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The Pamphlet Collection of Sir Robert Stout: Volume 73

Sulphur and Sulphuric Acid: A Practical Demonstration at Kempthorne-Prosser's New Zealand Drug Company's Works at Burnside, Dunedin

Sulphur and Sulphuric Acid: A Practical Demonstration at Kempthorne-Prosser's New Zealand Drug Company's Works at Burnside, Dunedin.

Sulphur is found in small quantities in all cultivated plants. It must therefore be taken as a necessary part of their mineral food which they take from the soil. In the soil it exists chiefly as sulphates of lime, magnesia, potash, soda, and ammonia. Superphosphate of lime—so largely used everywhere as one of the best kinds of artificial manures—contains from 20 to 45 per cent, of sulphate of lime, and nearly a fifth of this is sulphur, so that a ton of superphosphate contains from 90lb. to 200lb. of sulphur in a soluble state. This manure therefore supplies both sulphur and lime to the plants in an available form. There is another manure, under the name of kainit, in the market, which also offers sulphur in a soluble condition combined with potash, as sulphate of potash. Kainit will be described under the potash salts, as that element, and not sulphur, is its most valuable constituent. Sulphur itself, found in large quantities in old or recent volcanic regions—Sicily, Iceland, South Italy, and in New Zealand, at Tarawera and White Island, off the coast of Auckland—is now being made in nature by the meeting of two poisonous common colourless volcanic gases, of a powerful, penetrating, and most disagreeable odour.

These gases are sulphur dioxide—SO2—and sulphuretted hydrogen—H2S. The equation is
  • SO2 + 2H2S = 2H2O + S3.

The immediate result of the meeting of these two gases is the formation of liquid water and solid sulphur. Chemistry does not offer a better example of the wonderful transformations which take place when two substances act chemically on each other. The lecturer had seen the sulphur-manufacturing process in full operation in the Rotomahana district, just before the Tarawera volcano burst out. There the two gases named were issuing separately from their respective crevices and meeting in the air, the result being a yellow mist of sulphur perpetually forming and falling page 20 on the barren mixture of sulphur and sand that has to pass for [unclear: so] in that desolate region. Sulphur was made on the lecture-table by this process, constituting one of the prettiest experiments of the course.

The sulphur dioxide was made by boiling in a glass flask strong sulphuric acid with copper; and the sulphuretted hydrogen, by the action of dilute sulphuric acid on sulphuret of iron in another glass flask. From each flask the gas was led into a series of three clear glass globes, where they mingled, producing by their union, as if by magic, a dusty white cloud in each of the globes, which soon coated them with a coating of pure yellow sulphur.

The great part which sulphuric acid plays in the manufacture of artificial manures, as well as in many other very important industries, makes it necessary for us to know something of its own manufacture. Thought in was known to the alchemists of the Middle Ages, it was not till the present century had well begun that any considerable quantity began to be made. Its application to the manufacture of bleaching-powder (chloride of lime and washing-soda gave this acid a firm footing among the large industries of Europe; and then the increasing demand for nitric acid and acetic acid, which are products of its action on saltpetre and acetate of soda respectively, gave it a further impulse. Then, again, the more recent discovery or invention of the coal-tar colours (aniline, mauve, magenta, alizarin, picric acid, &c.), gun-cotton, and nitroglycerine, in all of which it is a prime factor, created for this acid a demand which is now supplied by the production of about a million tons per annum around the great industrial centres of Manchester, Glasgow, and Newcastle. So numerous and varied and vast are its applications in the great modern industries of civilised communities that it has been well said of this acid that the amount of a nation's consumption of it may be taken as a gauge of its industrial activity and a measure of its civilisation. In Otago there is one sulphuric-acid works, at Burnside, with an output of about 500 tons per annum. Taking the population of Otago at 160,000, this would give us about one-twelfth of the English production per head of the population.

The manufacture is carried on in large chambers made of heavy sheet-lead, 7lb. to the square foot. No solder can be used for joining the sheets of the metal, for no solder could withstand the action of the fumes that are concerned in the manufacture. The contiguous sheets are therefore joined together by a portion of the lead of each sheet being melted and run together by running the hot flame from a blowpipe slowly along the junction. This melted metal on hardening (which it does very readily) forms a homogeneous solder of lead itself, and is just as strong and effectual against the action of the fumes as any other part of the chambers.

