Acids, Alkalis and Salts :: CHAPTER II SULPHURIC ACID AND SULPHATES

CHAPTER II SULPHURIC ACID AND SULPHATES

Key Industries. The importance of the chemical industries depends mainly on the fact that they constitute the first step in a series of operations by which natural products are adapted to our needs. The materials which are found in earth, air, and water are both varied in kind and abundant in quantity, but in their natural state they are not generally available for immediate use. Moreover, very many substances now deemed indispensable are not found ready formed in Nature.

The end product of the chemical manufacturer is often one of the primary materials of some other industry. Soda ash and Glauber’s salt are essential for making glass; soap could not be produced without caustic alkali; the textile trade would be seriously handicapped if bleaching materials, mordants, and dye-stuffs were not forthcoming. Considered in this light, the preparation of chemicals is spoken of as a “key industry.”

Furthermore, very few of these indispensable substances can be made without using sulphuric acid. This acid is, on that account, just as important to chemical industries as the products of these are to other branches of trade. It may, therefore, be looked upon as a master key of industrial life.

Primary Materials. The composition of sulphuric acid is not difficult to understand. Air is mainly a mixture of oxygen and nitrogen; and when a combustible body burns, it is because chemical action between the material and oxygen is taking place. In this way, sulphur burns to sulphur dioxide. This gas, dissolved in water, forms sulphurous acid, which changes slowly to sulphuric acid by combination with more oxygen. Hence, sulphur, oxygen, and water are the primary materials required for making sulphuric acid.

Sulphur is the familiar yellow solid commonly known as brimstone. It is found native in the earth, and is fairly abundant in certain localities, notably in the neighbourhood of active and extinct volcanoes. Italy, Sicily, Japan, Iceland, and parts of the United States are the principal sulphur-producing countries. Though very plentiful and consequently cheap, only a relatively small quantity of sulphuric acid is made directly from native sulphur, because at the time when this industry was started in England, restrictions were placed on the export of sulphur from Sicily and, consequently, the plant which was then established was adapted to the use of iron pyrites.

Iron pyrites contains about 53 per cent. of sulphur combined with 47 per cent. of iron, and when this is burnt in a good draught, nearly the whole of the sulphur burns to sulphur dioxide, leaving a residue of oxide of iron which can be used for making cast iron of a low grade.

Iron pyrites is often supplemented by the “spent oxide” from the gas works. Crude coal gas contains sulphur compounds which, if not removed, would burn with the gas and form sulphur dioxide. The production of these pungent and suffocating fumes would be a source of great annoyance, and therefore it is necessary to remove the sulphur compounds. To do this, the gas is passed through two purifiers, the first containing slaked lime and the second ferric oxide, both in a slightly moist condition. After being some time in use, the purifying material loses its efficacy; the residue from the lime purifier is sold as “gas lime,” but that from the ferric oxide purifier is exposed to the air and so “revived.” At length, however, it becomes so charged with sulphur that it is of no further use for its original work. It is then passed on to the sulphuric acid maker.

Evolution of the Manufacturing Process. In dealing with the main processes for the manufacture of acids and alkalis, reference will frequently be made to the methods of bygone times. Although as an exact science Chemistry is comparatively modern, as a branch of human knowledge its history goes back to the dawn of intelligence in man. It is agreed that the higher types of living things are more easily understood when those of a simpler and more primitive character have been studied. In like manner, the highly specialized industries of modern times become more intelligible in the light of the efforts of past generations to achieve the same object.

Basil Valentine, who lived in the fifteenth century, states that the liquid which we now call sulphuric acid was in his day obtained by heating a mixture of green vitriol and pebbles. Until quite recent times, sulphuric acid of a special grade was made by precisely the same method, except that the pebbles were dispensed with. In passing, we may remark that the common name “vitriol,” or “oil of vitriol,” is accounted for by this connection with green vitriol. The second method, quoted by Basil Valentine, consisted of the ignition of a mixture of saltpetre and sulphur in the presence of water. This is actually the modern lead chamber process in embryo.

Fig. 2. PLAN OF SULPHURIC ACID WORKS

About the middle of the eighteenth century, “Dr.” Ward took out a patent for the manufacture of sulphuric acid, to be carried on at Richmond in Surrey. He used large glass bell jars of about 40-50 galls. capacity, in which he placed a little water and a flat stone to support a red-hot iron ladle. A mixture of saltpetre and sulphur was thrown into the ladle and the mouth of the vessel quickly closed. After the vigorous chemical action was over, the ladle was re-heated and the process repeated until at last fairly concentrated sulphuric acid was produced.

