Making AR

[bswartzwelder]

I will be dissolving some placer gold in the near future and would like to get things right rather than have to correct my mistakes later. I making AR, the ratio of 1 part nitric acid to 3 parts hydrochloric is commonly referred to as correct. However, I have also seen the ratio of 1 part nitric to 5.44 parts hydrochloric. This second ratio takes into account the fact that hydrochloric acid is usually between 31% and 32% and nitric acid is commonly between 69% and 70%. Do any of the people who have used this process agree or disagree.

I will calculate the amounts needed for a given weight of gold and place it in a beaker/flask. The add the nitric slowly until all the gold just disappears. This will save on the amout of nitric used and prevent me from having to neutralize the nitric later.

 

[element47.5]

Whether your hydrochloric acid is 31% or 32% and whether your nitric is 69% or 70%---these are rather small differences in concentration that cannot account for the much larger range of 1:3 all the way to 1:5.44 for a HNO3 to HCl ratio that you cite. More commonly, AR is thought of as between 1:3 and 1:4. Never heard of 1:5.4 but me not hearing of it is not the authority.

I have never done what you are proposing so I cannot speak from experience. However, I believe I would be washing, perhaps even near-boiling your placer gold with HCl before *any* kind of involvement with nitric, with the idea that any kind of base metals one can get rid of via (cheap) HCl before treating your material with AR is to your advantage.

Then, I would wash the material thoroughly, many times, to get rid of any HCl. Then, assuming the biggest pollutant of your gold is now silver, I would cover the material with nitric to dissolve silver, and I would dilute said nitric well under 69-70%. At that point, all HCl has been washed away. Decant and save the (presumably) silver nitrate solution for later refinement. What's left should be pretty clean gold.

I'm operating from the following principles:

1: You want to get rid of base metals and sulfides and carbonates and any odd traces of minerals ASAP, as cheaply as possible.
2: You do not want to try to place silver and gold into AR. From what I have read, silver is the most likely pollutant of placer gold. Thus, if you haven't removed it before heading for AR, you're headed for trouble.

I would not be that concerned with neutralizing nitric. It can be decomposed by mild heating, or, chemically neutralized with very cheap chemicals.

Corrections welcome.

 

[butcher]

bswartzwelder,

Here is my approach:
Since placer gold can be anywhere between 58% to 75% gold (sometimes a little higher),
I in-quarter my placer gold with silver (usually calculating the gold as 58%).
(I have never per-washed the placer gold in HCl) (now gold from ore can be different).

Melt with silver (sterling or your other silver) stirring with carbon rod and torch, and pour shot.

I measure out nitric acid figuring silver content 2.8mL/ gram of silver, and water 2.8ml/g

The gold and silver shot I put into a corning casserole dish and sit it on a hotplate solid burner, I will add about 3/4 of the of nitric acid and water, I will normally add some 3% hydrogen peroxide to further dilute and keep NOx fumes down, I let the nitric react until reaction seems to cease, then I will turn on the heat to the hot plate, heating the solution warming it up slowly over a period of time to just below a boil, let the solution react, as nitric is depleted dissolving silver, I will add more dilute nitric acid/H2O and splashes of peroxide as needed, considering the solution evaporates the free nitric gets stronger, before all of the shot changes to powders I will cease adding more HNO3, continue heating this will normally finish dissolving silver (or copper) out of the gold, heat may need adjusting to where solution fumes but does not boil, if after I figure the solution is concentrating and not dissolving more silver from the shot I will add more HNO3/H2O.

When I have dissolved all of the shot, and only gold powders remain I will lower heat and dilute just a little a little let powders settle (the gold powders will normally be fairly clean and stay on the bottom well), the solution I use a suction tool to remove, and filter it into clean quart canning jars, and cement out silver with a clean copper metal bar, the gold is washed with heated water until rinses are clear
This rinse water is filter into the other jar with the silver nitrate solution.

The gold now in fine powders in the casserole dish, I measure out 3.8 ml HCl per gram of gold and cover gold in this (you can add extra no harm).
Then measure .95ml per gram of gold of your 70% HNO3 in a clean measuring cup (we will add this small portion’s at a time).

