ON THE MANUFACTURE OF NITRATE OF POTASH (SALTPETRE.)

Previous to the middle of the seventeenth century, the chief part of the saltpetre consumed in this country was obtained from refuse animal matters, as is evident from the following edict, issued by James I., for the regulation of the “mynes of salt peter.”—“The King, taking into his consideration the most necessary and important use of gunpowder, as well for supply of his own royall navie, and the shippinge of his lovinge subjects, as otherwise for the strength, safety, and defence of his people and kingdoms, and how greate a blessinge it is of Almighty God to this realm, that it naturally yieldeth sufficient mynes of salt peter for making of gunpowder for defence of ittself, without anie necessitie to depend uppon the dangerous chargeable and casuall supply thereof from forraigne parts, hath sett downe certen orders and constitutions to be from henceforth inviolably kept and observed, for the better maynteyning of the breed and increase of salt peter, and the true making of gunpowder.

“Noe person doe from henceforth pave with stone or bricke, or floare with boarde, anie dove-house or dove-cote, or laie the same with lyme, sand, gravel, or other thing, whereby the growthe and increase of the myne of salt peter maie be hindered or ympaired, but shall suffer the floure or grounde thereof to lye open with goode and mellowe earth, apt to breede increase of the myne and salt peter, and so contynue and keep the same.

“That no innkeepers, or others that keep stables for travellers and passengers, doe use anie deceiptful meanes or devices whereby to destroy or hinder the growthe of salt peter in those stables. And that no stables at all be pitched, paved, or gravelled where the horse feete used to stand, but planked only, nor be paved, pitched, or gravelled before the plankes next the mangers, but that both places be kept and maynteyned with goode and mellowe earth, fitt and apt to breede and increase the myne of salt peter, and laide with nothinge which may hurte the same. {274}

“That all and every such person and persons as having had heretofore had anie dove-house, dove-cote, or stable (which were then good nurseries for the myne of salt peter) have sithence carried out the goode moulde from thence, and filled the place agayne with lyme, sand, gravel, rubbish, or other like stuff, or paved or floored the same, whereby the growthe of salt peter myne there hath been decayed and destroyed, shall and doe within three months next contryve to take up the pavements and boards agayne, and carrie out the said gravel, lyme, and offensive stuff from thence, and fill the place agayne with goode and mellowe earth fitt for the increase of salt peter, three foote deepe at the least, and so contynue and keepe the same for the breede of salt peter myne. No person, of anie degree whatsoever, was to denie or hinder the salt peter man workinge any earth; nor was anie constable to neglect or to forbeare to furnish him with convenient carriages necessarie for his worke; and every justice to whom the salt peter man should address himself for assistance was at his peril to fail to render it, that his majesties service might not suffer by his default. And no one was to give any gratuity or bribe to the salt peter man for forbearinge or sparinge of anie ground or place which may be digged or wrought for salt peter.”

To lessen the annoyance to the owners of these dove-cotes and stable beds of saltpetre, and to promote the comfort of the pigeons, the saltpetre man was “to dig and carrie away the earth in such convenient time of the daie, and work it in suche manner as maie give least disturbance and hurte to the pigeons, and encrease of their breede, and in the chief tyme of breeding, that it be not done above two howers in anie one daie, and that about the middest of the daie, when the pigeons use to be abroade. And shall in like seasonable tyme carrie in the saide earth after it shall be wrought, and spreade itt there, and make flatt the floure of the dove-house, and leave itt well and orderlie.”

In another proclamation, issued two years after this, it was ordered that whensoever anie ould building or house in London {275} within three miles, is to be pulled down and removed, notice is to be given at the king’s storehouse in Southwark, that the deputy may first take as much of the earth and rubbish as in his judgement and experience is fitted for salt peter for the King’s service.”

