────────────────────────────────────┬─────────────┬───────────── │ ORDINARY │ BORMETTES │ METHOD │ METHOD ────────────────────────────────────┼─────────────┼───────────── Coke, per cent. of the charge │ 14 │ 12 Blast pressure, water gage │12 to 20 cm. │12 to 14 cm. Tons of charge smelted per 24 hr │ 20 │ 25 Tons of ore smelted per 24 hr │ 8 │ 10 Lead assay of slag │0.80 to 0.90%│0.20 to 0.40% Matte-fall, per cent. of ore charged│ 5 to 10 │ 10 to 15 Lead extraction │ 90% │ 92% Silver extraction │ 95% │ 98% ────────────────────────────────────┴─────────────┴─────────────
[Illustration: FIG. 24.—Latest Form of Converter. (Section on C D.)]
The higher extractions of lead and silver are explained by the fact that the loss of metals in roasting is reduced, while, moreover, the slags from the blast furnace are poorer than in the ordinary process of smelting. The economy in coke results from the greater quantity of sulphur which is utilized as fuel, and from the increased fusibility of the charge for the blast furnace.
The new system of desulphurization enables the charge to be smelted with a less quantity of fresh flux, by the employment in its place of a greater proportion of foul slag. The reduction in the necessary amount of flux is due not only to the increased fusibility of the agglomerated charge, but principally to the fact that in this system the formation of silicates of lead (which are produced abundantly in ordinary slag-roasting) is almost nil. It is therefore unnecessary to employ basic fluxes in order to reduce scorified lead.
[Illustration: FIG. 25.—Latest Form of Converter. (Plan.)]
The losses of metal in the desulphurization are less than in the ordinary method, because the crude mineral remains only a short time (from one to three hours) in the apparatus for desulphurization and agglomeration, and the temperature of the process is lower. The blast-furnace slags are poorer, because there is no formation of silicate of lead during the agglomeration.
The Bormettes method, in so far as the treatment of lead ore is concerned, may be considered a combination process of roast-reaction, of roast-reduction, and of precipitation-smelting. It is not, however, restricted to the treatment of lead ore. It may also be applied to the smelting of pyritous copper-bearing ores. In an experiment with cupriferous pyrites, containing 20 to 25 per cent. sulphur, it succeeded in agglomerating and smelting them without use of any fuel for calcination, effecting a perfect smelting, analogous to pyrite smelting, with the production of a matte of sufficient degree of concentration.