Hello, GRF,
I’m new here, and my friend and I are electrical engineers in the consumer electronics industry. We recently cleaned out the EE lab at the office and were allowed to keep any of the electronics garbage that we wanted. So we spent a couple of weeks’ time, off-and-on, to pull out pins, cut out gold fingers, and desolder what components we could off the PCBs that were gold plated, for our gold recovery and refining project.
The lab has exhaust pipes with hoods to draw away the fumes from soldering, and so we’ll be using doing all of the chemistry underneath those.
We are not pressed for time or anything (this is a personal project, after all, probably once-in-a-lifetime-only), but we would like to be as safe and yet as efficient (in terms of gold recovered & purity achieved) as we can, all the while learning and having fun.
Toward those ends, I have drafted up the following procedure based on what we could understand from our research. The point of this post was to sum up our knowledge into a procedure we can safely follow to achieve our desired results, and to beg for your review and evaluation of it. Before that, here are a few side notes.
* I happen to have a reverse osmosis (RO) system at home so we’ll be using RO water throughout the entire process.
* When heating water, we would use a microwave, with the water inside a glass container that has been cleaned inside and out with soap, and nothing else would touch the inside of the container. When not in use, the container would be placed up-side-down on a rack.
* We are unable to obtain hydrochloric acid where we live, so we’ll just use the vinegar + table salt (iodine-free) + hydrogen peroxide (peracetic acid?) method. However, for simplicity’s sake, I’ll just refer to it as HCl (or hydrochloric acid).
* We also can’t obtain nitric acid, so no aqua regia for us; nor concentrated sulfuric acid, so we have to make our own from battery acid; we aren’t even going to consider cyanide due to the dangers associated with it; and we wanted to do something completely different from our work, so no reverse-electroplating.
* Thank you, Harold_V, for your multiple-washing instructions for gold powder. As well, thank you, Harold_V, Geo, Butcher, lasersteve, FrugalRefiner, 4metals, and all the experts whose names I have forgotten, for all your valuable info.
And now, on to the procedure that we have written up. The gold recovery process is already in progress and
Recover the Gold
a) Soak the electronics parts in a white vinegar, salt, and hydrogen peroxide solution for ~3 days and, with an old toothbrush, brush off all gold plating that hasn’t yet fallen off. I’m using an air bubbler, with a knot tied at the end of the tube but with holes poked through it with a sewing needle, to reduce the bubble sizes. This was used during my aquarium days and was great for injecting CO2 into the fish tank without having to deal with air stones with lousy suction cups and needing to be cleaned in bleach once in a while due to algae growth, etc.
b) Rinse gold flakes from larger components (eg. PCBs) with water into the metal solution.
c) Filter the solution using a plastic kitchen strainer to eliminate larger pieces while keeping the gold foils in the solution.
d) Rinse the larger pieces for final gold flakes. Use the toothbrush if needed.
e) Filter the gold flakes with 2 stacked coffee filters. Using a wash bottle, wash the gold flakes to the center of the filter.
Prepare Concentrated HCl for Dissolving, Washing and Purifying Gold
a) Place white vinegar in refrigerator (4 deg. C) for a couple of hours.
b) Filter for the solids formed (concentrated acetic acid).
c) Dissolve salt until saturated.
d) Filter out salt that was not dissolved.
Dissolve the Gold
a) Place entire filter paper with the gold flakes inside, into a glass container.
b) Add the concentrated HCl into the container with the filter paper + gold flakes.
b) Gently heat to keep mixture at ~65 deg. C.
c) Slowly add bleach, stir, until all gold dissolves.
d) Cover and allow to settle for half an hour. If more gold particles remain, reheat and add more bleach.
e) Repeat d) until all gold is dissolved.
Precipitate Platinum and Palladium
a) Saturate hot water with ammonium chloride.
b) Use filter paper to filter out trash.
c) Add to metal solution and stir.
d) Using excessive ammonium chloride may be helpful in precipitating all the Platinum and Palladium.
Q.: Harold_V has said that Platinum-group metals do not precipitate from dilute solutions, so I’m not sure about this step. Could I skip this step without the risk of contaminating the gold with the Platinum-group metals?