The large glass vessels used by Ward were costly and easily broken. They were soon replaced by chambers about 6 ft. square, made of sheet lead, but otherwise the process was just the same. The next advance consisted in making the process continuous instead of intermittent. An enormously increased output was thereby rendered possible, and the main features of the modern process gradually developed.

The Lead Chamber Process. We can now consider the actual working of the lead chamber process, aided by the diagrammatic plan of the works shown in Fig. 2. Sulphur dioxide is produced in a row of kilns (A-A) by burning iron pyrites in a carefully regulated current of air. The mixture of gases which leaves the pyrites burners contains sulphur dioxide, excess of oxygen, and a very large quantity of nitrogen. To this is added the vapour of nitric acid, generated from sodium nitrate and concentrated sulphuric acid contained in the “nitre pots,” which are placed at B. The mixture of gases then passes up the Glover tower (C) and through the three chambers in succession, into the first two of which steam is also introduced. Sulphuric acid is actually produced in the chambers, and collects on the floors, from which it is drawn off from time to time. The residual gas from the last chamber is passed up the Gay Lussac tower (D), and after that is discharged into the air by way of the tall chimney (J).

Fig. 3. GENERAL VIEW OF SULPHURIC ACID WORKS

The Oxygen Carrier. We have seen that sulphur dioxide, oxygen, and water are the only substances required to produce sulphuric acid. Why, then, is the nitric acid vapour added to the mixture? As described in a former paragraph, the combining of these gases was represented as being a very simple operation. So indeed it is, for it even takes place spontaneously. Yet, as a commercial process, it would be quite impracticable without the nitric acid vapour, for although the gases combine spontaneously, they do so very slowly, and it is the nitric acid vapour which accelerates the rate of combination.

It is not known with any degree of certainty how the nitric acid acts in bringing about this remarkable change. It has been suggested that reduction to nitrogen peroxide first takes place, and that sulphur dioxide takes oxygen from this body, reducing it still further to nitric oxide, which at once combines with the free oxygen present to form nitrogen peroxide again. So the cycle of changes goes on, the nitrogen peroxide playing the part of oxygen carrier to the sulphur dioxide; and since it is continually regenerated, it remains at the end mixed with the residual gases.

Recovery of the Nitrogen Peroxide. If the gases from the last chamber passed directly into the chimney shaft, there would be a total loss of the oxides of nitrogen, and the consequence of this would be that more than 2 cwt. of nitre would have to be used for the production of 1 ton of sulphuric acid. This would be a serious item in the cost of production, and it is therefore essential that this loss should be prevented.

The recovery of the oxides of nitrogen is effected in the Gay Lussac tower, a structure about 50 ft. in height, built of sheet lead and lined with acid-resisting brick. It is filled with flints, over which a slow stream of cold concentrated sulphuric acid is delivered from a tank at the top. As the gas from the last chamber passes up this tower, it meets the stream of acid coming down. This dissolves and retains the nitrogen peroxide. The acid which collects at the bottom of the tower is known as nitrated vitriol.

The next step is to bring the recovered nitrogen peroxide again into circulation. The nitrated vitriol is raised by compressed air to the top of the Glover tower, and as it trickles down over the flints in this tower it is diluted with water, while at the same time it meets the hot gases coming from the pyrites burners. Under these conditions, the nitrogen peroxide is liberated and carried along by the current of gas into the first lead chamber. The stream of cold acid coming down the Glover tower also serves to cool the hot gases before they enter the first chamber.

In order to complete the description of the works, it is necessary to add a note on the lead chambers themselves. The sheet lead used in their construction is of a very substantial character; it weighs about 7 lb. per square foot. The separate strips are joined together by autogenous soldering, that is, by fusing the edges together. In this way the presence of another metal is avoided; otherwise this would form a voltaic couple with the lead, and rapid corrosion would take place.

The size of the chambers has varied a great deal. In the early years of the nineteenth century, the capacity of a single chamber was probably not more than 1,000 cu. ft.; at the present time, 38,000 cu. ft. is an average size, and there may be three or five of these chambers. The necessity for this large amount of cubic space is easily accounted for. The reaction materials are all gases, and a gas occupies more than one thousand times as much space as an equal weight of a solid or liquid. Moreover, oxygen constitutes only about one-fifth of the total volume of air used in burning the pyrites; the other four-fifths is mainly nitrogen, which, though it does not enter into the reaction at all, has to pass through the chambers.

