Washing soda, then, contains nearly two-thirds of its weight of water. Some of this water is given off spontaneously when the soda is exposed to air; the water may even be said to evaporate. This accounts for the loss of weight observed and also for the formation of the white layer of partially dehydrated soda over the surface of the crystal. The property of losing water in this way is common to most crystals containing a high percentage of water of crystallization. The phenomenon is known as “efflorescence.” It may here be observed that crystals of washing soda which have become coated over in this way contain relatively more soda than those which are transparent.
Natural Soda.In Egypt, Thibet, and Utah, there are tracts of country where the soil is so impregnated with soda that the land is desert. The separation of the soda from the earth is a simple operation, for it is only necessary to agitate the soil with water and, after the insoluble matter has settled down, to evaporate the clear solution until the soda crystallizes out.
In addition to alkali deserts, there are also alkali lakes. Those in Egypt are small, nevertheless, about 30,000 tons of soda per annum are exported from Alexandria. Owens Lake in California is said to contain sufficient soda to supply the needs of North America; while in the East African Protectorate, beneath the shallow waters of Lake Magadi (discovered in 1910), there is a deposit of soda estimated at 200,000,000 tons.
The Leblanc Process.At the present time, the greater part of the world’s supply of soda is made from common salt by two processes. The older of these, which is known as the Leblanc process, was introduced in France towards the end of the eighteenth century. In those days soda was very dear, for the main supply came from the ashes of seaweeds; wherefore the French Academy of Sciences, in 1775, offered a prize for the most suitable method of converting salt into soda on a manufacturing scale. The prize was won by Nicholas Leblanc, who in 1791 started the first soda factory near Paris. These were the days of the French Revolution; the “Comité de Sûreté Général” abolished monopolies and ordered citizen Leblanc to publish the details of his process.
Fig. 12. SALT CAKE FURNACEFig. 12.SALT CAKE FURNACE
Fig. 12.SALT CAKE FURNACE
The first alkali works were established in Great Britain in 1814. The total amount of soda now made in this country every year is about 1,000,000 tons, of which nearly one-half is still made by the Leblanc process.
Salt Cake.The first stage of the Leblanc process consists in mixing a charge of salt weighing some hundredweights with the requisite amount of “chamber” sulphuric acid. The operation is carried out in a circular cast-iron pan (D,Fig. 12) about 9 ft. in diameter and 2 ft. deep. The pan is covered over with a dome of brickwork, leaving a central flue (E) for the escape of hydrochloric acid gas which is produced. At first, the reaction takes place without the application of heat, but towards the end the mass is heated for about one hour. The contents of the pan are then raked out on to the hearth of a reverberatory furnace (a,b) and more strongly heated. More hydrochloric acid gas is given off, and the reaction is completed. The solid product which remains is impure Glauber’s salt (sodium sulphate), and is known in the trade as “salt cake.”
Black Ash.In the second stage of the Leblanc process, salt cake is converted into black ash. The salt cake is crushed and mixed with an equal weight of powdered limestone or chalk and half its weight of coal dust. This mixture is introduced into a reverberatory furnace (Fig. 13) by the hopper K, and heated to about 1000° C. by flames and hot gases from a fire ata. During this operation, the mass is kept well mixed, and after some time it is transferred tohwhere the temperature is higher. The mixture then becomes semi-fluid and carbon monoxide gas is given off.
Fig. 13. BLACK ASH FURNACEFig. 13.BLACK ASH FURNACE
Fig. 13.BLACK ASH FURNACE
The formation of carbon monoxide within the semi-solid mass renders it porous. This is an advantage, because it greatly facilitates the subsequent operation of dissolving out the soluble sodium carbonate. The appearance of the flames of carbon monoxide at the surface of the black ash indicates the end of the process. The product is then worked up into balls and removed from the furnace.
