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by the mercury, and require more time for any chemical changes before amalgamation can take place. There is considerable resemblance between Nos. 3 and 4 of the Savage table, with a large excess of salt and sulphate of copper, and Nos. 6 and 7 with a solution of the sub-chloride of copper. The reason may be found in the fact that in the former the chloride of copper formed would be quickly reduced by the iron to the state of the sub-chloride, and similar conditions produced as in the case of the latter.
Chemistry of the process.-The action and value of common salt and sulphate of copper in the amalgamation of argentiferous ores, by what is known as the patio process, has always been a somewhat disputed question. Numerous theories have been advanced by metallurgists of long practical experience in Mexico, to account for the reduction of the sulphide of silver by the methods adopted in that country. The two which have obtained the most prominence, and which chemists have received with most favor, differ very widely in the manner the decomposition is supposed to be accomplished.
The most plausible theory, and the one now generally adopted, is that of Sonnenschmidt. He claims that the salt and sulphate of copper react upon each other, and produce sulphate of soda, which is neutral in its action, and chloride of copper. This Jatter salt then acts upon the argentiferous sulphide, and yields chloride of silver, subchloride of copper, and free sulphur. The sub-chloride reduces a second portion of the sulphide of silver, and causes the formation of an additional amount of the silver chloride, and sub-sulphide of copper. The silver salt is then attacked by the mercury; calomel, or sub-chloride of mercury, is produced, while metallic silver is set free, which combines with a second portion of the mercury, as amalgam. The following chemical equations show the reactions:
Bowring, an English metallurgist, on the other hand, denies that any of the sulphide of silver is chloridized, and asserts that before amalgamation takes place, metallic silver is first produced. He claims that chloride of copper, in contact with mercury, forms the sub-chloride of both metals. The sub-chloride of copper, in contact with the oxygen of air, is converted into an oxychloride, which, in turn, acts upon the sulphide of silver, and liberates the metal in a free state, by oxydizing the combined sulphur. These reactions are expressed as follows:
2 CuCl+2Hg=Cu_C1+Hg C1
Cu Cl+0=CuCl CuO
3 (CuCi CuO) +AgS=Ag+SO3 + 3 Cu2Cl.
Although oxychloride of copper may possibly be found at times, there does not appear to be any decided evidence that such is the case in practical operations, or that it decomposes the sulphide of silver, while the experiments already recorded show conclusively that both the chlorides of copper, under favorable circumstances, do chloridize the argentiferous sulphurets. The experiments, however, would seem to indicate that the action of the chloride of copper was much more intense than that of the subchloride.*
* Mr. Bowring's very ingenious arguments may be found in Ure's Dictionary of Arts, etc., vol. iii, p. 664. London, 1867. He urges the following considerations against the theory of the formation of chloride of silver and its decomposition by mercury in the patio:
1. Ores containing silver combined with chlorine only, are considered by Mexican miners most difficult of reduction, causing thrice the loss of mercury, and rendering 'the process much more tedious than those containing sulphides only. The amalgams from chloride ores and from sulphide ores are very different in appearance. The chlorides instantly attack the quicksilver, coating its globules with calomel-whereas sulphides leave it bright, proving, as Mr. Bowring claims, that they are not transformed into chlorides of silver during the process, and then decomposed by the mercury.
2. It is true that a strong solution of the chloride of copper, mixed with a solution of salt, and placed in contact with sulphide of silver, will, after some time, form chloride of silver and sulphide of copper; but the amount of sulphate of copper introduced in the patio process being sometimes less than one ounce and never more than eight ounces, to 70 pounds of water, could not give a sufficiently concentrated solution to permit this reaction.
3. An inspection of the formulæ given in the text shows that, on one hypothesis, sulphur is set free, and on the other, (Bowring's,) sulphuric acid is formed. That the lat ter is really the case, Mr. Bowring claims on the strength of the following experiment: Rich ore, containing sulphide of silver, is mixed with oxychloride of copper in a solution of common salt, and mercury is added at ordinary temperature. In about an hour the whole of the silver will have become amalgamated, when, after separating all the
The application and modification of the amalgamation process, as practiced in Washoe, has occasioned among experienced mill-men great doubt as to the beneficial results derived from the use of any chemical agents at present mixed with the ore. This doubt is occasioned, or at least strengthened, by the growing custom of late years of decreasing the quantity of salt and sulphate of copper added to the charge without apparently diminishing the product of bullion. Many amalgamators now abstain from the use of both reagents; others add a small quantity of the sulphate of copper, but no salt. In a few instances, the custom is to throw in only a little of the latter, while în many mills the rule is to employ a small amount of both substances, owing to a slight prejudice against the abandonment of "chemicals" altogether.
