A large furnace and a Bessemer converter, the pair capable of making a million pounds of copper a month from a low-grade sulphuretted ore, will not occupy a space of more than 25 ft.
The Alaska gold was derived almost wholly from the large low-grade quartz mines of Douglas Island prior to 1899, but in that year an important district was discovered at Cape Nome, on the north-western coast.
By ioo ft.; and whereas, in making metallic copper out of a low-grade sulphuretted ore, one day's labour used to be expended on every ton of ore treated, to-day one day's labour will carry at least four tons of ore through the different mechanical and metallurgical processes necessary to reduce them to metal.
In the case of the cheaper and more abundant minerals, such as coal and iron ore, and of large deposits of low-grade ores, the extent and character of the deposit can generally be determined by surface examinations at comparatively small expense.
In 1908 more than two-thirds of the total output was from the low-grade porphyry ores mined at Newhouse, Beaver county, and at Bingham, Salt Lake county.
In small works the cupellation is finished in one furnace, and the resulting low-grade silver fined in a plumbago crucible, either by overheating in the presence of air, or by the addition of silver sulphate to the melted silver, when air or sulphur trioxide and oxygen oxidize the impurities.
The output is to-day relatively small in comparison with that of many other fields, but there are one or two permanent gold mines of great value working low-grade ore.
This is due to improvements in mining methods and reduction processes, which have made profitable low-grade ores that were not commercially available in 1880.