Table of Contents
The specific gravities of blende (3.9 to 4.2) and siderite (3.7 to 3.9) are almost identical, and they may not be separated by any method based on this property. The most important application of magnetic separation in Europe has been the separation of siderite, or carbonate of iron, from blende. Many important ore bodies carrying galena and blende have siderite as their chief gangue mineral. The method followed in the treatment of these ores consists of the removal of the galena by water concentration, followed by magnetic separation of the middling products containing the blende and siderite. Formerly the siderite was removed after calcination to the oxide, but since the advent of the separators with intense magnetic fields direct separation of the raw siderite as a magnetic product has superseded the older process. Examples of the separation of siderite after calcination are given, as this method possesses advantages over the direct separation in the treatment of certain ores. Siderite has been separated magnetically after calcination for its value as iron ore: the separation of siderite from chalcopyrite will be taken up in a later chapter. The magnetic tailing from most European mills finds a market as iron ore.
Siderite, sp. gr. 3.7 to 3.9, FeCO3, is slightly magnetic. Delesse states that if the magnetic permeability of steel be taken at 100,000, that of siderite is 120. Crane obtained a permeability for a specimen from Roxbury, Conn., of 1.0234, and for a specimen from Allevard, France, of 1.0213. Siderite is readily transformed into the magnetic oxide of iron by calcination.
Ores in which the blende is in part magnetic, or the siderite accompanied by pyrite, should be roasted before separation. Important deposits of such ores are found in British Columbia, in the treatment of which magnetic separation would seem to be destined to play an important part. Ores carrying important amounts of strongly magnetic blende, or marmatite, may demand treatment in two stages: preliminary separation of the strongly magnetic blende on a high-intensity separator, followed by roasting and separation on low-intensity machines. The current on the magnets of the primary separator may be regulated to remove the blende down to a point where its permeability approaches that of siderite, then after calcination, the magnetic oxide may be removed by the low-intensity separators without affecting the remaining blende, as the increase in permeability on the part of the ferruginous blende, due to the roast. is slight as compared with the difference between raw siderite and the magnetic oxide into which it is transformed. This treatment, while requiring an additional separator, has the advantage of producing two clean zinc concentrates which may be marketed separately, the first product removed carrying the higher percentage of combined iron, and therefore being of lower zinc tenor.
Calcining Siderite to the Magnetic Oxide
Siderite heated to 800° C. breaks up into ferrous oxide and carbonic acid gas. If the roasting is carried out in a neutral atmosphere the ferrous oxide is transformed into the ignition oxide Fe6O7, or, if the atmosphere is moderately oxidizing, Fe3O4 results; both of these oxides are strongly magnetic. If, however, there is free access of air the nonmagnetic Fe2O3 is quickly formed. The whole success of the operation lies in the complete control of the air entering the furnace. If air is absent, the ferrous oxide reacts with the CO2 with the formation of Fe3O4, and the liberation of carbon-monoxide; this reaction does not take place, however, unless the air is completely excluded. After the CO2 is completely driven off, it is very easy to overroast the charge to the nonmagnetic red oxide, but if the operation is so controlled as to leave a small percentage combined the tendency to overroast is much reduced, without appreciably affecting the magnetic qualities of the product. Siderite, when pure, contains 37.9 per cent. CO2, and a large loss in weight results from the calcination. Siderite is usually mixed with from 3 to 5 per cent. of fine coal, or coke, before calcination, to aid in its decomposition and to insure a reducing, or neutral, atmosphere in the furnace: if coal is used it must be of a noncoking variety. In the treatment of ores which carry both siderite and pyrite a roast suitable to convert the carbonate into the oxide also suffices to transform the sulphide into the magnetic sulphide, in which state it is removed from the blende by the separators.
The calcined siderite is always very strongly magnetic, which may partially be due to the reduction of a small amount of metallic iron by the coal mixed with the charge. The roast is usually conducted at 850° C, at which temperature but 15 minutes is required for the production of magnetic oxide. The duration of the roast is also governed by the size of the particles treated: from 20 to 35 minutes is usually employed with fine material. Prolonged heating at the temperature of calcination affects the blende. With a properly conducted roast of normal duration the blende is covered by a white film due to incipient oxidation, which does not indicate a significant loss.
