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OREBRO , a See also:town of See also:Sweden, See also:capital of the See also:district (lan) of
Orebro, lying on both See also:banks of the Svarta a mile above its entrance into See also:Lake Hjelmar, 135 M. W. of See also:Stockholm by See also:rail. Pop. (190o), 22,013. In See also:great See also:part rebuilt since a See also:fire in 1854, it has a See also:modern See also:appearance. An See also:ancient See also:castle, however, with four See also:round towers, remains on an See also:island in the stream. It is used as a museum. There may be mentioned also the See also: A large See also:trade is carried on, by way of the Orebro See also:canal and lakes Hjelmar and See also:Malar, with Stockholm. Orebro was in existence in the 11th century. Its castle, erected by See also:Birger Jarl in the 13th century, played an important part in the See also:early See also: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 See also:diet of 1540 in which the See also:crown was declared hereditary, and that of 1810 when Bernadotte was elected crown See also:prince. ORE-DRESSING, one of the See also:principal processes in the See also:work of mining (q.v.). When the miner hoists his ore' to the See also:surface, the contained See also:metal may be either in the native uncombined See also:state, as, for example, native See also:gold, native See also:silver, native See also:copper, or combined with other substances forming minerals of more or less complex See also:composition, as, for example, telluride of gold, sulphide of silver, sulphide of copper. In both cases the valuable See also:mineral is always associated with minerals of no value. The See also:province of the ore-See also:dresser is to See also:separate the " values " from the See also:waste—for example, See also:quartz, See also:felspar, See also:calcite—by mechanical means, obtaining thereby " concentrates " and " tailings." The province of the metallurgist is to See also:extract the pure metal from the concentrates by chemical means, with or without the aid of See also: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 See also:form of mechanical preparation; among them are diamonds, See also:graphite, See also:corundum, See also:garnet, See also:asbestos and See also:coal. Ore-dressing, for the purposes of this See also:article, may be divided into three parts: (I) properties of minerals which render aid in their separation; (2) See also:simple See also:opera- tions; (3) operations combined to form processes or See also:mills. r. The specific gravity of minerals varies greatly, some being heavy, others See also:light. The See also:rate of settling in See also:water is affected by the specific gravity in this way: of two particles of the same properties. See also:size but different specific gravity, the heavier settles more
rapidly than the lighter, while of two particles of different specific . gravity which See also:settle at the same rate in water, that of higher specific gravity is of smaller See also:diameter than the other. The same statements are true in regard to settling in See also:air, and in regard to momentum in air when the particles are thrown out in a See also:horizontal direction. See also:Colour, lustre and fracture are of especial value in See also:hand-picking, to aid the See also:eye in selecting the mineral sought. Instances are, of See also:colours, the See also: In the See also:South See also:African diamond See also: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 See also:mined contains the valuable minerals attached to and enclosed in lumps of waste See also: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 See also:wise to progress by stages, coarse crushing being best done by one class of See also:machine, See also:medium by another, and See also:fine by a third. Coarse crushing is accomplished by breakers of the See also:Blake type (fig. I) or of a, Movable See also:jaw. 2). All of these See also:machines b, Fixed jaw. break by direct pressure, caused by a movable jaw, a (See also: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 See also:steam stamps. Rolls (fig. 3) crush by direct pressure caused by the ore being See also: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 See also:hammer, the pestle being forced down by steam pressure acting through See also:piston and See also:cylinder with great crushing force in the See also: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 See also:roller mills, by amalgamating pans, a, Movable jaw. by See also:ball mills, by See also:Chile edgestone mills, by b, Fixed jaw. See also:tube mills and by arrastras. The gravity c, See also:Gear with eccen- stamp (fig. 4) is a pestle of goo-lb See also:weight tric hub and with more or less, which is lifted by a revolving loose See also:fit on the See also:cam and falls by the force of gravity to spindle. strike a heavy See also:blow on the ore resting on the See also:die in the mortar and do the work of crushing; the frequent revolution of the cam gives a more or less rapid See also: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 See also:middle products,
namely by-products composed of grains See also:con-
taining both values and waste, since they
avoid making much fine slimes. They crush
by the See also:action of a roller, See also:rolling on the inside of a See also:steel See also:ring, both having See also:vertical axes. The amalgamating See also: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 See also:drag-See also: A considerable class of workable minerals, among which are surface ores of iron and surface See also:phosphates, contain worthless See also:clay mixed with the valuable material, the removal of which is accomplished by the See also:log washer. This is a disintegrator • consisting of a See also:long narrow cylinder revolving in a trough which is nearly horizontal. Upon the cylinder are knives or paddles set at an See also:angle, which serve the See also: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 See also:lower end. Roasting for Friability.—When two minerals—for example, pyrite and cassiterite (See also:tin ore)—one of which is decomposed and rendered porous and friable by heat and See also:oxygen—are roasted in a See also:furnace, the pyrite becomes porous See also:oxide of iron, while the cassiterite is not changed. A See also:gentle crushing and washing operation will then break and See also: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 See also: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 See also: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 See also:area double that of the one next below it; this may be called the screen ratio. A See also:process which does not need such close sizing might use every other screen of the above set, and in extreme cases even every See also: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 See also:flat shaking screens, and by fixed screens with a comparatively steel slope. Either See also:wire See also:cloth with square holes or steel plate punched with round holes is used. To remove the largest lumps in the preliminary sizing, fixed-See also:bar screens (grizzlies) are preferred, on See also:account of their strength and durability. Sizes smaller than can be satisfactorily graded by screens are treated by means of See also:hydraulic classifiers and See also: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 See also:regular intervals, are pockets or pits with hydraulic devices which hinder the outflowing See also:discharge