The chemical changes which take place in making black ash are probably as follows: Carbon (coal dust) removes oxygen from sodium sulphate, which is thus changed to sodium sulphide. This substance then reacts with the limestone (calcium carbonate), forming sodium carbonate (soda) and calcium sulphide.
Extraction of Soda.It now only remains to dissolve out the soda from the insoluble impurities with which it is mixed in the black ash. It is evident that the smaller the amount of water used for this purpose the better, because the water has subsequently to be got rid of by evaporation. The process of extraction is, therefore, carried out systematically. The black ash is treated with water in a series of tanks which are fitted with perforated false bottoms. The soda solution, which is heavier than water, tends to sink to the bottom and, after passing through the perforations, is carried away by a pipe to the second tank, and so on throughout the series. The fresh water is brought first into contact with the black ash from which nearly all the soda has been extracted.
The method of finishing off the black ash liquor differsaccording to the final product which the manufacturer desires to obtain, for the liquor contains caustic soda as well as mild soda. For the present, we will suppose that the end product is to be washing soda. In this case, carbon dioxide is passed into the liquor to convert what caustic soda there is into mild soda.
The clarified soda liquor is then evaporated until crystals of soda separate out. The first part of this process is carried out in large shallow pans (P.Fig. 13), using the waste heat of the black ash furnace, and finally in vats containing steam-heated coils. As the crystals separate out, they are removed, drained, and dried.
Alkali Waste.Black ash contains less than half its weight of soda, so that for every ton of soda produced there is from a ton and a half to two tons of an insoluble residue which collects in the lixiviating and settling tanks. This residue is known as alkali waste.
Alkali waste is of no particular value. It is not even suitable as a dressing for the land, and since it is not soluble in water there is no convenient means of disposing of it. Consequently, it is just accumulated at the works and, as the heap grows at an alarming rate, it cumbers much valuable ground. Moreover, it contains sulphides from which, under the influence of air and moisture, sulphuretted hydrogen is liberated. Alkali waste, therefore, has a very unpleasant odour.
The whole of the sulphur which was contained in the sulphuric acid used in the first stage of the process remains in the alkali waste, mainly as calcium sulphide. A plant for the recovery of this sulphur is established in some of the larger works. The alkali waste is mixed with water to the consistency of a thin cream, in tall, vertical cylinders. Carbon dioxide under pressure is forced into the mixture, and this converts the calciumsulphide into calcium carbonate and sets free hydrogen sulphide, which, when burnt with a limited supply of air, yields sulphur.
By this process, the most unpleasant feature of alkali waste, namely, the smell, is removed. The calcium carbonate which remains is of very little value. Some of it is used in making up fresh charges for the black ash process and some for preparing Portland cement, for which finely-ground calcium carbonate is required; the remainder is thrown on a heap.
Bicarbonate of Soda.Bicarbonate of soda can be easily distinguished from washing soda. It is a fine, white powder similar in appearance to the efflorescence on soda crystals. It does not contain any water of crystallization.
When bicarbonate of soda is heated, it does not melt, and, as far as its external appearance is concerned, it does not seem to undergo any change. If, however, suitable arrangements are made, water and carbon dioxide gas can be collected, and the sodium bicarbonate will be found to have lost 36·9 per cent. of its weight. The substance which remains is identical with that obtained by heating soda crystals, that is, anhydrous sodium carbonate. Sodium bicarbonate is, therefore, a compound of sodium carbonate and carbonic acid.
The most familiar use of this compound is indicated by its common names “baking-soda” and “bread-soda.” It is mixed with dough or other similar material in order to keep this from settling down to a hard solid mass in baking. The way in which bicarbonate of soda prevents this will be readily understood when it is remembered that an ounce of this substance liberates more than 2,300 cu. in. of carbon dioxide when it is heated. When the bicarbonate of soda is well mixed with the ingredients of the cake or loaf and disseminated throughout the mass, each particle will furnish (let us say) its bubble of gas. Since these cannot escape, a honey-combed structure is produced.
