rewalston said:
It depends on how you treat your waste and what material you dissolved. The spent acid will still contain some acid, dissolved lead and tin, iron, nickel, aluminium, zinc... and probably a bunch of other metals.
One way to remove most of the lead would be to add iron sulfate, copperas. The lead would precipitate as lead sulfate and the remaining solution would be less toxic. If you add sulfuric acid instead of copperas you could distill off hydrochloric acid.
There isn't one and only one way to treat waste. You could just simply evaporate the liquid and send the concentrate to a company dealing with waste, leave it on a city collection point with a detailed note of what it contains if you have small volumes, do the whole pH up - down and filter off hydroxides to end up with basically salt water and solid waste... and so on.
It all depends on where in the world you are and other circumstances.
The only thing I know is that I'll not flush any material, solid or liquid, down the drain at my house. My tap water is pumped up just 50 m from the septic tank. Biological toxins have a short lifespan but metals in solution stays forever. The ground below my house is quite permeable, the ground water table moves up and down about 1-2 meters over the year. The infiltrated water will probably flow away from the well but there are other houses downstream from me.
For the small amounts of waste I produce I first extract a dirty copper by cementing on iron (probably some lead and tin mixed in with the solid copper). The resulting liquid is mixed with NaOH until neutral. The hydroxides are mechanically separated and dried. The liquid is put in a plastic pan and the water evaporates over time. This way I reduce the waste to dry waste and very low volume and I leave it at the local recycling center for free.
I'm trying to be frugal in my refining (hi Dave!) so I don't produce too much waste and I cherry-pick the scrap I refine. Some I only stockpile until I find a better method that doesn't produce too much waste. One of my favorite methods is the sulfuric acid reverse plating cell for pins, then I can sell the metal as brass afterwards and doesn't have to mess with any dissolved metals.
Another method that I'm looking at is using cyanide. The same reason to keep down waste.
If I need to denox a solution then I use evaporation, less waste than other methods.
Dissolving gold powder... HCl and strong hydrogen peroxide rather than bleach, no additional solid waste is produced. Everything can be evaporated off. The same is true for aqua regia too.
Göran
