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OREBRO

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Originally appearing in Volume V20, Page 242 of the 1911 Encyclopedia Britannica.
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OREBRO, a town of Sweden, capital of the district (lan) of Orebro, lying on both banks of the Svarta a mile above its entrance into Lake Hjelmar, 135 M. W. of Stockholm by rail. Pop. (190o), 22,013. In great part rebuilt since a fire in 1854, it has a modern appearance. An ancient castle, however, with four round towers, remains on an island in the stream. It is used as a museum. There may be mentioned also the church of St Nicholas, of the 13th century; and the King's House (Kungsstuga), an old and picturesque timber building. In front of the modern town hail stands a statue, by Karl Gustav Qvarnstrom (1810-1867), of the patriot Engelbrecht (d. 1436), who was born here. The Swedish reformers of the 16th century, Olaus and Laurentius Petri, are commemorated by an obelisk. Orebro is in close connexion with the iron-mining district of central Sweden; it has mechanical works and a technical college. A large trade is carried on, by way of the Orebro canal and lakes Hjelmar and Malar, with Stockholm. Orebro was in existence in the 11th century. Its castle, erected by Birger Jarl in the 13th century, played an important part in the early annals of Sweden; and no fewer than twenty diets or important assemblies were held either in the castle or in the town. Such were the Orebro cancilium of 1537, the diet of 1540 in which the crown was declared hereditary, and that of 1810 when Bernadotte was elected crown prince. ORE-DRESSING, one of the principal processes in the work of mining (q.v.). When the miner hoists his ore' to the surface, the contained metal may be either in the native uncombined state, as, for example, native gold, native silver, native copper, or combined with other substances forming minerals of more or less complex composition, as, for example, telluride of gold, sulphide of silver, sulphide of copper. In both cases the valuable mineral is always associated with minerals of no value. The province of the ore-dresser is to separate the " values " from the waste—for example, quartz, felspar, calcite—by mechanical means, obtaining thereby " concentrates " and " tailings." The province of the metallurgist is to extract the pure metal from the concentrates by chemical means, with or without the aid of heat. There are also a number of non-metallic minerals which do not have any value, or at best do not reach their highest value until they have been subjected to some form of mechanical preparation; among them are diamonds, graphite, corundum, garnet, asbestos and coal. Ore-dressing, for the purposes of this article, may be divided into three parts: (I) properties of minerals which render aid in their separation; (2) simple opera- tions; (3) operations combined to form processes or mills. r. The specific gravity of minerals varies greatly, some being heavy, others light. The rate of settling in water is affected by the specific gravity in this way: of two particles of the same properties. size but different specific gravity, the heavier settles more rapidly than the lighter, while of two particles of different specific . gravity which settle at the same rate in water, that of higher specific gravity is of smaller diameter than the other. The same statements are true in regard to settling in air, and in regard to momentum in air when the particles are thrown out in a horizontal direction. Colour, lustre and fracture are of especial value in hand-picking, to aid the eye in selecting the mineral sought. Instances are, of colours, the white of quartz, the pale straw colour of felspar, the dull yellow of limonite, the brass yellow of chalcopyrite, the pale metallic yellow of pyrite; of lustres, the vitreous of quartz, the adamantine of diamond and cerussite, the resinous of blende, the earthy of limonite, and the metallic of pyrite; and of fractures, the cleavage planes of felspar and galena, the conchoidal fracture of quartz and pyrite, the granular of some forms of magnetite and blende. Magnetism is a most direct and simple method of separating minerals where it is available. The discovery that by the use of electro-magnets of great 1 The O. Eng. word was Ora, corresponding with Du. oer, the origin of which is unknown. The form " ore " represents the O.Eng. de, brass; cf. Lat. aes, Skt. ayas. power minerals formerly regarded as non-magnetic are attracted, has made it possible to separate several classes of minerals present in an ore; for example, the strongly magnetic mineral may first be taken out, then the mildly magnetic, and last the weakly magnetic, the non-magnetic being