Conversion of AgCl via NaOH and Karo Question

[publius]

I tried the method for reducing silver chloride using sodium hydroxide and sugar. I really like how fast it works. I would like to understand the process more. I know that sodium exchanges the hydroxide ion for the chloride ion and the silver takes an oxygen molecule to form sodium chloride and silver oxide and heat. (Lots of heat!)

The next step of the process is the one I am not sure of. Does there need to be an excess of sodium hydroxide? Is it possible that someone could show the reaction so I could have a better understanding of what is going on? Also is there a way to calculate the required amounts of NaOH and sugar to use so the waste of these to chemicals is kept to a minimum?

I am concerned with waste disposal and purity issues. If my silver chloride is not well rinsed, will this process also take the copper back to copper oxide and then to metallic copper?

All in all, this seems to be the fastest way to extract silver from solution without introducing additional base metals.

Thank you, lasersteve, for the outstanding video of the process.

Robert Jeffery, CWI

 

[kurt]

Rob - I have actually done quite a bit of the salt sugar lye method with silver (as well as cementing) they both have there pros & cons. (I have done over 400 T oz of silver in the last 16 months using both methods) I don't have time right now but will do my best to point out some of the things I have learned about going with the AgCl method tomorrow morning.

Kurt

 

[publius]

Rob - Snip... ut will do my best to point out some of the things I have learned about going with the AgCl method tomorrow morning.

Kurt

Thank you sir. I normally use iron and dilute sulfuric acid and was looking for a different way to avoid the contamination issues.

 

[philddreamer]

Hi Rob!
How pure do you need or want your silver? This will determine the best procedure to follow. If you want .999, I feel that cementing with copper is faster, with a purity of .99 or so, followed by processing thru a silver cell. My expirience with the lye & sugar is, its okay, but too much work.

Here are some ratios for the lye sugar method, that Juan Manuel shared with me some time ago for 500g of scrap silver:
600g of salt (without iodine)
1000g sodium hydroxide (Lye)
600g of sugar
200 ml. of HCl (for rinsing)
I find that, that's too much extra chemicals (& time) in order to end up with metallic silver, when just cementing with copper will do the job. Having the expirience of converting thru salt, lye & sugar prosess is great, if that's what you want.
Just my opinion...
Take care!

Phil

 

[goldenchild]

Here are some ratios for the lye sugar method, that Juan Manuel shared with me some time ago for 500g of scrap silver:
600g of salt (without iodine)
1000g sodium hydroxide (Lye)
600g of sugar
200 ml. of HCl (for rinsing)
I find that, that's too much extra chemicals (& time) in order to end up with metallic silver, when just cementing with copper will do the job.
Phil

Interesting. I recently processed a batch of nearly 500g of sterling and some of these numbers look a little high. The salt sounds about right if you use it to bring down the chloride but the sodium hydroxide and sugar seem excessive. I used HCL to bring down the chloride. Maybe 300-400ml. Then about 300-400 grams of sodium hydroxide with a few good squirts of kyro (5-6). I know Juan knows what he's doing though. Maybe there was something different in his process? I'd be curious to know what he has to say.

 

[goldsilverpro]

I have done a lot of conversion using Karo/NaOH. For each troy ounce of silver (41.3g of AgCl), it took 20g of NaOH and 13.3ml of Light Karo syrup. These figures were proven in a lab. In practice, I used 10% extra of each.

 

[goldenchild]

I have done a lot of conversion using Karo/NaOH. For each troy ounce of silver (41.3g of AgCl), it took 20g of NaOH and 13.3ml of Light Karo syrup. These figures were proven in a lab. In practice, I used 10% extra of each.

I definitely have to record these numbers for future reference and will convey them to a member that was recently asking about figures to use for AgCl. I was almost spot on just adding and eyeballing the reaction. According to GSP's figures it would take 197.8 ml of syrup and 297.4 grams of NaOH to get elemental silver from a 500g batch of sterling. Then add an extra 10% just to be certain. So the 1000g of NaOH and 600g of sugar would be about 3 times more than needed. Thanks for these numbers GSP.

