Problems with SMB Drop

[Lobby]

Folks,

I'm on my 4th batch of gold refining. 1st two batches (run concurrently) had some problems, but made 99+% gold. 3rd batch (10k scrap only) was just about perfect. In all these 3 batches, I've had no problems during the gold drop step. Either with copperas or SMB.

I'm in the middle of batch 4 (96.1 g of 14k scrap = 56.1 g gold)

During the inquartation, I had a bit of a problem. I had a heck of a time seasoning the new, 5 inch melting dish I just bought. I just couldn't get an even coating of borax on the dish; areas just didn't want to take the borax.

So I put too much on the dish, I suppose.

The resulting cornflakes contained some frozen borax on them (from dropping molten borax on top of the cornflakes in the water pot. You can see the purple borax specs in the pic.

I thought about remelting (especially since I searched through the forum and found a recipe for a better flux - thank folks!), but in the end, didn't. I used tooth cleaning pics the dentists use to clean the cornflakes. I got most of the borax off.

The HNO3 dissolution went uneventfully. After the reaction was ended, I poured off the solution and added fresh, concentrated HNO3 to the pot, enough to cover the cornflakes.

Boiled for a few minutes. Got some red fumes, but they cleared, so I turned off the hot plate and let the solution cool.

I then started the AR step. I added about 225 mls of conc HCl to the beaker and started heating.

Note the 100 ml beaker containing conc HNO3 with the plastic pipette. I normally add HNO3 in about 5 ml steps, to not overshoot.

The last HNO3 addition was 2 mls. With the hot plate on, the solution was slightly boiling, but no red fumes in the gas phase. I assumed the rxn was over, and I was merely boiling the solution.

Here I am about to filter the AR solution. Note how dark it is. Note the filter paper with the plastic tweezers on the right. Those pieces are unreacted gold from the AR solution.

Well, the SMB drop was horrible. I added a solution containing about 80 grams SMB (same SMB as the good Batch 3), dropped gold, but solution still showed gold (SnCl2). As I had no more solution, I started adding SMB powder. Every time I did, the solution would bubble and give off sulfur gasses. Yuck. I must have added about 100 grams more of SMB. Sigh. Finally, my solution showed clear of gold. I let it stand overnight. In the AM, it still showed clear of gold.

But big problem: I had white power mixed with the gold in the beaker. Obviously unused SMB, although perhaps some other sodium salt.

What did I do? I started washing the solution with water. 100 mls at a time, vigorous stirring, then settle for an hour or so. I think I did about 6 washes today. I'm about to do another.

And will continue washing tomorrow. The gold isn't free flowing. It feels, with the stirring rod, sorta muddy.


Questions:

- what the heck happened? Would remaining borax flux affect the AR reaction like that?

- can ya'll recommend a washing procedure to insure I remove the remaining SMB salts? I've been cautious about adding any acids to the mix.


Thanks.

 

[Geo]

SMB is soluble in water. use hot water, actually, boil the powder in water until the salts are dissolved. as the solution heats, you should stir often because powder will cause the glass to heat unevenly and when you do stir cooler water will hit the hot spots and could crack the glass. so stir until a boil is achieved.

for future reference, when you have excess borax either in your melting dish or on your material, place in a container and cover with dilute sulfuric acid (30%) and bring to a boil. cover with a watch glass and boil for at least 30 minutes and turn off heat. allow the solution to completely cool. decant solution into a container and cap tightly so you can re-use it later. rinse your dish or material well with water. any borax left should be soft and can be rubbed off. if there is any still stuck on you can repeat as necessary.

 

[Lobby]

Thanks.

You know, I experienced something a bit contrary to your advice re heating the SMB solution to aid dissolving. Room temp dissolving was taking too long. So I put it on the stove to heat it up. (yeah, wife wasn't home) :mrgreen:

And promptly forgot about it.

Went in a few minutes later and the flask was boiling vigorously, but instead of seeing all the SMB dissolved, some had actually dropped out of solution. I added some water to the hot flask and the SMB promptly dissolved. Amazing.

I threw out that SMB, and used fresh stuff for the actual drop, though.

 

[Geo]

Thanks.

You know, I experienced something a bit contrary to your advice re heating the SMB solution to aid dissolving. Room temp dissolving was taking too long. So I put it on the stove to heat it up. (yeah, wife wasn't home) :mrgreen:

And promptly forgot about it.

Went in a few minutes later and the flask was boiling vigorously, but instead of seeing all the SMB dissolved, some had actually dropped out of solution. I added some water to the hot flask and the SMB promptly dissolved. Amazing.

