AL′UM

K 2 SO 4 .Al 2 (SO 4 ) 3 .24Aq. Syn. Pot′ash-alum , Sul′phate of aluminum and pota ium , Common alum ; Alu′men , A. pot as′sicum , L.; Alun , Sulfate d’alumine et de pota e , Fr.; Alaun , Ger.; Alume , Ital. The principal alum-works in England, until recently, were those of Lord Glasgow, at Hurlett and Campsie, near Glasgow, and those of Lords Dundas and Mulgrave, at Whitby, Yorkshire (est. 1600); but those of Mr Spence, at Manchester, and at Goole (Yorkshire), and of Mr Pochin, at Manchester, are now among the largest, if they be not actually the largest in the world. There are also extensive alum-works at and near Newcastle-on-Tyne; but none of importance, that we know of, in any other part of these realms. Nat. hist. Alum is found native in some places ( NATIVE ALUM ), either effloresced on the surface of bituminous alum-schist (Göttwigg, Austria); or united with the soil in the neighbourhood of volcanoes (Solfatara, Naples); when it may be obtained by simple lixiviation and evaporation, a little potash being commonly added to convert the exce of sulphate of alumina present into alum. It is also found in certain mineral waters (East Indies). Sources. The alum of commerce is usually obtained from schistose pyritic clays, commonly termed alum-ores, aluminous shale, a.-schist, .; and from alum-rock, a.-stone, or alunite. At La Tolfa, Civita Vecchia, where the best Roman-alum is produced, the source is stratified alum-stone. On the Continent, and in Great Britain, it is generally pyritaceous clays, volcanic aluminous ores, aluminous shale, or alum-slate. These minerals contain sulphide of iron, alumina, bitumen or carbon, and frequently a salt of pota ium. Of late years large quantities of alum have been prepared on the banks of the Tyne from aluminous clay. Prep. The manufacture of alum is technically said to be conducted according to the natural proce when prepared from alum-schist or alum-ore; and according to the artificial proce when made by acting on clay with sulphuric acid, and adding a pota ium salt to the resulting lixivium. The manufacture of alum and of sulphate of alumina from such materials as contain only alumina, to which consequently sulphuric acid and alkaline salts have to be added, has come largely into practice in England. The materials employed are, in addition to clay, cryolite or Green land spar, a fluoride of aluminum and soda; bauxite, a hydrate of alumina, of more or le purity; and slag. The following are the details of these proce es:— a. From ALUM-ORE , ALUMINOUS SCHIST , or SHALE , .:— IMG:596405606043057898_i119.png: 1. The mineral (alum-ore, a.-schist, .) is placed in heaps, and moistened from time to time with water, when it becomes gradually hot, and falls into a pulverulent state. This decomposition commonly occurs either wholly, or partially, on the floor of the mine. If the ore does not po e this property on mere exposure to air and moisture, it is broken into pieces and laid upon a bed of brushwood and small coal, to the depth of about four feet, when the pile is fired and fresh lumps of the alum-mineral thrown on, until the ma becomes of considerable height and size. The combustion, as soon as established, is conducted with a smothered fire, until the calcination is complete; care being taken to prevent fusion, or the disengagement of either sulphurous or sulphuric acid, from contact between the ignited stones and the carbonaceous fuel. [35] To promote these ends the pile, at the proper time, is ‘mantled’ (as the workmen call it) or covered with a layer of already calcined and exhausted ore, in order to protect it from high winds and heavy rains; as also to moderate the heat, and let it proceed gradually, so that the sulphur present may not be lost or wasted by volatilisation. The roasting is finally checked by a thicker ‘mantling,’ and the whole allowed to cool. By this time the pile has usually lost about one half its bulk, and become open and porous in the interior, so that the air can circulate freely through the ma ; the latter, in dry weather, as the heap cools, being usually promoted by sprinkling a little water on it, which, by carrying down some of the saline matter, renders the interior still more open to the atmosphere. The whole, when cold, or nearly cold, is, if nece ary, still further exposed to the action of air and moisture. The time required to calcine the heap properly, including that taken by the burned ore to cool, varies, according to its size and the state of the weather, from three to nine, or even twelve months. The residuum of the calcination is next