Precipitate Lead
a) Around here, I can only get 1:1.28 strength battery acid. Concentrated sulfuric acid is out of the question.
b) Boil battery acid to remove water. Place inverted funnel on top to reduce splashing.
c) I would stop the heat once the boiling slows down to minimize dangers associated with overheating very concentrated sulfuric acid (hopefully this concentration is enough to precipitate all of the lead in the metal solution).
e) Allow both the concentrated sulfuric acid and the metal solution to cool to ambient temperature.
f) Drip sulfuric acid into the metal solution to precipitate lead, until no more lead sulfate forms with any additional sulfuric acid.
g) Add ice cubes to metal solution + lead to cool before filtering out lead sulfate and other particles.
Precipitate Gold
a) Heat metal solution to ~50 deg. C.
b) Sprinkle Sodium Metabisulfite (SMB) into metal solution and stir.
c) If Stannous Chloride test shows positive for gold, then sprinkle more SMB.
d) After Stannous Chloride test shows negative for gold in the metal solution, filter for the gold precipitate.
NOTE: Thanks in advance to lasersteve for his reply at Small precip particals. Good to know to avoid precipitating ultra-fine gold particles that pass right through the filter paper!
Wash and Re-Purify Gold
a) Hot water wash until the mixture becomes clear.
b) Into gold powder, add concentrated HCl prepared previously, and water. Boil mixture for about an hour.
d) Add water to cool, allow gold to settle, then decant.
e) Add 10% ammonium chloride solution (the best that I could get here), twirl to allow ammonium chloride solution to wash powder more thoroughly, then boil until ammonium is gone.
f) Add water to cool, allow gold to settle, then decant.
g) Wash with hot water, add water to cool, allow gold to settle, then decant.
h) Repeat through a) to g).
Q.: By Hoke (pg. 73, section entitled "Some Special Cases"), this should be enough to get rid of most of the tin, iron, copper, lead sulfate. However, she used aqua regia (which we can’t obtain because we can’t obtain nitric acid here, but tin would be dissolved by HCl anyway, so we believe that this process would still help). Is this correct?
Re-Dissolve and Re-Precipitate Gold
a) Into mixture, add concentrated HCl prepared earlier.
b) Gently heat to keep mixture at 65 deg. C.
c) Slowly add bleach, stir, until all gold dissolves.
d) Allow to settle for an hour. If more gold particles remain, reheat and add more bleach.
e) Repeat d) until all gold is dissolved.
f) Precipitate gold, using oxalic acid (Hoke said that this was good if the gold to be precipitated is already pretty pure, which we are hoping that this would be the case at this stage; Harold_V said that he never used 2 different precipitants and still got good results), which is available here as a bathroom and porcelain tile cleaner.
g) Filter gold powder, wash thoroughly with hot water while in filter.
Q.: Is this a bad idea, using a bathroom and porcelain tile cleaner containing oxalic acid to precipitate the gold?
Prepare Gold for Melting
a) Crunch filter-paper-with-gold into a ball, and gently blow-dry.
b) Wet ball with alcohol and allow to dry.
NOTE: This is so that I do not blow off any gold powder during the melting process. I don’t have the experience or the confidence to do it correctly (elevating the torch to 12+ inches initially, heating from the edges, etc.), and I don’t have the resources to practice with. Additionally, we have limited access to the torch.
Melt Gold
a) I’ll be using a newly-purchased ceramic melt dish, washed with soap and water. I will also abrasively clean the inside and outside of the torch head, as well as wiping it clean.
b) Heat with torch, sprinkling Borax powder now and then to see if it melts right away.
c) When Borax melts easily, cover lightly with Borax, and then place the filter paper ball inside, then sprinkle some more Borax over the ball.
d) Heat the ball, until all paper burns away and gold melts into a bead.
e) Very, very slowly lift the torch (taking eg. 5-10 minutes to get to a far-enough distance where the heat from the torch is no longer sufficient to keep the gold glowing-hot). The purpose of this is to see if I could make the gold form into crystals.
f) Allow gold and melting dish to cool completely, then add water to dissolve the Borax and retrieve the gold.
Q.: How could I make sure that the gold does not attach itself to the melt dish?