Fig. 14. THE SOLVAY PROCESSFig. 14.THE SOLVAY PROCESS
Fig. 14.THE SOLVAY PROCESS
Baking powder is a mixture of bicarbonate of soda and ground rice; the latter substance is merely a solid diluent.
The Solvay Process.Soda ash is one of the principal forms of mild alkali used in commerce. Large quantities of this substance are made by heating bicarbonate of soda. We shall now consider another alkali process in which this substance is the primary product.
For the greater part of the first century of its existence, the Leblanc soda process had no rival, although another method, known as the ammonia-soda process, was patented as early as 1838. In this case, however, as in many others, expectations based on the experiments carried out in the laboratory were not realized when the method came to be tried under manufacturing conditions. It was not until 1872 that Ernest Solvay, a Belgian chemist, had so far solved the difficulties, that a new start could be made. In that year, about 3,000 tons of soda were produced by the ammonia-soda or Solvay process, as it has now come to be known. Since then, however, the quantity produced annually has been steadily increasing, until at the present time it amounts to more than half of the world’s supply.
The Solvay process is very simple in theory. Purified brine is saturated first with ammonia gas and then with carbon dioxide. Water, ammonia, and carbon dioxide combine, forming ammonium bicarbonate, which reacts with salt (sodium chloride), producing sodium bicarbonate and ammonium chloride.
The principal reaction is carried out in a tower (Fig. 14(1),a,a) from 50 to 65 ft. in height and about6 ft. in diameter. At intervals of about 3½ ft. throughout its length, the tower is divided into sections by pairs of transverse discs, one flat with a large central hole, and one hemispherical and perforated with small holes (Fig. 14(2)). The discs are kept in position by a guide rod G.Fig. 14(3) shows a better arrangement of the guide rods. In modern works, the space between the discs is kept cool by pipes conveying running water. The ammoniated brine is led into the tower near its middle point. The carbon dioxide is forced in at E in the lowest segment, and as it passes up the tower it is broken up into small bubbles by the sieve plates. Sodium bicarbonate separates out as a fine powder, which makes its way to the bottom of the tower suspended in the liquid.
The perforated plates are necessary for the proper distribution of carbon dioxide through the brine. They are, however, a source of trouble, because the holes quickly become blocked up with sodium bicarbonate, and every ten days or so it is necessary to empty the tower and clean it out with steam or boiling water.
Recovery of Ammonia.The production of 1 ton of soda ash by the Solvay process involves the use of a quantity of ammonia which costs about eight times as much as the price realized by selling the soda. It is evident that the success of the process as a commercial venture depends largely on the completeness with which the ammonia can be recovered.
During the process, ammonia is converted into ammonium chloride, which remains dissolved in the residual liquor. From this ammonia gas is set free by adding quicklime and by blowing steam through the mixture. It is now claimed that 99 per cent. of the ammonia used in one operation is recovered.
Soda Ash.The bicarbonate of soda produced by the Solvay process is moderately pure. For all ordinary purposes, it is only necessary to wash it with cold water to remove unchanged salt, and after drying, it is ready to be placed on the market if it is to be sold as bicarbonate. The greater part of the Solvay product, however, is converted into soda ash by the application of heat. If soda crystals are required, the soda ash is dissolved in water and crystallized.
In many ways, the Solvay process compares very favourably with the older method. It is an advantage to start with brine, for that is the form in which salt is very often raised from the mines. The end product is relatively pure; moreover, it is quite free from caustic soda, which for some purposes for which soda ash is used is a great recommendation. There is no unpleasant smelling alkali waste. On the other hand, the efficiency of the Solvay process is not high, for only about one-third of the salt used is converted into soda. This would make the process impossible from the commercial point of view were it not for the cheapness of salt.
The Leblanc process, too, has its advantages. In the next chapter we shall see that it is adaptable for the production of caustic as well as mild alkali. The chlorine which is recovered in the Leblanc process is a very valuable by-product. In the Solvay process, chlorine is lost, for hitherto no practicable method has been found for its recovery from calcium chloride.
