ALKALI MANUFACTURE. The word “alkali” denotes both soda and potash, but by “alkali manufacture” we understand merely the manufacture of sodium sulphate, carbonate and hydrate. The corresponding potash compounds are not manufactured in the United Kingdom, but exclusively in Germany (from potassium chloride and from the mother-liquor of the strontia process in the manufacture of beetroot sugar) and in France (from vinasse). The term alkali is employed in a technical sense for the carbonate and hydrate (of sodium), but since in the Leblanc process the manufacture of sodium sulphate necessarily precedes that of the carbonate, we include this as well as the manufacture of hydrochloric acid which is inseparable from it. We also treat of the utilization of hydrochloric acid for the manufacture of chlorine and its derivatives, which are usually comprised within the meaning of the term “alkali manufacture.” A great many processes have been proposed for the manufacture of alkali from various materials, but none of these has become of any practical importance except those which start from sodium chloride (common salt); and among the latter again only three classes of processes are actually employed for manufacturing purposes, viz. the Leblanc, the ammonia-soda, and the electrolytic processes.
I. The Leblanc Process
The Leblanc process, which was invented by Nicolas Leblanc (q.v.) about 1790, begins with the decomposition of sodium chloride by sulphuric acid, by which sodium sulphate and hydrochloric acid are produced. The sodium sulphate is afterwards fluxed with calcium carbonate and coal, and a mixture is thus obtained from which sodium carbonate can be extracted by exhausting it with water.
Leblanc himself for a time carried out his process on a manufacturing scale, but he was ruined in the political troubles of the time and died by his own hand in 1806. His invention was, however, at once utilized by others in France; and in Great Britain, after a few previous attempts on a small scale, it was definitely introduced by James Muspratt (q.v.) in 1823. From that time onward the Leblanc process spread more and more, and for a considerable period nearly all the alkali of commerce was made by it. The rise of the ammonia-soda process (since 1870) gradually told upon the Leblanc process, which in consequence has been greatly restricted in Great Britain and Germany, and has become practically extinct in all other countries, except as far as its first part, the manufacture of sodium sulphate and hydrochloric acid, is concerned.
The production of alkali in Great Britain, soon after the introduction of the Leblanc process, became the most extensive in the world, and outstripped that of all other countries put together. With the rise of the ammonia-soda process, for which the economic conditions are nearly as favourable in other countries, the predominance of Great Britain in that domain has become less, but even now that country produces more alkali than any other single country. Most of the British alkali works are situated in South Lancashire and the adjoining part of Cheshire, near the mouth of the Tyne and in the West of Scotland. Various industries are carried on in Leblanc alkali works, as follows:—
- Manufacture of sodium sulphate.
- Manufacture of hydrochloric acid.
- Preparation of chlorine.
- Employment of chlorine for the manufacture of bleaching-powder and of chlorates.
- Manufacture of ordinary alkali from sulphate of soda.
- Manufacture of caustic soda.
- Manufacture of soda crystals.
- Recovery of sulphur from alkali waste.
1. Manufacture of Sodium Sulphate.—This is commercially known as salt-cake, and is made by decomposing common salt with sulphuric acid of about 80%, the reaction being 2NaCl + H2SO4 = Na2SO4 + 2HCl. This reaction proceeds in two stages. At first principally acid sodium sulphate, NaHSO4, is formed together with some normal sulphate; later, when the temperature has risen, the NaHSO4 acts with more NaCl so that nearly all of it is converted into Na2SO4. The gaseous hydrochloric acid evolved during all this time must be absorbed in water, unless it is directly converted into chlorine (see below, 2 and 3).
The process is carried out either in hand-wrought furnaces, or mechanical furnaces, both called “decomposing” or “salt-cake furnaces.” In the former case, the first reaction is produced in cast- iron pans or “pots,” very heavy castings of circular section, fired from below, either directly or by the waste heat from the muffle-furnace. The reaction is completed in a “roasting- furnace.” The latter was formerly often constructed as a reverberatory furnace, which is easy to build and to work, but the hydrochloric acid given off here, being mixed with the products of the combustion of fuel, cannot be condensed to strong acid and is partly, if not entirely, wasted. It is, therefore, decidedly preferable to employ “muffle-furnaces” in which the heating is performed from without, the fire-gases passing first over the arch and then under the bottom of the muffle. This requires more time and fuel than the work in “open” furnaces, but in the muffles the gaseous hydrochloric acid is separated from the fire-gases, just like that evolved in the pot, and can therefore be condensed into strong hydrochloric acid, like the pot-acid. This roaster-acid is, however, of less value than the pot-acid, as it contains more impurities.
It is not easy to keep the muffles permanently tight, and as soon as any leakages occur, either hydrochloric acid must escape into the fire-flue, or some fire-gases must enter into the muffle. The former is decidedly more objectionable than the latter, as it means that uncondensed hydrochloric acid is sent into the air. This drawback has been overcome by the construction of “plus-pressure” furnaces (figs. 1 and 2), where the fire-grate is placed 11 ft. below the top of the muffle. In consequence the fire-gases, when arriving there by the chimney shaft (a), have already a good upward draught, and when circulating round the muffle are at a lower pressure than the gases within the muffle, so that in case of any cracks being formed, no hydrochloric acid escapes into the fire-flues, but vice versa.
Figs. 1. and 2.—Salt-cake Furnace. (Sectional Elevation and Plan.)
Since the work with ordinary hand-wrought salt-cake furnaces is disagreeable and costly, many attempts have been made to construct mechanical salt-cake furnaces. Of these J. Mactear’s furnaces (fig. 3) have met with the greatest success. They consist of a horizontal pan, 17 ft. wide, which is made up of a central pan (e), and a series of concentric compartments (c1), (c2), (c3), and which is supported on a frame (d d), revolving round a perpendicular axis on the wheels (n n). It is with an arch and heated on the top from one side (l), either by an ordinary coal-grate or by a gas-producer. A set of stirring blades carried in the frame (b b), and driven by gearing, passes through a gap in the arch in such a manner that the gases cannot escape outwards. The salt is conveyed to the furnace by a chain of buckets running on the pulley (g), and passing into the hopper (h), and through the pipe (i) is mixed with the proper amount of acid supplied by the pipe (f). The mixture is fed in continuously to the central pan (e), whence it overflows into the compartments (c1), (c2), (c3) successively until it reaches the circumference, where it is discharged continuously by o and p into the collecting-box (q), being now converted into salt-cake. This furnace acts very well, and has been widely introduced both in Great Britain and in other countries, but it has one great drawback, apart from its high cost, viz. that all the hydrochloric acid gas gets mixed with fire-gases, and consequently is condensed in a weaker and less pure form than from ordinary pots and muffles. This has led some factories which had introduced such furnaces to revert to hand-wrought muffle-furnaces.
Much was expected at one time from the “direct salt-cake process” of Hargreaves and Robinson, in which common salt is subjected subjected in a series of large cast-iron cylinders to the action of pyrites-burner gases and steam at a low red heat. The reaction going on here is: 2NaCl + SO2 + O + H2O = Na2SO4 + 2HCl. This means that the previous manufacture of sulphuric acid in the vitriol-chambers is done away with, but this apparently great simplification is balanced by the great cost of the Hargreaves plant, and by the fact that the whole of the hydrochloric acid is mixed with nine or ten times its volume of inert gases. Owing to this, it is practically impossible to condense the gaseous hydrochloric acid into the commercial acid, although this acid may be obtained sufficiently strong to be worked up in the Weldon chlorine process (see below, 3). Therefore the Hargreaves process has been introduced only in a few places.
