November 7, 1895] 



NATURE 



17 



i.)ng run would extract the maximum percentage of gold per ton 

 at the minimum cost. It matters little to him how much gold 

 runs off in the tailings into the nearest stream, so long as enough 

 , extracted to pay expenses and yield a handsome jirofit. When, 



iwever, the industry becomes firmly established, the aspect of 

 lUiirs is changed. The richer mines can afford to spend some- 

 thing in endeavouring to improve their practice ; the poorer ones 

 have their very existence threatened by the loss of 30 or 40 per 

 cent, of the gold, which has been raised from a great depth, only 

 to be left on the dumping ground. 



This stage has long-been reached in the older gold fields of 

 Australia, such as those in Victoria, where the industry has 

 always been carefully nursed by the Government. One of the 

 latest proofs of the solicitude of the Victorian Department of 

 Mines is the issue of this report of Mr. Rosales, the veteran 

 expert on concentration, who gained a Government prize for an 

 essay on the subject as long ago as the year 1861. The report 

 deals with the concentration of tailings from the stamp battery, 

 and although it is specially applicable to Victoria, nevertheless 

 it contains much information and many suggestions which deserve 

 careful study by metallurgists in all parts of the world, and may 

 be particularly valuable to the workers in South Africa. 



In Victoria, as in many other countries, the majority of the 

 gold ores found are " free-milling," yielding a fair percentage of 

 their gold when amalgamated with mercury. The usual method 

 of treatment is to crush the ore in a stamp-battery, a little mer- 

 cury being added in the mortars, and to pass the pulp over 

 amalgamated copper plates, by which most of the free gold is 

 retained. The "battery sands," still containing a little free 

 gold and a varying percentage of auriferous sulphides of the 

 heavy metals, are then treated by various machines, such as 

 canvas tables, vanners, percussion tables, blanket and wooden 

 strakes, and revolving buddies, with a view to separate the hea\')' 

 particles in which the gold is contained from the lighter worth- 

 less gangue. The concentrates are treated by grinding to im- 

 palpable pulp with mercury in iron pans, by chlorination or by 

 smelting, according to the nature of the sulphides and to the 

 other conditions. 



The tailings from the orthodox concentrating machines would 

 be allowed to run to waste if it were not that, on almost every 

 mine in Victoria, they are compelled to run the gauntlet of the 

 simple contrivances of a few Chinamen, who pay tribute to the 

 mine-owner for the privilege of taking his leavings, and who 

 extract enough gold to provide themselves with a living. 

 Nevertheless it was stated in 1889, on the very high authority 

 of the late Government analyst, Mr. J. Cosmo Newbery, that the 

 tailings of the quartz-mining districts, even after passing the 

 Chinese tables, contained in general from two to two and a half 

 dwts. of gold per ton — some 15 per cent, of the amount origin- 

 ally contained in the ore. The gold thus lost is estimated as 

 being of the value of over £-},ip,ooo in the year 1894, and a 

 similar state of things is unfortunately only too prevalent in 

 other countries. 



Experts are agreed that it is the methods of concentration which 

 are chiefly answerable for the continuance of this unsatisfactory 

 state of things, not so much because the machines now at the 

 disposal of the metallurgist are defective, as that in many cases 

 they are set to do work for which they are inappropriate, 

 although capable of dealing effectively with certain classes of 

 material. In particular, the neglect on the part of metallurgists 

 to classify the crushed ore according to size has been fatal to 

 good concentration in a countless number of cases, and this 

 mistake has not yet been generally rectified. 



Let us suppose that a gold ore has been crushed so as to pass 

 through a screen equivalent to a wire-sieve with thirty holes to 

 tlie linear inch. The particles of ore are of all sizes, ranging 

 from those which can just pass through the screen down to 

 l>erfectly impalpable powder. From 20 to 50 per cent, would 

 easily pass through a lOO-mesh sieve, and a part of the ore, the 

 " slimes," is so finely divided that it settles in still water with 

 great difficulty. In spite of this, the whole mass, without any 

 classification, is perhaps, after treatment with blankets, hurried 

 over some one type of concentrator favoured by the manager, 

 and the tailings allowed to escape without further treatment. 