Q.: if the gold does get attached to the dish, would I be able to just break the dish to get the gold bead without damaging its surface? Hoke said to just break the dish (actually she used a crucible), but didn’t mention whether this would damage the gold bead’s surface.
Waste Solution
Q.: I’d like to grow cupric acetate crystals from the waste solution. Could I use a similar method to growing salt crystals? I’d probably add copper wires pieces to the solution until no more would dissolve (saturate), then using a long copper wire instead of the fishing line in the solution, then add small pieces of aluminum foil to the solution and let the cupric acetate form crystals on the copper wire? Would the cupric acetate precipitate too quickly for crystals to form on the wire, even if I add ~1cm^2 of aluminum foil at a time?
Q.: After growing the cupric acetate crystal to the size I’m happy with, we would first continue adding aluminum foil until it no longer dissolves. Then we’d slowly add lots of baking soda (slowly, so that the reaction isn’t so fast that the liquid spills) to neutralize the acid (and do a pH test). Then we’d filter the particles to seal in a transparent resin to serve as a record of this event (also for safety reasons). Afterwards, would the solution be safe to be poured down the drain? Would the sodium bicarbonate be enough to cement out the aluminum? By the reactivity series, it should... Right? We don’t have the sodium hydroxide or the magnesium hydroxide that 4metals mentioned in his waste treatment reply.
NOTE: Thank you to 4metals for his reply about waste treatment here.
NOTE: After I get the crystal, I would then cover it with a nice thick coat of clear nail polish to preserve it.
Additional Questions
Q.: How could I have recovered more gold, eg. from the waste solution?
Q.: How high could I expect the purity of the gold bead to be if I follow this process?
Q.: What steps did I miss? I’m sure I forgot something... What could I do better? Which steps may be unnecessary and could be skipped without affecting gold purity? Looking for an education here!
Q.: Are there any safety-related issues you see with this process? What could I do to further reduce damage to myself and my friend, and to the environment?
Thank you so very much for having read this far, and we really appreciate any comments, questions, what-ifs, and/or concerns that you might have for us!
I’m new here, and my friend and I are electrical engineers in the consumer electronics industry. We recently cleaned out the EE lab at the office and were allowed to keep any of the electronics garbage that we wanted. So we spent a couple of weeks’ time, off-and-on, to pull out pins, cut out gold fingers, and desolder what components we could off the PCBs that were gold plated, for our gold recovery and refining project.
The lab has exhaust pipes with hoods to draw away the fumes from soldering, and so we’ll be using doing all of the chemistry underneath those.
We are not pressed for time or anything (this is a personal project, after all, probably once-in-a-lifetime-only), but we would like to be as safe and yet as efficient (in terms of gold recovered & purity achieved) as we can, all the while learning and having fun.
Toward those ends, I have drafted up the following procedure based on what we could understand from our research. The point of this post was to sum up our knowledge into a procedure we can safely follow to achieve our desired results, and to beg for your review and evaluation of it. Before that, here are a few side notes.
* I happen to have a reverse osmosis (RO) system at home so we’ll be using RO water throughout the entire process.
* When heating water, we would use a microwave, with the water inside a glass container that has been cleaned inside and out with soap, and nothing else would touch the inside of the container. When not in use, the container would be placed up-side-down on a rack.
* We are unable to obtain hydrochloric acid where we live, so we’ll just use the vinegar + table salt (iodine-free) + hydrogen peroxide (peracetic acid?) method. However, for simplicity’s sake, I’ll just refer to it as HCl (or hydrochloric acid).
* We also can’t obtain nitric acid, so no aqua regia for us; nor concentrated sulfuric acid, so we have to make our own from battery acid; we aren’t even going to consider cyanide due to the dangers associated with it; and we wanted to do something completely different from our work, so no reverse-electroplating.
* Thank you, Harold_V, for your multiple-washing instructions for gold powder. As well, thank you, Harold_V, Geo, Butcher, lasersteve, FrugalRefiner, 4metals, and all the experts whose names I have forgotten, for all your valuable info.
