effluent: removing Pb, Sn, Sb

[scwiers]

I'm making reasonable efforts to contain effluent--storing spent liquor in sealed HDPE 5-gal containers. One thing I'm wondering though, is how contaminated the liquor still is with base metals, esp. lead. For example, I know Aluminum turnings effectively precipitate out most metals, because the liquor will go from dark emerald to 'almost clear', after a day or two, with the 'dropped' reduced metals at the bottom of the container. My guess is that Al steals the Cl from most metalic chlorides, including Pb, As, Sb. A likely factor, though, (and HS chemistry was more than 30 years ago!) is going to be the concentration of these metals in the initial solution.

 

[lazersteve]

You can displace more of the base metals by pH control. Many of the remaining metals can be precipitated as hydroxides (still hazardous and semi-solid) by increasing the pH above 9.

There are also special reagents for stripping base metals safely.

I'm not sure of exactly how much base metal is allowed in disposable solutions, but further treatment to reach 'acceptable' levels may be required and may include return the pH to 7 after hydroxide precipitation and filtering.

It would be great to have some hard facts from the EPA or a wastewater specialist.

Steve

 

[scwiers]

thanks, Steve

-Sam

 

[SilverNitrate]

The use of raw iron works good at absorbing heavy base metals. This is good for reducing most of that liquid volume, as the top layers are optically clear and quite free of heavy metal. This clear level can then be treated with aluminum provided the original solution contained a very little nitrates.
Aluminum is a poor choice as the reactivity creates aluminate jelly or liking to alum which is volume intense e.g. chelation

 

[scwiers]

I'll try out Fe, for sure. I'm starting to recognize that when the values drop from the green liquor after using scrap Al, I've also got to decant them off in a timely manner. As you say, and as I'm noticing, there's a defnitite 'sludging' point, after the liquor goes from emerald, to grey-brown, and eventually settling in a black sludge in the bottom of the bucket--no doubt these are the aluminates [?] you mentioned, now they're mixed in with my values, so I'll be careful to avoid doing so in the future. But junk iron I've got--i drive a heap to work every day :roll:

thanks again
Sam

 

[goldsilverpro]

The metal hydroxides are still very hazardous waste, based on the way the EPA tests waste. Adjusting the pH is a very easy but poor way of dropping metals as hydroxides or carbonates. When you drop metal with iron or aluminum, it drops as metal. If you can keep it as metal, it is far less hazardous.

 

[SilverNitrate]

Yes when working with dropping metals, its best to keep your solution nearer the acid side of the scale (<7pH) as H+ ions works better than OH- (base) in this type of substitution reactions.
OH- will work quickly on forming (sodium salts) then attacks the aluminum dropping nothing. --Thing is only neutralize basic solutions with acid. Also your waste base metal carboy should be of plastic only.

 

[Lino1406]

Which sodium salts will be formed?

 

[SilverNitrate]

Which sodium salts will be formed?

Suppose NaOH was used, the chloride and nitrate will become the ions of sodium and the OH ions become oxides of the heavier base metals. Aluminum ions added then this soaks up lots of H2O.

 

[Lino1406]

So all you need is adding more water, no?
I assume this what corrodes the Aluminium
thus enabling more dropping

 

[SilverNitrate]

Some salts attract water as part of their crystal structure. e.g. CuSO4-5H2O meaning 1mole of this annhydrous salt will absorb 5moles of water becomming a hydrate. Some aluminum salts can attract 12 or more water molecules per Al atom. Such substance can grab water from the air and even carbon dioxide.
I just say aluminum is a poor choice for dropping base metals because of the aluminum salts ability to soak up water thus becomming volume intensive.

 

[Lino1406]

Maybe you also negate this? It would
be interesting for me.