Freezing temperatures, Tested, won't drop

[Acid_Bath76]

Curious if anyone has had this experience. I had a 1000mL batch of AR that tested positive for Au. After the filtering and denoxing, I tried to drop with SMB. The SMB reacted, but nothing dropped. I was dumbfounded, seeing as this was the first time this has happened. I went ahead and put it back on the burner. Reduced the volume again, and then added fresh HCL to it's original volume. I had done this a couple times prior to ensure that there wasn't any nitric present. Even when adding the SMB the first time, there was an absence of the red fumes. Had there been an excess, I would have expected to see something like that. Regardless, after reducing and preparing for a second drop, I had the same results. The SMB reacted, but nothing came out of solution. And yes, I tested with Stannous. I keep it in a little eye dropper, in my coat pocket.
The SMB is good. I used it on another batch, and it worked just fine. To add another level of chaos. The temperature dropped overnight. When I came back the following morning, it looks like the power was out, and crystals had formed in my stubborn solution that would not precipitate. I tested again with stannous, positive. I reheated the solution. Having hit a roadblock, I just cemented the values out with a solid copper rod. I kept the solution warm, and the next day everything was out. Stannous confirmed the solution bare. What the heck happened?? Any ideas? Crystals? Stubborn solutions?

 

[Barren Realms 007]

Sounds like you still had some nitric in your solution. Some water added to your solution might have solved your problem and allowed the reaction to complete and let your gold drop.

 

[Acid_Bath76]

Water? Doesn't the SMB react with the HCL? Ive read here that some prefer to drop their values directly. Not using a water/SMB mixture. I could see where the values might drop faster with water present, buy in this case I don't see how that might have been the issue. If I understand it correctly, adding water increases the acid holding capacity. Is that what you meant? There wasn't enough acid to complete the reaction? Maybe I missed something you were trying to explain. If that's the case, please fill me in. Ive moved on since, but it still creeps into my brain. Kinda bothers me.

 

[philddreamer]

Acid, SMB reacts with water producing So2; do you add dry SMB, or do you dilute SMB in water before adding it to the solution? If you dilute, you don't need to add water to the solution, but if you have a concentrated solution, you need to add water so the SMB is "activated". SMB needs water in order to produce the So2 gas, that's why its recommended to add water 2 to 3 times the volume of the concentrated acid before adding the SMB.

Phil

 

[qst42know]

The SMB reacted, but nothing came out of solution.

Please explain what you mean by reacted?

Evaporation is somewhat subjective as it takes a degree of judgment.
In less that ideal lighting conditions you may not have seen any red fumes when adding HCL.

Full strength HCL (approx. 32%) has plenty of water in it to add SMB dry.

 

[butcher]

"Full strength HCL (approx. 32%) has plenty of water in it to add SMB dry."
True but you may have made gold salts with the acid, and heat may have removed much of the water (depending on conditions).

Way I see this is, that the 32% HCl acid (and 68% water) was used to dissolve metals (with oxidizer) now the acid has change to a metal salt in solution with water, (could still have some free acid, HNO3 giving problem but may not, depending on your evaporation procedure), HCL and water now formed these salts (not acid any longer unless excess was used, (gold chloride salts for example), if heated can easily drive out water, with more heat can eventually drive out excess HNO3, and excess free HCl if any (concentrating the gold chloride solution), (depending on your process, conditions, and concentration), now of course adding some fresh HCl will add some water but it may not be that much, after de-noxxing procedure, diluting with water will dilute the gold salts, and allow silver chloride to precipitate (which solution can be decanted from), and this will then give plenty of water for the SO2 gas to form from using the dry SMB, (excess water gives no trouble so using water to pre-dissolve SMB will cause no harm (except a little more waste to treat) SMB may even work better in the more dilute solution, as long as the gold is pure from other metals or salts of metals.

Other metals in the gold chloride solution may also give you trouble precipitating, tin comes to my mind, it can form colloids, the tin can reduce the gold to metal in solution, but these colloids are not positive or negative polarity, but of both polarity, and repel each other in solution and will not precipitate,(I am not saying this was your problem but if your treating electronic scrap it is highly likely), especially if incineration steps are not part of your processing, sometimes adding acid and strong boil can break these colloids, but other times cementing on copper may be the only option to recover your values.

Colloids in this solution will not reduce to metal using SMB, (as they are already reduced to metal in solution but will not fall to the bottom of vessel), So adding more SMB will have no further effect to get your gold to precipitate, more water just dilutes these colloids in solution and can actually make them harder to precipitate.

Tin can give us many problems, but even other metals like copper can give us troubles precipitating gold (if concentrations are high in solution), also organic compounds (oils or other trash) even in small quantity can give us problems with precipitation, using incineration and good washing practices can make your life much more simple, many times my materials are incinerated as a pretreatment, and may again be incinerated after some of the acid treatments (trouble like tin, oils, and trash is a monster we need to deal with, send them to the fire).

Also a stannous chloride test may miss gold colloid in solution, as gold is already reduced in solution, before adding stannous chloride (here I am speculating).

Incineration how can people who do not use this procedure get their values, I would like to have their stock-pots (probably high in values, missed values in their stock pot and waste).

 

[Dr. Poe]

I need to know if you were working scrap gold or a sample of gold ore. :?: Dr. Poe

 

[Acid_Bath76]

this was all e-scrap.... it was almost all AMD K6's.. a few other various ceramics.

 

[Dr. Poe]

Could you describe the crystals? Were they bright yellow needles or claret red prismatic 'monoclinic' crystals?
:?: Dr. Poe

 

[Acid_Bath76]

Dr.Poe,

I'll have to look up the terms after I submit this. Sorry for the delay. It's been hectic with the holidays, and getting ready for the semester. Tomorrow, it's back to the grind! As for the crystals, they were clear and their size ranged from half and inch to an inch. They were also fairly thin, and about the size of a #2 pencil lead. The issue has since been resolved. I removed the crystals, saved them, and carried on as I normally would. Regardless, it's something I want to figure out. I'll find the photos and post them ASAP. Thanks for looking into this.

 

[Dr. Poe]

Check out Oz's crystals on the ( Re: Filter then urea or urea then filter) post for Jan. 8. Does that look familiar? Dr. Poe
That's a monoclinic prismatic crystal formation. Notice that they seem to be clear on the top. within time, they will turn red, then brown. Dr. Poe