Not the question you asked, but this is an excellent example of a time when you can use AR to advantage in sorting values from non-values. It requires that you use excellent judgment, however, in that it's easy to toss values if you're careless, or you don't understand what you witness. Testing with stannous chloride is critical to success.
I occasionally would receive a batch of dental materials to process. Many of the white colored alloys contain gold, along with the platinum metals, although the vast majority do not have value to the refiner. In order to avoid sorting and testing each individual piece, my process revolved around an initial incineration, to remove any contaminants, then the lot was introduced to AR and heated. As the metals dissolve, everything that is present will go in to solution, which leans towards a dark brown color (assuming there are base metals present).
As the acid level is consumed, any base metals that are present will start cementing the values, so you'd notice the formation of dark particles, and the color of the solution shifts towards green. Given enough time (all the while being heated), all of the values are cemented, leaving only a solution of base metals. At that point, assuming the cemented particles are allowed to settle, the solution can be discarded, and a new, small batch of AR again introduced to the lot. This process is repeated until there is no further cementing of values, at which time the entire lot is dissolved, evaporated, filtered, then processed for values. A small percentage of base metals tends to follow the values by this method, with rare exception.
Harold