left behind. Adhesion acts when brightly burnished particles of gold issuing with the sand from the stamp mill come in contact with an amalgamated copper plate, for they are instantly plated with mercury and adhere to the copper, while the sand is carried forward by the water. In this way a very perfect separation of the gold from the sand is effected. In the South African diamond fields it has been found that if the diamond-bearing sand is taken in a stream of water over a smooth surface covered with a suitable coating of grease, the diamonds will adhere to the grease while the sand does not. 2. The concentration of ores always proceeds by steps or stages. Thus the ore must be crushed before the minerals can be separated, Slmpie and certain preliminary steps, such as sizing and classify-e Operations. must concentr tes. fin operati n more mpwhich ortatof produce simple operations will now be described. The ore as mined contains the valuable minerals attached to and enclosed in lumps of waste rock. The province of crushing or dis- integrating is to sever or unlock the values from the waste, so that the methods of separation are then able to part the one from the other. In crushing ores it is found wise to progress by stages, coarse crushing being best done by one class of machine, medium by another, and fine by a third. Coarse crushing is accomplished by breakers of the Blake type (fig. I) or of a, Movable jaw. 2). All of these machines b, Fixed jaw. break by direct pressure, caused by a movable jaw, a (figs. I, 2), approaching towards and receding from a fixed jaw, b. The largest size ever fed to a breaker is 24 in. in diameter, and the smallest size to which the finest crushing commonly done by these machines brings the ore is about s in. diameter. The machine is generally supplied with ore in lumps not larger than 9 in. in diameter, and crushes them to about 11 in. in diameter. Medium-size crushing is done mostly by rolls or steam stamps. Rolls (fig. 3) crush by direct pressure caused by the ore being drawn between two revolving rolls held closely together. They make the least fine slimes or fines to be lost in the subsequent treatment, and are therefore preferred for all brittle minerals. The steam stamp works upon the same principle as a steam hammer, the pestle being forced down by steam pressure acting through piston and cylinder with great crushing force in the mortar. Steam stamps have been very successful with the native Q copper rock, because they break up the little leaves, flakes and filaments of copper, and render them susceptible of concentration, B2.—G. gravity stamps, pneumatic stamps, by cen- trifugal roller mills, by amalgamating pans, a, Movable jaw. by ball mills, by Chile edgestone mills, by b, Fixed jaw. tube mills and by arrastras. The gravity c, Gear with eccen- stamp (fig. 4) is a pestle of goo-lb weight tric hub and with more or less, which is lifted by a revolving loose fit on the cam and falls by the force of gravity to spindle. strike a heavy blow on the ore resting on the die in the mortar and do the work of crushing; the frequent revolution of the cam gives a more or less rapid succession of blows. Gravity stamps are especially adapted to the fine crushing of gold ores, which they reduce to 4f-in. and sometimes even to -1s-in. grains. The blow of the stamp upon the fragments of quartz not only liberates the fine particles of gold, but brightens them so that they are quickly caught upon the amalgamatedplates. The centrifugal roller mills are suited to fine crushing of middle products, namely by-products composed of grains con- taining both values and waste, since they avoid making much fine slimes. They crush by the action of a roller, rolling on the inside of a steel ring, both having vertical axes. The amalgamating pan is suitable for grinding silver ores for amalgamation where the finest grinding is sought, together with the chemical action from the contact with iron. It crushes by a true grinding action of one surface sliding upon another. The Chile edgestone mill is employed for the finest grinding ever used preparatory to concentration. The arrastra or drag-stone mill grinds still finer for amalgamating. The ball millis a horizontal revolving cylinder with iron balls in it which do the grinding; the pulverized ore passes out through screens in the cylinder wall. It is a fine grinder, making a small amount of impalpable slimes. It is used for preparation for concentrating. The tube mill is of similar construction, but it is fed through the hollow shaft at one end and discharged through the hollow shaft at the other; the finely ground ore is floated out by water and contains a large proportion of impalpable slimes. It is used for preparation for cyaniding of gold. A considerable class of workable minerals, among which are surface ores of iron and surface phosphates, contain worthless clay mixed with the valuable material, the removal of which is accomplished by the log washer. This is a disintegrator • consisting of a long narrow cylinder revolving in a trough which is nearly horizontal. Upon the cylinder are knives or paddles set at an angle, which serve the double purpose of bruising and disintegrating the clay and of conveying the cleaned lump ore to be discharged at the upper end of the trough, the water meanwhile washing away the clay at the lower end. Roasting for Friability.