1902 Encyclopedia > Chlorine


CHLORINE, one of the chemical elements (symbol, CI), discovered by Scheele in 1774. It was long regarded as a compound ; Scheele termed it dephlogisticated muriatic acid, and Berthollet about 1785 gave it the name of oxygenized muriatic acid, which Kirwan contracted into oxymuriatic acid. In 1809 an abstract of a paper was published by Gay-Lussac and Thénard, in the 2d vol. of the Mémoires d'Arcueil, in which they demonstrated the possibility of the absence of oxygen from oxymuriatic acid. The atten-tion of Davy being drawn to the subject, he in 1810 com-municated to the Royal Society a paper in which he showed that there was no evidence in support of the opinion that oxymuriatic acid contained oxygen ; and in a paper published in the following year he comes to the conclusion that oxymuriatic acid is an uncompounded substance, and gives to it the name of chlorine, on account of the greenish colour it possesses. Chlorine and its combinations with other elements having been already treated of under the heading CHEMISTRY, it will be necessary here only to give some account of the more important commercial chlorine compounds hydrochloric acid, bleaching powder, and potassium chlorate, and of the methods of preparing them.

Hydrochloric Acid (HCl).—Practically, the whole of the hydrochloric acid now employed in the manufacturing arts is obtained as a collateral product in the celebrated soda process of Leblanc. The first stage in that process consists in treating common salt with sulphuric acid in the salt-cake roasting furnace, by which sodium sulphate and hydrochloric acid are formed :—2NaCl + H2S04 = Na,,S04 + 2HC1.

Till the year 1863 the acid fumes given off in alkali works were allowed to escape freely into the atmosphere, and being dissolved and brought down by every shower, destroyed or seriously damaged vegetation for miles around the works. In that year the Alkali Act was passed, by which manufacturers were obliged to condense not less than 95 per cent, of the total amount of hydrochloric acid evolved in their establishments; and since that time great attention has been paid to the condensation, so that now in many works practically no acid fumes escape. The hydrochloric acid gas liberated in the roasting furnace is conveyed through a range of stoneware pipes, in connection with which are various devices for cooling it in its passage. It is then conducted into condensing towers—long wide funnels packed with coke—through which a stream of water is made to percolate from the top, an enormous surface of moisture being thus presented to the acid fumes. The solution of the gas in the water constitutes the hydrochloric acid, muriatic acid, or spirit of salt of commerce. So prepared, the acid always contains several impurities, such as arsenious acid, ferric chloride, and sulphurous acid ; but these do not interfere with its application to the prepara-tion of bleaching powder, in which it is chiefly consumed. Without any purification it is also employed for " souring " in bleaching, and in tin and lead soldering.

Bleaching Powder, or Chloride of Lime.—The history of the application of chlorine to bleaching purposes before the introduction of the so-called chloride of lime will be found under the article BLEACHING. Bleaching powder is made by exposing pure slaked lime to an atmosphere of chlorine till the lime will absorb no more of the gas. Many plans for the preparation of the chlorine have been proposed, and various important processes adopted, since the manufac-ture of bleaching-powder was established by Messrs Tennant and Co. The original process was as follows. A mixture of native peroxide of manganese ground to a fine powder, common salt, and sulphuric acid was put into a large, nearly spherical, leaden vessel furnished at the top with an air-tight lid. In this vessel an agitator was placed by which its contents could be from time to time stirred up. From the lid a lead pipe conveyed the liberated chlorine into the chamber in which the lime to be saturated was spread in a thin layer. The exterior of the leaden vessel was cased with an iron covering, space for the circulation of a current of steam between it and the covering being left. Upon the charging of the still chlorine was at first given off without heat; but after some time a current of steam was made to circulate around the still, so as to maintain a sufficient temperature to disengage all the chlorine. The materials used consisted of common salt, manganese peroxide, and sulphuric acid, and the resulting products were manganous sulphate, sodium sul-phate, water, and chlorine:—

Mn02 + 2NaCl + 3H2S04 = MnS04 + 2NaHSO t + 2H20 + C12.

Through the development of Leblanc's well-known soda process large quantities of hydrochloric acid became available for the manufacture of chlorine, in place, as formerly, of a mixture of common salt and sulphuric acid; and for many years hydro-chloric acid alone has been used. Coarsely ground manganese oxide is placed within an oblong stone still, into which the necessary charge of strong hydrochloric acid is admitted. Steam is then allowed to circulate in the outer case of the still till the temperature of the mixture is raised to about 180° Fahr. When this point is reached, steam at a pressure of 20 to 25 lb. is blown through the charge at intervals for about six hours, after which the reaction is complete—the whole occupying about twenty-four hours. The chemical changes that take place are expressed in the following equation :—

Mn02 + 4HC1 = MnCl2 + 2H20 + Cl2.

