Separating various PM's from solution.

[dbrick81]

How do you go about separating different Precious metals if they are all dissolved in the same solution. For example if you had gold, silver, and palladium all dissolved in the same AR or AP solution. Actually lets make it a little harder... what if there is also copper dissolved with it.

Thanks

 

[Harold_V]

How do you go about separating different Precious metals if they are all dissolved in the same solution. For example if you had gold, silver, and palladium all dissolved in the same AR or AP solution. Actually lets make it a little harder... what if there is also copper dissolved with it.

Thanks

For starters, you need not worry about having silver and gold in the same solution. While traces of silver are known to behave like gold, once silver is in solution, the addition of chlorine (to dissolve the gold) will precipitate, for all practical purposes, all of the silver as silver chloride. Once any of the included elements are precipitated, they are separated from the solution by gravity, or filtration. Both gold and silver can coexist in a cyanide solution, ulike an acid solution.

In your example, you'd have gold and palladium remaining to be recovered. Assuming you had a large percentage of gold, with a small amount of palladium, you'd selectively precipitate the gold, using one of many precipitants. I used SO2, and also copperas (ferrous sulfate). Each of them will precipitate only the gold, but you'd get considerable drag down of both palladium and copper, so the gold wouldn't end up pure, although very close.

Palladium would be recovered after the gold had been precipitated. Because it doesn't precipitate from dilute solutions, you might end up concentrating the solution by evaporating, then you'd introduce ammonium chloride to the heated solution, then sodium chlorate. Palladium is recovered as a scarlet red salt. It, like the gold, would not be pure, again, due to drag down.

From this you can conclude that if you desire high quality metals, they should be recovered from solutions that do not contain large volumes of dissolved base metals, or from multiple types of values in the same solution. Assuming you have no choice, once recovered, the metals can be re-refined, raising their quality.

All of this can be gleaned by reading Hoke's book. If you don't have it,
GET IT!

Harold

 

[Platdigger]

First let me say, great answer, as always Harold.
But, let's just say he does have an appreciable amount of base metal in solution (for whatever reason) with the precious ones.
At this point the best thing to do is to treat it as a stock pot solution?

By this I mean, dropping or cementing everything out of solution with either iron or copper plates, or even zinc or zinc powder, and starting over?

I know you have probably addressed this issue many times on this forum............but for some reason I wanted to ask again....
Randy

 

[Harold_V]

In such a case, assuming I wasn't going to simply store the yield from the crock pot for future processing by furnace, I'd probably recover everything with scrap steel, washing the remaining steel well to remove all traces from the surface. I would then gather all of the precipitates and incinerate them to kill the chlorides. I would do that because the next operation would be to dissolve the precipitated base metals without dissolving the values, to separate them. Washing alone could work as well, assuming that there is a substantial amount of base metal present. It (the previously precipitated base metal) would re-precipitate any of the values that were dissolved. It's important to remember that you are working with a solution that is capable of dissolving the values, so you must work accordingly.

Assuming the chlorides are consumed, and the solution shows no values, one would then use the process of choice to dissolve the base metals, leaving behind the concentrated values. They would then be dissolved in AR for final separation by selective precipitation. In my case, incineration would have eliminated the chlorides, so I'd place the incinerated precipitates in a beaker and apply nitric and tap water. That would dissolve all base metals, leaving behind only the values as a finely divided powder, likely black in color.

I favored the furnace process, for it recovered everything with little effort, and became a wonderful savings plan for me. YMMV.

Harold

 

[Irons]

In 1803 Wollaston, the pioneer in Platinum Group Metals found that it was very difficult to recover pure Platinum because of the large amounts of Iron and other elements that are naturally alloyed with it. He deduced that the solution (excuse the pun) was to dissolve everything possible, precipitate using Iron and then leach out as many base metals as possible before attempting to recover any PGMs. Nothing much has changed in the last 200 or so years. The same year, he discovered Palladium.

Previously, in 1801, he demonstrated the electroplating of copper. That was cutting edge.