As has been mentioned on the forum ad nauseam, anytime you are depopulating circuit boards, you should get rid of as much in the way of base metals as possible. Aluminum heat sinks, small screws, nuts and spacers, as well as most any other visible metal attachments need to be removed before you ultimately use chemicals to further the process or recover values. These things just plain waste chemicals which most likely cost more than the scraps of metal are worth and create disposal problems.
Someone once suggested using AP to dissolve the copper under the gold plated foils AND get rid of the solder all in one operation. While it should work, it will exhaust your AP very quickly and you will end up with a lot of toxic chemical which will need disposal. AP is fairly cheap with 1 quart bottles of H2O2 going for about a $1 at WalMart and HCl going for less than $10 a gallon at Lowes or Home Depot. ALWAYS make sure you get the right HCl. Some of it is marketed as "GREEN" and contains other ingredients along with the HCl. This process can take several weeks to completely depopulate boards and you end up with chemicals which cannot be reused easily, if at all.
I have used a heat gun and it works fine. You have to hold the circuit board with pliers and the hot blast can reflect off the board onto you or your hands. When I did this, I wore welding gloves. Welding gloves can be purchased online for as little as $10 (sometimes even less) and the el cheapo heat guns go for anywhere from $15 to $20. I use the cheap ones because they invariably get dropped which shortens their life.
I have used the sand bath method and it is my preference at this time. Once heated up, it only takes about 10 to 15 seconds for the solder to become molten. If the sand bath is large enough, you can put several boards into it at once and just keep adding boards as you take the hot ones out. Then you smack the upright board onto a metal or ceramic container and everything falls off. The hotplate is about $20 from WalMart and the sand is a couple of dollars from many sources for a 25 pound to 50 pound bag. Very cheap in the long run as the hot plate seems all but indestructible.
The air chisel will work as many people on the forum like this method. But, while the air chisel itself is not too expensive, a good air compressor capable of keeping up with it can run into hundreds or even thousands of dollars. I have had a home made air compressor for probably close to 40 years and it is one of the most versatile tools in my shop, but buying one just to depopulate boards would probably never pay for itself. It is most likely the fastest method available but has some drawbacks. It is noisy, the air exiting the chisel can blow things all over creation, the chips and/or the boards can be damaged (but who really cares about that), and if the boards have gold plating on them, it can be scraped off and blown away if you don't exercise extreme caution. As with any method you choose, a face shield or goggles is a must.
Grinding up boards whole in a blender, and smashing them to pieces in a ball or roller mill have been suggested. However, aside from the noise (and the time a mill takes) other problems pop up. You still have the tin/lead in the solder and the resins used to make the circuit boards are very toxic. In my area, most of the landfills do not accept electronics. Blenders don't take kindly to circuit boards and therefore may not last very long.
While, I'm sure there are numerous individual ways to deal with it, according to Hoke, there is the wet way, the dry way and the electrical way to deal with recovery. Wet way is chemicals. Dry way is heat, magnetic, or mechanical separation. Electrical way which wont work here is the electrolytic cell. I'm sure there are other methods, but the only truly magic button is the one which works for you. Good luck and be safe.