I will add the measured nitric acid as generous portions at first, and may add a little water or 3% peroxide to keep fumes down, and when I have added about 3/4 of the nitric, and reaction slows I will slowly heat up the solution, until just under a boil, I will let this solution concentrate and dissolve more gold as the solution gets closer to being concentrated its color changes from a yellow to closer to an orange red, it will take time to get this far concentrated so keep an eye on the evaporation process (do not go take a nap).

This evaporates off water from aqua regia and the nitric and acid concentrates dissolving more gold, once I get fairly concentrated I will add the nitric with a pipette in very small proportions, waiting for reaction to finish dissolving more gold powder before I add more nitric acid, I also stop adding nitric acid with some gold is left un-dissolved, here I will concentrate the solution to an orange color, and if the gold still did not dissolve I will add a few more milliliters of HCl. and drops of nitric to finish dissolving gold (evaporating off the water in the newly added HCl, or nitric).

Just a little more heating after gold is dissolved and I let cool, add 4 time volume of water, a pinch of sulfamic acid, and decant into clean vessel with a watch glass cover, to sit overnight.

Next morning decant gold solution through filter to another clean jar to precipitate gold (wash little bit of silver chloride and add to you silver chloride jar or stock pot).

Remember to use your stannous chloride tests.
I will test before precipitating gold not only is it fun, but it can tell me the nitric is consumed), and after precipitating the gold, test again to determine gold precipitation was complete.
I will wash the precipitated gold powders using Harold's instructions.
Then the gold goes back for a second refining, being dissolved, and precipitated (using a different chemical to precipitate gold the second time, and again and washing the gold using the washing procedure before the gold is melted.

edited to clarify

 

[goldsilverpro]

With the strengths of acids you're using, 1:5 is about right, but there's some flexibility in this

 

[bswartzwelder]

I had some notes in Microsoft Word format and that is where I found the AR formulation. Upon much research, I found the original article at:
chemical-supermarket.com

If you enter the sire and do a search using aqua regia, the information comes up. It took forever to find the original. From now on, when I make notes, they will include the source of the information. Thanks to all.

 

[goldsilverpro]

The ratios of HCl and HNO3 in the equations you find are molar ratios and not volume ratios. For example, the equation given by T.K. Rose in his "Metallurgy of Gold" is:
Au + HNO₃ + 4HCl = 2H₂0 + NO + HAuCl₄
As you can see, the molar ratio of HCl:HNO3 in this equation is 4:1

You can also find an equation that gives a 3:1 molar ratio, but this is for combining HCl and HNO3 to make aqua regia only and doesn't consider the dissolving of gold or anything else.
HNO₃ + 3HCl = NOCl + Cl₂ + 2H₂O
The NOCl then breaks down:
2NOCl = 2NO + Cl₂

To compute the volume ratios, I first must determine the number of moles/liter (that ratio is called the Molarity) in the various strengths of the acids in question. I did this for 31% and 38% HCl and for 69% HNO3.
69% HNO3 = 15.43 Molar
31% HCl = 9.81 Molar
38% HCl = 12.4 Molar

From this, you can compute the volumes of each needed to give a 3:1 and a 4:1 molar ratio. For the HCl, I used 31% because that is the common HCl strength of Muriatic acid and 38% because it is the common strength of lab grade HCl.

For 3:1 molar ratio
31% HCl and 69% HNO3 = 4.7:1 ratio by volume
38% HCl and 69% HNO3 = 3.7:1 ratio by volume

For 4:1 molar ratio
31% HCl and 69% HNO3 = 6.3:1 ratio by volume
38% HCl and 69% HNO3 = 5:1 ratio by volume

So, what volume ratio should you use? Most of you know that, when dissolving 1 ounce of gold, it takes about 125ml of muriatic acid and about 25ml of nitric. I have proven these numbers to myself, with about a +/- 10% variation, many times. Thus, the practical ratio, by volume, seems to be about 5:1, which is slightly more than the 3:1 molar ratio given above. In practice, though, since I prefer an excess of HCl, I usually start with about 150ml of muriatic acid per ounce and end up with about a 6:1, by volume, ratio.

