“2. The limestone moderates the reaction temperature produced in the combustion of the sulphur, so that the fusion of the galena, the formation of dust and the separation of metallic lead are avoided, or at least kept within the limits permissible. The lowering of the temperature of reaction is due partly to the decomposition of the limestone into caustic lime and carbon dioxide, in which heat is absorbed, and partly to the consumption of the quantity of heat which is necessary in the further progress of the operation for the formation of a slag from the gangue of the ore and the lead oxide produced. “3. The limestone gives rise to chemical reactions. By its decomposition it produces lime, which, at the moment of its formation, is converted into calcium sulphate at the expense of the sulphur in the ore. The calcium sulphate at the time of slag formation is converted into silicate by the silica present, sulphuric acid being evolved. The limestone therefore assists directly and forcibly in the desulphurization of the ore, causing the formation of sulphuric acid at the expense of the sulphur in the ore, the sulphuric acid then acting as a strong oxidizing agent toward the sulphur in the ore.” The most conclusive proof for the correctness of the opinion which I expressed above, that it is very important to create at the beginning of the operation the conditions for the formation of as much sulphate as possible, has been furnished by Carmichael and Bradford. They recommend that gypsum be added to the charge in place of limestone. At one of the works of the Broken Hill Proprietary Company (where their process has been carried on successfully, and where lead ores very rich in zinc had to be worked up) the dehydrated gypsum was mixed with an equal quantity of concentrate and three times the quantity of slime from the lead ore-dressing plant, as in the table given herewith: ─────────────────┬────────┬─────────────┬──────────┬──────── │ OF THE │ OF THE │ OF THE │ OF THE CONTENTS │ SLIME │ CONCENTRATE │ CALCIUM │ WHOLE │ │ │ SULPHATE │ CHARGE ─────────────────┼────────┼─────────────┼──────────┼──────── Galena │ 24 │ 70 │ │ 29 Zinc blende │ 30 │ 15 │ │ 21 Pyrites │ 3 │ │ │ 2 Ferric oxide │ 4 │ │ │ 2.5 Ferrous oxide │ 1 │ │ │ 1 Manganous oxide │ 6.5 │ │ │ 5 Alumina │ 5.5 │ │ │ 3 Lime │ 3.5 │ │ 4.1 │ 10 Silica │ 23 │ │ │ 14 Sulphur trioxide │ │ │ 59 │ 12 ─────────────────┴────────┴─────────────┴──────────┴────────