placed in large stone or brick cisterns, and edulcorated with water, until all the soluble portion is di olved out; the solution is then concentrated in another stone cistern, so made that the flame and heated air of its reverberatory furnace sweep the whole surface of the liquor. (See engr. ) The evaporation is continued until it just barely reaches the point at which crystals are deposited on cooling; when it is run off into coolers. After the sulphate of iron, always present, has been deposited in crystals, the mother-liquor, containing the sulphate of aluminum, is run into other cisterns, and a saturated solution of chloride of pota ium, or of sulphate of pota ium, or (sometimes) impure sulphate or carbonate of ammonium, or a mixture of them, [36] is added until a cloud or milkine ceases to be produced on addition of more. [37] It is next allowed to settle and get thoroughly cold, and the supernatant ‘mother-liquor’ being drawn off with a pump or syphon, the precipitate, which is alum in the form of minute crystals (technically termed ‘flour’), is well drained, and subsequently washed by stirring it up with a little very cold water, which is then drained off, and the operation repeated a second time with fresh water. A saturated solution of the pulverulent alum (‘flour’) is next formed in a leaden boiler, and the clear portion is run or pumped off, while boiling hot, into crystallising ve els, called roaching casks (see engr. ), the staves of which are lined with lead, and nicely adjusted to each other. After the lapse of a week or ten days, the hoops and staves of these ‘casks’ are removed, when a thick crust of crystallised alum is found, which exactly corresponds in form and size to the interior of the cask. A few holes are then made in the sides of this ma , near the bottom, to allow the contained mother-liquor to drain off, after which the whole is broken up and packed in casks for sale. Sometimes the alum thus obtained, or the lower portion of it, is washed with a little very cold water, and, if discoloured, or small or slimy, is purified by a second crystallisation. [35] The generality of alum-minerals require roasting; and their own bituminous matter is, in many cases, sufficient to produce the heat required, which need not nece arily exceed 600 to 650° Fahr., provided it be continued for a sufficient period. It is only when they are le bituminous or carbonaceous that slack or saw-dust, ., is employed. [36] For pure Potash-alum a salt of potash only must be employed. When ammonia (usually in the form of gas-liquor or gas-sulphate) is used as the precipitant, the product is AMMONIA-ALUM . The ordinary alums of commerce are now generally mixtures of the two. [37] The respective quantities required to produce 100 parts of alum from the sulphate of alumina liquor are— | Chloride of | pota ium | 15·7 | | Sulphate of | ” | 18·4 | | ” | ammonium | 13·9 | In practice, the exact quantity required may be found by a previous trial of a little of the aluminous liquor; but the indications mentioned in the text will always show the operator when a sufficient dose is added. IMG:596405606043057898_i120.png: 2. As ammonia-alum (Spence’s proce ; see below ), but using a potash-salt as the precipitant, either wholly or in part, instead of ammonia; and, in the latter case, supplementing the deficiency of potash with ammonia, as there explained. b. From ALUMINOUS CLAY and OIL OF VITRIOL :— 1. Clay, free or nearly free from carbonate of lime and oxide of iron, is chosen for this purpose. It is moderately calcined (in lumps) in a reverberatory furnace, until it becomes friable; great care being taken that the heat be not sufficient to indurate it, which would destroy its subsequent solubility. It is next reduced to powder, sifted, and mixed with about 45% of its weight of sulphuric acid (sp. gr. 1·45), the operation being conducted in a large stone or brick basin arched over with brickwork. Heat is then applied, the flame and hot air of a reverberatory furnace being made to sweep over the surface of the liquor. The heat and agitation are continued for 2 or 3 days, when the ma is raked out and set aside in a warm place for a few weeks (6 to 8), to allow the acid the more perfectly to combine with the clay. At the end of this time the newly-formed sulphate of alumina is washed out, the solution evaporated until of a sp. gr. of about 1·38 (1·24 for ‘ammonia-alum’), and the salt of potash added. The remaining operations resemble those above described. rock or mine alum. 2. (Proce of Mr Pochin.) Fine China clay is heated in a