Cooley's Cyclopaedia of Practical Receipts Volume I Part 141

The sulphuric acid charged with nitric peroxide is used in the manufacture of chamber acid.

10. Weldon. _a._ The process by which the greater part of the chlorine employed in the manufacture of bleaching compounds is now obtained and which has. .h.i.therto proved the most practical is that of Mr Walter Weldon.

We have the authority of Mr Kingzett, in his work on 'The Alkali Trade,'

for the statement, that out of 90,000 tons of bleaching powder made in Great Britain in 1874, 50,000 were procured by Weldon's method; and turning to the Continent we find the process largely adopted in Germany, France, and Belgium. The utilisation and regeneration of the residual product left in the still after the evolution of the chlorine which it will be presently seen is accomplished by the above process, is not the only advantage accruing from it, since it has also been the means of removing an extensive source of contamination of many of our streams and rivers, into which the then useless chloride of manganese was thrown previous to Mr Weldon's invention. It is true that the only waste product formed in the course of the operations, viz., chloride of calcium, is got rid of by being run into the nearest waters, but it is stated by the Rivers Pollution Commission, beyond making these harder, no other objectionable effect is caused by it.

Mr Weldon's process is based upon the fact, that if protoxide of manganese be suspended in a solution of chloride of calcium, and an excess of lime be added, the protoxide will become readily converted into peroxide if air be forced into the liquor. It had long been known that it was possible to convert into a peroxide the protoxide of manganese obtained by treating the residual still liquors with an equivalent of lime, but all attempts to reduce this knowledge to practical account had proved unsuccessful until Mr Weldon attempted it.

Mr Weldon made the important discovery, that whilst protoxide of manganese is by itself, when treated in the wet way with air, only capable of being converted into peroxide, at the greatest, to the extent of one half; the addition to the protoxide so treated of a certain quant.i.ty of lime converted the whole of it into peroxide in less than a twentieth the time required to peroxidise half the protoxide if lime were absent. It will be seen that it is the employment of an _excess_ of lime which const.i.tutes the success of Mr Weldon's process, which is as follows:--

The residual liquors remaining in the still after the chlorine has been evolved by the action of hydrochloric acid on peroxide of manganese, and in which chloride of manganese is by far the predominating const.i.tuent, are run into a receptacle termed the _neutralising well_, which is usually six feet in depth by twenty in diameter. In this well the free hydrochloric acid of the still-liquor is neutralised by the addition of limestone or chalk, which at the same time serves to decompose the soluble ferric and aluminic chlorides present in the liquid, and to precipitate them as insoluble oxides. During this process the contents of the well are kept in a state of brisk agitation by means of a suitable stirrer. After this treatment the now neutral liquor consists of chlorides of manganese and calcium in solution, of a small quant.i.ty of suspended ferric and aluminic oxides and chalk. It additionally contains also in suspension a by no means small quant.i.ty of sulphate of lime, derived from the sulphuric acid always present in varying amount in the commercial hydrochloric acid used.

From the neutralising well the liquor is pumped to a height of some forty feet into tanks, called the _chloride of manganese settlers_, in which after from two to four hours it deposits the solid matters suspended in it, the supernatant clear liquor a.s.suming a pale rose-coloured appearance.

The next operation is to draw off by means of syphons, which can be lowered or raised in the settlers to any desired level, this clear liquid, containing the chlorides of manganese and calcium, into a vessel called the _oxidizer_; this latter being an iron cylinder, of from eight to twelve feet in diameter, and from twenty-two to thirty-five feet deep. Two pipes go down nearly to the bottom of the oxidizer; the larger one being used for conveying a blast of air from a blowing engine, and the smaller for the injection of steam. The introduction of steam is only had recourse to in case the liquor when drawn into the oxidizer should not have the requisite temperature, viz. 130 or 140 Fahr. Immediately above the oxidizer a reservoir containing milk of lime is placed. A great deal depends upon the careful preparation of the milk of lime, since on the degree of fineness in which the lime is added to the manganese chloride in solution depends the rapidity with which it acts in the oxidizer. The milk of lime is kept constantly agitated, to ensure its being of uniform consistency, and should contain from 15 lbs. to 20 lbs. of hydrate of lime in every cubic foot of the mixture.

