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amalgamate, here and there, the copper beneath. The result is a decrease in the ability of the plates to catch and hold the gold, and an excess of copper in the amalgam obtained. Of course, on the other hand, the addition of too little quicksilver diminishes the amount of gold caught on the plates.

6. Proper height of the charge in the mortar. This should not be allowed to rise higher than about three inches below the lower edge of the inner plates. If the quartz and pulp in the battery come nearer the plates, there is too much and too coarse stuff thrown upon the latter, preventing the accumulation of amalgam, or displacing it after it has collected.

7. Regular feeding. Occasional overcharging of the battery has the same effect as too little battery-water, besides of course producing the evil just alluded to, of too close an approach of the crushed or halfcrushed ore to the inner plates.

8. Care in keeping the plates clean.

Common accidents in crushing.-The usual accidents in stamp-mills (generally the result of defective management) are: the loosening of the various parts of the stamp, the breaking of the cam or the shank of the shoe, and the springing or bending of the stem.

All the parts of the stamp may work loose through its running "empty" or bare, in consequence of a lack of rock, or through the presence of pieces of broken dies or shoes beneath the stamps. The stem may be drawn out of the head if the battery box is filled too high, in which case the adhesion between the stamp-head and the pulp may be greater than that between head and stem. The shank of the shoe may break off, if a piece of cast iron from a die or shoe gets under the shoe in the neighborhood of its edge. Displacement or fracture of cams, and curvature of the stem may be caused by running the cam-shaft backwards, in which case the concave under side of the cam is pressed upon the upper part of the collar or tappet. A cam may also be broken by the fall of a stamp which has been hung up and is carelessly let go, so that the tappet strikes the cam with a blow. Neglect of proper lubrication (for which tar is generally employed) leads, here and there, to the premature wearing-out of a cam.

Workmen and their duties.-The immediate superintendent of the whole mill is usually the so-called foreman, who very rarely possesses theoretical knowledge, but who has himself discharged, at various periods, the duties of every position in such works, and is thoroughly familiar with them all, so that he can at any time take the place of an absent workman, or fully instruct a new hand. The foreman is responsible for the general regularity of operations, and particularly for those more immediately connected with the collection of amalgam, its safe keeping, and the final correct delivery of products to the owner. These processes (the cleanup, retorting, &c.) he performs as far as possible alone. (Sometimes the owner prefers to retort his own amalgam.) Complete trust is therefore necessarily reposed in him; and, since he is responsible for the proper conduct of every part of the operations, he usually has the power of engaging and discharging workmen. At least the owner makes no changes in the force contrary to the foreman's wishes. In stampmills, however, which are driven by steam-power, the care of the engines and gearing is usually more or less completely taken from his control, and intrusted to the two engineers. In small mills, up to say fifteen stamps, the foreman is commonly at the same time the first engineer, and the two feeders must, when he is prevented by absence or otherwise, tend the engine in addition to their other duties. In very

large mills, on the other hand, having more than fifty stamps, a special engineer with one or two assistants is required; and to these is given the care of all the machinery, the foreman being sufficiently occupied with the supervision and partly personal execution of the operations immediately connected with stamping, amalgamation, &c.

The necessity of supplies is reported by the engineer or foreman to the owner or his agent, that the required purchases may be made in time.

The foreman, like all the workmen, not excepting the engineers, is very rarely engaged for a specified period, but may at any time, without previous notice, resign or be discharged.

Wages are paid weekly or fortnightly. The pay of an ordinary workman, such as the feeder, per diem, is about $2 40; of the second engineer, $2 60 to $3; of the foreman or first engineer, from $3 20 to $4.

If a stamp-mill has a single foreman, he is usually at his post from 7 to 12 in the forenoon and from 1 to 6 in the afternoon, the owner or agent relieving him in the interval. In large mills, having two foremen, they change shifts, as do the engineers and other workmen, at the twelfth hour.

There has been hitherto no benevolent fund, of assessments or otherwise, for the relief of workmen injured or falling sick during service, or for the assistance of their families.

RESULTS.

Capacity of the stamps.-The normal average is about 1.09 cubic feet, or from 115 to 130 pounds of rock hourly, per horse-power developed by the stamp.

Wear. The wear of the die is nearly half as great as that of the shoe, both together amounting to about 185 pounds on the average for 1,000 hundred-weight of rock crushed.