The temperature of the roast must be carefully regulated, as the ferrous oxide has a strong tendency to slag with any siliceous particles of waste there may be in the material treated, forming aggregates of the several minerals. Calcination gives rise to marked decrepitation. The calcined ore is classified before separation.
Many types of shaft, reverberatory and cylindrical furnaces have been used for the calcination of siderite; almost any furnace in which the access of air may be completely controlled is suitable.
Separation of Raw Siderite
At Neunkirchen, Siegerland, Germany, the Lohmannsfeld Co. is operating a magnetic-separation plant on raw siderite-blende ores. The ores treated carry galena and blende, with rarely a little chalcopyrite, in a gangue of siderite and quartz; occasional accessory gangue minerals are calcite and barite. The siderite carries a varying quantity of manganese, which sometimes reaches 12 per cent. The blende is quite diamagnetic.
The material for separation consists of the middle products from water concentration, by which process the galena and the 2 or 3 per cent. of quartzose gangue which the ore carries are removed.
These middle products vary in size from 1 to 10 mm. and carry from 15 to 22 per cent. zinc.
The plant, which comprises six Humboldt-Wetherill separators,
was installed under a guaranty to produce a blende concentrate assaying from 42 to 46 per cent. zinc, and a tailing product carrying not more than 1 to 3 per cent. zinc, from a feed crushed to pass a 3-mm. screen, it having been determined that these ores free their component minerals at that size. The wet material from the concentrator is dried upon two end-less belts which transport it through kilns heated by waste steam from the engines; it remains in the kilns from 25 to 30 minutes and arrives at the first trommels quite dry. The dried ore is delivered from the belts to a trommel with 3-mm. screens, the fines are thence transported by elevator to the
classifying trommels, while the oversize is crushed in rolls and returned to the trommel. Before reaching the classifying trommels the ore is carried upon a conveyor belt beneath an electromagnet which attracts and removes any strongly magnetic particles it may contain. The classifying trommels divide the ore stream into the following sizes, which are fed separately to the separators: through 0.75 mm., from 0.75 to 1.4 mm., from 1.4 to 2.0 mm., from 2.0 to 3.0 mm. The separators are six in number, arranged in three series. The first separator of each series is a two-pole machine, while the second is of the three-pole type. The magnets of the first separator take 12 amperes at 65 volts and separate a clean siderite product; the belt speed on these separators is 40 meters per minute. The material passing unaffected from the two-pole separator is re-treated on the three-pole machines. Here the current is 5 amperes on the first magnet and 8 on the second; two middling products carrying blende and siderite are here removed, and the stream passing off the separator constitutes a finished blende concentrate. The belt speed on the three-pole separators is 25 meters per minute. The plant treats from 3 to 3.5 metric tons of crude ore
per hour. The crew required is one foreman, five boys, one engine-man, and one stoker. The cost of treatment (a year’s average) is 1.40 marks (33 1/3c.) per ton of crude material; no amortization is reckoned in this figure. The plant cost about 100,000 marks.
At Ems, Germany, the Emser Blei & Silberwerk Gesellschaft has been employing two Humboldt-Wetherill separators on blende-siderite concentrate since 1900. These machines are fitted with 280-mm. belts and each has two magnets. The feed is received from the concentration mill in the following sizes, which are treated separately on the machines: 3 to 4 mm., 2 to 3 mm., 1 to 2 mm., ½ to 1 mm., and two classes of fines. The average capacity of each machine per ten hours is, of the coarser sizes, 12 metric tons, and of the fines, 3.5 metric tons. The average material treated of all sizes is 6.25 metric tons. The feed carries 18.5 per cent. zinc and the finished zinc product 42.5 per cent.; the siderite tailing carries 2.4 per cent. zinc. The cost of separation foots up to 2.24 marks (average of a year’s run). This total, which does not include royalty or amortization of plant, is made up as follows:
The separators are fed from bins and deliver their products into cars. The current used on the magnets is from 8 to 9 amperes at 90 volts.
