Question 1

Applications of thorium oxide (thoria) as a refractory: Given that thoria (ThO2) is expensive and slightly radioactive, in which application is thorium oxide particularly used because of its exceptional refractoriness and purity?

Accepted Answer

Introduction / Context: Refractories must withstand high temperatures, thermal shock, and chemical attack. Thoria (ThO2) offers extremely high melting point and chemical stability, making it suitable for specialized, high-temperature applications where contamination must be minimized. Given Data / Assumptions: Thoria is costly and slightly radioactive. We seek the application that most benefits from its unique properties, justifying the cost and handling concerns. Concept / Approach: Thoria has very high refractoriness and low vapor pressure at elevated temperatures. These traits are critical when melting high-purity, reactive, or precious metals where crucible contamination would degrade product quality. General items like insulating bricks or muffles can use cheaper refractories (silica, alumina, mullite) without requiring thoria's premium performance. Step-by-Step Solution: Identify high-value application: contamination-sensitive metal melting in research or specialty metallurgy.Match material properties: ThO2 crucibles provide chemical inertness and temperature capability exceeding many alternatives.Conclude: thoria is well suited to crucibles for high-purity metals. Verification / Alternative check: Materials handbooks cite thoria’s use in crucibles and as an additive in tungsten electrodes; bulk structural uses are limited due to cost and radioactivity. Why Other Options Are Wrong: Seger cones: require calibrated softening behavior, not extreme refractoriness alone. Muffles/insulating bricks: typically alumino-silicates or high-alumina refractories suffice economically. Common Pitfalls: Choosing high-volume applications where cost would be prohibitive. Ignoring contamination control as a driver in choosing crucible materials. Final Answer: Crucibles for melting high-purity metals

Question 2

Seger cone correlation for silica refractories: A typical silica brick has refractoriness corresponding to Seger (Pyrometric) Cone No. 32. Approximately what temperature (°C) does this cone number represent?

Accepted Answer

Introduction / Context: Seger (Pyrometric) cone numbers correlate to equivalent thermal effects in ceramic firing and refractory testing. Knowing the approximate temperature associated with a cone number helps engineers select materials that can withstand service temperatures with adequate safety margins. Given Data / Assumptions: Material: silica refractory brick. Seger Cone No.: 32. We seek the corresponding temperature in degrees Celsius. Concept / Approach: Pyrometric cones deform at standardized heating schedules, approximating a temperature–time effect. For quick selection, engineers use tabulated conversions of cone number to approximate temperatures. Cone 32 is commonly associated with about 1710 °C for silica refractories. Step-by-Step Solution: Read the cone number (32) from specification.Consult standard associations: Cone 32 ≈ 1710 °C (commonly cited for silica softening range).Thus, the appropriate temperature is about 1710 °C. Verification / Alternative check: Ceramics references list cone-temperature tables; while exact temperatures vary with heating rate and atmosphere, 1710 °C is widely accepted for cone 32 in silica contexts. Why Other Options Are Wrong: 1380 °C and 1520 °C are low for high-grade silica refractories. 1915 °C is higher than the typical cone 32 correlation. Common Pitfalls: Confusing different cone systems or heating rates. Assuming a single fixed temperature; cones represent heat-work, not a single exact temperature. Final Answer: 1710 °C

Question 3

Chemistry of mullite refractories: Mullite, an important phase in alumino-silicate refractories, has which of the following idealized chemical formulas?

