One more application of blast-furnace slag is the manufacture of slag-wool, or silicate cotton, so called from its resemblance to cotton-wool. The first attempt at this manufacture was in 1840, by Edward Parry, in Wales, and a large quantity was made; but no effort appears to have been made to. confine the wool after production; con-sequently it floated about the works with the slightest breeze, and became so injurious to the men that the process had to be abandoned. About 4 years ago, Krupp of Essen, and a little later, Lurman of Georgmarienhutte in Hanover, both supplied a great deal to the market; but the precise methods of manufacture never transpired, having been kept a secret at the works; and until 2 years ago the wool had never been successfully made in this country.

As carried out by Wood at the Tees Iron Works, the process is exceedingly simple. A jet of steam is made to strike upon the stream of molten slag as it flows from the usual spout into the slag waggons or bogies. The steam scatters the slag into shot. As each shot leaves the molten stream, it draws out a fine thread, just in the same way as when you touch treacle lightly with the finger-if you lift it up you will see a fine thread attached. The consistency of molten slag is not unlike treacle; each shot makes a fine thread which, losing its heat, becomes set like glass. The shot being heavy, drops to the ground, but the thread is sucked into a large tube by an induced current of air caused by the steam jets, and the wool is discharged into a large chamber. The finer qualities float about and settle near the outside, whilst the heavier or larger fibres lie chiefly in the centre of the chamber. After each blowing, the chamber presents a most remarkable and curious as well as a beautiful appearance. The wool is of snow-white colour, and attaches itself to the sides and roof, or to anything which it can touch, in the same manner as a light fall of snow does in calm weather upon every tiny twig of a leafless tree. The wool is taken up daily with forks, and put into bags for sending away.

It is principally used for covering boilers or steam-pipes, for which purpose it is peculiarly adapted, as being a splendid non-conductor of heat, and incombustible. About 4 tons of this wool is produced per week, and as only 1/4 cwt. is made from each ton of molten slag operated upon, the process is not a very rapid one.

Repeated bursting of hot-water pipes encased in slag-wool induced Professor T. Egleston to examine into the cause. The results he obtained are set forth in the following abstract from his paper on the subject (Trans, Amer, Soc. Cw. Eng., cclix. 253-61):-

Slag transformed into wool does not differ in any respect from slag in a solid condition, except that its fibres become interwoven. It occupies, when not compressed, a maximum volume for a minimum weight, and thus retains a very large quantity of air. This is its only value: it is the air and not the slag which is required. The only value of the slag is its capability of holding this air when it is not compressed; when it is compressed, it has very little more value than slag in its solid form. This compression may be caused by the sagging of the pipes on it, if only the envelope and not the pipe is supported, or by its becoming soaked with water, when it mats together, water takes the place of the air, and it ceases to be a non-conductor of any value. Its great value is in its very fine fibres, but it is precisely this quality of fine division which makes it most dangerous, for in this condition it is most easily attacked by organic acids, alkalies, or moisture, which not only decompose it, but render the pipes liable to attack.

Even the commencement of decomposition causes it to sag and settle.

It would seem, therefore, that mineral wool, if made from slags containing sulphur, is, under certain conditions, a dangerous material. In one case of explosion, the moisture undoubtedly came from defective joints, which are likely to belong to any other system. In the other and far more dangerous one, this moisture was that of condensation; and as it was not expected, every precaution having been taken against it, it is by far the more dangerous one, as it would not be looked for, while every joint would from time to time be visited. In any system, moisture is likely to come from rain or snow leaking through the envelope, where the pipes are exposed above ground in the open air, or, when they are below ground, from drainage water, and, in both cases, from condensation, due to sudden cooling or too sudden heating of the pipes. The effects produced are likely to be all the greater in intensity as the pipes are hotter, this facilitating the liberation of the sulphuric acid, which produces a further decomposition of the slag, and keeps setting free new portions of acid to further decompose the slag and attack the iron.

The moment the silica commences to assume the gelatinous condition, the other constituents of the slag are set free to attack the pipes, and unless the leakage is found and stopped, an accident is sure to happen. It seems, therefore, a wise precaution, when this substance is used, to employ it only where leakage is not likely to occur or moisture to collect, and to carry the whole system above ground, under cover; or, when it is necessary to carry it below the surface, to have all parts easily accessible, so that it may be carefully examined. from time to time. Beneath the ground, where it cannot be examined, it becomes a real element of danger. Kept free from moisture, mineral wool is one of the best and cheapest materials that can be used for covering steam pipes. To employ it successfully, it must not become packed, for then it loses its non-conductive power. When it becomes moist, it packs; and if this moisture and packing are continuous, the slag is attacked, liberating its dangerous elements to act on and weaken the pipes.

