Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/771

Rh GOLD received special modiﬁcation in many details. Fig. 5 represents the ordinary Californian pattern of a stamp mill. The stamp is a cylindrical iron pestle faced with a chilled cast—iron shoe, removable so that it can be renewed when necessary, attached to a round iron rod or lifter, the whole weighing from 600 to 800 lb. The lift is effected by cams acting on the under surface of tappets cc, and formed by cylindrical boxes keyed on to the stems of the lifter about one—fourth of their length from the top. As, however, the cams, unlike those of European stamp mills, are placed to one side of the stamp, the latter is not only lifted but turned partly round on its own axis, whereby the shoes are worn down uniformly. The bed or mortar A is of cast—iron. The height of lift 1nay be between 8 and 10 inches, and the number of blows from 30 to 90 per minute. The stuff, previously broken to about 2 inch lumps in a Blake’s rock- breaker, is fed in through the aperture n at the back of the “battery box,” a constant supply of water being given from the channel /c, and mercury in a ﬁnely divided state is added at frequent intervals. The discharge of the com- minuted material takes place through the aperture cl, which is covered by a thin steel plate perforated with numerous 1 l" I 4 ‘IF g@‘V7/if / ; /. . FIG 5.—Stamp Mill. slits about _—,_’6th inch broad and illvth. to 33th inch long, a certain volume beinor discharrred at every blow and carried forward by the ﬂushoing watear over the apron or table in front, 212, covered by copper plates ﬁlled with mercury. Similar plates are often used to catch any particles of gold that may be thrown back, while the main operation is so conducted that the bulk of the rrold may be reduced to the state of amalgam by bringing the two metals into intimate contact under the stamp head, and remain in the battery. The tables in front are laid at an incline of about 8 degrees, and are about 13 feet long; they collect from 10 to 15 per "cent. of the whole gold; a further quantity is recovered by leading the sands through a gutter about 16 inches broad and 120 feet long, also lined with amalgamated copper plates, after the pyritic and other heavy minerals have been separated by depositing in catch pits and other similar contrivances. When the ore does not contain any considerable amount of free gold, mercury is not, as a rule, used in the battery. The pulverized stuff is received upon blanket tables or sluices. These are inclined boards covered with coarse 747 woollen cloth or sacking. The heavier particles become entangled in the ﬁbres of the cloth, while the lighter deposits are carried forward by the current. At intervals pf a quarter to half an hour the surface of the blanket 1S completely covered, when it is removed, and its con- tents are washed off in a tub of water and reserved for further treatment. This consists of amalgamation, in a contrivance analogous to the Hungarian mill subsequently described, and subsequent treatment in pan amalgamators somewhat similar to the arrastra in character, but with grinding surfaces of iron instead of stone. At Schenmitz, in Ilungary, quartz vein stuff containing a little gold, partly free and partly associated with pyrites and galena, is, after stamping in mills similar to those described above, but without rotating stami_)s, passed ll

FIG. 6.— Hungarian Mill. through the so-called Hungarian gold mill, fig. 6. consists of a cast—iron pan (1, having a shallow cylindrical bottom b, holding 50 lb of mercury, in which a wooden runner c, nearly of the same shape as the inside of the pan, and armed below with several projecting blades, is made to revolve by gearing wheels placed either above, or, as in This the ﬁgure, below. The connexion of the runner with the driving shaft is effected by the three-armed crutch shown in plan at e, which sits on the square part of the shaft. ly means of set screws analogous to those of a flour mill, the runner is adjusted at such a height that the knives just clear the surface of the mercury. The stuff from the stamps arrives by the gutter f, and, falling through the hole in the middle of the runner, is distributed over the mercury, when the gold subsides in virtue of its superior densit-y, while the quartz and lighter materials are guided by the blades to the circumference and are discharged at g, usually into a second similar mill, and sometimes to a third, placed at lower levels, and subsequently pass over blanket tables. The most advantageous speed is from 12 to 14 revolutions per minute. The action of this so-called mill is really more nearly analogous to that of a centrifugal pump, as no grinding action takes place in it. The amalgam is cleaned out about once a month. The average amount of gold collected from 50 tons of stuff stamped, is about 6 oz. in the mills, and in the subsequent dressing processes 1 lb of auriferous silver and 10 cwt. of lead. According to Rittinger, mercury that has been puriﬁed by distillation acts much more rapidly upon gold than such as has been saturated with the metal without losing its ﬂuidity, although the amount that can be so dissolved is very small. There are various forms of pan amalgamators of which space will not permit a description to be given. It may be stated, however, that experience of the great variety of pans that have from time to time been devisetl has led to