of $4.50 per ton is the basis rate calculated to cover profit and smelting losses as well as bare cost.

Some ores in special demand are, however, purchased by the smelting works on terms much more favourable to the producer. Thus, the gold bearing pyrites concentrates of Gilpin County, Colorado, containing under 10 per cent. SiO2, were for a long time eagerly purchased by Denver smelters at a smelting rate of $2.25 per ton, which barely covers the cost of roasting; and 95 per cent. of both gold and silver contents are paid for at New York prices.

Losses. The losses in smelting are (1) In slag thrown away; (2) in flue dust and fume incompletely recovered; (3) in bye-products, matte, speiss, flue-dust, &c., which suffer further treatment losses before they can be brought into a marketable condition.

When producing a lead bullion of about 300 ozs. Ag per ton, perfectly clean slags will run as low as per cent. lead by dry assay and oz. silver to the ton; the general average, however, in fairly good work is probably not much under 1 per cent. lead and oz. silver, while carelessness in working up the charges or neglect on the part of the furnace foreman may easily result in the production of slags which separate imperfectly from matte and contain 2 per cent. lead and 11⁄2 ozs. of silver; especially if the ore be somewhat rich in lead, and the furnace one of the old type with comparatively low column of charge and pressure of blast.

In some cases, where fuel and flux are dear (as, for example, at Broken Hill, N.S. W.); or where the ore charge is very rich in zinc, it may even be better policy to throw away slags containing 3 or 4 per cent. lead than to attempt to clean them by a further expensive consumption of fuel, and the addition of barren fluxes, with the attendant reduction of furnace capacity to which they give rise. The average percentage loss of silver and lead in slags on a charge containing 10 per cent. lead and 30 ozs. silver may be reckoned at not much less than 10 per cent. of the lead and 1 per cent. of the silver as calculated by the results of the commercial uncorrected dry assays, which are, as has been shown, far below the truth.

With regard to slag losses, it is desirable to bear in mind that losses which would be intolerable at Swansea, at Freiberg, or at Denver, may actually indicate the best possible practice in remote parts of the west, in the wilds of Mexico, or in South America. The object of the metallurgist should be to make the largest possible profit per ton of ore treated, and if it is more profitable to produce a slag with 6 per cent. lead and 4 ozs. silver per ton than to flux the charge in such a way as to make a cleaner slag, as is the case to-day in parts of Central and *The rate has been recently raised by $1 to $2 per ton.

Southern America, then the foul slag is the proper slag to make. As Lang puts it,*"It is always pecuniary profit, and not the perfection of processes, which is the criterion of metallurgical fitness."

With regard to the loss in uncondensed fume there is room for more doubt, and the real loss is always much greater than the apparent loss shown by the ordinary dry assays for lead and silver. In connection with this matter, reference should be made to Chapter ii., in which is recorded the fact that the ordinary dry assay for silver gives results which, in the case of a mixture of ordinary ores containing from 5 to 100 ozs. and averaging about 50 ozs., are below the truth to the extent of 7 or 8 per cent. of the total present; while the dry assay for lead with an ore mixture of 12 per cent. is on the average low to a greater extent, probably 10 or 12 per cent. of the total present. The real loss in uncondensed flue-dust is, therefore, as has been pointed out among others by Egleston,† much greater than it is usually supposed to be.

According to Hofman, ‡ the total apparent loss of lead in good work is 7 per cent., though it not uncommonly reaches twice this amount (corresponding with real total losses of 15 and 23 per cent. respectively). As regards the total apparent loss of silver, Hofman says it "rarely reaches 5 per cent.," which would indeed be quite an outside figure. That the real loss of silver is, however, much greater than that shown by assay, is proved by the fact that at some establishments the silver produced is more than that shown by the assays which have served as a basis for purchase. This, however, does not so much indicate perfection in the smelting as imperfection or bias in the assaying. As regards the loss of lead in uncondensed fume, which is so much greater than the corresponding loss of silver, a large proportion could be saved by filtering appliances, but in many places it would hardly pay to use them.

The following table shows the apparent percentages of silver and lead obtained from the ore at various metallurgical works, as admitted by the furnace managers.

Keen competition among smelters in America has led to greater care in assaying, American assayers commonly cupelling at a lower temperature than their confrères in Europe, with the object of partially compensating for the cupel loss, and so getting a nearer approximation to the truth. It is to this cause that the apparently higher loss of silver in American works should undoubtedly be attributed.

* Matte Smelting, New York, 1896, p. 72.

+ Trans. A.I.M.E., vol. xi., p. 379.

Op. cit., p. 293.

Grade of ore treated,. 40. 96 ozs. 50%. 23 ozs. 13. 40 ozs. 11%.

References.-1. Phillips, Elements of Metallurgy, 1874, p. 563. 2. Schnabel, Handbuch der Metallhüttenkunde, vol. i., p. 407. 3 and 4.