furnace, and mixed with a suitable proportion of sulphuric acid; the latter being considerably diluted with water, in order to moderate its action, which would otherwise be far too violent. The mixture is then pa ed into cisterns furnished with movable sides, where, in a few minutes, it heats violently and boils. The thick liquid gradually becomes thicker, until it bubbles of steam which are driven through it, owing to the heat resulting from the reaction of the ingredients on each other. This porous ma ( ALUM-CAKE ; CONCENTRATED ALUM ) appears perfectly dry, although retaining a large amount of combined water. It also contains all the silica of the original clay, but in such a state of fine division, that the whole appears homogeneous; whilst it imparts a dryne to the touch which can scarcely be given to pure sulphate of alumina. From this substance a solution of pure sulphate of alumina is easily obtainable by lixiviation, and allowing the resulting solution to deposit its silica before using it, but for many purposes the presence of the finely divided silica is not objectionable. The sulphate of alumina solution so obtained is adapted to all the purposes in dyeing for which alum is now employed; the sulphate of potash or of ammonia in the latter being an unnece ary constituent, and one merely added to facilitate the purification and subsequent crystallisation of the salt. To obtain ALUM from the porous alum-cake, the proper proportion of acid having been used in its preparation, or subsequently added, it is only nece ary to precipitate its concentrated solution with a strong solution of a salt of potash, or of ammonia, or a mixture of them, and to otherwise proceed as before. Ratio. In the above proce the sulphide of iron of the shale or schist is converted by atmospheric oxygen into sulphate of iron and sulphuric acid; the sulphuric acid decomposes the clay, setting silica free, and producing sulphate of aluminum. The sulphate of iron is mostly got rid of by concentrating the solution of the mixed sulphates, and the mother-liquors are converted into alum by the addition of the salt of pota ium. When chloride of pota ium is used, it yields chloride of iron and sulphate of pota ium, the latter combining with the sulphate of aluminum, and the former remaining behind in the mother-liquor. See Alums (in Chemistry). Comp. Pota ium alum has the formula K 2 SO 4 .Al 2 (SO 4 ) 3 .24Aq. c. From Cryolite . 1. (Thomson’s method.) Decomposition of cryolite by ignition with carbonate of lime. From the ignited ma the aluminate of soda is obtained by lixiviation with water, and into the solution carbonic acid gas is pa ed, when there result precipitated hydrated gelatinous alumina and carbonate of soda, which remains in solution. If it be desired to obtain the alumina as an earthy compact precipitate, bicarbonate of soda is used instead of carbonic acid. While the clear liquor is boiled down for the purpose of obtaining carbonate of soda, the precipitated alumina is di olved in dilute sulphuric acid; this solution is evaporated for the purpose of obtaining sulphate of alumina (the so-called concentrated alum), or the solution after having been treated with a pota a or an ammonia salt is converted into alum. 2. (Sauerwein’s method.) Decomposition of cryolite by caustic lime by the wet way. Very finely ground cryolite is boiled with water and lime, the purer the better, and as free from iron as po ible, in a leaden pan. The result is the formation of a solution of aluminate of soda, and insoluble fluoride of calcium (lime). When the fluoride of calcium has deposited, the clear liquid is decanted, and the sediment washed, the first wash-water being added to the decanted liquor, and the second and third wash-waters being used instead of pure water at a subsequent operation. In order to separate the alumina from the solution of aluminate of soda, there is added to the liquid while being continuously stirred very finely pulverised cryolite in exce , the result of the decomposition being alumina and fluoride of sodium, (soda). When no more caustic soda can be detected in the liquid, it is left to stand for the purpose of becoming clear. The clarified solution of fluoride of sodium is then drawn off, and the alumina treated as above described. The solution of fluoride of sodium having been boiled with caustic lime yields a caustic soda solution, which having been decanted from the sediment of fluoride of calcium is evaporated to dryne . Recently the fluoride of calcium occurring as a by-product has been used in gla -making. 