In these chambers—often 100ft. long by 20ft. wide and 18ft. high, and communicating with each other by wide lead pipes bent twice at right angles, and just protruding through the roofs of two contiguous chambers—sulphuric acid is made by the action of four gases on each other These gases are—(1.) Sulphur dioxide—SO2,—which is made by roasting eithur sulphur or pyrites (the mundic of miners) in a suitable furnace The sulphurous fumes are led by a short flue into the first of the chambers. (2.) The brown oxides of nitrogen. These are made in a small iron per containing Chili saltpetre and sulphuric acid, the pot and contents being heated by the heat of the sulphur-furnace, on a ledge of which it is placed so as to have the hot sulphurous gases streaming over it. These nitrons fumes pass into the lead chamber with the sulphurous fumes. (3.) Oxygen This gas is supplied by the air that feeds the sulphur in the sulphur furnaces. Part of the oxygen of the air is of course used up in burning the sulphur, and the rest of it goes into the chamber with the other fumes page 21 (4.) The last requisite is steam, which is projected into the chambers at various points from a boiler. The steam from the boiler is led in a 4in. pipe round outside the chambers, and from this pipe smaller branch-pipes project just in through the lead walls of the chambers, delivering the steam in such quantities as may be required.

The action on each other of these four gases, which results in the formation of sulphuric acid in these lead chambers, is something of this kind: (a) The sulphur dioxide—SO2—has room for another atom of oxygen, which would make it sulphur trioxide—SO3; but it is not disposed to take that extra atom of oxygen from the atmospheric air in the chambers; (b) the brown nitrous fumes, however,—NO2 and N2O3—have each one atom of oxygen that they can easily spare. They therefore pass this oxygen over to the SO2, as shown by the equations,—
  • SO2 + NO2 = NO + SO3.
  • SO2 + N2O3 = 2NO + SO3.
The sulphur trioxide thus formed has a most intense attraction for water in any form, and therefore instantly unites with the steam in the chambers, forming with it sulphuric acid by the equation,—
  • SO3 + H2O = H2SO4.
The sulphuric acid, H2SO4, thus produced, being heavy, falls on to the floor of the chamber, and mingles with the cold water there. Meantime, the oxide of nitrogen (NO) formed by the surrender of oxygen by the brown fumes, as shown above, takes another atom of oxygen from the air:—
  • NO + O = NO2,
  • and 2NO + O = N2O3,
thus becoming brown again, and ready to hand over the newly-picked-up oxygen to sulphur dioxide; and so the thing goes on, sulphur dioxide always pouring in from the sulphur-furnaces, perpetually taking oxygen from the brown fumes, and then uniting with the water sent in as steam. The excess of fumes passes from the first to the second chamber by the wide pipe connecting them through their roofs, and from the second to the third, so that there may be ample time and room for all the fumes to complete the formation of acid. The sulphuric acid is allowed to accumulate on the floors of the chambers until it is pretty strong, containing about 60 per cent, of acid and 40 per cent, of water. It is then drawn off by a leaden siphon-pipe as "chamber acid," in which condition it is strong enough to be used for many manufacturing processes. The chamber acid is brought up to a strength of 76 per cent, in leaden pans heated either below or above, the heat driving off part of the water in the form of steam, and leaving the acid itself behind in the pans. The acid thus strengthened to 75 per cent, is known in the trade as brown oil of vitriol, and labelled B.O.V.

To get the pure, strongest acid—the real oil of vitriol, containing 98 per cent, of acid—glass retorts or platinum stills are employed, as any of the cheaper metals—iron, lead, copper, silver, &c.—would be attacked and destroyed by the hot strong acid. In these distilling processes it is the water that is evaporated off and passes away, the sulphuric acid itself remaining behind in the stills.

This is just the opposite of what takes place in the distillation of alcoholic liquors, when it is chiefly the spirit that is evaporated off—the water mainly remaining in the stills.

The strong sulphuric acid or oil of vitriol thus made is a heavy oily liouid, nearly twice as heavy as the same bulk of water. It is about the page 22 strongest acid known, dissolving and destroying all the common metals except gold and platinum, charring wood, paper, leather, and all kinds of clothing materials, cottons, linens, woollen goods of every kind and quality; burning and charring the flesh of animals, and dissolving even their bones; breaking up sugar by separating out its charcoal from the elements of water, and appropriating the latter to itself. These and many other wonderful things this most powerful of acids does rapidly and silently, ever leaving destruction in its path. But this very power is, when properly applied, a source of incalculable benefits to the human race.

