Maximizing nitric use.

[glondor]

I am looking for hints, tips and tricks to get the most bang for the buck out of nitric.

I am dissolving base metal (Kovar) and I find the process uses a LOT of acid. 68% nitric cut 50/50 with tap water. I find that it takes a lot of heat to break down the Kovar, and when the reaction starts it goes fast and wild.

I am doing 1 pound of material at a time and it takes about 1800 ml nitric and 1800 ml water. Half to start and half after first reaction dies. Results vary from 80% to 95% disolution. Have to watch carefully for foam/ boil overs.Lots of red fumes.

I am working on a way to capture and condense the nox, however in the mean time I am looking to maximize the process I have. Would further dilution help? Slower longer preheat? Any ideas appreciated.

 

[samuel-a]

Hi Mike.

Cover the metal with water and heat to near boil.
Then add slowly, small portion of the acid to the hot water, the reaction will kick off immediately, but with no risk of spilling.

Also, you can 'scrub' the NOx through a series of two, three or four flasks filled with lye solution, where the NOx is bubbled underneath the liquid level.
The first one should probably be empty if you gonna leave it unattended to prevent back flow. (it will also act as a condenser)
The lye will convert NOx to sodium nitrate, test with pH paper, when it is on 7-8, the lye solution is exhausted. You could probably run few pounds before needing to change the lye solution.

Exhausted lye solution (now Sodium nitrate solution) can acidified slightly with nitric (5-6) and evaporated to produce NaNO3 crystals. (and re-used)

 

[philddreamer]

Thanks Samuel! 8)

 

[glondor]

Thanks Samuel! You gave me an idea, or a reminder rather about some sort of drip system. It would be great if there was a way to meter in 20 mil doses every 2 minutes. interesting.....

 

[samuel-a]

It would be great if there was a way to meter in 20 mil doses every 2 minutes

Burette / sep-funnel .... ?

If you are a technical guy, there are electrical valves that run by a simple programed controller.
Maybe Barren could suggest something here, though i haven't seen him here lately.

 

[goldsilverpro]

I am looking for hints, tips and tricks to get the most bang for the buck out of nitric.

I am dissolving base metal (Kovar) and I find the process uses a LOT of acid. 68% nitric cut 50/50 with tap water. I find that it takes a lot of heat to break down the Kovar, and when the reaction starts it goes fast and wild.

I am doing 1 pound of material at a time and it takes about 1800 ml nitric and 1800 ml water. Half to start and half after first reaction dies. Results vary from 80% to 95% disolution. Have to watch carefully for foam/ boil overs.Lots of red fumes.

I am working on a way to capture and condense the nox, however in the mean time I am looking to maximize the process I have. Would further dilution help? Slower longer preheat? Any ideas appreciated.

Actually, a pound of Kovar requiring 1800 ml of nitric is very close to the figure I always give for dissolving base metals. I have always said that it takes a gallon of nitric to dissolve 2 pounds of copper or most other base metals. Of course, you need to add water to the nitric. With 50/50 nitric, 1 gallon (3785 ml) will dissolve 1 pound of base metals. Whether you dilute it more or less, it will still take about 1/2 gallon (1893 ml) of the nitric, itself, to dissolve 1 pound of base metals. This can vary, somewhat, depending on the exact strength of nitric that you purchase.

The method that Sam mentioned - covering with water, heating, and then feeding the nitric in increments - is the same method I've been preaching since the forum started. However, when you allow the solution to cool, the metal salts can crystallize unless you have about 50% water. Therefore, when you finish dissolving, I would add some water before letting it cool.

When most of the metal has dissolved and the acid you've used to that point is fully spent (used up - no more reaction occurring when hot), I would pour the solution off into another container. Then, cover what metal remains with water, heat it, and feed in a little nitric in increments until it is dissolved. That last bit will dissolve much faster that way. When the solution gets too deep, the reaction slows down.

 

[philddreamer]

Chris, I read this on another of your resent posts & I'll be following this procedure from now on. I had that problem of crystals, & I added @ times water, but didn't realize the root of the problem. Thanks!

 

[goldsilverpro]

In case you didn't add enough water and it crystallizes when cooled, the crystals will dissolve by adding water. However, they will dissolve much faster if the water is first heated. Stirring helps. Your goal is to have enough water to prevent crystals forming at room temperature, yet not add too much water so you generate unnecessary volumes of waste solution. With winter approaching, it will probably take a bit more.

In certain cases, especially when I have a lot of metal in the dissolving vessel, I will add water, heat, and add nitric in increments until the solution level is about 2", or less, above the metal I am dissolving. Then, I let it cool off a bit, add a little water (this cools it off more and helps insure no crystals form), pour off the solution, add water to the remaining metal, heat, and again start feeding in the nitric in increments. I repeat this (sometimes, 4 or 5 times) until the metals are all dissolved. This actually goes faster since it accomplishes 2 things. (1) I can start with more metal in the container. (2) I have found that, when the solution level gets about 2" above the metal, the reaction slows down. This is mainly because the fresh increments of nitric I am adding are too diluted to dissolve the metal quickly. After you pour off and use fresh water and acid, you will find the dissolving goes very fast.

Before pouring the solution off, I let it cool, somewhat, for safety reasons - handling a very hot solution is not very safe. Also, make sure all the acid you've added is used up (fully reacted) before you pour the solution off. When doing this, I don't like to start with more than 20% metal, by volume, in the container. And, of course, don't pour off any of the solids with the liquid. When pouring off the spent solution, you don't have to pour off 100%. Just pour off what you can without pouring off any solids. Lastly, when I pour off, I put the receiving container (usually a bucket) in a plastic tray to catch any drips.

