Dissolving precious metals in HCl?

[lazylightning]

Check out this patent! It's quite shocking that it recommeds the possibility to dissolve precious metals in HCl. Can you make any sense of the mechanism here?

http://www.freepatentsonline.com/4722831.html

 

[Noxx]

It has now been discovered that the gold sand obtained from the nitric acid treatment of silver refinery slimes may be purified by contacting the gold sand with hydrochloric acid under controlled operating conditions. During the hydrochloric acid step, substantially all the metals other than the gold are selectively solubilized whereby a purified gold sand is obtained.

Where do you see that gold is being dissolved by HCl ?

 

[lazylightning]

Here they say that after leaching with nitric and then afterwards with HCl, that only 1.4% of the gold is dissolved while over 90% of the platinum (a contaminant) is in leach....

EXAMPLE I

Sixty pounds of gold mud containing about 54% water (12,491 grams on a dry basis) were contacted with 28.5 liters of nitric acid (having a concentration of about 230 g/l) for 5 hours at 90° C. in a stirred, ceramic coated, steam jacketed, 30 gallon kettle. The slurry was filtered using a Buchner funnel having a polypropylene filter cloth and washed 6 times with equal portions of room temperature water (total of 12.5 liters of wash water). The residue was transferred into the kettle and contacted with 16 liters of nitric acid (having a concentration of about 330 g/l HNO 3 ) and boiled for 4 hours, filtered and washed twice with a total of 4 liters of water.

The remaining residue (gold sand) was transferred into the kettle and contacted with 6 liters of 10N HCl for 30 minutes at 90° C., filtered and washed four times with equal portions of water (total of 8 liters of wash water).

Each of the filter cakes contained about 30% water.

Washing of the first HNO 3 residue was performed until the pH of the wash water was above 1. The second HNO 3 residue was washed until substantially no silver was present in the wash water as tested using concentrated HCl. The HCl treated residue was washed until substantially no chloride ions were present in the wash water as tested using a silver nitrate solution.

Analysis of the HCl leach liquor (including the wash water) and the final gold sand product (leached residue) showed the following results:
TABLE I
______________________________________
Total Metal in Metal in Leach Leach Liquor & % Metal Liquor (grams) Leached Residue Leached Metal (A) (grams) (B) (A) (100)/(B)
______________________________________


Gold 67.2 4766.0 1.4

Silver 12.0 122.5 9.8

Platinum

8.8 9.6 91.7

Palladium

2.4 4.5 53.3

Selenium

2.4 17.5 13.7

Tellerium

2.9 3.0 96.7

______________________________________

The results in TABLE I clearly show the selective removal of contaminant precious metals and impurities from the gold sand. Thus, only 1.4% of the gold is leached from the gold sand compared to significant amounts of the other metals. Over 90% of the platinum, for example, is selectively leached from the gold sand.

 

[goldsilverpro]

Interesting patent. I read it a couple of times, but it doesn't make much sense to me. If the Pt is in a metallic form, I can't see how it would dissolve in HCl, without an oxidizer present.

The Pt will be very finely divided in the slimes. Maybe enough Cl2 is generated from the HCl, when heated to near boiling, to dissolve it. However, if that were true, I would think that more than 1.4% of the gold would dissolve.

Maybe the Pt is mainly not in a metallic form, due to the oxidizing reaction at the anode. Maybe, it is in an oxidized form that is fairly insoluble in nitric but soluble in HCl.

It's also interesting to note that the Rh plus 10% of the Pt dissolved in the nitric. I do know that extremely small particles can react differently in acid than those in bulk.

Even if this does work I don't think I like it, since portions of the Pt ends up in 3 different places.

Lou, what's your take on this?

 

[lazylightning]

It mentions gold mud. Maybe this is all in the form of some semi-metallic slimes? I sure wouldn't want to have to try and make this one work, I just thought it would be interesting to see what anyone here would have to say about it =-)

 

[Lino1406]

initially present (about 12 L) contain chlorides?