AMALGAMATION. 215 the metallic muds are thrown into vats of wood or stone. Small mills provided with sails placed perpendicularly, turn round in these vats. These machines (tinas de cal y canto), which are particularly well executed at Guanaxuato, have a resemblance to those established at Freiberg for washing the remains of the amalgamation. The earthy and oxidated parts are carried away by the water, while the amalgam and the mercury remain in the bottom of the vat. As the force of the current carries away at the same time some globules of mercury, in the great works, Indian women are employed in gathering this metal from the water used in washing. They separate the amalgam collected at the bottom of the tinas del lavadero from the mercury, by pressing it through sacks; and they mould it into pyramids, which they cover with a reversed crucible in the shape of a bell. The silver is separated from the mercury by means of distillation. In the process which I have been describing, they lose in general from 1 to 1lb. of mercury for each pound of silver. In the process of amalgamation introduced into Saxony by MM. Gellert and Charpentier, the consumption of mercury is per pound of silver, or eight times less than the proportion used in Mexico. We have described the cold amalgamation (por crudo y de patio), without roasting the ores, and by exposing them in a court to the open air. Medina was only acquainted with the use of salt, and sulphates of iron and copper; but in 1586, fifteen years after his process was introduced into Peru, Carlos Corso de Leca, a Peruvian miner, discovered the beneficio de hierro. He advised the mixture of small plates of iron with the metallic powder, affirming that by this mixture more then nine-tenths of the mercury would be saved. This process, as we shall afterwards see, is founded on the decomposition of the muriate of silver by the iron, and on the attraction of this metal for the sulphur. It is now but very little followed by the Mexican azogueros. In 1590, Alonzo Barba proposed the hot amalgamation in copper vats. This process is called the beneficio de cazo y cocimiento, which was proposed by M. Born, 1786. The loss of mercury is much less by it than in the beneficio por patio, because the copper of the vessels serves to decompose the muriate of silver, while at the same time the heat favours the operation, either in rendering the action of the affinities more powerful, or in giving motion to the liquid mass which enters into ebullition. This hot amalgamation is used in several of the mines of Mexico, which abound in horn-silver and colorados. Juan de Ordoñez, whose work has been already quoted, even advised amalgamation by means of stoves. In 1676, Juan de Corrosegarra discovered a process which is very much in use at present, called the beneficio de la pella de plata; and in which silver already formed is added to the mercury of the amalgam. It is AMALGAMATION. 217 said, that this amalgam (pella) favours the extraction of the silver, and that the loss of mercury is so much less, as the amalgam disseminates itself with greater difficulty into the mass. A fifth method is the beneficio de la colpa, in which, instead of an artificial magistral, which contains much more of the sulphate of copper than of the sulphate of iron, they use colpa, which is a natural mixture of acid sulphate of iron and iron oxidated to the maximum. This beneficio de la colpa, extolled by Don Lorenzo de la Torre, offers part of the advantages which we have just pointed out in speaking of the amalgamation by iron. Since the practice of amalgamation of silver ores was introduced into Europe, and since the learned of every nation met at the metallurgic congress of Schemnitz*, the confused theory of Barba, and the Mexican azogueros, has been succeeded by sounder ideas, better adapted to the present state of chemistry. It is supposed that the practice of Freiberg, where a mass of roasted ores is amalgamated in a very few hours, will be gradually introduced into the amalgamation of Mexico, where the ores are generally not roasted, and where they remain exposed in the open air to the sun and the rain for several months. The enormous waste of mercury which we observe in the American process of amalgamation * Properly Szkleno or Glashutte, near Schemnitz. proceeds from several causes which act simultaneously. If in the process por patio all the silver extracted were owing to a decomposition of muriate of silver by mercury, there would be lost a quantity of mercury which would be to that of the silver in the muriate, nearly as 4:7. 6; for this ratio is that of the respective oxidations of the two metals. Another and perhaps the most considerable part of the mercury is lost, because it remains disseminated in an immense mass of the moistened powder (schlich); and this division of the metal is so great, that the most careful washing is not sufficient to unite the particles concealed in the residue. A third cause of the loss of the mercury must be sought for in its contact with the salt water, in its exposure to the open air and the rays of the sun for the space of three, four and even five months. These masses of mercury and schlich, which contain a great number of heterogeneous metallic substances, moistened by saline solutions, are composed of an infinite number of small galvanic piles, of which the slow but prolonged action is favourable to the oxidation of the mercury, and the play of chemical affinities. It results from these researches, that the use of fire will sensibly improve the process of amalgamation. If the minerals to be treated are only vitreous silver, iron filings alone would be perhaps sufficient to lay the silver bare, and separate it from the sulphur which retards its union with the mer AMALGAMATIÓN. 219 cury. But as in all the other silver ores there are, besides sulphur, different metals combined with the silver, the simultaneous employment of muriate of soda, and sulphates of copper and iron, becomes necessary to favour the disengagement of the muriatic acid, which combines with the copper, iron, antimony, lead and silver. The muriates of iron, copper, zinc, and arsenic, and even that of lead, remain dissolved; and the muriate of silver, which is next to insoluble, is decomposed by contact with the mercury. The amalgamation, such as we have described it, serves to extract all the silver from the ores which have been treated by mercury, provided the azoguero be experienced, and thoroughly know the aspect or external character of the mercury, by which to judge if the paste is in want of lime or sulphate of iron. At Guanaxuato, where this operation is best managed, ores are successfully amalgamated which contain only 15 ounces of silver per ton. M. Sonneschmidt found only of an ounce of silver in the residue of analgamation proceeding from ores of which the ton contained from 700 to 900 ounces of silver. In the works of Regla, on the other hand, the schlich frequently undergoes washing before the mercury has extracted all the silver in the paste; and it is believed at Mexico, that the father of the present proprietor of the famous mine of Biscaina threw, with the refuse, an enormous mass of silver into the river. |