t. high, fed
mechanically, and provided with stoves to heat the blast by the furnace
gases. The low percentage of sulphur in the roasted ore is little more
than enough to produce a matte of 40 to 45%, and therefore the escaping
gases are better fitted than those of most copper cupola furnaces for
burning in a stove. But as the slag carries on an average 46% of silica,
it is only through the utmost skill that it can be made to run as low on
an average as 0.3% in copper oxide. As the matte contains on an average
0.2% of silver, it is still treated by the Ziervogel wet method of
extraction, the management dreading the loss which might occur in the
Bessemer process of concentration, applied as preliminary to
electrolytic separation. Blast furnaces of large size, built of brick,
have been constructed for treating the richest and more silicious ores
of Rio Tinto, and the Rio Tinto Company has introduced converters at
the mine. This method of extraction contrasts favourably in time with
the leaching process, which is so slow that over 10,000,000 tons of ore
are always under treatment on the immense leaching floors of the
company's works in Spain. In the United States the cupola has undergone
a radical modification in being built of water-jacketed sections. The
first water-jacketed cupola which came into general use was a circular
inverted cone, with a slight taper, of 36 inches diameter at the
tuyeres, and composed of an outer and an inner metal shell, between
which water circulated. As greater size has been demanded, oval and
rectangular furnaces--as large as 180 in. by 56 in. at the tuyeres--have
been built in sections of cast or sheet iron or steel. A single section
can be removed and replaced without entirely emptying the stack, as a
shell of congealed slag always coats the inner surface of the jacket.
The largest furnaces are those of the Boston & Montana Company at Great
Falls, Montana, which have put through 500 tons of charge daily, pouring
their melted slag and matte into large wells of 10 ft. in diameter. A
combined brick- and water-cooled furnace has been adopted by the Iron
Mountain Company at Keswick, Cal., for matte concentration. In it the
cooling is effected by water pipes, interposed horizontally between the
layers of bricks. The Mt. Lyell smelting works in Tasmania, which are of
special interest, will be referred to later. (See PYRITIC SMELTING
below.)

_Concentrating Matte to Copper in the Bessemer Converter