The action exerted by these two reagents in the pan would appear clearly to indicate that the benefits derived from their use are partly to aid in converting the sulphide into chloride of silver, as in the patio, and partly to decompose such minerals as are but slightly attacked by the mercury. In the Washoe process, however, the large quantity of iron present must tend greatly to produce sub-chloride of copper almost as soon as the chemical agents are thrown into the pulp.
Notwithstanding the importance of common salt and sulphate of copper in the patio, and, under certain conditions, in the pan, their value must be considered as only setondary in the decomposition of a large proportion of the Comstock ores. The advaatages derived from their use are shown to be exerted chiefly upon such minerals as blende and galena, which are but slightly attacked by the mercury. But the amounts employed are in most cases too small to effect any very favorable results. On the other hand, if a sufficiently large proportion of the reagents are consumed in the pulp. in order to produce the beneficial returns, it is always at the expense of preserving the necessary purity of the mercury. The quantity of salt deemed necessary by mill-men varies from one-quarter of a pound up to seven or eight pounds per ton. Scarcely any two establishments have the same rule. Its action upon the ore, without sulphate of copper, in producing any marked results may well be doubted. The consumption of the sulphate of copper also depends upon the ideas of the amalgamators, but the amounts do not differ so widely as in the case of the chloride of sodium. It ranges from one-quarter of a pound to three pounds per ton.
The addition of the sulphate without salt is, of late years, a common practice. The opinion among those who work their ore in this way is, that it gives a little better yield than when mercury alone is employed, particularly where the ore indicates the presence of galena in any considerable amount, in which case it is said to quicken the mercury, and render it more energetic. Continued experience appears to determine this fact with a considerable degree of certainty. In working ores containing only a small percentage of lead, the quicksilver very soon becomes dull and inactive, or, as it is technically termed, it sickens, and the yield from the pan is consequently low. Lead is one of the most deleterious metals in destroying the amalgamating energy of mercury, and at the same time is very rapidly absorbed when the two metals are brought into contact. Sulphate of copper possesses, to a certain extent, the property of expelling lead from mercury, copper being amalgamated and sulphate of lead formed at the expense of the sulphuric acid of the copper salt. If a concentrated solution of sulphate of copper be allowed to stand upon lead-amalgam the action takes place quite rapidly, mercury containing lead acting much more energetically upon the copper solution than when perfectly pure. This salt, however, does not appear, under any circumstances, to possess the power of completely driving out the lead. Another advantage derived from the addition of a small quantity of the sulphate of copper is that mercury, under certain conditions, when exposed to the solution, forms a minute amount of copper-amalgam, which causes the metal to act with a somewhat greater intensity in the decomposition of the silver sulphide than when perfectly pure. Iroa, as a reducing agent, in the pan process, probably plays an important part in bringing about the favorable results obtained. This may occur in three ways: First, it aids, in a great measure, the decomposition of the chloride of silver; secondly, it reduces the calomel formed during the operation; the chlorine, combining with the iron, goes into solution, and the heavy metal is liberated. In this way it not only prevents a chemical loss of mercury but also serves to keep the surface of that metal bright and clean, which otherwise might be coated with a thin film of sub-chloride, which would greatly destroy its activity; thirdly, it undoubtedly assists directly in the amalgamasoluble salts by filtration, a test with chloride of barium precipitates sulphate of baryta equivalent in quantity to the sulphur which has become acidified.
Whatever may be the case in the patio process, it seems to me that Bowring's theory does not agree with the facts of the pan process. In this case, the ores of chloride of silver are considered the easiest of reduction; and the best method hitherto discovered for the treatment of refractory ores involves their chlorination preparatory to amalgamation. But the notion that the pan reactions are the same as those of the patio, though quite common, is not necessarily true. One great difference is in the amount of exposure to the air, and this alone would be sufficient to account for the presence of oxychloride of copper in the patio and not in the pan.-R. W. R.
tion, where the two metals are brought into close contact with the easily reducible sulphurets. The successful and continued operations in Washoe, without the aid of any other chemical agents, sufficiently prove this statement. The experiments already cited in treating argentite and iron filings with mercury confirm the fact. Humboldt, in speaking of the amalgamation problem in Mexico, draws attention to this point and remarks upon the rapidity with which amalgamation was secured when the two metals were triturated together with argentite. This action of iron is obtained not only from the constant agitation maintained, which brings the pulp and metal in contact with the sides and bottom of the pan, but also from the amount of iron dissseminated, in a fine condition, through the ore, produced by the wear of the stamps, shoes, and dies.