The visit of the lecturer and his students to the Burnside Chemical Works had a threefold object: First, to see the manufacture of sulphuric acid itself; second, to see the application of that acid to the manufacture of muriatic acid (spirits of salt) and nitric acid (aqua fortis), acetic acid, ammonia, and superphosphate, all carried on at Kempthorne, Prosser, and Co.'s chemical and manure works; and third, to see through the freezing-works, with its cold chambers and powerful air-compressing machinery.

The class mustered at Burnside to the number of about sixty members, including twelve lady students. Several of them had come long distances for the occasion, getting up at 4 o'clock in the morning, starting for the railway-station at Clinton, Lawrence, or Palmerston at 5 o'clock to catch the early train at 6 to 6.30, reaching Dunedin any time between 10 and 12, and getting back to their homes again between 7 and 10 p.m. This is the usual Saturday programme of several of the members of this chemistry class. They were received and heartily welcomed by Mr. Smith, the manager at the chemical works, who proceeded at once to guide them through the various deparments under his control. The lead-chambers here are three in number, 40ft. long by 14ft. wide and 12ft. high. There are four sulphur-furnaces, all contained in one piece of firebrick masonry, and fitted with sliding iron doors that can be adjusted so as to regulate the admission of air, much or little at pleasure, according to the requirements of the chambers. All the furnaces unite their gaseous sulphurous load in one common flue, along which they pour it into chamber No. 1. Exposed to these hot blue fumes the nitre-pots emit their deadly brown vapours, which are carried along with them into the chamber, there to mingle with the steam hurled in at various points from the boiler alongside. The air in the building outside the sulphur-furnaces is not at all uncomfortable, notwithstanding the deadly nature of the operations within. The sulphur burned is now got from the Tarawera volcanic hot-springs district, the Sicily importation having stopped some time ago. The nitre for the pots is the Chili salpetre (nitrate of soda), which is cheaper than ordinary potash-saltpetre, and yields more gas, weight for weight. Here nothing is wasted, the refuse of these saltpetre-pots being utilised afterwards for enriching some of the bonedust with soluble sulphate, and for rendering part of the bones themselves soluble. From the lead chambers they saw the acid led down in a leaden pipe to the platinum still. This still, made of one piece of pure platinum, is 4ft. long by 1ft. or 18in. wide, and is heated beneath by the flames from the furnace over which it is built. The water is expelled in vapour, and the strong acid is perpetually flowing out through a platinum siphon into a second smaller reservoir, from which it runs through a platinum tube into a cooler surrounded by cold water, and is thence conveyed in a perpetual, steady, slender, oily stream into the 3-gallon stone-jars, ready for use or for the market. The platinum still and appliances must have cost over £1,000, and would be worth page 23 now nearly double that sum, owing to the recent large rise in the price of that metal.

The next department visited was the muriatic-acid and nitric-acid plant. Here was to be seen, though not in operation on this particular day, the cast-iron pan which receives the common salt from which the muriatic acid is to be made. This pan is built into the masonry of a furnace in such a way that the flames will strike the bottom of it in a uniform manner. Strong sulphuric acid is led into the pan by a leaden pipe, and poured in a thin stream—slowly at first—over the salt. Chemical action of a very vigorous kind instantly is set up at the first contact of the acid with the salt; dense volumes of heavy grey muriatic-acid vapour are produced, and pour out through the pipe leading into the Woulffe's bottles placed in series alongside. These Woulffe's bottles have a capacity of 5 or 6 gallons. Each of them is about two-thirds full of clean cold water to begin with, and each of them has two necks. Through one neck—that nearest the furnace or pan—the delivery-tube conveying the fumes passes down into the cold water, and there delivers its muriatic-acid vapours. The vapour is condensed and dissolved by the water as fast as it comes in. This goes on for some time till the water in that first Woulffe's bottle is nearly saturated with the vapour, and then it begins to rise through the water into the roof of that bottle, whence it is led by another pipe (which just protrudes through the second neck) away into Woulffe's bottle No. 2. This, in its turn, gets charged and saturated with the muriatic acid, and the excess of the gas is in like manner passed into the third and fourth, and so on up to the twelfth Woulffe's bottle, the contents of each becoming converted into the "hydrochloric acid" or "muriatic acid" or "spirits of salt" of commerce. The cold water in these Woulffe's bottles absorbs five hundred times its own bulk of the acid fumes, and, when fully charged, contains about 40 per cent, of that acid.