For example, a percussion table with "end-blow" is used, 

 and the coarser particles of pyrites are readily separated from the 

 remainder of the ore by its action. It usually hapjxjns, how- 

 ever, that the valuable sulphides, being softer than the quartz 

 and other constituents of the gangue, are in the main more finely 

 pulverised than the latter, so that the slimes are the richest parts 



NO. 1358, VOL. 53] 



of the ore, and these, under the circumstances, will almost all 

 escape. Thus at the Johnson's Reef Mine, Eaglehurst, it was 

 found that the "slimes," though constituting only 3 per cent, 

 of the pulp, contained 21 per cent, of the gold, while 44 per 

 cent, of the pulp, which svas retained on a 60-mesh sieve, con- 

 sisted mainly of quartz grains, and was absolutely worthless. 



Or, in the alternative, the battery sands may be sent directly 

 to some travelling belt table, such as the P'rue vanner — a 

 machine capable of doing splendid work in saving rich slimes — 

 with the result that the coarser particles, valueless in themselves, 

 interfere with its efficiency. Mr. Rosales cites the case of a 

 mine at Ballarat, where the costly Frue vanners, which had lx;en 

 set to treat unclassified l^attery sands, were discarded in favour 

 of the cheaper percussion tables, the fact being that neither con- 

 centrator could be expected to save the pyrites properly, the 

 vanner being no more fit to treat coarse material than the per- 

 cussion table is adapted to concentrate slimes. It would have 

 been better to use the two machines successively on the same 

 material, although even then, in the absence of classification, 

 losses could not have been prevented. 



Mr. Rosales has not been content to criticise, but has added a 

 sketch of a complete system of concentration, which, with modi- 

 fications, would be applicable to almost every gold ore likely to be 

 met with. The keynote of the system is classification, and he 

 can hardly be accused of laying too much stress on it, seeing 

 that it has been neglected more generally than any other con- 

 sideration in the past. He favours hydraulic classifiers (inverted 

 pyramidal or pointed boxes of various forms) for separating the 

 slimes from the sand ; but, on the other hand, he considers that 

 the division of the sand itself into two or three classes, accord- 

 ing to the size of the grains, is best effected by revolving screens 

 or trommels. 



This view will undoubtedly be called in question. It is rare 

 that a finer screen than one containing twenty holes to the linear 

 inch is fitted to these machines, and although 60-mesh screens 

 have been employed, the smallness of the capacity of trommels 

 supplied with such fine sieves, and the great cost of repairs 

 caused by their rapid wear, seem to render it unlikely that they 

 will ever come into wide use. In revolving screens the effective 

 surface operating at any one time is only a few inches wide, and, 

 if they were fitted with lOO-mesh sieves, it is to be feared that 

 continuous clogging would reduce their capacity almost to the 

 proverbial teaspoonful. There seems no adequate reason why 

 the cheap, handy, rapidly-acting pointed boxes should be passed 

 over, and if Mr. Rosales would press these, instead of the 

 trommels, on Australian mill-men, he would perhaps find a more 

 ready acceptance of his suggestions. It is true that when 

 pointed boxes are used, the |iarticles of ore in each class are 

 " equivalents" {i.e. those falling at an equal rate in water), and 

 not equal in size, particles of high density being left mixed with 

 somewhat larger ones of lower density, but the classification is 

 usually sufficient for the purpose. 



For the rest, Mr. Rosales seems to lay more stress on efficiency 

 than on cheapness. When, as in his complete system, nearly 

 twenty different machines, without counting duplicates, are 

 at work, each with a differeru purpose, in removing the auri- 

 ferous sulphides from one kind of ore, the loss of gold may be 

 reduced to little or nothing, but it is evident that the extra 

 amount saved is not all clear gain. An additional percentage of 

 gold may often be obtained at a loss, even by an automatic 

 machine if it is costly to buy and to keep in repair. 



No sudden drastic changes, however, are proposed by Mr. 

 Rosales. The losses of gold in Victoria and elsewhere are 

 undoubted, and until it has been shown that they cannot 

 be profitably reduced, no shirking of the matter is admissible. 

 Tests on each mine by sieving and assaying in the laboratory 

 (which, alas ! too often is non-existent) can alone show in what 

 direction the practice may be improved, and, if proper attention 

 were paid to the slimes only, many gold mines would have a 

 much brighter outlook than at present. 



In conclusion, a word may be said in protest against the 

 unscientific and misleading Australian (and English) method of 

 reporting assays of gold ores. The actual weight of gold 

 extracted from the sample of ore is seldom recorded, and the 

 probable error is quite undiscoverable. An observed weighing 

 of 0001 grain may be reported as 15 grains per ton, or may 

 appear as two, three, or more times this amount. In every case 

 the unit in the report is much smaller than that used for the 

 observation. What analytical chemist would be guilty of such 

 practices in his other work ? T. K. Rose. 



