Mercury.

Silver.

Gold...

75.04 24. 18

.77

The proportions of silver-96.5 to gold-3.5, are here, it will be ob served, very different from those found in the bullion of the third-class ore spoken of above, and which was, silver 67.6, and gold 32.4.

In working the small pan used in making the experiments upon which the chapter upon the chemistry of the pan-process is based, it is to be noted that no such proportion of silver to gold was obtained as that which occurs in the regular returns of the Savage Company. In trying the first-class Savage ore, the bullion contained silver 934.7, to gold 49.5; or, in 100 parts, silver 94.9 and gold 5.1; and yet the yield of silver was only 32.55 per cent., while that of the gold was 70.6. Had the yield of silver equaled that of gold, the proportion of the latter would have been very much smaller. Probably in working in the large way gold collects in the bottom of the agitator and is taken up by the mercury of the next charge of pulp, a state of things that could not exist in the experiments, because the apparatus was there thoroughly cleaned after each pan.

CHAPTER XIII.

THE AMALGAMATION OF SILVER-ORES IN PANS, WITH THE AID OF CHEMICALS.

The attention of the mining public has, within late years, been largely attracted to the existence of numerous veins and deposits of silver-ore, the beneficiation of which was, however, apparently impracticable, as the ores they furnish cannot be worked with any degree of success by the ordinary amalgamation-process, and are not sufficiently rich to bear the expense of roasting, even after the improved and cheaper methods now in vogue. An over-confidence in the adaptability of the ordinary Washoe process to the working of ores of the most different characters has led to numerous failures, while, on the other hand, the caution bred of experience causes many properties to lie idle which would prove remunerative under a method of treatment equal in thoroughness to the ordinary process, without greatly exceeding it in expense. The demand for such a process has within the last three years been practically filled, but the suspicion with which all innovations are regarded, and the uninvestigative spirit of the "practical" millman, have prevented this new method from being more generally known and adopted. The facts now set forth should demonstrate its usefulness beyond a question. A test of its merits can be readily made. Simple as it appears, it can be made to solve many metallurgical and financial problems in the way of working ores to a fair percentage of their value, and with profit, which hitherto have, by their apparent rebelliousness, proven only sources of loss.

The first successful attempt at amalgamating rebellious silver-minerals in pans, and without roasting, was made on the Comstock slimes. By far the greater portion of these slimes was in former years allowed to run to waste. This loss was probably looked upon as an evil unavoidable in wet-crushing mills, and was partially excused by the fact that not more than 35 per cent. of their gross value could be extracted by the then known methods of treatment. There was consequently but little inducement to millmen to save slimes assaying $40 per ton, or thereabout, as they could barely be worked to a profit. Thousands of tons were thus lost, which at the present day would yield an enormous fortune to their

owner..

Some three years ago a series of experiments were made, which clearly proved that these slimes could be worked to a high percentage at a cost which, though greater than that of ordinary milling, still left a good margin for profit. Mill-owners were induced to save and sell what they had hitherto allowed to run waste, and, shortly after, the working of slimes became a distinct feature of milling on the Comstock.

All this was effected simply by adapting to the Washoe pan-process certain features of the Mexican patio, namely, the use of sulphate of copper and salt in sufficient quantities to decompose the rebellious silverminerals and leave them in a favorable state for amalgamation. The successful application of these chemicals to the ordinary Washoe process was entirely novel. It is true that in former years many millmen had tried these chemicals among a host of others, but their experiments were invariably carried on with such a want of knowledge that failure was inevitable. Some used bluestone without salt, and others salt with

out bluestone. When the two were used simultaneously, the amount was so small that the effect could not be appreciated. At the time referred to it was a maxim among millmen that no benefit was to be derived from the use of chemicals. Mr. Küstel, in his book on the Washoe process, after giving a list of different chemicals employed or recommended-none of which were probably ever largely adoptedexpresses strong doubts as to the efficacy of any or all chemicals; and in a later article, after admitting the extraordinary effects obtained on slimes, states that the same process would not be applicable to sands. So much to vindicate the practical originality of this process. It will be shown further on that it can be applied with equal advantage to the working of sands and tailings. There is no certainty as to the formulæ in obedience to which the chloride of copper-which is formed by the mutual action of salt and sulphate of copper-effects its purpose. Chemists are not agreed as to the various re-actions, and the subject is still under dispute. Its theoretical comprehension is, however, not essential to the practical operations which will be described.