Accepted Answer

Introduction / Context: Mullite is the principal crystalline phase that imparts strength and thermal stability to many alumino-silicate refractories and porcelain bodies. Identifying its stoichiometry is a foundational materials science fact useful in refractory selection and kiln operation. Given Data / Assumptions: Mullite is an alumino-silicate phase. We seek the idealized stoichiometric formula. Concept / Approach: The idealized composition of mullite is commonly written as 3Al2O3·2SiO2 (approximately 71.8 wt% Al2O3). Natural and industrial mullites may deviate slightly, but this formula captures the key alumina-rich nature that differentiates it from simple alumina or silica. Step-by-Step Solution: List options and eliminate non-alumino-silicate compounds (e.g., ZrSO4, ThO2).Recognize mullite as an alumina-rich alumino-silicate phase.Select 3Al2O3·2SiO2 as the correct formula. Verification / Alternative check: Phase diagrams of the Al2O3–SiO2 system show the mullite field near the 3:2 alumina to silica molar ratio. Why Other Options Are Wrong: Al2O3 is corundum (pure alumina), not mullite. ZrSO4 and ThO2 are unrelated compounds. Common Pitfalls: Confusing mullite with corundum or sillimanite group minerals. Assuming mullite equals “alumina” without silica; mullite is a mixed oxide phase. Final Answer: 3Al2O3·2SiO2

Question 4

Polymorphs of silica and temperature stability: Which crystalline form of silica is the stable phase below about 870 °C under standard pressure?

Accepted Answer

Introduction / Context: Silica (SiO2) has several polymorphs that are stable over different temperature ranges. Knowing which phase is stable at a given temperature is important for kiln design, refractory selection, and thermal shock management. Given Data / Assumptions: Atmospheric pressure conditions. Temperature threshold of interest: about 870 °C. Concept / Approach: At room temperature up to about 870 °C, the stable polymorph is quartz (with an alpha-to-beta transition near 573 °C). At higher temperatures, tridymite becomes stable, followed by cristobalite at still higher temperatures. Thus, for temperatures below roughly 870 °C, quartz is the stable form. Step-by-Step Solution: Recall stability sequence with rising temperature: quartz → tridymite → cristobalite.Compare the given temperature range (below 870 °C) to the sequence.Select quartz as the correct answer. Verification / Alternative check: Ceramic materials references show polymorph stability fields; quartz dominates at lower temperatures under standard pressure. Why Other Options Are Wrong: Tridymite and cristobalite are high-temperature forms stable above quartz’s range. “None of these” is incorrect because quartz fits the condition. Common Pitfalls: Confusing the alpha–beta transition of quartz (≈573 °C) with a change to a different polymorph. Assuming cristobalite forms readily at low temperature; it requires higher temperatures or catalysts/nuclei. Final Answer: Quartz

Question 5

Purpose of firing refractory bricks in manufacture: Why are refractory bricks fired after drying during manufacturing? Choose the most comprehensive reason.

Accepted Answer

Introduction / Context: Refractory bricks are manufactured by mixing, shaping, drying, and firing. Firing is not merely to remove moisture; it is a critical step that transforms the raw mix into a strong, stable ceramic capable of high-temperature service. Given Data / Assumptions: The brick has already been dried to remove free water. Firing temperatures are sufficient to promote phase development and sintering. We require the overall purpose of firing, not a single narrow effect. Concept / Approach: Firing performs three key roles: final removal of chemically bound water and organics, development of stable crystalline phases appropriate to the formulation (e.g., mullite in alumino-silicates), and sintering to create ceramic bonds between grains. These processes yield mechanical strength, thermal stability, and resistance to slag and spalling. Step-by-Step Solution: Consider dehydration: firing expels residual bound water and burn-out of binders.Consider phase development: temperature drives reactions to produce stable high-temperature minerals.Consider bonding: sintering creates ceramic necks/bonds, enhancing crushing strength and durability.Therefore, the comprehensive choice is “All (a), (b) and (c)”. Verification / Alternative check: Standard refractory processing texts describe these combined outcomes of firing, which are essential for service performance. Why Other Options Are Wrong: Each of (a), (b), and (c) alone is incomplete; the firing step achieves all simultaneously. Common Pitfalls: Equating “firing” with “drying”; drying occurs earlier at much lower temperatures. Ignoring sintering’s impact on mechanical strength and permeability. Final Answer: All (a), (b) and (c)

Question 6

High-alumina refractories: composition limit In commercial practice, the alumina (Al2O3) content of high-alumina refractories can reach up to approximately what maximum percentage by mass?