Product of gold.-The product of gold from the copper plates varies between 30 and 50 per cent. of that in the ore, and averages about 40 per cent. A smaller yield than this is usually due to imperfect disintegration (too coarse crushing) of the ore; a larger yield, chiefly on the inner plates, to the presence of more coarse gold.

About 15 per cent. of the gold remains on the average, under normal conditions of running, enveloped in the unredeemed portions of the ore, (i.e., the larger particles ;) hence, about 45 per cent. gold actually freed from this envelope still escapes the plates; and a small part only of this, say 7 per cent. out of the 45, is dissolved in the two-thirds of the quicksilver charged, which, as we have seen, likewise escapes from the plates. The remaining 38 per cent. escapes as unamalgamated gold.

Of the amalgam obtained the interior plates yield about 67 per cent., the outer 20 per cent., the skimmings 13 per cent.; or, distributing the latter in due proportion, the inner plates yield three-fourths, and the outer plates one-fourth of the amalgam obtained, or 30 and 10 per cent., respectively, of the gold contained in the ore. Reckoning by units of surface, the inner plates collect 36 times as much gold as the outer.

We will designate by M the total gold in the ore; A, the gold caught in the battery on the inner plates; B, the gold caught on outer plates, (A being about 3 B;) g, the coarse gold caught on the plates; f the fine gold caught on the plates; C, the free gold* escaping from the plates; D, the gold escaping not freed from ore or rock; K, the coeffi cient or percentage of yield.

* Including, apparently, that which escapes as amalgam.-R. W. R.

Assuming that no coarse gold is lost from the plates, and that the proportion of fine gold caught to that which escapes free is constant and independent of g and D, we may calculate from the value of C or D the proportion between ƒ and g.

From the equations

we have

(1) A+B+C+D=M, and A+B=KM

(2) C+D=(1—K) M

Making g and D=O, we have f= K M, and C(1-K) M (*)

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If we now designate with F and G the quantities of fine and coarse gold in the ore, (the whole of the latter being saved, or G=g,) we have F ___ƒ+ (1 — K) M

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Assuming further that no coarse gold escapes from the battery, but that all the gold passing the discharge is fine, and designating with f' and g' the fine and coarse gold caught on the inner plates, and by C' the total free gold carried through the discharge, and by F' and Gʻ the amounts (on this assumption) of fine and coarse gold in the ore; we may obtain the proportions and from equations (4) and (5) by substituting for K the value 0.75 K, and for C the value C'=B+C= 0.25 K M+C. This gives us :

ƒ' F
G'

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Since ƒ and C are independent of g and D, we may put the latter=0 without affecting the proportion between the former. In other words, by the original proposition, there being no coarse gold lost, and the gold lost by reason of envelopment in quartz, etc., not affecting the amalgamation, it may be assumed, in considering the proportions of the fine gold caught to the fine gold lost, that there is no coarse gold C_1-K and that the crushing is perfect. The proportion = is true on this hypothesis K for all values of g and D.-R. W. R.

From which it seems, that though the proportion between ƒ and C is independent D of the values of g and D, it is not independent of the proportion .-R. W. R. g

Mr. Reichenecker's calculation is so condensed that a little explanation may not be unacceptable. Since A is 3 B, or the gold saved on the inner plates is three-fourths of

For K=0.4, (40 per cent.,) and D=0.15 M, we have from equation (3)

f

C

F = 1.5; from equation (4) £ =3; from equation (5)

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equation (6) £=3.67; and from equation (7)

g'

F'

G'

=

G

14.6. (*)

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The loss in melting the retort metal is generally about 8 per cent.,f due mostly to the quicksilver retained in it. The regulus contains a varying amount of silver, seldom less than 15 per cent., and very little, say 1 per cent., of copper.

Cost of the gold extraction.-The cost of working, i. e., crushing and simultaneous amalgamation, is, for 1,000 hundred-weight, about as follows:

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This is, therefore, the minimum paying yield for a steam stamp-mill, aside from the cost of mining and transportation. These items amount for 1,000 hundred-weight seldom to less than $300, (freight being, on an average, about one-sixth of the running cost.) To obtain, therefore, the minimum yield which will cover all mining freight and reduction expenses, we must include this item, and increase the estimated loss of

the total saved, K, or the percentage of yield for all the plates, becomes 0.75 K when the inner plates only are considered. And C, the amount of free gold escaping from all the plates, becomes, when the inner plates alone are considered, B+C, or the whole amount passing the screens. But A+B KM; and A3 B; hence B=0.25 K M, and C'=0.25 K M+C. It will be seen that to determine the proportion of fine and coarse gold saved in the battery, the outer plates are assumed to be wanting, and the whole discharge is treated as a loss.-R. W. R.

