More recently, the use of organic salts of thorium and cerium for impregnation has been proposed by Dr. F. W. Wirth;[550] in fixing the impregnated fabric with hydrogen peroxide, the cerium is not removed in solution, since the weak organic acid formed will not dissolve cerium peroxide. The same author has also suggested[551] the addition to the fixing bath of substances which absorb oxygen from the air—e.g. sodium hydrogen sulphite, resorcinol, tannin—to prevent removal of cerium. He has also advocated impregnation with amorphous salts,[552] which will obviate the necessity for any subsequent fixing treatment, the hypophosphites and double compounds with ammonium oxalate being specified. Attempts have been made to achieve the same end by other methods. Thus Silbermann[553] has proposed a preliminary treatment with alkalies (mercerisation); the fabric is treated with concentrated sodium hydroxide solution in absence of air for half an hour, pressed through rollers, and plunged into the impregnating solution. Two years previously a patent was taken out by Drossbach[554] to protect the use of colloidal solutions of the hydroxides. To a boiling suspension of well-washed, freshly-precipitated thorium hydroxide, a solution of a small quantity of the nitrate is gradually added; after half an hour a colloidal solution is obtained, which, after the addition of the required quantity of cerium nitrate, and dilution to a suitable extent, is employed directly for impregnation. The patent states that this solution is more readily absorbed than the ordinary nitrate solution, but the statement has been questioned.

Artificial silk itself is of a colloidal nature, the solidification of the filaments of cellulose during the manufacture being rather in the nature of a coagulation than of a precipitation; it is to this fact that the fibres owe their solid, rod-like structure, and it is probably this circumstance also which determines the very intimate absorption of the hydroxides or peroxides produced by fixing. It is well known that colloidal substances under some circumstances possess the property of clinging tenaciously to foreign bodies, exhibiting the so-called phenomenon of adsorption. The strength and elasticity of the oxide skeleton, obtained when the fixed and dried fabric is subjected to the operation of burning off, are presumably to be referred to such a relation between the cellulose of the fibres and the insoluble thorium and cerium compounds, precipitated by one of the methods of fixing described.

The Final Stages.

—The treatment of the fabric after impregnation and fixing differs only slightly from that used for the impregnated ramie and cotton products. The dried strips are cut into suitable lengths, and the head is drawn together with asbestos and threaded across. No tulle or gauze is required, the end being simply turned down before threading. After the ordinary strengthening process for the head (vide p. 296) the process of manufacture was, until recently, finished, the goods being sent out in the unburnt condition, on account of the difficulties of collodinisation. These have now been overcome, so that the mantles are burnt off and collodinised as usual. Burning off and shaping are now frequently effected in one operation by machine; the nature of the methods by which the fibre is made produces a uniform fabric, and if the earlier processes are carefully carried out, a uniform product is obtained, which is therefore suitable for machine treatment.

CHAPTER XXI
OTHER TECHNOLOGICAL USES OF THE CERIUM AND YTTRIUM ELEMENTS, ZIRCONIUM AND THORIUM

The technical uses of the members of this group of the elements we are considering, apart from the employment in the manufacture of incandescent mantles, are at present very restricted. Innumerable proposals for the employment of the compounds of cerium and the allied metals, which are obtained in such large quantities as by-products in the thorium industry, have been put forward, but the actual extent to which they are utilised is so small that only an insignificant fraction of the available quantities is annually required. In the metallic form, a limited application is found for various alloys, e.g. the so-called pyrophoric alloys, misch metal, and the magnesium and aluminium alloys. Various compounds of the elements, as well as some alloys, have been suggested for use in arc-lamp electrodes, and the use of the metals themselves, as well as of various salts, for the manufacture of flashlight powders, is protected by several patents. Investigations have been made to determine the value of the oxides and sulphates as catalysts in the contact process for the manufacture of sulphuric acid, and one patent states that the yield obtained is equal to that given by platinised asbestos. Cerium salts have been proposed for tanning, and in the preparation of enamels; cerium sodium sulphate is used in the catalytic oxidation of aniline to aniline black. The oxalate has a very slight use in medicine. The oxidising power of ceric salts is of some use in photography; ceric sulphate in acid solution is also said to be an efficient oxidising agent for aromatic hydrocarbons. On account of the deep colour of the higher oxide of praseodymium, didymium salts find a limited application for marking textiles.