Native hinoxide of manganese being an impure and variable com-pound has to be used in quantities proportionate to the Mn02 it contains.

For each molecule of chlorine obtained by the above reaction, it will be observed that one of manganous chloride is formed, a sub-stance that was formerly run off as a waste product. Apart from the nuisance thereby created, the drain on the supply of manganese ore became serious, supplies diminished, and prices rose in proportion. It became therefore an object of much importance to obviate the waste of manganese, and this was sought in two different directions

Efforts in the first place were made to regenerate the manganese waste, so as render it continuously available for manufacturing pur-poses ; and secondly, it was frequently attempted to dispense alto-gether with the use of manganese in the process.

The first really successful plan for the recovery and reoxidation of manganese was devised by Mr Charles Dunlop, and was introduced at Messrs Tennant's works, at St Rollox, Glasgow, where it is still (1876) in operation. By Dunlop's process the residual still-liquor is first treated with carbonate of lime to neutralize its free acid and decompose the contained ferric chloride, which thus gives a pre-cipitate, ferric oxide. The clear liquor obtained is a second time mixed with carbonate of lime, then introduced into an enormous iron boiler, in which it boils at a pressure of four atmospheres for about twelve hours. Under the influence of the heat applied the manganese exchanges its chlorine for the carbonic acid of the car-bonate of lime, and a hydrated manganese carbonate, suspended in a solution of chloride of calcium, is produced, thus :—

MnCl2 + CaC03 + H20 = MnCO,. H20 + CaCl2.

The manganese carbonate is subsequently allowed to subside, and the solution of chloride of calcium is washed out. The manganese carbonate is drained till it becomes a white putty-like mass, when it is put into shallow iron trays, and roasted in a furnace at a heat gradually increasing to 550° or 600° Fahr. Under the influence of the heat the carbonic acid is driven off, and by degrees, from being a white powder, the manganese compound darkens till on the com-pletion of the process it is a soot-like mass. Owing principally to the expensive nature of the plant necessary, the Dunlop process never extended beyond St Rollox ; but it has satisfactorily main-tained its position in that gigantic establishment.

The method of artificially oxidizing manganese which is now almost universally adopted, and has during the past few years quite revolutionized the bleaching powder manufacture, is that invented by Mr Walter Weldon. Manganous chloride by treatment with lime becomes changed into the lower oxide of manganese (MnO), a body which with great difficulty is raised to the higher sesquioxide (Mn20:!), beyond which stage of oxidation no exposure to oxygen raises it. But when treated with excess of lime the manganous oxide rapidly unites with oxygen, and becomes transformed into the peroxide (Mn02), which is the chemical compound available for the preparation of clilorine. It was this fact which Mr Weldon dis-covered, and has turned to advantage in his process. The pheno-menon he accounts for on the assumption that the sesquioxide (Mn203) formed by the exposure of manganous oxide (MnO) to oxygen is really a manganous manganite, or a combination of MnO with Mn02, the former having basic, the latter acid functions. The lime added by Mr Weldon supplies a more powerful base, and enables the whole of the manganese to attain to the condition of peroxide, uniting as it does in such a manner with the lime as to form a calcium manganite (CaMn03). By employing a diminished quantity of lime along with an increased current of air, it has been found possible to effect the complete oxidation of the manganese, an acid manganite (CaMn03, H2Mn03) being probably formed.

In his treatment of the still-liquors Mr Weldon first neutralizes the acid, &c., as is done in Mr Dunlop's process. The clear rose-tinted liquor thereby obtained is passed into a vessel called the oxidizer, at a temperature of about 140° Fahr. A pipe for convey-ing a current of air passes down the centre of the vessel, terminating near the bottom in a series of distributing pipes, and connected at the other end with a blowing engine. The liquor being at, or brought by injected steam to the proper temperature, then, according to Mr Weldon [Soc. of Arts Lecture, May 1874), the injection of air is commenced, and there is rapidly added, in the state of very fine division, 1 6 times the quantity of lime equivalent to the manganese in the liquor. This converts the charge into a thin white mud, which consists of solution of calcic chloride holding in suspension manganous oxide, or MnO, and also holding partly in suspension partly in solution six-tenths of an equivalent—reckoned on the manganese present—of free lime. When a little of this white mud is thrown on to a filter, the clear filtrate is naturally found, owing to the quantity of free lime present, and to the powerfully solvent action upon lime of hot solution of calcic chloride, to possess a strongly alkaline reaction. As the injection of air goes on, the mud becomes gradually darker in colour, owing to the white hydrated MnO becoming converted into black Mn02 by absorption of oxygen from the injected air, what was originally a thin white mud being at length converted into a thin black mud. During the progress of this conversion of the originally white mud into a black mud, it is found that the alkaline reaction of the filtrate from the mud gradually diminishes in intensity, until at length it entirely dis-appears ; and it is found, too, that when this alkaline reaction ceases, the absorption of oxygen from the injected air ceases also. When this stage is reached, which is at the end of two, three, four, or five hours, according to the relation between the size of the oxidizer and the size of the blowing-engine employed, a little more liquor is run into the oxidizer from the settlers above, the injection of air is con-tinued for a few minutes longer, and the charge is then ran off from the oxidizer into one or other of a range of settlers placed below it. In these it separates, in the course of a few hours, into rather more than half perfectly clear solution of calcic chloride, and a little less than half black mud of rather more than twice the density of that which left the oxidizer. The solution of calcic chloride is now decanted, and the settled mud is then ready for use for the liberation of chlorine from hydrochloric acid.

