82

GOLD.—Extraction from Ores.

they are erected at intervals over the whole lengthof the valley of Anzasoa, from Chamgnaga to Pie diMulera.

The ore or mineral, when extracted from themine, is first subjected to a grinding or trituratingprocess in a machine resembling in its action andoperation the common millstone, and driven by awater-wheel. When thus reduced to powder, it iscarried off by a stream of water to the amalgamatingmills, represented in Fig. 18. Whilst held in sus-pension in the water, it is conducted first to theupper mill by the spout a, from this to the second

l'ig. is.

by the spout b, and so on in succession through thewhole series. Each mill consists of a cast-iron basin,D E G F, fastened by screws to the top of a strongframe or table, and fitted in its centre with a tube,traversed by the axis, II II', moved by the toothedwheel, 11 ', and with which revolves a wooden muller,k k, fixed to the upright spindle by the iron collar,m m'. This movable part of the mill has externallythe same form as the internal cavity of the fixediron casting, G, in which it revolves horizontally,at the distance of about half an inch from its sur-face ; at the same time it is furnished with severaliron ribs, which come nearly in contact with theinterior of the casting. The rotating muller is madeof hard wood, and is hollowed out like a funnel,in which the auriferous slime accumulates, andgradually penetrates into the interval between itsexternal surface and the bottom or interior surfaceof the iron basin, and from this it flows to the nextmill by the spout, B or c.

In the bottom of the fixed vessel is placed abouthalf a hundredweight of mercury, which has a depthof half an inch; and when the machine is set inmotion the pounded mineral is constantly agitatedand mixed with this fluid metal by means of the ironribs nailed to the bottom of the muller. The minuteparticles of gold are thus immediately dissolved andtaken up by the mercury, while those which escapeamalgamation in the first mill are arrestedand retainedby the second, third, or fourth of the series.

After working the mills continuously for severalweeks, the time required being longer or shorteraccording to the richness of the mineral underoperation, and when the mercury is judged by theworkmen to be sufficiently charged with the preciousmetal, the mills are stopped, and the amalgamated

mass is taken out. It is important not to protractthe process unnecessarily, otherwise a serious loss ofmercury is sustained, occasioned, perhaps, by itspartial conversion into a sulphide, which is carriedaway with the current. The mercury, when drawnoff, is washed, and then strained or filtered througha piece of chamois skin, for the purpose of ob-taining the solid amalgam, which usually containsabout one-third of its weight of pure gold. This isseparated from the quicksilver by distillation in aniron retort, the beak of which dips into a vessel con-taining water. The whole of the mercury unitedwith the gold is thus recovered, and theonly necessary loss is due to the carry-ing away, by the current, of mercury in

AUs the form of sulphide. Dumas statesthat the loss incurred in this manner isequal to about one-fourth of the quantityemployed; but so large a waste mustresult from careless manipulation.

At the Schemnitz works in Hungary ,the slimes, after leaving the stampingmills, pass directly into amalgamatorssimilar to those already described, ofwhich there are, however, only two ineach series, the slime passing out of thesecond on to inclined tables, as alreadymentioned. Only half the amount of mercury isemployed, and in eight mills the loss is only about8 ozs. of mercury per month.

In the Salzburg Alps the method adopted for theextraction of gold is peculiar, and consists of threedistinct operations, washing, roasting, and amalga-mation. As the annual produce, however, is quiteinsignificant (only about 65 lbs. per annum), it isunnecessary to describe its peculiarities further thanto say that, prior to passing into the amalgamators,the slime is ground in a mill very similar to thecommon flour mill.

Revolving barrels are employed for extracting goldfrom the concentrated slimes obtained, as alreadydescribed, at the Morro Velho mines in Brazil . Theseare very similar in construction to those used at silverworks, and which will be described in the article onthat metal. After the mixed slimes and mercury havebeen in the revolving barrels about thirty hours theyare run off into a trough or “ saxe,” divided by trans-verse partitions; in each division of which a rakemoves backwards and forwards, by which means therich amalgam collects on the bottom, the impoverishedslime being run off on to the “strakes” alreadydescribed, by which it is enriched prior to beingagain introduced into the barrels.

Amalgamation has been brought to a high state ofperfection in California, and as in Australia verysimilar arrangements are employed, a description ofthe American method will suffice. This descriptionis derived mainly from the annual report of theUnited States Commissioner of Mining Statistics forthe year 1872. On emerging from the stamping mills(where a preliminary amalgamation on copperplates frequently takes place), the slimes pass overan apron of amalgamated copper plates, by which