This consumption of metal from the batteries and pans varies very much in the different mills, depending partly upon the details of construction and grinding effects of the pans and partly upon the hardness of the castings employed. The following figures from two mills serve to show the quantity of iron reaching the pulp from this source, per ton of ore worked. The quantity of ore treated is sufficiently large to afford a very fair estimate of the metal consumed:
The fiue iron coming to the ore in this way is very considerable in proportion to the other minerals present. If ten pounds per ton are added from this source it is equal to one-half of one per cent. In the Kentuck ore, of which an analysis has been given, there is, including the iron from the batteries, less than 24 per cent. of ore-bearing minerals present.
Mercury and iron, under the proper conditions, undoubtedly are the principal agents in the extraction of the precious metals by the Washoe method. The results depend, however, in a great measure, upon the mechanical treatments employed to reduce the ore to an exceedingly fine state of division, and to maintain, with the proper degree of consistency, a constant agitation of the entire mass; the essential conditions of the amalgamation being that the mercury should be thoroughly incorporated in the pulp, and every particle of the reducible minerals brought in direct contact and triturated with the metal, in the manner so well accomplished by the friction and grinding action of the pan. The mercury should also at all times retain a bright, clean surface, free from any film of metallic salts, such as sub-chloride of mercury or sulphate of lead, and any coating of oil or grease. The slightest tarnish appears to retard very greatly the activity of the metal. The iron seems to act as an electro-chemical agent; the immediate contact of the two metals, aided by heat and friction, causing a local electric current, which renders the amalgamating energy of the mercury much more intense. Mercury, when perfectly pure, does not apparently possess to so great an extent the power of taking up other metals, or of decomposing mineral combinations, as when it holds a minute quantity of some foreign metal in solution. The experience among amalgamators in Mexico is that the yield of gold is increased by the presence of silver; also, that the latter metal is extracted with greater facility if a considerable proportion of the amalgam is already present. This opinion is held by most mill-men in Washoe.
It is stated by some writers upon the question that silver is absorbed with increased activity when copper is employed, and as the former is amalgamated the latter will be expelled. Both iron and copper cause the formation of copper-amalgam. On the other hand, sulphate of copper exhibits a tendency to drive out lead. Karsten mentions the property of this salt to purify the mercury from both zinc and antimony. Any one who has witnessed the intensity which sodium-amalgam exerts cannot fail to have been impressed with the rapidity with which it attacks gold, silver, and silver compounds; yet its application in Washoe, in practical operations, did not give such results as would warrant its general introduction in the process.
Although the presence of a small quantity of several metallic bodies enhances the amalgamating energy of the mercury, yet a slight excess "sickens" it; that is, it loses its fluidity and becomes dull and inactive. The peculiar phenomena attending the mercury, by which both electro-positive and electro-negative metals are absorbed, and the effects which they produce in increasing or neutralizing its action, are very little
The loss in quicksilver during the operation arises from two sources; the one mechanical, the other chemical. The former depends largely upon the manner in which
the final washing from the pulp is conducted; the separation being more or less perfect according to the skill and care with which it is executed. A considerable quantity of the metal, however, is so cut up and ground to such a fine state of division that it is impossible to save it. The chemical loss is occasioned by the formation of the chlorides of mercury, which escape with the tailings.
In the patio the chemical loss is frequently very considerable; the amounts of common salt and magistral employed are large, while, at the same time, there is no reducing agent present to act upon the calomel formed, as is the case in the pan. In the patio the loss is said to increase in proportion to the richness of the ore in the sulphurets of silver, owing to the fact that for every atom of chloride of silver reduced by the mercury a corresponding atom of the latter metal is consumed as sub-chloride.
In the Washoe process the chemical loss would seem to be small in proportion to the entire consumption. This is probably due to the beneficial effects of the iron, which combines with the chlorine of the calomel, setting the quicksilver free.
The more the metal is ground the more it must be cut up, and the greater the difficulty in recovering it. Now, if the consumption of iron is assumed to measure the grinding effect exerted by the pan, the relation between the loss of mercury and that of iron should be, in a certain degree, proportional.
The following table, compiled from the results of several mills, furnishes some interesting details in regard to the loss of mercury:
Part 1 shows that the loss of mercury is independent of the consumption of chemical agents.