The now practically universal use of regenerated m anganese oxide in a state of fine division has considerably modified the older operations for the manufacture of chlorine. Hydrochloric acid, according to the plan at present in use, is first run into the still, and the man-ganese mud is admitted gradually to it in a carefully regulated stream. Instead of the old oblong trough-like still, one much more capa-cious, of pentagonal form, is generally employed. The details of working vary in almost every establishment, but the following may be taken as an example of ordinary practice. A charge of hydro-chloric acid, equal to the amount yielded by about 55 cwt. of common salt, is led into the still, and to it is added from 14 to 15 cwt. of 70 per cent, manganese. Finely slaked lime to the amount of 40 cwt. is spread in a uniform layer on the floor of the chamber, and from these quantities 70 or 71 cwt. of bleaching powder containing from 85 to 37 per cent, of available chlorine is obtained. The lime, after receiving a first charge of gas, is left 24 hours, when it is turned by workmen. Another charge is then admitted, and on the expiry of twenty-four hours more the bleaching powder is ready to be drawn off and packed in barrels.

Of the many chlorine processes without manganese which have been proposed, the only plan which has stood the test of wide practical application is that introduced in recent years by Mr Henry Deacon, of Widnes, in Lancashire. Doubtless his is a process which would have been widely introduced but for the economy and satisfactory nature of Mr Weldon's method. A process for liberating chlorine from hydrochloric acid gas by exposing it with atmospheric air to a high temperature was patented by Mr Robert Oxlandin 1845. Mr Deacon discovered that in this process, when the air and gas are brought into contact with sulphate of copper the hydrochloric acid is decomposed much more completely and at a far lower temperature than without that salt. In practice hot hydrochloric acid gas and atmospheric air are passed over pieces of brick which have been dipped in a solution of sulphate of copper and sulphate of soda, and dried. Chlorine and water are produced by the decomposition, and some traces of hydrochloric acid also pass undecomposed. The water and acid are condensed, and the chlorine mixed with the residual nitrogen of the air passes on to the absorbing chambers. As the chlorine is greatly diluted by the nitrogen, the lime has to be exposed in thin layers over a large area, and Mr Deacon so arranges his chambers that the mixed gases as they enter meet almost com-pletely saturated lime, but as the chlorine becomes absorbed, less highly ehlorinized lime is met, till at the end it is almost fresh hydrate of lime over which they pass. Mr Deacon obtains from the acid of 1500 cwt. of salt, with the expenditure of 50 tons of small coal, upwards of 50 tons of 35 per cent, bleaching powder. The process is in operation in his own manufactory at Widnes, and in some Continental establishments Mr Deacon's plant was intro-duced, but it has been practically abandoned.

Bleaching powder when fresh is a dry, white lime-like powder emitting a strong odour of hypochlorous acid. Commercial samples vary considerably in strength, but when newly made and of good quality they should contain from 35 to 37 per cent, of available chlorine. On exposure bleaching powder parts with its chlorine with considerable rapidity, losing, according to some experimenters, at the rate of 0'63 per cent, per month. On the composition of bleaching powder see CHEMISTRY, page 494.

Potassium Chlorate or Chlorate of Potash.—The preparation of potassium chlorate is an example of the employment of chlorine on an extensive scale, its function, according to the ordinary process of manufacture, being to transform potassium chloride (KC1) into potassium chlorate (KC103) by supplying the necessary oxygen from lime. It is obtained by passing excess of chlorine into solution of potassium chloride and milk of lime, according to the equation

KC1 + 3CaO + 3C12 = 3CaCl2 + KC10S.

The operation is conducted in close leaden vessels, fitted with agitators and heated with steam. On the completion of the above reaction the liquid is filtered and evaporated to near dryness, and the residue is again dissolved in hot water. The two salts—calcium chloride and potassium chlorate—are then easily separated by crystal-lization ; the former, being exceedingly soluble, remains in solution, while the latter deposits in tabular crystals, which may be purified by a slight washing. The salt is very largely used in the manu-facture of lucif er matches and various detonating compounds, and in pyrotechny. It is also employed in calico printing as an oxidizing agent in the fixing of certain colours, and it is a convenient source of pure oxygen in laboratory work.

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