PART V. METALLURGICAL PROCESSES. CHAPTER LXXXIV. GENERAL REMARKS. It is quite generally believed by men of science abroad, and frequently repeated by those who ought to know better at home, that we are blundering along in this country in the treatment of our gold and silver ores, totally ignorant of the experience already gained in older countries; that somebody should be sent to Europe by our government to study the methods of gold and silver metallurgy there employed, and to enlighten us with the results. A large class of persons, on the other hand, scout the idea of all science from Europe, and point to the fact that we already surpass every country in the world, unless it be Australia, in the completeness and economy of our gold-extraction, while all the imported processes have hitherto failed. The truth lies between these extremes. American metallurgy is neither wholly in advance nor wholly in the rear of the same science in other nations. Out of our peculiar circumstances and conditions we have developed some distinctively American processes with which Europe cannot compete, because the experience of Europe has not led to the perfection of such devices. Thus the hydraulic system of mining, adapted to work cheaply large areas of auriferous earth, is native and peculiar to this country, and is employed on a larger scale and in greater perfection here than anywhere else. The California stamp-mill, with all its modern improvements, cannot be surpassed in the world for completeness of design and mechanical simplicity. Russia and Hungary cannot show such economical amalgamation as we do. Even in the treatment of tailings and sulphurets, we have employed and improved nearly all the European methods which our economical conditions will admit, and we have invented several of our own-such as the pan-process, the Brückner cylinder, &c., which seem to be much more promising than the plan of the eminent Professor Rivot, which has repeatedly failed among us, and never succeeded. Again, in the treatment of silver ores, the Washoe process of pan amalgamation is an outgrowth of American necessities, a hybrid from the arrastra and the patio, original with us, and not only remarkable in itself, but capable of modifications and applications which considerably diminish the realm of smelting. The Freiberg barrel-amalgamation is indeed adopted in many places; but we have, in place of laborious roasting in reverberatories, the cylinders of Brückner, and the shaft furnace of Stetefeldt, both American inventions; that is to say, the product of foreign science, stimulated and directed by actual experience in this country. All the instances just given testify that we are not without an American metallurgy. Even now, it is necessary for foreign governments to send agents here to study our methods and report at home. We know the processes of Europe better than Europe knows ours. But this American metallurgy which is springing up, is sadly desultory, incomplete, untrained, and unsystematized. Each step of progress costs more than it should. Thousands of experimenters waste their time in trying processes that have failed already, but of which they have no record. Many devices, even, that have been thoroughly tested and condemned in Europe spring up to enjoy a flourishing though brief career among our enterprising and ingenious people, not learned in their history. But many more originate among us, and, for want of harmonious action or organization among the mining interests, they are tried here and there, and the results of each trial smothered or magnified, so that it takes much more time and money than it ought to require to produce in the public mind the notion that a particular plan is beyond doubt good or bad. To furnish a nucleus of experience and an exchange of information, to settle many important points yet mooted in our incomplete American system, to consolidate and cheapen our progress, a national school of mines seems to me to be absolutely required. The California stamp-mill process for gold is described in a number of well-known books, perhaps most completely in that of Mr. J. Arthur Phillips on Gold and Silver. It is admitted to be the best method thus far developed in actual practice on a large scale, for the treatment of auriferous quartz. The idea of smelting the ordinary auriferous quartz (even of low grade) has been put forward at different times by the possessors of patent fluxes; but its absurdity will appear if we reflect that every five dollars' worth of gold in a ton of quartz constitutes but one one-thousandth part of one per cent., or one one hundred-thousandth of the whole mass. To smelt auriferous quartz, therefore, is to flux and remove, practically, one hundred per cent. of quartz. The case is different when the barren material is not quartz, which needs a base before it will flux, and still more widely different where the ore contains some other useful metal beside gold, such as lead or copper, and where the gold, for some reason, escapes the stamp process. There is a field for smelting which may become important and extensive, but it never can supersede amalgamation. This process, being principally mechanical, has been partially considered in a previous section. The causes of loss, and the direction in which greater perfection is to be desired and expected, will now be made subjects of brief comment. Alleged losses of gold in stamp-mills may be divided into two classes. Either the gold is mysteriously lost, and cannot be subsequently traced, or it is to be found by assay in the tailings. In the former case, I am satisfied, after many observations, that the loss is generally only apparent, and may be referred to incorrect sampling or assay of the ore. An ordinary assay of gold ore, made previous to crushing, is not nearly so trustworthy as an assay of the tailings, properly selected. Hence, if the two, taken with the yield of amalgam, do not agree, the quartz assay is probably in error. Sometimes, however, batteries leak, and gold is lost under and around the mortars. Sometimes, on the other hand, workmen are dishonest, and charge losses upon the mill which their own peculations have occasioned. Occasionally, where the tailings do not settle in a quiet spot, but are hurried away in a swift stream, more or less fine gold is swept off before it can be deposited. Assays to test this question should, of course, be made with due precaution as to these points; but one thing our experience denies, viz, that there is any pos sible mysterious loss of gold in the stamp-mill process. The loss is mechanical, definite, always traceable, sometimes avoidable or curable, and never supernatural, as the arguments of some would almost tend to make us believe. When the loss has been traced it is generally found to be due, apart from imperfections in the machinery, to the presence of fine gold, rusty gold, or pyrites, or to the flouring of the quicksilver. Floured or granulated quicksilver and fine particles of gold