Considering Refractories

Choosing a brick or refractory for your metalcasting operation sounds like it should be easy. But, overlooking the many details of refractory design and handling could result in unnecessary expenses, and possibly shutdowns. To help reduce these negative consequences, it is imperative that metalcasters be aware of the pitfalls and precautions associated with brick and refractory usage.

Typecasting

There are generally three types of refractory bricks:

• Silica brick: An acid-resistant brick with high resistance to thermal shock.
• High-alumina brick: Firebrick made with 50-90% alumina that has a high resistance to spalling caused by quick temperature changes.
• Basic brick: A general term used for brick made from magnesia. Basic brick typically has high resistance to thermal spalling and good mechanical strength.

  In the case of refractories there are either plastic or castable refractories. Among the plastic refractories there are five basic categories (high-duty fireclay, super-duty fireclay, 60% alumina, 70% alumina, and 80% alumina). Castable refractories are either alumina/silica-based or an insulating type of refractory.

  
  Mixing it up

  Preventing problems with refractory performance often centers on what occurs during the mixing phase. Just the right amount of water is imperative. Although a very wet mix may be easier to handle it decreases the refractory’s strength, while at the same time a mix that is too dry may be difficult to place.

  Another potential problem is contamination of the mix. Exposure to certain salts may cause reactions rendering the refractory useless. Using clean, potable water, clean mixing and handling equipment, and clean forms should minimize the possibility of contamination.

  Maintaining the correct temperature during mixing is another key. For maximum strength, the water and the dry material should be in the 60-70°F range, and air temperature should be 50°F. In fact, refractory manufacturers recommend that newly installed refractory should be protected against freezing until completely dried.

  
  Installation concerns

  Before deciding on the best mixing method, however, it helps to know how the refractory will be installed. For example, the gunning method allows the handling of a large volume of materials. In addition, it is a quick and versatile method of installation. However, water is added at the discharge nozzle or back at the charging chamber and the amount of water depends on the refractory’s characteristics. Troweling is done by hand and normally used for applying a thin lining. Enough water needs to be added to make the material sticky and stay in place. The ramming method is also “by hand”, and used for plastic refractory materials that require no water. The cast method is done by making the area completely watertight by using forms. Thus, the mix would contain more water than in a trowel application mix. Vibratory equipment can be used to remove internal air bubbles by allowing the material to flow freely and completely fill the form or box. Vibrating also works best with a stiffer mix.

  
  An ounce of prevention

  Adhering to the recommendations for mixing and installation could help prevent brick and refractory failure down the line. The two biggest causes of failure are ash and slag. Oil ash, for example, is a residual product that develops after burning fuel oil. Even the smallest amount of oil ash can cause major problems for refractories. Coal ash is also very corrosive to refractory materials.

  ‘Slagging’ is the term for the formation of molten, partially fused, or resolidified deposits (ash). Slag deposits can lower heat absorption, raise exit-gas temperature, and cause problems with ash removal. Air and fuel imbalances can cause slagging, particularly if there is a high iron content in the fuel or materials being burned or heated.

  Another case of refractory failure is spalling, a term that refers to the loss of fragments or spalls from the face of a brick or refractory through cracking and rupture, resulting in exposure of inner portions of the original structure. Thermal spalling occurs when there are rapid fluctuations in temperature that can cause unequal rates of expansion or contraction. Mechanical spalling is caused by changes in the physical structure normally caused by some outside force or object. Another type of spalling, chemical spalling, occurs by a reaction of one or more of the chemical or mineral components of the refractory to the chemical or mineral components of the fuel oil, fuel-oil ash, or coal ash.

  
  Safety issues

  Exposure to silica dust and other carcinogens remains the number-one safety concern when using refractories. Silica dust causes serious, and potentially fatal, health problems. Silica dust exposure is a particularly hazard during brick installation thanks to the dust produced during cutting of the bricks with power saws. Using wet saws minimizes this, but respirators and/or air masks are a must for employees.

  Other refractory products contain crystalline silica. To minimize the risk of exposure during installation employers need to make sure to provide adequate training, equipment-including protective suits to prevent dust from coming into contact with the skin, and disposal procedures that are in compliance with EPA regulations.

  Refractory ceramic-fiber insulation, often used in iron and steel foundries, is thought by many to have carcinogenic properties. Although there is no conclusive proof of this, there is evidence of a considerable threat so extreme care must be exercised when handling ceramic-fiber insulation. Ceramic fibers are very sharp and may cause skin and upper respiratory irritation. To minimize these risks, it is recommended to avoid ripping the insulation by hand. Wearing long sleeves and gloves, as well as head and eye protection such as a respirator or mask, should help reduce unwanted exposures.

  
  
  Extending Refractory Life
  Porous plugs are one way to improve refractory service life. The porous plug allows gas to pass with minimal resistance and permits close control of gas-flow rates. Due to the small average pore or slit opening, the refractory allows an absolute minimum of molten-metal penetration. Thus, there is almost no metal buildup in the refractory structure, which improves long-term efficiency. The resistance of the porous plugs to erosion and thermal shock also extends service life.

  However, for the porous plugs to work strict care and maintenance is critical. When the porous plug is used in a desulphur ladle, all gas flow must be up to operating pressure to prevent molten metal clogging upon iron introduction. Furthermore all pressure must remain on during the draining of the ladle to ensure no iron penetration into the porous plug. These steps must be followed continually to prevent buildup of desulphur agents that can damage porous plugs when the ladle cools.

  Never chip solidified iron from the surface of the porous plug, as it can cause refractory fracture. This would allow gas to flow in many directions, causing low pressure and the possibility of refractory clogging. In addition, buildup on the upper sidewalls should be monitored to prevent uneven gas flow and minimization of desulphur agents.
  
  Outlook for the Refractories Market
  According to a recent survey of members by The Refractories Institute (TRI), 2004 was a good year for refractories producers. A majority of respondents (82%) reported sales increased by at least 5%. And, there is more good news on the horizon. Growth in several areas around the world should promote refractory demand. The Brazilian foundry industry has begun to invest again after many negative or flat years. In fact, through most of last year Brazilian metalcasters were struggling to meet increased demands, especially from the automotive industry, suggesting that production levels should remain high for some time. Naturally, all eyes are on Asia, where refractory production has increased 51% since 2000, a figure mainly attributed to the refractories used by Chinese steelmakers, whose refractory consumption rate more than doubles their U.S. counterparts.
  
  Refractory Care Tips
  1. Carefully follow refractory manufacturers’ installation and curing instructions. 2. Check slag chemistry for compatibility with refractory lining. 3. Control furnace atmosphere to reduce oxidation. 4. Keep slag accumulation as low as possible and skim frequently. 5. Watch bath temperature carefully. Low temperatures may create buildup; high temperatures may result in excessive wear. 6. Keep records of lining life. Monitor vessel shell temperatures with infrared cameras or magnetic thermometers. Compare readings to refractory wear profiles during repairs to understand wear/shell temperature relationships. 7. Consider zoning linings. For example, install 90% alumina refractory in bottoms and moderate wear areas, aluminum chrome at the slag line, and mullite in the roof.