We will first consider the application of chloride of copper to the working of slimes, and, later, its extension to the treatment of sands or pulp.

It

Slimes constitute the clayey portion of the ore which flows from the battery in wet-crushing mills, and is imperfectly caught or settled in the tanks constructed for the purpose of catching the crushed ore. is needless to say that with a sufficient number of tanks these slimes could be saved, but lack of space generally renders this impracticable, and the usual plan is to allow them to flow into reservoirs outside of the mill, from which they are afterward dug out, allowed to dry, and hauled to the mill.

The mechanical treatment of slimes varies but little from that of ordinary ore. Their finely-divided condition renders grinding unneces sary, and calls merely for a sufficiently violent motion to thoroughly incorporate the quicksilver with the pulp.

The length of time devoted to the amalgamation of a charge of ore is influenced to a great extent by the amalgamating capacity of the mill, and by various reasons of economy. It often becomes advisable to sacrifice a certain percentage of the value for the sake of working a greater number of tons per diem. To insure good results a charge should not remain in the pan, subject to the action of quicksilver and chemicals, less than six to seven hours.

In working ordinary ores the quicksilver is not added until the pulp has been thoroughly ground. This is to prevent the flouring of the quicksilver; but since slimes require no grinding, one might suppose that it would be more advantageous to put the quicksilver into the pan on charging, thus allowing more time to the amalgamation.

Experience has, however, shown that better results are obtained by putting in the chemicals, i. e., sulphate of copper and salt, first, and adding the quicksilver from two to three hours later, thus allowing the intervening length of time for the decomposition or "preparation" of the refractory minerals. The reason of this is not immediately apparent, as the quicksilver charged simultaneously with the chemicals should attack the chloridized (?) silver-minerals in statu nascenti. The amount per ton of chemicals employed varies with the richness of the slimes. The quantity is always largely in excess of that called for by the chemical equivalents of the minerals to be acted upon, but in very large pans the relative proportions may be diminished. Taking ordinary slimes, assaying thirty dollars per ton, as a basis, the average quantities

of sulphate of copper and salt employed per ton are 10 to 12 pounds of the former and 20 to 25 pounds of the latter.

These quantities may be advantageously increased on slimes of higher grade. The exact amounts must be determined by conscientious assays. In working slimes of uniform character, the varying fineness of bullion furnishes an excellent empirical test of the amount of chemicals to be employed. The baseness of the bullion increases with the quantity of sulphate of copper used, owing to the precipitation of copper by the iron of the pan, and its consequent amalgamation. The millman soon dis.covers that beyond a certain point an excess of sulphate of copper produces no adequate results besides reducing the fineness of his bullion. Having once determined the average fineness of his bullion when working under the most favorable circumstances, he increases or diminishes. the amount of sulphate of copper as the bullion becomes finer or baser than his standard.

The same amount of chemicals with richer slimes will produce finer amalgam, which is due to the fact that a larger amount of silver is amalgamated while the same percentage of copper is precipitated. Should the value of the slimes and consequent fineness of the bullion increase materially, it becomes necessary to determine by assay the most advantageous proportion, and to adopt a new standard of fineness. A source

of great expense in the working of slimes is the excessive and apparently unavoidable loss of quicksilver. Working with all the advantages of settlers and agitators, this loss seldom falls below four pounds per ton, whereas in quartz-mills it does not as a rule exceed 14 to 11⁄2 pounds per ton of ore. This is undoubtedly due to the nature of the slimes, which have the same effect upon the quicksilver as oil or grease, forming a film or coating over the surface of the metal, and preventing the globules from uniting. The minute particles settle with difficulty, and to a great extent flow off with the tailings. The excessive loss in working slimes cannot with justice be ascribed to the action of chloride of copper, as in working the tailings by the same process the loss of quicksilver per ton is not greater than in ordinary quartz-mills, thus proving sufficiently that the deleterious action of chloride of copper upon quicksilver is not appreciable upon so large a scale.

