The Paddock pneumatic ore separator is a jig, in which air is used instead of the fluid more commonly employed—water.
Like the water jig, it consists of an inclined sieve, over which the crushed ore is passed, and through which the air is blown into the ore. The air current is intermittent, and being proportioned in strength to the thickness of the ore bed, the latter is lifted bodily, and falls again upon the sieve a great number of times during its passage through the machine. By this treatment it is separated in layers according to the specific gravity of its constituents, the heavier minerals being collected on the sieve with the lighter over them. This separation is not difficult, but it has been found on trial that in this simple form the machine is by no means perfect. Mr. Paddock, who is a miner of copper ores, built a concentrating mill, where he gradually devised one improvement after another to meet the difficulties encountered.
Where there is a change in the size of the sand, or in the minerals which an ore contains, or even in the proportion of the minerals which compose it, the time during which the ore remains under treatment must be altered, or there is danger of mixing the separated minerals again.
Mr. Paddock provides three modes of preventing this remixture. One permits the shortening or lengthening of the ore bed, another changes the inclination of the sieve on which the ore rests, and the third gives a horizontal movement in opposite directions to the rich and poor mineral.
- The Adjustable Hopper.—The ore is delivered upon the sieve through a hopper, and flows down to the tailboard with as much steadiness as if it were fluid, the separator being continuous in its action. As it flows down fresh supplies run in from the hopper, and it is evident that the position of the point of supply governs the length of the bed and the time of treatment. Mr. Paddock therefore makes the hopper adjustable, so that it can be moved down from the extreme head about one-third the length of the machine.
- Inclination of the Bed.—The air-chamber under the ore bed is made with flexible sides, so that its inclination can be altered without disturbing the motive parts of the machine. This change can be made either lengthwise, by which the rate of flow of the ore is increased or diminished, or sidewise, in which case the discharge of the separated minerals is controlled in favor of the rich or poor mineral. The first of these alterations evidently produces results of the same kind as the adjustment of the hopper. The operation of the second will be understood from the following paragraph.
- The Discharge Grating.—The final discharge of the ore is over a tailboard at the end of the machine, where heads, middlings,, and tailings run off side by side, instead of one over the other as in wet jigs, and to produce the horizontal movement necessary to place them side by side an inclined grating is laid on the cloth which in air jigs forms the sieve. This grating is made in two parts. The lower one, made of narrow strips of sheet brass inclined to the path of the ore, directs the concentrated mineral to one side of the bed, where it collects in a trough and runs down to the discharge without danger of mixing again with the gangue. Upon these strips there is a similar grating made of broader strips, which are inclined in the contrary direction.
In its progress down the bed the ore is separated into two layers, according to the specific gravity of its constituents, and each of these layers is led to its own side of the machine, forming the heads and tailings. Between them at the tailboard a mixture of the two is obtained in small quantity, and this composes the middlings. The whole bed is slightly inclined toward one of the lower corners, that on the side of the heads, and the flexible sides of the air-chamber permit this inclination to be altered until just that slope is formed which gives the best separation. All these changes can be made without stopping the work, and in a moment.
All these modes of adjustment are shown in the accompanying figure, but the elastic diaphragm, which answers the purpose of a bellows, and the motive parts are both concealed by their coverings. The frame is mostly made of wood, the corner standards only being of iron.
If we examine the system of wet jigs in Europe we shall be struck by the fact that they seem to have been made to satisfy previous calculations of what would be needed, but a very different impression will be obtained from American works. In Europe there is little adjustability, except in speed; in America the favorite machines permit the most complete adaptation to the work which is momentarily in hand, and this without a moment’s stoppage. Something of the same kind seems to have been needed in dry jigging, and Mr. Paddock has supplied it.
The number and force of the air puffs are also varied at will. The “action” of the machine is placed on the floor in a tight box, where it is kept from dust. It consists of cams that operate lever arms, the length of which can be altered at will. Air has been used for many years as a means of separating heavy and light, or coarse and fine minerals, but the older efforts seem to have been confined to attempts to winnow the ore as grain is winnowed. The finely crushed ore was blown into a long chamber with the expectation that the light particles would be carried farther than the heavy, but after repeated trials this system has received general condemnation.
Mr. Thomas J. Chubb, in attempting some years ago to devise a method for treating the gold sands of California, made the first air jig, and his ideas have been the source of all the systems of air concentration which now exist in America. He went to California with a machine, but died there before he could procure its adoption in the mines.
Mr. Paddock purchased Mr. Chubb’s rights from the administrator of the estate, and after great trouble succeeded in finding the original Chubb machine in San Francisco. It had “good ideas,” as Heine’s tailor said of the old overcoat which the poet took him to be made into a bed-gown, but it proved to be deficient in several necessary points. The whole machine was carefully studied and improved by Mr. Paddock, who is an experienced mechanician, and the result is an air jig, which is,
1st. Adjustable to different rates of work with the same ore.
2d. Adjustable to different kinds and sizes of ore.
3d. Extremely rapid in action and thorough in work.
In presenting this system of concentrating ores to the notice of the Institute, it is almost necessary to consider the objections which have been made to the use of air as a means of separation.
