Neutralization prior incineration

[stefano]

Hallo at all,

incineration is a great tool in the pm recovery after hcl-bath. I did it much times and know the importance of incineration for cleaning the material.

I followed always this formula:
- washing and rinsing the material
- adding naoh until pH 9,5
- bringing back to pH 7-8 with dilute hcl carefully
- washing and rinsing the chlorides
- ... incineration

The bringing back the pH to 7-8 carefully is very labour-intensive. Therefore i have a couple of questions.
- is there any possibility to reform other chlorides beside nacl in the neutralization process like unsoluble agcl ?
- is it threfore possible to jump the neutralization to pH 7-8 and do only the rinsing process of the alkaline solution ? And when so, are there any losses ?

Greatings from Stefano

 

[4metals]

Stefano,

What type of material are you processing and how are you incinerating this material?

 

[stefano]

Hallo 4metals,

you are right, it is important to know of the type of material to incinerate. Sorry i am not good in thinking and writing in english language.

The material i incinerate is always from electronic-scrap after eliminating all plastiks and aluminium, then cherry-picking, then washing the material whith hot water and dishwater detergent, then hcl-trating to eliminate tin or iron, then eliminating copper with ap, then hcl-boiling until hcl rests clear, then neutralization and incineration whith the formula from Harold and butcher. I do this incineration for passing then to nitric-leaching to dissolve silver.

I hope that i could it explane to understand adequate.

Greatings from Stefano

 

[butcher]

stefano,

If I understand you correctly, you are talking about rinsing and neutralizing the HCl, or rinsing the chlorides from your material before incineration, to remove chlorides that can make gold or silver volatile in the incineration process.

There is no reason to take the pH up to 9.5 (that is what we use for waste treatment).
The goal here is to neutralize acids and remove as much chloride salts as possible.
after rinsing the material of acids and soluble chlorides. You can use just a little water to cover, stir well, while adding the little bit of NaOH (or sodium carbonate), to neutralize the solution, keeping the powders stirred well, to try and get it close to pH 7 (neutral), then rinsing the salt water from your powders, before drying for incineration.

Raising the pH up to 9.5 in this instance would be counter productive, the goal here is to remove acids and chloride salts, not to precipitate base metals from solution as hydroxides, or form base metal hydroxides.

Raising heat slowly at first at the early stages of the incineration process, can also be helpful to drive off acids or salts, (similar to roasting), a little sodium carbonate can also be helpful during the process.

This is hard to explain, but I think you will know what I am talking about from your own experience.

Heating slowly also keeps the powders from bubbling small volcanoes and spitting out of the pan, adding some of the previously dried powders, (already incinerated), or adding a paper like tissue paper, shredded, paper towels, or dried filter papers (from previous projects) can also be helpful. To dry and separate the powders your working on. It can be helpful in opening space through the powders to the bottom of the pan where gases can escape without making those volcanoes and spitting everything out of the pot. Constant stirring, or being able to have an area of the bottom of the incineration pan clear of the powders, until the volatile acids or gases are removed, and the powders begin to fuse to form a solid mass, which will need crushing to fine powder before incinerating to red hot and exposing to air or oxygen...

 

[stefano]

butcher,

you are understanding correctly what am i talking about.

Most of the solubel chlorides i rinsed with cold an hot water washes before adding naoh. I where very happy if i could limit the neutralizing to pH 7. But what shall i do with the unsolubel silver chloride in the mix ? ( i dont want to use ammonium hydroxide to dissolve silver in this state ). But at pH 7 the silver chloride rests in the mix, and then gives the well known volatile problems for silver itself and gold too. Or am i thinking wrong ?
I thought to convert most of the silver chloride in the powder mix with naoh to hydroxide and oxide, and then with incinerating to elemental silver and oxide, and then to dissolve the elemental silver with nitric.

 

[butcher]

I say it would depend on what I was dealing with, how I would go about it.

If the powders I had were dealing with were mostly silver chloride (with some gold) then I would treat it like silver chloride, using one of the methods to convert the silver chloride, example using sulfuric acid an iron...

With powders recovered from electronic scrap, where the silver is not normally of high content. Or where the powders would not be mostly silver chloride ,where many other metals and their salts would contaminate the little bit of silver chloride involved. Where the process for converting silver chloride difficult with a method like NaOH and kayro corn syrup would not work well any way. then I would consider the method we are now discussing.

If this is powders recovered from electronic scrap (normally of low silver content), where silver is not the primary content or concern. Then using the hydroxide wash to neutralize acid and to help convert the little bit of silver chloride, and any other metal chlorides to oxides, as best as possible, and washing the powders of well of soluble soluble salts, before drying and slow roasting (again to drive off chlorides before raising the temperature to red heat), and using sodium carbonate to keep the minor amount of silver chloride in the powders from giving problems with volatile chlorides (much like how we can convert silver chlorides with sodium carbonate in a pre-melt) I believe your losses of silver or gold to volatile chlorides will not be much at all, or that much to worry about. Done carefully it should remove most of the problems you would have if you directly tried to wash the acids and roast the material without these steps or pre-treatments to try and convert or eliminate the problems of roasting powders containing chlorides...

 

[Geo]

After AP, very little of the silver will be in the form of silver chloride. The very fine silver metal is just metal. I neutralize with sodium bicarbonate and then do water rinses until the water stays clear. I then do a hot nitric acid bath which removes most of the Pd and almost all of the silver. No matter which route you take, it will contain about the same number of steps.

 

[stefano]

I now realize due to your comments that the neutralization processes can differ on the amount of base metal chlorides which are in the mix. Until now i used always the same process, no matter of the assumed content of chloride amount in the material, because i was afraid of possible losses in the incineration process.

Now i have your informations and so i am able to adapt the processes in every single case.

Thank you very much for your answers and informations.