Thus:-

Primary Reaction

2AgCl + Cu2S + NH2 = Ag2S + Cu2Cl2.NH2

Secondary Reaction

2AgCl + Cu2Cl2NH2 = Ag2 + CuCl2,NH2 It was also found that "reversal" takes place when a mixture of silver chloride and copper sulphide is treated with water or sodium chloride solution. No secondary reaction, liberating silver, however, takes place.

It follows from the foregoing that, in order to bring about double decomposition between silver sulphide and copper chloride, the copper chloride solution must be maintained at a certain strength, otherwise the reaction will cease, and anything which caused the solution to be still further diluted would undo a proportionate amount of the work already done.

Sometimes, before commencing to wash up a torta, a fresh quantity of mercury is added. The washing is effected in a triple series of circular tanks built of cemented masonry, measuring 9 1/2 ft. wide and 5 1/3 ft. deep, provided with discharge-holes at suitable levels, and with stirring apparatus which can be worked by mules. The mass of amalgam and residue (lama), previously softened by watering and treading, is introduced gradually into the first washing-tank in charges of about 4 tons, at the same time that water is admitted, and the stirring apparatus is set in rapid motion; this is continued till the whole of the charge is in, when the speed of the stirrer is reduced by degrees till the mules walk; as soon as the samples then taken from the upper portion of the mass appear pretty free from metal, the slimes are run off, and the operation is repeated on successive charges of lama till the torta is exhausted. Then the final accumulation of heavy matters at the bottom of the tanks is very carefully collected and washed out either on washing-tables or in bateas, to separate the clean amalgam from the other portions, the latter having to undergo further grinding and amalgamating. The treatment of the liquid amalgam will be described later on.

The Mexican process is obviously crude and wasteful, resulting in a loss of some 10 to 40 per cent. of the silver in the ore, with an accompanying consumption of 24 to 32 oz. of mercury for every 1 lb. of silver recovered.

(2) Stove Process

Some modifications of the patio process are in vogue in certain districts. In that known as the "stove " process, the operations are identical as far as to the treading, but when this is only half accomplished, the mass is removed to a chamber provided with heating flues beneath the floor, and here subjected to an increased temperature for 2 or 3 days, before completing the treatment on the patio. By this plan, the silver return is somewhat increased, but at the expense of a greater waste of mercury.

(3) Hot-Water Process

A hot-water process is employed on ores rich in native silver, or where the metal exists as a bromide, chloride, or iodide. The stamped and ground ore (not too fine) is washed, and charged into a vessel with wooden sides and copper bottom, with sufficient water to make it into a thin paste. The size of the vessel, which is tub-like in form, may vary from that of a cazo (3 ft. diameter at top, 2 ft. at bottom, and 18 in. deep), capable of holding about 100 lb. of ore, to that of a fondon (5 1/2 to 7 1/2 ft. diameter at the bottom), holding 1200 to 1500 lb. In either case, the contents are heated to boiling by a fire playing upon the copper bottom, and at this point 5 to 10 Per cent. of common salt is thrown in and thoroughly stirred up, by hand in the smaller vessel, but by rotating arms carrying lumps of copper, worked by mule power, in the larger vessel. After the salt is introduced, the attendant keeps up a gradual supply of mercury, until samples withdrawn at frequent intervals show that the silver is pretty completely extracted; the estimated requirement of mercury is twice the weight of silver present in the ore, and the cooking operation generally lasts about 6 hours.

The rationale of the process is that the copper of the vessel decomposes the silver bromide, chloride, or iodide, itself uniting with bromine, chlorine, and iodine, and liberating the silver in a metallic condition free for amalgamation by the mercury. But any sulphide of the metal is not so affected, consequently the residual mass is transferred to the patio to undergo treading in the usual manner, except that no addition of magistral is needed, as sufficient copper chloride is present to effect the conversion of the silver sulphide into chloride. In this process, the mercury loss is reduced to 2 or 3 per cent., mainly by reason of the fact that the reduction of the silver chloride is effected at the expense of the copper utensil instead of the mercury.

(4) Barrel Process

"Barrel amalgamation" is the term applied to a method introduced nearly 100 years ago at Freiberg, Saxony, for treating ores containing silver associated with antimony, arsenic, copper, iron, lead, sulphur, and zinc, with occasionally bismuth, cobalt, gold, and nickel. The ores are classified and assorted so that the lead present does not exceed a proportion of 4 Per cent. of the total bulk, nor the copper 1 per cent.; and they are divided into 2 grades,-a richer, in which the silver amounts to 90 to 130 oz. per ton, and a poorer giving only 30 to 40 oz. An essential condition in each case is the presence of about 25 percent. of iron sulphide, for conversion into sulphate during the roasting; if this is not sufficiently supplied by the iron-pyrites in the ore, the deficiency must be made up, whilst if the ore affords too much, part must be abstracted by previous roasting of the portions richest in sulphur. The prepared ore is spread in a thin layer, covered with 10 Per cent. of salt, thoroughly mixed together, passed through a coarse sieve, and piled in heaps of 4 1/2 to 5 cwt. each, ready for roasting in reverberatory furnaces.

For this purpose the charge is spread on the hearth, and gently heated for 20 to 30 minutes to drive off the moisture, which is facilitated by constant stirring. After breaking down the concretions formed at this stage the fire is urged till dense white fumes are evolved from the charge; this is continued for about 3 hours, during which time the stirring of the mass is kept up; and then the' fire is allowed to burn down, but the stirring is maintained till the charge becomes dark throughout and gives off no sulphurous odour. At this stage the mass has become oxidized, and is in a very friable state, occupying an increased space. On again raising the heat for about f hour, the iron sulphate formed from the pyrites decomposes the salt and liberates chlorine and hydrochloric acid, which attack the metals present, and form chlorides, together with some sodium sulphate. The roasted charge is withdrawn, cooled, and passed through sieves; the fine portion passing through is ground to impalpable powder ready for the amalgamation process, while those portions which do not go through the sieves are calcined anew.