A charge of the clear liquor having been drawn into the oxidizer, and raised if necessary to the requisite temperature, the blowing in of air is begun, whilst at the same time the milk of lime is run into the oxidizer as rapidly as possible, the flow of milk of lime being only discontinued when a sample of the filtrate drawn off, by means of a tap placed near the bottom of the oxidizer, ceases to give the manganese reaction when mixed with a solution of bleaching powder. This reaction consists in the production of a purple colour caused by the formation of permanganate of calcium. More milk of lime is then added, when the contents of the oxidizer are found to consist of a thin white mud, composed of a solution of calcium chloride, holding in suspension a mixture of protoxide of manganese and lime. The injection of air being continued the white mud rapidly becomes darker in colour, and soon changes into a thin black mud composed of solution of calcium chloride, holding in suspension certain compounds of peroxide of manganese partly combined with protoxide of manganese, but chiefly with lime, which compounds Mr Weldon terms "_manganites_." Mr Weldon suggests that the manganites so formed may be regarded as salts in which the basic radical is calcium or manganese, and the acid radical MnO_{3}; and may be represented by the formulae CaMnO_{3}, and MnMnO_{3}; and possibly also CaMnO_{2} (MnO_{2})_{2}. "The quant.i.ty of lime which has to be put into the oxidizer before the filtrate from a sample of its contents ceases to yield the manganese reaction varies very considerably. Recently precipitated protoxide of manganese dissolves very appreciably in neutral solution of chloride of calcium, its solution therein comporting itself with reagents exactly like solutions of manganese salts. It dissolves also in solution of oxychloride of calcium, that is to say, in solution of chloride of calcium containing dissolved lime; its solution in oxychloride of calcium not giving the ordinary manganese reactions."

"Hence even if all portions of the lime added to the chloride of manganese in the oxidizer were capable of acting on chloride of manganese equally readily, manganese could not cease to be so in solution as to be detectible by ordinary reagents, until more than an equivalent of lime had been added--that is to say, until enough had been added not only to decompose all the chloride of manganese, but also to form a certain quant.i.ty of oxychlorate of calcium. It is never the case, however, that all portions of the lime used are capable of acting on the chloride of manganese with equal readiness. The lime used always contains a larger or smaller proportion of particles coa.r.s.er than the rest, which coa.r.s.er portions cannot of course act so rapidly as the finer ones; and as the decomposition of the chloride of manganese requires to be completed as quickly as possible, those portions of the lime which will not act upon it instantly are scarcely allowed time to act upon it at all.

"These coa.r.s.er portions of the lime thus contribute very little to the decomposition of the manganese, though they afterwards dissolve completely in the hot solution of chloride of calcium, and then play their full part in the reactions which take place during the subsequent blowing. The proportion of lime which thus does not act on the chloride of manganese varies with the source of the lime, and with the manner in which it is prepared, so that the quant.i.ty of lime which has to be added to a charge of chloride of manganese liquor in the oxidizer, before the filtrate from a sample of the resulting mixture ceases to become coloured on the addition of solution of bleaching powder, varies from about 11 to 145 equivalent.

"The further quant.i.ty of lime which is added after that point has been reached is now usually so much as to raise the total quant.i.ty to about 15 to 16 equivalents, being from one half to six tenths in excess of the quant.i.ty which actually takes part in the decomposition of the chloride of manganese."[239]

[Footnote 239: 'Chemistry, Theoretical, Practical, and a.n.a.lytical, as applied to the Arts and Manufactures,'--Mackenzie & Co.]

As previously stated, Mr Weldon found that if only so much lime is employed as is necessary to precipitate the manganese, not more than half the protoxide of manganese will be converted into the peroxide, and that even this result will be accomplished very tardily. And, as has been already mentioned, a greater and more rapid yield of protoxide can only be obtained by using a larger proportion of lime. Any excess, however, of lime over that absolutely required for the peroxidation of the protoxide of manganese must be sedulously avoided, since a superabundance of lime leads to the formation of compounds that are not readily peroxidised.

Should such compounds be formed, it is necessary to destroy them, and this may be done by the addition of a fresh quant.i.ty of chloride of manganese.

The objectionable compounds in question are lime and protoxide of manganese, which are known in the process under the name of "bases;" and the reason why it is desirable to prevent as much as possible their formation will become evident when it is remembered that they cannot furnish chlorine when treated with hydrochloric acid, but that they merely dissolve in this latter.

The injection of air into the oxidizer, which const.i.tutes the blowing operation, varies from two to four hours.

The quant.i.ty required to be blown in is chiefly dependent upon the depth of the oxidizer, and upon the amount of protoxide of manganese contained in a given volume of the charge.