One more thing. When you only let the solution get 2", or so, deep, it is much easier and safer to pour it off than it is with a full container of solution.

This is really very simple and I hope I made it clear. If there are any questions, please ask.

 

[samuel-a]

The method that Sam mentioned - covering with water, heating, and then feeding the nitric in increments - is the same method I've been preaching since the forum started.

Yup, i agree.
I haven't invented this, it's all here on the forum.


Chris
Things that seems most fundamental to the experienced refiner, does sometimes allud the novice. I just wanted to say how much i appreciate your extremely detailed posts.

 

[glondor]

Thanks guys. The info is very clear. I will attempt 1375 grams this way. My reaction vessel is a 40 quart stainless pan. 12 inches wide 20 inches long and 6 inches deep. Came close to boil over several times adding nitric in 1 liter increments. I will add 11 lots of 500 ml and decant any solution over 2 inches as I go. I will adjust down as necessary. You all have been very helpful. It is quite a site to see the fume hood take the load. Thank god it works well. I am pleased my nitric use is on the mark. Thanks for the reassurance GSP. I based my usage on your excellent tutorials.

 

[goldsilverpro]

Mike,

Man, the shape of that big shallow pan seems pretty awkward to pour from. Maybe you could tilt it just a bit and use a siphon. The solution has to be pretty cool to do that, though. Sure would be better to have a taller round SS container.

Chris

 

[glondor]

Thanks Chris. I do tilt it and bail out most of the juice and foils with a 1 liter beaker. You are right tho it is awkward to pour from. I did not consider it shallow however I would like a deeper pan. I do work slow and carefull. I let it cool right down as well.

 

[goldsilverpro]

I forgot about those foils. Better to bail it than siphon it. One or two cup Pyrex measuring cups work well for that. I use the top side of a bucket lid under the measuring cup, at all times, to catch the drips while transferring the solution from one container to the other. When I finish, I just set the measuring cup on the bucket lid. I never lose a drop. You can tilt the SS container, a bit, near the end to bail more out. I usually use a small piece, or two, of 2 x 4 to tilt a container or a bucket.

 

[Grassbur]

In case you didn't add enough water and it crystallizes when cooled, the crystals will dissolve by adding water. However, they will dissolve much faster if the water is first heated. Stirring helps. Your goal is to have enough water to prevent crystals forming at room temperature, yet not add too much water so you generate unnecessary volumes of waste solution. With winter approaching, it will probably take a bit more.

In certain cases, especially when I have a lot of metal in the dissolving vessel, I will add water, heat, and add nitric in increments until the solution level is about 2", or less, above the metal I am dissolving. Then, I let it cool off a bit, add a little water (this cools it off more and helps insure no crystals form), pour off the solution, add water to the remaining metal, heat, and again start feeding in the nitric in increments. I repeat this (sometimes, 4 or 5 times) until the metals are all dissolved. This actually goes faster since it accomplishes 2 things. (1) I can start with more metal in the container. (2) I have found that, when the solution level gets about 2" above the metal, the reaction slows down. This is mainly because the fresh increments of nitric I am adding are too diluted to dissolve the metal quickly. After you pour off and use fresh water and acid, you will find the dissolving goes very fast.

Before pouring the solution off, I let it cool, somewhat, for safety reasons - handling a very hot solution is not very safe. Also, make sure all the acid you've added is used up (fully reacted) before you pour the solution off. When doing this, I don't like to start with more than 20% metal, by volume, in the container. And, of course, don't pour off any of the solids with the liquid. When pouring off the spent solution, you don't have to pour off 100%. Just pour off what you can without pouring off any solids. Lastly, when I pour off, I put the receiving container (usually a bucket) in a plastic tray to catch any drips.

One more thing. When you only let the solution get 2", or so, deep, it is much easier and safer to pour it off than it is with a full container of solution.

This is really very simple and I hope I made it clear. If there are any questions, please ask.

Chris I have tried a process similar to this. I read elsewhere on the forum that when disolving Silver (.925) for cementing, you need to dilute (after the reaction stops) with 10 times the Nitric with Distilled water. Is this correct?

So basically I have been doing the following:

3000ml beaker + 10ozt of .925 + 250 ml 70% nitric + 250 Dist. water.

It will typically only dissolve 6 ozt (leaving 4 to be started again).

I then, going by what I have read on the forum, add 2500ml of dist. water and filter it out and cement.

If the 10 times ratio holds correct, I can't very well fit more Nitric in the beaker because of the water requirements. What would you suggest?

I am hoping the 10x water requirement is false because right now I feel like I am dealing with alot of un needed waste.

Sorry if that is confusing.

Thanks,
Chris

 

[goldsilverpro]

I am hoping the 10x water requirement is false because right now I feel like I am dealing with alot of un needed waste.

I don't know where you got the 10X dilution from. Maybe you're thinking of sulfuric. The 50/50 nitric should be diluted about enough before cementation. You might add an extra 200 ml of water, though, to compensate for evaporation and make sure there's no crystallation.

 

[Grassbur]

I am hoping the 10x water requirement is false because right now I feel like I am dealing with alot of un needed waste.

I don't know where you got the 10X dilution from. Maybe you're thinking of sulfuric. The 50/50 nitric should be diluted about enough before cementation. You might add an extra 200 ml of water, though, to compensate for evaporation and make sure there's no crystallation.

Ok that is good to hear. I can't seem to find the thread I read the 10x on, but at any case. Both Harold and you suggest the same dilution so I will just go with that next time. Thanks again.