158. Limonite or brown hematite contains about 60 per cent of iron and is found in both compact and earthy varieties. Pipe or stalactitic and bog ore belong to this grade. The color varies from brownish black to yellowish brown, but they all leave a yellowish brown streak on unglazed porcelain. It is found in a belt lying between the red hematite and magnetite ores in the eastern United States. Formerly there was considerable of this mined in central Pennsylvania, Alabama, and the Lake Superior district.
159. Ferrous carbonate contains about 50 per cent of iron. It also is found in several varieties, called spathic ore, clay ironstone, and blackband. Spathic ore when quite pure has a pearly luster and varies in color from yellow to brown. The crystallized variety is known as siderite; this ore frequently contains considerable manganese and in some places is used for the production of spiegeleisen. When siderite is exposed to the action of the air and water, brown hematite is formed.
Clay ironstone is a variety that is found in rounded masses or irregular shapes and sometimes in layers or lumps, usually in the coal measures. It varies in color from light yellow to brown, but the light-colored ore rapidly becomes brown when exposed to the atmosphere. Like the former it also contains considerable manganese.
Blackband is also a clay ironstone, but it is so dark in color that it frequently resembles coal; hence the name. The ore is not very abundant in this country nor extensively used; it is generally found with bituminous coal or in the coal measures, therefore it is mined to some extent in western Pennsylvania and Ohio. It is an important ore in England.
160. The Value of Ores.—Ores are valued according to the amount of iron they contain, the physical properties, the cost of mining, the cost of transportation to the furnace, and their behavior during reduction. The ores of the Mesaba range in the Lake Superior district are very rich, and free from many impurities; they are soft and easily reduced, and as they are found near the surface, they can be mined with steam shovels. These are great advantages, but the greatest disadvantage is the fact that the ore is fine and some of it blows out of the furnace with the escaping gases; this also fouls the heating stoves and clogs the boiler flues.
161. Preparation of Ores.—Most of the ores are used just as they come from the mines, but in some cases they are put through a preliminary treatment. This is sometimes done as an advantage and at other times as a necessity. This treatment is very simple and consists of weathering, washing, crushing, and roasting.
162. Weathering is a common process. Sometimes ores that have been obtained from the coal measures and others that may contain pyrites or similar substances are left exposed to the oxidizing influence of the weather. This separates the shale and the pyrites. The former can easily be removed, and the latter is partly oxidized and washed away by the water or rain falling upon it. The ore piles shown in [Fig. 149] are exposed to the atmosphere and partly weathered before being used.
163. Washing is also done for the purpose of removing substances that would retard the smelting process. For instance, the limonite ores, which are generally mixed with considerable clay or earthy compositions, are put through an ore washer to remove those substances before they are charged into the smelting furnace.
164. Crushing is done with machinery to reduce to a uniform size such refractory ores as are mined in rather large lumps. If the ore were charged into the furnace as mined, the coarseness would allow the gases to pass through the ore too readily without sufficient action upon it. Smaller pieces will pack more closely together, thus offering greater resistance to the blast, and hastening the reduction.