The same plant serves, after a thorough cleaning-out, for the manufacture of nitric acid, or spirits of nitre, using in this case saltpetre instead of common salt, and leading the acid fumes into Woulffe's bottles containing only a very small quantity of cold water, or none at all.

The equation for making muriatic acid is—
  • 2NaCl + H2SO4 = Na2SO4 + 2HCl;
which reads: 117lb. of salt heated with 98lb. of strong sulphuric acid will yield 142lb. of salt cake and 73lb. of pure muriatic-acid gas. This would make 180lb. of the strong fuming muriatic acid of commerce.
For nitric acid the equation is—
  • 2KNO3 + H2SO4 = K2SO4 + 2HNO3;
  • or, 2NaNO3 + H2SO4 = Na2SO4 + 2HNO3
202lb. of common saltpetre (KNO2), or 170lb. of Chili saltpetre (NaNO3) yielding 126lb. of pure fuming nitric acid.
The visitors turned their attention next to the acetic-acid plant in another part of the works. Here they saw the steam-jacket surrounding and enclosing the still or retort in which the strong sulphuric acid attacks the acetate of soda, driving out the acetic acid in the form of fumes, while keeping the soda to itself. The equation is—
  • 2NaC2H3O2 + H2SO4 = 2C2H4O2 + Na2SO4;
meaning that 164lb. of acetate of soda will require 98lb. of sulphuric acid to make 120lb. of pure acetic acid, and leave in the retort a residue of 142lb. of sulphate of soda.
page 24

The arrangements and Woulffe's bottles for condensing and collecting the acetic acid are quite similar to those described above for collecting the muriatic acid. They next saw the whole process for making superphosphate of lime—the soluble phosphate of farmers. This manufacture is carried on in large, flat-bottomed, oblong wooden vats, about, say, 12ft. long by 7ft. wide and 2ft. deep. Into these vats is poured first a strong liquid nitrogenous material extracted from bones and other animal matters in another part of the works; over this are poured ten or twelve 3-gallon jars of the strong sulphuric acid already described; and then the bonedust, or guano, or mixture of both that is destined to be converted into superphosphate is delivered by barrowfuls and rapidly shovelled into the acid mixture, and rabbled through the heavy liquid by long-handled large wooden hoes. Immediately the mixture begins to effervesce and give forth clouds of steam and carbonic-acid gas. Very soon it becomes strongly heated by the chemical action of the acid; more and more raw phosphates are shovelled into it. The heat increases, the fumes become intolerable, and a clear space is soon created around the seething, puffing, sweltering vats. In the course of half an hour or so, however, the chemical process is finished; the sulphate of lime absorbs the spare moisture, and the clouds clear away; the air resumes its normal condition of comparative purity; the vats are ready to be discharged; and the superphosphate is hurled away, to ripen in the sheds and to await a purchaser.

The guano and crushed bones before this sulphuric-acid treatment contained their phosphate of lime in its insoluble or tricalcic state—Ca2P2O3. The sulphuric acid changed this into the soluble or monocalcic state, CaH4P2O8, and there was produced at the same time in the vats a quantity of sulphate of lime—CaSO4, which contains sulphur and lime, both soluble; so that now the superphosphate contains three soluble ingredients—namely, phosphoric acid, lime, and sulphur—all of which are required as plant-food.

The action of the sulphuric acid on the insoluble phosphate of the guano or bones is shown thus:—
  • Ca3P2O8 + 2H2SO4 = CaH4P2O8 + 2CaSO4;
which may be translated thus: 310lb. of insoluble tricalcic phosphate, acted on by 196lb. of sulphuric acid, will make 234lb. of soluble or monocalcic phosphate and 272lb. of sulphate of lime.

It is the sulphate-of-lime part of this manure that dries it up. It is desirable also that there should be a little carbonate of lime in the raw material, so as to furnish enough carbonic-acid gas in the vats to make the whole mass porous and loose in texture, like the sponginess of properly, baked bread, whose openness and lightness are due to the same cause-namely, carbonic-acid gas.

The students were then invited into the ammonia department, where they saw the boiler into which are put the sulphate-of-ammonia crystals and the slaked lime that is to expel the ammonia gas from that salt.

