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down to the lower hearth.* At this point it contains oxides and sulphates, with a small portion of undecomposed sulphurets. The previ ous charge on the lower hearth having been withdrawn, the charge nearest the flue (one ton) is pushed down and spread upon the lower hearth. Here at a lively heat, and with active stirring at intervals, the base metals are converted into oxides in about eight hours, and the charge is finally withdrawn into an iron car. As soon as a charge is drawn into the lower hearth, the following charges are moved forward in succession, and space is thus left nearest the chimney, at the remote end of the upper hearth for a new charge of one ton of raw sulphurets. The capacity of the furnace, with two roasters constantly employed, (or four in twenty-four hours, is therefore three tons, while that of an ordinary single reverberatory, employing one man, (or two in twenty-four hours,) is but little more than one ton.
The roasted ore is removed in the iron car to a cooling floor. In Mr. Deetken's Eureka Works this floor is in front of the furnace, and very near, so that the removal can be effected directly.
PREPARATION OF THE CHLORINE.
The chlorine gas is prepared from peroxide of manganese, salt, and sulphuric acid, in a leaden vessel. The proportions for a charge of three tons of roasted sulphurets are given as follows:
The water, salt, and manganese are introduced first into the generator, which is covered with a curved lid, fitting in an annular water-joint. Through this cover two lead pipes communicate with the interior, that is, with the open space above the mixture. One is a safety-tube or funnel-tube, bent twice upon itself and terminating above in a funnel, through which the sulphuric acid is introduced. The other is the pipe conveying the chlorine to the vat. A vertical shaft or rod through the center of the cover carries a pair of arms, with teeth, used by revolution as a stirrer of the charge.
The sulphuric acid is introduced in successive small quantities, as needed to maintain a lively generation of gas. The formula of the reaction is, Mn O2+Na Cl+2SO3 = Mn O SO,+Na OS 3+Cl, [or, according to modern chemistry, (NaCl)2+(H2(SO4))2+MnO2=Mn(SO1)+Na2 (SO1)+ (H2O)2+Cl2. See Barker's Chemistry, par. 140.] After all the acid has been added, when the action flags, it may be stimulated for a while by a moderate fire under the generator. Instead of the ingredients above named, the chlorine may be produced from one part peroxide of manganese, two parts muriatic acid, and one part sulphuric acid, diluted with one part water.
The gas escaping from the generator is purified of muriatic acid by
* Mr. Küstel says the charge is exposed to the preparatory roasting for about twenty-four hours on the upper hearth; but I cannot understand how this can be the case, since the rate of its advance depends entirely upon the removal of the earlier charges, and this is regulated by the final roasting on the lower hearth. With regard to the latter, he says (section 59, p. 250) a ton can be drawn out every eight hours, that is, three tons in twenty-four hours. As the charges are introduced at the same rate on the upper hearth, it follows that if the upper hearth carries, as he says, nine tons, each of them must remain upon it 9 x 8=72 hours. It is my impression that the quantity of ore on the upper hearth is not so large as this.
"washing" it through an arrangement almost exactly like a pneumatic trough, by which it passes through a stratum of half or three-quarters of an inch of water. This absorbs the muriatic acid, and a proportion of the chlorine, (about 23 volumes.) Warm water takes up less chlorine, and even a saturated solution of chlorine will still absorb muriatic acid. From the purifier the gas is conducted to the vats or tanks.
After the roasted ore has become sufficiently cool it is dampened with 4 or 5 per cent. of water, which, it is claimed, facilitates the mechanica! passage and the chemical activity of the chlorine. It is then sifted into the chlorination vat. The sieve used for this purpose need have no more than 7 to 8 meshes per linear inch.
The European authorities say that the vessels used for chlorination must not be of wood or metal, and recommend earthen pots or bottles,* This is troublesome and expensive, compared with the simple treatment in vats employed in this country. The reason for the usual prohibition of wood has been, I presume, the amount of gold solution which it would absorb; but Mr. Deetken has completely overcome this evil by the simple expedient of coating the inside with equal parts of pitch and tar. Thus, instead of small earthen pots, he is enabled to use large tanks, holding several tons of roasted ore. These are circular in form, and possess a false bottom, about 1 inch from the real one. Upon the false bottom, which consists of boards placed about one-eighth of an inch apart, and pierced with half-inch holes from 10 to 12 inches apart, is spread a layer of clean quartz, 1 to 2 inches thick. Any other indi ferent rock will do, but not a rock containing magnesia or lime. This first layer of quartz is coarse; over it smaller pieces are laid, and so on, decreasing in size till a layer of sand covers the whole, forming thus a filter from 4 to 5 inches thick. This filter remains in the vat. Upon it the ore is sifted, when duly prepared for chlorination, and the cover is put on. This is nearly flat, and of wood. It is suspended by a chain attached to its center from above, and can thus be swung to its place promptly. The edge all around is luted with wheaten dough. A small hole in the cover is left open, to allow the escape of air and to serve as a means of observing the moment when the chlorine appears on the top of the ore. When this moment arrives the whole is closed and plugged with dough.
