Iranian Journal of Materials Science and Engineering

---

Improved structures of MgO carbon bonded materials due to new binder systems and due to the application of electrical currents during operation have been achieved for advanced applications in the secondary metallurgy and during near net shape metal casting.

---

Being brittle and having low thermal conductivity, refractories suffer damage and sometimes fail in service as a result of thermal shock. While the approach of those making fine-grained technical ceramics is to make their products sufficiently strong to withstand thermal stresses the refractory technologist is more cunning. He uses, often little known, devices to provide resistance to thermal shock that minimise but do not eliminate damage to the component. In this paper the basic equations of thermal conduction and elasticity are presented and followed by some immediate results that should guide the designer of components subject to severe thermal environments. The influence of size and shape of the refractory components is then discussed along with ways in which refractory producers can engineer the thermal and mechanical properties. In particular, the methods used to tailor fracture behavior to optimize the thermal shock resistance are treated in some detail.

---

Hydration behavior and antioxidising effect of aluminium (AI) powder has been investigated. Bayerite Al (OH) 3 product layers formed on Al in pure water at 25-45°C were porous, so the hydration rate, although very slow at 25°C, increased rapidly with increasing temperature from 25 to 45°C. On further increasing temperature from 45 to 95°C, initial hydration rate increased, but changed little over long hydration periods due to formation of denser and more continuous product layers. At 100?C, due to rapid water-evaporation, hydration product layers (composed of Al (OH)3 and a small amount of boehmite AlO (OH) became detached from the Al surfaces, so offering less protection, so that the hydration rate of Al increased markedly. The presence of MgO or calcium aluminate cement (CAC) in water did not change the hydration product, but greatly accelerate the hydration rate of AI. Addition of even a small amount (e.g. 0.25 wt% of Al amount) of MgO or CAC to water accelerated significantly the hydration of Al, and with increasing level of MgO or CAC, the hydration extent increased markedly. Sol-gel Si02 coatings on Al were useful in improving the hydration resistance of Al, and did not have a negative effect on the behavior of Al as an antioxidant.

---

Abstract: Dolomite based refractories are widely used in Iranian steelmaking plants. In the present research, wear and corrosion of refractories used in steel-making converter lining in Esfahan Steel Company was studied. Post-mortem analysis of refractories clarified that the wear started with oxidation of carbon followed by chemical corrosion. Iron oxide from slag reacted with calcia, resulting in formation of low melting phase, and subsequent washout process, caused the refractory corrosion onset from the hot face. In addition, the effect of aluminum as an anti-oxidant and graphite on the corrosion resistance of refractory was investigated. Tar-dolomite samples containing different amount of graphite (0, 4, 7, and 10 wt. %) were prepared in order to study their physical properties, before and after coking. SEM micrographs employed to analyze the microstructures to determine the effect of graphite and antioxidant on corrosion behavior of the refractory. Results showed that oxidation process of carbon in the system was hindered and improved corrosion resistance by introducing graphite and antioxidant into the refractory composition.

---

gasification (BBLG). One particularly harsh application is linings for gasifiers used in the treatment of black liquor (BL). Black liquor is a water solution of the non-cellulose portion of the wood (mainly lignin) and the spent pulping chemicals (Na2CO3, K2CO3, and Na2S). Development of new refractory materials for the black liquor gasification (BLG) application is a critical issue for implementation of this technology. FactSage® thermodynamic software was used to analyze the phases present in BL smelt and to predict the interaction of BL smelt with different refractory compounds. The modeling included prediction of the phases formed under the operating conditions of high temperature black liquor gasification (BLG) process. At the operating temperature of the BLG, FactSage® predicted that the water would evaporate from the BL and that the organic portion of BL would combust, leaving a black liquor smelt composed of sodium carbonate (70-75%), potassium carbonate (2-5%), and sodium sulfide (20-25%). Exposure of aluminosilicates to this smelt leads to significant corrosion due to formation of expansive phases with subsequent cracking and spalling. Oxides (ZrO2, CeO2, La2O3, Y2O3, Li2O, MgO and CaO) were determined to be resistant to black liquor smelt but non-oxides (SiC and Si3N4) would oxidize and dissolve in the smelt. The other candidates such as MgAl2O4 and BaAl2O4 were resistant to sodium carbonate but not to potassium carbonate. LiAlO2 was stable with both sodium carbonate and potassium carbonate. Candidate materials selected on the basis of the thermodynamic calculations are being tested by sessile drop test for corrosion resistance to molten black liquor smelt. Sessile drop testing has confirmed the thermodynamic predictions for Al2O3, CeO2, MgO and CaO. Sessile drop testing showed that the thermodynamic predictions were incorrect for ZrO2.