Accepted Answer

Introduction / Context: High-alumina refractories span a wide composition range, from about 45% Al2O3 up to very high alumina contents approaching pure corundum. Knowing the upper limit helps in selecting materials for extreme temperature and corrosive environments. Given Data / Assumptions: We are considering commercial high-alumina refractories, not single-crystal alumina. Percentages are approximate mass percentages of Al2O3. Concept / Approach: As alumina content increases, refractoriness under load, slag resistance, and hot strength generally improve. Commercial bricks and monolithics are produced with Al2O3 contents in bands (50%, 60–70%, 80–90%). Near the top end, materials may be termed 90% alumina refractories, just below fused or sintered 99% alumina products. Step-by-Step Solution: Survey typical product grades: 45–50%, 60–70%, 80%, and ~90% alumina.Recognize that “high-alumina” includes very high Al2O3 contents short of technical alumina (99%).Thus, the practical maximum is about 90% Al2O3. Verification / Alternative check: Supplier datasheets and standards (e.g., ASTM classifications) list 90% alumina bricks and castables as common top-tier products in the category. Why Other Options Are Wrong: 30% and 50% are too low for the “maximum” of the class. 70% is mid-range. 90% matches the upper end before ultra-high-purity alumina ceramics. Common Pitfalls: Confusing “high-alumina” with “pure alumina”; 99% alumina is a different, specialty class. Overlooking impurities and glassy phases that limit performance at lower alumina contents. Final Answer: 90%

Question 7

Refractory engineering — Spalling of silica bricks occurs due to abrupt volume changes when cooled below approximately which critical temperature (°C)?

Accepted Answer

Introduction: Silica bricks are widely used in high-temperature furnaces because of their high refractoriness and load-bearing capacity. However, they are susceptible to thermal spalling near the quartz polymorphic transformation. This question checks understanding of the critical temperature at which sudden volume changes can damage silica refractories during cooling. Given Data / Assumptions: The brick composition is silica-rich (high SiO2). Cooling through the quartz inversion range is the concern. Question asks for the approximate temperature at which abrupt volume change causes spalling. Concept / Approach: Silica (SiO2) exhibits polymorphism. The alpha–beta quartz inversion occurs around 573°C. When a hot silica brick cools through this temperature, abrupt volume change occurs. If cooling is fast or temperature gradients are high, tensile stresses can exceed the brick’s strength, leading to cracking or layers flaking off (spalling). Step-by-Step Solution: Identify the inversion: alpha–beta quartz transformation at roughly 573°C (engineers commonly round to 570°C).Relate inversion to volume change: the structural change causes a significant, sudden specific volume shift.Link to spalling: thermal gradients during passage through ~570°C create internal stress and spall.Hence, the critical threshold is closest to 570°C. Verification / Alternative check: Materials handbooks and refractory datasheets consistently warn to slow-cool silica linings through ~600–500°C to avoid spalling—confirming ~570°C as the critical region. Why Other Options Are Wrong: 770°C: Above the critical inversion; not the main spalling trigger. 270°C and 70°C: Too low; most inversion-driven volume change has already occurred. Common Pitfalls: Confusing the cristobalite or tridymite transformations with the dominant quartz inversion; ignoring the rate of cooling’s influence. Final Answer: 570

Question 8

Refractory processing — Typical firing temperature range for magnesite (MgO) bricks is approximately which of the following (°C)?