—When two minerals—for example, pyrite and cassiterite (tin ore)—one of which is decomposed and rendered porous and friable by heat and oxygen—are roasted in a furnace, the pyrite becomes porous oxide of iron, while the cassiterite is not changed. A gentle crushing and washing operation will then break and float away the lighter iron oxide, leaving the cleaned cassiterite behind. Sizing.—This is the first of the preliminary operations of separation. It is found useful in concentration, for dividing an ore into a number of portions graded FIG. 4.—Gravity Stamp. from coarser sizes down to finer sizes. Each portion is made suitable for treatment on its respective machine. If crushed ore be sifted upon a screen with holes of definite size, two products will result—the oversize, which is unable to pass through the screen, and the undersize, which does pass. If the latter size be sifted upon another screen with smaller holes, it will again make oversize and undersize. The operation can be repeated with more sieves until the desired number of portions is obtained. P. von Rittinger adopted for close sizing the following diameters in millimetres for the holes in a set of screens: 64, 45.2, 32, 22.6, 16, II.3, 8, 5.6, 4, 2.8, 2, I.4, I. Each of these holes has an area double that of the one next below it; this may be called the screen ratio. A process which does not need such close sizing might use every other screen of the above set, and in extreme cases even every fourth screen. In mills the screen ratio for coarse sizes often differs from that for fine. Sizing is done by cylindrical screens revolving upon their inclined axes (fig. 5), by flat shaking screens, and by fixed screens with a comparatively steel slope. Either wire cloth with square holes or steel plate punched with round holes is used. To remove the largest lumps in the preliminary sizing, fixed-bar screens (grizzlies) are preferred, on account of their strength and durability. Sizes smaller than can be satisfactorily graded by screens are treated by means of hydraulic classifiers and box classifiers. The lower limit of screening and therefore the beginning of this work varies from grains of 5 millimetres to grains of i millimetre in diameter. A hydraulic classifier (fig. 6) is a trough-like washer through which the water and sand flow from one end to the other. In the bottom, at regular intervals, are pockets or pits with hydraulic devices which hinder the outflowing discharge of sand, b, by an inflowing stream of clear water, a. By regulating the speed of these water currents, the size of the grains in the several discharges can be regulated, the first being the coarsest and the overflow at the end the finest. Box classifiers (spitzkasten) are similar, except that the pockets are much larger and no inflowing clear water is used ; they therefore do their work much less perfectly. Classifiers do not truly size the ore, but merely class together grains-which have equal settling power. In any given product, except the first, the grain of high specific gravity will always be smaller than that of low. The Rolls. 240 box classifiers are suited to treating finer sizes than the hydraulic classifiers, and therefore follow them in the mill treatment. Picking Floors, the first of the final operations of separation, are areas on which men, boys or girls pick out the valuable mineral which is rich enough to ship at once to the smelter. The picking is often accompanied and aided by breaking with a hammer. Picking tables are generally so constructed that the pickers can sit still and have the ore pass before them on a moving surface, such as a revolving circular table or travelling belt. Stationary picking tables require the ore to be wheeled to and dumped in front of the pickers. Picking out the values by hand has the double advantage that it saves the power and time of crushing, and prevents the formation of a good deal of fine slimes which are difficult to save. Jigs treat ores ranging from 11 in. in diameter down to slu in. If an intermittently pulsating current of water is passed up through a horizontal sieve on which is a bed of ore, the heavy mineral and the quartz quickly