MINERAL PRODUCTS.
19
The actual gain in capacity is even more than is indicated by these
figures, for in the earlier years a number of small, antiquated
smelters that had been idle for several years were included in estimating
the total capacity, whereas in the later years almost all such
smelters have been dismantled or abandoned completely, and there
have been more new, modern additions than ever before in the same
time. It is to be borne in mind that a certain portion of the capacity
is taken up in the redistillation of zinc drosses, and that some plants
do not yield full capacity because they treat lower-grade ores, so
that the spelter produced from ore always falls much short of the
maximum capacity. On the other hand, the new zinc oxide plant
at Leadville, Colo., will take a great deal of the low-grade carbonate
ore away from the zinc smelters, so that the capacity per retort will
be increased because of the higher grade of the remaining supply;
and the introduction of ore flotation in the Western States will
probably raise the grade of zinc concentrates from that section, likewise
increasing the capacity per retort.
The following table shows that the production of zinc at the
mines is steadily increasing:
Joplin district
Now Jersey
Colorado
Montana
Upper Mississippi Valley
Other States
1911
137,633
77,445
47,304
21,905
33,939
27,034
345,260
152,465
69,755
66,111
13,459
37,115
39,911
378,816
1913
146,474
84,122
59,673
44,337
32,346
51,430
418,382
The largest increase in sight is the result of the application of ore
flotation concentration in Montana, but an increase due to the same
cause is to be looked for in Idaho and possibly in Utah. In Montana
the Butte & Superior mill is now producing at the rate of
50,000 tons of zinc a year, and the new Pilot Butte mill, which has
just gone into operation, should raise the State output to 70,000 tons
a year, or if, as planned, another unit is added to this mill the State
output may rise to 90,000 tons and more by 1915. In Colorado the
new zinc-oxide plant at Leadville is reported to be purchasing 11
per cent zinc carbonate ore, as against a minimum of about 20 per
cent ore purchased by zinc smelters. As the Leadville carbonate ore
is not amenable to concentration this means a large increase in the
available zinc-ore resources of that region. Another source of zinc
which will be made available in the future is zinc-bearing copper
ore, the zinc content of which now either makes the ore unsalable
or, when the ore is smelted for the copper, collects as flue and baghouse
dusts, for the recovery of the zinc from which there is at
present no satisfactory process. Much ore of this sort is mined in