3. The decomposition of cryolite by sulphuric acid yields sulphate of soda convertible into carbonate by Leblanc’s proce , and sulphate of alumina free from iron. This method of decomposing cryolite is, however, by no means to be recommended, as owing to the liberation of hydrofluoric acid, peculiarly constructed apparatus are required, whilst the sulphate of soda has to be converted into carbonate. d. From Bauxite. This mineral, occurring in some parts of Southern France, in Calabria, near Belfast, and in other parts of Europe, consists e entially (viz. 60 per cent.) of hydrate of alumina, more or le pure. In order to prepare alums and sulphate of alumina from it, the mineral is first disintegrated by being ignited with carbonate of soda, or with a mixture of sulphate of soda and charcoal; in each case the lixiviation of the ignited ma yields aluminate of soda, from which, by the proce es already described under “Cryolite,” alum, or sulphate of alumina, and soda are prepared. e. From blast-furnace slag. Lürmann recommends the slag to be decomposed by means of hydrochloric (muriatic) acid. From the resulting solution of chloride of aluminum the alumina is precipitated by carbonate of lime, any di olved silica being precipitated at the same time. The alumina is di olved in sulphuric acid, leaving the silica. IMG:596405606043057898_i121.png: Prop. Alum crystallises in regular octahedrons, often with truncated edges and angles; (see engr. ); and sometimes in cubes, but only when there is a deficiency of acid in its composition, with the alkali in slight exce of the proper quantity. (Löwel.) [38] It is slightly efflorescent in dry air: soluble in 18 parts of cold water, and in rather le than its own weight of boiling water; tastes sweet, acidulous, and very astringent; is styptic; and reddens litmus. When heated it melts, loses its water of crystallisation, and becomes white and spongy ( DRIED ALUM ); a strong heat, short of whitene , decomposes it, with the evolution of oxygen and a mixture of sulphuric and sulphurous anhydride; calcined with carbonaceous matter it suffers decomposition, and furnishes a pyrophoric residuum ( Homberg’s pyro′phorus ). Ignited with alkaline chlorides, hydrochloric acid is liberated; which also occurs when their concentrated solutions are boiled together. Ammonia precipitates pure hydrate of aluminum from pota ium alum; but only a sub sulphate from the simple sulphate of alumina. Sp. gr. 1·724; but, when containing ammonia, often so low as 1·710. [38] The ordinary alum, of commerce, consisting of large crystalline ma es, which do not present any regular geometrical form; but by immersion in water for a few days, octahedral and rectangular forms are developed on its surface. (Daniell.) Tests, . It is easily recognised by its crystalline form, its taste, and by its complete solubility in water. Its aqueous solution gives a white gelatinous precipitate soluble in exce ; a platinum wire moistened with the solution imparts a violet colour to the blowpipe flame; and chloride of barium gives a white precipitate insoluble in nitric acid. Pur. When pure, its solution is not darkened by tincture of galls, sulphuretted hydrogen or ferrocyanide of pota ium; neither does it give any precipitate with solution of nitrate of silver. Heated with caustic pota a, or quick-lime, it does not evolve fumes of ammonia. Adult., . The principal impurity, and one which renders alum unfit for the use of the dyer, is iron. This may be readily detected by the blue precipitate it gives with ferrocyanide of pota ium, or the black precipitate with sulphide of ammonium, which are very delicate tests. [39] Lime, another very injurious contamination, may be detected by precipitating the alumina and iron (if any) with ammonia, and then adding oxalate of ammonia to the boiled and filtered liquid. The liquid filtered from the last precipitate (oxalate of lime) may still contain magnesia, which may be detected by the white precipitate caused on the addition of an alkaline phosphate. Common alum frequently contains ammonia, from urine, or the crude sulphate of the gas-works, having been employed in its manufacture. Powdered alum is frequently adulterated with common salt, in which case it gives a white curdy precipitate with nitrate of silver, turning black by exposure to the light. [39] Good English alum contains le than 0·1% of iron. The best Roman or Italian alums seldom contain more than ·005% of iron-alum, notwithstanding their exterior colour. Phys. eff. . In small quantities alum acts as an astringent; in larger doses as an irritant. It acts