*These results may be thus expressed in words. If of the gold in the ore 40 per cent. is saved, 30 in the battery and 10 on the outer plates, 15 per cent. is lost, enveloped in quartz, and 45 per cent. escapes as free gold, then, assuming that no coarse gold is lost, and that the proportion of fine gold saved is independent of the amount of coarse gold saved and of the amount lost through insufficient crushing, it follows: 1. That two-fifths of the free fine gold is saved and three-fifths lost.

2. That of the gold saved three-fourths is fine gold.

3. That of the gold in the ore nine-tenths is fine gold. If we assume that no coarse gold passes through the screens, then,

4. Of the gold saved in the battery-box 387, or 78 per cent., is fine.

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5. Of the gold in the ore 18, or 93 per cent., is fine.

It will be seen that these results turn on a definition of coarse and fine gold. These terms are relative and vague. As the problem is here stated, however, in 1, 2, and 3, all gold fine enough to escape from the plates, and in 4 and 5 all gold fine enough to escape through the screens, is called distinctively fine gold.-R. W. R.

The average of 26 meltings, given by Mr. Hague, (U. S. Geol. Expl. Exped., vol. iii, p. 554,) is 5.42 per cent.-R. W. R.

And frequently, it might be added, to a great deal more. Six dollars per ton for mining and hauling is below the average.-R. W. R.

quicksilver in proportion, (i. e., from $2 35 on $187 25 to $6 60 on $491 50,) giving the following estimate for 1,000 hundred-weight:

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This yield represents $9 83 per ton, or an actual amount in the ore of $14 74 per ton, or $103 18 per cord. These estimates are based on the supposition that the tailings are run off without further attempt to save the quicksilver or gold which they contain.* When water is the motor, the minimum paying yield (possible profit from further concentration not being reckoned) is $406 30 per 1,000 hundred-weight, or $8 13 per ton. In this case the milling cost proper is $101 60, or $2 03 per ton, and is to the mining and hauling cost about as 1 to 3.

II. FURTHER CONCENTRATION AFTER CRUSHING.

The further concentration has for its chief object the saving of as large a portion as possible of the gold which has escaped from the plates. In the case of ores which contain considerable amounts of silver as well as gold, the saving of the argentiferous portions is desired at the same time. Both objects are sought by the separation of the cleaner portion of the battery slimes into two sorts, each containing gold ore, (enveloped gold,) free gold, auriferous quicksilver, and silver ore, the specific gravity of the two classes being, for the heavier, say, 3.6 to 4.6, and for the lighter, 3.0 to 3.5. The first class is subsequently subjected to further pulverization and amalgamation, and both classes are then concentrated until

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Subtracting from the total of $491 50 the cost of mining and freight, we have $191 50, or $3 83 per ton. The interest on capital being also subtracted, we have $158 90, or $3 18. The average of 1,300 tons at the Ophir Company's mill (see Mr. Hague's report, already quoted, p. 555) was $3 69. Mr. Hague's own estimates compare with those in the text as follows:

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Mr. Hague adds: "Making due allowance for other supplies not enumerated, extraordinary repairs and miscellaneous expenses, the estimated cost will accord closely with the figures just given as the result of the experience of the Ophir and the Sensenderfer mills. Of course the above items will vary considerably in different mills. The cost of fuel depends not only on the price paid for it, but on the economy with which it is used, the kind of boiler employed, and the proper adaptation of all the machinery to its purpose. The same remark applies to the economy of labor, while in all mills an essential condition of cheap work is constant employment at full capacity."

Mr. Hague takes no account of interest on capital; and I think it is difficult to include this item in estimates of running cost, though it certainly constitutes a large tem of expense, and is disastrously important when the mills are run too slowly, or on less than full capacity. Mr. Reichenecker reckons it very high; $32 60 on 50 tons of ore represents about 2 per cent. a month on the capital necessary to build a ten-stamp mill, crushing ten tons daily. I do not know, however, precisely on what basis his calculation is made.-R. W. R.

H. Ex. 10-23

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