have, in common with some. other polished metals, the property of condensing on their surfaces films of air, which decrease the specific gravity of the particle. As the amount of air, and hence the amount of decrease in specific gravity, is proportional to the surface exposed, and its ratio to the mass of the particle, and as the smallest particles expose always the largest proportional surfaces, (the cube roots of the volumes being as the square roots of the surfaces,) it follows that very fine particles will acquire a density less than that of water, and will, in fact, float upon it. Many more will be rendered so light as to settle very slowly. Perfect protection against this source of loss has yet to be found. The best expedients now known are to avoid that apparatus which tends excessively to flour the quicksilver, and to use sodium amalgam, cyanide of potassium, &c., to collect it when floured, though no mere chemical reaction will completely do this. With regard to the fine gold, it may be saved by careful manipulation, I am convinced, even in our present apparatus, to a greater extent than now. Of course, a swift current of water, escaping from the mill, must be avoided, if the gold is fine. Dry amalgamation has frequently been attempted, and never yet has succeeded. The use of quicksilver vapors is highly objectionable on account of their poisonous character. It is true that amalgamation can take place in tightly-closed retorts, but the regular discharging of these retorts, and, still more, their leaking, wearing out, or bursting, are perpetual sources of danger to health and life. The inhalation of the vapors of mercury, during experiments of this character, caused the twelve years' suffering and finally the lamented death of Dr. J. Adelberg, of New York, one of the most brilliant mining engineers and metallurgists of the country. He died a comparatively young man, after a gallant but ineffectual struggle against the poison that had taken insidious and unrelenting hold upon him; and the warning of his terrible, vain conflict, his sufferings and his premature death, should lead our metallurgists to beware of recklessly employing mercury in vapor form. Rusty gold is apparently the result of the decomposition of auriferous sulphurets of iron. The greater part of the fine gold found in our ores has been in such association, and generally the oxidation of the pyrites by slow natural processes leaves the gold in amalgamable form, but sometimes a coating of iron oxide is formed over the particles of gold. This is also the case when sulphurets are artificially roasted. We may perhaps suppose that slow oxidation leaves clean gold, while quicker oxidation, whether natural or artificial, leaves rusty gold. Various devices have been employed to remove this inconvenience. The rusty gold will not readily amalgamate with mercury, and the slight contact afforded by copper plates is not sufficient for its treatment. Pans which grind the ore together with mercury are more efficient; but these produce so much fine gold and floured mercury that it is never well to use them first. The best manipulation is that which catches all coarse fine gold, if possible, in battery or on plates, and passes the pulp or blanket-washings into close amalgamators of various kinds, or into pans. Of this the Eureka mill, at Grass Valley, is a good example. The treatment of sulphurets under the stamp-mill system has been a question of considerable difficulty. These are usually iron or copper pyrites, containing gold. The different practicable methods of treating them so as to secure the gold may be classified as follows. Concentration should in every case precede, where the amount of sulphuret is not large, in proportion to the gangue. 1. Natural decomposition by time. 2. Roasting and amalgamation. 3. Pan amalgamation with chemicals. 4. Smelting. 5. Chlorination. Of these methods the first is the cheapest, but it is not equally practicable for all varieties of pyrites. Some pyrites decomposes very quickly when exposed to the weather, especially if it is from time to time drenched from a hose. In favorable localities the manufacture of copperas might be combined with the elimination of the sulphur in this way. After a suitable time the heaps of tailings could be again run through the mill, and a considerable quantity of gold extracted. This slow oxidation does not seem to produce rusty gold. In the Southern States, this simple expedient was quite common thirty or forty years ago, and I have heard of instances in which the second or third milling of gold ores produced as much gold as the first. The plan of roasting the ore to remove sulphur, and then amalgamating it, seems quite rational, but it cannot yet be said to have succeeded. The roasting apparatus, invented in such abundance, has frequently been quite imperfect. In no other department of metallurgy have our inventors made such a brave show, with so little real success. Their plausible experiments turned on the fact that whatever is done to sulphurets, the first change produced is and must be a partial desulphurization. Hence the pyro-ligneous acid, the superheated steam, the cold bath, the hot bath, and numerous other fanciful inventions, comparing their results with the results of no process at all, could always claim to have desulphurized more or less of the original mineral. But other and more scientific appliances seemed equally to fail. The reverberatory, the Gerstenhöfer terrace furnace, the Brückner cylinder, have all been tried on gold sulphurets in Colorado, though it is claimed that they have not been thoroughly tried, and that the practical success of the reverberatory and the cylinder in treating silver ores for amalgamation promises better results with gold ores than have yet been achieved. However this may be, the fact is strongly indicated by careful experiments in San Francisco, that the roasting of auriferous sulphurets produces rusty gold, which will not amalgamate. Mr. Brückner's experiments at Central City go to show that, by the addition of salt to the roasting charges toward the end of the process, the coating of iron oxide may by converted into a chloride and removed. This expedient would, I suppose, be impossible in the Gerstenhöfer, where the addition of salt would cause the ore to cake on the terraces. Whether the Stetefeldt furnace can be adapted to such a purpose remains to be seen. This is a subject still requiring investigation; and the essential problem is not to invent a new desulphurizer, as many suppose, but to find out whether desulphurized ores can be successfully amalgamated, and if so, how. The system of pan amalgamation with chemicals is gradually gaining ground in California. The theory of the pan process, both for gold and silver ores, has been but little understood by those who, being skillful and practical mechanics, were, in spite of their ignorance of chemistry, our best mill-men. The use of chemicals, therefore, has been largely a |