Most problems with refractory materials can be traced to improper mixing. Attention to the four following points, however, should produce a serviceable lining:
Proper Proportions — The right amount of water is essential. True, a wetter mix handles more easily, but it robs the refractory of its needed strength. Too dry a mix, on the other hand, is difficult to place, and it may set to a weak, porous ‘popcorn’ structure. A proper mix will usually seem on the thick side when compared with conventional concrete. One good guide to follow when troweling refractory is the ‘ball-in-hand’ test. Make a small ball of refractory and toss it twelve inches into the air. If it breaks apart when it lands on your flat palm, it is too dry. If it flattens out, it is too wet. The ball should retain its size and approximate shape for the mix to be right.
Keep it Clean — Industrial compounds can easily contaminate a refractory mix and seriously affect its properties. Certain salts, for example, react with the binder to make it useless. Be sure to use clean water, clean mixing and handling equipment, and clean forms. Also, it is best to use potable water because it is free of minerals normally found in tap water. Those minerals can prevent the refractory from reaching its proper strength.
The Right Mixer — Although mixing can be done manually or in concrete mixers, mortar mixers usually give the best results. They are geared to handle fairly thick mixtures. Hand mixing and concrete mixers often require excess water. On big jobs, use two or three mixers to provide a continuous supply of fresh refractory. Refractories all have a ‘pot life,’ meaning the length of time after the material is mixed until it sets and becomes unusable. This will vary from twenty minutes to sixty minutes, depending on the refractory.
Don’t Over-Mix — Three or four minutes mixing time with a mechanical mixer should be plenty to insure a homogeneous mixture. The high iron aluminous cements and pure calcium aluminate cements have shorter setting times than do calcium silicate (Portland) cements. Over-mixing speeds the setting rate, reducing pot life and weakening the material. Remixing should never be done.
Cold Weather — In cold weather, the strength of a refractory will be adversely affected if the dry material used is in the freezing range of temperatures, and if mixed with cold water. It is desirable, that both the dry material and the water be in the 60-70° F range when maximum strength is a consideration. Provisions should be made to maintain a minimum ambient air temperature of 50° F when applying refractory materials. Also, any steel that comes into contact with the refractory should be maintained at a temperature not lower than 50° F. The freshly installed refractory should be protected against freezing for a minimum of 48 hrs., or until thoroughly dried. When the temperature is below 50° F, the maximum strength of the material can be improved by heating the mixing water.
The following formula may be used for estimating the temperature of fresh mixed refractory:
X = {(W T) + .22 (Wc Ts)}/ (W+.22Wc)
where:
W = weight of water (a quart of water weighs 2.08 lbs)
Wc = weight of dry castable refractory
T = temperature of water (°F)
Ts = temperature of solids (°F)
X = temp. of mixed castable (° F)
Care and Storage of Brick and Refractory
Care must always be taken when storing refractory materials. They should always be stored in dry, well-ventilated conditions. This will ensure that they will not lose any strength, give brick material an indefinite shelf life, and the refractory material a shelf life of up to one year. If the material is stored improperly, such as in a place where the brick or refractory bags can become damp or wet, the moisture may get into the material and cause a partial setting of the refractory or make it completely unusable.
Refractory material that is exposed to dampness will get hard and is easily recognized. However, when a brick is exposed to dampness it looks the same as if properly stored, so it is harder to recognize. A brick material should not be used if there is any doubt as to whether the brick is damaged because of where it is used. In an EAF, spoiled brick, when exposed to high temperatures, may spall or explode due to trapped water trying to escape the brick.
Curing and Drying
Brick material is a fired shape and does not require curing or drying. This is an important difference between brick and granular refractory. However, once it is properly installed, almost all granular material must be cured and dried to reach its proper strength.
Only after this process will the refractory be capable of doing its job. The term “curing,” as applied to refractory, means keeping the material wet or the surrounding atmosphere humid. The primary purpose is to create the most favorable conditions for the completion of the chemical reactions of the cement. Proper curing results in improved strength. Normally, a period of 24 hrs. is suggested to permit proper strength development before starting air drying or application of heat.
The term ‘dry out’ refers to the process of drying the cured refractory material by the use of heat. The process is very important to assure that the refractory reaches its full strength. Unlike the curing of refractory, which is done right after the installation, dry out is done later with no set time limit for when it has to be done. However, a phosphate-bonded material must be cured and dried at the same time and within two to three weeks after its installation. This is because the material will begin to absorb moisture from the surrounding atmosphere. Eventually, over a period of two or three weeks, the material will begin to slump and fall off.
If time permits, new linings should be heated gradually to let the moisture escape and reduce internal stresses. First, heat should be applied and raised at 75° F intervals and held at 250-400° F. A periodic raising of the temperature and holding it at the higher temperatures should follow.
The following heating schedule is of a general nature for ideal conditions:
Refractory thickness up to 9 in.:
- 8 hrs. hold at 250- 400° F.