Although the consumption of salt-cake for the manufacture of alkali is now much less than formerly, since the Leblanc alkali process has been greatly restricted, yet it is largely made and will continue to be made for the use of glassmakers, who use it for the ordinary description of glass in the place of soda-ash. Nor must it be overlooked that salt-cake must be made as long as there is a sale for hydrochloric acid, or a consumption of the latter for the manufacture of chlorine.
Fig. 3.—Mechanical Salt-cake Furnace. (Sectional Elevation.)
2. Manufacture of Hydrochloric Acid (commercially also known as “muriatic acid”). This unavoidable gaseous bye-product of the manufacture of salt-cake was, during the first part of the 19th century, simply sent into the air. When its deleterious effects upon vegetation, building materials, &c., became better known, and when at the same time an outlet had been found for moderate quantities of hydrochloric acid, most factories made more or less successful attempts to “condense” the gas by absorption in water. But this was hardly anywhere done to the fullest possible extent, and in those districts where a number of alkali works were located at no great distance from one another, their aggregate escapes of hydrochloric and other acids created an intolerable nuisance. This was most notably the case in South Lancashire, and it led to the passing of Lord Derby’s “Alkali Act,” in 1863, supplemented by further legislation in 1874, 1881 and later. There is hardly another example in the annals of legislative efforts equal to this, in respect of the real benefit conferred by it both on the general public and on the manufacturers themselves. This is principally the consequence of the exemplary way in which the duties of inspector under these acts were carried out by Dr R. Angus Smith (1817–1884) and his successors, who directed their efforts not merely to their primary duty of preventing nuisance, but quite as much to showing manufacturers how to make the most of the acid formerly wasted in one shape or another. Not merely Great Britain but all mankind has been immensely benefited by the labours of the British alkali inspectors, which were, of course, supplemented by the work of technical men in all the countries concerned. The scientific and technical principles of the condensation of hydrochloric acid are now thoroughly well understood, and it is possible to recover nearly the whole of it in the state of strong commercial acid, containing from 32 to 36% of pure hydrochloric acid, although probably the majority of the manufacturers are still content to obtain part of the acid in a weaker state, merely to satisfy the requirements of the law prescribing the prevention of nuisance. The principles of the condensation, that is of converting the gaseous hydrochloric acid given off during the decomposition of common salt into a strong solution of this gas in water, can be summarized in a few words. The hydrochloric acid gas, which is always diluted with air, sometimes to a very great extent, must be brought into the most intimate contact possible with water, which greedily absorbs it, forming ordinary hydrochloric acid, and this process must be carried so far that scarcely any hydrochloric acid remains in the escaping gases. The maximum escape allowed by the Alkali Acts, viz. 5 % of the total hydrochloric acid, is far above that which is now practically attained. For a proper utilization of the condensed acid it is nearly always imperative that it should be as strong as possible, and this forms a second important consideration in the construction of the condensing apparatus. Since the solubility of hydrochloric acid in water decreases with the increase of the temperature, it is necessary to keep the latter down—a task which is rendered somewhat difficult both by the original heat retained by the gases on their escape from the decomposing apparatus, and by the heat given off through the reaction of hydrochloric acid upon water.
Very different methods have been employed to effect all the above purposes. In Great Britain Gay-Lussac’s coke-towers, adapted by W. Gossage to the condensation of hydrochloric acid, are still nearly everywhere in use, frequently combined with a number of stone tanks through which the gas from the furnaces travels before entering the towers, meeting on its way the acid condensed in the tower. This process is excellent for effecting a complete condensation of the hydrochloric acid as prescribed by the Alkali Acts, and for recovering the bulk of the acid in a tolerably strong state, but less so for recovering nearly the whole of it in the most concentrated state, although even this is occasionally attained. On the continent of Europe, where the last-named requirement has been for a long time more urgent than in Great Britain, another system has been generally preferred, namely, passing the gas through a long series of stoneware receivers, and ultimately through a small tower packed with stoneware or coke, making the acid flow in the opposite direction to the gas. Great success has also been obtained by “plate-towers” made of stoneware, which allow both the coke-towers and most of the stoneware receivers to be dispensed with.
3. Preparation of Chlorine.—In this place we speak only of the preparation of chlorine from hydrochloric acid by chemical processes; the electrolytic processes will be treated hereafter. It is clear that free chlorine must be prepared from hydrochloric acid by oxidizing the hydrogen. This can be done most easily by “active” oxygen, such as is present in the peroxides, in chromic or permanganic acid. Practically the only agent employed in this way, and that already by C. W. Scheele, the discoverer of chlorine, in 1774, is the peroxide of manganese (manganese dioxide), found in considerable quantities in nature as “manganese ore” (the purest of which is called pyrolusite), and also artificially regenerated from the waste liquors of a former operation. Even now, where chlorine is required for immediate use in some other chemical operations on a comparatively small scale, it is obtained by the action of hydrochloric acid on native manganese dioxide, according to the equation: MnCl2 + 4HCl = MnCl2 + Cl2 + 2H2O. This action must be promoted by heating the mixture, but even then nothing like all of the hydrochloric acid employed is made to act as above, because the attack on the manganese ore requires a certain minimum concentration of the acid. Formerly, instead of free hydrochloric acid a mixture of common salt and sulphuric acid was sometimes employed, but this is never done on a manufacturing scale now. Owing to the impossibility of employing any metal in contact with the acid, the “chlorine stills,” where the above reaction is carried out, must be made of acid-proof stones or “chemical” stoneware. This process is very costly, as much of the acid and all of the manganese is wasted. Moreover it is of a most disagreeable kind, as the waste “still-liquor,” containing very much free hydrochloric acid and even some free chlorine, forms a most deleterious impurity when finding its way into drains or water- courses, apart from the intolerable nuisance caused by the escapes of chlorine from the stills and otherwise, which cannot be at all times avoided.
Many endeavours were made to avoid the loss of the manganese in this operation, but with only partial or no success. The difficulty was only overcome by the Weldon process, being the inventions of Walter Weldon from 1866 onwards, and his process up to this day furnishes the greater proportion of chlorine manufactured in the world. It begins with “still-liquor,” obtained in the old way from native manganese ore and hydrochloric acid. This liquor is first treated with carbonate of lime (ground chalk or limestone) in a “neutralizing-well,” made of acid-proof material and provided with wooden stirring-gear. Here the free hydrochloric acid is converted into calcium chloride, and at the same time any ferric chloride present is converted into insoluble ferric hydroxide: 2FeCl3 + 3CaCO3 + 3 H2O = 2Fe(OH)3 + 3CaCl2 + 3CO2. The sulphuric acid present is mostly precipitated as calcium sulphate. The mud thus formed is settled out, and the clear liquor, which is now quite neutral and contains both manganese and calcium chlorides, is mixed with cream of lime and treated by a strong current of air, produced by a blowing-engine. This is done in a tall iron cylinder, say 9 ft. wide and 30 ft. high, called the “oxidizer.” The air-pipe goes right to the bottom of the cylinder and there branches out into perforated side-pipes, so that the mass is thoroughly stirred up all the time. The first action of the lime is to convert the manganese chloride into manganous hydrate (Mn(OH)2) and calcium chloride; then more lime is added which greatly promotes and hastens the oxidizing process. The object of the latter is to convert the manganous hydroxide by the atmospheric oxygen into manganese dioxide, but this would take place much too slowly if there was not an excess of lime present ready to combine with the manganese dioxide to form a calcium manganite. Only so much lime is used that an acid manganite is formed corresponding to one molecule of calcium oxide to two of manganous oxide. This additional lime, which is called the “basis,” certainly takes up hydrochloric acid in the next stage of the process, but that causes no more waste of acid than the incomplete action on native manganese ore, mentioned before. The product obtained, called “Weldon mud,” is of such fine texture that it acts immediately with hydrochloric acid when mixed with it in the “Weldon stills” (fig. 4), and that this acid can be almost entirely neutralized thereby. The new still liquor formed in this manner is treated as above, so that the manganese does its work over and over again. There is only a slight mechanical loss, which is reduced in the best managed works to about 2 parts of manganese dioxide to 100 of bleaching- powder. There are also other advantages of this process which explain its wide extension, in spite of the fact that only from 30 to 35 parts of the hydrochloric acid employed is converted into chlorine, the remainder ultimately leaving the factory in the shape of a harmless but useless solution of calcium chloride.