form layers, the former beneath the latter. The machine by which this work is done is called a jig, and the operation is called jigging. In the hand jig the sieve is moved up and down in a tank of water to get the desired separation. In the power jig (fig. 7) i .' the sieve, a, is stationary Z _ _ (Ai and the pulsating current c.J is obtained by placing a ~/\~/ vertical longitudinal part tition, c, extending part of the way down to the 0 bottom of the jig box. fastened on one side of the partition, and on the other a piston or plunger, d, is moved rapidly up and down by an eccentric, causing an up-and-down current of water through the sieve, a. The sieve is fed at one end, e, with a, constant supply of water and ore, and the quartz over-flows at the other. Clear water (" hydraulic water") is brought by the pipe, i, into the space, g, below called the hutch, to regulate the condition of the bed of ore on a. The constantly accumulating bed of concentrates is either discharged through the sieve into the hutch, g, or by some special device at the side. On jigs where the concentrates pass through the sieve, a bed of heavy mineral grains too large to pass holds back the lighter quartz. The quartz overflow from one sieve, a, generally carries too much value to be thrown away, and it is therefore jigged again upon a second sieve, b. In jigging difficult ores, three, four, five and even six sieves are used. A succession of sieves gives a set of products graded both in kind and in richness, the heavier mineral, as galena, coming first, the lighter, as pyrites and blende, coming later. The best jigging is done upon closely sized products using a large amount of clear water added beneath the sieve. Very good jigging may, however, be done upon the products of hydraulic classifiers, where the heavy mineral is in small grains and the quartz is large, by using a bed on the sieve and diminished hydraulic water, which increases the suction or downward pull by the returning plunger. Bumping Tables.—Rittinger's table is a rectangular gently sloping plane surface which by a bumping motion throws the heavy particles to one side while the current of water washes down the quartz to another, a wedge-shaped divider separating and guiding the concentrates and tailings into their re-€-spective hoppers. The capacity on pulp of to 326 in. size is some 4 tons in twenty-four hours. In the Wilfley table (fig. 8) and those derived from it a gentler vanning motion is substituted for the harsh bump; they have a greatly increased width and a set of riffle blocks, b, at right angles to the direction of flow, c, tapering in height towards the side where the concentrates are discharged, d. This combination has produced a table of great efficiency and capacity for treating grains from * in. in diameter down to 21-,g in. or even finer. The capacity on 110. in. pulp is from 15 to 25 tons in twenty-four hours. Vanners are machines which treat ores on endless belts, generally of rubber with flanges on the two sides. The belt (fig. 9) travels up a gentle slope, a, on horizontal transverse rollers, and is shaken about 200 times a minute, either sidewise or endwise, to the extent of about 1 in. The lower to ft. is called the concentrating plane, b, and slopes 2.78% more or less from the horizontal; the upper 2 ft. of length is called the cleaning plane, c, and slopes 4.45 % more or less. The fine ore is fed on with water (technically called pulp) at the intersection of the two planes, d. The vibration separates the ore into layers, the heavy minerals beneath and the light above. The downward flow of the water carries the light waste off and discharges it over the tail roller e into the waste launder, while the upward travel of the belt carries up the heavy mineral. On the cleaning plane the latter passes under a row of jets, f, of clean water, which remove thelast of the waste rock; it clings to the belt while it passes over the head roller, and only leaves it when the belt is forced by the dipping roller to dip in the water of the concentrates tank, g. The cleaned belt then continues its return journey over the guide roller h to the tail roller e, which it passes round, and again does concentration f s ~-duty. Experience proves that for exceedingly fine ores the end shake with steep slope and rapid travel does better work than the side-shake vanner. For ordinary gold stamp-mill pulp, where clean- ness of tailings is the most important end, and where to gain it the engineer is willing to throw a little quartz into the concentrates, the end-shake vanner is again probably a little better than the side-shake, but where cleanness of concentrates is sought the side-shake vanner is the most satisfactory. The latter is much the most usual form. Slime-Tables are circular