Remove aluminum from etching & anodizing baths

A. Hi Edgar. As far as I know, decanting the solution (and removing the precipitated aluminum oxides) will not reduce the dissolved aluminum. Excess dissolved aluminum in the etch causes problems, including "Spangle and Shiny Metal" (topic 40098).

Luck & Regards,

A. The "dump point" or bleed/feed point has to be determined by you. The dump point varies from shop/shop depending on the desired finish. Keep a log sheet record and determine the level of dissolved aluminum that does not interfere with the particular finish you want, then over time bleed & feed to hold the aluminum constant. Each time pump out, dump the sludge, pump back in, titrate and bring up to desired concentration. If in the middle of the week you see "spangle", drop in a tiny bit of hex chrome and that will hold you until Saturday.

A. An effective method for removing and maintaining the Aluminum concentration at a constant level is by using a Diffusion Dialysis System. My company manufactures this type of system for Acid Recovery and is suited to your application. A majority of the aluminum can be removed and the operation of the bath can be maintained more consistently by removing aluminum as it builds up.

A. We do anodizing & hardcoating & have a special ion-exchange system for removing aluminum from a company called [name of ion exchange company deleted by editor] - It works great. We purchased it from [deleted by editor].

A. Aluminum can be selectively removed using a so called "ACID RETARDATION PROCESS". It is a relatively simple process and although there is an initial investment it pays back within a year.

Q. Ruud,

Can you explain me what is the 'acid retardation process'?

Which companies can I contact in Europe for this technique? [ed. note: Sorry, Francesco, we cannot post vendor or sourcing suggestions]

Thank you and best regards.

Francesco Cicchetti
- Sulmona, Italy

A. Francesco, Acid retardation is an ion-exchange process where the aluminium contaminated sulfuric acid is passed through a resin. After loading the resin it is eluted with water, resulting in a sulfuric acid stream with low Al and an aluminium sulphate solution with low sulfuric acid. A large number of European companies provide this system.

Ruud Gerritsen
- Simpelveld, Netherlands

A. I think that electrodialysis is used to recycle aluminum containing solutions but an even better way is to use an acidic etch instead of an alkaline one. The acidic etch for aluminum etches only 10-15% aluminum compared to an alkaline etch.

A. Electrodialysis would work, I think. The EPA ETV metal finishing center is in the process of verifying one unit being used to remove Al from a chromic anodizing bath.

A. Have you considered using alkaline cleaner prior to etching so you don't have to saturate your etch solution with lube? This will mean longer service life and less disposal.

A. Bravo Dado!

Many times I've seen a single alkaline tank doing double duty as both the aluminum cleaner and the etch but it's corner-cutting that comes back to bite you.

For example, when you get a batch of parts that needs extra cleaning, you'll automatically get extra etching that you didn't want; then your desmut isn't strong enough to remove the alloying ingredients left behind from etching that much aluminum. Then you're wondering why you have smut problems, and you're adding extra pickling steps or moving to more toxic pickles :-)

We should clean the parts in a non-etch cleaner first, THEN etch them (Sara could be right that an acid etch is preferable). Also, this way the organic contaminants are in one tank and most of the dissolved aluminum is in another and you have a much better chance of prolonging the solution life through one recovery mechanism or another.

When I went to work for Lancy Laboratories way back in the mid 1970's the centerpiece display in their building's atrium was a adsorbtion tower device for removing aluminum from caustic etches they'd been developing for years; people have been working on this issue 'forever'. But in my experience, many if not most of the recovery systems you see written up in the journals are decommissioned due to impracticality in actual operation once the grants expire. So if you try to remove both dirt & organic contaminants and dissolved aluminum simultaneously it will only be that much harder, and I'd bet you'll end up in that "most" category

A. For cleaning of oily aluminum work pieces we had the same problem and received 3 pieces of advice as follows:

1- changing lubricant to evaporative or water soluble type.
2- downward streaming to catch 90 percent lube and then cleaning and etching.
3- using double phase cleaner available in the market.

In practice we found No. 2 suitable and cheaper, but for you it depends if your lube being not viscose to be dripped.

About cleaning after lube removal, find a suitable low-attack silicate-free alkaline cleaner.

