Zinc as a Precipitant

[donnybrook]

What pH should a solution be to use zinc as a precipitant. Is it pH 3 ? It is said in some of the literature that the pH alters as the zinc is depleted (for want of a better word). Do you then filter out your PM and re-adjust the pH back to pH 3 ? Then add more zinc until no further reaction is taking place ? It has been said gold will precipitate only within a defined pH range. Can someone help with this question?

Donnybrook

 

[butcher]

solution would need to be acidic, I may be wrong, but I dont think PH here is not as important as having an acidic solution, as the zink is just replacing the metals which are less reactive in this acidic solution, and so the zink will precipitate out all the metals below it in the reactivity series. if acid content was very high you would just use more zink to use up any excess acid in solution, if acidity was very low it would take a longer time to exchange metals from solution. personally I would want a slight excess of acid.

 

[donnybrook]

Thanks Butcher,

I think to have a bit more acid than not enough is the way to go as it would find the so called ideal pH range. An article referring to CLS says the pH should be 2 and that it will move to pH 5 as the acid is obviously depleted or used up in the process. I agree with your thinking completely.

An aside to this I read another Post on the Chlorine Leaching system on the Forum earlier today. Don't have time to get back to it. The pH should be 8.5 according to the Late K Williams and information on another Web Site. They both recommend using Hydrochloric acid (or acetic) to remove iron and other base metals after roasting.

I have been using this method ( roasting and hydrochloric acid) with Iodine and it works well. Now just looking at alternative leaching processes to suit the material I am working with from an old mine discharge dam. I will also try the Chlorination method which was used in this area.

Precipitation with zinc is a method I will investigate and also try electro (anode cathode) recovery.

Kindest Regards,
Don

 

[manorman]

Don,
What do you use to drop your gold from your HCL Iodine leach? and is this Ore you are working with?
I have used lye, and this week used SMB that seemed to work ok but have not finished processing that drop yet.
Mike

 

[butcher]

I did not know you were leaching ore, and am still unclear of what process, or details of the process you are useing,

when I chemically leach an ore, I try to keep my solutions reusable, as much as possible. ( I try to streatch my chemicals as far as I can). once zink was added about the only way to reuse a chloride solution would be to distill it.

 

[donnybrook]

Mike and Butcher,
I'm looking at different leaches for different materials at mines. I had some assays done after doing my own. I purchased the CLS Manual which has a great deal of useful information and zinc is mentioned but more so as a way to check your solutions for residual metals. Many years ago they used this in Cyanide Recovery Plants and they lost a lot of gold as they say oxygen was needed to help the gold and zinc plate out (cement).

I started using Iodine and the late Ken Williams method. You must get rid of the iron and other base metals. I always roast first in a large electric hot water tank which I cut in half and put on legs with a good fire under it. I also made a good grinder out of another full one to break the sands down before roasting. It must run at about 30 revolutions a minute.

Iodine is selective. I purchased one lot of information which suggested roasting then removing base metals. The gold and silver were taken up as gold and silver iodides according to the literature. After filtering the red brown solution (and rinsing) first through a Calico Bag then a plastic food strainer lined with facial tissues I then added Clorox which I made using the Basement Chemiistry procedure which involved disolving in water,filtering, then using Bi-Carb to produce a strong solution. The Clorox is added to convert to a Chloride. I then adjusted the pH to 7 and added SMB. It dropped the gold or powders for further treatment. They say you cannot recover your Iodine with this particular method

Ken Williams just recommends Lye or Caustic which will drop the gold as a black or brown powder. Preparation is important. It will also bring down other metals. It looks like a great deal when wet in the filter but after drying and roasting in an iron container it comes back to a few grams. You can then recover your iodine to use again. I found some good info on this Forum concerning recovery from Iodine and also on how to recover the other PM's on a selective basis.

I have used the SWAN Leach and Iodine Leach. I am now going to use the Chlorine Leach. Make my own chlorine (or Clorox) and adjust the pH to 8.5. I always mesh out my sands and cone and quarter. I then work with one kilo or two pound samples. Zinc can be used when the pH is adjusted back from 8.5. Its not cheap.

I believe you can then re use your chlorine solutions.

Don

 

[manorman]

Don, thanks so much for the information.
MIke