THE great progress of new industrial applications, modern conceptions which tend to group around a main industry, subsidiary manufactures which increase its output, the excellent results obtained elsewhere, thanks to the utilisation of the by-products of the principal manufactures, have all brought to light in the sulphur industry the need of utilising in one way or another the enormous quantities of sulphurous acid which are at present lost in the extraction of sulphur from ores by partial combustion.

Given the situation of the mines, the possible variations in the output of sulphur in a given locality and the many variations in the cost of production and sale price of the raw material, the method of utilisation to be sought should fulfil the following conditions:

1. It should not require costly and unwieldy plant and apparatus.

2. It should be easy and safe to apply, and should be capable of adaptation to simple manufacture which only calls for a plant with few complications. 3. It should furnish a product which can find an immediate, easy, and extensive use.

Without stopping now to examine the different processes proposed, tried, or applied in analogous industries for the utilisation of the sulphurous anhydride contained in the furnace gases of various ores, it is worth while to study a method which has raised great hopes, and which is directed towards utilising the gases from sulphur extraction furnaces by combining the sulphurous anhydride which they contain with insoluble mineral phosphates with a view to transforming them into soluble products (superphosphates); This product would thus be obtained in a direct and economical way by the immediate combination of the sulphurous anhydride and the phosphorite without the use of lead chambers, &c., necessary for the manufacture of these substances in fertiliser factories.

It is unnecessary to emphasise the importance of such a practical application by which would be obtained a most useful product, for which the already great demand is continually increasing, while utilising directly the sulphurous gases in a simple and rational manner.

Since this idea presents such an attractive aspect, since it has even formed the subject of recent patents, and as, moreover, from the theoretical point of view there is no á priori difficulty of a chemical nature, I venture to think it would be interesting to deal with it briefly, to determine whether it would be capable of application and whether tests with a view to its realisation would justify themselves in the future.

Di-nietallic phosphates, e.g.

он

OP-ONa (di-sodium phosphate),
NONa

Tri-metallic phosphates, e.g.

ONa

O=P-ONa (tri-sodium phosphate).

ONa

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Physico chemical determinations have shown that the three acid functions of phosphoric acid present, in progressive saturation by bases, different degrees of affinity, that it to say the first behaves like that of a strong acid, the second like that of a weak acid, and the thir merely like that of a phenol. This is clearly shown, for example, in the determination of the heats of combination with the bases.

H3PO4 in solution + NaOH in solution = NaH2PO4 in solution 14,680 cal.

=

H3PO4 in solution + 2NaOH in solution Na2HPO4 in solution + 26,330 cal.

H3PO4 in solution + 3NaOH in solution Na3PO4 in solution +33 590 cal. (Berthelot and Longuinine).

It is also shown by the action of the salts on indicators. Thus, while a solution of mono-sodium phosphate is dis tinctly acid towards litmus and to phenol-phthalein, a solution of di-sodium phosphate is alkaline to litmus and neutral to phenol-pbalein, and a solution of the tri-sodium salt is alkaline to both these indicators.

Given this slight acidity of the second and third acid groups of phosphoric acid, it is evident that even a weak acid can displace it from its di- and tri-basic salts by removing a portion of the base with which it is combined, thus causing a state of equilibrium depending on different coefficients (relative affinity of the two acids for the base, concentration, &c.) with which we need not concern our

selves here.

The above mentioned fact is apparent not only in the presence of soluble alkaline salts, but also, for example, in the presence of precipitated tri-calcium phosphate, which is insoluble, or nearly so. If the latter is placed in suspension in water and the water is saturated with carbonic acid, it dissolves fairly well, giving rise to a certain extent to the following reaction, determined quantitatively by the laws of chemical equilibrium, and according to the state of molecular aggregation of the phosphate:" Ca3(PO4)2+4CO2+nH2O =

= Ca(H2PO4)2+2Ca(HCO3)2+(ו4)H2O. Mono-calcium phosphate. Calcium bi-barbonats. If the tri-calcium phosphate is treated with a dilute mineral acid in excess, even if the latter is weak, the salt dissolves completely, forming soluble mono-calcium phosphate. In the same way, if a solution saturated with sulphurous acid acts on it in excess, mono-calcium phos. phate and calcium bi-sulphite are formed and remain in solution (Pilter, Chem. Ind., 1878, p. 393; Jahr. Ber., 1878, p. 1124):

Ca3(PO4)2+4H2SO3 - Ca(H2PO4)2+2Ca(HSO3)2.