Weldon’s later attempts at superseding his classical process by other inventions which utilize a larger proportion of the chlorine, introduced as hydrochloric acid, have not been successful in the long run, although some of them were aided by the great technical skill of A. R. Pechiney. But the Deacon process, the invention of Henry Deacon (who was greatly aided by his chemist Dr Ferdinand Hurter), carried out since 1868, has attained to better, although nothing like complete, success in that direction.
|Fig. 4.—Weldon Chlorine Still. (Sectional Elevation.) C, Stone steam column resting in stone socket K.|
The Deacon process, like the Weldon process, effects its object by the oxidizing action of atmospheric air, but in a very different manner. Weldon retained the principle of the Scheele process by employing the active oxygen of manganese dioxide to convert hydrochloric acid into free chlorine, and he employed the atmospheric oxygen only indirectly, for the recovery of manganese dioxide from the manganese chloride formed. But Deacon worked on the direct reaction: 2HCl + O = H2O + Cl2. This reaction in ordinary circumstances is so slow as to be practically useless. If, however, a “contact-substance” is employed and that at the proper temperature, the process goes on at an immensely quickened rate and can even be carried out as a continuous operation. The only substance which possesses sufficiently strong catalytic properties for the reaction is cupric chloride. If pieces of porous clay are soaked in a solution of this salt and dried and kept at a temperature of 450° C. (in practice it is necessary to go to a rather higher temperature), it is possible continuously to convert a united stream of hydrochloric acid and atmospheric air, passed through the contact- substance in a “decomposer” (fig. 5), to a larger extent into chlorine and water, of course mixed with the excess of oxygen and all the nitrogen of the air. On a small scale it is possible to push the decomposition as far as 90% of the hydrochloric acid, but on the large scale only at most 60% is reached. The mixture of hydrochloric acid and air is taken directly from the “decomposing-pan” of an ordinary salt-cake furnace, is first cooled down in pipes sufficiently to condense most of the moisture present (together with about 8% of the hydrochloric acid), and then passed through a cast-iron superheater and from this into the “decomposer.” The gaseous mixture, issuing from the latter, is washed with water in the usual condensing apparatus, to remove the 40 or 50 parts of hydrochloric acid left unchanged, and can then be immediately employed for the manufacture of chlorate of potash.
Where (as is the more usual case) the chlorine has to serve for the manufacture of bleaching-powder, it must first be deprived of the great amount of moisture which it contains, by means of coke-towers fed with moderately strong sulphuric acid. As the gas issuing from these contains only about 5 volumes % of hydrochloric acid, it cannot be made to act upon lime in the ordinary bleaching-powder chambers, but specially constructed chambers must be provided (see fig. 4). The movement of the gases through all this complicated set of apparatus is produced by a Root’s blower placed at the end of it all.
The Deacon process makes cheaper chlorine than the Weldon process, but the plant is complicated and costly and the working requires a great deal of attention. In skilled hands it has been proved to yield excellent results.
The hydrochloric acid from the calcining-furnaces or “roasters” cannot be employed immediately for the Deacon process, as the sulphuric acid always contained in the roaster gases soon “poisons” the contact-substance and renders it inoperative. This acid must, therefore, be condensed in the ordinary way into liquid hydrochloric acid and formerly could be worked up only by the Weldon process. R. Hasenclever has overcome this drawback by running this impure acid into moderately strong sulphuric acid (140° Twaddell), blowing in air at the same time. This produces a mixed current of pure hydrochloric acid gas and air, which is carried into a Deacon decomposer where it acts in the usual manner. The sulphuric acid, of which 6 or 7 parts are used to one of impure liquid hydrochloric acid, is always reserved for use in the same process, by driving off the excess of water in a lead pan, fired from the top, so that the principal expense of the process is that of the fuel required for the last operation.
4. Applications of Chlorine.—Some of the chlorine manufactured (practically only such as is obtained by the electrolysis of chlorides) is condensed by cold and pressure into liquid chlorine. If this is anhydrous, as it must be in any case for this purpose, it does not act upon the metal of the compressors, nor upon the iron bottles in which it is sent out. It may even be sent out in tank wagons, similar to those which are employed for carrying sulphuric acid, holding 10 tons each.
Sometimes the chlorine is employed directly for bleaching purposes, especially for some kinds of paper. A number of organic chlorinated products are also produced on a large scale. But most of the chlorine is utilized for the production of bleaching-powder, of bleach-liquor, and of chlorate of potash.
Bleaching-powder is a compound obtained by the action of free chlorine on hydrated lime, containing a slight excess of water at ordinary temperatures or slightly above these. Its composition approaches the formula CaOCl2, and it is regarded as a double salt of calcium chloride and hypochlorite, which by the action of water splits up into a mixture of these salts. It always contains a certain quantity of chemically combined water and also an excess of lime. Usually this lime is regarded only as mechanically mixed with the bleaching-compound, CaOCl2, but some chemists adopt formulae in which this lime is equally represented.
For the manufacture of bleaching-powder, limestone of high degree of purity (especially free from magnesia and iron) is carefully burned so as to drive out nearly all the carbon dioxide without overheating the lime. The quick-lime is then slaked with the requisite quantity of water; the product is passed through a fine-meshed wire sieve and is spread in layers of 2 or 3 in. at the bottom of large boxes, the “bleaching-powder chambers,” made of lead, or sometimes of cast-iron protected by paint, of slate or even of tarred wood. Chlorine, generated in an ordinary or a Weldon still, is passed in and is rapidly absorbed. When the absorption becomes slow, the gas is cut off and the chamber is left to itself for twelve hours or more, when it will be found that all the chlorine has been taken up. Now the door of the chamber is opened, the powder lying at the bottom is turned over and the treatment with gas is repeated. Sometimes a third treatment is necessary in order to get the product up to the strength required in commerce, viz. 35% of “available” chlorine. The finished product is packed into wooden casks lined with brown paper. The work of packing is a most disagreeable and unhealthy operation which is best relieved by erecting the chambers at a higher level and placing the casks underneath, communication being made by means of traps in the chamber-bottom. so that the packers can do their work outside the chambers. The bleaching-powder casks must be kept in a dry place, as cool as possible, and never exposed to the direct rays of the sun, in order to prevent a decomposition which now and then has even led to explosions.