revolving tables (fig. to) with flattened conical surfaces, and a slope of 1J in. more or less per foot from centre to circumference; a common size is 17 ft. in diameter, and a common speed one revolution per minute. These tables treat material of ii Din. and less in diameter coming from box classifiers. The principle on which the table works is that the film of water upon the smooth surface rolls the larger grains (quartz) towards the margin of the table faster than the smaller grains (heavy mineral) which are in the slow-moving bottom current. The revolution of the table then discharges the quartz earlier at a, a, a, a, an intermediate middling product next at b, and the heavy mineral last at c. Suitable launders or troughs and catch-boxes are supplied for the three products. The capacity of such a table is 12 tons or more of pulp, dry weight, in twenty-four hours. Frames, used in concentrating tin ore in Cornwall, are rectangular slime-tables which separate the waste from the concentrates on the same principle as the circular tables, though they n run intermittently. They treat very fine pulp, and after being fed for a short period (about fifteen minutes) the pulp is shut off, the concentrates are flushed off with a douche of water and caught in a box, and the feed pulp is again turned on. Canvas tables are rectangular tables with plane surfaces covered with cotton duck (canvas) free from seams; they slope about 11 in. to the foot. They are fed with stamp-mill pulp, with the tailings of vanners, or, best of all, with very fine pulp overflowing from a fine classifier. The rough surface of the duck is such an efficient catching surface that they can run for an hour before the concentrates are removed—an operation which is effected by shutting off the. feed pulp, rinsing the surface with a little clean water, and hosing or brooming off the concentrates into a catch-box. The feed-pulp is then again turned on and the work resumed. They have been more successful than any other machine in treating the finest pulp, especially when their concentrates are finally cleaned on a steep slope end-shake vanner (the G. G. Gates canvas table system of California). Buddles act in principle like slime-tables, but they are stationary, and they allow the sand to build itself up upon the conical surface, which is surrounded by a retaining wall. When charged, the tailings are shovelled from the outer part of the circle, the middlings from the intervening annular part, and the concentrates from the inner part. They treat somewhat coarser sizes than the slime-table. The term buddle is sometimes applied to the slime-tables, but the majority confine the phrase to the machine on which the sand builds up in a deep layer. Riffles.—When wooden blocks or cobble-stones of uniform size are placed in the bottom of a sluice, the spaces between them are called riffles; and when gold-bearing gravel is carried through the sluice by a current of water, a great many eddies are produced, in which the gold and other heavy minerals settle. Kieves.—The kieve or dolly-tub is a tub as large or larger than an ordinary oil-barrel, with sides flaring slightly upwards all the way from the bottom. In the centre is a little vertical shaft, with hand-crank at the top and stirring blades like those of a propeller at the bottom. Fine concentrates from buddles or slime-tables are still further enriched by treatment in the kieve. The kieve is filled perhaps half full of water, and the paddles set in motion; concentrates are now shovelled in until it is nearly full, the rotation is continued a little longer and then the shaft is quickly withdrawn and the side of the kieve steadily thumped by a bumping-bar as long as settling continues. When this is completed, the water is siphoned off, the top sand skimmed off and sent back to the buddle, and the enriched bottom shovelled out and sent to the smelter. 3. In designing concentration works, the millwright seeks so to combine the various methods of coarse and fine crushing and of Combined preliminary and final concentration that he will obtain the operations. maximum return from the ore with the minimum cost. Some of the more important of these mill schemes will now be described. The hand-jig process used for the zinc and lead ores of Missouri is first to clean the ore from adhering clay by raking it back and forth in a sluice with a running stream of water, and then shovel it upon a sloping screen with holes of about i in., where it yields oversize and undersize. The former is hand-picked into lead ore, zinc ore and waste, while the latter is jigged upon a hand-jig and yields several layers of minerals removed by a hand-skimmer. The top skimmings are waste, the middle skimmings come back with the next charge to be jigged