Modern Improvements. Among the modern innovations in the lead chamber process, the following are worthy of note. “Atomized water,” that is, water under high pressure delivered from a fine jet against a metal plate, has certain advantages over steam. In order to bring about a more rapid mixing of the gases in the chamber, it is proposed to make these circular instead of rectangular, and to deliver the gases tangentially to the sides. Another suggestion is to replace the lead chambers by towers containing perforated stoneware plates set horizontally. By this arrangement, since the holes are not placed opposite one another, the gases passing up the tower must take a zig-zag course. This makes for more efficient mixing.

THE CONTACT PROCESS

Sulphur Trioxide. When elements are combined in different proportions by weight, they produce different compounds. Thus, in the case of sulphur and oxygen, there are two well-known compounds, namely, sulphur dioxide and sulphur trioxide. In the former, a given weight of oxygen is combined with an equal weight of sulphur; in the latter, this same weight of sulphur is combined with 50 per cent. more oxygen. On this account, sulphur trioxide is spoken of as the higher oxide.

We can now state in general terms another method by which sulphuric acid can be built up from its elements. Sulphur, as we have seen, burns in oxygen, forming sulphur dioxide. This substance can then be made to unite with more oxygen to give sulphur trioxide, which, with water, yields sulphuric acid. There are three steps in this synthesis. The first, namely, sulphur to sulphur dioxide, has already been considered; the last, sulphur trioxide to sulphuric acid, only requires that sulphur trioxide and water shall be brought together: we can, therefore, confine our attention to the intermediate step, namely, the conversion of sulphur dioxide into trioxide.

This operation, when carried out in a chemical laboratory, is a very simple one. Fig. 4 shows the necessary apparatus. Sulphur dioxide from a siphon of the liquefied gas and air from a gasholder are passed into the Woulff’s bottle A, containing concentrated sulphuric acid; this removes moisture from the gases. The drying process is completed in the tower B, which contains pumice stone soaked in sulphuric acid. The mixed gases then pass through the tube C, containing platinized asbestos heated to about 400° C.: the sulphur trioxide collects in the cooled receiver D.

Fig. 4. SULPHUR TRIOXIDE—THE CONTACT PROCESS

Platinized asbestos is made by soaking long-fibred asbestos in a solution of platinum chloride. The material is then dried and subjected to a gentle heat. In this way, metallic platinum in an exceedingly fine state of subdivision is deposited on the asbestos fibre, which merely serves as a convenient support.

Catalytic or Contact Action. The influence of the finely divided platinum is a very important factor in the reaction. It cannot, however, be said to cause the union of sulphur dioxide with oxygen, for the gases combine to a very slight extent when it is not present. What the platinum actually does is to influence the rate of formation to such a degree that, under favourable conditions, practically the whole of the sulphur dioxide is changed to sulphur trioxide instead of an exceedingly small fraction of it.

The most interesting, and at the same time the most perplexing, feature of the reaction is that the platinum itself does not appear to undergo any change. It is not diminished in quantity, for only a very small amount is necessary for the conversion of a very large amount of the mixed gases. Its activity lasts for a very long time, and even when it does become inactive, it can be shown that this is due to some external cause, such as the presence of dust and certain impurities in the gases.

Many other similar cases are known in which the presence of a small quantity of a third substance greatly influences the course of a chemical reaction without appearing in any other way to be necessary to the reaction. These substances, which are often metals in a very fine state of subdivision, are called catalytic or contact agents.

The Contact Process for making sulphuric acid is nothing more nor less than the simple laboratory operation which we have described above, carried out on a larger scale.

The sulphur dioxide is produced as in the lead chamber process by roasting iron pyrites in a current of air. This gas, together with the excess of air, is passed into the contact furnace, which consists of four tubes, each containing platinized asbestos, supported on perforated plates. The union of the two gases is said to be almost complete: an efficiency of 98 per cent. of the theoretical value is claimed for this process. The sulphur trioxide, or “sulphuric anhydride”[1] is either condensed in tin-lined drums or absorbed in ordinary concentrated sulphuric acid.

The proposal to manufacture sulphuric acid by this method was first made in 1831 by Peregrine Phillips, of Bristol. The early attempts were not successful, and it was not until about forty-four years later that the difficulties arising in the working of the contact process were overcome sufficiently to enable the sulphuric acid produced in this way to be sold at the same price as that made by the lead chamber process. Since 1890, the total quantity of acid made by the contact method has increased very rapidly, so that it now furnishes about one-half of the world’s supply, and seems likely in time to displace the lead chamber process altogether.