The weak chlorine from the Deacon process cannot be treated in this manner, as chambers of impossibly large dimensions would be required. Originally the absorption of the Deacon chlorine took place in a set of chambers, constructed of large slabs of stone, containing a great many horizontal shelves superposed over one another. About sixteen such chambers were combined in such manner that the fresh gas passed into that chamber which had been the longest time at work and in which the bleaching-powder was nearly finished, and so forth until the gas, now all but entirely exhausted, reached the last-filled chamber in which it met with fresh lime and there gave up the last of the chlorine. These “Deacon chambers” occupied a large space, besides being expensive to build and difficult to keep in repair.
They are now mostly replaced by an apparatus, the invention of R. Hasenclever, consisting of four horizontal cast-iron cylinders with internal stirring-gear. The fresh lime is continually charged into the top cylinder, is gradually moved towards the other end, falls down into the next lower cylinder and thus gradually makes its way to the lowest cylinder. The weak chlorine gas from the Deacon apparatus travels precisely the opposite way, from the bottom upwards, the result being that finished bleaching-powder is continually discharged at the bottom and air free from chlorine leaves the apparatus at the top.
Bleaching-powder is manufactured to the extent of several hundred thousands of tons annually, almost entirely for the use of papermakers and cotton bleachers. Smaller quantities are used for disinfection and other purposes. It is usually sold in “tierces,” that is, casks containing about 10 cwt.
Bleach-liquors.—If the chlorine is made to act on cream of lime, care being taken that the temperature does not rise above 35° and that the chlorine is not in excess, a solution is obtained containing a mixture of calcium chloride and hypochlorite which is a very convenient agent for bleachers, but which does not bear the expense of carriage over long distances. Similar liquids are obtained with a basis of sodium (“eau de Javel”), by passing chlorine into solutions of sodium carbonate. The former kind of bleach-liquor is mostly used in the industry of cotton, the latter in that of linen.
Chlorate of Potash.—Formerly all chlorate of potash, as some is still, was obtained by passing chlorine into milk of lime, allowing the temperature to rise almost to the boiling-point, and continuing until the bleaching-solution, originally formed, is converted into a mixture of calcium chlorate and chloride, the final reaction being 6Ca(OH)2 + 6Cl2 = 5CaCl2 + Ca(ClO3)2 + 6H2O. On adding to this solution, after settling out the mud, a quantity of potassium chloride equivalent to the calcium chlorate, the reaction Ca(ClO3)2 + 2KCl = CaCl2 + 2KClO3 is produced, the ultimate proportions thus being theoretically 2KClO3 to 6CaCl2, though in reality there is rather more calcium chloride present. When this solution is concentrated by evaporation and cooled down, about five-sixths of the chlorate of potash crystallizes out. It is purified by redissolving and crystallization, and is sold either in the state of crystals or finely ground. During these operations care must be taken lest a spark should produce the inflammation of the chlorate on contact with any organic substance. Large quantities of potassium chlorate exposed to strong heat in contact with the wood of casks or the timber of a roof have produced violent explosions.
Most of the chlorate of potash is now prepared by electrolysis of potassium chloride (see below). It is employed for fire-works, for some descriptions of explosives, for safety matches and as an oxidizer in some operations, especially in dyeing and tissue printing. For the last-named purpose it is sometimes replaced by sodium chlorate. The chlorates are usually sold in wooden kegs containing 1 cwt. each.
|Fig. 6.—Black-ash Furnace and Boiling-down Pan.|
5. The Manufacture of Soda-ash from Salt-cake by the Leblanc process.—This process consists in heating a mixture of commercial sulphate of soda (salt-cake) with about the same weight of crushed limestone and half its weight of coal, until the materials are fluxed and a reaction has taken place, the principal phase of which is expressed by the equation Na2SO4 + CaCO3 + 2C = 2CO2 + Na2CO3 + CaS. A number of secondary reactions, however, occur, owing partly to the excess of calcium carbonate and coal and partly to the impurities present, so that the solid product of the process, which is called “black-ash,” has a somewhat complicated composition. Its principal constituents are always sodium carbonate and calcium sulphide, which are separated by the action of water, the former being soluble and the latter insoluble.
The furnace in which the reaction takes place is shown in fig. 6 in a sectional plan. It is called a “black-ash” furnace, and belongs to the class of reverberatory furnaces. A large fire-grate (ab), having a cave (c) to facilitate stoking and stepped back at (d), is bounded on one side by a fire-bridge (e); on the other side of this, separated by an air-channel (g), there is first the proper fluxing bed (h), and behind this the “back-bed” (i) for pre-heating the charge. The flame issuing from the furnace by (o) is always further utilized for boiling down the liquors obtained in a later stage, either in a pan (p) fired from the top and supported on pillars (qq) as shown in the drawing, or in pans heated from below. The charge of salt-cake (generally 3 cwt.), limestone and coal is roughly mixed and put upon the back-bed; when the front- bed has become empty it is drawn forward and exposed to the full heat of the fire, with frequent stirring. After about three- quarters of an hour the substances are so far fluxed or softened that the reaction now sets in fully, as shown by the copious escape of gas. This is at first colourless carbon dioxide, but later on inflammable gases come out of the mass, which at this stage has turned into a thicker, pasty condition, showing that the end of the reaction is near. The inflammable gas is carbon monoxide, which, however, does not burn with its proper purple flame, but with a flame tinged bright yellow by the sodium present. This carbon monoxide is formed by the action of coal on the lime formed at this stage from the original limestone. When the “candles” of carbon monoxide appear, the pasty mass is quickly drawn out of the furnace into iron “bogies,” where it solidifies into a grey, porous mass, the “black-ash.” Care must be taken to heat it no longer than necessary, as it otherwise turns red and yields bad soda.
|Fig. 7.—Revolving Black-ash Furnace. (Elevation.)|
The hand-wrought black-ash furnace has been mostly superseded in the large factories by the revolving black-ash furnace, shown in fig. 7. These furnaces possess a large cylindrical shell (e), lined with fire-bricks, and made to revolve round its horizontal axis by means of a toothed wheel fixed on its exterior; (ff) are tire-seats holding tires (gg), which work in friction rollers (h). The flame of a fixed fireplace (a) enters through an “eye” (b) in the centre of the front end of the cylinder and issues in the centre of the back end, first into a large dust-chamber (m), and then over or under boiling-down pans (p). These mechanical furnaces do the work of from four to ten ordinary furnaces according to their size. with comparatively very little expense for labour, but they must be very carefully managed and the black-ash from them is more difficult to lixiviate than that from hand-wrought furnaces, because it is less porous. The lixiviation of the black-ash requires great care, as the calcium sulphide is liable to be changed into soluble calcium compounds, which immediately react with sodium carbonate and destroy a corresponding quantity of the latter, rendering the soda weaker and impure. This change of the calcium sulphide may be brought about either by the oxidizing action of the air or by “hydrolysis,” produced by prolonged contact with hot water, the use of which, on the other hand, cannot be avoided in order to extract the sodium carbonate itself. The apparatus which has been found most suitable for the purpose was devised by Professor H. Buff of Giessen, and first practically carried out by Charles Dunlop at St Rollox. It consists of a number of tanks or “vats,” placed at the same level and connected by pipes which reach nearly to the bottom of one tank and open out at the top into the next tank. The vats are also provided with false bottoms, outlet cocks, steam pipes and so forth. Tepid water is run in at one end of the series, where nearly exhausted black-ash is present; the weak liquor takes up more soda from the intermediate tanks and at last gets up to full strength in the last tank, charged with fresh black-ash and kept at a higher temperature, viz. 60° C. When the first tank has been quite exhausted, the water is turned on to the next, the first tank is emptied by discharging the “alkali- waste,” and is filled with fresh black-ash, whereupon it becomes the last of the series. In spite of all precautions a certain quantity of impurities is always formed, but this should be kept down as much as possible by strictly watching the temperature in the vats and by taking care that the black-ash in the wet state is never exposed to the air. The unavoidable contamination with muddy particles of vat-waste is removed by allowing the vat- liquor to rest for some hours in a separate tank and settling out the mud.
