Pay to dispose of AgCl?

[MGH]

Does anyone produce silver chloride waste that they do not refine themselves? Do you know of any waste handling operation that will take the waste free of charge – or even pay out on it – rather than charge a fee for waste disposal?

The laboratory where I work performs salt analysis in food using both the Volhard technique and an autotitrator with potentiometric determination. The Volhard waste is 5-10% nitric acid and is a mix of primarily food, silver chloride, and silver thiocyanate. The autotitrator waste is comprised of the food sample itself and silver chloride. It is otherwise pH neutral. We combine waste from both analyses together.

We’ve been disposing of about one 55 gallon barrel of this waste once per year at a cost of almost $2000 for Safety Kleen to come take it away. I need to run an estimate of how much silver is in there. I’m going to look into the practicality of collecting this waste in a little smarter manner (rather than dumping it all in a barrel) and recovering the values ourselves. However, my boss [understandably] won’t tolerate much in the way of extra steps needed to recover ourselves if the time spent outweighs the cost of disposal in the first place. I just have to think, surely, it would be worthwhile to recover the values, neutralize the silver-less stream, and pay nothing to send it down the drain.

Barring a workable in-house solution, does anybody know of an outfit that would pay out on this silver chloride waste, or at least be cost-neutral? And for that matter, how about organics waste (methanol, hexane, ether, etc.)? It’s probably much less likely, but any way to get rid of that without a fee?

Thanks,
Matt H.

 

[GotTheBug]

Still a beginner here, but it seems you could neutralize the nitric, drop any silver in solution, and decant the barrel, leaving Ag and AgCl in the bottom. Should end up with very dilute sulfuric solution.

 

[Geo]

You can use anaerobic digestion to break down organics or go with straight NaOH which will dissolve the organics and convert the silver chloride to silver oxide. The sugars in the food samples should be enough to convert the silver oxide to metallic silver. After a year, the silver should collect in a layer at the bottom of the container. Regular additions of NaOH along should take care of material as it is deposited.

 

[Lou]

yeah, just so everyone knows, this material sucks. Horrible stuff.

We got a large lot in on a refining basis that was a few K oz Ag and it was the nightmare from hell.

Horrible stuff.

 

[MGH]

Lou, would you be willing to share any tips on how you dealt with the difficulty, if there are any?

I ran a trial on this material as a proof of concept about a year ago. I used NaOH and glucose for the conversion to elemental silver. I do not have much experience with this technique in the first place, but it just didn’t seem to progress as expected. I did end up with a few grams of nice looking silver after the final melt, but I didn’t keep good enough records to estimate a yield (especially since the starting material is mostly liquid).

In my trial I tried to filter after the conversion – both with gravity and a Buchner funnel under vacuum. Both were very difficult and I can’t imagine it would be feasible doing large volumes. I encountered quite a bit of fatty material, and I’m not sure if that added to the filter clogging, or if it was just the nature of particulate matter.

Reclaiming values from this waste stream does sound like a simple proposition, but as I’ve seen with what might appear to a simple analytical method, there are any number of variables in the sample matrix or restrictions on a process flow that can really mess things up.

Also Lou, does the difficulty you encountered mean that you couldn’t justify handling the material again in the future? Not enough profit in it for a refiner, even if simply taking the material at no cost to the party who wishes to dispose of it? If so, that must mean this stuff really is a nightmare.

Thanks

 

[MGH]

I just ran the numbers on how much silver nitrate we use on average. In silver value, it's only about $100 per year. That's not worth going after in our scenario when we're supposed to be doing a lot more valuable things with our time. Still, it would be interesting to know why this kind of waste is so much more difficult to deal with than other sources of silver chloride.

 

[Lou]

$100 worth seems way too low.

How many samples a day do you do?

 

[MGH]

It’s not a huge volume of samples - about 600 samples, per year, for each test method. So I guess that does only average a couple samples per day. The Volhard analysis uses a constant 25 mL of AgNO₃ per test. The autotitrator varies based on sample size and actual salt content. The table doesn't insert well, so I've attached an Excel file with the calculations. If I’ve messed up the math, that’ll be embarrassing.

Calculation for grams of silver per test: (mL AgNO3 used)/1000*(Normality of AgNO3)*(molecular weight of silver) ; [MW of AgNO3 cancels out]

 

[Geo]

Evaporate to dry and store solids which take up much less room. Save it up and worry about it when silver hits a hundred dollars an ounce.

 

[MGH]

Evaporate to dry and store solids which take up much less room. Save it up and worry about it when silver hits a hundred dollars an ounce.

Yes, that is another thought, though evaporation is probably not the most practical method. Each Volhard sample turns into about 150 mL of solution by the end of the analysis, and each autotitrator sample is about 45 mL. But I had thought about siphoning/decanting the acidic liquid, then neutralizing, then sending it down the drain, and then yes, letting the solids build up until it’s a better proposition to refine them. All that will still depend on how much leeway our boss is willing to give to even fiddle with this in the first place.

 

[goldsilverpro]

Does everything settle out over time or does it stay suspended? Are you able to filter the waste, as is, before attempting any conversion?

What indicator are you using? Are you using potassium permanganate? What about chloroform, diethyl ether, or nitrobenzene? Anything else added?

 

[MGH]

The solids (silver chloride and silver thiocyanate) do settle out relatively easily. The liquid portion is then clear, maybe a little yellow depending on the specific sample, and there is often some fat floating on the top. I haven’t tried filtering before conversion. I think the fatty portions would slow things down, and make a mess in general, but perhaps I could try skimming it off the top before filtering. I’ll need to look into this.

The indicator for the Volhard titration is ferric ammonium sulfate (reacts with excess thiocyanate in the back-titration to make a salmon colored endpoint). 2 mL of an 8% solution is added to each sample, so that puts the final concentration in the waste stream at about 0.1% or less. There is sometimes potassium permanganate added to oxidize organic matter that is not digested by the nitric acid. An over addition of permanganate will result in the characteristic purple color, which is then dealt with by small additions of sucrose. There is no chloroform, ethers, or nitrobenzene. And there’s nothing else from the autotitrator waste as that is just the sample, DI water, and AgNO₃ – the endpoint is determined potentiometrically.