# Use of Nitric to get rid of base metals



## chemgal68 (Oct 5, 2012)

So just wondering if anyone knows the answer to this question. I am dealing with ore right now. There are an abundance of base metals which are causing difficulties with leaching and recovery. This appears to be a sulfide/sulfate based ore. If the ore was pre-treated prior to leaching with a nitric wash, would this also dissolve the PGMs. I was thinking of roasting the ore at 1000 degrees for 15 minutes prior to adding the nitric and then washing really well with DI water before performing the HCl/Cl leach. 

Thanks for any thoughts on this. Also, what strength of nitric might work best.


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## butcher (Oct 5, 2012)

Roasting sulfide is a must, adding iron (like nails) to the roast can help to convert the sulfides and drive off the sulfide as SO2 gas, sulfide can lock up you're gold in the ore keeping a leach from getting to it.

sulfide can be handy when concentrating the ore as they like to float in a froth of air and oil, but for leaching sulfides are trouble.

I see no reason to use nitric at this point ($$$), when you can leach most base metals with another acid like sulfuric acid, or hydrochloric acid, or even weaker acids like citrus acid or even vinegar depending on the base metal and ore.


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## chemgal68 (Oct 5, 2012)

I think the base metals are most likely iron, copper, nickel, zinc and aluminum in small quantities.

My thought was to try and get rid of the base metals in the ore after roasting prior to leaching. Since I am leaching right now with HCl/Cl and I know that AR digests the PGMs I am looking for something that might be selective for the base metals and leave the PGMs alone. My thought was based upon the idea that in Hoke's book after leaching with AR, one is recommended to get rid of the nitric acid and base metals to help aid in ppt and PGM metals recovery. Did I miss something in there? 

You mentioned vinegar (acetic acid) - we use a dilute solution of this to simulate acid rain or acidic landfill groundwater in a leaching procedure for soil disposal. I know that this would leachout any mobile metals species, what about the PGMs?


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## butcher (Oct 6, 2012)

Yes I believe you missed an important theme of Hoke's book, the base metals are removed from the values (like gold) before the values are dissolved into the aqua regia.

The base metals are not dissolved into the aqua regia solution with the gold.

Although we cannot eliminate all traces of the base metals in the recovery stages we then refine the gold by putting it into solution (traces of base metals), of course remove nitric from solution, and precipitate the gold leaving the traces of base metals in solution, and wash as best as possible what traces of base metals that may have followed with our gold precipitant, and then refine the gold again putting it back into solution and repeating the refining process for higher purity (similar to re-crystallizing salts of metals for higher purity).

Working with Ore can be similar but it is also a whole different ball game, rock ore can be a very complex chemistry of metals and acids that formed the rock, other material like silicates arsenic and so on, also complicate matters, there are many different combinations of chemicals that make up rock and there are not all alike, the acids heat Etc, will not react to every rock the same way, as the ore actually changes the chemistry depending upon its composition, the same way as one flux recipe will not work the same with every ore in an assay (the flux recipe is chosen by the chemistry of the ore), with ore many times pretreatments like roasting to remove sulfides are needed even before we even think of using acids or an assay, elemental metals or alloys like jewelry or electronic components are different we can pick acids or processes depending on the metal or alloy, although we may also pre-treat these with processes like roasting or an acid treatment to remove certain base metals, but most of the time with these materials we do not have other chemistry or chemical reactions involved from acids or salts or bases that you will have with ore in the leaching (or melting) processes.

With ore free gold (or metals) should be separated from the ore as much as possible grinding to fine powder if needed and gravity separated, gold bound up as a sulfide (usually with another metal like iron or even copper) can be concentrated by froth floatation and separated from the tons of relatively worthless ore, many times the gold or platinum group metals in ore are a byproduct of mining for another metal like copper.