Compounds of the yttrium group have at present no technical importance. They were formerly used to some extent for the manufacture of filaments for Nernst lamps, but with the introduction of metal filament lamps in electric lighting, the demand for Nernst lamps and consequently for the yttria oxides, has to a very great extent died away.

Zirconium and its compounds, on the other hand, promise to become of some technical importance. The metal received considerable attention in the earlier stages of experimental work on metallic filaments for electric lighting, but it has been shown that its melting-point is not sufficiently high to allow of extended use in this direction. The carbide has been proposed for the same purpose, but is even less suitable; this compound, however, on account of its great hardness, is likely to find employment as an abrasive, and in glass-cutting. The oxide, which occurs in nature in an impure form as the mineral Baddeleyite (q.v.), is employed in the manufacture of ‘Siloxide’ glass and of enamels, as a pigment and polishing agent, and in various forms of lamps, e.g. the Nernst and Bleriot lamps, the Drummond light, etc. Far more important, however, is its use for fire-resistant crucibles, furnace linings and supports, etc., for which its refractory nature renders it particularly suitable. On account of its high specific gravity and non-poisonous character, it has been proposed for use in the Röntgen ray examination of the human body. Quite recently, metallic zirconium has been employed in metallurgy; addition of small quantities, in the form of suitable alloys, is said to secure sound castings, with increased strength and resistance to acids.

The Cerium Group

Pyrophoric Alloys.

—It has long been known that the metals of the cerium group possess the property, when scratched or struck, of throwing off glowing particles; this power of emitting sparks is not lost when the metals are alloyed, so long as the percentage of foreign metal is not allowed to become too high. In a patent[555] protecting the use of various ‘pyrophoric alloys,’ as these spark-giving alloys are called, Auer states that the pure metals do not show this property, which only appears when foreign metals are present; he accordingly patents alloys of the cerium metals with iron, specifying particularly the alloy with 30 per cent. of the latter element. Auer’s statement has been contradicted,[556] and it seems to be generally accepted that misch-metal[557] of ordinary technical purity has the property of sparking when scratched. This alloy of the cerium metals, however, is far too soft to be useful for the purpose, and the addition of some foreign element is required to obtain the strength, hardness, and brittleness necessary in the various forms of ‘lighters.’ Besides the addition of iron, the use of tin, lead, zinc, cadmium, silicon, etc., has been patented.[558]

Various forms of these lighting devices are manufactured;[559] in all of these the sparks produced by scratching the pyrophoric alloy with hardened steel, by means of some simple mechanical device, is caused to ignite a fragment of tinder, or a wick supplied with a suitable liquid, e.g. methyl alcohol, benzene, or petrol. In the numberless forms of cigarette-lighters at present before a somewhat indifferent public, the friction is obtained by means of a toothed wheel, actuated by a spring which is released when the device is opened. Many forms of gas lighter are also on the market, but the demand for them is very small. Many attempts have been made to adapt the device to the ignition of the Davy miners’ lamp, but none have been successful, since it is impossible to prevent the sparks flying through the gauze. Much work has also been spent in efforts to utilise the pyrophoric alloys for the automatic ignition of incandescent gas-lamps, but these have been equally unsuccessful, so that it may be said that important technical applications of this interesting property have still to be made.

Auer prepared his alloys by addition of iron, or other heavy metal, to the fused mixture of cerium metals obtained in the electrolytic apparatus employed for the production of the latter. They can, however, be prepared by fusing together the required quantities of foreign metal and misch-metal, the latter being obtained by processes other than those of electrolysis usually employed. The rare metals were obtained by the earlier chemists in a very impure state by reduction of the halogen or double halogen compounds with sodium or potassium. More recently[560] much purer products have been obtained—especially in the case of zirconium—by the action of metallic calcium, in the form of powder, on the oxides. Another method,[561] which has been employed in the preparation of metallic filaments for lamps, consists in heating the oxides with powdered magnesium in an atmosphere of hydrogen or nitrogen; by this means, hydrides or nitrides are obtained, which on heating decompose into the gas and the metal.