over, and the bottom skimmings go to a second jig with finer screen. The coarsest of the hutch product, i.e. the product which passed through the sieve and settled at the bottom of the tank, goes to the second jig, the finest is sold to a sludge mill to be finished on buddies. The second jig makes top skimmings which are sent back to the first jig, middle skimmings which are zinc concentrates, and bottom skimmings and hutch, which are lead concentrates. In the Missouri zinc-concentrating mill the ore carrying blende and calamine with a little galena is in very large crystallizations and contains, when crushed, very little in the way of included grains. It is crushed by Blake breaker and rolls, to pass through a sieve with holes a in. in diameter, and is then treated on a power jig with six consecutive sieves, yielding discharge and hutch products from each sieve, and tailings to waste. The earlier discharges are finished products, while the later are re-crushed and re-treated on the same jig. The hutch products are treated on a finishing-jig with five sieves, and yield galena from the first discharge and hutch, and zinc ore from the others. The capacity of such jigs is very large, even to 75 or 100 tons per day of ten hours. In the diamond washing of Kimberley, South Africa, the material taken from the mine is weathered by exposure to the air and rain for several months, and the softening and disintegration thus well started are completed by stirring in vats with water. Breaker and rolls were tried in order to hasten the process, but the larger diamonds were broken and ruined thereby. The material from the vats is screened and jigged, and of the jig concentrates containing about 2% of diamonds the coarser are hand-picked and the finer are treated on a greased surface. Lead and copper ores contain their values in brittle minerals, and are concentrated in mills which vary somewhat according to local conditions; the one here outlined is typical of the class. The ore is crushed by breaker and rolls, and separated into a series of products diminishing in size by a set of screens, hydraulic classifier and box classifier. All the products of screens and hydraulic classifiers are jigged on separate jigs yielding concentrates, middlings and tailings; those of the box classifier are treated on the slime-table, vanner or Wilfley table, yielding concentrates and tailings and perhaps middlings. The coarser middlings contain values attached to grains of quartz and are therefore sent back to be re-crushed and re-treated. The finer middlings contain values difficult to save from their shape only, and are sent back to the same machine or to another to be finished. The native copper rock of Lake Superior is broken by powerful breakers, sometimes preceded by a heavy drop-hammer weighing a ton, more or less. The operation is accompanied by hand-picking, yielding rich nuggets with perhaps 75% of copper ready for the smelter; at some mines a second grade is also picked out which goes to a steam finishing-hammer and yields cleaned mass copper for the smelter and rich stamp stuff. The run of rock which passes by the hand-pickers is of a size that will pass through a bar screen with bars 3 in. apart, and goes to the steam stamps. The stamp crushes the rock and discharges coarse copper through a pipe 4 in. in diameter, in which it descends against a rising stream of water which lifts out the lighter rock. The copper is let out about once an hour by opening a gate at the bottom. The rest of the rock is crushed to pass through a screen with round holes 4 in. in diameter, more or less. This sand is treated in hydraulic classifiers with four pockets, the products from the pockets being jigged by four roughing-jigs yielding finished mineral copper for the smelter, included grains for the grinder, partially concentrated products for the finishing-jigs, and tailings which go to waste. The overflow of the hydraulic classifier runs to a tank of which the overflow is sent to waste in order to diminish the quantity of water, while the discharge from beneath, treated upon slime-tables, yields concentrates, middlings and tailings. The middlings are re-treated. All the finished concentrates put together will assay from 6o to 8o% of copper according to circumstances. The extraction from the rock is from 50 to 8o° of the copper contained in it. Cornwall Tin.