of sand, b, by an inflowing stream of clear water, a. By regulating the See also: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 See also:grain of high specific gravity will always be smaller than that of See also: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 See also:rich enough to See also: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 See also: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 See also:advantage that it saves the power and See also:time of crushing, and prevents the formation of a See also:good See also:deal of fine slimes which are difficult to See also: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 See also:sieve on which is a See also: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 See also: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 See also:longitudinal part tition, c, extending part of the way down to the 0 bottom of the jig box. fastened on one See also:side of the See also:partition, and on the other a piston or plunger, d, is moved rapidly up and down by an See also:eccentric, causing an up-and-down current of water through the sieve, a. The sieve is fed at one end, e, with a, See also:constant See also:supply of water and ore, and the quartz over-flows at the other. Clear water (" hydraulic water") is brought by the See also:pipe, i, into the space, g, below called the hutch, to regulate the See also: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 See also:special See also: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 See also:kind and in richness, the heavier mineral, as galena, coming first, the lighter, as See also: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 See also:plane surface which by a bumping See also:motion throws the heavy particles to one side while the current of water washes down the quartz to another, a See also: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 See also: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 See also: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 See also: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 See also: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 See also:row of jets, f, of clean water, which remove thelast of the waste rock; it clings to the belt while it passes over the See also:head roller, and only leaves it when the belt is forced by the dipping roller to See also:dip in the water of the concentrates tank, g. The cleaned belt then continues its return See also:journey over the See also:guide roller h to the tail roller e, which it passes round, and again does concentration f s ~-See also: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 See also: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 See also:foot from centre to circumference; a See also: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 See also: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 See also:short See also: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. See also:Canvas tables are rectangular tables with plane surfaces covered with See also:cotton See also:duck (canvas) See also: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 See also: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. See also:Gates canvas table See also:system of See also:California). Buddles See also: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 See also:outer part of the circle, the middlings from the intervening See also:annular part, and the concentrates from the inner part. They treat somewhat coarser sizes than the slime-table. The See also:term buddle is sometimes applied to the slime-tables, but the See also: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 See also:uniform size are placed in the bottom of a sluice, the spaces between them are called riffles; and when gold-bearing See also: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-See also:barrel, with sides flaring slightly upwards all the way from the bottom. In the centre is a little vertical shaft, with hand-See also:crank at the See also:top and stirring See also: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 See also: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 See also:zinc and See also:lead ores of See also:Missouri is first to clean the ore from adhering clay by raking it back and forth in a sluice with a See also:running stream of water, and then See also: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-See also:skimmer. The top skimmings are waste, the middle skimmings come back with the next See also: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 See also: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 See also: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 See also:day of ten hours. In the diamond washing of See also:Kimberley, South See also:Africa, the material taken from the mine is weathered by exposure to the air and See also: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 See also: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 See also:local conditions; the one here outlined is typical of the class. The ore is crushed by breaker and rolls, and separated into a See also: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 See also: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 See also: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 See also:gate at the bottom. The See also: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, See also: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 See also:groups according to richness. Gravity or pneumatic stamps then crush the ore to h in., and stripes (a See also: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 See also:main treatment begins with the finest size, which is much the largest product. It is fed to a See also: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 See also: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 See also:sulphur, See also:tellurium, &c. The usual gold-milling See also: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 See also: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 See also: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 See also: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. See also: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 See also: A special magnet of very high power, known from its inventor as the Wetherill magnet, has been designed for treating the See also:franklinite of New See also: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 See also:mica. Asbestos, when of good quality, is in compact masses, which by suitable bruising and beating are resolved into fine flexible See also:fibres. The See also:Canadian asbestos is associated with See also: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 See also: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 See also:fan to a settling-chamber with air outlets covered by fine screen cloth. This fibrous product is the clean mineral for the See also:market. A special treatment separates the fibres of different lengths. The usual method of dressing corundum and See also: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 See also:talc, See also:chlorite, mica, &c., are worn off from the harder mineral. A constant current of water carries off the light impurities. This is called the ",See also: 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 See also: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. Additional information and CommentsThere are no comments yet for this article.
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