The ease with which misch-metal and its alloys with iron and other elements throw off glowing particles when struck is due to the low ignition temperature of cerium, and the energy with which it combines with oxygen. When such alloys are scratched, small fragments are struck off, which are raised to the ignition temperature by the heat of friction. It is generally accepted, however, that this explanation is by no means a complete one, and the existence of a pyrophoric suboxide was suggested.[562] The theory was advanced that the pyrophoric properties of the alloys were due primarily to the formation on the surface of a film of this sub-oxide, and the partial oxidation of cerium alloys, protected by patent (loc. cit.), was said to cause a marked increase in the ease with which sparks could be obtained. In this connection, an experiment of Hirsch, who has thoroughly investigated the properties of metallic cerium,[563] is of interest. He found that when the element is warmed in a sealed glass bottle, a black powder forms on the surface, which, when the bottle is opened, ignites spontaneously. It is probable that this black sub-oxide plays an important part in the production of sparks from the ordinary pyrophoric alloys.

Other Alloys.

—On account of the great affinity of the cerium metals for oxygen, misch-metal has been suggested as a reducing agent,[564] as have also the alloys of cerium and magnesium;[565] the formation of the latter is endothermic, so that they act much more vigorously than either metal separately. The alloys of cerium with tin and aluminium have been thoroughly investigated from the stand-point of metallography, by Vogel.[566] It has recently been claimed that the addition of very small quantities of cerium to aluminium has a very marked effect, the rare earth metal acting as a purifying agent,[567] and greatly improving the properties of the aluminium. The cerium may be introduced as fluoride, either to the electrolytic bath in which the aluminium is being prepared, or to the latter metal, after preparation, in the fused state. The most favourable effect is said to be produced by 0·2 per cent. of cerium.

Applications of the Cerium Compounds.

[568]—One of the earliest investigations in which cerium compounds were examined with a view to technical employment, was that of Kruis,[569] who made experiments on the comparative value of the salts of different metals as catalysts in the manufacture of aniline black. He showed that a solution of aniline with an oxidising agent (potassium chlorate or chromate) develops no colouration unless a salt of a heavy metal is present. In the case of the fabric impregnated with the solution, the only metals of which compounds were found suitable for producing a colour were copper, which was then generally used for the purpose, and cerium, iron, and manganese. Of these, cerium, used in the form of the double sulphate, was found to be by far the most suitable, and moreover to have the advantage that only small quantities are required; the price was at that time too high to allow of its use, but it has since been employed.[570] Cerium compounds have also been proposed as mordants for alizarin,[571] but they do not appear ever to have come into general use.

In photography, ceric sulphate has been employed for some time for the purpose of ‘reducing,’ i.e. removing silver from over-developed negatives.[572] It is said to act very evenly and rapidly, the small quantity of free sulphuric acid required to hold the salt in solution having no bad effect. More recently, cerium salts have been proposed for use in colour photography.[573] An emulsion is obtained by adding salts of iron, uranium, or cerium to a colloidal solution of albumen in ammonia, borax solution, or sodium carbonate solution; this is spread on the paper or negative, and is said to be readily sensitive to light.

The crude mixture of cerous sulphate with basic sulphates of other elements of the cerium group, which has been patented for use as a catalyst in the contact process for the manufacture of sulphuric acid,[574] is prepared from the earth compounds obtained as by-products in the treatment of monazite. These are converted into the sulphates, and, after evaporation of the excess of sulphuric acid, heated for several hours at a low red heat (300°-600°C.). The porous mass is then broken, and is ready for use. It is stated that a nearly quantitative yield of sulphur trioxide is obtained, and that the mixture acts more efficiently than pure cerous sulphate. The reaction is said to depend on the continuous formation and decomposition of the ceric salt, thus:

This process does not appear to have come into general use.