Many attempts have been made to effect a saving of quicksilver in milling, but so far only mechanical appliances have met with success. Although sodium, sodium amalgam, cyanide of potassium, &c., are very effective in temporarily "enlivening" the quicksilver, their action speedily dies out after the latter has been transferred to the pan, and experience has sufficiently demonstrated that when employed in practicable quantities they have no effect in aiding the amalgamation. The only thorough method of cleansing quicksilver is to retort it, and the use of proper straining-sacks will render a recourse to the method unneces

sary.

The following table will show results obtained in working on a large scale two different lots of slime:

1. Slimes from various mills.

2. Slimes from Savage ore.

Number.

Assay value per ton.

Yield per ton.

Per cent. extracted.

Fineness of bullion.

Gold. Silver. Total. Gold. Silver. Total. Gold. Silver. Total.

The results obtained in the working of lot No. 2 are somewhat above the average, which can be estimated at about 80 per cent. of the assay value.

The method of treatment now in vogue is susceptible of modification and improvement. The yield can be increased by charging the chemicals at two different periods, say two-thirds at first and the remaining third two or three hours later. This is due to the fact that the action of the chloride of copper becomes more feeble as time elapses, and is revivified by the addition of this second portion.

A saving in the amount of salt consumed can be effected by dissolving it together with the sulphate of copper in vats in proportions approximating to their chemical equivalents, and charging the resulting chloride of copper in the liquid state. The plan of putting the sulphate of copper and salt into the pan separately, necessitates a large excess of the latter to insure the complete decomposition of the former. An excess of salt was originally deemed advantageous on the supposition that it would act beneficially in holding the chloride of silver in solution, thus presenting more surfaces of contact to the action of the quicksilver, and, moreover, would convert the Hg Cl formed to Hg Cl2, in which state it would be precipitated by the iron of the pans in the metallic form. It is, however, improbable that such re-actions should occur on a large scale where the re-agents represent so small a proportion of the entire mass.

The discovery of a cheap method of working slimes led to important results. The same process was soon applied to the working of low-grade tailings, of which vast quantities had accumulated on the plains bordering on the Carson River, at Dayton, and at other points. Several mills, with a capacity of working from 250 to 300 tons per diem each, are now in operation in Dayton and vicinity. The tailings, which were formerly allowed to run into the Carson River, are now carefully saved, and in fact to such an extent has the re-action set in against the wasteful practices of the past, that many mill-owners save the tailings after they have passed through the pans, with a view to reworking them at some future time. The tailings being mostly very poor, it becomes necessary to work a large amount daily in order to obtain a fair interest on the capital invested. This is effected by using very large pans and by rapid amalgamation, Economy in the time devoted to amalgamation cannot, however, be effected without sacrificing a portion of the precious metals which might otherwise be saved. It is, however, considered policy to suffer this loss. The method of treatment is practically the same as in working slimes, with the exception that a smaller proportion of sulphate of copper and salt is used, and that in other respects less attention is paid to the extraction of a high percentage, the value of the tailings not justifying too careful a manipulation. For the same reason the extra expense of grinding the tailings, which would undoubtedly increase the yield, is dispensed with.

As a rule the bullion produced is very base, not exceeding on the average .450. and often going as low as .350.

In the mill of Birdsall & Co., in Dayton, where the crude bullion rarely exceeds in fineness the latter figure, an inexpensive process has been adopted for partially refining it.

As the bullion comes from the retort a great portion of the copper is found in a crust on the surface of the slab. This slab-crust, which contains some silver and gold, is separated from the finer metal beneath, and the latter is cast into bars 500 to 600 fine. The cupreous crust is roasted in a reverberatory furnace and crushed, and the copper-oxide separated by