---

Abstract: Refractory materials containing cordierite (2MgO.2Al2O3.5SiO2) and mullite (3Al2O3.2SiO2) are used as support in furnaces, because of their low thermal expansion properties which confer them a very good ability to thermal shock resistance. Composed of two phases presenting very different CTE (1.5–3×10-6 for cordierite and 4–6×10-6 K-1 for mullite), these materials can develop damage during thermal cycling due to internal stresses. The resulting network of microcracks is well known to improved thermal shock resistance of materials, since it usually involves a significant decrease in their elastic properties. This paper is devoted to the characterisation of the damage generated by this CTE mismatch, thanks to the application of a specific ultrasonic device at high temperature.

---

Partially Lateritised Khondalite (PLK) rocks are the waste materials generated as a result of mining of bauxite. The major discolouring elemental impurity in the PLK is iron oxides, which render it unsuitable for its use as a refractory material. The iron can be removed by suitable beneficiation methods. The main aim of this present investigation is to prepare a value added material from the mining waste by preparation of PLK rock containing less than 2% Fe2O3 which finds application in the refractory industries and then preparation of brick suitable for industrial applications. The feed sample containing 4.31 % Fe2O3 subjected to hydrocyclone for refractory of low iron content product in the underflow. The results of these studies reveal that the hydrocyclone underflow sample contains 1.9% Fe2O3 is suitable for making bricks due to presence of low iron content. However, bricks are made from a feed sample as well as from hydrocyclone underflow and overflow products. Physical, chemical and thermo-mechanical properties of these bricks are evaluated. Mineralogical properties of these bricks are also correlated with the thermomechanical properties. The developed bricks are compared with the standards for their suitability in industrial applications. Hence the bauxite mining waste can be a value added material but not a waste material which at present
creates environmental pollution at the mining site.

---

In order to improve the corrosion resistance of aluminosilicate refractories by molten aluminum, alkaline fluoride NaF and cryolite Na3AlF6 powders were studied. Both physical and chemical properties are known to influence wetting and corrosion behavior. This paper devoted to determine the influence of alkaline fluoride and cryolite added to andalusite based castable on the reaction with aluminum alloys. These additives led to the in-situ formation of celsian phases within the refractory matrix that led to improved corrosion resistance at 1300°C. Phase analysis revealed that celsian formation suppressed the formation of mullite within refractories, thereby reducing Penetration

---

In this study the effect of nano meter size ZrO₂ particles on the microstructure, densification and hydration resistance of magnesite –dolomite refractories was investigated. 0, 2, 4, 6 and 8 wt. % ZrO₂ particles that were added to magnesite –dolomite refractories containing 35 wt. % CaO. The Hydration resistance was measured by change in the weight of specimens after 72 h at 25℃ and 95% relative humidity. The results showed with addition of nano meter size ZrO₂ particles_, the lattice constant of CaO increased, and the bulk density and hydration resistance of the specimens increased while apparent porosity decreased. With the addition of small amount ZrO₂ the formation of CaZrO₃ phase facilitated the sintering and the densification process. The mechanism of the nano meter size ZrO₂ particles promoting densification and hydration resistance is decreasing the amount of free CaO in the specimens.