With a view to economy, the cheapest iron matters were long used ; but this plan has been given up on account of the excessive corrosion caused, and the rapid destruction of the twyers. It was only in 1866 that attention was given to the employment of streams of cold water in the twyers or in contact with the sides. The experiment was made on a Ratchetts furnace, and succeeded perfectly. The Raschetts furnace (Fig. 139) is used at Altenan, in the Harz. Its length is about 7 ft.; Its breadth, which descends to 3 ft. at the level of the twyers, reaches 4 ft. 9 in. at the level of the throat; each face is provided with 5 twyers arranged in quincunx order. It is claimed that this system largely increases the production of metal, reduces the lead contents of the mattes, and permits the use of iron cinders instead of cast or scrap iron. This latter point once accomplished at Altenau, further experiments were made with other furnaces when provided with water twyers and tanks. At Clausthal, in particular, these efforts were crowned with success.

The employment of a Raschette furnace with this object is thus not an essential; but it is well to approach at least its bell •mouthed form, in order to reduce the losses by priming, and to obtain mattes and cinders poor in lead by reason of the complete reduction of the iron oxides.

Fig. 139.

The use of ferruginous cinders has much reduced the cost of the precipitation process; nevertheless, the losses by slagging and volatilization have always remained high, and it is conceded that pure and rich ores should by preference be treated by roasting and reaction. If these ores are silicious, this latter method becomes inconvenient; but the precipitation method offers another difficulty - the necessity for subjecting the mattes obtained to long operations, whence result new - leeses of metal. Therefore in many works, the plan is adopted of - having a preliminary roast-ing for silicious ores. Precipitation by ferruginous matters entails special difficulties if the ores are blendic; the zinc sulphide generally concentrates in the mattes, passing equally into the cinders, both which become very pasty; but at the contact of the lead cinders there is produced zinc silicate, and during this transformation a part of the zinc oxide formed is reduced by the fuel, giving metallic zinc, which vaporizes. By employing cast iron, the direct action of the iron on the zinc sulphide suffices to bring about the formation of metallic zinc.

Finally, abundant vapours of zinc arise, of which a large proportion arrives at the mouth after having escaped a new - oxidation, and carry away at the same time much lead and silver. There are hardly any but the slightly argentiferous copper ores which lend themselves well to the precipitation process. The copper remains in the state of sulphide in the mattes, where it concentrates, and these are subjected to a special treatment for the recovery of the copper; but it is still necessary that these mattes be not too abundant; that is to say, that the proportion of pyrites contained in the ore remains within certain limits, otherwise a preliminary roasting would still be required to eliminate part of the sulphur.

Slag Smelting

In smelting the slags and cinders resulting from the foregoing operations, the object aimed at is the quantity of lead recovered without regard to its quantity; hence a very .high temperature is adopted. The fur-nace employed, as will be seen from Fig. 140, bears a strong resemblance in general outline to the ore - hearth (Fig. 136, p. 337). Its dimensions are 36 in. high, and 26 by 22 in. in internal area. The bottom a is a cast - iron plate covered with 16 in. of closely packed porous cinders, which latter permit the molten lead to escape further oxidation while restraining the slag.' The sides b are of sandstone. The molten metal escaping from a gains an outer reservoir c, which is cinder - lined in like manner, and thence Tuns into the iron melting - pot d, heated by a distinct fire. The accumulating slag flows off the cinder lining in a and c, and collects in a tank of water e, which causes it to break up so that the lead caught in it can be washed out. The fuel used is peat, and a blast is introduced from the rear about 4 in. above the cinder bed. Coke is employed to urge the heat before charging in the slags.

Prof. Huntington gives the ordinary charge as 100 parts reverberatory slag, 20 of coal ashes, 13 of lead - impregnated clay hearth of old furnaces, and 5 of rich slag from a previous operation. The rationale of the process is as follows:- The silica and alumina of the clay and in the coal ashes combine with the lime and iron oxide in the reverberatory slag to form a very fusible slag; the lead is rendered metallic chiefly by the reducing action of the hot carbon on the lead oxide. In arranging the charge an effort is made to keep the coke in front and the slag at the back. In order to carry the draught up the middle of the charge, the twyer is allowed to form a prolongation of itself, termed a "nose," which it does by gathering molten slag on its end. and chilling it to the solid state; this is only effected when the blast is at the right temperature, about 300° F. (149° C), for if too hot no solidification will take place, and if too cold the nose will grow too long.

Fig. 140.

. Another form of furnace is used for treating slags in and around Newcastle. It is variously known as the "econo - mico," or Richardson's, and is a modification of the Castilian, being a blast furnace provided either with a blowing engine and 3 twyers, or a chimney and half - a - dozen draught - holes. The body is of firebrick, circular in form, 8 1/2 ft. high and 2 1/2 ft. in diameter; the bed is covered with tightly rammed clay and coke - dust, dished in the middle. The ore, previously roasted, and containing not over 30 Per cent. of lead, is charged in at the top; the lead collects in depression, and is tapped out at intervals, while the slag runs over the side wall of the hearth into cast - iron trucks.