The position with regard to the future supply of alkali is very interesting. The competition between the Leblanc and the Solvay processes for supremacy in the market is very keen. At the same time, both processes are in some degree of danger of being supplanted by the newer electrical methods, which will be mentioned in the last chapter.
The following table shows very clearly the rapid progress made by the Solvay process in ten years. The quantities are given intonnes(1 tonne = 0·9842 ton).
Mild Potash.Potassium carbonate (mild potash) was formerly obtained from wood ashes. The clear aqueous extract was evaporated to dryness in iron pots, and the substance was on this account calledpotashes; later, potash. A whiter product was obtained by calcining the residue, and this was distinguished aspearl-ash. Chemically pure potassium carbonate was formerly obtained by igniting cream of tartar (potassium hydrogen tartrate) with an equal weight of nitre. It is for this reason that potassium carbonate is sometimes called “salt of tartar.”
About the middle of last century, natural deposits of potassium chloride were discovered in Germany. The beds of rock salt near Stassfurt are covered over with a layer of other salts, and for many years these were removed and cast aside as “waste salts” (abraumsalze). When at a later date they were examined more carefully, they were found to contain valuable potassium compounds, notably the chloride. After that discovery,mild potash was made by the Leblanc process., and Germany controlled the world’s markets for all potassium compounds.
At the outbreak of war, the German supplies of potassium compounds ceased as far as the allied nations were concerned, and an older method of making potassium chloride fromorthoclaseor potash-felspar was revived. This involves the heating of the powdered mineral to a high temperature after mixing it with calcium chloride, lime, and a little fluorspar. The potassium chloride is then extracted from the fused mass with water. This method has been worked with great success in America, and it is claimed that potassium chloride can be made in that country at a cost which is lower than that formerly paid for the imported article.
Mild potash and soda are so very similar in chemical properties that in most cases it is immaterial which compound is used. In all cases in which there is this choice, soda is employed, both because it is cheaper and because it is more economical, for 106 parts of soda ash are equivalent to 138 parts of potash. There are, however, some occasions when soda cannot be substituted, notably for the manufacture of hard glass and soft soap, and for the preparation of caustic potash, potassium dichromate, and other potassium salts.
Potassium Bicarbonate.This resembles the corresponding sodium salt in nearly every respect. It is, however, much more readily soluble in water, so much so, that it is not possible to obtain this substance by the Solvay method. It is made from potassium carbonate by saturating a strong aqueous solution of that substance with carbon dioxide.
The Alkali Metals.The discovery of current electricity in 1790 furnished the chemist with a very powerful agency for bringing about the decomposition of compounds. Hydrogen and oxygen were soon obtained by passing an electric current through acidulated water; and in 1807, Sir Humphry Davy, who is perhaps better remembered for his invention of the miners’ lamp, isolated the metals sodium and potassium by subjecting caustic soda and caustic potash respectively to the action of the current.
Sodium and potassium are very remarkable metals. They are only a little harder than putty, and can easily be cut with a knife or moulded between the fingers. When exposed to the air, they rust or oxidize very rapidly, so much so that they have to be preserved in some mineral oil or in airtight tins. They are lighter than water, which they decompose with the liberation of hydrogen, and under favourable circumstances the hydrogen takes fire so that the metals appear to burn on the surface of the water. After the reaction is over and the sodium or potassium has disappeared, a clear colourless liquid remains which has a strongly alkaline reaction, and when this is evaporated until the residue solidifies on cooling, caustic soda or potash is obtained. For very special purposes, the caustic alkalis are sometimes made by the action of the metals on water, but for production on a large scale, less expensive methods are adopted.