 

[goldsilverpro]

Sounds like Manuel was using a solid form of sugar instead of Karo syrup. That would make the numbers different and they would most probably depend on the type of sugar he was using.

You can also use only the NaOH to convert the silver chloride to silver oxide and then decompose the silver oxide to silver metal with heat. According to Wiki, the decomposition temperature is about 540F. In practice, I would get it somewhat hotter than this.

 

[lazersteve]

I never measure when performing the sugar-lye method. I simply go by the color of the silver chloride and proceed to the next step as the color of the powder matches the correct shade and cosistency.

First I cover the wet silver chloride with a volume of water equal to twice the volume of silver chloride.

Secondly, I slowly add dry lye pellets and stir. This step produces lots of heat so proceed very slowly and stir a lot. I add the pellets until the entire mass of powder is a dark brown, almost black color.

Now I mix up a saturated solution of table sugar and warm water. I use more sugar than the water will hold and stir until the water is no longer cloudy. A small amount of solid sugar remains in the bottom of the vessel that the sugar is mixed in. I make up a volume of this solution equal to the volume of the silver chloride. The cooled clear liquid (no solids) from this solution is slowly stirred into the dark brown silver oxide until the color shifts to light gray and the beaker becomes coated with a mirror of silver. I scrape the mirror off of the beaker glass with a glass road while stirring.

Finally I allow to the solids to settle and rinse (three cycles of settling and rinsing with fresh water). The rinsed solution/sludge is then vaccum filtered and rinsed one final time with clean water. The final rinse should run clear.

The keys to sucessful conversion are making sure the silver chloride starts out wet (never allowed to dry), the color of the solids is 100% the same color throughout after each step, and good stirring.

I love this method as it is very fast and clean.

Steve

 

[philddreamer]

I never measure when performing the sugar-lye method. I simply go by the color of the silver chloride and proceed to the next step as the color of the powder matches the correct shade and cosistency.

This is how I do it too. The ratios Juan Manuel gave me, helped me see what the transition would look like, plus more information shared by other fellow members. So, I just look for the right color.
But, since trying cementing with copper, I prefer cementing method. I recover plenty of copper for my "batches", so I found a good use for it & re-cycle it. I have accumulated AgCl from batches of AR and other processes and that's where the salt, lye, sugar comes in handy. It's good to have experience with both methods.

Phil

 

[goldsilverpro]

I should mention that, for each tr.oz. of silver, I used 135ml of water. In most cases, I was converting large batches, usually in a 55 gal cutoff plastic drum. I used a power stirrer (usually a Lightin' mixer with a prop blade which provided a chopping action) and constantly stirred it for at least an hour after adding everything. Using the stirrer, I found it wasn't necessary to wait until the NaOH converted it to silver oxide to add the Karo. I usually added the NaOH first and then immediately added the syrup. I don't like doing things in the dark, so I always analyzed (using the Volhard titration) the silver in the nitric before precipitating the AgCl with HCl. That way, I always knew how much NaOH, Karo, and water to use when converting. Using this method, I always got 100% conversion.

 

[samuel-a]

2 AgCl + 2 NaOH = Ag2O + 2 NaCl + H2O

That means that every gram of expected silver would need 0.37g of dry NaOH prills to convert.
To accomodate the resoulting NaCl in the solution, you need to have 1.42 ml of water (at least) for each gram of expected silver.
(this would have been much simpler to present in moles rather then grams)

These are the masurments i'm using in the rare cases is do need to produce and process AgCl... Seems to be working just fine.
Similar to GSP, i too prefer to add masured excess of Water and Lye.


I don't take the extra step to reduce the oxide with sugar, i just make dry cakes and melt as it is going to the cell anyway.
Also, i'm not so sure about the actual redox between the sugar and the silver oxide so i can't calculate it. Sucrose baing a molecule composed of glucose and fructose make thing all that confusing to me.

It is mentioned in this report, that Sucrose (table sugar) in it self is not a good reducing agent, but rather its individual components are (i.e. glucose and fructose), they mention therw the need to acidify the Sucrose solution to promote hydrolysis into the two simple sugars that actually perform as reducing agent and in turn oxidized by the silver oxide (to... Formic acid??)