Part 2 shows that the loss of mercury is, in some measure, dependent upon the consumption of the iron of the pan.
The following is the result of an analysis of some artificial crystals of Washoe amalgam:
They have the composition, very closely, of three atoms of mercury to one of silver. From the foregoing considerations of the principal features of the Washoe process it
That the ore consists chiefly of native gold, native silver, and argentiferous sulphurets, associated with varying proportions of blende and galena.
That the action of chloride of sodium and sulphate of copper in the pan produces chloride of copper.
That the presence of metallic iron necessarily causes the formation of the sub-chloride of copper.
That both the chlorides of copper assist in the reduction of the ore by chloridizing the sulphurets of silver, and in decomposing the sulphurets of lead and zine.
That sulphate of copper enhances the amalgamating energy of mercury, by causing the formation of a small quantity of copper-amalgam. It also tends to expel the lead. That notwithstanding the importance of chemical agents, as above indicated, the quantities added to the pulp, in the ordinary practice of Washoe mills, are too small to effect any very beneficial results.
That mercury and iron, aided by heat and friction, are the principal agents in the extraction of the precious metals by the Washoe process.
That the essential conditions in the amalgamation of the gold and silver are that the mercury be kept perfectly bright and pure, in order to produce a direct contact of that metal with the iron and sulphide of silver.
That the consumption of mercury in the Washoe process may be considered chiefly a mechanical, and, only to a limited extent, a chemical loss.
The Washoe process in Owyhee, Idaho.-I am indebted to Mr. John M. Adams, the superintendent of the Owyhee and other mills at Silon City, Idaho, for interesting notes upon certain details of the pan process as practiced by him.
The chemicals employed for different purposes connected with the amalgamation are salt, sulphate of copper, sulphate of iron, sal ammoniac, sulphuric acid, potash, gum catechu and cyanide of potassium, of which sal ammoniac and sulphate of iron are used by some mill-men of the district, but not by Mr. Adams. Chemical action is also due to the quicksilver, the iron pans, the friction of the grinding surfaces, and the heat given by the introduction of steam.
The exact effect of some of the chemicals is a moot question. Mr. Adams, whose scientific training and practical experience entitle his opinion to great respect, speaks substantially as follows concerning the subject, as connected with the Owyhee ores.
He does not think that salt alone chloridizes the ore in the pans, though it exercises a stimulating effect of some kind upon the amalgamation, as he has proved to his own satisfaction by working different charges of the same ore with nothing but quicksilver, and with nothing but salt and quicksilver. He finds, however, that the effect, though decidedly beneficial, is not very strong. I am inclined to infer from these observations that the salt does decompose, and therefore chloridize, certain minerals in the ore, possibly blende and galena, and that these minerals are minor elements of the ore.
Sulphate of copper, when added alone to the quicksilver in the pan, assists the amalgamation. This Mr. Adams explains as follows: the sulphate of copper is decomposed in contact with the iron of the pan, forming sulphate of iron and metallic copper; the latter continues the decomposition of already partially decomposed silver sulphurets in the ore. But this effect does not amount to a complete reduction of the silver sulphurets; which, indeed, cannot be accomplished with either salt or sulphate of copper alone.
These two chemicals together, however, give rise to a strong reducing agent, the sub-chloride of copper. Mr. Adams finds that this substance, employed in the proper proportion and for a sufficient time, will entirely reduce any of the minerals of silver, except those containing antimony, which salt and sulphate of copper, even employed together, fail to
Any effect from sulphate of iron or sal-ammoniac he has failed to discover. Sulphuric acid will, to a certain extent, decompose sulphides of iron and copper, thus freeing some gold; and it attacks in a similar manner argentiferous compounds of iron, lead, and copper which do not contain antimony. Moreover, if kerosene, tar, or machine-grease, gets into the pan with the pulp, sulphuric acid will destroy it, and thus prevent the contamination of the quicksilver, which is detrimental to amalgamation. This acid also serves to keep clean the surface of the iron of the pan, which is thus enabled to exert continuously the reducing action.
Frequently tallow, grease, and candle-ends are brought in the ore from the mines, and pass into the pulp, where, if not counteracted, they will coat the mercury. The use of potash in the pan destroys the grease, and frequently, in drawing a charge into the settler, a thick scum, like soft-soap, will be seen floating on the surface of the liquid, while the quicksilver comes out perfectly clean and as bright as a mirror.
Cyanide of potassium has a beneficial effect in the pan; but its use in adequate quantity is too expensive at present prices. It may be em