Caustic Alkaliis obtained from the correspondingmild alkali in the following way. The substance—washing soda, for example—is dissolved in water and the solution is warmed. Lime is stirred into this solution, and from time to time a small test portion of theclearsupernatant liquid is removed and mixed with a dilute mineral acid. When this ceases to cause effervescence, the change is complete. The clear liquid is now separated from the solid matter (excess of lime together with calcium carbonate) and evaporated in a metal dish. Since the caustic alkalis are extremely soluble in water, they do not crystallize as do most of the compounds previously described. Evaporation is, therefore, carried on until the liquid which remains solidifies when cold.
Caustic Soda.To describe the process by which caustic soda is manufactured, we must return to the making of black ash. The mixture from which black ash is made contains limestone. It is heated to 1000° C., which is a sufficiently high temperature to convert limestone into lime. When the black ash is subsequently treated with water, the lime which is present converts some of the mild alkali to caustic; consequently, black ash liquor always contains both alkalis.
When the manufacturer intends to make caustic soda and not soda crystals, the composition of the black ash mixture is varied by adding a larger proportion of limestone, so that there may be an excess of lime in the black ash produced. The treatment with water is carried out as described under washing soda, and then more lime is added to convert the mild soda into caustic soda. After the excess of lime and other suspended matter has settled down, the clear caustic liquor is evaporated in iron kettles until it becomes molten caustic, which will solidify on being allowed to cool.
There are various grades of caustic soda on themarket differing one from another in purity. The soap manufacturer uses caustic liquor or lye containing about 40 per cent. of caustic soda. For other purposes, the solid containing from 60 to 78 per cent. is used. Sometimes the product is whitened by blowing air through the strong caustic liquor or by the addition of a little potassium nitrate. Finally, for analytical purposes, caustic soda is purified by dissolving it in alcohol and subsequently evaporating the clear liquid.
Caustic Potash.The methods for the preparation of the corresponding potassium compound are precisely the same as those described for caustic soda; in fact, wherever the words sodium and soda occur in this chapter, the reader can always substitute potassium and potash respectively.
Caustic Lime.Apart from its use in making mortar and cement, lime is very often employed to neutralize acids. For this purpose, a suspension in water, called milk of lime, is generally used, for lime itself is not very soluble. Probably it is only the soluble part which reacts; nevertheless, as soon as this is used up, more of the solid dissolves, and in this way the action goes on as if all the lime were in solution.
Lime is also a very valuable substance in agriculture, especially on damp, boggy land, where there is much decaying vegetable matter, and on land which has been liberally manured. The soil in these cases is very likely to become acid and is then unproductive. Lime is added to “sweeten” the soil; in other words, to neutralize the acid.
Ammonia.The pungent smelling liquid popularly known as “spirits of hartshorn” is a solution of ammonia gas in water. It is a caustic alkali and, as such, is sometimes used to remove grease spots. Here, however, we shall consider ammonia only in connectionwith ammonium salts, some of which are used in very large quantity as fertilizers.
The principal source of ammonia at the present time is the ammoniacal liquor obtained as a by-product in the manufacture of gas for heating and lighting. Coal contains about 1 per cent. of nitrogen, and when it is distilled, some of this nitrogen is given off as ammonia, which dissolves in the water produced at the same time. This liquid is condensed in the hydraulic main and in other parts of the plant where the gas is cooled down.
Gas liquor contains chiefly the carbonate, sulphide, sulpho-cyanide, and chloride of ammonia, together with many other substances, some of which are of a tarry nature. It would not be practicable to evaporate this liquid with a view to obtaining the ammonium salts, because it is only a very dilute solution. Hence, after the removal of tar, the liquor is treated in such a way that ammonia is set free.
In some cases the liberation of ammonia is accomplished by blowing superheated steam into the liquor, which sets free the ammonia which is combined as carbonate, sulphide, and sulpho-cyanide, but not that which is present as chloride. In other works, the gas liquor is mixed with milk of lime, which liberates all the combined ammonia. The ammonia is then expelled from the mixture by a current of steam or air and steam. In both cases, the gas which is given off is passed into sulphuric acid, whereby ammonium sulphate is formed in solution and afterwards obtained as a solid by evaporation.