Chiorine is now conducted into the ore and permitted to operate from twelve to eighteen hours. Leakages of gas from the apparatus may be detected by the odor, and by the formation of white fumes when ap proached with a glass rod previously dipped in ammonia.
The coarser the gold the longer the chlorination. After, as a maximum, eighteen hours, the cover is taken off and water is introduced. Usually, if the process has been effective, free chlorine has passed through the body of the ore, and makes its appearance as a green gas on the surface. When the gold is fine this may take place after twelve hours.
LIXIVIATION AND PRECIPITATION.
When the gas appearing on the surface indicates that the whole mass is permeated with chlorine, the cover is removed, and water is introduced until the surface of the charge is covered. Then a cock at the
* See Crookes and Röhrig's Kerl's Metallurgy, vol. i, p. 637.
bottom, communicating with the space under the false floor, is opened, and the solution is drawn off and conveyed to the precipitation vat. This is a wooden tub or cistern, but without a false bottom. Küstel recommends a rectangular form, with a half-round, somewhat inclined. bottom, and a lining of sheet lead. The precipitant employed is a solution of sulphate of the protoxide of iron, (copperas, or green vitriol,) which is usually prepared fresh at the works from wrought iron and sulphuric acid. This part of the process is so simple as not to require detailed description here. The mixture generally remains undisturbed over night, giving the gold precipitate an opportunity to settle. The supernatant liquid is then carefully removed by tapping the tank at successively lower levels, until little is left with the precipitate. The latter is dipped out with care, by means of a dipper or scoop, into a clean porcelain or enameled dish, the final residuum being washed out through the lowest stop-cock of the vat, and the vat is cleansed from adhering particles with a stream of water, in the same manner as precipitates are washed upon filters, or breakers are cleaned of adhering portions of precipitate in the chemist's laboratory.
The gold obtained is in the form of a brown powder or "cement." This is filtered upon paper, and subsequently dried in an iron or porcelain vessel. Finally, it is smelted to a metallic regulus in clay crucibles, a little salt, borax, and nitrate of potash (saltpeter) being used as fluxes. Küstel gives the following expense of the cost of treatment, for a capacity of three tons daily from a double furnace. His figures refer to Grass Valley in 1867, since which time some items of expense have been reduced in that locality. The results obtained in this table are, however, in my opinion, not far from the present cost, since Mr. Küstel has made no allowance for incidental outlays, which are inevitable. I have added a column, giving estimates (of less authority) for the same items in Arizona, where expenses are much higher:
Four roasters, at $3 50..
Three cords of wood, at $4..
Thirty pounds manganese, at 61 cents..
Forty pounds salt, at a cent...
Seventy-five pounds sulphuric acid, at 24 cents.
One man at the vats two days, at $3 50...
Sulphate of iron...
Total for three tons...
Or, in Grass Valley, $14 55 as the average cost per ton, and $20 33 in Arizona.
Much more could be written in detail concerning the manipulations of this process, and the combinations of other kindred processes, such as the plan of Calvert, who recommends the production of "nascent chlorine" in the chlorination tank, instead of the introduction of ready-made chlorine gas; of Roeszner, who employs a salt solution saturated with chlorine; and of Patera and others. Many of these processes are intended to save the silver as well as the gold. But I must refer the reader to the books on the subject for all these matters. None of the processes, save simple chlorination, is, so far as I know, employed in the United States. I have confined myself, therefore, to a brief and general description of what is universally known as the Plattner process.
There is no doubt of the metallurgical perfection of this process. The reasons why it is not more frequently employed in this country
1. The cost of treatment per ton, excluding all low-grade ores from profitable reduction by it.
2. The real scarcity, except in one or two mining districts, of ores suitable for chlorination. Even perfectly effective chlorinating works suffer almost everywhere from lack of ore, and scarcely any in the country are run continuously at full capacity. Yet this "full capacity" would require but from one to three tons of ore daily.
3. The lack of metallurgical skill in the construction and operation of furnaces for the preliminary roasting. On this everything depends, and it is perhaps in this department that several failures in Colorado have occurred. It certainly seems strange that the Territory offering apparently most suitable conditions for the process should witness so many failures in it. The works of Cash & Co. at Central City are not here referred to; they are reported to be successful; but the owners are reticent as to their methods and results, and there is consequently nothing certain to be said of them.