If the solution obtained is boiled the excess of sulphu. rous acid is set free, the bi-sulphate is decomposed, and a bi-calcium phosphate and netural sulphite of lime, almost insoluble in water, separate out (Rotondi, Ann. di Chim., lxxiv. p. 129; Jahr. Ber., 1882, p. 272) :Ca(H2PO4)2+2Ca(HSO3)2=

:

=2CaHPO,+CaSO3+2H2O+3SO2, and also hexagonal crystals of the composition (Gerland, Jahr. Ber., 1870. p. 312; 1871, p. 280)—

Ca306(PO/2SO2.2H2O.

There is an appreciable difference between newly precipitated tri-calcium phosphate and the dry salt, which depends on the more or less pronounced state of molecular aggregation. The former is gelatinous and dissolves in water in the proportion of 8: 100,000; the latter, on the other hand, is a white powder soluble only in the pro portion of 3: 100,000.

It would appear then that the reaction which is produced when the precipitated phosphate is employed should take place in an industrial process in which mineral phosphates are used, under certain conditions, and that it would be possible to transform them into soluble salts by the aid of pure sulphurous acid in place of the solution of sulphuric acid which is employed at present, the manu facture of which necessitates special and costly plant. However, in spite of the fact that theory and scientific experiments made with the pure product in a state of feeble aggregation offer prospects of a possible realisation of the process, it must be admitted that little hope remains when natural products and questions of a practical nature are considered.

Tri-calcium Phosphates and Phosphorites.

The difference which exists between precipitated tricalcium phosphate and the phosphorites is due essentially to the other products which the latter contain, and to the mineralisation which the tri-calcium phosphate, their principal constituent, has undergone.

Although it is generally stated that the phosphorites consist of amorphous phosphate of lime, it should be pointed out (and this fact is proved by microscopic examination) that a portion has been changed to a complex state of aggregation, even to the extent of producing a crystalline form in variable quantity, actual specific minerals mingled with the mass being thus produced (ornithite, xyplite, dahlite, apatite, cacoxene, &c.). This state of aggregation is more or less pronounced according to the nature, quantity, and duration of the action of the agents which have modified the original product (fossilised remains of animals) during different periods and in special conditions of formation. The result is that there is a notable difference between the original substance and the phosphorite, not only as regards form, molecular aggregation, hardness, composition, &c., but also as regards the manner in which they behave in the presence of chemical reagents; this difference resembles from this point of view that which distinguishes precipitated silica from mineralised silica.

Industrial Treatment of Phosphorites.

The transformation of the mineral phosphates into soluble phosphates is carried out ordinarily by means of sulphuric acid, acting upon the finely crushed substance. It is necessary

(a) That the ore should be mechanically crushed as finely as possible, so as to allow access for the acid, for disaggregation and reaction, to each particle, and to facilitate thus molecular contact of phosphates and acid; (b) That the acid used should be in sufficient quantity, and of a given concentration (48° 54° Bé.) to suit the greater or less amount of calcium carbonate, earthy and metallic phosphates, fluorides, phosphosilicates, &c., contained in the phosphorites, so that disintegration and reaction with the phosphate occur at a temperature which is such that these actions may be complete, and so that the water present, whilst sufficient for the reaction and for the bodies which are formed, may not form too moist a product;

(c) That the duration of the reaction should be long enough and the mixture homogeneous;

(d) That the superphosphate should not under any circumstances be exposed to a high temperature (90-100°). If any one of these conditions be not carried out, the process will not be satisfactory, and the manufacturer will suffer losses which may be considerable, for the profits are wiped out if all the phosphoric acid is not transformed.

In industrial practice disintegration and transformation of the phosphorites into soluble salts is carried out in two stages:

(a) Mechanical pulverisation, as complete as possible. (b) Action of the quantity of sulphuric acid necessary under the above-mentioned conditions.

Passing over the mechanical process, we may note that

the action of the sulphuric acid also takes place in two stages:

(a) Reaction on and disintegration of the mass. (b) Reaction and molecular disintegration of the phosphate.

The action of the sulphuric acid on the particles of the phosphorite, which forms a more or less compact agglomerate of phosphates, carbonate of lime, fluorides, silicates, &c., causes first the immediate conversion of the carbonate of lime into hydrated sulphate :

CaCO3 + H2SO4 + H2O = CaSO4.2H2O+CO2.

