The samples are generally taken 3 times daily, viz: before, during, and after the day's treading, and they must be chosen from all parts of the mass. The treading increases and hastens the action of the majistral, and is usually completed in 15 to 45 days. Towards the end of the operation, the mercury globules on being rubbed show little amalgam, and finally they become-almost altogether free from silver, and run together in the bottom of the bowl, when the mass is considered to be exhausted of its silver, and is subjected to a washing process.
Before proceeding to describe the next step, it will be interesting to notice some remarks made by Prof. Huntington (JI. Soc. Chem. Ind.) on the nature of the reactions which take place in the torta on the patio. The following are some of the main points investigated:-
1. Mercury worked up with silver sulphide, sodium chloride, sand and water, extracted about -7/8 of the silver present, and 3 times as much as when sodium chloride was absent. The presence of iron oxide causes loss of mercury when the mixture contains sodium chloride, owing to ferric chloride being formed, which is reduced to ferrous chloride by the mercury, with formation of calomel. A very little iron oxide has a marked effect, as the ferrous chloride, as fast as formed, re-oxidises, in contact with the air, to ferric chloride, and so on:
2. When, in addition to the sodium chloride, the mixture was made up with copper sulphate, which would produce copper chloride by double decomposition, rather less silver was obtained, and the loss of mercury was greater. However, on substituting the mineral proustite (silver 65.5, arsenic 15.1, sulphur 19.4) for the artificial silver sulphide, twice as much silver was extracted by the mercury when the mixture contained copper chloride. The decomposition of the silver sulphide is very imperfect, unless the mixture be frequently and thoroughly agitated.
3. Zinc sulphide, in the presence of cupric chloride, causes the formation of copper sulphide and zinc chloride. It is well known that ores containing blende amalgamate badly.
4. A number, of experiments were made to establish the action of cupric and cuprous chlorides on silver sulphide. The generally accepted explanation of the action of the copper chloride in the Mexican amalgamation process is embodied in the following equations:-
In the experiments made by Prof. Huntington, it is shown that the liberation of the sulphur is entirely due to a secondary reaction, which takes place only to a limited extent. It may be represented as follows:-primary : Ag2S+CuCl2 = 2AgCl +CuS;
CuS+CuCl2=Cu2Cl2+S. The secondary reaction takes place pari passu with the primary, at the moment double decomposition is occurring between the cupric chloride and the silver sulphide, and is brought about by the copper of the chloride concerned in the reaction reducing to a lower chloride a further quantity of. cupric chloride, in preference to combining with the sulphur of the silver sulphide; so that in the result silver chloride, cuprous chloride, and free sulphur are produced. The experiments, as a whole, lead to the conclusion that the amount of cuprous chloride formed and of sulphur set free is directly dependent on (a) the strength and quantity of a solvent for cuprous chloride present, such as NaCl and CuC2, (6) the temperature, (c) the presence of air. The secondary reaction is limited by the power of the solution to dissolve cuprous chloride. Obviously, if the cuprous chloride in solution can by any means be removed, the solvent power of the solution will be to a certain extent renovated.
The action of air in facilitating the secondary reaction is therefore due to its converting the cuprous chloride into insoluble oxy-chloride, according to the equation-3Cu,Cl2+3H20+30= 3CuO.CuCl2.3H20+2CuCl2
5. In the course of these experiments it was found that when silver sulphide is treated with a strong solution of cupric chloride at a high temperature (boiling), in a stout closed bottle, cuprous chloride and free sulphur are formed in large quantities. If the heating be continued for some time, all the free sulphur disappears, and sulphuric acid is formed. Under similar circumstances, copper sulphide strongly heated with cupric chloride, air being excluded, yielded copper sulphate and cuprous chloride, no free sulphur being formed. Sulphur strongly heated in a hermetically sealed tube with water, produced sulphuretted hydrogen, but no sulphuric acid. When a 300 Per cent. solution of cupric chloride was similarly heated with sulphur, cuprous chloride and copper sulphate were formed. The following equations explain what takes place:-
S+H20 = H2S+0; H2S+CuCl2=CuS+2HCl; CuS+40=CuSO4. Also, CuS+CuCl2=Cu2Cl2+S
6. The next point investigated was the action of cuprous chloride, dissolved in a solution of sodium chloride, on silver sulphide. This is a case of simple double decomposition, represented by the formula-
Ag2S+Cu2Cl2 = 2AgCl+Cu2S. A great many erroneous statements have been made by Malaguti and Du-rocher, and by others, as to the action of cuprous chloride on mixtures containing silver sulphide. They say that silver is liberated in the metallic state. On this supposition they have built up various theories relating to amalgamation processes and to geological phenomena. These errors have arisen through their having employed ammonia as a reagent in the investigation of the action of Cu2Cl2 on Ag2S. Ammonia being a solvent for silver chloride, they thought that if the latter were formed by the action of cuprous chloride on silver sulphide, it could be dissolved out from the copper sulphide by ammonia, and then detected by precipitation in the ordinary way. But when ammonia is added to a mixture of AgCl and Cu2S, it causes double decomposition to take place, Cu2Cl2 being re-formed; and Cu2Cl2+AgCl, in the presence of ammonia, react upon one another with the formation of CuCl2 and metallic silver.