AR and heavy plated pins

[SapunovDmitry]

Hi everyone!
Yesterday i've got a lot of heavy plated pins (about 650 gramms). There is a "hunt" for them in Russia, cause they have 5-7% gold by weight.
AR was used for them (1,5 l of HCl and 0,5 l of nitric).
The first thing i didn't like about them was a big amount of foam (more than usually).
It was foaming(it was also fuming, but 10 meters of brown/white fumes is a normal practice) so much that i had to divide the whole solution into three parts( though i've used a new 5 l bucket ) cause i didn't want to lose even a ml of the solution. Of course i did that only when reaction stopped due to the lack of acid (AR was added 3 times, 400 ml at a time).
Well, two of them were filtered, dilluted and urea/smb was used for them.There was a colour change from green/blue to dark green in them (but no seen precipitate).
The third one was still running, but i finally ran out of chemicals and reaction stopped (not everything was dissolved).
So here is a question, i have the whole solution now ( except maybe for 15-20 ml spilled solution) in three glass buckets, and i think the first two don't have much gold in them (but they do have some gold though cause i've tested them with stannous and it was positive) and the reason i didn't see much gold is that it is too fine (particles are small) and will settle down later). There is a great amount of black powder on the bottom of the third bucket among the remnants of pins.I think that copper replaced gold in the solution (thats why i think two first buckets don't have much precipitate). All of the gold is in those three buckets but i don't know what to do next to drop it all (I think an once of gold is a good reason to do that ). My biggest fear is that remnants of those pins didn't dissolve cause they had a silver underplating and thick sivler chloride film was formed (thopugh it was shining like the nickel one). Am i right with this logic? Shall i add some more AR next weekend to disslove everything? Or there are other ways?

 

[goldsilverpro]

By my calculations, it would take about 2.5 liters, or a little more, of aqua regia to dissolve 600 gm of copper. As long as there is any Cu left, I would think that any dissolved gold will cement on the undissolved copper, just as you suspected. I don't think you will be guaranteed complete recovery until all the copper is gone.

Silver could definitely cause problems when using AR. However, by your description of the white layer being shiny, it doesn't sound like AgCl. To make sure, rinse off a pin or two and see if the white material will dissolve in a little warm ammonia. Then acidify the ammonia with a little HCl. A white ppt would indicate AgCl.

It would have been nice to first use only nitric to dissolve the Cu and Ag, if it is present. However, since the gold is so heavy, the nitric would probably never penetrate through it to get after the copper. If there were some simple way to scar the gold, so the copper was exposed on each pin in a place or two, the nitric probably work. Also, you probably could have melted and shotted the pins and then gone into nitric. That's how I would have done it if I were using acids. I know that doesn't help your present situation but, you might consider it in the future.

I have run pins of that gold value that had a heavy silver underplate. I used sodium cyanide plus oxidizers (m-NBSS + H2O2) at about 140 F .

 

[SapunovDmitry]

I almost always use cell for pins (cause it is cheaper) but thats the special case. I thought of nitric before the use of AR but i expected a heavy plating so i skipped that part, though i usually do the same u do.
So i will try to continue to dissolve pins till they are all gone and use a standard AR method.
Any other suggestions are welcome.

 

[goldsilverpro]

You're right. The sulfuric cell would have worked great, especially if the parts were tumbled.

 

[eemtek]

I have run pins of that gold value that had a heavy silver underplate. I used sodium cyanide plus oxidizers (m-NBSS + H2O2) at about 140 F .

silvergoldpro, why did you quit using cyanide?

 

[goldsilverpro]

silvergoldpro, why did you quit using cyanide?

The last time I used cyanide was about 5 years ago. I'm retired now so I don't use anything. Were I to refine again, I would definitely use cyanide, if the material called for it (anything with exposed gold plating).

 

[SapunovDmitry]

I never used cyanides guys. And i will try never to use them.
It's kind of "bad for health" stuff, though i bet it is effective in this case.

 

[eemtek]

silvergoldpro, why did you quit using cyanide?

The last time I used cyanide was about 5 years ago. I'm retired now so I don't use anything. Were I to refine again, I would definitely use cyanide, if the material called for it (anything with exposed gold plating).

thanks for your honesty. I've been working with it for a while. Its just too taboo, all i hear is bad things about it.. I have good equipment and building a new lab devoted to it.. most say it is unheathy, and they choose to work with chlorides and acids?

please tell me your opinion, do you drop the silver before the gold. Some references say to add sulfide or chloride first.. I perfer to add zinc, buffer, and drop all the metals at once.. Tried ion exchange and carbon, but they foul out too fast with more material to handle..

 

[goldsilverpro]

thanks for your honesty. I've been working with it for a while. Its just too taboo, all i hear is bad things about it.. I have good equipment and building a new lab devoted to it.. most say it is unheathy, and they choose to work with chlorides and acids?

If you know what you're doing and obey all safety and waste requirements, I think it's safer than working with acids.

May I ask what equipment you are using, the type material you're processing, and the cyanide formulations you're using? I might be able to make some suggestions. If you don't feel comfortable posting this information, PM me.

please tell me your opinion, do you drop the silver before the gold. Some references say to add sulfide or chloride first.. I perfer to add zinc, buffer, and drop all the metals at once.. Tried ion exchange and carbon, but they foul out too fast with more material to handle..

Most of what I ran was gold only. With a mix of gold and silver, I would zinc it all together and remove the excess zinc with dilute sulfuric. What is the "buffer" you are using?