Expert: Profile Closed - 1/3/2010

Question
QUESTION: I'm not sure whether or not this is within your area of expertise, but perhaps even if you are unfamiliar with the topic you could recommend where I might find help.

I am a 10th grade student taking honors chemistry. We have a project in school (you might consider this a homework question, but please keep reading and then decide) where we have to come up with an innovative "green" idea: an idea that hasn't been thought of before to help the environment.
My idea has to do with incinerators and how their lead emissions are poisonous to the environment. Many people are saying just to get rid of them, but incinerators are up to 5 times less environmentally damaging than landfills, which is where the waste would go if it isn't incinerated.
I've been researching how to purify lead, and I've found several sources that say you can purify lead with wax. Most of them are blogs, so I'm looking for an expert opinion on the subject. There's an alchemy website (http://levity.com/alchemy/leyden.html), that gives a procedure on how to create a substance that looks like gold. The first step in the procedure is to purify lead with wax. It says to "Melt [the lead], spread on the surface lamellose alum and copperas reduced to a fine powder and mixed, and it will be hardened."

The other problem is that my initial idea was to put a wax coating on an incinerator ventilator where the emissions are released and it would purify the emissions. But it seems like if purifying lead with wax is even possible, you'd need to do it with lead as either a solid or a liquid. So is it possible to turn lead emissions into solids or liquids?

Thank you so much for your time.

ANSWER: Hi there, and thank you for your question; I think your choice of project sounds very interesting.

There are a few methods by which lead and similar heavy metals can be purified, most of which rely on electrochemical processes similar to those involving copper which you may have studied (the Betts process being the most common industrial lead purification method). The only references I can find to the Lead / Wax method are alchemical, and whilst some of the principles used may be sound, it is not used on an industrial scale at all as far as I'm aware. (I'm guessing that the logic behind wax purification is that melting the impure lead gives a layer of liquid pure lead and a crust of solid impurities  -which will mostly be oxides and so have very high melting points. If wax is used to capture those impurities, and if they can be made to remain in solution in the wax, the "dirty" wax  / waste layer and the "clean" lead layer would then be separated by their varying melting points.)

However, there are two major points which you may want to consider before fixing the direction of your research. These are:

1) The situation you have in an incinerator is very different from the situation of purifying a solid metal. Even an "impure" metal will be, say, 90-95% lead, in very concentrated form. In an incinerator, you will have lead vapor and particles in dilute form as part of a huge, complex reaction mixture (mostly products of combustion). In other words, a totally different situation exists in the incinerator, where the lead is much less that 1% of a very hot, fast-moving stream of waste gas. In this instance, you're not trying to purify lead, you're removing it as an impurity itself, from the waste stream.

2) Depending on the incinerator type, is lead the key issue to focus on? It is, of course, important to remove lead from the environment, but have you considered other types of toxin that may be present in ever greater amounts (e.g. Dioxins from burning municipal waste)? That's not to say that there is not a need to remove lead - there is, but it's important that the process you use doesn't interfere with other processes that may be used to clean the waste stream.

So, how will you remove lead vapors from a relatively massive stream of very hot and complex gases? The standard method, which works for heavy metals like lead, and for many other materials, is to "scrub" the furnace emissions as they leave the waste stream and before they are released from the chimney. In its simplest form, this involves blowing all of the vapors from the furnace through a bath of water (or other absorbing solution), and then passing the scrubbed gas out to the environment through a chimney.

The majority of the gases produced by a furnace will be products of combustion, like CO2. These will be cooled by the bath, then continue out up the chimney. However, materials like lead, other heavy metals, unburned plastic vapors etc that are in the waste stream will condense as soon as they hit the water, and sink to form a solid layer at the bottom of the  scrubbing tank, which can later be removed. In this way, the emitted gas is cleared of a large proportion of the impurities it carries. Scrubbers are the chief way of cleaning gases from both incinerators and chemical processes, and analytical reports on the sludge recovered from a number of household waste incinerators indicate that it does indeed contain a proportion of lead, demonstrating that the scrubbing process has effectively removed lead from the gases being emitted from the incinerator.