A. Allen, you pay some people to take away your waste and dispose of it in a proper and responsible manner. That's what I do (I'm a research student working in a lab' on baths where 2 liters would be big) how the chemicals are treated is something I'm interested in, but so far I've not learned much except it's all specific to application. Ian

A. Hi Allen. Users attempt to operate their plating and anodizing systems as "equilibrium processes" which are maintained rather than disposed of. Some plating baths have been in use, never dumped, for many decades. But sometimes that can't be done because despite best efforts the baths grow or get contaminated.

Some processes like electroless nickel can't be equilibrium processes because phosphates build up inexorably with use. Some of the support processes like cleaning, etching, desmutting, and dyeing cannot be maintained in equilibrium and do become exhausted. This is generally a matter of neutralization of acid/alkali, with consequent settling out of solids. However, anodizing facilities must be registered and disposal permits obtained. These are generally issued by the POTW serving your area.

Hydrogen evolution is generally not a serious problem, but the tiny droplets of solution that evolve from the tank along with the hydrogen, in ginger ale or champagne style, must be captured with an exhaust system and scrubbed. The exhaust system keeps the hydrogen below the lower explosive limit (although I've never heard of what most people would consider an 'explosion', just the 'popping' we demonstrated to ourselves back in high school chemistry class). Good luck.

Regards,

A. Hi Noah. Dissolved aluminum and precipitated aluminum are two different things: you can filter out that precipitated aluminum sludge.
Luck & Regards,


Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Probably not.

The difference between diffusion dialysis for acids is that you are trying to recover an anion, which is a large molecular compound in most cases, and also usually divalent, which gives it greater propensity to "attach" itself to water passing thought the membrane. You're trying to recover caustic in the cation application - which competes with hydrogen ions for transport (even in the highly alkaline background). Sodium is a poor transport ion as it is stable and only monovalent.

Have you tried to seed the material and filter it out?

A. Yes you can recover NaOH using cation membrane if your solution is NaOH and sodium aluminate. The problem is that as you recover the sodium hydroxide the sodium aluminate precipitates, therefore you can only recover as much as 40 % during dialysis. If you crystallize your high aluminum discharge, you can recover the rest of your NaOH. You have to play with the water inflow to reduce the dilution of your NaOH. I had the chance to do this process about 11 years ago at the University of Rhode Island.

Best regards

A. Hi. Unfortunately it can be risky to use your waste in someone else's products. For example, that 8% nickel could be a serious issue in cement. And even if the cement factory plans to use only some much smaller percentage of your waste in their cement to keep the nickel levels down, they may accidentally or deliberately use more and produce cement that is hazardous.

Malaysia is not the USA and your rules are different, but here's what happened here: Companies used to pay other companies to dispose of their waste, but some of those 'other companies' turned out to actually be illegitimate 'midnight dumpers' who contaminated the land, leaving the citizenry with huge cleanup costs :-(
The problem was 'solved' by new rules which said the original generator of the waste was responsible for their waste forever, no matter how much they paid anyone to dispose of it ("cradle to grave"). One unintended result was that the rapidly growing business of "industrial waste exchanges" totally collapsed almost overnight. Who in America would be dumb enough to let their waste be used for concrete in a bridge support if there was any risk of being responsible for removing & replacing that bridge support? One thing I would suggest is that if the concrete company pays you even a token amount, rather than you paying them, you may have reasonable grounds for claiming it was not waste.

Luck & Regards,

A. Diffusion dialysis is probably the only known commercial mechanism. I am currently working on developing a process for a client, but do not have any results. Diffusion dialysis turns out to be very, very expensive, but has been done. You should be able to find manufacturersthrough the Metal Finishing Guidebook.

A. You seem to be providing analytical data for a liquid waste. So, it's a bit hard to tell.

Aluminium hydroxide based sludge from the treatment of such a wastewater might be suitable for blending with cement. Even with the high price of zinc (the main heavy metal in the water you describe) these days, 0.14% is not enough to interest a recycler.

A. Hi. It may depend to some extent on your country's waste laws. It's ideal to use waste products in gainful fashion, but if they are regulated wastes and you are responsible for them forever like in the USA, that can be very risky. Google "Times Beach, MO".

Luck & Regards,

Hi Ghada, people wouldn't continue to use the poorer method if another method was better in any and all circumstances. Please give us actual details of your own situation because lack of detail encourages canned sales pap rather than tailored advice. Thanks!

