A Dictionary of Arts (supplement): Alum.

(A Dictionary of Arts, Manufactures, and Mines; containing A Clear Exposition of Their Principles and Practice)
Recent improvements in
Arts, Manufactures, and Mines:
Being A supplement to his Dictionary
by Andrew Ure, M. D.,
F.R.S. N.G.S. M.A.S. LOND.; M. ACAD. N.S. PHILAD.; S. PH. SOC.N. GERM. HANOV.; MUHL. ETC., ETC.

Illustrated with one hundred and ninety engravings.

New York: D. Appleton & Company, 200 Broadway.  Philadelphia: George S. Appleton, 148 Chestnut St.
MDCCCXLVII
1847

ALUM. In the alum works on the Yorkshire coast, 8 different liquors are met with.

1st. "Raw Liquor." The calcined alum shale is steeped in water till the liquor has acquired a specific gravity of 9 to 10 pennyweights, according to the language of the alum-maker.

2d. "Clarified Liquor." The raw liquor is brought to the boiling point in lead pans, and suffered to stand in a cistern till it has cleared: it is then called clarified liquor. Its gravity is raised to 10 or 11 pennyweights.

3d. "Concentrated Liquor." Clarified liquor is boiled down to about 20 pennyweights. This is kept merely as a test of the comparative value of the potash salts used by the alum-maker.

4th. "Alum Mother Liquor." The alum pans are fed with clarified liquor, which is boiled down to about 25 or 30 pennyweights, when a proper quantity of potash salt in solution is mixed with it, and the whole run into coolers to crystallize. The liquor pumped from these rough crystals is called "alum mothers."

5th. "Salts Mothers." The alum mothers are boiled down to a crystallizing point, and afford a crop of "Rough Epsom," which is a sulphate of magnesia 6th and 7th.  "Alum Washings." The rough crystals of alum (No. 4), are washed twice in water, the first washing being about 4 pennyweights, the second about 2½, the difference in gravity being due mother liquor clinging to the crystals.

8th. "Tun Liquor." The washed crystals are now dissolved in boiling water, and run into the "roaching tuns" (wood vessels lined with lead) to crystallize. The mother liquor of the "roach alum" is called "tun liquor;" it is, of course, not quite so pure as a solution of roach alum in water.

The alum-maker's sp. gr. bottle holds 80 pennyweights of water, and by 10 pennyweights he means 10 more than water, or 90.

The numbers on Twaddle's hydrometer, divided by 2.5, gave alum-maker's pennyweights.

The alum-maker tests his samples of potash salts comparatively by dissolving equal weights of the different samples in equal measures of alum liquor at 20 pennyweights, heated up to the boiling point, and weighing the quantity of alum crystals produced on cooling.

For the above information I am indebted to my friend Mr. Maurice Scanlan, who superintended for some time the Mulgrave alum works.

He informs me that 61½ tons of the alum rock at the Mulgrave works to the north of Whitby, yield, after calcination, &c., one ton of alum.

It has been computed that with sulphur at 6l. per ton, sulphuric acid of spec. grav. 1.750 can be produced at 3l. per ton, including the mere cost of making: this acid contains 2 atoms of water: 174 tons of this acid, and 87½ tons of sulphate of potash, with the pipe-clay, will form 474 tons of alum; so that the neat cost would be 522l., for the acid+1047l. for the sulphate of potash =1569l.; which sum, divided by 474, gives a quotient of 3l. 6s. for the neat cost of 1 ton of alum by the direct process.

At the pit 1 ton of alum, rock or mine, coast 3l. 4s.; to which, adding the cost of the potash salt for 1 ton of alum, 3l. 5s., they constitute together an amount of 6l. 19s. From the latter sum 1l. 10s. must, however, be deducted for the value of rough Epsom salt produced, leaving a balance of 5l. 9s. for the cost of a ton of mine-alum, prior to evaporation and crystallization.

A patent was obtained in November, 1839, by Mr. William Wiesmann, of Duesburg, for improvements in the manufacture of alum. He subjects potter's clay to a moderate red heat, grinds it, and subjects the powder, in leaden pans, to the action of concentrated sulphuric acid (66° Beaumé), taking care to use excess of clay and a moderate heat. This mixture is to be stirred till it is dry, then treated with boiling water, in order to dissolve the sulphate of alumina formed. This mixture is to be stirred till it is dry, then treated with boiling water, in order to dissolve the sulphate of alumina formed. So far the process is old and well-known. The novelty consists in freeing the saline solution from iron by ferrocyanure of potassium (prussiate of potash). When the iron has been all thrown down in the form of prussian blue, the liquor is allowed to settle, the supernatant pure sulphate is drawn off, and evaporated till it forms on cooling a concrete mass, which may be moulded into the shape of bricks, &c., for the convenience of packing. He proposes to crystallize his alum; but he will find this process rather difficult. The prussian blue obtained may be reconverted by any alkaline solution into a ferrocyanide, and again employed on a fresh quantity of the raw sulphate. How he is to precipitate the iron by sulphate of lime, as he states, I can not comprehend.