One is that the ore must be dried artificially. In practice this has not been found to be true, but I cannot say that ores from unusually wet mines have been worked. The machines are in constant use, and no means are employed for drying the ore, which is worked just as it comes from the mine, and the mines at which the machine has been used demand the usual means of drainage.
Another is the production of dust. It is a cardinal principle of wet-ore dressing that the ore shall not be broken finer than is absolutely necessary to separate the rich mineral from the lean rock. This lessens the production of dust, which in treatment with water is always liable to float off. It is now not uncommon to find water jigs that treat ore of ¾ and 1 inch diameter.
But this precaution is impossible in air jigs, for the ore must be fine to be moved by the air puffs. Mr. Paddock has usually treated ore that had been crushed fine enough to pass through a 40 or a 60 mesh, but this fineness is not absolutely required by the machine, which has treated 1/16-inch material, and it is thought can treat ore of 1/12-inch size. It has also treated with success ore which passed through a mesh of 100 to the linear inch.
With ore of 40 and 60 size the amount of dust is about 15 to 17 per cent. This goes on the bed with the coarser ore, and being blown out by the air is drawn by a suction fan and thrown to a cloth-chamber, where the air is discharged and the powder collects. This powder is then washed upon a slime machine, and Mr. Paddock prefers that of Mr. Hooper, the able manager of the concentration at the Ticonderoga Graphite Works.
There is no doubt that the crushing for dry concentration produces more dust than the crushing for wet jigs, provided the work is done by rolls in both cases. But the amount of dust is not very great, and there are advantages in air jigs which must go far in any situation to counteract the disadvantage of the dust. These advantages are cheapness, quantity of product, and freedom from loss by float mineral. A plant suited to the treatment of 30 to 50 tons of ore daily can be built in the Eastern States, with all its breakers, rolls, sieves, and jigs, for about $10,000. One machine will treat from 1 1/3 to 1½ tons per hour, and three machines will treat 50 tons daily, and work up the middlings. The present cost of treatment is reported at 50 cents per ton, the work being done on the scale of more than 30 tons daily, and at such a cost it is evident that extremely poor ores can be made available.
This is the true test of mining. It is well known that the costs of work are greatly increased when the ground is “ pockety,” and all mines become pockety when none but ores of the highest grades can be taken out and treated. The conditions for safe and profitable mining almost necessarily include the economical removal and treatment of the middle grade ores. Frequently this can be done only by joining some system of concentration to the mine, and all countries contain innumerable situations where this can be done best, or done only by means of air.
The advantages of dry concentration do not depend upon a careful balance of what it will do in comparison with the wet treatment. There are great numbers of mines, both West and East, where water cannot be had in quantity, except at considerable and constant expense. Many mines are lying idle, or only worked for their richest ore, because no means of concentration, successful in itself, and adapted to their conditions, has been presented. In such a case there is no need of urging the claims of any system of concentration to be acknowledged the best in the world. The problem is to produce one that will work profitably in that situation. Wherever water sufficient for making steam can be had these machines will do good work on ores of iron, lead, and copper containing gold and silver, or not. I do not go further and place gold sands and other ores of the precious metals in the list, simply because the Separator has not yet been tried on them, but there is no reason known for excepting them from the list of minerals that can be profitably treated by this system.
Air Jig Notes
When this paper was read before the Institute I was asked by Professor Monroe for comparative analyses of the ore and tailings, but was unable to give them. Since then the results obtained at Blue Hill, Maine, where six Paddock separators are working on ores of copper and lead containing silver, have been supplied me. Mr. C. W. Kempton, who first brought this district to the notice of the Institute, wrote October 1st, 1879, as follows: “The dry machines separated the ore (galena and copper, with some iron) from the rock most completely, giving tailings assaying from 7/10 oz. per ton up to 1¼ oz. [Probably this refers to silver in the tailings.] I am of the opinion that nothing would prevent working even closer with sizing arranged especially for silver ore. We worked on ore which assayed only 4 ounces per ton with very profitable results, and found no more loss in higher grades. The No. 3 size machine took from 60 mesh to 100 mesh, and separated finely, but should use an intermediate machine, say 80 to 100, for steady silver work. After separating the rock from the ore we ran the ore through the dry machines a second time, separating the galena from the copper and iron most completely.” In working the slimes (passing through 100 mesh) the loss in tailings was found to average 2 ounces per ton, and Mr. Kempton thinks this can be reduced.
These results are sufficiently favorable to indicate that the important problem of thorough ore concentration by air has been solved. The cost of treatment at Blue Hill was reported by a New York Tribune correspondent at 50 cents per ton. The field for this method of concentration is not confined to that vast region in the West, where water is so scarce as to be sold for 10 and 20 cents a gallon. Even in New England there are many places where the conveyance of water to the mines in sufficient quantity would cost more than the mill machinery, and in such localities the engineer has hitherto been confined to a balance between the cost of transportation to the nearest water and the cost of an extensive water service. The introduction of dry concentration will permit the erection of the works at the mine’s mouth, where the ore will be delivered to the mill without adding any cost to the mining expenses.