There is much erroneous information about this on the internet. The Wikipedia article on aqua regia, for example, says to use a 3:1, by volume, ratio.

 

[ayeyou]

As a placer gold miner and thirty year prospector I feel the need to say that I have never heard of placer gold as low as 58% and in fact placer gold that is only 75% would be on the low end of the scale. The gold I am currently mining is 93% and in B.C. the gold runs from 830 to 960 fine. I just thought since the measurement of AR used is based on Gold to be processed it may be important for someone to know going in what his golds fineness really is. It would seem guessing somewhere between 58% and 95% could affect the outcome of the process could it not? I attach a page of a study on placer gold fineness averages in Alaska and my refinery receipt from B.C.

 

[butcher]

Gold here I pan is usually around 75% to 80% gold to base metal usually silver and possibly some PGM, but placer gold can be as low as 68%, or be fairly pure closer to 90%.

I really was not posting on what the gold content in placer gold was in different areas as much as I was trying to explain How I figure the silver I add when quartering the gold. (I prefer to have more silver than not enough for me it is a lot less trouble and I feel benificial to my processes).

(placer gold can also have particals of quartz rock, or quartz rock with particles of gold so here the gold ratio is lower).

The figures I gave 58%-75% or more pure, where just for discussion and for figures I use to calculate how much silver i would use to inquarter my gold with, they were given as a general guide, and were not indicative of what percentage your gold would be,.

(where when buying or selling or dealing in placer gold where that fact of gold percentage would be more important ).

You can keep a tally on a batch of placer gold you process and then determine what percentage the gold was after you finish a process, but I was never really concerned what percentage my placer gold was, as I do not count eggs till they are hatched as pure gold.

I do not buy or sell placer gold (so true percentage is no concern to me), so when I use a lower percentage to determine how much silver I will use to inquarter my gold with, calculating my gold at 58% when working with placer gold, this assures me of a little extra silver in the melt, a little extra silver is not a problem when inquartering my gold, the gold mixes in the melt and is more finely devided, and sometimes adding even more silver I feel is benifical, as if any platinum group are involved (which can be in my area the extra silver can help these to be removed in the acids), the nitric gets in and removes silver easier, and the finely devided powdered gold afer parting with nitric is not a problem for me, and actually I like it better than having lumps that may still contain silver, and the fine gold powder reacts better to acids, even if I do not use aqua regia or use another method like HCl/bleach or HCl/30% H2O2 to dissolve my fine gold powders, but if I do not add enough silver it makes more problems for me in my veiw, having to remove undissolved gold from the nitric and melt it again with more silver, is a pain.
Also if my gold did not make shot well, the extra silver makes it easier to dissolve the larger lumps of metal in nitric acid.

this is the reason I calculate my placer gold as a lower gold ratio (58%) in the process, I know it is closer to somewhere around 80% gold, but my goal is to inquarter with a little on the extra silver side.

 

[g_axelsson]

this is the reason I calculate my placer gold as a lower gold ratio (58%) in the process, I know it is closer to somewhere around 80% gold, but my goal is to inquarter with a little on the extra silver side.

How can underestimate the amount of gold get you a little extra silver, or did I misunderstand you?

When inquartating, if you have a high gold content you have to add more silver, if the gold content is low you need to add less silver.

/Göran

 

[butcher]

g_axelsson,

Thank you I see where I am wrong here in my thinking, now that you made me think about it more, under estimating the gold content will not get more silver in the melt.

I guess when I do this, I did not have as much silver compared to gold in the melt as I was thinking.

I never really thought about it that much when I work with placer gold, I just figured the percentage low and added sterling silver and went from there (thinking I had excess silver all along), being happy with my misconception as the process went as expected.

Thank you for getting me to see the error of my ways.
:lol:

 

[g_axelsson]

No problem, just glad to help. 8)

/Göran

 

[butcher]

Now I Have use more silver, and work on that math more.

 

[nickvc]

Now I Have use more silver, and work on that math more.

If it isn't broke don't fix it Butcher, it worked for you that's all that matters.