The boiler is an iron one built into a firebrick furnace so that the flames may be equally distributed over the bottom of it. The mixture of lime and sulphate when heated gives off voluminous but invisible vapours of ammonia, along with some steam. These hot vapours are led up into a metallic drum at the ceiling, where most of the steam is condensed to liquid water, whilst the ammonia gas travels on through the pipe that leads from the drum up through the roof, where it takes the form of a spiral coil or worm, which, being kept cold by the air, condenses the rest of the water, leaving the ammonia gas to pursue its way down through the descending pipe into cold water contained in a series of large Woulffe's page 25 bottles, similar to those described for the manufacture of muriatic acid. The ammonia gas—NH3—dissolves in the cold water just as the muriatic-acid gas does, and the excess of it passes from one Woulffe's bottle to the next till they are all charged and saturated with it.

The water absorbs about a thousand times its own volume of the gas, and, in doing so, becomes heated, and expands till the specific gravity is 0.88. It is then ready for the market as "ammonia fortissima."

The sulphate of ammonia used is got from the company's Auckland works, where it is made from the ammoniacal liquor of the gasworks. The Newcastle coal used at Auckland for gas-making contains more nitrogen than our West Coast coal, and that nitrogen appears in the gas liquor as ammonia in such quantity that it can be profitably extracted.

The equation for getting ammonia from sulphate of ammonia is—
  • (NH4)2SO4 + CaOH2O = CaSO4 + 2H2O + 2NH3;
which reads: 132lb. of sulphate of ammonia, heated with 74lb. slaked lime, will produce 136lb. sulphate of lime, 36lb. water, and 34lb. ammonia gas.

A good deal of sulphate of ammonia is used at these works for putting into the manures to raise their percentage of ammonia. This is a most commendable use to make of that salt, as in the state of sulphate the ammonia is not only ready-made, but readily soluble, so that the plants may have the immediate use of it.

An outstanding feature of these chemical and manure works at Burn-side is the fact that, since they make their own sulphuric acid for the manufacture of superphosphates, they can very well be liberal in the use of that necessary material in their vats. Their position—adjacent to the freezing-works and stock saleyards also—is a most favourable one for getting large supplies of blood and bones, and other animal matters rich in nitrogen, for their manufacture.

When the party had had their fill of the chemical and manure works they adjourned to the freezing-works, and were heartily received by Mr. Scott, the manager, who at once put them on a home-footing in the large establishment under his charge. Their first experience there, however, was a very cold one, as Mr. Scott, with a good deal of dry humour, insisted on putting them all, in two detachments, right through the freezers—ladies first. Being under his own guidance, however, lantern in hand, with which several of them were also furnished to help to guide them on their chilly journey, they proceeded with some misgiving into Mr. Scott's polar regions.

As they pursued their shivering way, with sheep, dead and disembowelled, by the hundred hanging hard as iron on each side of them, with the crystalline snow inches deep, hard and glittering in the candle-light, clinging to the walls that narrowed their frozen path, with their very breath freezing into a fine white mist of snow at every exhalation, and with the intimation (which they at first took for a grim joke) that after doing the whole long length of this arctic parade they must, in order to get back to the light of day, retrace their freezing footsteps and do it all over again, their state of mind as well as of body may be imagined; it cannot be described. They were haunted, too, by the benumbing dread that some of their number might wander in the darkness and get left behind in these glacial regions. The first detachment (including all the ladies of the party), having emerged into the light and warmth of the outer air, hurried the others, in a very encouraging way, into the cadaverous and blood-curdling labyrinth of frost and snow from which they themselves had just escaped alive. Their safe return, after five minutes' experience of the frozen horror, however, equalised matters, and the whole party were page 26 hurried round by Mr. Smith into the engine-room to thaw; and there they saw the throbbing machinery at work compressing the air which by its expansion in the chamber is to produce the searching cold that sends these carcases across the equator to England safe from the touch of the putrefaction fiend. Mr. Scott very clearly explained the wonderful way in which the heat is squeezed out of the air when, as here, a great volume is compressed into a small space; and how, when this same air (now cold) is allowed to resume its original volume, it demands the native heat which had been squeezed out of it, and, this having now been lost to it for ever, it draws out of all surrounding objects (carcases, &c.) the heat that properly belongs to them, and thus reduces them to the frozen state in which they are seen in these frigid chambers.