Crushing Machinery And Gold-Saving Appliances.

The number of crushing machines erected and in course of erection at the different goldfields I visited, amounts to twenty-one, eighteen of which—including two near completion—are for quartz crushing, whilst the remaining three—including two in course of erection—are for crushing the cement of the Blue Spur.

Crushing Machinery.—Speaking of the crushing machinery proper, all these machines are, with but slight variations, constructed after the same model, viz., they consist of batteries of revolving stamps, four and five in a battery, working in cast-iron coffers on false bottoms, and being partly fed by hand, partly supplied with self-feeding hoppers. As far as I could see, they are well and substantially erected, the comparative freedom from jar of those I found at work proving their stability, and that due care had been bestowed upon the preparation of their foundations. At one or two machines the discs on the shanks were not quite in order, and the wipers too long, but the managers knew of these defects, and were going to repair them. On the, at one time in Victoria, much discussed question, whether round stamps are equal in crushing power to square ones, I need only remark that careful experiments under equal conditions have proved that they are nearly, if not quite, as effective as the latter, whilst the wear and tear, in consequence of the turning, is in the average less. In the weight and lift of the stamps and the speed at which they were driven, there were considerable differences between the machines, the weight ranging from four to seven cwt., the lift from five to eight inches, and the speed fifty-six to eighty- page 174five blows per minute. As regards the most advantageous weight for stamps, opinions are still divided in Victoria; still, heavy stamps are, on account of greater efficiency on the generally hard quartz, the most in use. In my opinion, a medium of about 5½ to 6 cwt. would best suit the character of the stuff of most of the Otago reefs, though for such, consisting principally of hard, more or less solid, quartz—as, for instance, Logan’s Reef, &c.—stamps up to 8 cwt. would no doubt, be preferable.

Touching the height of drop and speed of the stamps, the former should not be less than seven inches, and might advantageously be increased to nine inches, especially if the stamps are light; whilst, touching the speed, it is generally considered best at the rate of 75 to 80 blows per minute. On the amount of stuff crushed during a certain time I could not obtain any definite information, but the figures given seem d to me to be rather low, and in order, therefore, to show what ought to be done under certain conditions, I may mention that at the Port Phillip Company, Clunes, stamps of 6 cwt., driven at a speed of 75 blows per minute, and with a drop of eight to nine inches, crush in the average 2 tons 4 cwts. per 24 hours; whilst others of 8 cwt., with the same fall and speed as just given, are expected to reduce each up to 4 tons per diem. Regarding the iron coffers, they seemed to me rather, if not too shallow for both economic and effective working. They allowed hardly one inch of loose quartz to be put beneath the false bed plates, and it would, therefore, require the greatest care in the placing of these plates, the feeding, &c, to prevent the plates from working unevenly into the shallow gravel bed. In fact, I think it can hardly be avoided that they (the plates) come frequently in contact with the iron bottom, the result of which, of course, is, as the sound of the blows already indicates, ineffective working and great wear and tear—sometimes even an unexpected breaking of the coffers. On this account I think it would certainly be advisable to have the latter, say, about two inches deeper, so as to allow a gravel bed of three inches beneath the false bottoms. Considering that the office of these latter is not only the saving of the coffers from wear and tear, but mainly to prevent the gold from being smashed too fine or "beaten dead," as it is called, they should only be of the same size as the stamp-heads, in order to leave sufficient space around them for the liberated gold particles to get into the gravel out of reach of the stampers.

The provision of self-feeding hoppers—a great desideratum for saving labour—has been neglected at a great number of the machines, and would deserve early attention. Many practical quartz crushers consider, and no doubt rightly, that hand feeding, if properly executed, is more effective; still it is extremely doubtful whether, especially in the case of small machines, the value of the increase in the quantity of stuff crushed is equal to that of a man’s page 175labour; in my opinion, this labour is far more profitably applied to attending upon the gold-saving appliances. With only one or two exceptions, I found the great defect of the coffers having only front discharge; for it must stand to reason that, as it is, or ought to be, the aim to get the finely crushed material quickly out of the boxes, large escapes both in front and at the Back are the most effective. Of course, such a double discharge arrangement necessitates a corresponding increase in the quantity of water to be supplied per stump-head—a supply of from five to eight gallons per head per minute, regulated according to the weight of the heads and the nature of the crushing stuff, would however, satisfy all requirements. The guage of the gratings, varying at the different machines from 122 to 144 holes per square inch, is, I think, scarcely well adapted to the nature of the stuff treated; for, as the gold is mostly very fine, the gratings should be very fine also, in order to ensure the necessary degree of reduction for a satisfactory liberation of the gold particles. Gratings with 169 to 196 holes per square inch would certainly be safer.

