Process and Technology Sloss Furnace - Sloss-Sheffield Steel & Iron Company, Birmingham Alabama

The Furnace

The furnace is the key element in the process of iron making. Modern furnaces, large cylindrical stacks of sheet metal and fire brick construction, came into widespread use in the last third of the 19th century replacing smaller stone furnaces. Within the furnace, iron ore, coke, and a fluxing material - usually limestone or dolomite - are combined with hot air blown into the furnace through openings called tuyeres. The combination of ore, coke, flux, and heat produces molten iron. Coke, a high carbon residue of refined bituminous coal, is the primary blast furnace fuel. Its combustion produces CO, which acts on the iron ore as a reducing agent to produce metallic and CO^. The fluxing material combines with non-metallic ore properties and coke ash to produce slag which, because it is lighter than iron and floats on top, can be easily separated. Iron and slag leave the furnace through separate notches - the iron to be cast, either in sand or by machine the slag to be either dumped, hauled away, or more recently, processed for use as cement aggregate, railroad ballast, or soil conditioner.

The Cast House

If sand-cast, the iron ran in channels formed in the sand floor of the casting shed. These channels resembled a series of large combs on either side of a central channel. (They also resembled pigs suckling at the sow - hence the term pig iron.) The central channel carried the molten iron directly from the furnace and distributed it first, to narrower channels, and then to the sand molds.

Work in the casting shed was physically arduous, intense, and hot. The sand molds were formed by hand as were the sand dams used to skim excess slag from the molten iron. The opening of the iron notch at the base of the furnace required six to eight men working with hand drills and sledges from ten to sixty minutes. Because of the heat, the men at the notch had to be relieved every two to three minutes. Closing the notch was also done by hand - a process employing clay balls and a ram, or stopping hook, that might take fifteen to twenty-five minutes. But the heaviest, most disagreeable work of all was breaking and loading the pig iron. In the South, the iron was generally allowed to cool first. This reduced the intensity of the heat, but made the iron harder to break. Heavy sledges and crowbars were used, and the men worked under intense time pressure for the furnace was continually charged, and could be expected to make its next iron run within four to six hours of its last.

The iron carriers were unskilled laborers who occupied a crucial point in the production process and acquired a reputation for independence. One early 20th century furnace manual claimed that: The entire plant depended upon iron carriers, since the furnace could not be operated unless the iron were carried out, not any time but within a very limited period, so as to permit the beds to be made ready for the next cast. These conditions had the result of making this class of labor extremely hard to handle.

The work was so demanding, according to Edward Uehling, that "the extraordinary muscular exertion required bars four-fifths of the laboring class from standing up under the strain at all...". Bars of iron, weighing 100 to 125 lbs., had to be carried six to then paces over "loose hot sand" and loaded onto a railroad car. This process was repeated 250 to 300 times in four to six hours. Uehling interpreted the work succinctly: The task of breaking and carrying out the iron from the casting-beds of even a moderate-sized furnace is not a fit one for human beings. If it were possible to employ horses, mules, or oxen to perform this work, the Society for the Prevention of Cruelty to Dumb Beasts would have interfered long ago, and rightfully so.

For the large Northern furnaces, the increased rate of production in the 1890s made it almost impossible for the men to keep pace. It became harder to maintain full work crews, even with increased wages, and the men, growing more conscious of their critical role, became more difficult to "handle." As a direct result, labor-saving expedients including the Killeen Skimmer, which replaced the hand-made dams used to divert excess slag; cast iron molds, which eliminated the need for hand-formed sand molds; and pig breakers and cranes, which eliminated a large number of iron carriers were introduced. The latter two innovations were less effective than the major breakthrough of the period, the Uehling casting machine.

In its original form, the machine consisted of two endless chains arranged in tandem. The first carried the molds into which the hot iron was poured. (A ladle car transported the iron from the furnace to the pig machine.) The molds were cooled by immersion in water, and the solidified iron was then discharged to a second chain, which also ran under water, before it delivered its product to a waiting railroad car. As the molds returned for the next pour, they were cooled and coated with lime to keep the iron from sticking. A later modification eliminated the need for two chains by merging their functions into one.