Accepted Answer

Introduction: Magnesite (MgO) bricks are basic refractories used in steelmaking and other high-temperature basic environments. Their manufacturing involves high-temperature firing to achieve desired densification, strength, and slag resistance. The question focuses on the representative firing temperature range. Given Data / Assumptions: Product: fired magnesite refractory (not chemically bonded cold-set types). Objective: sufficient sintering and densification for service in steel and cement kilns. Temperatures listed are broad ranges. Concept / Approach: Sintering of magnesia grains requires high temperatures because MgO has a very high melting point (~2800°C) and relatively sluggish diffusion at lower temperatures. Industrial firing for standard dense magnesia bricks commonly occurs around 1600–1800°C to achieve proper microstructure and bond. Step-by-Step Solution: Consider MgO properties: high melting point implies high sintering temperature.Industrial practice: kilns for MgO bricks operate ~1600–1800°C for adequate densification.Exclude too-low ranges (≤1200°C): insufficient sintering.Exclude 2400–2600°C: technically unnecessary and uneconomical for standard products. Verification / Alternative check: Textbook firing schedules and vendor datasheets typically cite 1600–1800°C for ordinary fired magnesite bricks; special ultra-dense grades may use higher but still below 2400°C. Why Other Options Are Wrong: 800–1000°C / 1000–1200°C: Too low for MgO sintering. 2400–2600°C: Excessive; not standard practice for typical fired bricks. Common Pitfalls: Confusing firing temperatures of clay/silica refractories with basic magnesia; assuming “higher is always better” without considering energy and microstructural goals. Final Answer: 1600-1800

Question 9

Applications of fireclay bricks — Identify the most appropriate typical use from the following high-temperature installations.

Accepted Answer

Introduction: Choosing the right refractory for each furnace zone is essential. Fireclay bricks (alumino-silicate with moderate alumina content) serve in intermediate temperature regions and structural areas where extreme silica or basic slag resistance is not mandatory. This question checks the awareness of typical placement. Given Data / Assumptions: Fireclay bricks suit moderate to moderately high temperatures and neutral to slightly acidic conditions. Different furnace zones (coke ovens, LD converters, blast furnaces) have distinct chemical/thermal demands. We seek the most appropriate among provided options. Concept / Approach: Coke oven main walls and checkerwork at very high temperatures are commonly silica due to high-temperature strength and thermal expansion behavior. Basic oxygen (LD) converters and blast furnace hearths demand basic or carbon-based refractories due to basic slags and severe thermal/chemical attack. Fireclay is often used in ancillary or somewhat cooler structural areas such as certain regenerator settings or lower-temperature zones associated with coke ovens. Step-by-Step Solution: Evaluate coke oven walls: predominantly silica bricks in the hottest zones → not the best match for fireclay.Evaluate LD converter outer lining: primary wear linings are basic (magnesia/dolomite); fireclay is not typical.Evaluate blast furnace hearth bottom: typically carbon/graphite or high-alumina/carbon blocks for severe conditions.Evaluate coke ovens regenerator: fireclay can be used in certain regenerator structures or lower-temperature sections → most plausible option provided. Verification / Alternative check: Industry layouts and refractory handbooks show silica dominance in coke oven hot zones; basic or carbon materials in steelmaking vessels; fireclay is reserved for less severe areas, including some regenerator structures. Why Other Options Are Wrong: Coke oven walls: typically silica at very high temperatures. Outer lining of LD converter: basic refractories needed due to basic slag. Hearth bottom of blast furnace: carbon/graphite or very high alumina/carbon systems used. Common Pitfalls: Assuming a single refractory fits all furnace zones; overlooking chemical compatibility with slag. Final Answer: Coke ovens regenerator

Question 10

Refractory testing — The “panel test” for refractories primarily determines which performance property?

Accepted Answer

Introduction: Refractory selection depends not only on composition and refractoriness but also on resistance to thermal shock and spalling. The panel test is one of the standardized ways to evaluate how a refractory endures rapid heating and cooling cycles that cause surface layers to flake off. Given Data / Assumptions: Test type: panel test. Objective: determine resistance to damage from thermal shock and cyclic temperature changes. Other properties like RUL and fusion point are measured by different tests. Concept / Approach: Spalling resistance reflects a material’s ability to withstand abrupt temperature gradients without cracking or delaminating. Panel tests expose a refractory panel to cycles of heating/cooling to simulate service conditions, and the degree of surface damage or cracking is assessed. Step-by-Step Solution: Identify what panel test stresses: thermal cycling → spalling behavior.Map other properties to tests: fusion point by pyrometric cones; RUL by load-bearing under temperature; slag penetration by crucible/slag tests.Thus, panel test aligns with spalling resistance. Verification / Alternative check: Standards and refractory testing manuals explicitly classify panel/thermal shock tests under spalling resistance evaluations. Why Other Options Are Wrong: Fusion point: measured via cone fusion or high-temperature microscopy. Slag penetration: assessed by slag cup/immersion tests. RUL: determined under load at temperature in a dedicated apparatus. Common Pitfalls: Confusing general “high-temperature tests” with specific performance evaluations; assuming one test covers multiple properties. Final Answer: Spalling resistance