The clear vat-liquor, if allowed to cool down to ordinary temperature, would separate out part of the sodium carbonate in the shape of decahydrated crystals. As these do not come out sufficiently pure, they would not be marketable and therefore they are not allowed to be formed, but the liquid, while still hot, is either run into the boiling-down pans, or submitted to one of the purifying operations to be described below. If it is boiled down without further purification, the resulting soda-ash is not of the first quality, but it is sufficiently pure for many purposes. The boiling down is most economically performed by means of large iron pans covered with a brick arch and heated from the top by the waste flame issuing from the black-ash furnaces (see figs. 6 and 7). It is continued until the contents of the pan have been converted into a thick paste of small crystals of monohydrated sodium carbonate, permeated by a mother-liquor which is removed by draining on perforated plates or by a centrifugal machine, and is always returned to the pans. The drained crystals are dried and heated to redness in a reverberatory furnace; when “finished,” the mass is of an impure white or light yellow colour and is sold as ordinary “soda-ash.” It is not easy to make it stronger than 92% of sodium carbonate, which is technically expressed as “52 degrees of available soda” (see next page). If purer and stronger soda-ash is wanted, the boiling down must be carried out in pans fired from below, and the crystals of monohydrated sodium carbonate “fished” out as they are formed, but this is mostly done after submitting the liquor to the purifying operations which we shall now describe.
The dried or “finished” soda-ash is ground to a pretty fine powder and is packed into wooden casks or “tierces,” holding from 10 to about 20 cwt. each, according to the way of filling them.
The principal impurities of crude vat-liquor are sodium hydrate and sulphide, the latter of which always leads to the formation of soluble double sulphur salts of sodium and iron. The other impurities are of minor importance. The sulphides can be removed by “oxidizing” them into thiosulphates by means of atmospheric air, with or without the assistance of other agents, such as manganese peroxide; or by “carbonating” them with lime-kiln or other gases containing carbon dioxide; or by precipitating them with lead or zinc oxide. The last mentioned is the best but costliest method, and is employed only in the manufacture of the highest strengths of caustic soda. The most usual process, where soda-ash is to be made, is the “carbonating.” This is usually effected either by forcing lime-kiln gas through the liquor, contained in a closed iron vessel, or by passing the gases through an iron tower filled with coke or other materials, suitable for subdividing the stream of the gases and that of the vat-liquor which trickles down in the tower. The same apparatus is used for “oxidizing” by means of atmospheric air passed through by means of an injector; sometimes both air and carbon dioxide are passed in at the same time. The operation is finished when all the sodium sulphide has been converted into normal sodium carbonate, partly also into acid sodium carbonate (bicarbonate) NaHCO3; at the same time a precipitate is formed, consisting of ferrous sulphide, alumina and silica, which is removed by another settling tank, and the clear liquor is now ready either for boiling down in a “fishing-pan” for the manufacture of white soda-ash, or for the process of causticizing.
Soda-ash (as well as caustic soda) is sold by degrees of “available soda.” This means that portion which neutralizes the acid employed for testing, and the degrees mean the percentage of Na2O thus found, whether it be present as Na2CO3, NaOH, or sodium aluminate or silicate. The purest soda-ash, equal to 100% Na2CO3, would be 581 degrees of available soda. The ordinary commercial strength of Leblanc soda-ash is from 52 to 54 degrees (in former times much was sold in the state of 48%).
6. Manufacture of Caustic Soda.—Most of the Leblanc liquor is nowadays converted into caustic soda, as white soda-ash is more easily and cheaply made by the ammonia-soda process. We shall therefore in this place describe the manufacture of caustic soda. This is always made from the carbonate by the action of slaked lime: Na2CO3 + Ca(OH)2＝CaCO3 + 2NaOH. The calcium carbonate, being insoluble, is easily separated from the caustic liquor by filtration. But as this reaction is reversible, we must observe the conditions necessary for directing it in the right sense. These are: diluting with water so as not to exceed 10% of sodium carbonate to 90% of water; boiling this mixture; and keeping it well agitated. At the best about 92% of the sodium carbonate can be converted into caustic soda, 8% remaining unchanged.
The operation is performed in iron cylinders, provided with an agitating arrangement. This may consist of a steam injector by means of which air is made to bubble through the liquid, which produces both the required agitation and the heating, and at the same time oxidizes at least part of the sulphides; but this method of agitation causes a great waste of steam and at the same time a further dilution of the liquor. Many, therefore, prefer mechanical stirring by means of paddles, fixed either to a vertical or to a horizontal shaft, and inject only sufficient steam to keep the mass at the proper temperature. Some heat is also gained by the slaking of the caustic lime within the liquor. After from half an hour to a whole hour the conversion of sodium carbonate into sodium hydrate is brought about as far as is practicable. The whole mass is now run into the filters, which are always constructed on the vacuum principle. They are iron boxes, in which a bed is made of bricks, above them gravel, and over this sand, covered on the top by iron grids. The space below the sieve thus formed is connected by means of an outlet tap with a closed tank, and this again communicates with a vacuum pump. By this means the filtration is quickened by the atmospheric pressure, and goes on very rapidly, as also does the subsequent washing. The filtered caustic liquor passes to the concentration plants; the washings are employed for diluting fresh vat-liquor for the next operation, or for dissolving solid soda-ash for the same purpose. The washed-out calcium carbonate, which always contains much calcium hydrate and 2 or 3% of soda in various forms, usually goes back to the black-ash furnaces, but it cannot be always used up in this way, and what remains is thrown upon a heap outside the works. Attempts have been made to use it in the manufacture of Portland cement, but without much success.
|Fig. 8.—Caustic Soda Concentration Boat-pan. (Sectional Elevation.)|
The clear caustic soda liquor must be concentrated in such a way that the caustic soda cannot to any great extent be reconverted into sodium carbonate, and that the “salts” which it contains, sodium carbonate, sulphate, chloride, &c., can be. separated during the process. Formerly the most usual concentrating apparatus was the “boat-pan” (fig. 8). This is an oblong iron pan, the bottom of which slopes from both sides to a narrow channel. The latter rests on a brick pillar; the remaining part of the sloping bottom is heated, either by the waste fire from a black-ash furnace or by a special fireplace. This arrangement has the effect that the salts, as they separate out, slide down the sloping part and arrive in the central channel, which is not exposed to the fire-gases, so that they quietly settle there, without caking to the pan, until they are fished out by means of perforated ladles. These boat-pans were for many years almost everywhere employed, and did their work quite well, but rather expensively. At many works they have been replaced by either Thelen pans or vacuum pans.
