Fever,
Pins vary in their substance quite a bit. They can range from zinc to brass plated with gold and every other base metal in between. This variable makes a single technique good only with certain types of pins. The nitric is a good overall choice, but it's expensive and hard to come by for most folks. The pins, when all piled together, tend to protect one another from the etchant you choose as well.
Fortunately, there are plenty of great substitutes.
Here are a few:
Straight HCl
HCl + H2O2
Ferric Chloride
Persulfate compounds
Reverse Plating
Another side affect of processing pins is the salts that form. These can show up as solids, sludges, or even emulsions. If you don't have the concentration of solvents right you will get a variety of salts in the gold. Some salts are insoluble in water or acid or both. If you mix a bunch of types of pins together you only complicate the dissolution process with more varieties of metals. These mixtures tend to interact with each other to make one big mess if you don't know how to handle the percipitates you encounter.
Yet another problem arises from debris. These can come in many forms as well. Insoluble ceramics, fiberglass, board laminates, plastics, rubbers, etc.
The list above is just a short summation of the problems associated with doing batches of pins.
If you encounter a specific difficultly, post a question with the details of the experiment and I'm sure the forum can help you out.
Steve