Some Nitric Acid Questions

[FrugalRefiner]

I assume that the reason you want to reduce the copper that gets dissolved into solution is because those 6" diameter copper tubes you rotate in the solution are pricey and you don't want them to dissolve too quickly. I do not understand why you rotate the tubes, is there some sort of scraper that dislodges the cemented Silver from the rotating cylinder? I usually aerate a cylinder that copper slabs hang inside of inside the tank of liquid to be cemented to guarantee circulation and allow the aeration to dislodge the Silver.

You can see a tutorial of this technique in When In Doubt, Cement It Out. 4metals described his method, and I created a mini version. 4metals also provided a picture that's at the end of that post of his larger version.

Dave

 

[orvi]

On this scale, I would suggest (as many more experienced refiners before me) just depleting acid beforehand using more silver. As 4metals suggested, treating the solution with silver cement you already have gives you fastest reaction. If speed is not a big deal, you can just siphon the solution onto good excess of more silver feedstock and wait for it to react. It will, even at room temperature, but it would be much slower than with cement powder.

Raising pH with hydroxide is also viable technique, if you have good mechanical stirring device. Equilibration of temporary crushed hydroxides can take some time, so it need to be considered beforehand.

BTW, what is the average purity of silver cement that you are producing by copper cementation ? I am just curious, because it has a lot to do with concentration of silver nitrate and leftover acidity (at least from my observations). Through my career I produced cements ranging from 95-99.8% Ag, and there are some patterns regarding Ag concentration and free acid.

 

[jeff@performance-construc]

This so amazing so many interesting responses, suggestions and ideas I am still trying to digest everything, and we are going to test out some of these ideas and see what may work for us. To answer your question. on average we range 95-98+%. Would like to understand more about the patterns that you talked about. Depleting or consuming the acid beforehand is very interesting and makes sense. Could this be as simple as keeping the silver anode materials in the cell for extra 24 hrs. or more make difference in consuming excess nitric. Or even dropping in some solid refining bars into the solution to help consume excess acid?

 

[4metals]

Could this be as simple as keeping the silver anode materials in the cell for extra 24 hrs. or more make difference in consuming excess nitric. Or even dropping in some solid refining bars into the solution to help consume excess acid?

You mentioned speed as you need to completely treat about 25 gallons a day. In dissolving Silver to consume any excess nitric acid the 2 factors that have the greatest effect on reaction rate are temperature and surface area. Putting the waste into a tank you can easily heat may not be possible but the surface area of the Silver you add will be the greatest if you use cement Silver.

 

[grainsofgold]

You mentioned speed as you need to completely treat about 25 gallons a day. In dissolving Silver to consume any excess nitric acid the 2 factors that have the greatest effect on reaction rate are temperature and surface area. Putting the waste into a tank you can easily heat may not be possible but the surface area of the Silver you add will be the greatest if you use cement Silver.

In reading this i wonder if you could take say 250ml of the silver nitrate that contains free nitric , put it on below boiling heat then add cemented silver from a container that has X grams of it. Once you see an accumulation of cemented silver left in the bottom of the beaker stop. Weigh what’s left of the cemented silver container and subtract that weight from the start weight to give you the amount of cemented silver needed to consume the free nitric per 250 ml. This could then be used to know roughly the amount of cemented silver you will need to add to the remaining silver nitrate to consume the free nitric present before adding Cu to cement the Ag out.

 

[4metals]

You can use silver you have on hand in your shop and use the same trick that Harold used with gold. (if you recall Harold used gold to kill free nitric in aqua regia) If you are running a silver cell and you can always use cement Silver especially when you dissolve it for free while killing off the extra nitric from your dissolve. And doing it as I just said has the added advantage of adding anode material with very low copper which makes the cell run longer between solution maintenance for accumulated copper.

No need to do it on a small measured volume, you can get a stainless steel basket used by electroplaters and weigh out an excess of silver to dissolve and immerse the basket and let it react until it stops blowing red. Then remove the. basket, rinse it, and re-weigh what didn't dissolve if you need to track how much Silver was recovered from the first dissolve.

 

[grainsofgold]

You can use silver you have on hand in your shop and use the same trick that Harold used with gold. (if you recall Harold used gold to kill free nitric in aqua regia) If you are running a silver cell and you can always use cement Silver especially when you dissolve it for free while killing off the extra nitric from your dissolve. And doing it as I just said has the added advantage of adding anode material with very low copper which makes the cell run longer between solution maintenance for accumulated copper.

No need to do it on a small measured volume, you can get a stainless steel basket used by electroplaters and weigh out an excess of silver to dissolve and immerse the basket and let it react until it stops blowing red. Then remove the. basket, rinse it, and re-weigh what didn't dissolve if you need to track how much Silver was recovered from the first dissolve.

good idea