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In addition to reducing the labor from forty or fifty men to five, the pig machine provided a cleaner, more uniform product than before. It was a decided advantage not to have sand adhere to the iron, and it was much easier to maintain chemical consistency if the iron were thoroughly mixed in a ladle car rather than running freely from the furnace to the sand molds. It was a manifestly superior process, but one which the Sloss Company did not adopt until labor scarcity made it necessary.

Charging

The work of charging the furnace with its burden of ore, coke, and flux was also done by hand until the mid-1890s. The earlier stone furnaces were generally built into the side of hills to facilitate top loading. The first metal-plate furnaces employed steam-driven vertical hoists (Figure 5). Loading at the bottom, and unloading at the top, however, were purely manual. The top loaders job was more dangerous and more responsible. Gas leaks, which were inevitable, were a constant cause for wariness, as well as a potential cause of serious occupational illness. The job required attentive and responsible workers to circle the furnace rim while charging. If the stock were unevenly distributed in the furnace, it would not reduce evenly. One furnace manual stated, "slight variations in dumping more than any other cause, derange the works of the furnace."

Particularly when the furnace was "driving" the top fillers might be tempted to dump all the stock on one side in an effort to "keeping-up."

The industry responded by introducing automatic charging devices consisting of inclined, steam-driven skip-hoists which carried stock from the stock bins to the furnace top with a minimum of human labor. Improved stock bins were the key element in the system. The bins discharged their stock by gravity feed to traveling scale cars installed in a tunnel below the bins. The scale cars were designed to automatically weigh the stock and then dump it into one of the skip cars operating on the inclined hoist. The car traveled up the hoist and mechanically deposited its stock into the furnace. The system was in use by the mid-1890s as part of the new era in furnace building inaugurated at Duquesne in the Pittsburgh district. It allowed for a reduction in labor from twenty men to three.

With automatic charging and the replacement of top fillers came a need for a new furnace tops with would effectively distribute the stock. These generally consisted of a double bell and hopper arrangement. The two inverted cone-shaped bells were designed to provide a gas seal and a method of distribution. The McKee top came to be the one "almost universally used," In its improved form, the McKee top consisted of a stationary receiving hopper and a revolving small bell, placed directly over the large bell. The small bell would receive the stock, discharge it to the bell below, and rotate sixty degrees before it received the next charge. In this was, the stock would be evenly distributed on the large bell before being discharged to the furnace. The two bells functioned as a gas seal by operating in series. One of then was always closed at any given time.

Gas Cleaning

The operating furnace produced three products: iron, slag, and gas. In the older stone furnaces the gas was allowed to escape into the atmosphere. But with the introduction of improved gas cleaning devices, it was possible to recirculate the gas and use it to fuel the boilers and hot-blast stoves. The gas was cleaned in two stages. It was first drawn off from the top of the furnace and carried in a pipe called the downcomer. The downcomer introduced the gas into a large cylindrical tank, suspended vertically above the ground. The tank, called a dust catcher, was designed to reduce the velocity of the gas, by increasing the larger dust particles to settle at the bottom.

With the larger particles removed, the gas then passed to the washers. Many different types of washers existed: stationary, either vertical or horizontal, revolving, and some operating on centrifugal force. Most washers contained vertical chambers fitted with water sprays. The water cleaned the gas of the remaining coke ash and ore dust, making it suitable for use in the boilers and stoves. Uncleaned gas would have clogged boiler flues and the interior brick work of the stoves. After 1900, new types of gas cleaners, using electrodes to remove suspended matter, came into increasing use.

Boilers and Blowing Engines

The boilers, operating with coal and natural gas as well as furnace gas, provided steam for the blowing engines. They also provided steam for the skip hoist elevator, the revolving furnace tops, and an assortment of water pumps. Because of the demands of blast furnace operation, large, high-pressure water tube boilers were a necessity. The types in most common use were the Cahall, the Sterling, and the Babcock & Wilcox. The Rust boiler, invented by E. G. Rust of the Colorado Fuel & Iron Company, found increasing favor by the early 20th century. This was a boiler with straight, vertical tubes, and was consequently easier to clean - a clear advantage for a continuously used boiler.