The greater the depth of this latter the more rapidly does the peroxidation take place; and the greater the number of molecules of protoxide in a given volume of the charge, the larger is the total surface presented to the action of injected air, and consequently the greater is the proportion of the oxygen absorbed.

"In one instance 175,000 cubic feet of air were blown in during five hours, and of the oxygen contained in this, 148 per cent. (equal to rather more than 4 cwt.) was absorbed in the production of 22 cwt. of peroxide of manganese."[240]

[Footnote 240: Weldon.]

The expenditure of mechanical power in forcing the air into the oxidizer averages between seven and eight horse-power for every 100 lbs. of peroxide of manganese obtained. In theory, to produce the quant.i.ty of chlorine contained in a ton of bleaching powder containing 37 per cent. of chlorine, 1020 lbs. of peroxide of manganese would be required; but it is found in practice that instead of this quant.i.ty of peroxide giving the above result, 1100 lbs. are needed.

"The consumption of lime averages 14 cwt. per ton of bleach. By this process 1 ton of bleach is made, using 2832 lbs. of hydrochloric acid, generated by the decomposition of 475 cwt. of salt, viz. a quant.i.ty which theoretically yields 3334 lbs. of hydrochloric acid. There is therefore a loss of acid of 15 per cent. The loss of manganese varies from 4 to 10 per cent. The whole of the lime is lost, and two thirds of the total chlorine (in combination with calcium) contained in the acid used.[241]

[Footnote 241: Kingzett.]

When sufficient air has been blown into the oxidizer, the contents which consist of a solution of chloride of calcium, holding in suspension peroxide and protoxide of manganese and lime, are run into one of a range of settling tanks placed below the level of the oxidizer. These tanks are known as _mud settlers_. In these the manganese mud is left to deposit until about half its volume has become clear. It generally requires 3 or 4 hours to deposit. The clear part, which consists of chloride of calcium, being then decanted by means of a swivel-pipe, is usually thrown away. The mud remaining in the settlers, which contains in a cubic foot from 4 lbs.

to 5 lbs. of peroxide of manganese, is now in a fit condition to be placed in the still, where it is to be exposed to the action of the hydrochloric acid. The stills, which are made of slabs of hard siliceous sandstone or of Yorks.h.i.+re flagstones, and are usually in the shape of an octagonal prism, are about 8 feet square, and 10 feet in depth. Mr Kingzett says "the new Weldon stills are polygonal in shape, about 12 feet across, and 7 feet to 8 feet deep." Contrary to the course formerly followed, when native manganese was used in the Weldon process, the still is charged with hydrochloric acid first, and the manganese mud is run in upon the acid in a small, steady stream, the flow of which can be regulated by a stopc.o.c.k.

Steam being carefully admitted into the still at the same time, the mud dissolves very rapidly in the acid, and the chlorine is evolved in an even current, the force and flow of which is dependent upon and can be very accurately regulated by the admission of the mud.

The time occupied before the reaction between the acid and the manganese is completed varies in different works from two to six hours. At the end of this time the contents of the still are run off into the well placed below it, and are afterwards submitted to the various operations already described, which we have seen to accomplish the regeneration of the residue and effect its reconversion into peroxide of manganese. The process is a continuous one, and theoretically the original quant.i.ty of manganese should be capable of being used over and over again for an unlimited number of operations. In practice, however, there is always found to be a loss of a small per-centage of manganese, arising from some of the chloride of manganese being carried down by the sulphate of lime and the ferric and aluminic oxides in the settlers, and not being thoroughly recovered when the deposit is washed; for, though an exhaustive was.h.i.+ng of the precipitated matters could be easily managed, the bulk of the wash waters would render the recovery of the chloride of manganese from them a non-paying affair. This loss of chloride varies according to the statements of different manufacturers from 2 to 10 per cent.

It is stated that not only is the chlorine yielded by Mr Weldon's process of very pure quality, and the bleaching powder manufactured from it very high of strength and excellence; but that over from 20 to 25 per cent.

more bleach is obtained from a given quant.i.ty of hydrochloric acid, when artificial peroxide of manganese is used instead of the native. This advantage is chiefly owing to the artificial manganese (of the manganese mud) from its physical condition being much more accessible than the native form of manganese to the action of the hydrochloric acid, and from its dissolving in the acid so much more readily and thoroughly, and neutralising as much as from 95 to 99 per cent. of it, a much larger amount than the native ores are capable of neutralising.