Ammonium Chloride.Like all other alkalis, ammonia solution neutralizes acids, forming salts. With hydrochloric acid, it produces the white solid known assalammoniacor ammonium chloride. This compound is familiar as the one required to make the liquid used in a Leclanché cell, which is generally used as the current generator for electric bells.
Ammonium Carbonate, which is also called stone ammonia and salt of hartshorn, is made by subliming a mixture containing two parts chalk and one part ammonium sulphate. It is a white solid which gives off ammonia slowly and is, therefore, used as the basis for smelling salts.
Ammonium Nitrateis obtained by passing ammonia gas into nitric acid until it is neutralized. It is a white solid, which melts easily on being heated, and breaks up into water and nitrous oxide (laughing gas), which is the “gas” administered by dentists. Ammonium nitrate is also used in the composition of some explosives: for example, “ammonite” is said to contain 80 per cent. of this substance.
Ammonium Sulphateis used chiefly as an artificial manure; the amount required for this purpose throughout the world is over 1,500,000 tons every year.
Synthetic Ammonia.Though the soluble compounds of nitrogen are fairly abundant, the supply is by no means equal to the demand, because such enormous quantities are required for agricultural purposes. It has been already said that ammonia is obtained as a by-product in the distillation of coal, and it has been repeatedly pointed out that our coal supplies are far from inexhaustible; moreover, coal gas may not always be used for lighting and heating. It, therefore, becomes a very important question as to how the future supply of ammonium salts is to be maintained.
Ammonia is a very simple compound formed from the elements nitrogen and hydrogen, and, as before mentioned, the supply of free nitrogen in the air isliterally inexhaustible. In recent years, the efforts of chemists have been directed towards finding a method of converting the free nitrogen of the air into some simple soluble compound. This problem is usually spoken of as the “fixation of nitrogen.”
In the Haber process, nitrogen obtained by the fractional distillation of liquid air is mixed with three times its volume of hydrogen, and this mixture is heated to between 500°C. and 700°C. under a pressure of 150 atmospheres (nearly 1 ton to the square inch) and in the presence of a contact agent. Under these conditions, nitrogen and hydrogen combine to form ammonia, which is condensed by passing the mixed gases into a vessel cooled with liquid air, any unchanged nitrogen and hydrogen being passed back again over the contact substance.
The problem of making ammonia from the air is closely connected with that of making nitric acid from the same source. In some experiments the two are combined, and ammonium nitrate is produced directly. Ammonia made by the Haber process, or some modification, is mixed with atmospheric oxygen and passed through platinum gauze heated to low redness. This results in the formation of nitric oxide, which is further oxidized by atmospheric oxygen; and finally, from a mixture of oxides of nitrogen, water vapour, and ammonia, synthetic ammonium nitrate is obtained.
One of the most noteworthy developments of modern chemical industry has been the increasing use of electricity as an agent for bringing about changes in matter. This has followed naturally from the reduction in the cost of electricity, due in great measure to the utilization of natural sources of energy which for untold ages had been allowed to run to waste.
This last achievement is likely to produce such a change in economic conditions that it is worth while giving a little thought to what may be called a newly-discovered asset of civilization. One example will make this clear. In the bed of the Niagara river, which flows from Lake Erie to Lake Ontario, there is a sudden drop of 167 ft. over which the water rushes with tremendous force and expends its energy in producing heat which cannot be utilized. This is a waste of energy, but it cannot be circumvented because no method has yet been found to control the waters of the Falls themselves. Nevertheless, by leading the head waters through suitable channels from the high level to the low, it is possible to use the energy to drive turbines, which, in their turn, drive dynamos which produce the current. This is merely the conversion of the energy of running water into electrical energy; and while the sun remains, this supply of energy will be forthcoming in undiminished quantity, because by the heat of the sun the water is lifted again as vapour, which descends as rain to replenish the sources from which the Niagara flows.