The boiler-fed blowing engines propelled air through the hot-blast stoves into the furnace. These engines replaced earlier water-powered bellows. In the late 19th century they gradually increased in size and capacity. The years 1880-1905 marked the greatest development of these reciprocating steam blowing engines. Most of the engines used were vertical, though a few were horizontal or combined vertical and horizontal properties. The three types most commonly used were the "long-crosshead," with a flywheel on either side; the single flywheel, cross-compound, which because of its height became known as the steeple engine; and the single flywheel, "quarter-crank" with its steam and air cylinders placed on separate pedestals. The "long-crosshead" was probably the most widely used. It achieved prominence in the 1880s as the first of the modern blowing engines, and continued in use into the 20th century (Figure 8). Steam blowers were supplemented, or replaced, during the first quarter of the 20th century, by gas-driven blowers or turbo-blowers. The latter, operated by air drawn to the center of "rotating impellers" and discharged by centrifugal force, gradually came into prominence by mid-century.

Hot-Blast Stoves

The hot-blast stoves were tall cylinders, with spherical caps, constructed of metal plate and firebrick. The interiors consisted of a large combustion chamber either in the middle or on one side, running the height of the stove. Arranged around the semi-circular combustion chamber were networks of brick checkerwork partitions designed for heat retention. Stoves were classified as two, three, or four pass, depending on the number of partitions. A two pass stove consisted of a combustion chamber and one set of brick checkers; a three-pass, a combustion chamber and two sets of brick checkers; etc.

Each type operated on the principle of heat regeneration. Blast furnace gas entered at the bottom of the stove where it was immediately subject to combustion through the action of a gas burner. The products of combustion passed up the chamber and down and over the brick partitions, imparting heat to the brick checkers. Waste gases passed out of the stove through the chimney valve. The gas main was closed, and the cold blast main was opened. The cold air, driven by the blowing engines was heated as it passed through the stove, after which it was propelled through the hot blast main to the furnace. A stove of this type was patented by the English engineer, E.A. Cowper, in 1857. The basic technology was brought to the U.S. by Cowper's associate, Thomas Whitwell, in the 1870's. Regenerating stoves replaced earlier iron pipe stoves, which had provided the first successful method of heating the blast. Prior to the use of pre-heating stoves, the blast was introduced to the furnace cold.

Furnace companies employed from three to six stoves per furnace. Extra stoves were necessary because a certain number were always heating, or "on heat", while one, and sometimes two, were "on blast" giving up their heat to the furnace. Generally, the number of stoves used per furnace increased over the 20th century. The additional stoves functioned as spares, helped to reduce the wear, and when operated in conjunction with others, assisted in equalizing blast temperature.

The tending of stoves, blowing engines, gas cleaning equipment, and boilers was not subject to extensive labor-saving innovation. In general, jobs in these areas were more desirable (though the cleaning of boilers and stoves could be both disagreeable and hazardous) and better paid than work in the cast house or stock bins. Stove tenders and blowers clearly perceived cast house labor as low status. One blower at a northern furnace around 1920 commented that "only Hunkies" (Poles and other Slovakian immigrants) worked in the cast house. The jobs, he said, were "too damn dirty and too damn hot for a 'white" man."

Auxiliary Processes

In addition to the major blast furnace equipment, there were also important auxiliary processes. The maintenance of an initial adequate water supply, with its necessary pumping machinery, cooling ponds, and cooling and storage towers for recycling water, was imperative for cooling furnaces, boiler operation, and steam condensing. Even after electricity came into widespread use, steam machinery was retained for the operation of skip hoist elevators and furnace tops as a hedge against the disruptive consequences of electric power failure. Some furnaces were also fitted out with additional heating elements and iron pipe recuperators designed to pre-heat the cold blast before its introduction to the stoves. Other furnaces experimented with moisture control devices. These were essentially air-conditioning units whose purpose was to freeze the moisture out of the air before it was blown to the furnace. The process was first developed by Jaraes Gayley, c. 1890, but its utility remained a subject of controversy into the 20th century.