Regards,

A. Sorry, Claude, you will not be able to plate the aluminum out of this waste. Aluminum is too active a metal, and you will electrolyze the water into hydrogen and oxygen without the aluminum coming out. Recovery of aluminum is a very difficult proposition that is done by ion exchange, absorption contactors, etc. Please try to contact companies specializing in this and see if there is an affordable solution for your situation. Good luck.

Regards,

A. Either Diffusion Dialysis or Acid Retardation can remove aluminum from sulfuric acid. Unfortunately, neither will remove all of it.

Lyle Kirman

A. You may want to look into acid filtration units, I don't want to mention specific company or product names here - it is against the forum guidelines - but you should be able to find information relatively easily via google.

It won't remove all of your aluminum, but if you are just trying to get it back down to an acceptable level, it should work quite well.

Good Luck!

A. This is a job for an APU (Acid Purification Unit). If you're looking to reclaim the aluminum, that will be more difficult. Metal Hydroxide precipitation could be used but then you'll have a cake about 75% water and 25% Aluminum Hydroxide.

A. On the initial aluminum and sulfuric acid, if there are no other contaminants of concern you may want to look at it for potential product sale. You have just made Alum which is used for wastewater treatment. There are potential outlets that may buy the material as a product instead of having to perform hazardous waste disposal.

A. Hi Brian,

I think you will find few, perhaps zero, "waste chemical exchanges" in operation anymore because of the EPA's solution to the problem of "midnight dumpers". There was a time when waste generators would pay someone to take their waste (probably at cut-rate prices) but the small hauling companies that took it would keep the money, dump the waste somewhere, and disappear ... leaving the public to bear the cost of cleanups.

To fix this problem, the EPA came up with their 'cradle to grave' rule where the waste generator is responsible for their waste products forever regardless of who they pay or how much they pay to dispose of it. Today it unfortunately feels safer to get your wastes into a secure landfill than to risk them, for example, being made into toxic bricks in a skyscraper which you are then responsible for rectifying :-(

Google 'Times Beach, MO' to fully appreciate the point.

Even when a waste contains no hazardous ingredients it might still be regulated as a 'categorical' waste (a waste generated by a particular industry like plating shops) ... so I strongly suggest that an environmental lawyer be consulted before having your wastes incorporated into someone else's products.

Regards,

A. I used to run our acid recovery unit 3 days a week, about 6 hours at a time.

The advantage is that you can control Al without doing periodic batch dumps of electrolyte. That requires neutralization, with the generation of massive amounts of Al(OH)3 sludge which must be dewatered, and disposed of. The costs and labor associated with this, not the cost of the acid, is why the unit is worth the money.

A. Diffusion Dialysis is my recommendation for smaller applications, and acid retardation by ion exchange resins for larger applications. If your dump frequency averages less than 50 gallons per day I would definitely recommend Diffusion dialysis, and if it is greater than 100 gallons per day I'd recommend acid retardation. In either case, you should try to maintain an aluminum concentration of 7-8 g/L, since this results in consistent performance.

A. Hi JH. I'd be very cautious about "find someone that wants to use it" due to EPA regulations, per this aside:

"Midnight dumping" was a very serious environmental problem for several decades. Companies would pay someone (frequently the lowest bidder) to dispose of their waste, and those disposal companies would take the money, illictly dump the waste, and then disappear, leaving the government and the taxpayers to foot the cleanup bill. The EPA solved this problem with "cradle to grave" regulations: if you generate the waste, you are responsible for it forever, no matter how much you pay anyone to dispose of it. So if you can get someone to BUY it, you perhaps have a case that it's not a waste; but if you pay someone a little to take it and "reuse" it I think you're on dangerous ground. Please consult an environmental lawyer about this in any case.

Dealing with aluminum hydroxide wastes from etching & anodizing has been a thorny problem absolutely forever. When I joined Lancy Laboratories almost 50 years ago, Dr. Lancy (the world's first designer of waste treatment systems for metal finishing wastes) already had an sorbtion tower which he had been working on for decades for that purpose as the centerpiece in the atrium. It's possible but it is not easy, cheap, or 100% efficient to recover aluminum or caustic.

We've appended your question to a very long thread discussing all of the alternatives, which hopefully will offer you a direction while we await someone responding directly to your situation.

Luck & Regards,