- 8 hrs. raise at 100 °F/hr. to 1050-1200° F.
- 8 hrs. hold.
- Raise temperature at 150 °F/hr. to operating temperatures.
Refractory thickness more than 9 in.: - 16 hrs. hold at 250 - 400° F.
- 8 hrs. raise at 100 °F/hr. to 1050-1200° F.
- 8 hrs. hold.
- 8 hrs. raise at 100 °F/hr to 1850-2000° F
- 8 hrs. hold.
- Raise temperature at 150 °F/hr. to operating temperatures.
The rule of thumb is that hold time is based on the greatest refractory lining thickness found on the entire project. If the thickest area is 4 in. thick, then the hold time is 4 hrs. — not 8 hrs. as given in the above example.
In many applications, conservative heating rates can be followed without great penalty. But in some cases, such rates are uneconomical from a production standpoint. Each material has its own allowable deviation from conservative heating schedules. Check with the refractory manufacturer for a compromise between safe heating rates and operating costs.
Health and Safety
Brick — During brick installation silica dust is created by the use of power saws when cutting the bricks. Silica dust is a serious, and potentially fatal, health threat. To prevent this, one should use wet saws wherever possible to cut the brick, as this will decrease the dust generated. Also, respirators or air masks should be used, and in some cases where a lot of brick is being cut, exhaust fans will need to be installed.
Ear and eye protection should always be used because of exposure to high noise levels from the power saws and from flying particles. Again, use wet saws wherever possible when cutting brick. This will cut down on almost 95 percent of the dust. The high level of noise made from saws must be addressed. Wet cut saws generate 112-114 dB. Without proper ear protection, serious damage can occur when exposed to such noise levels for even moderate lengths of time. Also, respirators or air masks should always be used.
Refractory — Many medium weight refractory products contain crystalline silica. Crystalline silica, when converted to dust (when cutting brick), presents a potential health hazard if inhaled over a period of years. It is recommended by most refractory manufacturers that when mixing refractory or when installing crystalline silica refractory in a gun application that you do the following:
- Provide training, education, and equipment to any personnel who will or may be in contact with the refractory.
- Provide proper monitoring of the removal process.
- Wear proper masks to prevent inhaling the refractory dust.
- Wear proper protective suits to prevent the refractory dust from coming into contact with the skin.
- Dispose of the refractory material in accordance with EPA regulations for the disposal of Group 1 Toxic Substances.
Ceramic Fiber — Refractory ceramic fiber (RCF) insulation is commonly found in iron and steel foundries (i.e., between the roof and an electric arc furnace). Most RCF products have been classified by the Seventh Annual Report on Carcinogens as products with sufficient evidence for the carcinogenicity of ceramic fibers in experimental animals (IARC V.43, 1988). However, no data have been made available on the carcinogenicity of ceramic fibers to humans (IARC V.43, 1988).
What this means is that ceramic fiber insulation must be handled with care. The ceramic fibers in the product are extremely sharp and can cause skin and upper respiratory irritation. Skin irritation may result if the broken ends of the ceramic fibers become embedded in the skin. The upper respiratory irritation is a reaction your body has to the sharp ends of the broken fibers. Whether you are installing or removing refractory ceramic fiber insulation, avoid tearing or ripping any of the insulation by hand. The insulation material should be cut with a sharp knife. Use extreme caution at all times, especially if fibers accumulate on the skin. Do not rub or scratch. Never remove fibers from the skin by blowing with compressed air. If the fibers can be seen penetrating the skin, they may be removed by applying, then removing, adhesive tape. The fibers will adhere to the tape as it is pulled out of the skin with the least amount of harm to the body.
To prevent the skin and respiratory irritation you should do the following when handling or removing ceramic fiber and refractory materials:
- Wear long-sleeved clothing and gloves.
- Wear head and eye protection, including respirator or masks to prevent inhaling dust.
- Wash any exposed skin surface with soap and water after handling the ceramic fiber material.
- Wash RCF-soiled clothing frequently and separately from other clothing.
- Refrain from smoking, eating or drinking while working near RCF.
- Keep RCF work areas clean to prevent accumulation of debris on the floor surface.
- Use High Efficiency Particulate Air filters (HEPA) for clean-up tasks. If the use of HEPA filters is not feasible, wet sweep or use dust-suppressing compounds.
- Refrain from using compressed air to clean work clothes and other contaminated surfaces.
- Refrain from using power tools to cut or drill RCF products.
Conclusion
Compared with most components found in iron and steel foundries, the cost of materials and installation of brick and refractory are relatively small. Yet, brick and refractory are arguably the most important component found in the iron and steel foundry, because if improperly designed, specified, stored, installed, cured or dried, brick and refractory will have an adverse affect upon your energy usage and operation. So remember, even though brick and refractory may be one of the smallest of components at your facility, it pays to be attentive to your brick and refractory practices.