PREPARATION OF ORE FOR SMELTING Transport.—In England and on the Continent much of the iron ore is smelted in furnaces erected nearby—even at the mine mouth. Freight is a minor matter. That which is transported from overseas is handled for the most part in ordinary cargo vessels. In America, however, the long haul down the Lakes from the ranges to the furnaces has caused the development of very specialized handling equipment. At the mines, hopper-bottom railroad cars of 5o to 6o tons capacity are loaded by steam shovels in the pit, or from storage bins at the mine, and hauled to the nearest port. Here long lines of ore bins are built out into the water; whole trainloads of ore are dumped in a few minutes by pneumatic devices. When these bins are full an ore boat docks alongside, opens her hatches, and a load of ro,000 tons of ore is spouted into her in less than a half hour. Ore boats that are used for service on the relatively calm Great Lakes are of special construction and are essentially long, self-propelling barges with a movable deck. Arrived at its destination, the hatches are again opened, and enormous grab-buckets reach through into the hold, scooping out the ore and dropping it inshore, either in railroad cars, or on a stock pile. If the furnace plant is inland, trainloads of ore are hauled to the site, and dumped in bins or placed in stock piles. Hopper-bottomed cars are used almost exclusively for this purpose, although for stockpiling it is generally found better to install a car-dumper, which turns the car upside down, and then to move the ore to the pile by a grab-bucket and overhead bridge. One cannot comprehend the magnitude and power of this unloading machinery, unless it has been seen in operation. Large stock piles, which are almost unknown elsewhere, are the general rule in America, where water transportation is entirely suspended by ice during the winter months. Limestone and coke are also stocked in sufficient quantity to provide against irregular deliveries from a distance.
Screening is the simplest method. Some 17 million tons of Lake ore were improved by this method in 1927. In England as well the miner frequently digs marlstone without removing the loamy overburden; the quarried material is then crushed, and the fine particles screened out and discarded. In this way the average
iron content of the ore is increased, and the suitability for smelt ing improved, for too many fines in the charge slow down blast furnace operations.
Washing is also common in America. This is an application of the water concentration methods so highly developed in ore dressing (q.v.) used at copper, lead and zinc mines. Some five million tons of Mesabi ore were treated in log washers in 1927. The washer is an inclined trough ; from end to end is a stout shaft or log, with plow blades so set on its circumference that when the shaft revolves the plows clear the trough bottom and move any solid material gradually along. The fine screenings enter at one end, and travel against a copious current of water, which washes out the finer, clay bearing particles, lean in iron.
Calcining is common in England, particularly on the Cleveland carbonate ores. A Gjers kiln is ordinarily used, a short cylindrical steel shell set vertically, and lined with fire brick. A mixture of ore, with about 5% slack coal, is charged through the top as the burned ore is withdrawn at the base. Such a kiln will drive moisture and carbon dioxide from about i,000 tons of ore in a week; Cleveland ores lose 27 to 30% in weight, but the resulting ore has had its iron percentage increased thereby to about 31%; still a very lean ore, viewed by American standards. Lorraine ores contain much moisture, but cannot be calcined, because this preliminary heating causes them to fall to powder.
Sintering is a method of converting fine ores or flue dusts into a porous clinker which can easily be smelted. American and Swedish magnetite ores, low in grade, are frequently concentrated by fine grinding, and drawing the iron bearing minerals out of waste rock with strong magnets. This fine magnetite is then moistened and mixed with fine coal and a little flux or with blast furnace flue dust, and spread over a travelling grate, the coal ignited and burned in a strong down-draft. The resulting clinker smelts so much more easily than the dense magnetic ores that the latter, even of high grade, are frequently sintered at the Amer ican plants.