That's of course, somewhat contradicts our experience as we seen with our own eyes that plain sugar water does in fact reduce Ag2O...
So the questions remains... does the water itself hydrolyse Sucrose ?

I'm out on a limb here, so feel free to currect me if i'm wrong.
I'm struggling barely with inorganic chemistry as it is... So it's safe to say me and organic chemistry are not such a good friends

 

[goldenchild]

As far as the reduction of chloride to oxide goes I never really worry about getting it 100% In fact unless you are using a blender, tumbler or stirrer of some sort I think you would be hard pressed to reach 100% reduction. I let the additions of the karo do all the work. With the heat and boiling that occurs with the karo additions, any bits of AgCl that weren't reduced will then come in contact with the NaOH and of course immediately convert to elemental Ag. As long as there is enough NaOH in solution everything should convert to elemental Ag in the end.

 

[Geo]

when im doing this process, i have quite a bit of fine brown powder that forms and takes some time to settle. ive been collecting it because i dont know what it is. any ideas what it might be?

 

[qst42know]

It is mentioned in this report, that Sucrose (table sugar) in it self is not a good reducing agent, but rather its individual components are (i.e. glucose and fructose), they mention therw the need to acidify the Sucrose solution to promote hydrolysis into the two simple sugars that actually perform as reducing agent and in turn oxidized by the silver oxide (to... Formic acid??)

That's of course, somewhat contradicts our experience as we seen with our own eyes that plain sugar water does in fact reduce Ag2O...
So the questions remains... does the water itself hydrolyse Sucrose ?

In the lye the sugar is simply carbon source to form CO2 from the oxide isn't it?

Unrelated but in the food industry citric acid is often used to form the sweeter tasting simple sugars from sucrose.

 

[publius]

I understand, now, the conversion from AgCl to AgO. Can someone show the formula for the reaction of the AgO with the sugar?
:roll:

 

[samuel-a]

In the lye the sugar is simply carbon source to form CO2 from the oxide isn't it?

I thought so too, but the more i go about it, it seems it is not that simple as we might think.
For me, it is just a theory, but it seems that the reaction of simple sugars to Ag2O, like Glucose for example, will not couse decomposition as you suggested, but rather an oxidation to Gluconic acid or some gluconate salt....

It will be wonderful if a chemist could chim in here and explain to us what's really happning in this reaction and and therefore we could calculate stoichiometric amounts instead of guessing...

Some more related info i think may be relevant:
Reducing sugars

 

[qst42know]

No carbon in that glucose model.

 

[Juan Manuel Arcos Frank]

Here is the trick:

Common sugar(sucrose) is a non reducing sugar,I mean,if you add common sugar to black silver oxide then the reducing reaction will not take place.When you add an excess of NaOH to the white silver chloride you will convert it to black silver oxide and a lot of NaOH remains in solution,this NaOH excess reacts with common sugar breaking it in glucose and fructuose,glucose is a reducing sugar so the reaction with silver oxide will reduce it to metallic silver.

Karo syrup is glucose but it is very expensive and not founded in some countries.The difference between Karo syrup process and sugar process is time and money,Karo syrup is faster than sugar process but more expensive,sugar process is slower than Karo syrup but it is cheaper...When you add sugar to your silver oxide/NaOH solution you have to mix the solution very hard ,the reduction reaction takes some time to start but once started it is very very exothermic and a lot of foam is released,like a volcano explotion,so be careful.

I hope it helps

Kindest regards.

Manuel

 

[samuel-a]

According to this paper:
http://www.jbc.org/content/264/17/9901.full.pdf
And several other resources i have found across the web, Hydrolysis actually do ouccur once sugar is dissolved in water. And might even be promoted further with lowring of pH with weak acids....

Cool, now we know why it works.

According to wiki (and if i understand correctly), both the Glucose and fructose (eventually) will have aldehyde group which are capable to reduce.
Anyone knows how to expess this redox in a balanced equation? Ag2O + Glucose + fructose = ??