I threw out that SMB, and used fresh stuff for the actual drop, though.

that's simple enough to explain. you saturated the water with SMB and then set it to boil. water evaporated to the point it couldn't hold the the salt in suspension, so it precipitated out as salt. you could have used it as there was nothing wrong with it really.

 

[samuel-a]

- can ya'll recommend a washing procedure to insure I remove the remaining SMB salts? I've been cautious about adding any acids to the mix.

Start your acid (HCl) washes. Boil if possible.
Eventually the 'mystery' salt will be gone.

 

[Lobby]

that's simple enough to explain. you saturated the water with SMB and then set it to boil. water evaporated to the point it couldn't hold the the salt in suspension, so it precipitated out as salt. you could have used it as there was nothing wrong with it really.

:mrgreen: Of course that's right. The solution got supersaturated.

- can ya'll recommend a washing procedure to insure I remove the remaining SMB salts? I've been cautious about adding any acids to the mix.

Start your acid (HCl) washes. Boil if possible.
Eventually the 'mystery' salt will be gone.

I will. I can't imagine after all the water washes I've done that there's anything in there that can cause problems when I start the HCl washes.

I'm tied up the rest of the day and can't work on it. (Imagine having 2 ounces of gold sitting in a beaker and having higher priorities) :roll:

I'll take pics and post results.

 

[goldsilverpro]

How much total nitric did you use. Is the nitric 70%, 67%, or 61%?

What strength HCl?

Great to see you using that Corning Ware Pyroceram dish.

 

[Lobby]

How much total nitric did you use. Is the nitric 70%, 67%, or 61%?

What strength HCl?

Great to see you using that Corning Ware Pyroceram dish.

Thanks.

For the AR step, I added approx 225 mls 31% HCl (from Lowes). I calculated a total addition of 45 mls of 67% HNO3, but I didn't add that much. (Heck, every batch I've done so far has only required about 60% of calculated HNO3)

I added HNO3 in steps:

16:55 6 mls
17:10 6 mls
17:25 6 mls
17:55 6 mls
18:03 reaction slowing, so added 2 mls of HNO3

I had the hot plate on. Solution was slightly boiling, but barely. No appreciable red fumes in watch glass covered 1 L beaker. I turned off the heat to the hot plate, and the solution stopped boiling. I did NOT add more HCl.

Solution was dark. I could not see through it. After filtering, I saw a few pieces of gold still present. I captured them in the filter paper.

And then the fun started with SMB.

 

[Lobby]

Oh, I got a PM from a helpful GRF participant, asking whether I'd added ice cubes to the solution prior to SMB addition.

I did not.

Each SMB addition caused significant fizzing (and evolution of stinky sulfurous smells ) on the surface of the solution.

 

[Geo]

Oh, I got a PM from a helpful GRF participant, asking whether I'd added ice cubes to the solution prior to SMB addition.

I did not.

Each SMB addition caused significant fizzing (and evolution of stinky sulfurous smells ) on the surface of the solution.

the smell is to be expected. thats the SO2 being generated.

 

[Dr. Poe]

Being conservative with the nitric acid in your aqua regia solution (dissolving gold) is a good thing. However, seeing the black solution tells me that you are being too conservative. Dr. Poe :roll:

 

[Lobby]

Being conservative with the nitric acid in your aqua regia solution (dissolving gold) is a good thing. However, seeing the black solution tells me that you are being too conservative. Dr. Poe :roll:

Thanks. I guess I'm not sure what the solution is supposed to look like. I've taken the advice on this forum to heart about not adding too much nitric acid, so perhaps I erred on the safe side. But I didn't choose the volumes to add arbitrarily.

Hoke says: "4 fl oz of HCl + 1 fl oz of HNO3 dissolves 1 oz gold" That translates to 118 mls of conc HCl and 30 mls of concentrated HNO3 per ounce.

Mr Gold-n-Scrap advises "120 mls of HCl and 25 mls of HNO3 per oz gold."

Yet when I calculated the stoichiometry, I got roughly half that amount.

Regardless, I used the generally accepted volume of HCl. For all 4 batches I've run, I've only had to use about 60% of the recommended HNO3 volumes. If there's advice ya'll wish to impart re HNO3 volumes, what the solution needs to look like or any other visual cues, or on my stoichiometry calcs, I'm all ears.

I'm also all ears on my problem with the SMB drop. So far, the clearest theory is that I let my solution overheat when I added the SMB, thus allowing the SO2 gas to vent and not drop the gold.

 

[eeTHr]

I did an experiment one time where I added a conservative amount of SMB, directly after evaporating the AR with gold in solution. Nothing dropped.