The history of the rise of the contact process is interesting because it illustrates in a striking manner the very great difference that there is between a successful laboratory process and a successful manufacturing process, though seemingly identical.

The first and possibly the most serious difficulty encountered in the working of the contact process was the frequent interruption caused by the loss of activity of the contact substance. Iron pyrites always contains arsenic which volatilizes on heating, and this quickly caused the platinum to lose its activity, or, as it was sometimes rather fancifully expressed, “poisoned the catalyst.” Dust also is inevitable, and this, carried forward mechanically with the stream of gas, settled on the contact substance and caused the action to cease.

To get over this difficulty it is necessary to purify the gases. They are first passed slowly through channels in which the coarser particles of dust settle down. Steam is injected into the mixture to wash out the finer particles of solid, and also to get rid of arsenic, and then the gases are passed through scrubbers. Before being admitted to the contact furnace, the moist gas is submitted to an optical test. It is passed through a tube, the ends of which are transparent; a bright light is placed at one end and viewed from the other through a column of gas of considerable length. If the purification process is working satisfactorily, there is a complete absence of fog. The gases are then dried by passing through concentrated sulphuric acid and admitted to the contact tubes.

In all operations carried out on a large scale, the regulation of temperature is a matter of some difficulty. In the case which we are considering, the most suitable temperature range is a rather narrow one, and the difficulty of keeping within the limits is very much increased by the large amount of heat given out when the sulphur dioxide and oxygen combine. The result of the failure to maintain the temperature at a fairly constant level was that the process worked in a very irregular manner, for as soon as it was working really well and sulphur trioxide was being formed rapidly, the heat given out by the reaction itself was also great, and consequently, the higher temperature limit was exceeded.

The method of controlling the temperature in the contact process is worth noting, because it is really ingenious. The tubes containing the platinized asbestos are surrounded by wider concentric tubes. The gases which are about to enter the contact furnace pass through the annular space between the two tubes, and are thereby heated to the required temperature, while at the same time they serve to cool the inner tubes. The most satisfactory temperature is about 400° C. The tubes are first warmed to 300° C. to start the reaction, and thereafter the heat evolved by the reaction itself is sufficient to keep it going.

The absorption of the sulphur trioxide also caused some difficulty at first. This substance reacts most violently with water, dissolving with a hissing sound like that produced when a red-hot poker is plunged into water. At the same time great heat is developed, and consequently, much of the sulphur trioxide is vaporized, and in that way lost. This difficulty was got over by using 98 per cent. sulphuric acid for the absorption, the acid being kept at this strength by the simultaneous addition of water.

The contact process has some very distinct advantages over the older lead chamber process. The plant covers a much smaller area than the bulky lead chambers. Although the preliminary purification of the gases is somewhat tedious and costly, this is in great measure compensated by the purity of the acid produced. No separate plant is required for concentration and purification, as in the older process. Finally, sulphuric acid of any concentration can be produced at will, including the fuming acid, which is required as a solvent for indigo, and in the manufacture of artificial indigo and other organic chemicals.

The lead chamber process produces what is called chamber sulphuric acid very cheaply. Although this is only a 60-70 per cent. solution and very impure, nevertheless, it is quite good enough for the heavy chemical trade, particularly for the first stage of the Leblanc soda process, and for making superphosphate. These two industries alone consume many thousands of tons of this sulphuric acid every year. Probably for some years to come the two processes will continue to exist side by side, but it may be doubted whether new works will now be installed to make sulphuric acid by the lead chamber process.

Properties of Sulphuric Acid. The pure non-fuming acid is a colourless oily liquid whose density is 1·84. It mixes with water in all proportions, yielding dilute sulphuric acid, and it also dissolves sulphur trioxide, yielding the fuming acid.

The mixing of sulphuric acid and water is accompanied by an evolution of heat and by contraction in volume. It is an operation which must be carried out with great care, the acid being always poured into the water, otherwise the water floats on the heavier acid, and so much heat is developed at the surface of separation that some of the water will be suddenly converted into steam, and this, escaping from the liquid with explosive violence, may cause the contents of the vessel to be scattered about.