Soon after, we find that this enactment which caused much complaint, was repealed. “The manufacture of salt peter,” says the king, “had hitherto produced much trouble and grievance to the lieges, by occasioning the digging up the floors of their dove-cotes, dwelling-houses, and out-houses, and had also occasioned great charge to the salt peter men for removing their liquors, tubbes, and other instruments, and carrying them from place to place, but now, divers compounds of salt peter can be extracted by other methods, for which Sir John Brooke and Thomas Russell, Esq., have received letters patent.

“To encourage so laudable a project, all our loving subjects,” continues his majesty, “inhabiting within every city, town, or village, after notice given to them respectively, shall carefully and constantly keep and preserve in some convenient vessels or receptacles fit for that purpose, all the urine of man during the whole year, and all the stale of beasts which they can save and gather together whilst their beasts are in their stables and stalls, and that they be careful to use the best means of gathering together and preserving the urine and stale, without any mixture of water or other thing put therein. Which our commandment and royal pleasure being so easy to be observed, and so necessary for the public service of us and our people, that if any person be remiss thereof, we shall esteem all such persons contemptuous and ill affected both to our person and state, and are resolved to proceed to the punishment of that offender with that severity we may.”

Sir John agreed to remove the liquid accumulations from the houses once in every twenty-four hours in summer time, and every forty-eight hours in winter time.

About the year 1670, the importation of saltpetre from the East Indies (where it is obtained as a natural product, being {276} disengaged by a kind of efflorescence from the surface of the soil) had so increased as to affect the home manufacture, which has since gradually declined and become extinct. The manufacture of saltpetre from sources of the kind above mentioned, is not followed in this country at the present day, and it will be unnecessary to indicate here the process employed in France, Sweden, Germany, and other countries for obtaining it by the decomposition of animal refuse, the more especially as full accounts are given in Knapp’s Technology, Ure’s Dictionary of Arts and Manufactures, and other standard chemical works; we shall therefore confine our attention to an account of the processes which have been proposed for obtaining nitrate of potash by the decomposition of nitrate of soda and other sources.

The first of these processes is that of adding nitrate of lime to a solution of sulphate of potash; sulphate of lime is precipitated, and nitrate of potash obtained in solution, which, on evaporation yields crystals of that salt.

Mr. Hill’s method of manufacturing nitrate of potash is by decomposing nitrate of soda by means of muriate of potash. For this purpose the nitrate of soda is put into a suitable vessel, made of wrought or cast iron, and dissolved in as much water as is required, and then the equivalent quantity of muriate of potash is added; decomposition ensues, with the formation of nitrate of potash and muriate of soda; the greater portion of the latter is separated during evaporation, as it is equally soluble at all temperatures. The nitrate crystallizes on the cooling of the solution. Specimens of this nitre were shown at the Great Industrial Exhibition.

Mr. Rotch’s processes for converting nitrate of soda into nitrate of potash are as follows:—

First process with American potashes, (caustic).—In a suitable round-bottomed iron boiler, he dissolves 2000 lbs. of the ashes in 1000 quarts of water, and then applies heat for three hours, at the end of which time the solution ought to be of a density of 45° Baumè, (sp. gr. 1.453). In a similar boiler he dissolves 1300 lbs. of nitrate of soda in 1200 quarts of water, {277} applying the heat as before, until the solution becomes of the density of 45° Baumé. Both solutions are then allowed to stand for twelve hours to cool and settle. They should be heated to from 175° to 200° Fah., and then both poured into a third vessel or crystallizing pan, when the double decomposition will take place, and the crystals of nitrate of potash be deposited, this first deposition giving from 700 to 900 lbs. of good merchantable saltpetre.

Care must be taken not to let the heat fall below 85°, at which the crystals form; and the better and more regularly the heat is kept up, the speedier will be the deposition of the crystals. The mother-liquor should then be poured off, and the crystals collected and thrown into the centrifugal drying machines, where they may be washed with weak mother-liquors. The portion of nitrate of potash that is left in the mother-liquor may be obtained by cry­stal­li­za­tion as before.

