Waste treatment for the small refiner.
Most of the practicing alchemists on this forum generate waste liquids which, if you have the time and desire, can be neutralized and made free of toxic metals rather simply. The required supplies are simply a pH meter, a few plastic tanks (15 gallon size is ideal), an aquarium air pump and some tubing, slab of copper, some scrap iron. The advantage you have in being a small producer is that you have the time to let the solutions sit and react before proceeding to the next step. Because of this you do not need expensive pH controllers or a filter press and the associated pumps.
The procedure that follows is for your acid waste only, it is not for cyanide or leaching type wastes, only the spent acids used to digest the metals you have refined.
Select a plastic tank to use as a holding tank for your spent acid, if it is translucent it makes it easy to see how full it is. This tank should have a lid to keep out rainwater and it should be kept where it will not freeze. What we will do in this tank is a process called cementation, whereby you hang a slab of copper metal in the waste acid to displace any precious metals which may still be in solution. When the tank is approaching full, maybe a few inches from the top, the copper is suspended in the tank and the solution is agitated to expose the copper to as much solution as possible. An easy and inexpensive trick is to buy an aquarium pump and some tubing and weigh the end of the hose down so it is close to the bottom of the tank and let it bubble away. Overnight should suffice. The copper bar will have displaced any PM’s in the solution which will settle to the bottom. Don’t expect a lot of material to settle from each load, but in time the material on the bottom is collected and refined for values. Allow the solution to settle after removing the bubbler and the copper bar, a few hours should do it. Now siphon the acid into another tank, being careful not to suck out any of the settled material. Usually a few inches of solution left on the bottom will prevent you from sucking out the solids and it is just left in the tank as you start to accumulate more waste acid.
A simple siphon device to prevent siphoning all the way to the bottom can be easily made as follows; cut a length of ½” CPVC pipe about 24” long, glue on a tee, followed by a 6” piece of pipe and a cap on the end. I prefer to use half inch “copper pipe sized” CPVC because it is nice and slim and easy to find tubing to slip over the end. Now you will need some flexible tubing to slip snugly over the end of the pipe opposite the capped end, about 4 or 5 feet will do. You will now have a hose and a short length of pipe which has an opening (because of the tee) 6” from the capped end. To start a siphon, place a finger over the hole in the tee and fill the entire hose and pipe with fresh water, then quickly submerge the capped end and place the end of the hose lower than the solution level in the tank. With this setup you don’t have to worry about sucking the solids off the bottom of the tank. One drawback is the tank you are siphoning from has to be higher than the tank you are siphoning to, so you’ll have to lift the full cementation tank on to a table to siphon it completely.
This second step can boil over if you’re not careful so I would transfer the liquid from the 15 gallon cementation tank into a 30 gallon tank if you’re going to process this all at once. Add your air bubbler to the 30 gallon tank and start bubbling. Check your pH, it should be close to zero when you start. Slowly add soda ash to the bubbling liquid, it will foam and generate CO2 so be careful not to boil it over, since an aquarium pump doesn’t move the liquid aggressively, add the soda ash slowly and stir with a pvc pipe occasionally to make sure it all is dissolving. Slowly over the course of a day or two raise the pH to 2.5.
Now you will add scrap iron to the tank, it can be done by placing the pieces of iron directly in the tank and fishing them out later, (needless to say with rubber gloves on) or you can get a plastic tank that fits into the 30 gallon tank, drill holes in it on the sides and bottom, and immerse the iron in all at once. The iron will displace copper and other metals from solution and slowly dissolve in the liquid. The iron will get coated up and a good shaking will remove the coating from the iron. After a few hours, the solution will have a green tea look to it which is ferric chloride. If you leave the iron in longer it will continue to dissolve so remove it and rinse it well. The copper and other metals will accumulate on the bottom of the 30 gallon tank where they are from time to time collected and dried.
You now use the siphon device to siphon the liquid to a final tank. Your primary contaminant now is iron. You can raise the pH with sodium hydroxide and the iron will drop out of solution which is quite easy to recognize. Unfortunately this liquid is a bit gel like and doesn’t filter well. A better way is to raise the pH with Magnesium hydroxide, the resultant sludge settles better and dries nicely. You cannot measure the pH of the reaction while adding magnesium hydroxide because it is slow to change the pH and you will over add, just go slowly and when you get the color change you’ll know the iron has dropped out. Let the iron settle and you can siphon off the now clear liquid to discard.
Note; in the time since this was originally posted some members have had issues with incomplete dropping of the values from their waste. This is usually because there is not enough exposure of the values in solution to the copper. The key here is time and surface area. As this process only works when the metals beneath copper on the electromotive series chart come in contact with the copper, it is important to have a lot of circulation (as in the air bubbler) and enough time for the reaction to go to completion. I have had great success using copper in the powdered or even dust form to carry out this cementation very quickly. (the metals below Copper on the electromotive series are basically all of the precious metals and Mercury)
Please remember your stannous chloride test will indicate when all of the values have been removed from the solution.
. 
