Hi
new short refining here
To start new year, I chosen rather interesting and not very common material - gold plated metal discs from old power semiconductors (most probably power tyristors/triacs or diodes). These are either from pure molybdenum, or tungsten/aluminium sintered alloy. Silicon wafer is on the one side of the disc (actual semiconductor), and both the Mo and Si side is heavily gold plated. By heavily, I mean like very thick plating. That kind of plating that XRF on the best pieces cannot see the underlying base metal and shows 999 gold (this is good informative test which approximately tell you the thickness of the plating - how much gold your XRF see in the sample of eg pins or fingers). Usually, thickness of the plating was bigger on the silicon wafer side, and thinner on the Mo/W side.
As molybdenum could be attacked by AR (at least I read it from the literature), I chosen I2/KI leach the method for this stripping - due to the fact that Mo and W should both be quite resistant to iodine.
I prepared stock solution of leach from 15 g I2, 60g KI and 150 ml of water. Discs were propped in the beaker by magnetic stirbar - which allowed both sides of the disc to be stripped at same time. As long as the base metal/substrate of gold plating is inert to the iodine, this method is very, very comfortable and practically safe to perform even without fumehood (of course, be aware that iodine stains nearly anything - penetrate nitrile gloves relatively easily etc.). Some iodine smell is unavoidable during weighing of the solid iodine (sublimation), but as long as the iodine is covered by the KI solution, the solution of formed active KI3 has just very subtle iodine smell - not saying that it is healthy practice, and essentially any chemistry is best done in fumehood. If you just don´t pour this solution over your skin, or worse eyes, you will be fine.
So as I had 11 pieces, I decided to speed up the leaching by setting three independent small beakers, all with same setup. Leach solution was poured to the beakers in a way to cover the discs completely, and occasionally swirled to mix the solution around. It took 5-25 minutes to fully strip gold plating from the pieces - varying duration due to variating thickness of gold plating. It was also somewhat consistent with preliminary findings of XRF - more shown gold = thicker plating = longer leaching time. This method is also very nice due to the fact you can easily observe how the stripping is going - visually. So when you see gold is gone to the solution, you can then remove the discs using some plastic tweezers, submerge it in the beaker with a bit of water as the wash solution, and continue by immersing the next piece into that very same leaching solution.
As time progressed, I realized that I made unnecessarily big ammount of leaching solution - and I would be good to go also with like 2/3 of it. No worries, I will use it later to strip some more low grade stuff
When all of the discs were de-plated, I poured all of the solutions to one beaker, washed all beakers firstly with that aforementioned rinsing water, and then pure water (not to unnecessarily grow up in volume).
In order to drop gold from this solution - there is hardly anything more simple You just take regular sodium sulfite (cheap and plentiful, SMB works as good, if not better) and start spooning it in, simultaneously mixing the solution with glass rod. As the sulfite reacts with iodine first - you will see discoloration and fading of the iodine colour, and immediately as iodine is all reduced, gold start to also be reduced and drop out of solution. I should also mention that this method is consistently yielding some of the heaviest gold precipitates I have ever created. After mixing enough sulfite to reduce all of the gold (stannous negative + SO2 smell evident), it took no more than 20 minutes or so for solution to completely clarify and settle - leaving very heavy gold deposit on the bottom.
As usual with classical gold recovery procedures from AR, I then decanted the gold (simultaneously filtering the poured solution through the Buchner funnel to avoid losses of few "floaters" and eventually some very tiny particles of gold that gone "floaty"). Washed with cold HCL with good swirling for few minutes, followed by rinsing with water till neutral pH is reached. Gold is dried on the hotplate in the beaker and weighed.
Discs issued. Finger for scale medium-sized ones has diameter of around 4cm. Smallest ones I didn´t included in the batch due to the fact they are just flash-gold plated over thick silver. Not much value there, and simultaneously contamining the dropped gold.
Disc in the beaker supported with magnetic stirbar to enable simultaneous leaching from both sides. Anything suitable can be used to prop the disc not to lay flat on the bottom, stirbars were just in hand, so I used these
Leaching in progress simultaneously in three beakers (left) and disc showing in the leach on the right. Notice that they were probably hand-plated, as there were distinct "smears" shown upon leaching - pointing out the various thickness of the gold layer in different places.
Deplated discs. There were some small bits of gold left, mainly on the circumference of the area on the silicon side, where bits of silicon polymer were probably still attached and covered the gold. But overall, this was just very tiny ammount. I will be more diligent in the future
Washed gold on the left and dried gold powder weighed on the right. Nicely pale brown coloured, pointing out big grain size
Result was 0,953 g of very nice, very heavy gold precipitate. Occupying the space less than 0,3-0,4 mL in the vial, this is one of the densest gold powders I ever created.
I was expecting decent ammount since preliminary measurements were good, but nearly gram of gold amazed me. We will see if I will ever stumble across this interesting material once again. As added bonus, I was left with the de-plated Mo and W discs - which are somewhat cool and also like very heavy No use for them at the moment, but cool thing to have for sure.