I added a little water, and it instantly precipitated. It was a very small test amount of solution, like a few tablespoons. I added the water with a dropper. I was watching the drops, about four, and by the time I looked down at the solution it had gone from transparent yellow to opaque gray.

My conclusion is that it won't precipitate without some water. Most people recommend adding three times as much water as AR solution. Hoke mentioned three to five times the amount of solution, and that some jewelers went as high as ten times the amount.

 

[butcher]

I agree water is needed in the reaction as well as some HCl; sodium metabisulfite is not like the SO2 gas that Harold used with concentrated solutions, and some ice.

Also you could of had some un-reacted nitric acid, and so you may of needed an excess of SMB to overcome the excess HNO3 and get the gold to precipitate, (or the lack of water and or free HCl acid in solution the SMB may not have reacted fully).

(From Your description, all of the gold was not dissolved and no evaporation process, then positive test with SMB and having to add excess SMB).

And or a possibility the solution was not acidic enough as you did not add a little HCl after concentrating (which by the way the 32% HCL is also 68% water).

Adding water 3 to 4 volumes can also let silver chloride precipitate from solution, which can be settled overnight, and solution decanted, before you filter and then use the reagent to precipitate the gold.

I think I would revaluate the method you used.

Looking good just some modifications to the techniques.

 

[goldsilverpro]

Lobby,

I don't think you used enough HCl and that's why all the gold didn't dissolve. Typical reagent grade HCl is 37% and that's what Hoke's numbers are likely based on. The 31% is only 81.7% as strong as 37%. If it takes 120ml of 37%, it will take 147ml of 31%. When using 31% muriatic, I usually start with 150ml-160ml, or a little more. An excess of HCl will cause no problems.

Starting with too little HCl makes things a little more complicated. At the end, when an addition of nitric produced no reaction, you stopped. At that point, had you added more HCl (say, 20ml) the reaction would have likely started again. When that died down, a small addition of nitric would probably have started it reacting again. In other words, when you start with too little HCl, you can end up doing this back and forth thing with the acids. This can be confusing and it makes the nitric harder to control. All this can be totally avoided by using a definite excess of HCl to start with. Or, if you discover that you've started with too little HCl, just add enough to give a good total excess and then get it hot.

If you search the internet, you will find 3 different equations for the dissolving of gold with aqua regia. One is the stoichemetric equation, that only works under perfect controlled conditions. The second is for very dilute AR and is 1.5 times stoichemetric. The third is twice stoichemetric. When you have a hot, concentrated AR in an open container, the 3rd equation will apply. Simply stated, half of the acid used dissolves the gold and the other half goes up in the fumes (NO, for example).

 

[Geo]

of the two acids, hcl is by far the least expensive. for someone to be shy about the amount of hcl used in dissolution is hard to understand. use an overabundance of hcl and remove all doubt of the issue. ive never read of an example of adding too much hcl when dissolving metals.

 

[Lobby]

I've really appreciated the info ya'll have given me in this thread. I find it interesting that you quickly focused on two things: why the gold didn't drop, and the problems with my AR solutions.

It's interesting that in the best of the 4 gold batches I've run so far (where I achieved 100% gold yield), my procedure (which was a bit different than this one) used both of the major suggestions you've made in this thread:

- I added additional HCl towards the end of the AR reaction, which restarted the reaction, causing me to add a bit more HNO3 when the HCl rxn had died

- I added ice cubes prior to the SMB drop, both cooling and diluting the solution.

Why I didn't repeat the procedure for batch 4 I can't answer. Cocky, perhaps. I'll take your advice about not being shy with my HCl additions. I'll also dilute and cool my gold solution prior to dropping. :mrgreen:



P.S. My caution on water dilutions comes from the waste water I created on my 1st two batches. For about 3 oz gold, I created about 2.5 gallons waste water + about 3/4 gallon stock pot water. That seemed stupidly crazy to me, so I was more careful on the 3rd and 4th batches.

 

[goldsilverpro]

A couple of comments.

I see no reason for scraping the small amount of slag off the cornflakes. It should not interfere in the dissolving process. At least, I've never had any problems with it.

There are 2 reasons to use ice in AR solutions.
(1) To cool the solution when dropping gold from a highly concentrated (in gold) solution using SO2 gas. This will generate a lot of heat and this will quickly expel the SO2 gas from the solution. Harold has discussed this in detail.
(2) In lieu of diluting 3 times with water to more fully precipitate the silver chloride, some people accomplish the same thing by chilling the solution with ice. I prefer diluting.