A general patent had been taken out in 1901, protecting the use of oxides of the rare earth elements for ‘high-temperature catalysis’ in the manufacture of sulphuric acid,[575] but the oxides do not seem to be very efficient.[576]

It has also been proposed to utilise the oxidising power of ceric salts in acid solution[577] for the preparation of aldehydes, quinones, etc., from aromatic hydrocarbons, for which purpose they are claimed to be more efficient than chromates. By the use of a crude cerium dioxide (60-70 per cent. CeO₂) obtained by the ignition of the by-products of the thorium industry, good yields were obtained of benzaldehyde, naphthaquinone and anthraquinone from toluene, naphthalene and anthracene respectively.

Garelli[578] has examined the action of cerium salts in tanning; he states that with neutral solutions, effects very similar to those produced by aluminium salts are obtained, but Eitner, who has also examined the question,[579] is of opinion that the cost of isolating and purifying the salts from the monazite residues renders their employment for this purpose impossible.

The fluoride, silicofluoride, and dioxide have also been proposed for the preparation of enamels,[580] but do not give satisfactory results.

Several patents protect the use of rare earth compounds for flashlight powders. For most of the mixtures covered, it is claimed that the usual defects of fumes, slow firing, etc., are absent. The usual recipes[581] are for magnesium or aluminium powder with chromates, nitrates, or similar salts of thorium, cerium, etc.; in one case[582] the rare earth metals, alloyed with barium, silicon, uranium, or titanium, are to be used with ‘an oxidising agent which leaves a non-volatile residue.’ None of these mixtures appears to have been successful.

Cerium compounds have also been proposed for use in arc-lamp electrodes; it is claimed that they give a very intense light, one patent[583] adding that the presence of cerium peroxide and a little fluorspar causes the arc to burn evenly and quietly. In another, the use of a mixture of tungstates or molybdates of the alkaline earths, with fluorides of the rare earth elements is protected;[584] the use of pyrophoric alloys, either entirely, or for the core of the electrode, has also been suggested.[585]

The Nernst Lamp.

—The first efforts which were made for the employment of electricity in illumination endeavoured to utilize the heat produced, when a current traverses a very thin metallic filament, to raise the conductor to incandescence. Numerous efforts were made to adapt platinum to this purpose, but its melting-point was finally admitted to be too low; at length it was found possible to produce carbon filaments, and the well-known carbon lamps came into use. Numerous attempts were made to effect improvements;[586] one plan was to coat the carbon filament, after its production, with a skin of metallic conductor, and zirconium and thorium were among the metals proposed in this connection.[587] The first really important advance, however, was effected by Nernst, who took up the study of ‘conductors of the second order,’ and within a few months succeeded in adapting these to the purposes of illumination (1897-1898). The Nernst lamps gave a very intense white light with considerably less consumption of electricity than the carbon filament lamps; they enjoyed a very considerable vogue for some years, but have been almost entirely displaced by the cheaper metal filament lamps, which were occupying the attention of Auer von Welsbach at the time Nernst perfected his invention.[588]

In his first patent,[589] Nernst proposed the use of a rod of magnesia or zirconia as filament; these oxides, which belong to his class of conductors of the second order, are non-conductors at ordinary temperatures, but their resistance decreases as the temperature rises, so that at high temperatures they will conduct electricity at the ordinary voltage. The preliminary heating was at first effected by means of a Bunsen burner, but a later patent[590] of the same year protects a method of heating by means of a platinum spiral in an auxiliary circuit, which is automatically cut out when the current in the main circuit, bearing the filament, attains its required strength. In the following year[591] it was found that filaments composed of mixtures of oxides were far more suitable than the earlier magnesia or zirconia rods; yttria, thoria, and zirconia were the chief oxides used, small quantities of ceria being occasionally introduced. With these filaments, the increase of conductivity with temperature is far more rapid than with the pure oxides; the preliminary heating required, therefore, is less and the light obtained more intense. The filaments used were in the form of rods or spirals obtained by compressing the powdered oxides.

