specific material. Depending on who you are talking to ‘alum’ can either mean aluminum sulfate or (probably more commonly) one of the hydrated double salts of aluminum sulfate with another metal (like potassium). The potassium one is usually either called ‘alum’ or ‘potassium alum’.

      1. It’s not THAT MUCH harder.

        And since we are talking about significant quantities, specialist equipment, several hours of time, and potential waste products; distillation makes more sense.

        1. For home use, aluminum sulfate seems much better. Having done quite a bit of chemistry in my life I wouldn’t like the idea of many many people (having essentially no clue of what they’re doing) distilling IPA in some makeshift stills in their private rooms.

          Stirring in some white powder, letting it coagulate over night in an airtight (no fumes!) container and filtering it the next day is much safer. Still best done outside…

          I like the idea.

        2. A still is just as specialist as a washing station, and you’d end up with the same waste products minus a tiny amount of aluminum sulfate – so where’s the upside?

          IMHO, distilling a highly flammable liquid in residential setting is a very bad idea.

  2. I had to step through the video to find the amounts. Here it is (partly so I can find it later!)
    25 grams Aluminum Sulfate + 150 mL distilled water -> Treats 1 L of resin wash

    He also says “Alum” sold in grocery stores is often potassium aluminum sulfate, which he did not test.

  3. A couple of things: 1. It’s probably worth getting a couple of inexpensive lab devices to supplement this: A Buchner Funnel Filtering Kit + filters (essentially a flask with a suck-through filter apparatus on top) and a vacuum source – easiest being a water faucet suction adapter (ebay/amazon/lab supply). And 2. Minor… water is infinitely soluble in IPA, so there won’t be a fraction to pour off in a separate layer (hence a better idea is to filter what’s left).

    1. This actually could be a nice option indeed! As i wrote here also, if looks like the resin doesnt settle but only the pigments. The Resin seems to stay in solution (Density doesnt change from stirred to settled).
      If we can cure and solidify the resin in solution, a simple filtration could be all thats needed…
      Hm! Interesting idea!
      Maybe i steal some dirty IPA from the wash-station and try to “cure-out” the resiin in there… Interesting idea!

  4. I’d still be concerned about the water. Perhaps specific gravity measurements of new IPA compared to cleaned and dirty IPA can give a measure of how much water is present, and how it accumulates over several use-dirty-clean cycles.

    Then there’s the thought – If there’s enough water in the dirty IPA, maybe you can add the Aluminum Sulfate directly to the dirty IPA.

  5. I actually worked on IPA recycling in industry, and I’m glad to see new approaches. This method could benefit a lot from some proof that it works. Several options:

    1) The sludge *does* cure in sunlight or in a postcure chamber as was promised to try (shows there is resin in it)
    2) The weight of the solid precipitate at the end of the process is more than the weight of the alum used (shows the alum captured anything rather than just precipitating back out due to it not liking IPA)
    3) Density of the dirty IPA solution is changed before vs. after the process (shows the composition of the solution changed; can be measured by hydrometer). This test is useful because even if it looked like the IPA cleared, well a) it’s now 10% distilled water and b) clear or not, what the user cares about is if it will clean prints well, so the requirement is == high purity IPA

    The reason I am skeptical is that he’s added 25g of an IPA-insoluble white powder to 1000g of IPA and then at the end there is what looks to be 25g of an IPA-insoluble white powder.

    1. Given some of the excellent feedback I’ve received on the video I may try some additional experiments and I’ll include suggestions like this in the next video. There is far more sludge than the input but to your point would be nice to show those results. I’m also planning to measure the specific gravity before and after to see the actual reduction in IPA concentration. Another thought as well I heard from a viewer was to use table salt to further clear the mixture and extract more IPA from the water itself. All great suggestions for a follow-up video.

  6. Hm… I have at work such a IPA-Station.
    We also got a little densitiy-meter to check if the IPA is saturated with resin and needs to be replaced.
    And yes: If it doesnt clean well anymore, the density is at or slightly above where we should replace it. And fresh IPA really cleans MUCH better.

    I also was sadened to see all the IPA go to waste and let it settle over a long weekend.
    The density didnt change at all really but the gunk was all at the bottom. Looks like the visible stuff are “only” the pigments and they settly slowly by themseves or fast with such a coagulant. But in the end, i think the resin forms a well mixed and non-separating solution with the IPA.

    Destilling could be a viable process thou… Hm…

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