Question 11

Steel vacuum degassing — The refractory lining typically used in vacuum steel degassing units is:

Accepted Answer

Introduction: Vacuum steel degassing involves exposing molten steel to low pressure to remove dissolved gases like hydrogen and nitrogen. The lining must withstand thermal shock, steel/slag chemistry, and vacuum operation. High alumina refractories are a common choice for their chemical stability and temperature capability. Given Data / Assumptions: Service: contact with steel and slag at high temperature. Environment: reduced pressure (vacuum) with potential for thermal cycling. Need: chemical compatibility and mechanical integrity. Concept / Approach: High alumina bricks (e.g., 60–90% Al2O3 grades) offer higher refractoriness under load and better corrosion resistance to basic slags than ordinary fireclay. Silica bricks are acid refractories better suited to silica environments and can be problematic with basic slags. Graphite is used where carbon linings are intended (e.g., blast furnace hearth), but not typically as the general lining for vacuum degassing vessels. Step-by-Step Solution: Evaluate chemistry: steel slags often basic → high alumina performs better than silica or low-duty clay.Consider temperature and mechanical strength: high alumina exhibits superior RUL and hot strength.Assess vacuum service: high alumina bricks are well-established for vacuum degassing linings. Verification / Alternative check: Industrial practice and supplier catalogs list high alumina as the default material for ladles and vacuum tanks unless specific chemistries require magnesia-carbon or other specialized refractories. Why Other Options Are Wrong: Silica bricks: acid nature; not preferred with basic slags. Low duty firebricks: inadequate hot strength and corrosion resistance. Graphite blocks: used in different applications (e.g., BF hearth), not standard for vacuum degassing vessel linings. Common Pitfalls: Assuming one refractory type fits all steelmaking vessels; overlooking slag chemistry. Final Answer: High alumina bricks

Question 12

Basic refractories — The fusion (softening) point of a basic refractory is affected how by the addition of acid oxides?

Accepted Answer

Introduction: Refractory selection often follows the acid–basic classification. Mixing incompatible chemistries typically forms low-melting eutectics, reducing service temperature. This question focuses on how acid oxides influence the fusion point of basic refractories (e.g., MgO, CaO based). Given Data / Assumptions: Basic refractory matrix (e.g., magnesia, doloma). Acid oxides introduced (e.g., SiO2). Fusion point refers to softening/melting onset under heat. Concept / Approach: Acid–basic reactions create compounds or eutectic mixtures with lower melting points than the parent refractory. For instance, MgO with SiO2 can form forsterite or low-melting phases, degrading hot strength and lowering the effective fusion point. Step-by-Step Solution: Identify chemistry: basic refractory meets acid oxide → potential eutectic.Thermodynamic implication: eutectic formation lowers melting/softening temperature.Therefore, the fusion point is reduced when acid oxides contaminate or react with basic bricks. Verification / Alternative check: Phase diagrams (e.g., MgO–SiO2) show eutectics below the melting point of pure MgO, confirming the reduction. Why Other Options Are Wrong: Increase/no effect: contradicts well-known acid–basic incompatibility. Always less than 1000°C: false; basic refractories have far higher fusion points unless severely contaminated. Common Pitfalls: Ignoring slag/refractory interactions; assuming purity despite service contamination. Final Answer: Reduced by the addition of acid oxides.

Question 13

Basic electric furnace design — The roof of a basic electric furnace is commonly constructed from which type of bricks?

