ON THE LOSING AND SAVING OF FINE GOLD.

Otago Witness, Issue 1892, 15 May 1890, Page 14

ON THE LOSING AND SAVING OF FINE GOLD.

By J. A. Millee.

111. THE SAVING OF FINB GOLD FROM ALLDVIAL

DEPOSITS,

Juat a word, by way of confidential oxplana- 1 tion, may bo permitted kere. If there is one ] •chiag upon which miners pride themselves more than another it is the virtue of their own ■and particular plans they have adopted for the saving of fine gold ; everyone claiming that Mb method is the best, and that all other methods are more or leas faulty and ineffective. in such a comparison it is too often quite forgotten that all the methods may be the beßt £or the particular wash to ba treated, for different kinds of wash »nd different kinds of gold require different tastboda of treatment, and by promulgating tUa tail race of the future it ia not intended to infere with or reflect in any way upon private experiments, the whole object being to look at the question of saving fine gold in a philosophic manner. Alluvial minerß deserve every credit for the ingenuity and perseverance with which they havo worked out methods to suit their in*

dividual cases and requirements. But to return to the business of these articles. As

THE OOEVED BOTTOM of the tail race of the future will obtrude itself frequently upon the discussion, the properties in favour of its introduction may as well be considered in detail in order to simplify future descriptions. When the water is suddenly turned off after running off stuff, every miner must have noticed that little beaches have formed alternately on the opposite Bides of the tail race, and that a scour channel has run down the centre of the tail race. This is not owing to the current being diverted in zig-zag form by the sides of the tail race, but to the impetus of running water. What is meant by tho impetus of a current is beßt explained by an experiment which every miner can easily «make for himself. If a sluice box is fixed at an angle about 20deg from the vertical and about half a Government head of water turned down the box, it will be noticed that the water flows down the box in waves, or rathar falls down the box in lumps, as it were, at measurable intervals of several seconds, though the water ia paid into the box in a continuous and unbroken stream. Wator in all channels flows in the same manner by ixnperua, but tho demonstration is lessoned with tho reduction in the rate of fall, until it becomes imperceptible, but is present in all running water confined in channels or boxes. This flow and halt, alternating, gives a rotary motion to a current confined between two unyielding walls, and instead of the water having zig-zagged down the race it has moved in convolutions, in form like the flanges of a carpenter's augur, bo that in a tail race of say 40ft long the water and gravel are made to travel a distance of 50ft — that is in diagonal lines from side to side of the tail race. Anyone who has watched the action of water in natural streams must be familiar with the , convolutions of currents. These convolutions are due to the impetus of the currents, and are greatly assisted by a curved bottom — nay, in natural streams the convolutions are the cause of the trough-like shape of river or creek channels. Now, it so happeos that it is also this peculiar action of running water which carries off the stones and sand in the direotion of the discharge, and at the same drops the specß of gold and the heavier particles of the wash into the crevices and catches providedfortheir reception ; therefore anything that increases the duration of the impetns and the intervals aids in the saving of fine gold. This is chiefly attained by increasing the fall, as is done in the natural streams by bars at right angles with their course, and in artificial tail raoes by a curved bottom. To obtain a good rousing current, the centre of the curve should be in depth not less than one -third of the width of the tail race ; that is to Bay, if a a tail raoe is 18in wide, the depression in the centre of the bottom should be Gin deep in vertical depth. In tail races constructed upon this prinoiple the gravel and sand are washed up the curved sides during the impetus of the flow, and fall back again during the interval, while the water goes on revolving, and these actions are repeated for the whole length of the tail race.

THE ADVANTAGES GAINED by favouring and inducing these convolutions are — first, that thdy keep the catches between the paving atones open and the gravel already in them in a loose condition ; second, that if scaly float gold is present the speps or scales will be turned repeatedly upon their edges, inducing them to sink to the bottom, being the only position in which they can sink ; and third, the convolutions will keep the bottom of the tail race continually clear, allowing no part to be oovered with dead gravel or stones. A further advantage obtained by a curved bottom is that in running off debris consisting of flat shingly 3tonea and gravel it will not by any chance be allowed to lodge upon the bottom and choke the rail race, but if the curve be sufficiently deep the convolutions will be strong enough to keep the shingle, which, in the majority of cases is of low specific gravity, in a constant state of floating down the race until discharged from it. When the tail race haa but little fall, and the bottom but a slight ourve, so that the depression amounts to only 3in, the convolutions are weak and long drawn ; when the bottom is flat, the convolutions, though still present, are no longer strong enough to affect the gravel and stones in the spaces coming in between the twists, but when there is a good fall of say, 1 in 12 or even 1 in 8, and the aro of the bottom iB 6in or more in a tail race 18in wide, the gravel and Band goes down the race merrily ; there are then no beaches of dead gravel at the aides of the raoe t no choke anywhere, but the whole of the paving has aa equal chance of saving the finest gold, and the finest float gold is deprived of the quality by which it can, and bo often does, escape and gets itself lost. The greatest difficulty in the saving of fine gold is met in

DREDGING OPERATIONS,

and consequently the loss of gold is considerable, although dredges offer two of the three conditions whioh are necessary for the arrest of fine gold. These two conditions are fall and an unlimited supply of water ; the missing one, length of boxes or tail race, is sought to be gained by adding to the width ; the object of this being to bring the sand and gold into the moat direct and prolonged contact with the gold-saving catches of the oocoanut matting or other material used for the same purpose in its place. In boxes of great width, such as are used in connection with dredges, the sides of the boxes can have no effect upoD the current, which is also much reduced in speed by giving the boxes the smallest amount of fall at which they can clear themselves. It must be admitted that a great deal is to be said in favour of this method, and for the treatment of small quantities, suoh aa are discharged by quarts crushing machinery, no better is known at present, provided that the boxes are of such a width as not to affect the current, which in this case moves simply by the specific gravity of the water. However, for this method to be effective it is necessary that the stuff treated must be sifted and reduced to as nearly uniform a degree of evenness as possible. Taking all things into consideration, therefore, the ordinary bucket and ladder dredge is perhaps best served with wide tables instead of boxes for the saving of fine gold ; but in the case of the Wolman dredge, in which there appears to be no limit to tba quantity of stuff to b3 raised, Borne application of the sluice, aftdr tho largest of the stones have been separated from the wash, commands an amount of attention that should induce those interested to submit the question to praotical experiment.