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According to Weed' the distribution of costs is as follows:
The above distribution applied to the 1917 figures, but are more or less true for other years. The total cost of mining, milling, etc., in the Missouri region is given by Weed as $1.32 a ton in 1917, a rise of 28 cents per ton in two years; the cost per ton of ore was a little higher in the vicinity of Miami (Oklahoma), averaging $1.60. The cost on the basis of concentrate, however, is in favor of the Miami field because the ore in that section is higher grade (8 to 10 per cent) as compared with the Joplin ore (2 per cent). Assuming Weed's figures to be correct, the cost of concentrate in 1917 was approximately $42 per ton in Joplin and $16 per ton in Miami.
In May, 1918, the high-grade ore producers submitted a brief 2 to the War Industries Board, showing the cost of producing concentrates in the Joplin district to be $58.37 per ton (including a $10 charge for depreciation and depletion and an average royalty of $8.37). Late in the summer it was claimed that the cost of production had increased 10 per cent bringing the total cost in the Joplin area to $63 per ton of concentrates. It was further claimed that the depletion charge of $10 (which had been allowed) was inadequate.
In the early part of 1919 only the higher grade mines were able to operate. Most of the Missouri properties were idle, the production coming almost wholly from Oklahoma. Labor and supplies were adjusted to a more nearly normal cost basis. F. B. Hyder, of the Bureau of Mines, after a thorough investigation, has determined the average cost of mining and milling in May, 1919, was $2.10 per ton of ore hoisted, or between $28 and $30 per ton of 60 per cent concentrate. This, however, is only operating cost. The average royalty is 17 per cent on the price of ore in producers' bins, and depletion and depreciation charges are estimated at $12 per ton.3
MILLING PRACTICE AND METHODS.
KOMSPELTER (JOPLIN, MO.) DISTRICT.
The only variations in ores of the various parts of this district that are of importance in milling are those due to differences in grade and in the relative proportions of lead and zinc. Differences in grade affect the flow sheet in degree rather than in kind of treatment. In only rare instances are zinc ores of these districts sufficiently high grade to smelt without concentration other than sorting at the mine. As a rule the ratio of concentration is fairly high as the sheet-ground ores that now furnish the major part of the production are low-grade. Under 1918 conditions, ores yielding 2 per cent of combined recoverable sulphides are close to the line of demarcation between waste and ore.
1 Mines Handbook. XIII, p. 1256 and p. 947.
2 The data which follow are taken from report published in Mining Congress Journal.
These data are to be published in detail in the Monthly Report of War Minerals Investigations of the Bureau of Mines.
Preliminary crushing in jaw breakers and rolls, followed by roughing and cleaning of coarse concentrates in Cooley jigs, is standard milling practice. The finer material is roughly classified for table concentration. At a few of the larger plants regrinding and flotation of the tailing is being introduced, but is not as yet generally adopted.
On account of the custom of leasing in small tracts, a mill is required for each lease and its capacity is limited by the hoisting capacity of the mine. Recently there has been a tendency toward the elimination of the smaller operators and a concentration of the industry among comparatively few large companies. The average capacity of the mills is from 200 to 500 tons per day, although some of the new mills can treat from 1,000 to 2,000 tons daily. Most mills are now run on a 24-hour basis, except Sundays, working three 8-hour shifts.
All material is crushed to three-eighths to one-half inch and treated in a "rougher" Cooley jig of the fixed-sieve type, with separate plunger compartments, and having five or six cells per unit. The first three or four hutch products, locally known as "smittem," run from 10 to 25 per cent zinc and also contain lead. They are treated without regrinding in a 7-compartment "cleaner" jig. The "chats," or material from the last two or three beds are reground to pass a one-eighth-inch screen opening and treated on a separate jig (sand jig) together with the sand from the last two hutches of the rougher and the reground tailing from the cleaner.
The cleaner jig makes a clean lead concentrate in the first hutch and finished zinc concentrates in the last five hutches. In the second hutch, the product is almost pure sulphide, but as it carries both lead and zinc, it must be retreated, usually returning to the feed of the same machine. The cleaner overflow or tailing is reground and treated in the sand jig as stated above.