For Insulating

Buttgenbach, the well-known metallurgist, gives the following method for the utilization of blast-furnace cinder as an insulator for steam-pipes, &c;-Mix 150 parts cinderdust, 35 of fine coal-dust, 250 of fire-clay, and 300 of flue-dust, with 10 of cows' hair; add 600 parts water into which 10 to 15 of raw sulphuric acid have been poured, and make a stiff dough of the whole. This is thrown in small amounts upon the warmed pipe, hardening rapidly. -Upon this rough coat a second, third, etc, is laid, according to the thickness which is to be used. By the action of sulphuric acid gypsum is formed, and the silica, rendered free, hardens. The mass becomes as hard as porcelain, and is still porous. It adheres firmly and never cracks. Buttgenbach states that he has tested its merits by 10 years' use, and has found it to meet all requirements.

For Manure

A material containing so much lime, silica, alumina, sulphur, and magnesia, in a condition like the white soft slag sand, suggested its application as a fertilizer for some kinds of land. Three years ago it was brought before the Royal Agricultural Society, and Dr. Voelcker reported that the result of his examination showed that it might be usefully employed upon moorland and peaty soils as a cheap and effective substitute for lime. Since this report was made, many hundreds of tons have been sold for this purpose, and although there was only 32 per cent. of lime in the slag supplied, the results have been very satisfactory, particularly on land growing potatoes. Had it been Bessemer slag, containing 40 to 50 per cent. of lime, there cannot be a doubt that the results would have been still more satisfactory; and Wood feels sure that it must in some localities find a large outlet for this purpose.

The successful attempts made in recent years, to remove the phosphorus from iron in the course of manufacture by the basic process, have attracted the attention of chemists to the possibility of recovering the valuable manure, phosphoric acid, from the slag. A patent has been recently obtained by Thomas and Twynam for the manufacture of phosphoric acid and phosphates from slag, especially adapted to their recovery from slags produced in the basic Bessemer and Siemens processes. The slag is first finely ground, and the particles of iron are picked out by means of magnets; it is then treated with sufficient hydrochloric acid, either in aqueous solution or in vapour, to dissolve out the phosphoric acid, and part of the iron oxides. The powdered slag may first be roasted in a calciner, to oxidize the ferrous oxide. If sufficient ferric acid be not present in the solution to combine with all the phosphoric acid, it will be necessary either to add some ferric oxide or puddlers' slag (which should preferably have been first roasted), or to oxidize any ferrous oxide present, either by chlorine gas or by the addition of manganese binoxide (in which latter case there must be sufficient free acid in the solution to cause the decomposition of the manganese binoxide and set free chlorine), or the ferrous oxide may be oxidized by other well-known means.

The solution is then run off from any insoluble residue, and sufficient lime or (preferably) chalk is added to cause precipitation of ferric phosphate (magnesian limestone may be used in place of ordinary limestone for this purpose, the magnesia dissolved being afterwards precipitated by lime). The precipitation of the ferric phosphate may be effected in the cold, keeping the solution well agitated; but a gentle heat causes the precipitate to settle down better. When the precipitation is complete, the ferric phosphate is separated from the solution by filtration, and washed to free it from calcic chloride. The precipitate is then dried, so as to drive off all its water; and digested with a considerable excess of sulphuric acid, so as to decompose it into phosphoric acid and iron sulphate, which latter is insoluble in the excess of sulphuric acid employed. Ordinary chamber acid is able to effect this decomposition, but a stronger acid, such as is obtained when using a Glover tower, is better adapted for the purpose. The ferric phosphate should be kept well agitated in the liquid, which may be gently heated to facilitate the decomposition.

When the ferric phosphate is considered to be completely decomposed, the insoluble ferric sulphate is separated from the solution containing the phosphoric acid and excess of sulphuric acid by means of a filter-press. The filtrate will be found to contain nearly all the phosphoric acid in a free state, together with the excess of sulphuric acid, from which it may be separated by the aid of heat; the sulphuric acid being driven off and condensed, leaving the phosphoric acid practically pure, or the solution containing the mixed acids may be used in place of ordinary sulphuric acid for making super-phosphate of lime. The iron sulphate precipitate after being washed with sulphuric acid may be decomposed by heat into ferric oxide and fuming sulphuric acid, or it may be mixed with salt and heated to form sodic sulphate. In some cases, .when there is no lime in the slag employed (as when treating puddlers' cinder), the phosphoric acid is thrown down as a mixed ferrous and ferric phosphate by means of lime without previous oxidation, and is then dried and treated in the same way as ferric phosphate.

For Casting-Beds

In the Siegen district, Belgium, granulated slag is used for casting-beds, and gives clean pigs, preferred by the puddlers even to those cast in iron moulds.