Electricity is employed in chemical industry in two ways. In the first place, it may be used to produce very high temperatures required for the reduction of some metallic ores, for melting highly-refractory substances, and for making steel. It is, however, rather with the second method, called electrolysis, that we are here mainly concerned.
Fig. 15. THE ELECTROLYSIS OF SALT SOLUTIONFig. 15.THE ELECTROLYSIS OF SALT SOLUTION
Fig. 15.THE ELECTROLYSIS OF SALT SOLUTION
Solutions of acids, bases, and salts, and in some cases the fused substances themselves, conduct the electric current; but at the same time they suffer decomposition. This method of decomposing a substance is known aselectrolysis, or a breaking up by the agency of electricity.
The apparatus required in a very simple case is shown inFig. 15. It merely consists of some suitable vessel to contain the liquid; two plates—one to lead the current into the solution, the other to lead it away again—and wires to connect the plates to the poles of a battery,storage-cell, or dynamo. Each plate is called anelectrode, and distinguished as positive or negative according as it is joined to the positive or negative pole of the current generator. By convention, electricity is supposed to “flow” from the positive pole of the battery to the positive electrode oranode, and then through the solution to the negative electrode orcathode, and so back to the negative pole of the generator, thus completing the circuit external to the battery.
When acids, alkalis, and salts are dissolved in water, there is strong evidence to show that they break up to a greater or less extent into at least two parts calledions. These are atoms, or groups of atoms, which have either acquired or lost one or moreelectrons.[5]They move about quite independently of one another and in any direction until the electrodes are placed in the liquid. Then they are constrained to move in two opposing streams—those which have acquired electrons all move towards the negative electrode, and those which have lost electrons towards the other. At the electrodes themselves, the former give up and the latter take up electrons, and become atoms again. Let us now consider a concrete example. Common salt is composed of atoms of sodium and atoms of chlorine paired. When a small quantity of this substance is dissolved in a large quantity of water, the pairing no longer obtains. The chlorine atoms move away independently accompanied by an extra satellite or electron, and the sodium atoms move away also but with their electron strength one below par. When the current is introduced into the liquid, the sodium ions travel towards the cathode and chlorine ions towards the anode, and when they reach the goal, sodium ions gain one electron and chlorine ions lose one, and both become atoms again. Chlorine atoms combine in pairs forming molecules and escape from the solution in the greenish yellow cloud that we call chlorine gas. The sodium atoms react immediately with water, forming caustic soda with the liberation of hydrogen.
To return now to practical considerations. The electrolysis of salt solution appears to be an ideally simple method of obtaining caustic soda and chlorine from sodium chloride. As a manufacturing process, it would seem to be perfect, for the salt is broken up directly into its elements and a secondary reaction gives caustic soda automatically. There is no “waste” as in the Leblanc process, and it does not require the use of any expensive intermediary substance afterwards to be recovered, as in the Solvay process. But, as very often happens when working on a large scale, difficulties arise, and these up to the present have only been partially overcome.
Some of the chlorine remains dissolved in the liquid and reacts with the caustic soda, forming other substances which, though valuable, are not easy to separate from the caustic soda. It is possible to get over this difficulty to some extent by placing a porous partition between the anode and the cathode, and in that way dividing the cell into cathodic and anodic compartments. As long as the partition is porous to liquids, it will allow the current to pass, but at the same time it will greatly retard the mixing of the contents of the two compartments. Porous partitions or cells which are in common use for batteries are made of “biscuit” or unglazed porcelain.
It must be remembered, however, that porous partitions only retard the mixing of liquids; they do notprevent it. Moreover, a further difficulty arises from the fact that chlorine is a most active substance, and therefore it is difficult to find a material which will resist its corrosive action for any length of time, and the same difficulty arises in the case of the anode where the chlorine is given off.
Castner Process for Caustic Soda.The following is the most successful electrical process for the manufacture of caustic soda yet devised. It was introduced in 1892, and is known as the Castner process. It should be noted that the use of the porous partition has been avoided in a very ingenious way.