—Tinstone in Cornwall occurs associated with sulphides, wolfram, quartz, felspar, slate, &c., and is broken by spalling-hammers to 3-in. lumps. Hammers make less slimes thanthe rock-breakers, and they also break the ore more advantageously for the hand-picking. The latter rejects waste, removes as far as possible the hurtful wolfram, and classes the values into groups according to richness. Gravity or pneumatic stamps then crush the ore to h in., and stripes (a species of long rectangular buddle) yield heads, middlings, tailings and fine slimes: the first three are sent separately to circular buddies, and the last to frames. The buddles yield concentrates, middlings and tailings: the middlings are re-treated, the tailings are all waste; the concentrates are still further enriched by kieves, which yield tops to the buddle again and bottoms shipped to the smelter. The fine slimes are treated on frames, the concentrates of which go to buddies; of these the concentrates go to kieves. The Missouri zinc-lead sludge mill takes the finest part of the hutch product of the hand-jigs. The treatment begins on revolving screens with two sizes of holes, 25 mm. and 1 mm.: these take out two coarser sizes, of which the coarser is waste and the other is jigged, yielding concentrates and waste. The main treatment begins with the finest size, which is much the largest product. It is fed to a convex circular buddle (first huddle), and yields a coarser product at the outer part of the circle and a finer product in the inner. The finer product is treated by a series of buddlings which vary somewhat, but in general are as follows: fed to a second buddle it yields zinc and lead ore in the centre, next zinc ore, next middlings which come back, and, outside of all, tailings. The zinc-lead ore is set on one side until enough has accumulated to make a buddle run, when it is run upon a third buddle yielding in the central part pure lead concentrates, next lead ore (which is returned to this treatment), next zinc ore, and outside of all a zinc product which is fed to the second buddle. The coarse outside product of the first buddle is treated in much the same way as the fine, but it yields practically no lead zinc product, which simplifies the series of buddlings necessary. Gold Mill.—Gold ores usually contain their value in two conditions—the free gold, which can be taken out by mercury, and the combined gold, in which the metal is either coated with or combined with compounds of sulphur, tellurium, &c. The usual gold-milling scheme is to crush the ore by rock-breaker to about ii in. diameter, and then to crush with water by gravity stamps, a little mercury being added to the mortar from time to time,to begin the amalgamation at the first moment the gold is liberated. The pulp leaves the mortar through a screen with holes or slots sra to & in, in width, and is then passed over amalgamated plates of copper or silver-plated copper. The free gold, amalgamated by the mercury, adheres to the mercurial surface on the plate; the rest of the pulp flows on through mercury traps to catch any of the mercury, which drains off the end of the plate. The plates and mortar are periodic-ally cleaned up, the plates being scraped to recover the amalgam and leave them in good condition to do their work: if plates are used inside the mortar, they are cleaned in the same way. The residue of partly crushed ore in the mortar, with amalgam 'and free mercury scattered through it, is ground for a time in a ball mill, panned to recover the amalgam, and returned to the mortar. The pulp flowing away from the mercury traps flows to a Frue vanner or Wilfley table, on which it yields concentrates for the chlorination plant or smelter and tailings: these are waste when the heavy mineral is of low grade, but if the vanner concentrates are of high grade, they still contain values in very fine sizes which can and should be saved. Recent improvements in California for saving this material have been made. The vanner tailings are sent to a fine classifier, from which the light overflow only is saved; this is treated upon canvas tables yielding concentrates and tailings, and these concentrates, treated upon a little end-shake vanner with steep slope and rapid travel, give clean, very fine, high-grade concentrates for the chlorination works. Iron Ores.—The brown ores of iron from surface deposits are contaminated with a considerable amount cf clay and some quartz. The crude ore from surface pits or shallow underground workings is treated in a log-washer and yields the fine clay, which runs to waste, and the coarse material which is caught upon a screen and hand-picked, to free it from the little quartz, or jigged if it contains too much quartz. The magnetic oxide of iron occurs associated with felspar and quartz, and can often be separated from them by the magnet. The ore, after being broken by breaker and rolls to a size varying from s to Ttg of an inch in diameter, goes to a magnetic machine which yields (1) the strongly magnetic, (2) the weakly magnetic, and (3) the non-magnetic portions. The second or middlings product contains grains of magnetite attached to quartz, and is therefore