Many people think the can just leach a bunch of rocks and get rich, not understanding that most of the gold ever recovered was free gold very little in history came from the leaching processes, and these were always complicated processes, usually developed at the mine specifically for the type of ore at that mine, what leach or process that would work at one mine, would be a waste of money and would not work at another mine, rarely were all of the values recovered from the ore as loses could sometimes be high, and they always looked into ways for better recovery, mountains were crushed burned and moved for ounces of gold.

I know you understand a lot of what I am trying to say, but I have seen people who get a gold pan or a few bottles of acid and think they are going to get rich mining, when in reality most miners died broke and almost starved or worked themselves to death while alive, although in the area where I live it is estimated 6,000,000 troy ounces of gold were mined and that is not counting what the Chinese miners shipped back to china, most miners barely could afford to buy beans and bread to eat with the game they killed for supper, many mountains and river beds were moved to get at the few grains of gold, but all of them grains did add up.

If gold mining were easy gold would be cheap.


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## chemgal68 (Oct 6, 2012)

I agree with you that lots of people think gold and PGM recovery is easy, however, experience is a great teacher. I have people bring samples to me all the time that they are just sure contain lots of precious material, however, that is usually wishful thinking on their part. 

This particular ore I am working with definitely is unique in it's chemistry. The PGMs seem to be complexed within the iron sulfide and the standard methods of recovery only work so well - I am estimating that currently we are only getting leaches of around 30-40% of the available. 

I recently read a patent on the Mintek mining process which seems to have encountered very similar obstacles, I will try some different approaches and see what happens.


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## Irons2 (Oct 6, 2012)

chemgal68 said:


> So just wondering if anyone knows the answer to this question. I am dealing with ore right now. There are an abundance of base metals which are causing difficulties with leaching and recovery. This appears to be a sulfide/sulfate based ore. If the ore was pre-treated prior to leaching with a nitric wash, would this also dissolve the PGMs. I was thinking of roasting the ore at 1000 degrees for 15 minutes prior to adding the nitric and then washing really well with DI water before performing the HCl/Cl leach.
> 
> Thanks for any thoughts on this. Also, what strength of nitric might work best.



It also depends on where you live. In my State, Maine, it's against the Law to treat ore with Nitric Acid. It caused so much Pollution at one time, that it was banned.


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## DarkspARCS (May 31, 2013)

> This particular ore I am working with definitely is unique in it's chemistry. The PGMs seem to be complexed within the iron sulfide and the standard methods of recovery only work so well



I too am working with several dynamic ore types that possess high percentages of iron, copper, and arsenic sulphides that precipitate the right colors at a very pleasing weight ratio yet when fired produce a disappointing result... there's definitely an elemental containment that's blocking my progress.

ggggrrrrrrr.... :twisted:


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## Traveller11 (Jul 5, 2013)

In the late 1890's, chlorine was used to leach gold from ores until cyanide was introduced about 1900, at which time chlorine was largely abandoned. This is not to say chlorine was not an effective method for leaching gold, it was simply that cyanide was a simpler and more inexpensive method.

Finely ground ores were roasted in the presence of oxygen for two reasons: a) to break the bonds of the sulphides, turning the iron to oxides and the sulphur to sulphur dioxide and b) to convert base metals to oxides.

The most important thing to know about the HCl/Clorox method is that this was NOT the chlorine method employed to recover gold from ores in the 1890's. By using so much hydrochloric acid, the pH is lowered so far that the bonds of the oxides are broken and base metals are put into solution instead of gold. This is why the only solution many have is to completely remove base metals from the ore, a pointless and cost prohibitive effort, to say the least. As I stated, the entire purpose of converting all of the base metals to oxides is that the pH of the chlorine solutions they were employing was close enough to neutral that the solutions could not break the bonds of the oxides, preventing any base metals from going into solution.

As a matter of fact, according to all of the old papers I have studied, these people did not use HCl at all. Also, in the instances they did make chlorine gas, it was pressurized into neutral pH water, making hypochlorous acid, BEFORE it was used to leach gold.


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