Gold Saving Appliances.—As regards the gold saving appliances in use, they consist, with the exception of two machines, which have drop-ripples attached, of amalgamated copperplate tables, in some instances with improvements in the arrangement of the ripples, and all, except the cement crashing machines, have various lengths of blanket-strakes succeeding. During working, it is the regular custom to put quicksilver into the stamper boxes. For the treatment of the blanket sand serve the common revolving barrel with shaking table or ties attached, dolly tubs, small berdan machines and simple ties, though the latter inferior appliances at a few machines only. Although generally well constructed and carefully attended to, as I found these appliances at the machines in work, and as they respectively are, and were said to be at those at a standstill, most of the managers I came in contact with were w[gap — reason: illegible]ll aware that they lost a considerable per-centage of the gold, and in some instances notable quantities of quicksilver besides. Considering this general loss, which in some establishments was occasionally much increased through an insufficient supply of, and the use of muddy water, I attribute it mainly to three causes, viz., the use of amalgamated copper plates, too strong an inclination and insufficient length of the blanket strakes, and last, though not least, to the introduction of mercury into the stamper boxes. It would lead too far here to enter into a discussion upon the merits or otherwise of copperplate tables generally suffice it to say that tables of the same pattern as those under notice were at one time in high favour in Victoria, but careful trials soon proved their inefficiency in many respects, and they have at all the principal crushing establishments been long ago discarded in favour of more perfect appliances which I shall mention further on. According to Rüstel and other authori- page 176ties on the subject, they have also suffered a similar fate long since in California—the country where they were first introduced. The putting of quicksilver into the boxes is no doubt a great improvement in case of copperplates being used, but it is fundamentally detrimental in the crushing of stuff so highly charged with pyrites as most of the Otago reefs produce. For it is a well-known fact that pyrites generally, though certain kinds more than others (and these abound in the latter reefs), cause a flouring of the mercury and amalgam in the boxes, and for the saving of such floured stuff no method has as yet been discovered. On this account I would therefore strongly advise to abandon the practice, even in case of copperplates being retained: for the loss both in quicksilver and gold caused by it alone is, perhaps, much larger than what would be sustained by less efficient working of the plates through its disuse. I may at this place take the opportunity to remark, that I hold the practice of special harm with regard to the cement crushing machines at Bluespur, on account of the abundant occurrence in the bottom portions of that cement of secondary pyrites, i.e. such formed in the drift, a kind that through easy decomposition is very prone strongly to flour the mercury. Considering also that the generality of the gold crushed from the cement is dirty, i.e., more or less coated with oxyde of iron, or pyrites, and that, therefore, the copperplate tables (which I found only in use) have but a poor chance of retaining it, I think the attachment of blanket-strakes, or perhaps better of a well constructed tail-race, would be found very profitable. Touching the blanket-strakes, they are, at most machines, from ten to fourteen feet in length, and their inclination is seldom less than one and a half inches, and reaches two inches per foot—both figures which, considering that the supply of water is mostly rather copious are certainly not calculated to ensure a satisfactory saving of the fine gold and amalgam escaping from the copperplates; irrespective of that, at some machines, the blankets are not, or have not been, washed frequently enough. Having herewith given my opinion on the point from whence at least the greater per-centage of the loss in gold and mercury proceeds, I would recommend the exchange of the present appliances and system for, or respectively their modification according to those for years successfully in use at the Port Phillip Company’s works, Clunes—an establishment which in Victoria occupies the foremost place in satisfactory gold extraction, mainly because the practice there introduced of daily taking and assaying samples of the tailings server not only for controlling and guiding the working of the appliances adopted, but in the case of any new invention in gold saving being tried, it affords also the best proof of the merits or otherwise of the latter. The system of appliances used at Clunes simply consists—starting from the battery—of three connected quicksilver troughs—the first with a 10-inch drop, the second with page break

Geology of Otago. Ulrich. Plate VIII

page 177a 9-inch, and the third, or lowest, with an 8-inch, drop—through which the material passes in succession, to ran next over 24 to 27 feet of blanket strakes, laid at a pitch of only one foot in 16, and ultimately to pass from the blankets through another quicksilver trough before it reaches the waste channel. This last trough is only cleared, however, at intervals of several months, whilst the upper troughs are cleared every week. In order to keep any coarse stuff from entering these latter, and also for even distribution of the material, a perforated plate is fixed right in front of the battery, through which both Back and front discharge pass on to an apron which leads it (the material) into the first trough. An important part of each trough is the splash-board, which, reaching down to within about one and a half inches of the bottom (of the the trough), near to the surface of the quicksilver compels the material, in its drop, to pass more or less through the latter before rising over the lip of the trough. All the troughs are supplied with tap-holes on one side, by means of which the quicksilver and amalgam can be drawn off when required. The whole system will be easily understood by reference to appended Plate VIII., Fig. 1, which represents a longitudinal section, whilst Fig. 2 is a section of the troughs on a larger scale with the principal measurements marked. As to the blanket-strakes, their small inclination requires the supply of water to be up to eight gallons per stamp-head per minute, according to the nature of the stuff, in order to keep them free from sandy settlement. The rate at which the blankets are washed at Clunes is generally the upper row every hour, the second row every two hours, and of the remaining length of the strakes the blankets of the upper half every six, those of the lower half every twelve hours. Considering the nature of the stuff of the Otago reefs, I think, however, that partly because the more or less slimy stuff from the mullock reefs renders the surface of the blankets quickly inactive, partly on account of the great amount of pyrites contained in the quarts, a more frequent changing of the blankets than the above is advisable.