The Nernst filaments differ markedly from those of the ordinary electric glow lamp in that they are not conductors in the ordinary sense (or of the first order, as Nernst puts it) but electrolytes, the passage of the current being actually attended by an electro-chemical change in the filament.[592] The oxide is ionised; the ions of the metals travel to the cathode or negative pole, where the liberated atoms of metal instantly recombine with the oxygen of the air, whilst oxygen ions travel to the anode, from which the gas is liberated. There is thus a gradual redistribution, resulting in accumulation of oxide at the cathode with a corresponding loss at the anode, which is balanced, after some time, by diffusion, so that equilibrium is attained. In consequence of this redistribution the filament glows more brightly at the anode, where it is thinnest, than at the cathode.

Industrial Applications of Zirconium

It has already been mentioned that zirconium received considerable attention as a suitable substance for the preparation of metallic filaments during the early stages of their development. Whilst at the present time this element has been abandoned for the purpose,[593] several zirconium lamps were at one time on the market, and a brief mention of some of the work done in this direction may not be out of place.

One of the general methods for the preparation of the metallic filaments may be illustrated by a patent taken out in 1902 by Sander,[594] for the preparation of filaments of zirconium, with or without addition of zirconium carbide. The metal, or a compound which on heating will yield the metal and a volatile substance which can be removed, is prepared in a finely divided condition, and made into a paste with some organic binding material; the paste is then forced through a tiny aperture, and the resulting thread is shaped and heated to a high temperature in vacuo or in an inert atmosphere. If an organic substance be used to form a paste with metallic zirconium, the final process of heating results in the formation of the carbide; the same compound is also obtained by another process protected by Sander (loc. cit.) in which the hydride of zirconium, prepared by the reduction of the oxide by means of powdered magnesium in an atmosphere of hydrogen, is mixed with a cellulose solution, and the liquid treated as in the manufacture of artificial silk, the threads obtained being then heated to remove all organic matter as far as possible.

The carbide is also probably obtained by the process of the British Thomson-Houston Company, in which advantage is taken of the fact that zirconium oxalate is a pasty gelatinous substance, which can be forced through a die to form a thread without addition of any agglutinating agent. The oxalate, precipitated by addition of ammonium oxalate to a solution of a zirconium salt, is mixed with finely divided carbon, and the threads obtained from the pasty mass heated to a very high temperature in a furnace.[595] Zirconium oxalate is also proposed as a binding material for powdered tungsten, in the preparation of filaments from that metal.[596]

The compounds of zirconium and thorium with elements of group VB, according to two German patents,[597] are suitable for the preparation of metallic filaments in much the same way. Thorium, titanium, and zirconium are also among the metals which, it is claimed, can be obtained in the pure fused state by heating in an electric arc in vacuo, so that filaments can be drawn directly.[598]

Metallic zirconium and its alloys have recently been employed in metallurgy. The pure metal can be obtained by the calcium reduction of Kuzel and Wedekind (vide p. 316); zirconia is not reduced by powdered aluminium (Goldschmidt’s process), but alloys of zirconium and iron can be easily obtained by the reduction of mixtures of the two oxides by this method. Alloys can be obtained containing up to 35 per cent. of zirconium; this ferro-zircon, as it is called, has been used to some extent recently in place of ferro-titanium (vide infra) for the purification of steels.[599] Addition of small quantities of zirconium to steels, brass, copper, etc., is said to secure sound castings, and to increase considerably the strength and resistance to acids of the metal.

The Technical Uses of Zirconia.

—Since the discovery of Baddeleyite, the natural oxide of zirconium (vide p. 75), which occurs in large quantities in Brazil, many proposals have been brought forward for the employment of this compound. Its application to the manufacture of glasses and enamels will be referred to in the next chapter. Patents have been taken out protecting its use for the preparation of white pigments,[600] as a toilet-powder,[601] and as a polishing powder,[602] for it is extremely stable towards chemical reagents, very voluminous, and at the same time very hard. It has long been employed for coating the lime and magnesia pencils used in the Drummond or ‘lime’ light; and recently it has been employed for the headlights of automobiles, in the Blériot lamp,[603] in which a rod of zirconia is heated in a blowpipe flame fed with oil vapour and oxygen.