Again, the bleaching powder produced by the above process stands not only very high in point of strength, but varies very little in the amount of chlorine it contains, as may be gathered from the following table, which shows the average strength for thirteen consecutive weeks of the bleaching powder made at six large and different manufactories:--

------------------------------------------------- FACTORY.

WEEK. ----------------------------------------- I. II. III. IV. V. VI.

------- ------ ------ ------ ------ ------ ------ 1st 369 378 367 363 366 366 2nd 361 368 367 362 351 374 3rd 365 380 366 361 372 379 4th 359 371 359 362 368 371 5th 358 369 360 363 365 378 6th 360 370 362 359 365 372 7th 365 360 363 366 358 368 8th 363 368 363 363 350 371 9th 364 363 368 357 352 370 10th 365 367 363 360 362 376 11th 368 368 362 359 360 372 12th 368 359 356 356 361 372 13th 367 352 359 361 369 375 -------------------------------------------------

Spite of the expensive plant required to work Mr Weldon's process, it is said to possess very decided advantages over the old methods as far as regards cost of production.

In connection with Mr Weldon's process may be mentioned Mr Valentin's modification of it, for which a provisional patent was taken out by this latter gentleman. Instead of adding more lime, after the neutralised still liquors have been precipitated by an equivalent of lime, as is done in the above process, Mr Valentin adds a solution of pota.s.sium ferricyanide, and air being blown in, the peroxidation of the manganese is effected much more quickly than in Mr Weldon's process. It was also calculated that, by Mr Valentin's method, bleaching powder could be produced at a cost of about ten s.h.i.+llings per ton less than when made by Mr Weldon's.

For the successful working of Mr Valentin's process it is necessary that the ferricyanide should be recovered, not only because of its cost, but also because its presence gives rise to the production of cyanogen compounds, which would enter the chamber with the chlorine. Hitherto no economical plan for the recovery of the salt has been devised, and consequently Mr Valentin's proposed modification of Mr Weldon's process has failed to be adopted.

_b._ A second process for obtaining chlorine, called "_the magnesia process_," has been devised by Mr Weldon. In the previous method, or "_lime process_," two thirds of the chlorine contained in the hydrochloric acid, as we have seen, is lost, pa.s.sing away in the form of waste chloride of calcium.

In the "magnesia process" all the chlorine is utilised, the acid employed being made to yield the whole of its gas in the free state. The regeneration of the manganese peroxide being likewise accomplished, and the process being a continuous one, theoretically no loss of material should take place.

Beyond the employment of liquor pumps, no machinery is requisite for carrying out the operation, which, being very simple in its details, requires the employment of little skilled and, consequently, expensive labour. Further, the inventor claims for it the production of bleaching powder at a less cost per ton than by any other process. The "magnesia method" is worked as follows:--

The spent liquors of the still, consisting of chloride of manganese and free hydrochloric acid, are neutralised with magnesite, or, as it is sometimes called, Greek stone--a very pure native form of carbonate of magnesia. Sometimes the magnesite is calcined, and the magnesia thus obtained used instead.

The neutralisation may be effected either in the still itself, or in a well made of cast iron. The liquid is next pumped into the settlers, in which it deposits its ferric and aluminic oxides and sulphate of lime. The clear liquor containing the chlorides of manganese and magnesia is then run into an iron evaporating pan, where it is concentrated by boiling until it reaches a temperature of between 300 and 320 F. At this point the magnesium chloride begins to be decomposed by the water, and hydrochloric acid is given off. When it has reached the above degree of concentration, it is conveyed into a m.u.f.fle furnace. This furnace is divided into two compartments, separated by an iron door, which can be opened or shut by means of a pulley placed outside. The desiccation of the ma.s.s which is accompanied with the evolution of a little chlorine and a large amount of hydrochloric acid, having been completed in one of the divisions of the furnace, it is broken up by constant stirring into thin cakes and raked into the second division, where it is gently heated with access of air; when the operation is complete the residue which left the first compartment as a mixture of manganese and magnesium chloride becomes converted into manganate of magnesia (MgMnO_{3}), its chlorine having been driven off partly in the free state and partly as hydrochloric acid. "As long as water is present in the furnace hydrochloric acid is evolved, and as the main evaporation takes place in the first division of the furnace, it is chiefly hydrochloric acid that is there generated. In the second division it is chiefly chlorine which is evolved, but it is, of course, mixed with some hydrochloric acid. It is, indeed, doubtful whether much manganese chloride is decomposed by the water so long as there remains any chloride of magnesium, as this body is far more readily decomposable."[242]

[Footnote 242: Kingzett.]

