Parting is the separation of silver from gold and a process during which the base metals are separated from both, but, as the presence of a high percentage of these base metals is injurious to the successful conduct of the processes which are chiefly in use, a preliminary refining by one of the methods already described is usually necessary. Only about 10 per cent, of base metals is permissible in the alloys when sulphuric acid is used, although a somewhat larger quantity does no harm if nitric acid is employed.
The processes of parting may be tabulated as follows:
- Melting with sulphide of antimony.
- Melting with sulphur, and precipitation of the gold from the regulus by silver, iron, or litharge.
- Parting by nitric acid.
- Parting by sulphuric acid, sometimes called “refining.”
A combination of these last two methods.
- The Gutzkow process.
- Parting by chlorine gas.
- Parting by electrolysis.
- Parting by aqua regia.
The first two of these methods were known to the ancients and the third was described as early as the beginning of the 11th century.
In this ancient and obsolete process, gold was freed from small quantities of silver, copper, &c., contained in it. The method was mentioned by Pliny and described by Geber, who wrote in Arabic, probably in the eighth or ninth century; it is possibly still in use in some parts of the East and of South America. It consists in heating granulations of argentiferous gold mixed with a cement, consisting of two parts of brick-dust, or some similar material, and one of common salt, in pots of porous earthenware. The temperature used is a cherry-red heat which is insufficient to melt the granulations. After about thirty-six hours’ treatment, the greater part of the silver is converted into the state of chloride, and this, together with the cement, can be removed from association with the granulations by washing with water. The gold can in this way be raised to a fineness of about 850 or 900. The silver is recovered from the cement by amalgamation with mercury.
Parting by Using Sulphide of Antimony
This process was also used to purify gold which contained only small quantities of silver. The alloy was repeatedly melted with sulphide of antimony, upon which the gold became alloyed with the antimony and sank to the bottom of the mass, while the silver was converted into sulphide and floated on the top, mixed with the excess of antimony sulphide added. The gold was subsequently refined by a blast of air directed upon it, the antimony being thus oxidised and volatilised. The method is now obsolete, but was in use at the Dresden Mint up to the year 1846, and gold of the fineness 993 was said to be produced in this way.
Parting by Means of Sulphur
This method was formerly used for the purpose of concentrating the gold contained in auriferous silver in order to obtain a richer alloy. The granulated alloy was melted with sulphur and some of the silver was thus converted into a matte. The gold was then precipitated from the matte and collected in a smaller quantity of silver by fusion with pure silver, or with iron, or litharge. No attempt was made to obtain pure gold in this way, and the enriched alloy of gold and silver was parted by nitric acid. The silver was recovered from the matte by fusion with iron. The method was in use in several refineries in Europe at the beginning of the last century. The employment of sulphur in refining at the United States Mints has been already noticed.
Parting by Nitric Acid
The first clear mention of the use of nitric acid for parting silver from gold is made by Albertus Magnus, who wrote in the thirteenth century, but the process does not appear to have been employed on a large scale until two centuries later in Venice. Here, according to an old tradition, some Germans were employed in separating gold from Spanish silver in the fifteenth and sixteenth centuries, the art being kept secret. These refiners were not inaptly named “ gold makers ” by those who were unacquainted with their methods. The process was fully described by Biringuccio in his treatise, published in 1540, and by Agricola in 1556. It was first used in the Paris Mint about the year 1514, and in London at least as early as 1594, but for a long period the operations were conducted in secret in both countries, and it is supposed that this method of refining was not fully practised in England until about the middle of the eighteenth century.
Parting by means of nitric acid is conducted on the large scale in the same general manner as in the assaying of gold bullion.
It consists of the following operations:
- Granulation of the alloys.
- Solution of the silver in nitric acid.
- Treatment of the gold residues:—Sweetening by washing with water, drying, melting, and casting into bars.
- Precipitation of the silver as chloride by salt solution.
- Reduction of the silver chloride by zinc and sulphuric acid.
Treatment of the Gold Residue
The pulverulent gold is “sweetened ” by being washed thoroughly in perforated earthenware dishes with boiling distilled water, stirring being performed with a spatula of wood, platinum, or porcelain. The gold is thus freed from nitric acid and nitrate of silver, the operation being continued until the washings show no signs of turbidity on the addition of salt. The washings are added to the first silver solutions, serving to dilute them, the dilution, as has already been observed, being necessary to prevent crystallisation on cooling. The sweetened gold is generally pressed, dried, melted, and cast into bars, which are now made of a weight of either 200 or 400 ozs. The gold thus obtained is usually of a fineness of about 997 or 998, the remainder being chiefly silver, which would not pay for extraction although part of it could be separated with a further expenditure of time, fuel, and acid. The gold is pressed by a hydraulic ram, the pressure exerted being about 800 lbs. to the square inch. The cakes of metal are dried at a cherry-red heat, and then broken up for melting.
Treatment of the Silver Solution
The solution of nitrate of silver is diluted with water, allowed to cool, and then treated with a strong solution of salt which is regulated so as not to be in large excess, continuous agitation being kept up by revolving wooden agitators driven by steam power, or by hand paddles. When all the silver has been precipitated as chloride, the whole is allowed to settle overnight, and, in the morning, the clear solution of nitrate of soda, containing most of the base metals originally present in the alloys, is drawn off and filtered. The precipitated chloride is washed several times by decantation and agitation, and finally sweetened in wooden filters by boiling water, which incidentally dissolves out the chloride of lead. The filters are usually lined with linen or some similar material.
Common Parting Process
At the Philadelphia Mint a combined process is used, nitric acid and sulphuric acid being employed in succession. The alloys are granulated and digested with concentrated nitric acid for six hours in the same manner as has already been described; the solution is then siphoned off, and the gold washed two or three times with distilled water, by decantation, subjected to a second boiling with strong nitric acid, and subsequently sweetened in lead-lined filters with boiling water. The gold is then introduced into cast-iron cylindrical kettles and boiled for five hours with strong sulphuric acid, the gold being stirred up with an iron rod every ten or fifteen minutes to prevent agglomeration, and the solution is then ladled out and treated as already described. For a charge of 190 lbs. of metal, 175 lbs. of nitric acid are used in the first boiling, and 50 lbs. in the second. Some nitre is added to the sulphuric acid.
The gold is washed thoroughly and sweetened in wooden filters, boiling distilled water being poured through it until the washings will no longer redden blue litmus paper. The silver is precipitated from these washings as chloride by the addition of salt. The gold is then pressed, dried, melted, and cast into bars, which are from 998 to 999 fine.
The process is much cheaper than the nitric acid process, costing 20 per cent, less for acids, and saving some fuel. The granulations contain 100 parts of gold in 333 of the alloy. After the boiling in nitric acid very little silver is left with the gold. The cost of refining is a little over one cent per ounce.
- The Gutzkow Process
- Miller’s Chlorine Process
- The Crucibles
- Refining Machine
- Purification of Gold by Electrolysis
- Gold Refining Process by Aqua Regia