Like any other sector, the refractory industry is developing to become more environmentally conscious. New efforts and technologies are driving this mentality in the sector. However, there is always room for improvement. We are going to go over some of the major technology improvements in the refractory industry.
Environmental issues
Refractory materials can in general be considered to be ‘energy-concentrated’ materials (e.g. fusion-cast materials, production of high-fired bricks such as direct-bonded bricks). The trend towards the use of more monolithic materials and fewer high-fired bricks contributes to energy conservation in refractory production. However, the entire cycle, covering the production of the refractory material, as well as its application, must be considered, as highly energy-consuming refractory materials may be preferred if they save energy in their application. Refractory materials have, after removal from service, historically been landfilled. Change in the refractory industry has been driven strongly in terms of energy conservation and environmental protection. Examples thereof include: reduction in energy consumption (development of binders that lead to high strength when refractories are fired at low temperatures, the formation of spinel grain through mechanical alloying), reduction in emissions (e.g. change of air-fuelled firing technology to oxy-fuelled firing technology, whereby NOx emissions are reduced), development of non-toxic and environmentally friendly resins for carbon-containing refractory materials, and the replacement of coal tar pitches for impregnation by other benzo-α-pyrene (BaP) free carbonaceous agents. Recycling is expected to become a key factor in saving energy and resources, protecting environments, addressing the raw materials shortage and soaring raw material prices in the industry. The recycling of spent refractory materials is considered to be a matter of survival that will enable the industry to achieve an environmentally and socially sustainable way of doing business.
carbon-containing castables
Because LCCs and ULCCs consist of oxides, they suffer the same shortcomings as oxide-based refractory bricks, e.g. Al2O3-based castables have poor corrosion resistance (specifically in basic slag environments) and spalling resistance, and MgO-based castables have poor slag penetration and spalling resistance. The improvement of the properties of oxide bricks through the addition of carbon suggested that graphite addition to oxide-based castables would improve properties. However, problems arose as graphite has poor aqueous wettability and dispersion, Albased antioxidants have a tendency to hydrate, and they have lower strength after curing and drying. Research on C-containing castables for iron and steelmaking applications was initiated by CIREP (Centre for Industrial Refractories at École Polytechnique, Montréal) in 1996. The commercialization of carbon-containing castables has been hindered by poor water-wettability and dispersion ability of graphite, which result in castables that flow poorly, requiring a high water content for placement, which leads to high porosity after drying, and subsequent lowering of mechanical strength and corrosion resistance. To overcome these problems, the use of surfactants or dispersants, micropellets of graphite and coating techniques concerning the graphite have been examined. The coatings can be oxides such as Al2O3, TiO2, SiO2, MgO, ZrO2 or carbides such as SiC, TiC. Different techniques through which these coatings can be produced include mechanical impact treatment, sol-gel and molten salt synthesis. The main requirement is the formation of a uniform, crack-free, thick and strong bonded coating. It was found that antioxidants play multifunctional roles in MgO-C castables: they protect carbon against oxidation, enhance hot strength, and play a role in the formation and deposition of secondary carbon, which improves slag penetration resistance. Trials have already been conducted in Brazil with a developed MgO-C self-flow castable for hot repairing of the BOF converter and steel ladle slag lines. This material is reported to be compatible with MgO-C bricks and basic steelmaking slags.
NANOTECHNOLOGY
Nanotechnology aims to achieve enhanced material properties and functionality by dealing with matter on the atomic and molecular scale. The first papers on nanotechnology and refractories that appeared during UNITECR 2003 created widespread interest. These papers addressed the formation of a nanostructured matrix in MgO-C bricks. At UNITECR 2007 the papers on nanotechnology expanded to alumina-based refractory materials, ZrO2-C materials and the production of nano-size MgAl2O4 particles. A variety of nanoscale materials are already used or have the potential to be used in refractory products. These include nanoscale carbon black (pure elemental carbon in the form of nanoscale particles with a semi amorphous molecular structure), carbon nanotubes (long seamless cylinders of one-atom-thick layers of graphite with diameters of a few nanometres), metallic nanoscale materials (for use as antioxidants), colloidal (nanoscale) silica (which has been used in the refractories industry for many years) whereby a nanostructured matrix is produced. Technology also exists for the production of almost any oxide, such as MgO, Al2O3, ZrO2, Cr2O3 and spinel, on nanoscale. The development of nano - graphitized black containing MgO-C bricks means that lower levels of carbon can be used in these bricks with a reduction in the modulus of elasticity, improved thermal shock resistance, excellent corrosion resistance and good oxidation resistance. The use of highly reactive nanoparticles opens up a vast range of possibilities as sintering agents and participants in matrix phase formation through in situ reactions. Problems associated with these nanoscale materials are related to their cost, availability, handling, dispersion and mixing, as well as health and safety aspects associated with the handling of these very fine materials.
MgAl2O4-containing refractories
Due to their high refractoriness, mechanical strength, thermal shock and corrosion resistance MA (MgAl2O4), spinel containing refractories are increasingly finding new applications, and new types of spinel-containing refractories are being developed. It is anticipated that continuous improvement in their synthesis techniques will take place, but novel techniques such as mechanochemical alloying and molten salt synthesis may become significant methods whereby high quality spinels can be produced at lower temperatures and cost. An extensive research effort on Al2O3-MgO-based castables and bricks is currently also driven by FIRE (The Federation for International Refractory Research and Education).
Conclusions
The refractories industry has developed from a trial and error approach, using mainly natural materials, into a highly innovative industry, which uses composite oxide-carbon metal systems, based on innovative application methods. It has grown in response to developments in particularly the iron and steel industry. The driving force for change has been improved process technology and a desire for higher productivity via longer campaign lives, but increasingly also better use of energy, the need to protect the environment, as well as the reduction, reuse and recycling of refractory waste materials. The field of refractory materials has become very broad, and the technology increasingly sophisticated. The evolutionary process is far from complete, and will develop further through the continuous interchange of ideas between the research teams of refractory manufacturers, the users of refractory materials, the contribution from researchers at academic institutions and the development team of raw material and additive suppliers as well.