chemically on the animal ti ues and fluids, is absorbed, and has been discovered in the liver, spleen, and urine (Orfila), the last often becoming acid (Kraus). Externally, it is astringent. The almost general use of alum by the English bakers is one of the most fertile sources of dyspepsia and liver and bowel complaints in adults; and of debility and rickets in children. Bad teeth and their early decay is another consequence of the daily use of alum in our food. The bone matter (phosphate of lime) of bread, instead of being a imilated by the system, is either wholly, or in part, converted into a salt of alumina, which is usele and incapable of appropriation. When alum has been taken in poisonous doses an emetic should be given, followed by warm diluents and demulcents, containing a little carbonate of soda; and subsequently by a purgative. Uses, . The applications of alum in the arts and manufactures are numerous and important. It is used to harden tallow and fats; to render wood and paper incombustible; to remove greasine from printers’ blocks and rollers; to prepare a paper for whitening silver and silvering bra in the cold; to help the separation of the butter from milk; to purify turbid water; to dre skins; to fix and brighten the colours in dyeing; to make lake and pyrophorus, ., . It is also extensively used for clarifying liquors, and for many other purposes connected with the arts and everyday life. In medicine , alum is used as a tonic and astringent, in doses of 5 to 20 gr.; as a gargle (1 dr. to 1 ⁄ 2 pint of water); and as a collyrium and injection (10 to 15 gr. to 6 oz. of water). In lead colic, 1 ⁄ 2 to 1 dr. of alum (di olved in gum-water), every 3 or 4 hours, is said to be infallible. Powdered alum is frequently applied with the tips of the fingers, in cases of sore throat and ulcerations of the mouth, . A teaspoonful of it is said to be one of the very best emetics in croup. (Dr Meigs.) Alkalies, alkaline carbonates, lime, magnesia, acetate of lead, astringent vegetables, ., are incompatible with it. Gen. commentary. In addition to the particulars of its manufacture given above, we may add, that the plan of getting rid of the ferric salts there referred to has to some considerable extent been succe fully replaced by that of precipitating the alum, instead of the sulphate of iron, by adding alkaline matter to the lixivium. The crystalline precipitate is purified by draining, re-solution, and re-crystallisation; whilst the sulphate of iron and Epsom-salts contained in the mother liquor are obtained by subsequent evaporation and crystallisation; after which a fresh crop of alum may be got from it, by the use of an alkaline precipitant, as before. In estimating the strength of his solution the alum-maker takes as a standard a measure or sp. gr. bottle capable of holding exactly 80 pennyweights of distilled water. The exce of the weight of liquor, in pennyweights, over 80, or that of water, is called so many ‘pennyweights strong.’ Thus one of 90 pennyweights (90 dwt.) is said to be ‘10 dwt. strong,’ or simply, ‘one of 90 dwt.’ These numbers correspond to 2 1 ⁄ 2 degrees of Twaddle’s hydrometer, and may easily be found by dividing Twaddle’s degrees by 2·5 or 2 1 ⁄ 2 ; or by multiplying them by 4, and pointing off the right-hand figure of the product for a decimal. The result is in alum-makers’ pennyweights. By a patent now expired (We is man’s, 1839) the ferric salts are precipitated by the addition of a solution of ferrocyanide of pota ium (pru iate of potash); after which the supernatant clear liquor, which is now a solution of nearly pure sulphate of alumina, is decanted, and evaporated for future operations, until it either forms, on cooling, a concrete ma , which is moulded into bricks or lumps, for the convenience of ‘packing,’ or until it is sufficiently concentrated to be converted into ALUM by the addition of a salt of potash or of ammonia in the usual manner. The product, in each case, is perfectly free from iron. By a like addition of the ferrocyanide to a solution of ordinary sulphate of alumin i a or alum, the dyer may himself easily render them free from iron, or iron-alum; when, as mordants for even the most delicate colours, they are equal to the very best Roman alum. Another proce has been patented (Barlow & Gore, 1851) for the manufacture of alum from the ash or residue of the combustion of Boghead-coal, which, though hitherto regarded as almost valuele , actually contains about 30% of alumina. It has not, however, been found a convenient material for the purpose. By the latest and most