In the production of pig iron, brick and refractory materials are used to form the very large ovens or retorts (coke ovens) in which coal is converted to coke. Refractory and brick materials are needed to line huge shaft-like blast furnaces in which the molten pig is formed. The auxiliary furnaces or hot blast stoves, which heat the air for the blast furnace, are also lined with refractories. The molten pig iron from the blast furnace is then sent to a brick- or refractory-lined ladle. The molten pig iron is then poured into a brick- or refractory-lined pig casting machine, where the pig iron is formed into large castings and cooled.
Molten pig iron is a relatively impure alloy of iron, with about 4 percent carbon and other materials. It is used chiefly as a primary ingredient of steel, but is also used for many finished products such as pipe fittings and structural castings. However, these products are rarely made directly from blast furnace iron. In most cases, the cold pig iron is re-melted in brick or refractory lined cupolas or malleable iron furnaces. In these lined furnaces the quality and analysis of the iron can be more accurately adjusted and controlled, before casting the metal into final form.
Most pig iron from the blast furnace is used to make steel. To do this, pig iron — usually molten — is combined with molten steel scrap in open hearth furnaces, or in Bessemer converters, to be refined and purified. Regardless of process , the refined and purified molten steel is poured into ladles, made into ingots, put into a soaking pit, sent to roughing mills, passed through a reheating furnace, and finally sent to the finishing mills. Almost regardless of where you look, you will find brick and refractory.
Although this article started with a general overview of the iron and steel industry, we will refer only to one process for the rest of this feature, a gray iron foundry using an electric arc furnace. We will see how important brick and refractory are and why everyone should pay attention to their brick and refractory designs and installations.
History and Background of the Electric Arc Furnace
The electric arc furnace (EAF) has been around since the early 1900s and is primarily used to produce high quality alloy steels. EAFs have refractory-lined steel shells and vary in size from only a few feet in diameter to twenty feet or more. The bottom, or hearth, of the furnace is dished to hold the metal charge. The vertical side walls include one or more doors and a tapping spout for removing the melted charge. The roof over the furnace is dome-shaped, and incorporates two or three openings near its center for the admission of electrodes into the furnace. The roof is usually detachable and is lifted off or turned aside for charging. The whole furnace structure is tilted forward and backward for slag skimming or tapping. The furnace capacity will depend upon the size of the furnace and can be less then one ton per charge to up to eighty-five tons per charge.
The operation of an EAF is a batch process in which steel scrap, pig iron, a source of iron oxide such as roll scale or iron ore, and limestone or burnt lime are charged into the furnace. The roof is closed over the furnace and electrodes are lowered until arcs are established between them and the metallic charge. The heat generated by the arc and the heat developed from the resistance to the flow of current in the metal melts the charge. When the heat (meaning the metal in the furnace) is finished, the furnace is tilted and poured into a tap ladle through a tap hole or spout in the furnace side wall. After each heat, repairs to the bottom and slag line are made by shoveling in a refractory bottom patching material.
There are two types of electric arc furnaces, acid or basic, which refer to the type of slag present in the furnace. Both types are lined with a combination of brick and refractory. Practically every advance in foundry melt shop practices has put greater demands on the brick and refractory used. Higher operating temperatures and other desirable conditions, which are increasingly destructive to refractory materials, are called for by the plant operators. Over the past ten to fifteen years, a lot of expertise in the iron and steel foundry has been lost. Therefore, it would serve the industry well if some fundamentals of brick and refractory applications are reviewed here.
Brick and Refractory Basics
Brick Materials — In an EAF, the term ‘brick’ can apply to a silica brick, a high alumina firebrick, or a basic brick:
- Silica brick is an acid resistant brick usually made from ganister, bonded with a small amount of lime. Silica brick has high resistance to thermal shock.
- High-alumina bricks are standard firebricks made with a high percentage of alumina ranging from 50 to 90 percent. A high alumina brick has high resistance to spalling due to rapid temperature changes.
- Basic brick is a general term used to designate a brick that is made from magnesia. A basic brick has high resistance to thermal spalling and excellent mechanical strength.
Brick Installation — There are so many things to know when installing brick that entire books are needed to cover the many things pertinent to the task. It is not the intent of this article to rewrite those books, but rather to indicate some of the main points for proper installation. The most important of these are mortar and temperature.
To start, one should understand that the main intent in the correct laying of brick is to obtain a strong, solid wall or roof. To achieve that, brick must be laid up in an air setting mortar matching the characteristics of the brick. Mortar will cause bricks to adhere to each other and to distribute the pressure or weight uniformly across the bed.
Proper brick installation means that the horizontal and vertical joints between the bricks or tiles be filled with mortar. This will ensure a strong wall impervious to penetration by molten metal. It is recommended that all be bricks be laid with the thinnest possible joint. Bricks should be dipped in the mortar and laid with a thin brick-to-brick joint (less than 1/8 in. thick). The result will be a strong, monolithic structure. All brick, except that which is steel clad, is to be laid in mortar. Steel clad brick is laid dry. The steel cladding will melt during the initial furnace firing and will act as mortar.
