Silica Refractories Research Articles

Thermal cycling damage of silica refractories for high-temperature thermal energy storage (HT-TES) – Can it be healed?

Silica refractories are promising materials for high-temperature thermal energy storage (HT-TES), because they exhibit excellent thermal cycling properties, unless cooled below a critical temperature (usually assumed to be 600 °C). When cooling down to room temperature severe damage can occur, which can be conveniently monitored via the impulse excitation technique (IET). This damage is most severe when the cycling maximum temperature is low. The question is whether this damage can be healed again. In this short contribution we show that severe damage in silica refractories, caused by heating from room temperature to 300 °C and back again to room temperature, can indeed be healed by thermal cycling to 1300 °C. The healed material is actually better than the pristine material.

Assessment of Microsilica as a Raw Material for Obtaining Mullite–Silica Refractories

The possibility of using microsilica in the production of mullite–silica refractories was assessed. The chemical and mineralogical compositions of the raw materials, refractory Arkalyk clay and microsilica, were studied. It has been found that primary mullite and quartz formation occurs due to dehydration of kaolinite with the formation of intermediate metakaolinite. The introduction of alumina and microsilica into the charge composition promotes the formation of secondary mullite due to the interaction of aluminum oxide and highly dispersed chemically active microsilica. Free silica in compositions undergoes polymorphic transformations with the formation of cristobalite and tridymite. Mullite–silica refractories with an open porosity of 21%, a compressive strength of 42 MPa, and a thermal deformation temperature under the load of 0.2 MPa–1350 °C were obtained.

Refractory materials for construction based on quartzite

Information about the presence and characteristics of existing deposits of high silica and refractory clay raw materials of the Republic of Uzbekistan is given. It is shown that the main raw materials for the production of silica refractories are veined quartz, quartzites, quartz sands containing not less than 92 wt.% 8i02, and also for packing masses kaolin clays are used, which are promising raw materials. It is established that the studied quartzite rocks are of great interest and are a promising raw material base for the refractory industry of the Republic of Uzbekistan.

Evaluation of thermo mechanical and corrosion properties of uniaxially pressed refractory blocks used in the transition zone of cement rotary kilns

Alkali resistance is of great importance for the refractories used in the transition zone of cement rotary kiln. In this present work, Andalusite were selected to replace the bauxite in refractories used for transition zone of cement rotary kiln. The effect of Andalusite is enhanced by incorporating micro silica. The originality of the work is that it brings a new platform of using Andalusite incorporated with micro silica for cement rotary kiln application. The thermo mechanical properties like Permanent Linear Change, Refractoriness Under Load, Thermal Shock Resistance, abrasion resistance and alkali resistance of the developed andalusite incorporated with micro silica refractories was compared with the existing bauxite and SiC based. The results confirm that andalusite is a better refractory material and also reveals that micro-silica incorporated andalusite refractories have excellent alkali resistance against K2CO3 / K2SO4 at 1250 °C for 24 h and also it can able to withstand a temperature of 1444 °C. Addition of micro-silica up to 1% forms glassy phase and increases the compressive strength and also improves densification than other refractories. The andalusite – micro silica composite refractories manages to survive 30 + cycles on water quenching from 950 °C. 1% addition of micro silica in andalusite shows better alkali resistance, abrasion resistance and refractoriness under load.

Analysis of the Quasi-Static and Dynamic Fracture of the Silica Refractory Using the Mesoscale Discrete Element Modelling.

Computer modelling is a key tool in the optimisation and development of ceramic refractories utilised as insulation in high-temperature industrial furnaces and reactors. The paper is devoted to the mesoscale computer modelling of silica refractories using the method of homogeneously deformable discrete elements. Approaches to determine the local mechanical properties of the constituents from the global experimental failure parameters and respective crack trajectories are considered. Simulations of the uniaxial compressive and tensile failure in a wide range of quasi-static and dynamic loading rates (102 s−1) are performed. The upper limit of the dynamic loading rates corresponds to the most severe loading rates during the scrap loading on the refractory lining. The dependence of the strength, fracture energy, and brittleness at failure on the loading rate is analysed. The model illustrates that an increase in the loading rate is accompanied by a significant change in the mechanical response of the refractory, including a decrease in the brittleness at failure, a more dispersed failure process, and a higher fraction of the large grain failure. The variation of the grain–matrix interface’s strength has a higher impact on the static compressive than on the static tensile properties of the material, while the material’s dynamic tensile properties are more sensitive to the interface strength than the dynamic compressive properties.

