COBALT

This metal being difficult to reduce from its ores, is therefore very little known, and has not hitherto been employed in its simple state in any of the arts; but its oxide has been extensively used on account of the rich blue colour which it imparts to gla , and the glaze of porcelain and stone-ware. The principal ores of cobalt are those designated by mineralogists under the names of arsenical cobalt and gray cobalt . The first contains, in addition to cobalt, some arsenic, iron, nickel, and occasionally silver, . The other is a compound of cobalt with iron, arsenic, sulphur, and nickel. Among the gray cobalts, the ore most esteemed for its purity is that of Tunaberg in Sweden. It is often in regular crystals which po e the lustre and colour of polished steel. The specific gravity of cobalt pyrites is 6·36 to 4·66. The Tunaberg variety afforded to Klaproth, cobalt, 44; arsenic, 55·5; sulphur, 0·5: so that it is an arseniuret. Others, however, contain much sulphur as well as iron. It imparts at the blowpipe a blue colour to borax and other fluxes, and gives out arsenical fumes. The ore being picked to separate its concomitant stony matters, is pounded fine and pa ed through a sieve; and is also occasionally washed. The powder is then spread on the sole of a reverberatory furnace, the flue of which leads into a long horizontal chimney. Here it is exposed to calcination for several hours, to expel the sulphur and arsenic that may be present; the former burning away in sulphurous acid gas, the latter being condensed into the white oxide or arsenious acid, whence chiefly the market is supplied with this article. This calcining proce can never disengage the whole of these volatile ingredients, and there is therefore a point beyond which it is usele to push it; but the small quantities that remain are not injurious to the subsequent operations. The roasted ore is sifted anew; reduced to a very fine powder, and then mixed with 2 or 3 parts of very pure siliceous sand, to be converted into what is called zaffre . With this product gla es are generally coloured blue, as well as enamels and pottery glaze. In the works where cobalt ores are treated, a blue gla is prepared with the zaffre, which is well known under the name of smalt or azure blue. This azure is made by adding to the zaffre 2 or 3 parts of potash, according to its richne in cobalt, and melting the mixture in earthen crucibles. The fused ma is thrown out while hot into water; and is afterwards triturated and levigated in mills mounted for the purpose. There remains at the bottom of the earthen pot a metallic lump, which contains a little cobalt, much nickel, arsenic, iron, . This is called spei . As it is the oxide of cobalt which has the colouring quality, the calcination serves the purpose of oxidize ment, as well as of expelling the foreign matters. A finer cobalt-oxide is procured for painting upon hard porcelain, by boiling the cobalt ore in nitric acid, which converts the arsenic into an acid, and combines it with the different metals present in the mineral. These arseniates being unequally soluble in nitric acid, may be separated in succe ion by a cautious addition of carbonate of soda or potash; and the arseniate of cobalt as the most soluble remains unaffected. It has a rose colour; and is easily distinguishable, whence the precipitation may be stopped at the proper point. The above solution should be much diluted, and the alkali should be cautiously added with frequent agitation. IMG:4147767755307473660_illo0302a.png:Cobalt ore furnace The cobalt ores, rich in nickel, are exposed to slow oxidize ment in the air, whereby the iron, cobalt, arsenic, and sulphur get oxygenated by the atmospheric moisture, but the nickel continues in the metallic state. This action of the weather must not be extended beyond a year, otherwise the nickel becomes affected, and injures the cobalt blue. The ore hereby increases in weight, from 8 to 10 per cent. Fig. 291. is a longitudinal section of the furnace: fig. 292. , a horizontal section upon a level with the sole of the hearth. It is constructed for wood fuel, and the hearth is composed of fire-bricks or tiles. The vapours and gases disengaged in the roasting, pa off through the flues a a , into the channels b b , and thence by c into the common vent, or poison chamber. See the representation of the poison tower of Altenberg , under the article Arsenic . The flues are cleared out by means of openings left at suitable situations in the brick-work of the chimneys. The azure manufacture is carried on chiefly in winter, in order that the external cold may favour the more complete condensation of the acids of arsenic. From 3 to 5 cwt. of Schlich (pasty ore), are roasted at one operation, and its bed is laid from 5 to 6 inches thick. After two hours, it must be turned over; and the stirring must be repeated every half hour, till no more arsenic is observed to exhale. The proce being then finished, the ore must be raked out of the furnace, and another charge introduced. The duration of the roasting is regulated partly by the proportion of sulphur and arsenic present, and partly by the amount of nickel; which must not be suffered to become oxidized, lest it should spoil the colour of the smalt. The latter ores should be but slightly roasted, so as to convert the nickel into spei . The roasted ore must be sifted in a