At Musen bei Creuzthal, Germany, the Gewerkschaft Grube Staalberg is operating Humboldt-Wetherill separators on blende-siderite concentrate. The capacity of the plant is 1.23 metric tons per hour. Two men and two boys constitute the total working force. The cost of separation varies from 1.50 to 2.50 marks and averages 2.20 marks per ton of feed. This figure includes labor, coal, lubricants, repairs, and supervision, but does not include royalty.
At Lauenburg, Germany, The Rheinische-Nassauische Aktien-Gesellschaft is operating a plant on blende-siderite ores, employing the Mechernich separator. The ore, pulverized to pass a screen with 4-mm. openings, is dried in a revolving kiln, with the expenditure of 1 lb. of coal per 15 lbs. moisture evaporated. The dry ore is passed through a dry-screening apparatus which removes all material below 50 mesh. The coarse product is passed through a trommel with 2-mm. and ½-mm. screens. The fines are treated in a dust trommel, and everything passing 120 mesh is removed. The larger sizes are separated on motor type separators and the fines on the Mechernich separator. The machines are enclosed in dust-tight housings and the dust exhausted by fans.
The average recovery of zinc in the concentrate is given at 85.72 per cent. of the total zinc in the feed.
Separation of Calcined Siderite from Blende
Friedrichssegen, Germany. The ore from the Friedrichssegen mines has been treated by magnetic separation for some 25 years. Up to within a few years ago the method followed included calcination and separation on low-intensity separators; the ores are
now separated direct on Humboldt-Wetherill machines. The description of the old process is included, as it is a standard method for the treatment of blende-siderite ores, the direct separation of which is not always advisable.
The ore treated at Friedrichssegen was a mill product, assaying from 11 to 15 per cent. zinc (as blende) and 18 to 23 per cent. iron (as siderite). This was heated to redness in a furnace of the McDougal type, which put through from 20 to 25 metric tons per 24 hours, according to the size of particles; the coal consumption was 1.2 metric tons. The plant comprised two furnaces, each of which required the attention of one man, who also trammed the calcined ore to the cooling floor. When the ore had cooled to 50° C, or lower, it was elevated to a trommel which di-
vided it into sizes as follows: over 4 mm.; between 2 and 4 mm.; and through 2 mm. The material which was refused by a 4-mm. screen was sent to a set of rolls and reduced to 4-mm. and elevated again to the trommel. The two sizes passing the screens dropped into separate bins from which they were fed to the primary separators. There were twelve primary separators, arranged in three groups of four each; the four machines of each group were set up in pairs, tandem. The arrangement of the separators in each group is shown in the above figure. The ore diverted to a group was divided equally between machines A and B, which made two products, one enriched in zinc and one enriched in iron. The iron product of both machines was led to D and the zinc product to C. The two lower machines made a zinc product with 38 to 42 per cent. zinc and 6 per cent. iron at the most a mixed product, and an iron product which still retained 6 to 8 per cent. zinc. The mixed product was re-treated by a group of two machines and the iron product by another group of four machines, which yielded a finished iron product containing 40 per cent. iron and 3 to 4 per cent. zinc, representing the entire loss of the process. Out of a total of eighteen separators, twelve were employed on original ore, while six were used cleaning the products of the primary machines; the general arrangement of the plant is shown in the accompanying figure.
At Maiern, Austria, there is a magnetic separation mill separating siderite from blende after calcination.