Development of a quality control system for the supply of silica refractory material from China

This document describes how to perform a quality control management of refractory silica material purchased in China. Silica refractories are key actors in the lifetime of the equipment and they are fundamental to ensure the adequate operation of the plant. The objective is to plan the sampling, minimising the number of tests to obtain an assessment of the quality of the consignment with the greatest precision. The types of destructive and non-destructive tests, the number of pieces to be inspected per batch and the acceptance criteria will be defined in this article. The quality control will be carried out at three levels, one to be performed by the manufacturer and the others by the buyer and the final client. The approval of the lots by the three parties ensures that the quality of the product is in accordance with the specifications. It will be achieved by dividing the order into different batches to treat them as independent units for acceptance or rejection. To avoid the loss of “see freight time” in case of rejection of any consignment, the customer’s quality control must be performed before shipping the batches.

Failure of refractory masonry material under monotonic and cyclic loading – Crack propagation analysis

Refractory masonry (refractories) is exposed to in-service loads of different types. To rationalise the masonry design and failure analysis, differences of failure under cyclic and monotonic loading were studied. For samples of silica refractories tested in wedge splitting set-up global failure parameters and crack trajectories were assessed. Under cyclic loading, higher fracture energy and lower brittleness at failure were seen. Cracks of different modes had similar non-linearity and branching. However, the size and microstructural characteristics of the fracture process zone was different. In addition, higher energy dissipation during cyclic loading is promoted by repetitive friction events along the crack trajectory.

Role of fatigue in damage development of refractories under thermal shock loads of different intensity

Refractories insulation of industrial furnaces often fail under repetitive thermal shock. Degradation of silica refractories under thermal shock loads of different intensity was studied. The load variation was achieved by utilisation of geometrically similar samples of different dimensions. Finite element method modelling predicted loads developing during the test. Resulting damage was determined by the ultrasound velocity and crack patterns. Tests involving up to 150 cycles demonstrated the role of fatigue in enabling sub-critical crack formation and countering the crack arrest. Repetitive cycles reduce crack wake friction and intensify loading due to crack debris re-location. Damage saturation, sigmoidal and near-exponential damage growth was typical for low, intermediate and high loads, respectively. Similar trends of damage accumulation were observed in mechanical displacement controlled cyclic fatigue tests performed in wedge splitting set-up. Strain and strain energy based criteria of thermal shock intensity seem to have complimentary value in predicting the crack formation and growth. Thermal shock damage after the first cycle seems to be an effective parameter to predict overall resistance to the degradation in the sample. Load reduction due to previous crack formation related to the fatigue potential for subsequent crack development can explain the crack size variation typically observed in refractories after multiple thermal shocks. For thermal shock tests, the variation of sample size, instead of the temperature interval, is a suitable alternative for refractories with strongly temperature dependant material properties.

Thermal and mechanical cyclic tests and fracture mechanics parameters as indicators of thermal shock resistance – case study on silica refractories

The paper discusses alternative methods to assess thermal shock in refractories. Thermal shock performance of two conventional silica bricks and two novice fused silica materials has been studied. Methods involving fracture mechanical tests of monotonic loading, repetitive thermal shock tests and cyclic strain controlled fatigue tests have been utilised. The amorphous fused silica is rather brittle. However low thermal expansion guarantees its superior thermal shock resistance. In-service crystallisation is to happen rather quickly. For crystallised fused silica of the studied morphology the tests coherently predict superior thermal shock resistance due to less brittle failure. Strain controlled fatigue test allows assessment of strain limits in the conditions of cyclic loading and thus combines the benefits of the thermal shock and monotonic fracture mechanics tests. Strain tolerance seems to be the property to correlate the results of the alternative test methods and those with the service loads.

Effect of the phase transformation on fracture behaviour of fused silica refractories

In this paper, the influence of phase transformation on the properties and fracture behaviour of fused silica refractory was investigated. The virgin fused silica refractory is amorphous, and possible failure is attributed to the propagation of a single crack in the structure. Due to the crystallization and phase transformation of low-/high- temperature cristobalite subpolymorphs occurring during the heat treatment, microcracks are formed especially in the matrix and at the grain boundary. This microcracking enables the development of sizable fracture process zone, which is responsible for the increase of specific fracture energy even with the decrease of strength. Therefore, the heat-treated specimens exhibit lower brittleness and higher strain tolerance before failure compared with the virgin fused silica refractory. All of these properties represent a better thermal shock resistance. Furthermore, microcracking causes a characteristic temperature dependence of Young’s Modulus due to phase transformation and partial crack closure at increased temperatures.