For applications in which brick is laid in two independent layers (backup and working lining), such as in ladles, the brick linings must be impervious to penetration for longevity of use. This means that there must be a minimum of open joints, and to accomplish this all joints must be staggered so that vertical joints do not fall one above another. When a working lining is laid over a back-up or safety lining, all horizontal and vertical joints of one lining should be staggered so that no common seam (joint) exists between the two layers. It is recommended that the mortar be applied between the working and back-up lining. This should be a thin wash coat over the face of the back-up lining (about the thickness of paint). This will allow the working lining to be mortared on three sides. It is also recommended that the mortar should be applied over the entire face of the working lining. This should also be a thin wash coat over the face of the working lining. This will help fill any gaps or cracks on the working lining.
Type of Mortar — The mortar should match closely with the type of brick being laid. Using the proper mortar will help eliminate expansion differences between it and the brick. If the brick inside a ladle is laid with too thick of a mortar joint, and the mortar does not match closely with the chemical characteristics of the brick, gaps or cracks will result when the ladle is preheated to 1500° F.
Many have asked about the strength of the mortar versus that of brick or tile. Mortar is definitely stronger. A crack will almost always form in the brick itself before it begins in the mortar. Anyone that has tried to reuse brick after a demolition of an existing brick structure knows how difficult it is to separate the brick and mortar from each other. The bricks usually break or crack and, for the most part, cannot be re-used. That is why a thin coating of mortar is better than a thick coat and why dipping is specified in application of mortar to brick.
Cold Weather — The temperature of surrounding air is critical for proper brick installation. No brick structure should be ever done when the air temperature is below 28° F (when temperatures are rising), or 32° F (when they’re falling). The air temperature must be measured at the point of installation. No frozen materials should be used. On a warm day, bricks should be wet just prior to being laid. However, when the air temperature is at or near freezing, the brick must be thoroughly dry. Laying brick in freezing weather seriously affects the strength of the brick construction.
Refractories
The term ‘refractory’ can apply to a plastic or a castable type material. According to ASTM C-64 there are five basic categories of plastic refractory (high duty fireclay, super duty fireclay, 60 percent alumina, 70 percent alumina, and 80 percent alumina). A plastic (meaning molded, or shaped) material will be made from either fireclay or alumina, with the rest made of other materials. According to ASTM C-64 there are two classifications for castable refractory. Either it is an alumina-silica base or an insulating type refractory.
There are three basic types (dense, medium weight and light weight). All use the same common chemicals, yet each varies considerably to meet specific use requirements. Some of the most common chemicals for refractory materials used today are: alumina, silica, ferric oxide, titanium oxide, calcium oxide, magnesium oxide, and alkalies.
Refractory Installation — Refractory materials like brick, when properly installed, can add longer service life to your furnaces and ladles and reduce maintenance costs.
The first step in assuring proper installation, after proper material selection, is mixing the refractory. However, in order in to properly mix it you must know how you intend to install it. There are four fundamentally different ways to install refractories:
The gunning method permits the handling of a large volume of materials. Water is added at the discharge nozzle or back at the charging chamber. The amount of water is determined at the point of installation by the nozzle man and also by the characteristics of the refractory. Forms are not required. The advantages, of course, are speed and versatility. Rebound losses mean that more material will be needed than for casting. Overage will vary considerably depending on whether a ceiling, a wall or a floor is being installed.
The trowel method is a hand-applied application generally used for thin lining (i.e. ladle tops at the pour areas). The amount of water will vary with the type of refractory and must be just enough for the material to be sticky and stay in place. An important thing to pay attention to when troweling is to avoid over troweling the finished surface. This will bring cement to the surface and have the effect of sealing the refractory, thus restricting the escape of water vapor, which occurs during dry out. Sealed-in moisture can turn to steam during dry out and explode the lining.
The cast, or pour, method is one in which the area is made completely watertight by use of forms (i.e. ladle walls and floor). The mix would contain more water than a trowel application mix. A cast refractory should be thoroughly worked into place, with internal air bubbles removed by rodding or vibrating. The best method is to use vibratory equipment, since it encourages the material to flow freely and completely fills the form or box. Vibrating also permits working with a stiffer mix (less water). However, hand rodding — with a shovel, rod, ice scraper or pole — will suffice if done thoroughly. Working out all the air strengthens the material and reduces porosity. The action of vibration or rodding sets the refractory particles in motion, reducing the friction between the particles and giving the mixture the texture of a thick fluid. Internal vibrating or rodding does the most consistent job, but external vibration is also effective. Although vibration equipment is available for refractories, it can often be improvised with equipment already on hand to vibrate the mass externally. An air hammer held against the form can do a good job in many cases. So may a nut runner or other pneumatic vibratory tool. Even hammering the form with a hand sledge will do if accompanied by hand rodding. In all cases, be sure the material is worked near the forms. That is where air bubbles collect.
Finally, the ramming method of application is a hand approach similar to trowel application for plastic refractory materials. Plastic material will require no water and once the material is applied by hand a pneumatic ramming tool is used to finish the job.
Gary Bases is president of BRIL, inc., an independent consulting firm specializing in brick, refractory, insulation and lagging, where energy saving solutions are possible.