So that´s it. I hope you find these threads interesting. I will keep you updated with more "tutorials" in the future
orvi
new short refining here
To start new year, I chosen rather interesting and not very common material - gold plated metal discs from old power semiconductors (most probably power tyristors/triacs or diodes). These are either from pure molybdenum, or tungsten/aluminium sintered alloy. Silicon wafer is on the one side of the disc (actual semiconductor), and both the Mo and Si side is heavily gold plated. By heavily, I mean like very thick plating. That kind of plating that XRF on the best pieces cannot see the underlying base metal and shows 999 gold (this is good informative test which approximately tell you the thickness of the plating - how much gold your XRF see in the sample of eg pins or fingers). Usually, thickness of the plating was bigger on the silicon wafer side, and thinner on the Mo/W side.
As molybdenum could be attacked by AR (at least I read it from the literature), I chosen I2/KI leach the method for this stripping - due to the fact that Mo and W should both be quite resistant to iodine.
I prepared stock solution of leach from 15 g I2, 60g KI and 150 ml of water. Discs were propped in the beaker by magnetic stirbar - which allowed both sides of the disc to be stripped at same time. As long as the base metal/substrate of gold plating is inert to the iodine, this method is very, very comfortable and practically safe to perform even without fumehood (of course, be aware that iodine stains nearly anything - penetrate nitrile gloves relatively easily etc.). Some iodine smell is unavoidable during weighing of the solid iodine (sublimation), but as long as the iodine is covered by the KI solution, the solution of formed active KI3 has just very subtle iodine smell - not saying that it is healthy practice, and essentially any chemistry is best done in fumehood. If you just don´t pour this solution over your skin, or worse eyes, you will be fine.
So as I had 11 pieces, I decided to speed up the leaching by setting three independent small beakers, all with same setup. Leach solution was poured to the beakers in a way to cover the discs completely, and occasionally swirled to mix the solution around. It took 5-25 minutes to fully strip gold plating from the pieces - varying duration due to variating thickness of gold plating. It was also somewhat consistent with preliminary findings of XRF - more shown gold = thicker plating = longer leaching time. This method is also very nice due to the fact you can easily observe how the stripping is going - visually. So when you see gold is gone to the solution, you can then remove the discs using some plastic tweezers, submerge it in the beaker with a bit of water as the wash solution, and continue by immersing the next piece into that very same leaching solution.
As time progressed, I realized that I made unnecessarily big ammount of leaching solution - and I would be good to go also with like 2/3 of it. No worries, I will use it later to strip some more low grade stuff
When all of the discs were de-plated, I poured all of the solutions to one beaker, washed all beakers firstly with that aforementioned rinsing water, and then pure water (not to unnecessarily grow up in volume).
In order to drop gold from this solution - there is hardly anything more simple
You just take regular sodium sulfite (cheap and plentiful, SMB works as good, if not better) and start spooning it in, simultaneously mixing the solution with glass rod. As the sulfite reacts with iodine first - you will see discoloration and fading of the iodine colour, and immediately as iodine is all reduced, gold start to also be reduced and drop out of solution. I should also mention that this method is consistently yielding some of the heaviest gold precipitates I have ever created. After mixing enough sulfite to reduce all of the gold (stannous negative + SO2 smell evident), it took no more than 20 minutes or so for solution to completely clarify and settle - leaving very heavy gold deposit on the bottom.As usual with classical gold recovery procedures from AR, I then decanted the gold (simultaneously filtering the poured solution through the Buchner funnel to avoid losses of few "floaters" and eventually some very tiny particles of gold that gone "floaty"). Washed with cold HCL with good swirling for few minutes, followed by rinsing with water till neutral pH is reached. Gold is dried on the hotplate in the beaker and weighed.
Discs issued. Finger for scale
medium-sized ones has diameter of around 4cm. Smallest ones I didn´t included in the batch due to the fact they are just flash-gold plated over thick silver. Not much value there, and simultaneously contamining the dropped gold.Disc in the beaker supported with magnetic stirbar to enable simultaneous leaching from both sides. Anything suitable can be used to prop the disc not to lay flat on the bottom, stirbars were just in hand, so I used these
Leaching in progress simultaneously in three beakers (left) and disc showing in the leach on the right. Notice that they were probably hand-plated, as there were distinct "smears" shown upon leaching - pointing out the various thickness of the gold layer in different places.
Deplated discs. There were some small bits of gold left, mainly on the circumference of the area on the silicon side, where bits of silicon polymer were probably still attached and covered the gold. But overall, this was just very tiny ammount. I will be more diligent in the future
Washed gold on the left and dried gold powder weighed on the right. Nicely pale brown coloured, pointing out big grain size
Result was 0,953 g of very nice, very heavy gold precipitate. Occupying the space less than 0,3-0,4 mL in the vial, this is one of the densest gold powders I ever created.
I was expecting decent ammount since preliminary measurements were good, but nearly gram of gold amazed me. We will see if I will ever stumble across this interesting material once again. As added bonus, I was left with the de-plated Mo and W discs - which are somewhat cool and also like very heavy
No use for them at the moment, but cool thing to have for sure.So that´s it. I hope you find these threads interesting. I will keep you updated with more "tutorials" in the future
orvi
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