usernameguy,
First of all, you need some zinc powder. A pound should be enough to drop 7 to 14 oz of gold. I would suggest using 325 mesh. This guy on Ebay sells it. Don't buy the 5000 mesh or 2500 mesh offered by other sellers - it can ignite under certain conditions.
http://stores.shop.ebay.com/FINE-POWDER-AMERICA__W0QQ_armrsZ1
If the pH of the bath is 7, or so, there is no free cyanide present. To have good success in zincing out all of the gold, you will need to add an ounce/gallon of either potassium or sodium cyanide. If you don't use it, you may have problems getting all the gold out.
You need sodium hydroxide, a bucket, about a 12" long stainless or plastic stirring rod, and filtering equipment. You also need HCl and HNO3.
SAFETY. Wear gloves and a full face shield. You mainly don't want to get this stuff in your eyes or mouth. USE A FUME HOOD, especially in step (9)!
I would strongly suggest that you try this with a 500 ml sample in a beaker to start with.
(1) Put solution in a 5 gallon plastic bucket.
(2) Adjust the pH to 12 with sodium hydroxide (NaOH). It won't take much, unless the solution is strongly buffered - which it very well may be. Once the NaOH overcomes the buffer, the pH will rise quickly. When you do the 500 ml sample, keep track of how much NaOH it took and use this as a guide. Try not to use an excess. The more NaOH you add, the more zinc it will take, and, also, the more zinc you will have to get rid of at the end. A slight excess won't hurt. A larger excess will work but you will use more chemicals. A big excess may cause problems in dropping the gold.
(3) AFTER THE pH IS ADJUSTED TO 12!, add and dissolve about 1 oz of KCN or NaCN per gallon of solution, with stirring.
(4) Add about 1 or two times as much zinc as you have gold, with good stirring. The zinc tends to clump and I usually add it from one of those small squeeze-type flour sifters to break it up. If you don't break it up, the gold will coat the lump and you will use a lot more zinc.
How to tell when all of the gold is dropped. This is difficult without analyzing it. However, you can get a very good idea by the color of the foam created when stirring and by the indication of excess zinc.
When adding the zinc, whip up the solution vigorously with the stirrer in order to create a little foam. Before adding the zinc, the foam will probably be light yellow. When the gold is out, the foam should be white.
Also, when you add zinc, the gold will drop as a dark brown powder. When all the gold is dropped, the excess zinc will not dissolve and will appear as a gray powder mixed with the brown gold powder. I would add zinc until the powder is almost totally gray or, totally gray. You want some excess zinc. Better a little too much than not enough. Don't add way to much, though, or you'll pay the piper in step (9).
(5) Filter the solution ASAP. Try to get it filtered in a couple of hours, or so. If you wait around too long, the excess zinc can dissolve and then some of the gold can also re-dissolve. When doing large amounts of solution that are too much to quickly filter, you can hang a couple of zinc bars in the solution, allow the gold/zinc powder to settle overnight, siphon off the top solution, and then filter the remaining sludge. The zinc bars prevent the gold from re-dissolving.
(6) Rinse the powder in the filter paper several times with hot tap water. The main purpose is to rinse all the cyanide out.
(7) Put the filter paper (if there is only 1 paper) and powder back into the bucket. The paper will fall apart in the acid and will cause no problems. Actually, it will help since it will act as a filter aid.
(8) Add enough distilled water to cover.
(9) UNDER THE HOOD, add small amounts of nitric acid, in increments, to dissolve the excess zinc powder. The solution will get quite hot. Let each addition work until the reaction slows down considerably. Stir and add a little more nitric. Repeat until an addition of nitric produces no reaction. Doing this under the hood is critical, since any cyanide remaining will react with the acid to produce the deadly hydrogen cyanide gas.
(10) Filter and rinse.
(11) Purify the gold using aqua regia