safety apparatus. The lo of weight in the roasting amounts, upon the average, to 36 per cent. The roasted ore has a brownish gray hue, and is called safflor in German, and is distributed into different sorts. F F S is the finest safre ; F S, fine; O S, ordinary; and M S, middling. These varieties proceed from various mixtures of the calcined ores. The roasted ore is ground up along with sand, elatriated, and, when dry, is called zaffre . It is then mixed with a sufficient quantity of potash for converting the mixture into a gla . IMG:4147767755307473660_illo0302b.png:Smalt furnace Figs. 293. and 294. represent a round smalt furnace, in two vertical sections, at right angles to each other. The fire-place is vaulted or arched; the flame orifice a , is in the middle of the furnace; b is the feed hole; c , a tunnel which serves as an ash-pit, and to supply air; d , openings through which the air arrives at the fuel, the wood being placed upon the vault; e , knee holes for taking out the scoriæ from the pot bottoms; f , working orifices, with cast-iron plates g , in front of them. Under these are the additional outlets h . The smoke and flame pa off through the orifices i , which terminate in expanded flues, where the sand may be calcined or the wood may be baked. Eight hours are sufficient for one vitrifying operation, during which the gla is stirred about several times in the earthen melting pots. The preparation of the different shades of blue gla are considered as secrets in the smelting works; and marked with the following letters:—F F F C, the finest; F C, fine; M C, middling; O C, ordinary. A melting furnace, containing 8 pots of gla ; produces in 24 hours, from 24 cwts. of the mixture, 19 cwts. of blue gla ; and from 1 ⁄ 2 to 3 ⁄ 4 cwt. of scoriæ or spei ( speise ). The composition speise , according to Berthier, is,—nickel, 49·0; arsenic, 37·8; sulphur, 7·8; copper, 1·6; cobalt, 3·2 in 100. Nickel, arsenic, and sulphur, are its e ential constituents; the rest are accidental, and often absent. The freer the cobalt ore is from foreign metals, the finer is the colour, and the deeper is the shade; paler tints are easily obtained by dilution with more gla . The presence of nickel gives a violet tone. The production of smalt in the Pru ian states amounted, in 1830, to 7452 1 ⁄ 2 cwts.; and, in Saxony, to 9697 cwts.; in 1825, to 12,310 cwts. One proce for making fine smalt has been given under the title Azure ; I shall introduce another somewhat different here. The ore of cobalt is to be reduced to very fine powder, and then roasted with much care. One part, by weight, is next to be introduced, in succe ive small portions, into an iron ve el, in which three parts of acid sulphate of pota a has been previously fused, at a moderate temperature. The mixture, at first fluid, soon becomes thick and firm, when the fire is to be increased, until the ma is in perfect fusion, and all white vapours have ceased. It is then to be taken out of the crucible with an iron ladle, the crucible is to be recharged with acid sulphate of potash, and the operation continued as before, until the ve el is usele . The fused ma contains sulphate of cobalt, neutral sulphate of pota a, and arseniate of iron, with a little cobalt. It is to be pulverized, and boiled in an iron ve el, with water, as long as the powder continues rough to the touch. The white, or yellowish white residue, may be allowed to separate from the solution, either by deposition or filtration. Carbonate of pota a, free from silica, is then to be added to the solution, and the carbonate of cobalt thrown down is to be separated and well washed, if po ible, with warm water; the same water may be used to wash other portions of the fused ma . The filtered liquid which first pa es, is a saturated solution of sulphate of pota a: being evaporated to dryne in an iron ve el, it may be reconverted into acid sulphate by fusing it with one half its weight of sulphuric acid: this salt is then as useful as at first. The oxide of cobalt thus obtained, contains no nickel; so little oxide of iron is present, that infusion of galls does not show its presence; it may contain a little copper, if that metal exists in the ore, but it is easily separated by the known methods. Sometimes sulphuretted hydrogen will produce a yellow brown precipitate in the solution of the fused ma ; this, however, contains no arsenic, but is either sulphuret of antimony or bismuth, or a mixture of both. It has been found advantageous to add to the fused ma , sulphate of iron, calcined to redne , and one tenth of nitre when the residue is arseniate of iron, and contains no arseniate of cobalt. There is then no occasion to act upon the residue a second time for the cobalt in it. This proce is founded on the circumstances that the sulphate of cobalt is not decomposed by a red heat, and that the arseniates of iron and cobalt are insoluble in all neutral liquids. It is quite evident that to obtain a perfect result, the exce of acid in the bisulphate of pota a must be completely driven off by the red heat applied. 110,646 lbs. of smalts were imported into the United Kingdom in 1835, and 96,949 were retained for home consumption. In 1834, only 16,223 lbs. were retained. In 1835, 322,562 lbs. of zaffres were imported, and 336,824 are stated to have been retained, which is obviously an error. 284,000 lbs. were retained in 1834.