approved proce es the least po ible quantity of boiling water or liquor is employed for making the solutions, so that they may crystallise without evaporation, and thus economise fuel; and the mother-liquors of previous operations are constantly employed for this purpose, when po ible. Nor is anything which is convertible to use, from the drainage of the heaps, to the liquor and slime of the roaching casks, allowed to be wasted. By whatever proce , or from whatever materials alum is obtained, it is absolutely nece ary for the succe ful and economical conduct of its manufacture, that the precise composition of the mineral or minerals employed should be exactly known. This can only be determined by actual analysis, which should be extended to several parts of the same bed, and particularly to the upper and lower strata, which frequently differ in composition from each other, and thus require different treatment, or may be most advantageously employed in combinations with each other. The nece ity of this will be seen by reference to the composition of the following minerals, of which the top contains a larger proportion of iron-pyrites than the bottom, and the two require to be mixed, to equally diffuse the sulphuric acid generated by the calcination, ., to which they are subjected. The following is the per-centage composition of certain alum shales:— | Whitby, Yorkshire. ( Richardson. ) | | Top rock. | Bottom rock. | | Sulphide of iron ( pyrites ) | 4·20 | 8·50 | | Silica | 52·25 | 15·16 | | Protoxide of iron | 8·49 | 6·11 | | Alumina | 18·75 | 18·30 | | Lime | 1·25 | 2·15 | | Magnesia | ·91 | ·90 | | Oxide of manganese | traces | traces | | Sulphuric acid (SO3) | 1·37 | 2·50 | | Pota a | ·13 | traces | | Soda | ·20 | traces | | Chlorine | traces | traces | | Coal | 4·97 | 8·29 | | Water | 2·88 | ·00 | | Lo | 4·60 | (?) | | ——— | ——— | | 100· | 100· | | Campsie, near Glasgow. ( Ronalds. ) | | Top rock. | Top rock. | Bottom rock. | | Sulphide of iron ( pyrites ) | 40·52 | 38·48 | 9·63(?) | | Silica | 15·40 | 15·41 | 20·47(?) | | Protoxide of iron |... |... | 2·18 | | Alumina | 11·35 | 11·64 | 18·91(?) | | Lime | 1·40 | 2·22 | ·40 | | Magnesia | ·50 | ·32 | 2·17 | | Oxide of manganese | ·15 |... | ·55 | | Sulphuric acid |... |... | ·05 | | Pota a | ·90 |... | 1·26 | | Soda |... |... | ·21 | | Carbon or bituminous matter | 27·65(?) | 28·80 | (?) | | Coal |... |... | 8·51 | | Water |... |... | 8·54 | | Lo | 2·13(?) | 3·13 | 1·59(?) | | ——— | ——— | ——— | | 100· | 100· | 100· | Alum-rock, or alum-stone, is a species of impure alunite, and is not of very common occurrence. That of Tolfa, near Civita Vecchia, according to Klaproth, consists of— | Silica | 56·5 | | Alumina | 19· | | Sulphuric acid (SO3) | 16·5 | | Pota a | 4· | | Water | 3· | | Lo | 1· | | ——— | | 100· | which exhibits an exce of about 3% of sulphuric acid, and about 14% of alumina, more than are requisite to form alum with the 4% of pota a; proportions which, therefore, require to be supplemented with a pota ium salt during the proce of manufacture. The alum-stone of Mont d’Or contains, according to Cordier, 1·4% of oxide of iron. The presence of lime in alum-ore is most prejudicial, owing to its affinity for sulphuric acid being greater than that of either alumina or iron. Ores containing it in any quantity are, therefore, unfitted for the manufacture of alum. Magnesia is also prejudicial; but in this case the sulphate of magnesia left in the mother-liquors is not wholly valuele , as it may be crystallised and sold as ‘Epsom-salt,’—a thing which is actually done in some English alum-works. The potash-salt employed by the alum-makers is either the sulphate or the chloride—chiefly the latter; its sources being the waste liquor of soap-works, saltpetre refineries, and gla -houses. Wood-ashes, although rich in potash, do not answer well unle freed by lixiviation from the large amount of carbonate of lime which is always present in them. The ammonia-salt used in making alum is generally the crude sulphate prepared from the ammoniacal liquor of gas-works, or that from the manufacture of sal-ammoniac by the destructive distillation of animal matter. Both these liquors may be used without previous conversion into sulphate of ammonia whenever there is an exce of sulphuric acid in the aluminous solution. Soda-salts are seldom, if ever, used as precipitants in the manufacture of alum, on account of the easy solubility of the resulting SODA-ALUM —a property which unfits them for this purpose. See Alums , Ammonia , Dyeing , Mordants , Potash , Sulphuric Acid , . (also below ).