All the other jig tailings are combined and passed over dewatering screens (1 to 2 millimeter apertures). The oversize is stacked on the dump and the undersize goes to settling tanks. The overflow from these tanks is thickened in a Dorr thickener and treated on slime tables (usually Deister-Overstrom). The settling-tank product is screened to 1 millimeters in a trommel (the oversize being reground to pass this opening) and classified. The hydraulic classifier has six spigots. The spigot products are tabled. Arbuthnot tables with the Overstrom head motion are used for the coarser material and Wilfley tables for the finer sands. Each table makes lead concentrate, zinc-sand middling, and a tailing. Each of the two middling products is retreated either by separate tables or returned to the classifier.
MILLING OF ORES CONTAINING SULPHIDES OTHER THAN ZINC AND
The treatment of the Joplin ores is comparatively simple as it involves the separation only of sphalerite from lead sulphide (galena) and a gangue composed almost entirely of hard flint or of limestone. The specific gravity of the zinc sulphide is intermediate between the gravities of the galena and waste. Furthermore, the minerals are
coarsely crystallized and separate easily in crushing and are not intergrown. These comparatively ideal conditions, however, do not exist in most of the zinc producing districts of this and other countries. Even in Wisconsin where the occurrence is most nearly comparable to that in the older field, a large amount of iron sulphide (pyrite or marcasite) is invariably present. As the specific gravity of pyrite is but little more than that of zinc sulphide, any method of gravity separation results in the contamination of the zinc product with the (usually) valueless mineral. A certain amount of iron may sometimes be accepted by the smelters. But in excessive amounts it is always penalized and so lowers the grade of the product that it is not accepted by the smelter. In addition to iron sulphide, copper minerals, also of practically the same specific gravity as sphalerite, are often present in the ore. The value of the zinc product at the smelter is affected by association with these minerals in the same way as when contaminated with barren iron sulphide. In addition, the copper has an inherent value of its own and is quite generally accompanied by appreciable values of precious metals. The values other than zinc are often not paid for in a zinc concentrate and are always subject to a heavy discount.
The separation of this complex middling has been the subject of considerable study and the failure to solve the problem has been the cause of disaster to many operators.
The method in most general use involves magnetic separation. Pyrite, in spite of its large iron content, is nonmagnetic. However, only a light superficial roast renders it sufficiently magnetic so that it can be removed by one of the different types of magnetic separators, leaving a more or less pure zinc concentrate. Except in remote districts, the lightly roasted pyrite is a marketable product and may be sold to makers of sulphuric acid, only a small portion of its sulphur having been removed.
Using exceptionally strong magnetic machines (e. g. the Wetherill) black jack or sphalerite containing small amounts of iron can be lifted from nonmagnetic material and this method is occasionally used as a last resort.
Electrostatic separation (Huff machine) is of quite general application. Next to roast-magnetic treatment, it is probably the most common special process and has been adopted by some of the largest operators.
Preferential flotation has proved highly successful on many ores, especially in Australia. Active research is under way and is expanding this promising field. This system of treatment, involving no drying, and fitting naturally into the general wet concentration, may be expected to become a more and more important factor in the treatment of complex zincy ores.
Igneous concentration is described on page 42. Leaching (lixiviation) is coming into some vogue as a process for treating zinc ore, but as it now forms only a stage in the electrolytic production of spelter it will be discussed in another report.1
1 Report in files of the Tariff Commission.
APPRAISEMENT OF ZINC ORES.
The duties on zinc in zinc-bearing ores can not be assessed on the foreign market value, since the ore in which the zinc is contained is the merchandise imported. Duties are accordingly taken upon American selling prices in accordance with paragraph L of Section III of the act of 1913, as shown by the following instructions of the Treasury Department (T. D. 36446, superseding T. D. 34280 and 35624):
(1) From the ascertained assay deduct 8 units for sulphide and 6 units for nonsulphide ores. The remainder will represent percentage of recoverable zinc in the ore, which multiplied by 2,000 will give the number of pounds of zinc recoverable from a ton of ore.
