The process of parting gold with nitric acid is old, and probably dates from the discovery of nitric acid itself; it is one of the simplest parting processes, and does not require a costly plant or much manipulative skill. The main objection to it is the cost of the acid. The operation comprises the preparation of a suitable alloy. It was formerly considered that three parts of silver to one of gold was necessary, hence the term inquartation process; it has been found, however, that a very much lesser quantity suffices if more than one treatment with acid is given. Kerl states that if all the silver is to be dissolved from gold by one boiling with nitric acid the amount of silver must be at least eight times the weight of the gold. When the proportion, 1:2½, is parted, the resulting gold is left as a porous sponge, if dilute acid is used for the first treatment. Silver is not essential as an alloying metal, for other metals soluble in nitric acid may be used. However, if silver contains even a small amount of gold it would be better to use the metal rather than an equivalent amount of any other. When another metal is used, the weight need not necessarily be so great as that of silver. Parting appears to depend upon the space occupied by the molecules of the alloying metal, also upon the molecules of the acid and of the products which form and their relative dilution; the temperature and time of action are also essential factors. Dealing with the first point, if approximately two of silver are used to one of gold we would have an alloy whose approximate formula is Au Ag 4. When the silver is dissolved from this alloy the mass holds together. The particles of gold are therefore close enough to cohere. Since the atomic volume of gold and that of silver are approximately equal, by choosing some other metal whose molecules occupy the same space as the silver, it should be possible to remove this in the same way. Taking zinc for example. If the gold zinc alloy be made according to the formula Au. Zn.4, the amount of zinc to be added need only be 1.3 to 1 of gold.
If sodium were used the alloy Au. Na4 would have only one-half the sodium as the gold. In this case also the high atomic volume of the sodium would serve to separate the molecules of gold further from each other. Admixtures of metals such as silver and copper also serve the same end as silver by passing into solution, allowing the acid to pass between the particles of gold.
An amalgam of mercury, silver, and gold can also be parted by means of nitric acid, but if the gold-silver alloy is rich in gold the separation of silver is incomplete. When gold is parted from such an amalgam it will be found to be in fine crystals.
Since no authentic information as to the ratio of other metals required for parting is to be found in current literature, a number of experiments were made to determine whether lesser quantities of metals might be used. The old term inquartation assumed a ratio of 3 of silver to 1 of gold, and this ratio is quoted even now as being necessary for parting operations. It is well known to assayers that or 2½ or 2¼ of silver to one of gold will part as well as 3 to 1, but how much less is sufficient was determined experimentally.
The acid used was nitric, 68 per cent, strength, and two strengths were used, the dilute being made by mixing two volumes of water with one volume of acid, the second by taking one of each; the term strong acid means the 68 per cent, acid. The gold taken in the three cases was 10 grains, the silver 10, 15, 20 grains, or in ratios of 1 to 1, 1.5 to 1, 2 to 1. The alloy was made by wrapping the metals in lead and cupelling; the buttons were afterwards flattened and rolled into strips of uniform thickness.
A, B, and C were then boiled with 5cc. of strong acid, washed, dried, and weighed. The weights were:
It is therefore clear that 2 of silver and 1 of gold are sufficient for parting, and that 1.5 to one is ample if the resultant gold is subsequently treated by sulphuric acid or bisulphate of sodium. The action on 1 of silver to one of gold was not complete in the time allowed, the weaker acids had practically no effect, and there is little doubt had the strong acid been allowed to act for a longer time that much more silver would have been removed.
Parting of Zinc and Gold
In order to find what proportion of zinc would be required, 10 grains of gold and 10 grains of zinc were melted together in a porcelain crucible, using potassium cyanide as a cover to exclude air. The alloy melted quietly at a comparatively low temperature. The KCy was washed out and the solid button was treated with 40cc. of 1 to 1 nitric acid. The zinc was violently attacked, and the button became fissured through the rapid evolution of gas. After boiling for half an hour the solution was poured off and the bead washed with water; 20cc. of strong nitric was then added, and the solution boiled for five minutes.
The button was washed and weighed. The weight was 10.586. It is probable had the action of the acid been continued for a few minutes longer that the zinc would have been removed.
The button was next fused with NaHSO4, washed, heated, and the weight was 10.054 grains. This on cupellation gave a button weighing 9947 grains. It would appear therefore that some gold dissolves in the strong nitric acid, even though some zinc is still left in the button.
Parting of Zinc, Silver, and Gold
Since the removal, not of zinc, but of silver is aimed at in parting, an alloy consisting of:
was fused in a porcelain crucible under a layer of potassium cyanide. After cleaning the button, it was boiled for half an hour with 20cc of 2 of nitric acid to 1 of water. The zinc dissolved first, and white silver-like crystals appeared on washing with water after first treatment by acid. The button became fissured. The acid was poured off, and 10cc of 1 to 1 acid put on and boiled for about 15 minutes; finally 5cc of strong nitric were applied. The button was washed, dried and weighed. The weight of gold was 9023. It was then wrapped in sheet lead and cupelled. The final weight was 8885. In this case, as in the previous one, some solution of the gold takes place, even although the whole of the zinc was not removed.
The ratio of the gold-zinc alloy would give a formula approaching Au. Zn3, which seems to be sufficient for parting. It therefore follows that zinc could be substituted for silver in this operation. It has the advantage of being cheap, of forming an alloy at a low temperature, and even when in small proportions it reacts vigorously with nitric acid.