The ore, after calcination, is crushed to pass 4 mm. and then classified into the following sizes: on 3 mm.; on 2 mm.; on 1 mm.; and through ½ mm. The two larger sizes are treated upon Separator No. 1, and the smaller on Separator No. 2. These separators make clean blende and a middling product taken out by the magnets. The middling from Separator No. 1 is recrushed and classified upon a 1-mm. screen; the sand and finer sizes from this classification are treated at different times upon Separator No. 3, which produces a clean iron product and a blende product which is subjected to a repassage over the same machine; the products from this second passage are a clean blende and a finished iron product. The middling from Separator No. 2 is passed over Separator No. 4; from this a clean iron product is obtained and a zinc product which is repassed over the same machine, giving on the second pass a finished iron product and a zinc product which is cleaned by tables.
Heberle separators are employed, the capacity on the two coarser sizes being tons per hour, and of the fines, 1 ton per hour. The speed at which these separators may be operated is limited, owing to the fact that beyond a certain point centrifugal force is sufficient to overcome the magnetism and throw off magnetic particles; this limiting speed is, for the 60-cm. drum, 45 R.P.M.
At Allevard France, there is an installation for calcining and magnetic separation of siderite for its value as an iron ore. The ore consists of siderite in a gangue of sandstone, slate, and quartz. The raw ores are screened on a grizzly with bars spaced 1½ ins. The coarse ore is hand picked, the waste thrown out, and the balance calcined in shaft furnaces. After calcination the lumps are broken up and the pieces of waste which have not been rendered friable in the furnaces are thrown out. The coarse ore is not subjected to magnetic separation. The final product from this material carries in excess of 50 per cent. iron and manganese. The fines are screened, and material passing 1/18 in. is sent directly to shelf-calcining furnaces, the material between 1/18 and 1½ ins. is sized and jigged and the concentrate calcined in reverbatory furnaces and separated. The calcining is conducted at a temperature of 1000° C. The loss of weight in the furnaces is 28 per cent., and the calcined charge still retains 2 per cent. CO2. The separators used consist of wooden drums upon which a number of small magnets are mounted. The fines, after separation, are briquetted after addition of 5 per cent. slaked lime. The capacity of the plant is from 210 to 220 metric tons in 10 hours.
At Krompach, Austria, the Hernadthal Ungarische Eisenindustrie Aktien-Gesellschaft has been operating a magnetic-separating plant of 10 metric tons daily capacity since 1901. This plant was installed to try out a method for treating the Szlovinka ore bodies, and has also been operated as a custom plant. On the basis of the results obtained a mill of 500 metric tons daily capacity is now being built.
The Szlovinka ores consist of siderite occurring with quartz and schist, and lesser amounts of finely divided chalcopyrite, pyrite, and tetrahedrite; the latter mineral occurs finely disseminated. The high copper and sulphur content has hitherto rendered this class of ore unworkable, and the object of the enterprise is to produce a commercial iron concentrate through the elimination of these impurities, while also obtaining a marketable copper concentrate as a by-product.
The run of mine ore after hand picking is crushed to 2.5 mm. in breakers and rolls, and is then passed through a revolving dryer before classification. The dry ore is next divided by shaking-screens into the following sizes: 2.5 mm. to 2 mm., 2 mm. to 1.5 mm., 1.5 mm. to 1 mm., 1 mm. to 0.5 mm., 0.5 mm. to 0.25 mm. and through 0.25 mm. The coarser sizes from 2.5 mm. down to 0.25 mm. are treated separately on 24 Primosigh dry separators, while the material passing 0.25 mm. is treated on four wet separators of the same make.
The magnetic product from these machines is sent to a battery of revolving furnaces where it is calcined and sintered, removing all but traces of sulphur, and preparing the product for the iron furnaces. The middling product from the dry separators is recrushed dry in a ball mill to 0.25 mm. and fed to the wet separators; it amounts to less than one half on one per cent. of the feed. The nonmagnetic product from the dry separators is concentrated on tables which deliver a copper concentrate and tailing; the nonmagnetic product from the wet separators is similarly treated after dewatering and classification in spitzkasten, etc.
The low iron content of the magnetic concentrate is due to the fact that the siderite is contaminated by magnesia.
The separators are fed and their products removed by a system of belt conveyors, thus avoiding manual labor and rendering the passage of the ore through the mill automatic.