Second process with carbonate of potash (Pearlash).—The pearlash is dissolved in water, and the solution brought to a density of 40° Baumé (sp. gr. 1.384). This will cause whatever sulphate of potash may be contained in it to be deposited. The solution should then be left to stand for five or six days, after which it should be poured off, and diluted with water, until its density becomes 15° Baumé (sp. gr. 1,116). Caustic lime should then be added in the proportion of one-fourth of the weight of the original quantity of carbonate employed. It should then be poured off from the carbonate of lime formed, heated and mixed with the solution of nitrate of soda, as above described. The precise proportions that the caustic alkali should bear to the nitrate of soda, are forty-eight parts of the former to eighty-six parts of the latter. The materials to be used should be tested, so as to enable the just proportions to be arranged according to the formula just given. The patentee states that by this means a nitre is produced which is equal to the Bengal saltpetre, after the latter has gone through the expensive process of refining.

A Stockholm manufacturer says:—“On dissolving nitrate of {278} soda in excess of caustic potash solution, and evaporating to 28° or 32° Baumé (sp. gr. 1.241 or 1.285), the chief part of the saltpetre crystallizes, contaminated by the magnesia which is precipitated, and a small quantity of carbonate of lime. In order to obtain the whole of the saltpetre, the solution must be concentrated to 45° or 50° Baumé (sp. gr. 1.453 or 1.530). Here however, a difficulty arises; the cast iron crystallizing vessels are not impermeable to the liquor, which, whatever the thickness of the vessels, oozes through them, thus occasioning great loss. The saltpetre which still remains in solution after cry­stal­li­za­tion in the caustic solution at 30° Baumé (sp. gr. 1.263), cannot be collected, and if it be employed in the manufacture of soap, this will be found to contain so large a proportion of saltpetre, that it deliquesces and falls to pieces in a few days.”

“A method employed in the Russian manufactories is first to dissolve the fine pearlash, and the nitrate of soda in the relative proportions of water required for their mutual decomposition, or rather with an excess of potash in such a quantity of water that the resulting product remains dissolved at 50° Reaumur. The solution is then allowed to settle, whereby the carbonates of lime and magnesia are deposited, after which the liquor is run off into wooden crystallizing vessels. As soon as the temperature is lower than 50° Reaumur, the principal part of the nitrate of potash crystallizes. The cry­stal­li­za­tion must now be very attentively watched, for as soon as the soda begins also to crystallize, the mother-liquors should be run off into other vessels, where a small quantity of nitrate of potash will crystallize, though the principal part will be soda. The nitrate of potash and the soda must then be purified by new cry­stal­li­za­tions. The salts formed from the mother-liquors must be redissolved with the nitrate of potash or the soda, according to which of the two most predominates.”

Messrs. Crane and Jullion patented in 1848 the following method of manufacturing the nitrates of potash and soda:—The oxides of nitrogen evolved in the process of manufacturing oxalic acid, are mixed with oxygen gas or atmospheric air, and {279} made to pass slowly through a chamber or other apparatus containing an alkali placed on trays (similar to the lime in a dry lime purifier), the mixed gases combine with the alkali, forming a nitrate of potash or soda, whichever alkali may have been employed.

De Sussex’s process for the manufacture of nitrate of potash is as follows:—A solution is made of 166 pounds of nitrate of lead, and another of 76 pounds of chloride of potassium. The two solutions are then mixed, when double decomposition takes place, chloride of lead being precipitated, and nitrate of potash obtained in solution. In order to avoid the presence of lead in the nitrate of potash, a small portion of caustic or carbonated lime or magnesia is added, by which means any portion of the chloride of lead remaining in solution is precipitated. The solution of nitrate of potash is then evaporated and crystallized.

Nitrate of soda is obtained in the same way, by substituting sixty-six pounds of chloride of sodium for the chloride of potassium above mentioned.—Phar­ma­ceu­ti­cal Journal and Transactions, July 1, 1852.