REFRACTORIES REVIEW<br>Refractories Technology and Engineering
November 29, 2002
The last decade has been especially active for the refractory and affiliated industries, with many important technical and engineering improvements being developed. Following are a few examples of innovations in refractories that provide a brief glimpse of the current activities, past successes and future needs of this vital and challenging industry.
Materials Technology Magnesia-carbon refractory bricks and shapes are highly sophisticated composites containing magnesia (natural or synthetic), carbon/graphite, high residual carbon organics, powdered metal(s) and other proprietary additives. Technical knowledge of these refractories has reached a level where they can be made on a prescription basis specifically for the application conditions involved. Based on the success of these bricks, active work is under way to develop magnesia-carbon castables, which will allow faster installation and service life equivalent to or better than the bricks.
Although there still may be skeptics, it is now possible to make and install castable refractories with properties that are better than pressed/fired bricks. The rebound rate (material loss during shotcasting installation) can be as low as 2%, compared with 10% or more for traditional gunning. Advanced castables (low-, ultra-low and no-cement types) and other monolithic refractories, used in conjunction with continually improving installation methods, are now the dominant refractory type in most of the industrialized countries, surpassing bricks in terms of the tonnage used.
The permeability of castables can now be measured at high temperatures, which will allow the development of dryout schedules specific to each castable, rather than the traditional generic schedules. This change will help eliminate one of the major problems for castable installations: cracking, spalling and premature failure caused by improper curing/dryout. Additionally, microwave-assisted dryout of pre-cast shapes is now being done; field trials have shown up to a 50% increase in service life for microwave-dried castable shapes.
Knowledge and control of particle sizing has increased greatly, and software is available to help achieve optimized formulations. Mixing capabilities have also improved—equipment is available that can uniformly disperse additives present in very small quantities (< 2 wt%).
Engineering Advances It has become standard practice throughout industry to use finite element analysis (FEA) and/or simulation modeling to guide refractory development and product selection, optimize lining design, quickly evaluate “what-if” scenarios and interpret premature lining deterioration/failure. The validity of such analyses will continue to increase as more experience is gained in correlating mathematical predictions with actual field performance. Also, there are numerous applications where thermographic and laser-scanning measurements are done routinely to show the refractory lining condition inside process vessels.
In brick manufacturing, fully automated robotic techniques can be used to insure consistent, high-quality production that meets or exceeds customers’ specifications. Sonic and laser measurements, as well as automated weighing, can be used to document the quality of a high percentage of the production lot, and the movement/stacking of bricks throughout the process—from pressing to final storage—can be done entirely by robots.
I recently observed the automated relining of the upper 80 feet of a blast furnace (stack) after seven years of life. A suspended robotic spray unit, monitored from a computerized mobile control room, was used. The blast furnace was placed on hot standby, and the bed was lowered to the mantle/bosh level. The lining was cleaned with a 7000 psi water jet, and the remnant lining profile was measured with a laser scanner. These data were used for computer control of the robotic spray unit to install the needed thickness of castable, between roughly 2-30 in., in all regions of the lining.
A total of 440 tons of colloidal silica bonded castable (two types) was installed in less than 48 hours, followed by a 24 hour dryout, using three high velocity gas burners. The repair returned the stack lining to the original design profile. This lining repair is expected to provide 18-24 months of maintenance-free life. It is envisioned that future blast furnace stack linings (and many other applications) can be installed and periodically repaired whenever needed by spraying castable, which is much faster than bricking.
Summary Technology and engineering continue to play a major role in the advancement of refractories. As the sophistication levels continue to increase, aided by the incorporation of more input from other fields, the potential for continued advancement will be enhanced. No doubt there will be many more exciting new materials and engineering innovations in the coming years.