Fig. 16. THE CASTNER PROCESSFig. 16.THE CASTNER PROCESS
Fig. 16.THE CASTNER PROCESS
The cell (seeFig. 16) is a closed, rectangular-shaped tank divided into three compartments by two non-porous partitions fixed at one end to the top of the tank, while the other end is free and fits loosely into a channel running across the tank. The floor of the tank is covered with a layer of mercury of sufficient depth to seal the separate compartments. The two end compartments contain the brine in which are the carbon anodes; the middle compartment contains water or very dilute caustic soda in which the cast-iron cathode is immersed.
The current enters the end compartments by the carbon anodes and passes through the salt solution to the mercury layer which in these compartments are the cathodes. The current then passes through the mercury to the middle compartment, and then through the solution to the cathode, thence back to the dynamo. It is important to note that in the middle compartment the mercury becomes the anode.
Chlorine is liberated at the carbon electrodes, and when no more can dissolve in the liquid it escapes and is conveyed away by the pipe P. Sodium atoms are formed at the surface of the mercury cathodes in the outside compartments and dissolve instantly in the mercury, forming sodium amalgam.
While the current is passing, a slight rocking motion is given to the tank by the cam E. This is sufficient to cause the mercury containing the dissolved sodium to flow alternately into the middle compartment, and there the sodium amalgam comes into contact with water; the sodium is dissolved out of the mercury and caustic soda is formed. Water in a regulated stream is constantly admitted to the middle compartment, and a solution of caustic soda of about 20 per cent. strength overflows.
The production of caustic soda by an electrical method still remains to be fully developed. A process which gives only a 20 per cent. solution cannot be looked upon as final. In the meantime, other methods have been tried, in some of which fused salt is used in place of brine in order to give caustic soda in a more concentrated form. For a description of these methods, the reader must consult some of the larger works mentioned in the preface. Here we can only say that very great difficulties have been encountered, particularly in the construction of a satisfactory porousdiaphragm or, alternately, in devising methods in which this can be dispensed with.
Another interesting application of electrolysis is furnished by the use of copper sulphate in industry. When this salt is dissolved in water, it breaks up into copper ions (positive) and an equal number of negative ions, composed of 1 atom of sulphur and 4 atoms of oxygen (SO″4). Under the influence of the current copper ions travel to the cathode, and there by the gain of two electrons become copper atoms. Now, since copper is not soluble in copper sulphate solution, and is not volatile except at very high temperatures, it is deposited on the cathode in a perfectly even and continuous film when the strength of the current is suitably adjusted. This film continues to grow in thickness as long as the conditions for its deposition are maintained. If the current employed is not suitable, the metallic film is not coherent, and the copper may appear as a red powder at the bottom of the cell. Any other metal or impurity which might be present in the unrefined copper falls to the bottom of the tank.
Other metals are deposited electrolytically in exactly the same way. The metal to be deposited is joined to the positive pole and the article to be plated to the negative pole of the battery. Both are suspended in a solution of salt, generally the sulphate, of the metal which is to be deposited. Thus, for nickel plating, a piece of sheet nickel would be used in conjunction with a solution of sulphate of nickel or, better, a solution of nickel ammonium sulphate, made by crystallizing ammonium and nickel sulphates together. The current required is small; indeed, if it is too strong, the deposit adheres loosely to the article, and the result is, therefore, not satisfactory.
Electrotype blocks are also made by a similar process.An impression of the article to be reproduced is made in wax, or some suitable plastic material, and polished with very fine graphite or black lead, in order to give a conducting surface. It is then suspended in a solution of copper sulphate and joined to the negative pole of the battery; a plate of copper connected with the positive pole is suspended in the same solution. When a weak current is passed, copper is deposited on the black-leaded surface and grows gradually in thickness, until at length it can be stripped off, giving a positive replica of the object.