This rapid reaction, which gives rise to considerable heat and sets free an appreciable quantity of gas (CO2), assists not only the disintegration of the particles of phosphorite, but also the subsequent reactions, which require a certain temperature, and by which the sulphuric acid replaces and sets free hydrofluoric and hydrosilicic acids, &c., which thus assist to a considerable extent in the disintegration of the most refractory compounds. The importance of this reaction between calcium carbonate and sulphuric acid for the success of the operation is c early shown by practical experience, which proves that phosphorites which contain an insufficient quantity of calcium carbonate disintegrate slowly and with difficulty; they remain "cold," in technical language, and yield a wet product of poor quality. These phosphates require treatment with a warmer and more concentrated sulphuric acid, or (a less satisfactory method) to be mixed as intimately and homogeneously as possible with phosphates rich in lime, such as phosphatic chalks.

The action of sulphuric acid on these secondary compounds assists contact with the phosphate, whilst the carbon dioxide which is set free renders the mass porous. and the heat which is generated increases the power of substitution of the sulphuric acid and assists its action on the compounds of iron and aluminium, phosphosilicates, &c., which are present.

The reaction between the tri-calcium phosphate and the sulphuric acid, which produces soluble phosphate, does not take place as is generally shownCa3(PO4)2+2H2SO4+SH2O =

= Ca(H2PO4)2H2O+2CaSO4 2H2O, but also takes place in two distinct stages—in the first, which is instantaneous, free phosphoric acid is formed, and a part of the tri calcium phosphate remains unattacked,

3Ca 3(PO4)2+6H2SO4+12H2O =

= 4H3PO4Ca3(PO4)2+6CaSO4.2H2O. In the second, the reaction is slowly completed, in the presence of excess of water, producing mono calcium phosphate,

4H3PO4+C2 3(PO4)2 H2O - 3CaH4(PO4)2. H2O. The reaction takes place in two stages not only because sulphuric acid has an affinity for the bases which is double that of phosphoric acid, but above all because phosphate of lime, whilst mechanically broken up, is not in a molecular state, as is for instance a salt in solution; it is composed, on the contrary, of more or less complex molecular aggregates, so that the interior molecules are protected from contact with the acid, whilst the exterior molecules are exposed to the action of an excess of acid which would suffice practically for the conversion of the whole mass. It is to the presence of these aggregates and groups, and to the fact that the penetration of the acid is to some extent hindered, that we must attribute the fact that, even when in practice more acid is used than is strictly necessary for the treatment of the mass, a portion of the phosphorie acid exists in the form of a bi-basic salt, soluble only in citrate. In fact, the molecules which do not come into direct contact with the free acid (sulphuric or phosphoric) can only react with the molecules in which conversion into a mono calcium salt has taken place, and

CHEMICAL NEWS.}

Feb. 14, 1918 bi-calcium phosphate is formed according to the equation,

Ca(H2PO4)2+Ca3(PO4)2 = 4Ca HPO4.2H2O.

The phosphates of iron and aluminium, which occur in variable quantity in phosphorites, behave in an analogous fashion, yielding variable quantities of free phosphoric acid, according to the degree of concentration of the sulphuric acid, the temperature, duration of the reaction, &c. As these phosphates are more refractory, the reaction must be as complete as possible, so that no insoluble phosphoric anhydride remains to cause a decrease in percentage solubility.

The phosphosilicates and silicates are attacked partly by the hot sulphuric acid, partly by the hydrofluoric acid, which carries down silica in the form of silicon fluoride, which is subsequently found again in the gas washing

chambers.

Phosphorites and Sulphurous Anhydride. Having studied, in their successive developments, the principal phenomena which occur between sulphuric acid and phosphorites, and the conditions indispensable to the smooth operation of the process, we can easily understand that it is very difficult in practice advantageously to replace the sulphuric acid by sulphurous anhydride. Were it not so, it would be strange that in so prosperous an industry as that of the superphosphates, such a substitu tion should never have been contemplated and tried.

It is evident, à priori, that the use of liquefied and dry sulphurous anhydride cannot be considered, if only by reason of the fact that in these conditions it has no acid action; we are therefore limited to the use of an aqueous solution of sulphurous anhydride, either ready prepared or produced by the contact of the sulphurous gases with the phosphorites in the presence of water or moisture. It is known that sulphurous acid in solution behaves towards bases like a weak acid and that its salts may be decom posed by weak and dilute acids. The action of soluble salts on indicators shows the weak acid qualities of sul phurous acid. The acid salts (for example, bisulphite of soda, NaHSO) are neutral to methyl-orange, while neutral salts (for example, sul, bite of soda, Na2SO3) are alkaline. If we admit for a moment that the power of combination of sulphurous acid is equal to that of phosphoric acid (that is to say half as great as that of sulphuric acid) these two acids, when in the presence of a base in insufficient quantities for saturation, will divide it amongst them in direct proportion to their respective concentration, or rather to their mass.