Private notes.

Purchase of Ores-Western Smelters' Sliding Scale.-The system of ore-purchase varies greatly in accordance with local conditions and local customs; but the schedules in use throughout the Western States of America are typical and may be described in detail as capable of almost universal applica tion with slight modifications. All the assays are uncorrected dry assays, and the basis of the calculation is the assumed mean price of lead in New York ($4 per 100 lbs. about £9, 10s. per ton in England). At this price for lead, ores are bought according to the following scale :

Under 5 per cent. lead

=

at nothing.

25 cents per unit of 20 lbs.

35

The gross value of a 50 cent lead ore at the price of $4 per 100 lbs. is 80 cents per unit, but the smelter retains 25 cents per unit (or £5, 4s. 2d. per ton) to cover smelting losses, refining charges, and freight to Eastern markets. It is obvious that

these losses and charges will be greater in proportion on the poorer ores; hence the above scale. It is further provided that for every 5 cent per 100 lbs. fluctuation in the New York price of lead an allowance shall be made of 1 cent per unit in the

above scale up or down. When lead ores are scarce the above scale is modified, sometimes to the extent of paying 50 cents per unit on everything over 20 per cent. in the case of good carbonate ores which are very much in demand.

As to silver, 95 per cent. of the assay contents of all ores is paid for at New York quotations on the day of settlement, which means that lead ores have to pay the bulk of the refining charges on the silver of the dry ores. Gold is paid for without deductions at $19, or now usually $20 per oz. fine, which is 96.6 per cent. of the full Mint value ($20.67 per oz.).

As a rule, copper is not paid for in lead ores, but of late years competition has forced some of the smelters to pay for this metal when over 5 per cent., at about $1.20 or $1.30 per unit, which is something like one-half of its gross value.

As an example, the value of the ore already quoted a few pages back may be calculated according to the above schedule:—

Lead, 21 units at 40 cents per unit,

$8.40 Silver, 95 per cent. of 60 ozs. = 57 ozs. at, say, 66 cents, 37.90 Gold, oz. at $20,

4.00

$50.30

The true cost of smelting the above ore having been already ascertained to be $9.10 per ton, the smelter would probably quote a "treatment charge" of $12 per ton, and the price at which the ore was purchased would be:

Gross value per ton as above,
Treatment charge,

Net value per ton,

$50.30

12.00

$38.30

The smelters' profit may vary from $1 to $3 per ton, according to the nature of the ore and the amount of competition for that class of ore at the moment; thus carbonate lead ores with ferruginous gangue are frequently treated without charge, or even sometimes at a positive loss, which is made up by increasing the treatment charge on the dry ores.

As a rule, throughout the Western States outside of Colorado, no attempt is made to calculate the actual cost of treatment of true lead ores at the particular locality; but, in order to prevent ores from being shipped to Colorado, it is customary to take as the normal smelting charge the Denver or Pueblo rate plus the -cost of transit to either of these points, when a small deduction will turn the scale and prevent ores being shipped away from local works.

CHAPTER XIV.

THE TREATMENT OF ZINC-LEAD SULPHIDES.

REFERENCE has been made in Chapter viii. to the difficulty of treating, by the ordinary roast and reduction process, such argentiferous lead ores as contain 20 per cent. and upwards of zinc, except by diluting or smothering them with an admixture of purer ores. The object of almost all attempts at treating such material has been to remove the zinc in solution so as to smelt the plumbiferous residue more easily and with less loss.

Concentration.-It is probable that very few, if any, of the zinc-lead sulphide ores contain galena and blende so intimately mixed as put them entirely beyond the reach of the ordinary dressing processes to separate a notable proportion of the galena in a condition suitable for smelting. Concentration. therefore, should always be the first step in the treatment of such ores; unfortunately, however, not only is a large amount of "middlings" produced, which it is quite beyond the power of ordinary dressing machinery to separate, but there is a great loss of both lead and silver in fine slimes, the whole of which have, therefore, to be carefully settled and collected for smelting.

Concentration at Broken Hill. As regards Broken Hill sulphides, the dressing plant is generally arranged much on the lines familiar in European and American practice, except that the crushing is not so fine; that the slime treatment is quite insignificant owing to the fact that the slimes are, in any case, so rich in silver as to be worth collection and smelting; and that a small number of very large jigs, treating practically unsized pulp, are found to do quite as perfect and more economical work than the large number of small jigs working on carefully classified sizes which have been the characteristic of German and American practice.

Thus the concentrating plant of the Broken Hill Block 14 Co., the capacity of which is 2500 tons per week, consists of two Blake jaw-crushers, each followed by a pair of rolls, 33 inches diameter and 20 inches face, crushing through 10-mesh with the help of a stream of water, any coarse particles being separated by a trommel and returned to the rolls by an elevator. From the rolls the pulp passes without classification to four large jigs of the "Hancock percussion" pattern, the hutches of which are