Can somebody explain electrowinning?

Red-

Electrowinning is the process with which a pure metal is formed with electricity being the work put into the system. Without delving into reduction/oxidation reactions too much, this process changes the ore (an oxidized form of the metal to be obtained) into the pure metallic form. This process is used for metals such as Cu, Zn, Al, and Mg most often in industrial production. Using copper as an example the half reaction follows as:

Cu^2+ + 2e^- --> Cu

Where a copper sheet is used as the cathode.

(Note: In this specific process often a lead sheet is used as the anode. This is used partially because the lead is insoluble in the Cu2+ H20 SO4 electrolyte.)

To get to your specific question about platinum and gold, the answer is more complicated than you might think (and more complicated than what I know how to solve). First of all, having more than one cell would make it more than one process, but I digress. I think that the problem lies within what sort of electrolytic solution you would place the cathode and anode. At the cathode, electrons are being provided by the current source in order to form the pure metal. At the same time, at the anode, something must be oxidized, or taking on electrons. With this in mind, you have to think about how you can reduce two metals in one solution (in one cell) at the same time. I have to admit that electro chemistry is not my strong point, but I would suggest that you take a look at how gold and platinum are found in nature and why/how it would be useful to use electrowinning to purify them. It would also be advantageous to take a look at the standard reduction potentials (tables of these values are easily found) for each of these metals. (Hint: Au = +1.5 and Pt =

Although I wrote quite a bit, I don't think I really answered your question. I guess that is what individual research is for. (Damn those Thermodynamics professors for giving us equations for solving these problems but making us provide the thought and work eh?.)

In terms of places to look, the internet gives a broad range of topics but very little in terms of in depth information. I would suggest you go on down to your nearest library and pick up a chemistry or thermodynamics book which covers electrochemistry. I personally have "Introduction to the Thermodynamics of Materials" by Gaskell. It is helpful for relationships, equations, and general knowledge of the subject but will provide no specific answer to your question.

Seth Imhoff

____________________ I take no responsibility for incorrect information. Who knows whether I am qualified or even sane/sober enough to respond to your question.

To add to this for the original questioner, sometimes what is ismportant is what does NOT dissolve. The classic example is in purifying the copper that comes from smelting. The copper dissolves then plates out on the cathodes until the anode is too thin to be useful. The stub and it's top that was out of the solution are then returned to be re-melted and cast into newer anodes.

Once a month or whenever the cells are drained and cleaned. The mud at the bottom is the material that did not dissolve. From the copper electrowinning cells the world extracts 30% IIRC of the annual gold production, significant silver, other precious metals, and often many other metals. The point here is that electrowinning is also an indirect way to concentrate metals.

Where several metals are in solution it can be difficult to seperate them. Gold, silver, copper, zinc, and lead can be in the same cyanide solution and all will plate out together.

To the best of my knowledge platinum is not recovered by electroplating except perhaps as a final cleaning step. A significant reason is the there is far too little of it anywhere in any industrial process and electrowinning must start with percentage level concentrations of metals to be effective. The commercial supply is seperated from the country rock by froth flotation, sometimes in nickel bearing concentrates, then recovered with base metals by smelting, then from the base metals by chemical processes.