(2) Multiply the result as above ascertained by the average price of prime western spelter in East St. Louis for the week in which the ore was exported; that is, the week including the date of sailing of the ship or day the car leaves the foreign country. This will give the gross value of the zinc in the ore at the time of its exportation. (3) Deduct from the gross value of the zinc in the ore as above ascertained the following:
(a) The freight actually paid from the foreign mine to the domestic smelter receiving same in the United States.
(b) The insurance actually paid.
(c) The actual shipping charges.
(d) Foreign export duties and charges, if any.
(e) Treatment charge (as explained in par. 4). Penalties for iron, as stipulated in paragraph 5. Duty on lead contents, if any.
Duty on zinc.
(4) Treatment charge. (a) For sulphide ores the treatment charge will be ascer tained as follows: From the value of the recoverable spelter in a ton of 2,000 pounds medium grade Joplin sulphide ore, 60 per cent base, deduct the average of the quoted prices for such ore and $1.50 per ton of 2,000 pounds as representing the average freight on Joplin ores from Joplin, Mo., to common Kansas smelting points.
(b) For nonsulphide ores the treatment charge will be ascertained as follows: From the value of the recoverable spelter in a ton of 2,000 pounds of 40 per cent calamine ore deduct the average of the quoted prices for such ore and $1.50 per ton of 2,000 pounds as representing the average freight paid from Jolpin, Mo., to common Kansas smelting points.
(5) Penalties. On iron ore deduct penalties as follows: $1 on each unit of iron in excess of 1 per cent up to and including 6 per cent; 50 cents per unit on each unit of iron in excess of 6 per cent up to and including 12 per cent; 25 cents for each unit of iron in excess of 12 per cent.
(6) The average market price of Joplin zinc ore and prime western spelter to be taken in accordance with quotations contained in the Engineering and Mining Journal for the calendar week including the date of the sailing of the ship or day the car leaves the foreign country.
(7) Recoverable spelter, wherever that term is used in this memorandum, means for sulphide ores the assay minus 8 units, and for nonsulphide ores the assay minus 6 units.
EXAMPLE: SULPHIDE ORE.
Assume: Assay 48 per cent zinc, 6 per cent lead, 14 per cent iron; freight mine to smelter, $13 per ton of 2,000 pounds; insurance, $1.50 per ton; loading and expenses, $1 per ton; prime western spelter, 15 cents per pound; medium grade Joplin ore, 60 per cent base, $100 per ton.
48 per cent-8 per cent×2,000 pounds=800 pounds at 15 cents...
60 per cent-8 per cent=52 per centX2,000=1,040 at
Lead, 2,000 pounds X6 per cent=120 pounds at cent..
$41.60 110 per cent=$37. 82 dutiable value.
EXAMPLE: NONSULPHIDE ORE.
Assume: Assay 36 per cent zinc, 3 per cent lead, 2 per cent iron; freight, mine to smelter, $10 per ton of 2,000 pounds; export duty, $1; prime western spelter, 15 cents per pound; Joplin calamine, 40 per cent base, $50 per ton.
36 per cent-6 per cent=30 per cent×2,000 pounds-600 pounds at 15 cents. $90.00 Deductions:
Freight, mine to smelter...
Iron penalty, 2 per cent-1 per cent=1 per cent at $1... Treatment:
40 per cent-6 per cent=34 per cent X2,000 pounds=689 pounds at 15 cents...
Owing to the fact that the several factors necessary to determine the market value or purchase price of zinc ores are usually not known at the time of shipment, and importers can not ascertain the value of the zinc in the ore, collectors are authorized to permit entry by appraisement of zinc-bearing ores.
ANDREW J. PETERS,
These instructions were supplemented on June 29, 1916 (T. D. 36534) with administration directions on August 31, 1916 (T. D. 36652) the method set forth in T. D. 36446 was made inapplicable to sulphur ores assaying 40 per cent or less of zinc, the zinc in such ores to be appraised at not less than the contract or purchase price.
ZINC BURNING (IGNEOUS CONCENTRATION).
The manufacture of zinc oxide direct from the ore is a distinctly American achievement in the metallurgy of zinc. Hitherto, this has been done chiefly for the production of lead and zinc pigments and has been known as the Wetherill process. The principles of this process are, however, of wider application. They may be applied in either a blast or in a reverberatory furnace, and the final product,