Commercial Scale Gold Parting with Nitric Acid

Commercial Scale Gold Parting with Nitric Acid

The method of gold parting with nitric acid usually consists in:

  1. Granulating the alloy, which should he free from antimony, tin, or sulphides.
  2. Treating with dilute nitric acid, and decanting the solution.
  3. Boiling with strong nitric acid.
  4. Washing the parted gold free from alloy.
  5. Smelting the parted gold.
  6. Recovering the silver from solution.

The granulating is done by melting the alloy and pouring it into cold water, in such a way that the granules do not stick under the water. This operation can be carried on by allowing the stream of molten metal strike the edge of an inclined plate before falling into water. In this way more shell-like granules will be formed. It is inadvisable to have shotty and solid lumps amongst the granules, for the time of solution will be governed by the diameter of these. The water should be kept cool, for the molten metal can sometimes be seen to be red hot below its surface, and if it is all poured in one place an adherent pyramid of the granules may be lifted out. The pots used for parting consist of porcelain, or well glazed earthenware. Those used in the American Mints were about 21 inches in diameter, and 22 inches deep, provided with handles and a spout. 125lb. of granulated bullion is put in each of the pots. The pots are set in tanks containing water, which is heated by steam coils. Each pot is provided with a wooden lath, which acts as a stirrer.

The acid used on a large scale appears to have been much too strong. Johnson and Matthey boiled the alloy with three lots of acid, 50 pints, 30 pints, and 20 pints respectively, for 4, 3, and 2 hours, when parting 800oz. of bullion.The last acid was used over again, as it contained very little silver. The strength of the acid being 1.4 sp. gr., diluted with its own volume of water. The acid used in the San Francisco Mint was ordinary strong acid, 1.4 sp. gr. In neither case was any attempt made to re-convert any of the oxide of nitrogen escaping into fresh acid.

The amount of nitric acid required to dissolve a metal depends on the amount of acid present, and temperature, as well as the nature and weight of metal. If the acid is strong and hot the higher oxides of nitrogen are evolved, but if weak and cold no gas at all may come off; in the latter case nitrous oxide, or even ammonia may form. Even when hot concentrated acid is used the reactions which go on cannot be expressed by one formula, for, on dissolving zinc with such an acid, although nitrogen peroxide is abundantly given off, nitrate of ammonia will be found on evaporating the solution to dryness. Copper and zinc, on account of their lower equivalent weights, will consume more nitric acid than silver; zinc also appears to have a greater reducing power on the acid than either of the others. The most economical strength of acid for any given alloy is readily determined by taking a known weight, and estimating the free acid left over after dissolving as much as will pass into solution in a given time. As a rule the lower the products of oxidation evolved the less will be the acid consumption; for instance, with silver, if nitrogen per¬oxide, nitric oxide or nitrate of ammonia is found according to the following equations:

  • Ag + 2HN03 = AgNO3 + N02 +H2O.
  • 6Ag + 8HNO3 = 6AgN03+ 2NO + 4H20.
  • 8Ag + 10HNO3 = 8AgN03 + NH4N03 + 3H20

In other words, the amount of acid used up or neutralised would be in the proportion of 1 for the first case, 0. 66 for the second, and 0.62 for the third case. After boiling in the first acid, and after the action has ceased, as may be observed by the cessation of the evolution of brown fumes, hot water should be added, to prevent the deposition of silver nitrate crystals, and to give more liquor for the decantation of the silver solution. It is preferable to remove as much of the silver nitrate in solutions as possible by decanting it by means of a siphon, and replacing with water free from chlorine and again decanting, before adding the second lot of acid. Very little silver is removed by the second acid, and this solution, as well as a third, may be again used on a fresh lot of bullion. After the acid treatment, the gold is washed with hot water and well stirred, allowed to settle, and the supernatant liquid removed. When the reaction for silver is slight, the metal is removed and washed on a filter until the silver nitrate is removed—sometimes a wash with ammonia water is given in order to remove any chloride which may have remained. The gold precipitate is then pressed, dried, and heated to redness, and smelted, the bars usually running 995 to 998 fine.

Parting Zinc Cyanide Bullion

A modification of the process is sometimes made use of in treatment of the bullion obtained from the precipitation of gold and silver on zinc filaments in the cyanide process. The operations are as follow:—

  1. The removal of the bullion from the zinc filaments.
  2. Smelting the same, so as to obtain a low-grade bullion.
  3. Granulation of the alloy so obtained.
  4. Parting with nitric acid.
  5. Recovering silver from the solutions.

The zinc filaments, as well as any finely divided precipitated metals, are removed from the zinc boxes, and passed through a sieve having about 50 to 60 holes per linear inch. The finely divided precipitated metals pass through—the coarser zinc fragments are retained and replaced in the box for further use; the fine material is allowed to settle, the water is decanted, and the slimes are washed by filtration or in a filter press. They are then removed and gently dried in a cast-iron tube set in brickwork, and heated externally. The cylinder is open one end, and having an exhaust pipe at the other leading to a pipe which acts as an atmospheric condenser—mercury is often caught in this. The slimes are thus dried and partly oxidised. They are then mixed with suitable fluxes so as to slag off the oxides present, and collect the bullion on melting. During the fusion some of the zinc is volatilised, but part of it alloys with the other metals, and melts down. So long as about 45 per cent, is present the alloy will part with nitric acid when granulated.

The composition of such an alloy may be indicated by the following proximate analysis:—

Gold 40—45%
Silver 15—20%
Lead 2—4%
Copper 1—2%
Zinc 40—30%

This alloy is granulated, then parted first with dilute nitric acid, the nitrates formed are partly removed by washing with water; strong nitric acid is then added, and the granules boiled for some time. The liquid is decanted, the granules washed and smelted. The silver is precipitated as chloride with salt, washed, treated with zinc, and the reduced silver washed, dried and smelted.

Double Acid Gold Parting

A double system for parting was introduced in the San Francisco and Philadelphia Mints. Nitric acid, as previously described, was used for the preliminary removal of silver: 135lb. of the granulations were heated with 125lb. of strong nitric acid; the alloy stirred with a wooden lath every twenty minutes. The water bath surrounding the pots was kept boiling for twelve hours; water was added, and the nitrate of silver decanted. An additional quantity of acid was added, and after twelve hours boiling this was removed and applied to a fresh lot of bullion, since it contained very little silver. The gold remaining was washed first by decantation in the pot, and was then removed and washed in a shallow vat provided with a false bottom, covered with a filter. Hot water was allowed to run through until all the soluble nitrate of silver had been removed. The gold now containing only a small proportion of silver was ladled into cast-iron pots provided with a hood to carry away fumes, sulphuric acid sufficient to cover the gold was added, and the mass evaporated almost to dryness; a small amount of nitre was added to make the gold denser; more sulphuric acid. 92 per cent, in strength, was added and heated, and the gold stirred with an iron rod. The acid was poured or ladled out, the gold removed, washed on a filter with hot water, dried and smelted into bars which assayed 998 fine.