677

the preparation of chlorine from hydrochloric acid by chemical | can be almost entirely neutralized thereby. The new stillprocesses; the electrolytic processes will be treated hereafter. 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 bleachingpowder. 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 Weldon's later attempts at superseding his classical process by the shape of a harmless but useless solution of calcium chloride. 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. Péchiney. 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. The Deacon process, like the Weldon process, effects its Weldon retained the principle of the Scheele object by the oxidizing action of atmospheric air, but in a very different manner.

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: MnO2+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
chemical" stone-
out, must be made of acid-proof stones or
ware. 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.

66

The

Many endeavours were made to avoid the loss of the manganese in this operation, but with only partial or no success. 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 It begins with "still-liquor," manufactured in the world. It begins with " 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+3H2O= 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

FIG. 4.-Weldon Chlorine Still. (Sectional Elevation.) C, Stone steam column resting in stone socket K.

is

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: 2HC1+0=H2O+Cl2.
This reaction in ordinary circumstances is so slow as to be
practically useless. If, however, a "contact-substance"
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 hydro-
chloric 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 "decom-
posing-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

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

FIG. 5.-Deacon " Decomposer." (Sectional Elevation.) a, a, Upright cast-iron cylinders; b, b, brick jacket; c, c, flues; d, e, iron plates arranged like venetian blinds, between which the contact-substance is contained; f, charging hole; g, discharging hole; h, entrance pipe for gas; i, exit pipe for gas.

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 inmediately for the Deacon process, as the

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

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.

bed has become empty it is drawn forward and exposed to the full heat of the fire, with frequent stirring. After about threequarters 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.

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 The hand-wrought black-ash furnace has been mostly superfluxing bed (h), and behind this the "back-bed "(i) for pre-heating seded in the large factories by the revolving black-ash furnace, the charge. The flame issuing from the furnace by (o) is always shown in fig. 7. These furnaces possess a large cylindrical shell further utilized for boiling down the liquors obtained in a later (e), lined with fire-bricks, and made to revolve round its horizontal stage, either in a pan (p) fired from the top and supported on axis by means of a toothed wheel fixed on its exterior; (f) are pillars (qq) as shown in the drawing, or in pans heated from below. tire-seats holding tires (gg), which work in friction rollers (h). The The charge of salt-cake (generally 3 cwt.), limestone and coal flame of a fixed fireplace (a) enters through an eye" (b) in the is roughly mixed and put upon the back-bed; when the front-centre of the front end of the cylinder and issues in the centre of

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 vatliquor to rest for some hours in a separate tank and settling out the mud.

noitulos otai snitold the back end, first into a large dust-chamber (m), and then over | of impurities is always formed, but this should be kept down as 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 blackash 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 " alkaliwaste," and is filled with fresh black-ash, whereupon it becomes the last of the series. In spite of all precautions a certain quantity

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 | filters, which are always constructed on the vacuum principle. 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 66 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.