Another point I would make to you is that the environment of a furnace is actually like a chemical reactor; hot and fast-moving mixtures of many substances. So the lead that vaporizes and escapes to the waste stream will not be your only problem. A lot of the lead present will be oxidized, or otherwise chemically bonded to another material. Lead oxide will not melt in a normal furnace, and so will remain behind as part of the ash after the fire has burnt itself out. In other words, any process you look at in which you're clearing lead from a furnace will have to take account of the fact that some lead is discharged as a vapor, and some is retained as solid compounds in the ash.

Hope that this is of some use to you; in essence, your problem isn't purifying lead, it's removing lead vapor from a waste stream, something that a scrubber will do relatively well. The process is not selective, so the sludge will consist of lead mixed with other heavy metals and residues, which is a plus as far as cleaning the emissions is concerned. Even if the wax purification method worked well on an industrial scale, it would only be of value for recovering lead from the scrubber sludge or ash, not for the initial removal.

I haven't gone in to more detail on any of the points I raised because I don't want to interfere with the direction you choose to take the project, but if the information above is of use to you and you have further questions on any points, follow-ups are very welcome.

Best wishes for success in chemistry :-) Happy new year.

---------- FOLLOW-UP ----------

QUESTION: Thank you so much for your help, it's exactly the kind I was looking for.

I guess now I'm sort of at a loss as to where to take this project. I hadn't realized that "scrubbers" existed, and from what you've said they do a pretty good job of "cleaning" the emissions. What still confuses me, however, is if scrubbing is a proven method of removing lead, why is there still so much controversy over incinerators? Because from what you've said, it seems like incinerators are fairly "clean," and I don't know what other steps could be taken to purify them even more--they seem to be pretty "pure" already.

I'm not sure if you're familiar with this field of science, but our teacher was recommending we look into nanotechnology and the environment, because nanotech is a relatively new branch of technology. Is there any environmental "problem" that you can think of that might be helped by using nanotechnology? I've heard of clothing lines using nanotechnology to install tiny particles into clothing to make said clothing stain resistant. Is there such thing as an environmentally hazardous "stain"?

I'm not sure if I should change my project topic completely or if I should just work on taking the incinerator idea in a new direction.

Thank you again, I don't know where I would be without your help :P

ANSWER: Hi there, glad I could be of assistance :-)

To answer your follow up, there are two broad reasons that incinerators are still controversial.

The first are legitimate environmental concerns, e.g.

A) Scrubbers only remove particulate matter; gases that aren't very soluble like CO, CO2 and some dioxins can still pass through, and present an environmental and possibly a health hazard.

B# The residues #ash# often contain harmful levels of heavy metals which must be disposed of; this is obviously not a plus.

There are other similar issues that still exist around the use of incinerators; looking in to solutions to these might be a valid project path?

The second kind of incinerator controversy results from mis-informed journalists using scare tactics to excite the public!

As for the direction of your project, whilst stains are not hazardous, a similar technology can be used to capture particles of hazardous materials. The simplest everyday material that is a near-nanomaterial is Activated Charcoal, whose molecular structure makes it ideal for adsorbing harmful gases; it is the material used in gas mask filters, and also for absorbing impurities in fish-water. Perhaps you could look at some of the more modern, designed nanomaterials (e.g. buckminsterfullerene), and find out how they are being used in environmental protection? My own research has concentrated recently on using nanomaterials to absorb heavy metals from water, so you could even tie in the use of nano-materials to finding a safe way to dispose of the heavy metals left over after incineration?

Whichever direction you decide to take the project, I am more than happy to assist in any way I can, and can provide some additional background information regarding nanotechnology in environmental protection in particular, if that would help?

Best wishes for the project! Apologies for the "no homework" rider at the top of my profile, but we get lots of "can't be bothered" students just posting their homework! It's clear you are working very hard and thinking logically about the project you're doing, and in such cases we're always really happy to give any help we can. Happy New Year!

---------- FOLLOW-UP ----------

QUESTION: Thanks again for all your help.

I've decided that I'm still going to focus on incinerators, specifically the fly ash that's left over from the scrubbing (I'm assuming the fly ash is hazardous?). I've learned that the fly ash is enriched with lead and cadmium halides and that you can't just dump it into a landfill because the groundwater in the landfill will cause the halides to leach substantial amounts of lead and cadmium, which wouldn't be very good for the environment. Does that sound about right?