Dr. Turner's process for making alum from felspar is thus described by him in the specification of his patent, sealed October 8, 1842. If it be desired to make a potash alum, the best substance to operate upon is a potash felspar. This felspar is ground in a common edge-stone mill to the consistency of fine sand (a process which is much assisted by first heating it to redness, and then plunging it in cold water); it is then mixed with its own weight of sulphate of potash, and placed in the upper part of the inclined bed of a reverberatory furnace (being such a furnace as is known in the potteries as a frit furnace), and which furnace has previously been brought to a full white heat. When by the action of the heat a glass has been produced, and is obtained to flow down the inclined bed of the furnace, to such glass is to be added gradually, at the lower end of the furnace, as much carbonate of potash as was before used of sulphate of potash. And this process of placing the mixture of felspar and sulphate of potash at the upper part of the bed of the furnace is to be repeated, adding at the lower part of the bed, gradually and proportionally, as the glass flows down from the upper part, the carbonate of potash, as before mentioned. This is continued until the sack of the furnace is filled with the glass; this glass is then fit for the next process. The preparation of the glass may also be affected in a reverberatory furnace with a flat bed; and the facility of removing the glass from such a furnace is an advantage. In this case no carbonate must be added to the mixture until the sulphate of potash is observed to be completely decomposed. On boiling in water the glass thus obtained, the  same quantity of potash as was added to the felspar and two thirds of the silica contained in the felspar are dissolved, while the remaining one third of the silica and the alumina, and an equal quantity of potash as the felspar originally contained, are left in the form of a light porous substance, similar in chemical composition to the mineral commonly called elæolite; this porous substance is carefully separated from the said solution, and washed with water until freed from the silicate of potash, then placed in an open leaden cistern or boiler, and boiled with dilute sulphuric acid of the specific gravity 1.2 (one and two tenths). This acid will contain about the quantity of water required for the solution and crystallization of the alum produced by the decomposition of the elæolite the quantity of the dilute sulphuric must be such as will contain about 160 lbs. of dry sulphuric acid for every 285 lbs. of felspar rock (if that rock be used), and in like proportion to the silica and alumina contained in the substance, if any other substance be used, as it is important that the alum solution thus obtained should not contain an excess of acid. I recommend that only four fifths of the proposed quantity of dilute sulphuric acid should be used in the first operation, which will leave a portion of the elæolite undecomposed; but by acting upon this undecomposed potion, after the solution has been drawn off, with the full quantity of dilute acid to be used in the next operation, it will be completely decomposed, and the alum thus formed becomes part of the next batch. In this way a neutral solution of alum is obtained at each process. The boiling solution, after the sediment subsides, is drawn into coolers, such as are commonly used for the crystallization of alum; here about four fifths of the alum held in solution will form into crystals. The mother-liquor from the coolers is boiled in any convenient boiler to dryness, in order to render the silica it contains insoluble; the residuum is boiled either in water or in the mother-liquor from the roaching tubs, so as to dissolve the alum it contains, and the process of crystallization repeated. Had the above process been performed with the salts of soda instead of potash, a soda alum would have been formed. For this purpose the soda felspar or albite should be selected. The potash or soda (as the case may be) contained in the liquor, drawn as aforesaid from the elæolite (or nepheline, which is formed when soda is used), may be recovered by either of the following processes: The strong solutions which are obtained, about the specific gravity of 1.2 are placed in any convenient vessel in which a stream of carbonic acid gas, obtained in any convenient method, may be driven through them, the carbonic acid becomes absorbed, and the solution assumes the form of a gelatinous mass: this mass consists of carbonate of potash or soda and hydrate of silica. On drying this mass in a furnace, which must never be allowed to rise to red heat even in the dark, the silica loses its water and becomes insoluble; the potash or soda may then be separated from it in the form of a sesquicarbonate of  potash or soda, by solution and evaporation to dryness. The other process, which under most circumstances will be found more economical and convenient, is to allow the boiling solution of silicate potash or soda to filter through a bed of caustic lime, when it will be found that the lime has combined with the silicate, and a caustic potash or soda ley is obtained. This process may be conveniently conducted in an apparatus similar to that used by soap-makers for the preparation of their caustic leys. The potash or soda may then be readily obtained as caustic potash or soda, or as carbonate, by the known processes used in making soda. The weak solutions of silicate of potash or soda are used to decompose another portion of the glassy substance.

now I do not claim as new, or as my invention, any particular form of vessel or apparatus in which or with which my operations may be conducted; nor do I claim any particular proportions in which the alkaline salts may be used. but I claim as new, and as my incention, the improvements aforesaid, and the production of substances similar to elælite and nepheline artificially, by the decomposition by water of the glassy susbtances produced by the fusing of felspar as aforesaid, or other mineral substances containing silica and alumina, with salts of potash and soda, as aforesaid, and the use and application of such artificial elælite and nepheline in the production and manufacture of alum, as aforesaid. I also claim the process, as above described, for separating the alkalies from silica by means of caustic lime.