It is stated that all the manganese is not converted into peroxide in the furnace, but that a certain portion of it is left as protoxide; which, with the magnesia, const.i.tutes the useless "bases." The completion of the process is known when portions of the cake drawn from time to time from the furnace cease to indicate any increase in the quant.i.ty of peroxide of manganese.

The finely-divided black powder--manganate of magnesium--thus obtained, after leaving the furnace and when sufficiently cold, is ready for the stills--where, in contact with hydrochloric acid, it is again employed in the generation of chlorine.

The chlorine leaves the furnace mixed with much hydrochloric acid, nitrogen, and air. The gaseous mixture is drawn by a chimney draught through the c.o.ke towers, as in the making of salt-cake. By this contrivance the hydrochloric acid is recovered, yielding a solution strong enough to react upon fresh manganese in the still. The diluted chlorine may be made to ascend leaden towers, where it comes in contact with a shower of milk of lime, which absorbs the gas and forms ordinary bleaching liquid, whilst sometimes it is employed in the production of pota.s.sium chlorate.

We have seen that the chlorine yielded by the "magnesia process" is partly in the concentrated, and partly in the dilute condition. The ratio of strong chlorine generated in the still to that of weak chlorine produced in the furnace may be anything between one to one, and one to about four, at pleasure.

"When working so as to obtain strong chlorine and weak chlorine in about equal proportions, the quant.i.ty of the liquor to be boiled down per ton of total bleaching powder made was about 105 cubic feet. As the proportion of the weak chlorine increased, the quant.i.ty of liquor to be boiled down diminished until, when the proportion of the weak chlorine to that of the strong became as four to one, the quant.i.ty of liquor to be boiled down per ton of total bleaching powder made was only about 40 cubic feet."[243]

[Footnote 243: 'Chemistry, Theoretical, Practical, and a.n.a.lytical,'--Mackenzie & Co.]

11. (Deacon.) As we have already seen, Vogel proposed to obtain chlorine by the decomposition by heat of cupric chloride, and to reconvert the resulting cuprous chloride into the cupric salt by treatment with hydrochloric acid.

Chlorine may be produced by pa.s.sing a mixture of gaseous hydrochloric acid and air over heated bricks or other porous substances, a reaction which Oxland unsuccessfully attempted to turn to account for the production of chlorine for manufacturing purposes. The cause of failure appears to have been the great heat necessary to effect the decomposition of the acid atmospheric oxygen.

In the late Mr Deacon's process both Vogel's and Oxland's methods are combined. He discovered that to be able to generate chlorine and water from gaseous hydrochloric acid and air, a very much lower temperature than that employed by Oxland was necessary, and he found that this diminished temperature could be attained, if the gas and air to be decomposed were pa.s.sed over porous bricks saturated with a solution of sulphate of copper, and heated to a temperature of 700 to 750 F.

Beyond this point he found the heat ought not to be carried; for at 800 the cupric chloride formed begins to volatilise, and to condense in the cooler parts of the apparatus (presently to be described), thereby interfering with the draught through it, and delaying the working, since its removal becomes necessary. It was found that below 400 the reaction does not take place. Experience has demonstrated that the best temperature to effect this decomposition is 625 F.

The hydrochloric acid obtained either from a soda furnace or evolved from an aqueous solution is immediately mixed with a quant.i.ty of air containing an excess of oxygen over that required for liberating all the chlorine from the evolved hydrochloric acid, and pa.s.sed through heated U-shaped tubes of cast-iron, from which the gaseous mixtures obtain the necessary temperature. The original plant was so contrived that the heated gases were conveyed from the U-shaped tubes into a series of nine towers made of iron or other suitable material. Entering by a pipe at the bottom of the first tower, and pa.s.sing on to the second, the gases came into contact with a series of ordinary agricultural drain pipes of small bore arranged with vertical s.p.a.ces, these pipes being saturated with a solution of sulphate of copper and sulphate of soda, it being subsequently found that this latter addition increased the efficacy of the copper sulphate, as well as its power to resist decomposition. From the first two towers of the series the mixed gases traversed the remaining ones, where they encountered small pieces of common brick, fire brick, or burnt clay also impregnated with the copper and soda sulphates, after reacting upon which they pa.s.sed out of the apparatus, called the '_decomposer_,'