Expert: Profile Closed - 1/12/2010

Question
It's me again :) I haven't had much time to work on this project over the past week, which is why I haven't been coming to you with questions. Unfortunately, our teacher is making us hand in the project THIS FRIDAY, so I'm a little worried. Hopefully I'll be able to pull it together in time.

So now I know (thanks to you) that it's possible to purify the heavy metals leftover from incinerator emissions with nanomaterials. I just need to understand how that process works, so I can come up with a plan to apply to my topic. I don't want you to do my research for me, but I'm a little lost as to where to begin. Nanotech seems like such a broad category, where do you suggest I start my research?

The other problem was that the nanomaterials plan on an industrial scale would be really expensive. Luckily for me, in this project it's more the idea that counts, not so much the economics. But I was thinking about it anyway, and it occured to me that there have to be people (like scientists) who would need those heavy metals that would be removed from the rest of the ash. Does that make any sense? Maybe I've got the whole principal wrong. The way I understand the issue, though, is that you've got all this leftover fly ash that's partly just plain ash and partly harmful materials, and you purify that with nanomaterials, so you've got a pile of harmless ash and a pile of lead and all the other heavy metals that were in the ash. Wouldn't you be able to SELL all those heavy metals for people who need them? I mean, how do people get things like lead in the first place? It'd be like recycling :)

Answer
Hi again, and thank you for the follow-up!

I hope the project goes ok; seems to have been sprung on you a bit!

I'll discuss the question of recovery and sale of lead first, and then give links a bit of general background information about nanomaterials that I hope will help.

The primary aim of your scheme would be to remove the lead from the ash so that it could be disposed of separately and safely. This will involve some way of filtering out the lead from the ash.

There are several reasons why it probably won't be a good plan to try and sell the metals you recover:

1) The materials you separate out will be a very impure mixture of heavy metals. It will cost a huge amount to purify them, more than you could get for selling the purified products. The impure metals will be of no use to anyone.

2) The process of recovery isn't like a lab-scale filter; the material you recover won't be in the form of big lumps of metal, it will be a finely dispersed clinker (that is, ash that has been fused). Again, you would have to use a lot of time and energy to recover workable metal from this residue.

3) Most importantly, as has been said before, remember that you're not recovering lead itself, you're recovering lead compounds like lead oxide and lead chloride (there are very few metals that would not react in the intense heat and chemically rich mixture of a furnace). Oxides in particular are incredibly stable and therefore it would be very difficult to come up with even a theoretically workable method for actually recovering the metal, rather than the lead compound.

In summary, when your purification is complete, you will have small amounts of a powdery residue of reacted heavy-metal compounds, mixed together. You won't have small amounts of the metals themselves, which you would need to have if you wanted to start a recovery.

To ensure I'm not misleading you, I put a small amount of lead powder on some paper in a special fume cabinet at work, and set fire to the paper. I washed off the ash after the flames had died down, ands was left with a hard residue of melted lead and lead oxide (identified by mass spectrometer). The lab-cost of reducing the lead oxide and other lead compounds back to a workable metal worked out at about $10 for an ounce of material, no counting time, equipment, fuel etc costs. In industry, the process would be more difficult and you would also have to remove the material from a nanomaterial first. There is no way I can see that you could do this without a huge cost.

In summary then, the reason that your idea of selling the recovered lead is that it's recovered as an impure compound in small amounts, not a pure-ish metal element in large amounts. Remember that you're designing the process to remove lead from the ash, not to recover the lead. Obviously if you could recover and sell the material that would be an added benefit, but in this case I can't see how it would work. Remember though that in science, a negative result is just as important as a positive one, and the fact that you have considered sale and recovery of the metal would be an important part of the project.

Lead is normally sourced by reducing a mixture of lead ore (rock containing lead), and scrap lead metal in a furnace. Electrolytic processing is then used to refine the metal further for applications that need a high purity.

As for the general use of nano-materials, there are two separate things to consider, each of which raises a few questions. At this point, I’ll give you a list of the sort of things you might want to thin about and a couple of weblinks, but if you need any more info or you want me to discuss any of the points with you, don’t hesitate to send a follow up,

A)   – How will you prepare the waste ash for nanomaterials?

This is not like a sieve, you will need to remove as much other material as possible before filtering. Nanomaterials work on a molecular level; just dumping the ash on them won’t work. Can you separate off as much waste as possible before nanofiltering? Could you wash the ash by dumping it in a tank of water so that all the metal residues sink and the rest of the waste floats and can be skimmed off? What physical state do the materials need to be in to be recovered?

B)   – How do nanomaterials work? How will you know if the filter is doing its job when it works on such a small scale? How will you know when it’s full of waste metal? How will you clean out the nanomaterial when it’s full so that you can use it again?

The links below may be a good place to start.

http://www.sigmaaldrich.com/materials-science/nanomaterials/tutorial.html
(ignore the pop-up, just put your country info in and continue).

http://www.watertreatmentguide.com/nanofiltration.htm

http://www.eurodia.com/html/nab.html



Again, apologies for the lack of solid info, but I’m trying not to interfere with the direction of your work. Any further points you have to discuss I will be more than happy to help with.

Best wishes for your project work, fingers crossed that it all gets done for Friday! 

George