The “Thélen pan” (thus named from its inventor, a foreman at the Rhenania works near Aachen) is a mechanically worked fishing-pan, which requires considerably less labour and coal than ordinary boat-pans. It is a long trough, of nearly semicircular section, the whole bottom being exposed to the fire- gases. A horizontal shaft runs length-ways through the trough, and is provided with stirring blades, arranged in such a manner that they constantly scrape the bottom, so that the salts cannot burn fast upon it, and are at the same time moved forward towards one of the ends of the trough where they are automatically removed by means of a chain of buckets.
The most efficient evaporating apparatus, as far as economy of fuel is concerned, is the vacuum-pan, of which from two to five are combined to form a set, but it has the drawback that the removal of the salts is much more difficult than with the older pans, described above. In this apparatus only the first of the pans is heated directly, usually by means of ordinary boiler- steam circulating round a number of pipes, containing the liquid to be concentrated. The steam rising from the latter is passed into a similar pan, in which it circulates round another set of pipes, but as it could not bring the liquid in the latter to boil under ordinary conditions, the second pan is connected with a vacuum-pump so that the boiling-point of the liquid in this pan is lowered. This pan may be followed by a third pan, in which a stronger vacuum is maintained, and so forth. By this means the latent heat of the steam, issuing from all pans but the last, is utilized for evaporating purposes, and from half to three-fourths of the fuel is saved.
|Fig. 9.—Caustic Soda “Finishing-pot.” (Sectional Elevation.)|
After being concentrated up to a certain point, and after the separation of nearly all the salts, the caustic liquor is transferred to cast-iron “finishing-pots” (fig. 9), holding from ten to twenty tons. Here it is further boiled down until the greater part or nearly all of the water has been removed, and until the salts on cooling would set to a solid mass. This requires ultimately a good red heat. Before the mass has reached that point the sulphides still present have been destroyed, either by the addition of solid nitrate of soda or by blowing air through the red-hot melt. Before finishing, the molten mass must be kept at a quiet heat for some hours in order to settle out the ferric oxide which it always contains, and which becomes insoluble (through the destruction of the sodium ferrite) only at high temperatures. When it has completely cleared, the liquid caustic is ladled or pumped out into sheet-iron drums, holding about 6 cwt. each, where it solidifies and forms the caustic soda known to commerce.
The best caustic soda tests from 75 to 76 degrees of “available soda”; this is only a few per cent removed from the composition of pure NaOH, which would be = 77.5 degrees Na2O. Most of the caustic soda is sold at a strength of 70 degrees, sometimes as low as 60 degrees.
Caustic soda is used in very large quantities in the manufacture of soap, paper, textile fabrics, alizarin and other colouring matters, and for many other purposes.
7. Soda-Crystals.—Another product made in alkali works is soda-crystals. Their formula in Na2CO3, 10H2O, corresponding to 37% of dry sodium carbonate. They are made by dissolving ordinary soda-ash in hot water, adding a small quantity of chloride of lime for the destruction of colouring matter and the oxidation of any ferrous salts present, carefully settling the solution, without allowing its temperature to fall below the point of maximum solubility (34° C.), and running the clarified liquid into cast-iron crystallizers or “cones,” where, on cooling down, most of the sodium carbonate is separated in large crystals of the decahydrated form. This process lasts about a week in winter, and up to a fortnight in summer. In France the crystallization of soda is performed not in large tanks but in sheet-iron dishes holding only about 1 cwt., and requires only from 27 to 48 hours in the cool season; it is not carried on at all in warmer climates during the summer months. The mother-liquor, drained from the soda-crystals, on boiling down to dryness yields a very white, but low-strength soda-ash, as the soluble impurities of the original soda-ash are nearly all collected here; it is called “mother-alkali.”
Although the soda-crystals contain the alkali combined with such a large quantity of water, they are made in large quantities, because their form, together with their complete freedom from caustic soda, makes them very suitable for domestic purposes. Hence they are best known as “washing-soda.” Sometimes they are made, not from soda-ash, but from Leblanc soda-liquor before “finishing” the ash, or from the crude bicarbonate of the ammonia-soda process by prolonged boiling, until nearly half of the carbonic acid has been expelled.
Formerly bicarbonate of soda was made from Leblanc soda-crystals by the action of carbonic acid, but this article is now almost exclusively made in the ammonia-soda process.
8. The Recovery of Sulphur from Alkali-waste.—For many years all the sulphur used in the Leblanc process in the shape of sodium sulphate, and originally imported into the manufacture in the shape of brimstone or pyrites, was wasted in the crude calcium sulphide remaining from the lixiviation of black-ash. This “alkali-waste,” also called tank-waste or vat- waste, was thrown into heaps where the calcium sulphide was gradually acted upon by the moisture and the oxygen of the air. The sulphur was by these converted partly into gaseous sulphuretted hydrogen, partly into soluble polysulphides, thiosulphates and other soluble compounds, and in all shapes caused a nuisance which became more and more intolerable as the number and size of alkali works increased. Both the air and the water in their neighbourhood were contaminated thereby.
Both this nuisance and the loss of the sulphur (whose cost sometimes amounted to more than half of the total cost of the soda-ash) led to many attempts at extracting the sulphur from the alkali-waste. This was first done with a certain amount of success by the processes of M. Schaffner (1861) and L. Mond (1862), but as these required the use of hydrochloric acid, and as they only recovered about half of the sulphur, they were superseded by another—a process which had been originally proposed by W. Gossage in 1837, but has been made practicable only by the inventions of C. F. Claus, in 1883, and from 1887 onward by the technical skill of Messrs Chance Brothers, of Oldbury. The Claus-Chance process, as it is called, comprises the following operations. The wet alkali-waste as it comes from the lixiviating vats, is transferred into upright iron cylinders in which it is systematically treated with lime-kiln gases until the whole of the calcium sulphide has been converted into calcium carbonate, the carbon dioxide of the lime-kiln gases being entirely exhausted. The sulphur issues as sulphuretted hydrogen, mixed with the nitrogen of the air. It is mixed with fresh air containing sufficient oxygen for the combustion of the hydrogen, and the mixture is passed through red-hot iron oxide (burnt pyrites) which by its catalytic action causes the reaction H2S + O = H2O + S to take place. By cooling the vapours the sulphur is condensed in a very pure form, and about 85% of the whole of it is recovered, the remaining 15% escaping in the shape of sulphur dioxide (SO2) and H2S. Unfortunately it has been hitherto found impossible to deal with these gases in any profitable way.It should be noted that this “recovered sulphur,” which is equal in purity to the “refined brimstone” of commerce, has a far higher value than the sulphur contained in the originally employed pyrites, so that the recovery is a paying process, in spite of the somewhat considerable cost of the plant and of the working operations. It has been introduced at most large Leblanc alkali works, and has, so to say, given them a new lease of life.
II. The Ammonia-Soda Process
From Thorpe’s Dictionary of Applied Chemistry, by permission of Longmans, Green & Co.
AA, Tower; B, ammoniacal brine main; E, gas-inlet; Z, vacuum filter; V, pipe to air-pump.