Touching the treatment of the blanket-sand, the method in use at most of the machines, viz, by revolving barrel and shaking table, gives, if properly carried out, the most satisfactory results, and deserves, therefore, general adoption. In the proper working of the barrel, upon which depends most, certain rules require to be followed, however, and as I had no opportunity of judging of the mode of operation at any of the machines, I give the following particulars on this head for comparison and guidance. Assuming the barrel to be about 4 feet long by 2 feet in diameter, it should be charged with 8 to 10 cwt. of damp sand, and 2 to 300lbs of mercury and set to revolve for about 8 hours at a speed of from 14 to 16 revolutions per minute. After this, it should be filled with hot water and set to revolve again for another 4 hours at the rate of 5 page 178to 6 revolutions per minute, when the operation is finished, and the charge—quicksilver first—may be drawn off. Having been informed that at several of the machines the practice is followed of putting round stones or pieces of iron into the barrel, in order to grind the sand finer and aid the amalgamation of the gold, I have to remark that I consider this proceeding likely to prove more harmful than advantageous, on account of the large quantity of pyrites generally present in the sand, which through the grinding is very liable to sicken or flour the mercury and amalgam, and this invariably is followed by a loss of mercury and gold afterwards. A determination of the exact loss of mercury in this and the main gold-saving process by carefully weighing the metal at short intervals, is not practised, as far as I could glean, at any of the machines, but as it is of the greatest importance in testing the comparative efficiency of the amalgamating appliances, it ought certainly not to be neglected in future.

Having herewith noticed all the principal points touching the saving of gold from the crushed material, it remains to draw attention to the saving and treatment of the pyrites, which, as already mentioned, occurs in greater or less abundance in most of the quartz reefs of the Province. Although small experiments have as yet been made of the pyrites of but a few of the reefs, and trials on the large scale of that of only one reef (Southberg’s Reef, Skipper’s Creek) proving the payably auriferous character of the ore, still I think there can hardly be a doubt, judging from Victorian experience, that the pyrites of all the other auriferous reefs of the Province is more or less payably auriferous also, and it would be highly advisable, therefore, after the truth of this supposition has been established by fire assays^*, to take early steps towards the con- page 179centration of and the extraction of the gold from the ore. Both these operations are not, however, very easy ones, but require, for profitable and satisfactory execution, much care, favorable natural conditions, and a considerable amount of capital in the construction of the necessary works. The saving operation has to be effected at each single crushing mill, and for it the so-called Borlase’s buddle with Munday’s patent scrapers, is the most approved in Victoria, but where capital and favourable conditions are wanting for the erection of these rather cumbrous machines, the use of a good length of blanket strakes—say 20 to 30 feet —and careful attention to, and more frequent washing of the blankets than hitherto practised, would at least save the greater quantity of the pyrites. With regard to the after treatment of the latter for the extraction of the gold, roasting furnaces and certain kinds of amalgamators are required, the working of which has to be specially learned. In fact, it may be said to constitute a special industry which not every company or reefowner might care or be able to enter into. In Victoria nearly all the larger companies have their own pyrites works, but there are also special private establishments of the kind, unconnected with crushing mills, at which miners or companies can either have their pyrites treated at a fixed rate per ton, or can sell it at a certain reduction on its gold-value, which latter is ascertained by careful metallurgical sample assay. A similar course must be left for private enterprise to follow in Otago.

In view of the fact that in the reefs of Victoria—and this as already stated, will most likely also happen in the reefs of Otago—the pyrites generally increases in quantity in depth, whilst the free gold correspondingly decreases, and that, moreover, the latter is the more difficult to save, the more pyrites the stuff contains: the pyrites question, as it is called, referring to the modes of concentration and after treatment of the ore, sanitary precautions connected with the roasting, etc., has there for years been one of increasing importance, and the Government appointed some time ago a commission to fully investigate the subject in all its bearings. The recently published report of this commission contains full information, with drawings of machinery, etc., on the best methods of pyrites concentration, and gold extraction; and as, if I entered upon a description of the respective processds, I should merely have to recount what is stated in it, I append a copy of this report instead. Considering, in conclusion, the general working effect of the crushing machines throughout the gold-fields of the Province, there are two serious natural disadvantages under which all suffer, though some in a stronger degree than others. I refer to the hard frost in winter, and the general low temperature of the water throughout the greater part of the year. The former compels the actual stoppage of the works for several months (five to six months at Skipper’s Creek), the latter affects the liveliness of the quick- page 180silver, and thereby impairs, as it were, its amalgamating power. Against the first nothing can be done, and to ameliorate the second difficulty the introduction of hot water into the coffers can only conveniently be practised at those machines driven by steam engines. However, I think that, whilst the introduction of long blanket-strakes would prove a great safeguard against any loss of gold caused by imperfect amalgamation, the most rational way to meet the frost difficulty would be to have all parts of the machinery sound and in best working order, so as to guard against interruptions in crushing day and night through the summer months, in order to work up as well as the crushing material accumulated during winter stoppage, as also that concurrently produced.