After a lot of reading and experimenting I've come to a question I cannot find the answer to.
I've carried out test runs on Sterling, using both the precipitation by Copper method, and the Silver chloride way and Hoke's Zinc method to reduce it to Ag metal. The latter seems to work best for me as filtration and cleaning of the chloride is easier, especially when using large amounts of hot water as Hoke recommends ( Karo syrup is not available here - so that way is ruled out ). The downside is the cost of Zinc, and the large volumes of diluted solutions which cannot be re-used.
I've now got a fair quantity of "qauternary metal" ( 50%Ag, 40%Cu, 5% Zn & Ni ea ). The problem is the amount of Nitric needed to dissolve the metal ( Goldsilverpro's 1.22ml/g of Ag & 4.3ml/g of base metal is spot on ) - about 2.75l per kg of the metal.
I see two methods to proceed:
1. Precipitation by Cu, and then using electrowinning of the Cu to restore some of the Nitric, a bit like this thread by lazersteve;
https://www.goldrefiningforum.com/phpBB3/viewtopic.php?f=50&t=2868
I would then use the "restored" nitric to dissolve more metal and repeat until the Nitric is "used up". The Cu removed from the Nitric could be re-used to drop more Ag, that way any Silver that gets electro-deposited with the copper would return to the process and get removed with the rest of the Ag.
2. Precipitation of Silver Chloride using HCl ( only 90-95% as mentioned here - https://www.goldrefiningforum.com/phpBB3/viewtopic.php?f=50&t=26660 )
The copper would also have to be removed electrochemically as it builds up as in the previous method. ( Loss of some Silver seems inevitable here if any of it deposits along with the Copper when it is removed, as it will be mixed with large amounts of Cu ).
The problem I see with both processes is that the Nitric acid eventually becomes "used up" through the loss of Nitrogen as NOx and NO2 when the source metal is dissolved.
Other than regeneration of Nitric acid by bubbling the NOx and NO2 through water with a wash bottle is there another way to regenerate the HNO3?
Some research seems to suggest that the addition of Hydrogen Peroxide to the solutions might help, as per the following:
2NO + 3H2O2 - 2HNO3 + 2H2O and,
2NO2 + H2O2 - 2HNO3
See dangers here:
http://hydrogen-peroxide.us/chemical-catalyst-decomposition/Solvay-Hydrogen-Peroxide-Nitric-Acid-Hazards-2005.pdf
I would believe the time to add the H2O2 would be when the gasses are created ( ie. when dissolving the metal ), and when regenerating the Nitric during electrochemical removal of the Cu, as well as when dropping AgCl with HCl.
So, eventually I get to the questions....
1. Will addition of some dilute H2O2 to the Nitric acid help in making the acid "go further" by regenerating some Nitric acid as per the above 2 equations while dissolving the source metal?
2. Will addition of some dilute H2O2 to the solution when adding HCl ( to drop AgCl ) help in recapturing some of the NO3?
3. Will addition of some dilute H2O2 to the solution when electrolytically removing Cu from Copper Nitrate help in regenerating the HNO3?
Anything I'm missing? Has anyone tried adding Peroxide at any time during Silver processing? Any comments or refinements to the processes would be appreciated.
I also apologise in advance for the length of the post....
Pete.
I've carried out test runs on Sterling, using both the precipitation by Copper method, and the Silver chloride way and Hoke's Zinc method to reduce it to Ag metal. The latter seems to work best for me as filtration and cleaning of the chloride is easier, especially when using large amounts of hot water as Hoke recommends ( Karo syrup is not available here - so that way is ruled out ). The downside is the cost of Zinc, and the large volumes of diluted solutions which cannot be re-used.
I've now got a fair quantity of "qauternary metal" ( 50%Ag, 40%Cu, 5% Zn & Ni ea ). The problem is the amount of Nitric needed to dissolve the metal ( Goldsilverpro's 1.22ml/g of Ag & 4.3ml/g of base metal is spot on ) - about 2.75l per kg of the metal.
I see two methods to proceed:
1. Precipitation by Cu, and then using electrowinning of the Cu to restore some of the Nitric, a bit like this thread by lazersteve;
https://www.goldrefiningforum.com/phpBB3/viewtopic.php?f=50&t=2868
I would then use the "restored" nitric to dissolve more metal and repeat until the Nitric is "used up". The Cu removed from the Nitric could be re-used to drop more Ag, that way any Silver that gets electro-deposited with the copper would return to the process and get removed with the rest of the Ag.
2. Precipitation of Silver Chloride using HCl ( only 90-95% as mentioned here - https://www.goldrefiningforum.com/phpBB3/viewtopic.php?f=50&t=26660 )
The copper would also have to be removed electrochemically as it builds up as in the previous method. ( Loss of some Silver seems inevitable here if any of it deposits along with the Copper when it is removed, as it will be mixed with large amounts of Cu ).
The problem I see with both processes is that the Nitric acid eventually becomes "used up" through the loss of Nitrogen as NOx and NO2 when the source metal is dissolved.
Other than regeneration of Nitric acid by bubbling the NOx and NO2 through water with a wash bottle is there another way to regenerate the HNO3?
Some research seems to suggest that the addition of Hydrogen Peroxide to the solutions might help, as per the following:
2NO + 3H2O2 - 2HNO3 + 2H2O and,
2NO2 + H2O2 - 2HNO3
See dangers here:
http://hydrogen-peroxide.us/chemical-catalyst-decomposition/Solvay-Hydrogen-Peroxide-Nitric-Acid-Hazards-2005.pdf
I would believe the time to add the H2O2 would be when the gasses are created ( ie. when dissolving the metal ), and when regenerating the Nitric during electrochemical removal of the Cu, as well as when dropping AgCl with HCl.
So, eventually I get to the questions....
1. Will addition of some dilute H2O2 to the Nitric acid help in making the acid "go further" by regenerating some Nitric acid as per the above 2 equations while dissolving the source metal?
2. Will addition of some dilute H2O2 to the solution when adding HCl ( to drop AgCl ) help in recapturing some of the NO3?
3. Will addition of some dilute H2O2 to the solution when electrolytically removing Cu from Copper Nitrate help in regenerating the HNO3?
Anything I'm missing? Has anyone tried adding Peroxide at any time during Silver processing? Any comments or refinements to the processes would be appreciated.
I also apologise in advance for the length of the post....
Pete.