The ammonia-soda process was first patented in 1838 by H. G. Dyar and J. Hemming, who carried it out on an experimental scale in Whitechapel. Many attempts were soon after made in the same direction, both in England and on the continent of Europe, the most remarkable of which was the ingenious combination of apparatus devised by J. J. T. Schloesing and E. Rolland. But a really economical solution of the problem was first definitely found in 1872 by Ernest Solvay, as the result of investigations begun about ten years previously. The greater portion of all the soda-ash of commerce is now made by Solvay’s apparatus, which alone we shall describe in this place, although it should be borne in mind that the principles laid down by Dyar and Hemming have been and are still successfully carried out in a number of factories by an entirely different kind of apparatus.
The leading reaction of this process is the mutual decomposition of ammonium bicarbonate and sodium chloride: NaCl + NH4HCO3＝NaHCO3 + NH4Cl. It begins, however, not with ready-made ammonium bicarbonate, but with the substances from which it is formed—ammonia, water and carbon dioxide—which are made to act on sodium chloride. In practice the process is carried out as follows. A nearly saturated solution of sodium chloride is obtained by purifying natural or artificial brine, i.e. an impure solution of common salt, especially removing the alkaline earths and so forth by addition of sodium or ammonium carbonate and settling out the precipitate formed. This solution is saturated with ammonia, produced in the recovery plant (see below), in vessels provided with mechanical agitators and strongly cooled by coils of pipes through which cold water is made to flow. These vessels, as well as all others which are used in the process, are not open to the air, but communicate with it through washers in which fresh salt solution is employed for retaining any escaping vapours of ammonia. The ammoniacal salt solution is now saturated with carbon dioxide. This is employed in the shape of lime-kiln gases, obtained in a comparatively pure and strong form (up to 33% CO2), in very large kilns, charged with limestone and coke. The kilns are closed at the top, and the gases are drawn out by powerful air-pumps, washers being interposed between the kilns and the pumps for the purpose of purifying and cooling the gas. The heat evolved by the compression in the air-pumps (which rises to four atmospheres or upwards) is again removed by cooling, and the gas is now passed upwards in the “Solvay tower” (fig. 10). This is a tall iron erection, built up from superposed cylinders, which are separated from one another by perforated horizontal diaphragms, constructed in such a way that the gases are over and over again subdivided into many smaller streams and are thus thoroughly brought into contact with the ammoniacal salt solution with which the tower is about two-thirds filled. There the reaction mentioned above takes place, and owing to the concentration of the liquid the sodium bicarbonate formed is to a great extent precipitated in the shape of small crystals, forming with the mother-liquor a thin magma. This takes place with considerable evolution of heat which is removed by internal and external cooling with water. The temperature must not be allowed to rise beyond a certain point, for the reaction NaCl + NH4HCO3＝NaHCO3 + NH4Cl is reversible, and at a temperature of about 60° or 70° C. it is in fact practically going the wrong way, viz. from right to left. On the other hand the cooling must not be carried too far, for in this case the crystals of sodium bicarbonate become so fine that the muddy mass is very difficult to filter. The best temperature seems to be about 30° C.
Either at certain intervals, or continuously, a portion of the contents of the tower is withdrawn and fresh ammoniacal salt solution is introduced higher up. The muddy liquid running out is passed on to the vacuum filters (Z, fig. 10). Here a separation takes place between the crystals of sodium bicarbonate and the mother-liquor. The former are washed with water until the chlorides are nearly removed, and are then carried into the drying apparatus.
This must be constructed in such a manner that the bicarbonate, which always contains some ammonium salts, is first freed from these by moderate heating (of course taking care that the ammonia is completely recovered), and later on, by raising the temperature, it is decomposed into solid sodium carbonate and gaseous carbon dioxide. The former needs only grinding to constitute the final product, ammonia- soda ash; the latter is again employed in the process of treating the ammoniacal salt solution with carbon dioxide. Various forms of apparatus are employed for this treatment of the crude bicarbonate—sometimes semi-circular troughs with mechanical agitators on the principle of the Thelen pan (see above)—all acting on the principle that the escaping ammonia and carbon dioxide must be fully utilized over again. The soda-ash obtained in the end is of a high degree of purity, testing from 98 to 99% Na2CO3, the remaining 1 or 2% consisting principally of NaCl.
A very important part of the process has still to be described, viz. the recovery of the ammonia from the mother-liquor coming from the vacuum filters and various washing liquors. Unless this recovery is carried out in the most efficient manner, the process cannot possibly pay; but so much progress has been made in this direction that the loss of ammonia is very slight indeed, merely a fraction per cent. The ammonia is for the major part found in the mother-liquor as ammonium chloride. A smaller but still considerable portion exists here and in the washings in the shape of ammonium carbonates. These compounds differ in their behaviour to heat. The ammonium carbonates are driven out from their solutions by mere prolonged boiling, being thereby decomposed into ammonia, carbon dioxide and water, but the ammonium chloride is not volatile under these conditions, and must be decomposed by milk of lime: 2NH4Cl + Ca(OH)2＝2NH3 + CaCl2 + 2H2O. The solution of calcium chloride is run to waste, the ammonia is re-introduced into the process.
Both these reactions are carried out in tall cylindrical columns or “stills,” Consisting of a number of superposed cylinders, having perforated horizontal partitions, and provided with a steam-heating arrangement in the enlarged bottom portion. The milk of lime is introduced at a certain distance from the bottom. The steam causes the action of the lime on the ammonium chloride to take place in this lower portion of the still, from which the steam, mixed with all the liberated ammonia, rises into the upper portion of the column where its heat serves to drive out the volatile ammonium carbonate. Just below the top there is a cooling arrangement, so that nearly all the water is condensed and runs back into the column, while the ammonia, with the carbon dioxide formerly combined with part of it, passes on first through an outside cooler where the remaining water is condensed, and afterwards into the vessels, already described, where the ammonia is absorbed by a solution of salt and thus again introduced into the process.
The reversible character of the principal reaction has the consequence that a considerable portion of the sodium chloride (up to 33%) is lost, being contained in the waste calcium chloride solution which issues from the ammonia stills. This is, however, not of much importance, as it had been introduced in the shape of a brine where its value is very slight (6d. per ton of NaCl). It is true that all the chlorine combined with the sodium is lost partly as NaCl and partly as CaCl2; none of the innumerable attempts at recovering the chlorine from the waste liquor has been made to pay, and success is less likely than ever since the perfection of the electrolytic processes. (See CHLORINE.) For all that, especially in consequence of the small amount of fuel required, and the total absence of the necessity of employing sulphur compounds as an intermediary, the ammonia-soda process has supplanted the Leblanc process almost entirely on the continent of Europe and to a great extent in Great Britain.
In theory by far the simplest process for making alkalis together with free chlorine is the electrolysis of sodium (or potassium) chloride. When this takes place in an aqueous solution, the alkaline metal at once reacts with the water, so that a solution of an alkaline hydrate is formed while hydrogen escapes. The reactions are therefore (we shall in this case speak only of the sodium compounds): (1) NaCl＝Na + Cl, (2) Na + H2O＝NaOH + H.
The chlorine escapes at the anode, the hydrogen at the cathode. If the chlorine and thehydrate can act upon each other within the liquid, bleach-liquors are formed: 2NaOH + Cl2＝NaOCl + NaOH + H2O. The production of these for the use of papermakers and bleachers of textile fabrics has become an important industry, but does not enter into our province.
If, however, the action of the chlorine on the sodium hydrate is prevented, which can be done in various ways, they can both be collected in the isolated state and utilized as has been previously described, viz. the chlorine can be used for the manufacture of liquid chlorine, bleaching-powder or other bleaching compounds, or chlorates, and the solution of sodium hydrate can be sold as such, or converted into solid caustic soda. Precisely the same can be done in the electrolysis of potassium chloride.