Temperature dependence of damping in silica refractories measured via the impulse excitation technique

Using high-temperature impulse excitation the temperature dependence of damping, quantified by the so-called inverse quality factor, has been measured for silica refractories with porosities of 13 − 17% up to 1200 °C. Damping-temperature curves exhibit closed loops between heating and cooling and do not show any indication of damage accumulation after two complete heating-cooling cycles. The temperature dependence of damping exhibits features that are due to an interplay between phase transitions (between the subpolymorphs of tridymite and cristobalite) and microstructure (reversible closure re-opening of preexisting microcracks) and are also known from resonant frequency measurements. Additionally, however, the temperature dependence of damping reveals some details that are undetectable by resonant frequency measurements and may tentatively be attributed to the competition between microcrack closure and increased dry friction (damping peak at approximately 325 °C) and to a phase transition of the dicalcium silicate phase shannonite (damping increase in the range 700–800 °C).

Preparation and characterization of clay bonded high strength silica refractory by utilizing agriculture waste

Clay bonded silica refractory was prepared by utilizing agriculture waste called rice husk ash (RHA) and refractory grog. Various samples were prepared with different compositions based upon partial replacement of quartz by RHA. Rectangular samples were prepared by following semi dry process prior to pressing in a uniaxial hydraulic press and sintering at a temperature of 1200°C in air atmosphere. Various physical, mechanical and thermal characterizations were done like X-ray diffraction (XRD), scanning electron microscope (SEM), apparent porosity (AP), bulk density (BD), cold crushing strength (CCS), refractoriness and thermal conductivity measurement. The sample utilizing 30% of RHA was considered most optimum composition which produced cold crushing strength of 38MPa and thermal conductivity of 2.08W/mK at 800°C with a considerable good refractoriness. Enhancement in the mechanical as well as thermal properties may be considered as attributed to the amorphous silica which has reacted more easily and efficiently with other material surrounding giving rise to the densification and produced stable crystalline phase to the refractory material. These promising characteristics suggests that the RHA may lead to be used as a potential material for the preparation of clay bonded high strength silica refractories.

Cyclic fatigue of silica refractories – effect of test method on failure process

Silica refractories serving in high temperature industrial installations fail due to thermo-mechanical cyclic loads. The failure process was investigated by performing cyclic fatigue tests of several methods. In uni-axial compression the samples were tested either with constant force or displacement amplitude or with fixed upper displacement limit. In bending constant displacement amplitude tests were done. The investigation was supported by monotonic loading tests and microstructural analysis. It was determined that the fatigue failure occurs due to the degradation of interlocking in the crack wake. Cracking of larger grains is important for the crack initiation. The test set-up and the loading procedures significantly influence the potential to resist the crack propagation. Cyclic loading produces less brittle failure than monotonic loading. The displacement controlled method allows more gradual, less brittle, failure than the force controlled method. The potential of the crack arrest is less developed in bending than in compression.

Refractories for Coke Oven Wall – Operator’s Perspective

After a successful campaign of more than 40 years, the walls of Coke Plant 2 of Tata IJmuiden are being replaced. Our paper addresses the issues of the selection and sourcing of the refractory materials for the new walls. Material properties formalised in the specification are reviewed. Those are compared with the properties of alternative silica refractories available on the market. Both conventional silica bricks and relatively new fused silica blocks are evaluated. The discussion is based on thermo-physical data obtained in-house and from plant experience. It is argued that, due to specific lining design and material properties, the coke oven wall refractories are not off-the shelf products that can be sourced on a short notice from any supplier. An extensive pre-order effort is mandatory before the acceptable material and supplier are selected and the order is placed.