Parting of Copper and Gold
If parting depended on the formula alone, assuming that the alloying metal was dissolved by nitric acid, then copper should do as well as zinc, since the atomic weights are nearly equal. In order to determine this an alloy of 10.008 of gold and 10.080 of copper was made and placed in 1 of nitric to 2 of water and boiled. The solution was almost unaffected, showing that this alloy is not attacked by very dilute acid; the alloy was then immersed in a solution of 1 of nitric to 1 of water and boiled; the solution became light green colour, showing that the attack of the acid had commenced; the action, however, was so slight that the strong acid was used. This solution was colored green almost immediately, and brown fumes were evolved. Boiling was continued for some hours, and the fresh acid added from time to time as long as it became green. When the solution did not react for copper the button was washed, dried and heated. It weighed 9789. On cupellation this went 9746, showing that gold dissolved in the nitric acid, even though copper remained in the bead. Thus, although the action took place with copper as with zinc, it was much slower, and stronger acid had to be used to start the attack.
Sodium and Gold
In order to find out the effect of metallic sodium on gold in parting, an alloy of gold, silver and sodium was made by melting:
- 4.4 grms. gold, or 62 per cent. Au.
- 1.7 grms. silver, or 24 per cent. Ag.
- 1 grm. sodium, or 14 per cent. Na..in a hard glass tube.
- The mixture melted at a dull red heat.
On removing the same, and placing in water, a copious evolution of hydrogen took place, and the alloy, instead of being silvery white, like amalgam, became light bronze yellow, and appeared to be made of a mass of coherent crystals, which would crumble to powder under pressure. An assay of this product, after water treatment, gave:
A portion of the alloy was boiled with dilute, then strong nitric acid, but it did not wholly part, the gold button remaining being only 964 fine.
By taking a somewhat greater proportion of sodium, parting can be effected by means of nitric acid. This alloy, as will be shown, will part with sulphuric acid, followed by fusion with nitre cake.
Tellurium in Gold
Some time ago the author was struck with the fact that a comparatively low amount of tellurium in gold bullion enabled nitric acid to dissolve out a large proportion of the silver, even when the latter was present in a lesser extent than half the gold. The general explanation must be in this case, as well as in that of sodium, that small proportions of these elements occupy comparatively large spaces between the individual particles of gold, thus enabling the solvents to penetrate. The temperature of the acid, and its strength, and the time of application are also governing factors.
Thanks for posting your leaching information, it gives me a better idea of what you are trying doing. You are on the right track, BUT (and it sticks out like a sore thumb) the problems you are having on the back-end are caused by an OVERKILL on you chemistry formula. If you would think of chemistry as the same as baking a cake, it makes more sense.
If your Grandma’s favorite chocolate cake recipe called for 2 eggs, you would not put in 12 eggs to make it taste better, would you? All leaching chemistry is the same thing. Based on the ore to be leached, you start with a good recipe (formula), mixed in the proper sequence, used at the correct temperature, for a specified period of time, producing the desired end results. Sounds simple, huh?
Anyway, the two major mistakes with your leach formula is; (1) you are creating a supersaturated salt solution by boiling the water and dissolving all the salt (rock, not table salt) it will take, and (2) you are using way too much nitric acid, (remember the 12 eggs). Your leach is way too HOT and that is why when adding the zinc, nothing good was happening. Let’s back your leach chemistry back down a bit and still get you some good results.
To a gallon of tap water, add 3 lbs of rock salt, stir occasionally and let it dissolve. Decant the brine water, or filter through a coffee filter of there is trash in the rock salt. The ideal gold leaching formula is 7:1 (7 parts brine water to 1 part nitric acid), at a leaching temperature between, a low of 70 F and a maximum of 176 F. The higher the heat, the faster the reaction. Empirical tests have shown that about 1500 milligrams of 18K gold (about the same purity of a nugget), will go into solution per hour, per liter at this formula and maximum temperature. That is getting after it big time.
There are several ways to recover your gold from the leachate, some of them expensive and dangerous, but with a little preparation, zinc will work. Dilute the leach solution with 2 times the amount of tap water, and check the pH. It should be between 1.5 and 2. If not, use a 10% sodium hydroxide (lye) solution to raise the pH to about 2. Weight out 5 grams of zinc per liter of leach (21 grams/gal). Divide the zinc into 3 glasses 1/4 – 1/4 – 1/2 and cover with water. While stirring the leach liquid, add the first shot of zinc, after a minute add the second shot of zinc. After all reaction has stopped, slowly add the last half of the zinc. If done correctly, this will drop all of the metals, including gold, as a black/gray material. Allow everything to settle and check the pH, the liquid should be close to normal (pH 7), and clearer. Filter and recover the precipitant for firing.
Your idea of using charcoal to recover the gold is a good one (thumbs up to you), it’s cheap and almost hassle free. Just a couple of things to keep in mind with this recovery system. When you ash the charcoal, it’s in a reducing atmosphere which returns the metals ions back to their metallic form (a good thing), but your leach has put the gold (and other metals)in a chloride form which are water soluble. Let the charcoal dry really well before ashing so the gold won’t follow the water up the flue. Also, after recovering the gold from the ashes, soaking it in a mild (15%) HCl or nitric acid solution will rid it of most base metals recovered by the charcoal.