Now whilst phosphoric acid may be mixed with water in any proportion, sulphurous acid forms, above 40°, solutions of variable concentration according to the temperature and pressure, while below 40° the concentration of the solution is below that determined by the pressure according to the law of Henry and Dalton. With pure sulphurous anhydride at 15 and 760 mm., we obtain an aqueous solution containing 43'56 volumes of sulphurous anhydride per volume of solvent, that is to say about 12'4 per cent by weight of sulphurous anhydride. It corresponds from the acidimetric point of view to a solution containing about 19 per cent of sulphurous acid; from the point of view of its avidity of combination with the bases,

it is much less active than an equivalent solution of sul

phuric acid.

Up to the present we have dealt with sulphurous anhydride of 100 per cent purity. In practice we have to deal with sulphurous gases produced by the combustion of an ore containing sulphur, such as are given off in smelting furnaces; the average richness of this gas in sulphurous anhydride does not exceed 5 to 6 per cent by voluine. The solution obtained by the contact of these gases with water, if the sulphurous anhydride obeyed the law of Henry and Dalton at temperatures below 40°, would be of a concentration at 15° and 760 mm. of

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litres per litre of water used, that is to say o'62 to 0.74 per cent of sulphurous anhydride by weight. Therefore in order to obtain a solution of the same concentration as should have to exert upon the gases which contain 5 per we obtain with 100 per cent sulphurous anhydride, we cent of sulphurous anhydride a pressure greater than 20 atmospheres.

As, on the other hand, the sulphuric acid which the should be of a concentration of 52 to 53° Bé., it is obvious treatment of ordinary phosphorites demands in practice that we are far from the conditions indispensable to the cific difference of combination and substitution between the industrial process, even if we assume that there is no spesulphuric scid to the extent of only one degree Beaume two acids. It is known that in practice the dilution of yields an inferior product, and that the strength of sulphuric acid increases within certain limits with the temperature, even with relation to stronger acids (HCI, HNO3), whilst the strength of sulphurous acid varies inversely with the temperature of the solution. Let us admit even that when working with high pres sures one can obtain a solution of sulphurous anhydride sufficiently concentrated and active to be able to produce the desired effects of disaggregation and molecular disintegration upon the particles of phosphorite; but when the pressure employed is relieved and the excess of water necessary to bring about contact between the sulphurous anhydride and the phosphorite is removed, the removal of the excess sulphurous anhydride and the decomposition of the bisulphite formed will immediately cause the reconversion of the acid phosphate of lime to bi-calcium phosphate.

Experiments and Experimental Tests.

The industrial conversion of mineral phosphates into soluble phosphates by means of sulphurous anhydride has been attempted repeatedly, and has continually given rise to various patents, of an essentially theoretical nature, taken out by people who are obviously not familiar with

the above-mentioned facts.

One of the first patents of this sort was that of the "Pilter" Company (D R.P. 2661) which did not lead to Nor did those which followed it. any practical result. We remain therefore limited to the use of sulphurous anhydride either simply to enrich in tri-calcium phosphate the phosphatic chalk (Ramsay and Gouthier, D.R.P. 105, 387) by eliminating the excess of carbonate of lime in the form of soluble bisulphite, &c., or to attempt to convert the bone phosphate into soluble products.

If we consider that bones contain phosphate of lime in a non-mineralised form, it is natural that after the failure experimenters should have been directed towards bone of all attempts to transform the phosphorites the hopes of phosphates, all the more because the use of liquid sulphurous anhydride as a solvent for fats and oils gave first for the extraction of fats, and afterwards for the promise of the possibility of using sulphurous anhydride extraction of phosphate of lime by means of water or steam; the residue would then only consist of cartilage, osseïn, &c.

Dr. Max Schroeder has particularly gone into this

interesting question, but his tests have not led to any result, as is shown clearly by the conclusions of a work published at the time of the Chicago Exhibition. (Dr. Max Schroeder, "Die wasserfreie flüssige schwefl ge Säure und ihre Verwendung in der Industrie," Buchdruckerei Kühne, 1894).

"The extraction of mono-calcium phosphate by means of an aqueous solution of sulphurous acid has frequently been proposed and attempted, without the discovery up to the present of a profitable use.

"There is no doubt that neither of the two processes mentioned, the extraction of fats from bones by means of liquid sulphurous anhydride under pressure, and the extraction of acid phosphate of lime from bones by