Chlorination, in its present form, cannot supersede amalgamation for ordinary milling ores. It can compete with smelting where nothing is to be extracted but gold, (or, by Roeszner's method, gold and silver,) and in any case it is quite likely that the process will be best administered by custom works, receiving the ore from different mines, and running continuously.
But, since the cost of roasting is more than half the total cost, it is possible that improvements in the mechanical means of roasting, such as the introduction of the O'Hara, the Gerstenhöfer, the Brückner, or the Stetefeldt patents, may considerably reduce the expense, and thus enlarge the field of the Plattner chlorination. The capacity of the Stetefeldt and the Gerstenhöfer furnaces is very great, and the evil of a short supply of ore might be aggravated by their employment; but, on the other hand, the reduction of the cost of treatment by a little would increase the amount of ore chlorinated by a great deal.
GOLD REFINING BY CHLORINE GAS.
The following interesting paper, read before the Royal Society of Victoria, by F. B. Miller, F. C. S., Assayer in the Sydney Branch of the Royal Mint, describes a new method of refining gold, which, it is reported, will be tried by the Mint of the United States, Mr. Miller having visited this country for that purpose:
There is no recorded instance of gold having been found in an absolutely pure state. Every natural alloy of gold (or native gold, as it is called by mineralogists) contains more or less silver; and in almost all bullion resulting from the melting of Australian alluvial gold, the portion that is not gold consists chiefly of silver, with only a very small proportion of foreign metals, usually copper and iron, with occasionally a little lead or antimony, and sometimes a trace of tin, iridium, etc. This, however, though true generally, is not always the case with gold obtained from quartz veins by amalgamation, as the mercury occasionally reduces aad takes up other metals as well as the gold, which appear in the bullion on melting. The accompanying table will give some idea of the proportion of the precious metals contained in the gold from the various districts of New South Wales, after melting. It will be seen that the most argentiferous is that from Boonoo Boonoo, in the north, containing as much as 34 per cent. of silver. This approaches in composition the gold from the productive Thames district of New Zealand; while the gold from Nerrigundah, in the south, only contains 1.5 per cent. of silver, the remaining 934 per cent. being gold with a trace of copper:
Table showing the proportion of gold and silver in characteristic samples of gold dust from various localities in New South Wales, (after melting.)
An interesting, and as yet unanswered question here arises: Is this argentiferous character in any way connected with the geological structure of the district?
It is a fact, and certainly a very curious one, whether it arises from accidental causes, or whether it may hereafter be traced to peculiarity in the rocks whence the gold of the different districts is derived, that its quality or fineness deteriorates the further north we go; in other words, it contains more silver and less gold.
Thus, the average fineness of Victorian gold is about 23 carats; that is to say, it contains about 96 per cent. of gold and 34 of silver, with a per cent. of base metals; while, on passing north, we find the average fineness of New South Wales gold to be only 22 carats 14 grain, or to contain 934 per cent. of gold and 6 per cent. of silver. On going still further north, to the colony of Queensland, the average fineness is little more than 21 carats, (considerably below standard,) or it contains 87 per cent. of gold and 12 per cent. of silver; that from Maryborough containing as much as 14 per cent. of silver and only 85 per cent. of gold.
These are averages only. It is not to be supposed that there is a regular and consecutive diminution in fineness with every degree of latitude we go north. There are exceptional localities in the north of this colony, where the gold found is of a high degree of purity, as at Rocky River, where it is over 23 carats fine, or 96 per cent.
Possibly at a future time our geologists may be able to throw some light on these curious facts, and the exceptional cases may then even help in explaining the apparently general rule. The point, however, of principal interest, as far as regards the subject of this paper, consists in the fact that, as the alloy obtained by the gold miner is poorer in gold, it is proportionally richer in silver.
According to the published returns, 6,820,198 ounces of gold have been received for coinage in the Sydney Mint between its establishment, in May, 1855, and December 31, 1868. The average assay of this quantity would be about 943; in other words, it contained 94 per cent. of gold, 5 per cent. of silver, and per cent. of base metals. Allowing an average loss of 2 per cent. in melting the gold dust, there would remain, after smelting, 6,683,795 ounces of gold bullion; and as the silver it contained amounted to 5 per cent. of this quantity, the gross amount of silver in the gold received for coinage was 334,190 ounces; being at the rate of 24,720 ounces per annum.
The average proportional quantity of silver contained in the gold arriving in