One of Crompton's indicators, registering up to 250 amperes, was obtained for measuring the strength of the current, and detecting flaws or breakages in the same. Some such instrument is invaluable for such work, and this proved very reliable.
The arrangement of the plant having been detailed, it remains to give the results of the working of the same. The critical point had been arrived at; the working capacity of the structure was to be determined; that which calculation could not indicate was about to be demonstrated. Acting in accordance with the principle, " Aspire high," the baths were first arranged in series. The connections being complete, the engine of 8 horse-power was set in motion; the dynamo increased rapidly in speed, while the needle of the indicator followed tremulously until it stood vacillating within a few degrees of 240 amperes, the dynamo then making about 9()0 revolutions per minute.. Thus the most economical conditions of working had luckily been hit upon; the machine bore the current bravely, showing no signs of over-heating.
The tin deposited was at first of a spongy nature, owing to the great acidity of the bath. Soon, however, it began to be precipitated in a more dense, extremely fine, granular, and partially crystalline state, which indeed was preferable, as it fell to the bottom of the bath, and was not in danger of forming a communication with the anode. I will not say that the tin was "chemically pure, but it was purer than commercial tin, and, when thoroughly washed, contained no trace of iron. It fused readily, and almost completely, and that withont any addition, provided it had previously been thoroughly washed and dried. The rapidity with which it dissolved in hydrochloric acid was not to be compared to the slow action of that liquid upon granulated tin, and this rendered it peculiarly suitable for the production of stannous chloride. From the data already given, the theoretical amount of tin deposited can readily be calculated. The electro-chemical equivalent of tin as a dyad, compared with silver is (117.8+2)/(107.66)=0.546 and this is equivalent to the precipita tion of 67.65 x 0.546 = 36.94 mgm. of tin per minute per ampere. For 240 amperes, working through 8 baths arranged in series, we obtain a total precipitate of (36.94x240x8x60)/1,000,000=4.25 kilo, per hour.
As a matter of fact, little more than half this quantity was obtained, the discrepancy being due to part of the current being absorbed in dissolving the iron as well as the tin, as soon as the former began to get bare. This, together with the natural solution of the iron in the acid, led to the rapid accumulation of sulphate of iron in the baths. The acid here employed took about seven weeks to become saturated. On analysis, the baths were found to vary in a very remarkable manner, first one and then another containing the largest quantity of iron. The tin. on the contrary, remained very constant in amount, both in the individual baths and in the total, the average being 1.5 grm. per litre. Pure tin was deposited until the acid was saturated and no more tin was present in solution; then iron hydrate began to form. This might be avoided for a time by the addition of more acid, but it was better to run the liquid into the "green vitriol" tanks, and add fresh solution. It was not at all necessary to continue the action of the current until all the tin had been removed; in fact, after a certain time the action on the iron was even stronger than on the tin.
It was found in practice that after the passage of the current for the space of 5-6 hours the quantity of scrap referred to was sufficiently free from tin to be dissolved in the iron sulphate tanks with the greatest ease; the tin remaining unacted upon in the presence of the large excess of iron always provided for, and it was not difficult to recover that tin, and utilise it with the rest.
Cost of plant and expense of working of course would depend upon the neighbourhood fixed upon, and other circumstances; but, in any case, they would be little compared with the value of the tin capable of being recovered. I think I have given sufficient details for any one to be able to calculate approximately their amount for any particular locality. Generally, this scrap is obtained for a mere nominal sum, frequently just for the expense of transport. One stoker and two or three labourers would be quite sufficient to work three tons per week, using one of the small dynamo machines manufactured by Siemens Bros. The 3 cwt. of metallic tin obtained therefrom, at 3l. 18s. per cwt., will compare favourably with the cost of fuel necessary to maintain the production of 7 or 8 horse-power, wages of workmen, interest on the plant, and occasional repairs. If it were worked on a larger scale, and the tin and iron further worked up into salts, as in the instance detailed, the profits would be increased in a much greater ratio.
When we consider that in Paris alone 3000 tons of this scrap are produced annually,.it is no unimportant matter to determine the best method of utilising it.
In addition to the consideration of economy, this method possesses advantages which are well worthy of attention, especially where iron sulphate and "iron mordant" are marketable products. As has been observed, the tin is precipitated in a pure form, and in a state of fine division. It may, therefore, be either fused and sold in the metallic form, or it may be converted into stannous chloride, or other salts, for which it is exceedingly well suited, owing to the ease with which it is dissolved. In the process in which the scrap is acted on by chlorine gas, no choice exists as to the form in which the tin shall be sent to the market, tetrachloride being the constant product. The same remark applies to the method where caustic soda and litharge are employed, and in other respects this latter method has little to recommend it, judging from my own observations. The other process which I mentioned is also unsatisfactory. In employing a mixture of hydrochloric and nitric acids, considerably more iron than tin is dissolved, and when the tin is precipitated by metallic zinc, the contamination of the iron chloride with zinc chloride renders it of little value. I do not claim perfection for the method I have advocated. It is capable, however, of considerable modification.
A better electrolyte could doubtless be found; probably a solution of stannous sulphate would be the best. I think, however, I have proved that this process is practical, simple, and economical, and further, that it presents the additional advantages of purity of product and variety of utilisation, thus rendering it well worthy of a wider application.
The Cost Of The Tin Scrap At The Locality Spoken Of Was Only 2 Francs Per Ton
(Dr. J. A. Smith.)