And now, on to the procedure that we have written up. The gold recovery process is already in progress and
Recover the Gold
a) Soak the electronics parts in a white vinegar, salt, and hydrogen peroxide solution for ~3 days and, with an old toothbrush, brush off all gold plating that hasn’t yet fallen off. I’m using an air bubbler, with a knot tied at the end of the tube but with holes poked through it with a sewing needle, to reduce the bubble sizes. This was used during my aquarium days and was great for injecting CO2 into the fish tank without having to deal with air stones with lousy suction cups and needing to be cleaned in bleach once in a while due to algae growth, etc.
b) Rinse gold flakes from larger components (eg. PCBs) with water into the metal solution.
c) Filter the solution using a plastic kitchen strainer to eliminate larger pieces while keeping the gold foils in the solution.
d) Rinse the larger pieces for final gold flakes. Use the toothbrush if needed.
e) Filter the gold flakes with 2 stacked coffee filters. Using a wash bottle, wash the gold flakes to the center of the filter.
Prepare Concentrated HCl for Dissolving, Washing and Purifying Gold
a) Place white vinegar in refrigerator (4 deg. C) for a couple of hours.
b) Filter for the solids formed (concentrated acetic acid).
c) Dissolve salt until saturated.
d) Filter out salt that was not dissolved.
Dissolve the Gold
a) Place entire filter paper with the gold flakes inside, into a glass container.
b) Add the concentrated HCl into the container with the filter paper + gold flakes.
b) Gently heat to keep mixture at ~65 deg. C.
c) Slowly add bleach, stir, until all gold dissolves.
d) Cover and allow to settle for half an hour. If more gold particles remain, reheat and add more bleach.
e) Repeat d) until all gold is dissolved.
Precipitate Platinum and Palladium
a) Saturate hot water with ammonium chloride.
b) Use filter paper to filter out trash.
c) Add to metal solution and stir.
d) Using excessive ammonium chloride may be helpful in precipitating all the Platinum and Palladium.
Q.: Harold_V has said that Platinum-group metals do not precipitate from dilute solutions, so I’m not sure about this step. Could I skip this step without the risk of contaminating the gold with the Platinum-group metals?
Precipitate Lead
a) Around here, I can only get 1:1.28 strength battery acid. Concentrated sulfuric acid is out of the question.
b) Boil battery acid to remove water. Place inverted funnel on top to reduce splashing.
c) I would stop the heat once the boiling slows down to minimize dangers associated with overheating very concentrated sulfuric acid (hopefully this concentration is enough to precipitate all of the lead in the metal solution).
e) Allow both the concentrated sulfuric acid and the metal solution to cool to ambient temperature.
f) Drip sulfuric acid into the metal solution to precipitate lead, until no more lead sulfate forms with any additional sulfuric acid.
g) Add ice cubes to metal solution + lead to cool before filtering out lead sulfate and other particles.
Precipitate Gold
a) Heat metal solution to ~50 deg. C.
b) Sprinkle Sodium Metabisulfite (SMB) into metal solution and stir.
c) If Stannous Chloride test shows positive for gold, then sprinkle more SMB.
d) After Stannous Chloride test shows negative for gold in the metal solution, filter for the gold precipitate.
NOTE: Thanks in advance to lasersteve for his reply at Small precip particals. Good to know to avoid precipitating ultra-fine gold particles that pass right through the filter paper!
Wash and Re-Purify Gold
a) Hot water wash until the mixture becomes clear.
b) Into gold powder, add concentrated HCl prepared previously, and water. Boil mixture for about an hour.
d) Add water to cool, allow gold to settle, then decant.
e) Add 10% ammonium chloride solution (the best that I could get here), twirl to allow ammonium chloride solution to wash powder more thoroughly, then boil until ammonium is gone.
f) Add water to cool, allow gold to settle, then decant.
g) Wash with hot water, add water to cool, allow gold to settle, then decant.
h) Repeat through a) to g).
Q.: By Hoke (pg. 73, section entitled "Some Special Cases"), this should be enough to get rid of most of the tin, iron, copper, lead sulfate. However, she used aqua regia (which we can’t obtain because we can’t obtain nitric acid here, but tin would be dissolved by HCl anyway, so we believe that this process would still help). Is this correct?