Do you think it's at all possible to use nanotechnology to clean that lead and cadmium halides? I also want to look into using nanotech in such a way that the cleaned lead and cadmium is useful in some way.

Just a side note: Please don't feel like you need to do my research for me; that's not what I'm asking for at all. I just need expert opinions on my ideas just to make sure they make sense.  :)
And also, for this project we're each supposed to make a website on our idea, and I'd love to mention your name in mine, just to give you credit for all your help. I just want to make sure that's alright with you, and if it is, if you have a title or something that you'd want me to use when referring to you :)

Answer
Hi there, and thanks for the follow-up; hope the work is going ok?

I think the new project title sounds great, and should give you scope to do a lot of interesting research :-)

A couple of points that may guide you on your way;

1) Make sure you are happy with explaining how the lead / cadmium etc ends up in the ash in the first place; e.g. which products and equipment that go in to the incinerator might it come from? And remember that the lead that ends up as ash is only part of the lead that was in there to start with; some of it will have either escaped to the atmosphere or been caught in the scrubber.

2) What's the chemistry going on? The lead and cadmium in products like batteries is normally the pure metal. There must be a reaction or reactions that get it from this form to being present in the ash.

3) Remember that the ash itself is the residue from organic material (wood, paper etc). The ash isn't made of the heavy-metal compounds, it just traps particles of the compounds in it.

4) You also need to figure out what sort of compounds will be present. You mention halides, which are simply compounds in which the metals are bonded to halogens (e.g. PbCl2). These will form when vaporized lead in the furnace reacts with free halogen molecules, which will be present from burning plastics and similar items (PVC, for example, releases large amounts of  compounds when it burns). However, there are many many different heavy metal compounds present in the ash (as you would expect, given the large number of different materials in the hot reaction zone), and they each have their own properties; the method you use will depend on what compounds are present. Remember also that there may be other dangerous residues, including other heavy metal residues, and dioxins, depending on the material that went in to the furnace.

5) How are you going to remove them? Using a nanomaterial will mean using the structure of a material that is "well-defined on a nano-scale" to selectively trap the molecules you want to filter out. This will be very effective and selective, but how expensive would it be on an industrial scale? How easy to perform on tonnes of ash a day? Are there other methods in use at the moment? I can tell you for certain that it is possible to use nanomaterials to selectively filter heavy metals, because I've done it myself on the lab scale; that doesn't always mean, though, that it would be possible or economical on an industrial scale!

6) You said you wanted to possibly recover the metals for re-use; how easy is that to do with each of the possible methods? Is it worth it? (E.g., if the recovery process is electrolytic, it may well cost more to do the recovery than you could ever get back by selling the material). Also, what form will the recovered metal be in? As I mentioned above, there may be many different heavy-metal compounds in the ash; you might have to use different processes to recover the metal from each (e.g. you could easily electrolyse any PbCl2 that is present from lead that has reacted with chlorine, but if some of the lead has also reacted with oxygen at the high temperature of the furnace and formed lead oxide, an electrolytic process wouldn't work on the oxide). Also, what will you do with the recovered material? If it is re-used, won't that just mean there is more harmful lead in the environment? Alternatively, if you are not going to re-use it, how could it be stored or destroyed safely?

The description of groundwater leaching you give above is spot on, so you've identified the problem clearly. You might also want to include information about the permitted levels of heavy metals in water, to show that groundwater leaching would exceed these limits, and so would be dangerous.

I'm sorry that the above information won't be of much direct use to you, and that there are a lot of blank questions! I just wanted to give you some ideas to think about; I don't want to interfere with how you choose to do your project and what you decide to focus on. Have a look over the paragraphs above, and if anything's not clear, or if you decide that more in-depth information on a particular topic would help, drop me a note and I'll be happy to help. I'm really finding your project and ideas interesting, so it really will be a pleasure if there's anything I can do to help.

Thank you again, best wishes, George

PS: Oh wow, fame on the web at last :-) I would be honored to be included as a background info source on your website if you so wish. I hardly ever get called by my title (except when I'm being told off by the boss....) but on paper, I'm Dr George Maxwell, a clinical toxicologist in the UK health service.