Strong sulphuric acid chars most organic substances. From substances such as wood, sugar, paper, starch, it withdraws the elements of water, liberating carbon. Since it acts in the same way upon human flesh, it is clear that the concentrated acid must be handled with very great care, for it causes most painful burns. For this reason, vitriol throwing has always been regarded as a most serious and dastardly offence. A simple first-aid remedy for burns produced by sulphuric acid is the liberal application of an emulsion of linseed oil and lime water. The lime, being an alkali, neutralizes the acid, and the oil excludes air from the wound.

The readiness with which sulphuric acid combines with water is often made use of both in the laboratory and in industrial Chemistry for the purpose of drying gases. One illustration of this use has already been given in describing the contact process. Another instance which may be fairly familiar occurs in the case of liquefying air, where the gas must be thoroughly dried before being passed into the refrigerating apparatus, otherwise this would soon become blocked with ice.

The position which sulphuric acid occupies in Chemistry is due mainly to three outstanding features. In the first place, it is a strong mineral acid and displaces all other acids from their salts. Secondly, it has a high boiling point (338° C.), and consequently, the displaced acid with the lower boiling point can be distilled from the mixture. Lastly, sulphuric acid can be made very cheaply from materials which are very abundant in Nature, and, therefore, it meets all the requirements of an acid which is to be used for general purposes.

SULPHATES

All the common metals, except gold and platinum, dissolve either in concentrated or in dilute sulphuric acid, forming sulphates. These salts are highly important and interesting substances. They are all soluble in water, with the exception of the sulphates of calcium, strontium, barium, and lead.

Ferrous Sulphate, also called green vitriol and copperas, is obtained by dissolving iron in dilute sulphuric acid. The solution is green, and when it is evaporated, the crystals which separate out look like bits of green glass. It was because of this that the substance was first called green vitriol (vitrum = glass). It is used very largely in dyeing as a mordant. Writing ink and Prussian blue are also made from it.

The Alums are double sulphates. They are made by crystallizing solutions of potassium, sodium, or ammonium sulphate together with solutions of iron (ferric), chromium, or aluminium sulphates. In this way, we may have potassium aluminium alum, or iron ammonium alum, and so on, but whichever combination of elements is present, the salt which is formed always crystallizes in octahedra. The chief use of the alums, as also of aluminium sulphate, is as mordants in dyeing.

Since a great many metallic salts, particularly acetates and sulphates, are used in the dye industry as mordants, it may be well to explain here very briefly what a mordant is.

It must be remembered that almost all the dyes are solids which dissolve in water, yielding intensely coloured solutions. Hence, in most cases, if a fabric is merely dipped in the dye and then dried, the colouring is not permanent, but can be washed out with water. In order to fix the colouring matter, the material is first dipped in the mordant, usually a bath of some metallic salt, and then, generally after exposure to air or after steaming, into the dye bath, with the result that the colour becomes fixed. The first part of the process is called “mordanting” the material. The mordant either adheres to or combines with the fibres, and the dye forms with the mordant a coloured compound called a “lake,” which resists the action of water. The colour is then said to be “fast,” that is, firmly fixed.

For printing on calico, the mordant is thickened with gum arabic or other glutinous substance. The design is then stamped on the material with the thickened mordant liquor. The subsequent treatment consists of dipping the material in the dye and afterwards in water, when the colour comes away from those parts which have not received the impress of the mordant.

Sodium Sulphate, or Glauber’s salt, is made from common salt by the action of concentrated sulphuric acid. It is one of the raw materials used in making glass.

Ammonium Sulphate. (See p. 99.)

Calcium Sulphate, or gypsum, occurs in large quantities in Nature. The salt contains 20·9 per cent. of combined water, and when carefully heated to 120° C, it loses about two-thirds of this water, yielding a white powder known as plaster of Paris. This substance, when made into a paste with water, gradually sets to a hard mass, because the partially dehydrated gypsum re-combines with the water.

Lead Sulphate, the chief impurity of commercial oil of vitriol, is a white powder which is very often used for making white paint in place of lead carbonate (white lead). The sulphate has the advantage over the carbonate in not being so readily discoloured; its disadvantage is that it lacks “body.”

Copper Sulphate, or blue vitriol, is frequently found in the drainage of copper mines, where it is formed by the oxidation of copper pyrites. It is made on a large scale by roasting sulphide ores of copper in a current of air. Oxygen combines with copper sulphide, forming copper sulphate, which is extracted with water and crystallized. It forms large blue crystals containing 36 per cent. of water. This salt is put to many different uses. Very large quantities are used for dyeing and calico printing; some of the green pigments, such as Schweinfurt green, are made from it.

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