Re-Dissolve and Re-Precipitate Gold
a) Into mixture, add concentrated HCl prepared earlier.
b) Gently heat to keep mixture at 65 deg. C.
c) Slowly add bleach, stir, until all gold dissolves.
d) Allow to settle for an hour. If more gold particles remain, reheat and add more bleach.
e) Repeat d) until all gold is dissolved.
f) Precipitate gold, using oxalic acid (Hoke said that this was good if the gold to be precipitated is already pretty pure, which we are hoping that this would be the case at this stage; Harold_V said that he never used 2 different precipitants and still got good results), which is available here as a bathroom and porcelain tile cleaner.
g) Filter gold powder, wash thoroughly with hot water while in filter.
Q.: Is this a bad idea, using a bathroom and porcelain tile cleaner containing oxalic acid to precipitate the gold?
Prepare Gold for Melting
a) Crunch filter-paper-with-gold into a ball, and gently blow-dry.
b) Wet ball with alcohol and allow to dry.
NOTE: This is so that I do not blow off any gold powder during the melting process. I don’t have the experience or the confidence to do it correctly (elevating the torch to 12+ inches initially, heating from the edges, etc.), and I don’t have the resources to practice with. Additionally, we have limited access to the torch.
Melt Gold
a) I’ll be using a newly-purchased ceramic melt dish, washed with soap and water. I will also abrasively clean the inside and outside of the torch head, as well as wiping it clean.
b) Heat with torch, sprinkling Borax powder now and then to see if it melts right away.
c) When Borax melts easily, cover lightly with Borax, and then place the filter paper ball inside, then sprinkle some more Borax over the ball.
d) Heat the ball, until all paper burns away and gold melts into a bead.
e) Very, very slowly lift the torch (taking eg. 5-10 minutes to get to a far-enough distance where the heat from the torch is no longer sufficient to keep the gold glowing-hot). The purpose of this is to see if I could make the gold form into crystals.
f) Allow gold and melting dish to cool completely, then add water to dissolve the Borax and retrieve the gold.
Q.: How could I make sure that the gold does not attach itself to the melt dish?
Q.: if the gold does get attached to the dish, would I be able to just break the dish to get the gold bead without damaging its surface? Hoke said to just break the dish (actually she used a crucible), but didn’t mention whether this would damage the gold bead’s surface.
Waste Solution
Q.: I’d like to grow cupric acetate crystals from the waste solution. Could I use a similar method to growing salt crystals? I’d probably add copper wires pieces to the solution until no more would dissolve (saturate), then using a long copper wire instead of the fishing line in the solution, then add small pieces of aluminum foil to the solution and let the cupric acetate form crystals on the copper wire? Would the cupric acetate precipitate too quickly for crystals to form on the wire, even if I add ~1cm^2 of aluminum foil at a time?
Q.: After growing the cupric acetate crystal to the size I’m happy with, we would first continue adding aluminum foil until it no longer dissolves. Then we’d slowly add lots of baking soda (slowly, so that the reaction isn’t so fast that the liquid spills) to neutralize the acid (and do a pH test). Then we’d filter the particles to seal in a transparent resin to serve as a record of this event (also for safety reasons). Afterwards, would the solution be safe to be poured down the drain? Would the sodium bicarbonate be enough to cement out the aluminum? By the reactivity series, it should... Right? We don’t have the sodium hydroxide or the magnesium hydroxide that 4metals mentioned in his waste treatment reply.
NOTE: Thank you to 4metals for his reply about waste treatment here.
NOTE: After I get the crystal, I would then cover it with a nice thick coat of clear nail polish to preserve it.
Additional Questions
Q.: How could I have recovered more gold, eg. from the waste solution?
Q.: How high could I expect the purity of the gold bead to be if I follow this process?
Q.: What steps did I miss? I’m sure I forgot something... What could I do better? Which steps may be unnecessary and could be skipped without affecting gold purity? Looking for an education here!
Q.: Are there any safety-related issues you see with this process? What could I do to further reduce damage to myself and my friend, and to the environment?
Thank you so very much for having read this far, and we really appreciate any comments, questions, what-ifs, and/or concerns that you might have for us!