There is a third way of conducting the action, viz. so that the chlorine can act upon the caustic soda or potash at a higher concentration and temperature, in which case chlorates are directly formed in the liquid: KCl + 3H2O＝KClO3 + 3H2. This has indeed become the principal, because it is the cheapest, process for the manufacture of potassium and sodium chlorate. Perchlorates can also be made in this way.
In all these cases the chlorine, or the products made from it, really play a greater part than the alkali. From 58.5 parts by weight of NaCl we obtain theoretically 23Na＝40NaOH＝53Na2CO3, together with 35.5 Cl, or 100 bleaching-powder. As the weight of bleaching-powder consumed in the world is at most one-fifth of that of alkali, calculated as Na2CO3, it follows that only about one-tenth of all the alkali required could be made by electrolysis, even supposing the Leblanc process to be entirely abolished. The remaining nine-tenths of alkali must be supplied from other sources, chiefly the ammonia-soda process. As long as the operation of the Leblanc process is continued, it will supply a certain share of both kinds of products. Trustworthy statistics on this point cannot be obtained, because most firms withhold any information as to the extent of their production from the public.
The first patents for the electrolysis of alkaline chlorides were taken out in 1851 and several others later on; but commercial success was utterly impossible until the invention of the dynamo machine allowed the production of the electric current at a sufficiently cheap rate. The first application of this machine for the present purpose seems to have been made in 1875 and the number of patents soon rapidly increased; but although a large amount of capital was invested and many very ingenious inventions made their appearance, it took nearly another twenty years before the manufacture of alkali in this way was carried out in a continuous way on a large scale and with profitable results. A little earlier the manufacture of potassium chlorate (on the large scale since 1890) had been brought to a definite success by H. Gall and the Vicomte A. de Montlaur; a few years later the processes worked out at the Griesheim alkali works (near Frankfort) for the manufacture of caustic potash and chlorine established definitely the success of electrolysis in the field of potash, but even then none of the various processes working with sodium chloride had emerged from the experimental stage. Only more recently the manufacture of caustic soda by electrolysis has also been established as a permanent and paying industry, but as the greatest secrecy is maintained in everything belonging to this domain, and as neither patent specifications nor the sanguine assertions and anticipations of interested persons throw much real light on the actual facts of the case, nothing certain can be said either in regard to the date at which the profitable manufacture of caustic soda was first carried out by electrolysis, or as to what extent this is the case at the present moment.
We shall here give merely an outline of those more important processes which are known to be at present working profitably on a large scale.
(1) The Diaphragm process is probably the only one employed at present for the decomposition of potassium chloride, and it is also used for sodium chloride. A hot, concentrated solution of the alkaline chloride is treated by the electric current in large iron tanks which at the same time serve as cathodes. The anodes are made of retort-carbon or other chlorine-resisting material, and they are mounted in cells which serve as diaphragms. The material of these cells is usually cement, mixed with certain soluble salts which impart sufficient porosity to the material. The electrolysis is carried on until about a quarter of the chloride has been transformed; it must be stopped at this stage lest the formation of hypochlorite and chlorate should set in. The alkaline liquid is now transferred to vacuum pans, constructed in such a manner that the unchanged chloride, which “salts out” during the concentration, can be removed without disturbing the vacuum, and here at last a concentrated pure solution of KOH or NaOH is obtained which is sold in this state, or “finished” as solid caustic in the manner described in the section treating of the Leblanc soda.
(2) The Castner-Kellner process employs no diaphragm, but a mercurial cathode. The electrolysis takes place in the central compartment of a tripartite trough which can be made to rock slightly either to one side or the other. The bottom of the trough is covered with mercury. The sodium as it is formed at the cathode at once dissolves in the mercury which protects it against the action of the water as long as the percentage of sodium in the mercury does not exceed, say, 0.02%. When this percentage has been reached, the cell is rocked to the other side, so that the amalgam flows into one of the outer compartments where the sodium is converted by water into sodium hydrate. At the same time fresh mercury, from which the sodium had been previously extracted, flows from the other outside compartment into the central one. After a certain time the whole is rocked towards the other side, and the process is continued until the outer compartments contain a strong solution of caustic soda, free from chloride and hypochlorite.
(3) Aussig process.—Here the anode is fixed in a bell, mounted in a larger iron tank where the cathodes are placed. The whole is filled with a solution of common salt. As the electrolysis goes on, NaOH is formed at the cathodes and remains at the bottom. The intermediate layer of the salt solution, floating over the caustic soda solution, plays the part of a diaphragm, by preventing the chlorine evolved in the bell from acting on the sodium hydrate formed outside, and this solution offers much less resistance to the electric current than the ordinary diaphragms. This process therefore consumes less power than most others.
(4) The Acker-Douglas process electrolyses sodium chloride in the molten state, employing a cathode consisting of molten lead. The latter dissolves the sodium as it is formed and carries it to an outer compartment where by the action of water the sodium is converted into caustic soda, while the lead returns to the inner compartment. This process is carried on at Niagara Falls, but it is uncertain to what extent.
(5) The Hargreaves-Bird process avoids certain drawbacks attached to other processes, by employing a wire diaphragm and converting the caustic soda as it issues on the other side of this, by means of carbon dioxide, into a mixture of sodium carbonate and bicarbonate, which separates out in the solid state. This process is but little used.
It stands to reason that the electrolytic processes have been principally developed in localities where the electric current can be produced in the cheapest possible manner by means of water power, but this is not the only condition to be considered, as the question of freight to a centre of consumption and other circumstances may also play an important part. Where coal is very cheap indeed and the other conditions are favourable, it is possible to establish such an industry with a prospect of commercial success, even when the electric current is produced by means of steam-engines.
Natural Soda.—This is the term applied to certain deposits of alkaline salts, or their solutions, which occur, sometimes in very large quantities, in various parts of the world. The oldest and best known of these are the Natron lakes in Lower Egypt. The largest occurrence of natural soda hitherto known is that in Owen’s Lake and other salt lakes situated in eastern California. The soda in all of these is present as “sesquicarbonate,” in reality 4′3 carbonate: NaHCO3·Na2CO3·2H2O, and is always mixed with large quantities of chloride and sulphate, which makes its extraction more difficult than would appear from the outset. Hence, although for many centuries (up to Leblanc’s invention) hardly any soda was available except from this source, and although we now know that millions of tons of it exist, especially in the west of the United States, there is as yet very little of it practically employed, and that only locally.
References.—The principal work on the manufacture of alkali is G. Lunge’s Sulphuric Acid and Alkali (2nd ed., vols. ii. and iii., 1895–1896). This work has also appeared in a German and a French edition. The same author wrote the articles on the manufacture of sodium and potassium compounds and on chlorine in Thorpe’s Dictionary of Applied Chemistry (3 vols., 1890–1893). The subject is also treated, very much more briefly, in Sorel’s Industrie chimique minerale (1902), and of course in every other general treatise on chemical technology. A special treatise on the manufacture of ammonia soda ash has been published in German by H. Schreib. Consult also the official Annual reports on Alkali, &c., and, from 1864 onwards, Journal of the Society of Chemical Industry, Fischers Jahresberichte der chemischen Technologie, and Zeitschrift fur angewandte Chemie. (G. L.)