re-crushed and sent back to the magnets; the strongly magnetic portion is shipped to the furnace; and the waste to the dump heap. In concentrating by water certain zinc sulphides, siderite (carbonate of iron) follows the zinc, and would seriously injure the furnace work. By a carefully adjusted roasting of the product in a furnace the siderite is converted into magnetic oxide of iron, and can then be separated by magnet from the zinc ore. A special magnet of very high power, known from its inventor as the Wetherill magnet, has been designed for treating the franklinite of New Jersey, a mineral which is non-magnetic in the usual machines. The ore, crushed by breaker and rolls and hand-picked to remove garnet, is treated upon a belt with a roughing magnet to take out the most magnetic portion, and then very closely sized by screens with '6, 24, 30 and 5o meshes per linear inch. The several products are treated each on its own magnetic machine, yielding the franklinite for the zinc oxide grates, and followed by spiegel furnace; the residue, which is jigged, yields the zinc silicate and oxide for the spelter furnaces, and waste carrying the calcite, quartz and mica. Asbestos, when of good quality, is in compact masses, which by suitable bruising and beating are resolved into fine flexible fibres. The Canadian asbestos is associated with serpentine, and is crushed by breakers to a in., screened on A-in. screens to reject fines. The values are removed by hand-picking and are crushed by rolls carefully set so as not to break the fibre; this product is then sized by screens and the various sizes are sent to the Cyclone pulverizer, which by beating liberates the individual fibres. It then goes to a screen with eleven holes to the linear inch, and yields a granular undersize and oversize, and a fibrous oversize which is drawn off by a suction fan to a settling-chamber with air outlets covered by fine screen cloth. This fibrous product is the clean mineral for the market. A special treatment separates the fibres of different lengths. The usual method of dressing corundum and emery, after the preliminary breaking, is to treat the material in an edge-stone mill fitted with light wooden rollers. The action is that of grinding one particle against another, whereby the talc, chlorite, mica, &c., are worn off from the harder mineral. A constant current of water carries off the light impurities. This is called the ",muller " process. At Corundum Hill, North Carolina, the first step in removing the impurities from "sand"corundum is to subject it to the scouring action of a stream of water while it is being sluiced from the mine to the mill, the action being increased by several vertical drops of 5 to 10 ft. in the sluice. After reaching the mill all that will not pass through a 14-mesh screen is crushed by rolls, and the undersize of the screen is treated in a washing trough; this removes part of the light waste, and the " mullers " mentioned above complete the cleaning. Graphite occurs in schist, but being of less specific gravity than the other minerals which enter into the composition of the schist, it settles later than they do. It also breaks into thin scales, which reduces its settling rate still further. The ore is broken by breakers, and by Chile edge-stone mills or by gravity stamps, to a size varying with the character of the minerals from perhaps 131 to iu in. diameter. The pulp is then conveyed through a series of settling tanks of which the later are larger than the earlier. The quartz and other waste minerals settle in the earlier tanks, while the graphite settles later: the latest tank gives the best graphite. In the Dixon Company's works in New York some forms of concentrators are believed to have replaced the slower settling tanks. The phosphates of Florida are of four kinds: hard rock, soft rock, land pebble and river pebble. The hard rock is crushed by toothed rolls, and cleaned in log washers. The washed product is screened; the sizes finer than ,'s in. are thrown away because too poor; the other sizes are dried and sold, some waste having been picked out of the coarsest. The soft rock is simply dried, ground and sold. Land pebble is treated by log washers, any clay balls remaining being re-moved by a screen, and the phosphate dried and sold. In special cases land pebble is treated by hydraulicking, followed by a log washer, and this again by a powerful jet washer, to remove the last of the clay. River pebble is taken from the river by centrifugal pumps, and screened on two screens with 1-in. and A-in. holes respectively; the oversize of the first sieve and the undersize of the second sieve are thrown away because of too low grade. (R. H. R.)
End of Article: OREBRO
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