High-temperature Young’s moduli and dilatation behavior of silica refractories

Silica refractories with cristobalite–tridymite ratios between 0.83–0.86 and 1.43–1.47 and porosities of 13–17% are characterized by impulse excitation up to 1200°C and by dilatometry up to 1300°C. During heating, Young’s moduli start to decrease from their room temperature values (9–12GPa) to about 5–7GPa at 200°C, followed by a very steep increase at around 230°C to values higher than the room temperature values and a nonlinear increase to their final high-temperature values. During cooling, Young’s moduli increase even further, exhibit a maximum and a nonlinear decrease that does not follow the heating curve, a very steep decrease at around 200°C and a final increase to the intial room temperature values, so that a closed loop results during thermal cycling. Dilatometric measurements confirm that the strong increase of the Young’s modulus at around 230°C (during heating) can be attributed to the phase transition between low- and high-temperature cristobalite, while the more gentle—but still very significant—changes in Young’s moduli below 200°C are due to the more diffuse transitions between tridymite subpolymorphs. Microcracks are identified as the primary cause of the low Young’s moduli and their increase with temperature.

Temperature dependence of Young׳s modulus of silica refractories

The temperature dependence of Young׳s modulus of silica refractories exhibiting pore volume fractions in the range 18.6–20.9% is investigated via impulse excitation up to 800–1000°C and four-point bending up to 500°C. This temperature dependence exhibits a broad valley approximately below 200°C, where Young׳s modulus values decrease to 55–62% of their room temperature magnitude. Upon further heating, Young׳s modulus increases steeply within a few degrees around 230°C, followed by an S-shaped increase to maximum values corresponding to 174–282% of the room temperature values. Cooling branches are different from heating branches and do not form closed loops, indicating damage accumulation. Dilatation measurements, polarization microscopy, micromechanical calculations and X-ray phase analysis are used to underpin the conclusion that the observed elastic effects are not caused by phase transitions alone, but are combined effects of phase transitions and microcrack opening and partial closure.

Corrosion of refractory materials in fluidised bed gasification of alkali rich fuels

Gasification of renewable fuels is not common practice due to the high costs of technologies and the absence of reliably working refractories. Refractory degradation is of such high significance that improved refractory durability was ranked first by industry experts in a list of 20 research and development areas related to the economic viability of gasification. Therefore, for improvement of the reliability and durability of refractory linings, this work is dealing with the corrosion resistance of nine commercial refractories to a variety of emissions from potential fuels. The refractories were exposed to a gasifier-like, water vapour and alkali rich atmosphere. Exposures with a duration of 250 h produced corrosion effects that were investigated by scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. Furthermore, thermodynamic calculations were included to further explain the equilibrium chemistry. The results show that extremely low silica refractories are promising candidates for gasifier utilisation.

Waste catalyst as raw material in alumina–silica refractories

This work aims for the development of methodology for the utilization of the waste catalyst from hydrocarbons catalytic cracking process in the production of alumina–silica refractories. This residue was used in substitution of part of conventional raw materials, with the consequent ambient and economic profit. Some formulations were selected based on the chemical and mineralogical compositions of refractory bodies. They were fired in similar temperatures to those used for the production of commercial refractories. The analysis of the technological properties of the bodies revealed that the composition containing 15 wt% of catalyst waste had suitable characteristics. The final product showed technical parameters similar to that of a commercial refractory material.

Silica crown refractory corrosion in glass melting furnaces

The critical parameters of silica refractories, such as compressive strength, bulk, density, quantity of silica, microstructure and porosity were evaluated of unused and used bricks to line the crowns of glass furnaces, when the rate of corrosion of crowns were about 2 times greater. The change of these parameters, the chemical composition and formation of the microcracks in the used silica refractories material were studied. It was established that the short time at service of container glass furnace crown can be related to low quality of silica brick: high quantity of CaO and impurities, low quantity of silica, low quantity of silica, transferred to tridymite and cristobalite and formation of 5-10 ?m and more than 100 ?m cracks in the crown material. The main reason of corrosion high quality silica bricks used to line the crown of electrovacuum glass furnace is the multiple cyclic change of crown temperature at 1405 - 1430?C range in the initial zone of crown and at 1575 - 1605?C range in the zone of highest temperatures.

Determination of SiO<sub>2</sub> Content in Boron-Containing Silica Refractories by Hydrofluoric Acid Gravimetry

The co-volatilization of boron and silicon has been studied for determination of SiO2 content in boron-containing silica refractories. The co-volatilization of boron may cause positive interference for determining of SiO2 content. The interference can be eliminated by the volatilization of boron with 5mL and 2mL of ethanol by separately volatizing, before volatilization of SiO2 with hydrofluoric acid. The method for determination of SiO2 content in boron-containing silica refractories has been established. The scope and precision of this method are equaled to that of the conventional method by hydrofluoric acid gravimetry.