Accepted Answer

Introduction: Furnace roofs see intense radiative heating and repeated thermal cycling. Historical and practical designs often use silica bricks in roof constructions because of their high refractoriness and specific thermal expansion behavior that can lock joints at service temperatures. This question targets typical material choice for a basic electric furnace roof. Given Data / Assumptions: “Basic electric furnace” refers to basic slag practice, but roof material selection also considers thermal profile and structural needs. Options include silica, fireclay, and chromite bricks. We seek the commonly used roof material. Concept / Approach: Silica bricks are noted for excellent performance at very high roof temperatures and for their tendency to expand and tighten joints, improving gas tightness at heat. Although the bath/slag is basic, roof chemistry exposure is different from the slag line, permitting the use of silica in many traditional designs. The slag line and walls may use basic bricks, but the roof often remains silica. Step-by-Step Solution: Consider roof duty: high radiant heat, thermal cycling → silica performs well.Basic practice does not automatically mandate basic refractories for every zone; compatibility is zone-specific.Thus, silica bricks are a conventional choice for the roof in many basic electric furnaces. Verification / Alternative check: Engineering texts on furnace design cite silica roofs for open-hearth and electric furnaces, with basic refractories applied at slag lines and hot spots. Why Other Options Are Wrong: Superduty fireclay: lower hot strength than silica at extreme roof temperatures. Chromite: used in specific corrosion environments; not the common default roof material. None of these: incorrect because silica is typical. Common Pitfalls: Assuming “basic furnace” means every part must be basic; roof choices are driven by thermal and structural considerations. Final Answer: Silica

Question 14

Insulating refractory bricks — Which of the following is NOT typically used to induce porosity during manufacture (high porosity and low thermal conductivity desired)?

Accepted Answer

Introduction: Insulating refractory bricks achieve low thermal conductivity largely through high, controlled porosity. During manufacturing, temporary pore-formers are added that burn out or decompose, leaving behind fine, closed or interconnected pores. This question asks which additive is not used for that purpose. Given Data / Assumptions: Desired properties: high porosity, low density, low thermal conductivity. Common pore formers: organic materials or foaming agents that volatilize on firing. Candidate materials listed include organic burnouts and inert mineral. Concept / Approach: Organic pore formers such as cork and sawdust combust during firing, generating gas and voids. Chemically prepared foams expand within the wet body to create a pore structure. Sand, however, is inert silica grains; it does not volatilize or burn away and thus does not create new pores by removal—rather, it can fill space and may even reduce porosity if added as a solid filler. Step-by-Step Solution: Assess cork/sawdust: organic → burn out → leave pores.Assess chemical foam: generates bubbles → pore network upon setting/firing.Assess sand: remains as solid particles; does not disappear → not a porosity inducer. Verification / Alternative check: Manufacturing literature on insulating bricks lists sawdust, paper pulp, polystyrene beads, and foaming processes as pore formers; mineral sand is not used for this purpose. Why Other Options Are Wrong: Cork, sawdust, foam: all are classic porosity-inducing agents. Common Pitfalls: Assuming any granular addition increases porosity; forgetting that pore formers must be removed by burning or decomposition. Final Answer: Sand

Question 15

Manufacture of hollow refractories — Hollow refractory bricks are commonly produced by which forming method?

Accepted Answer

Introduction: Shaping methods for refractories include pressing, casting, and extrusion. Hollow bricks require continuous formation of uniform wall sections with internal voids, which strongly favors extrusion using a die configured to produce the desired hollow profile. Given Data / Assumptions: Product: hollow refractory bricks/blocks. Requirement: continuous, uniform sections with controlled void geometry. Methods listed: slip casting, hand moulding, pressing, extrusion. Concept / Approach: Extrusion forces a plastic ceramic body through a die that shapes both external dimensions and internal cavities via mandrels, enabling efficient production of hollow sections (similar to structural clay tile). Slip casting suits complex, thin-walled shapes but is slower and not optimal for regular hollow bricks. Dry pressing is common for dense, solid shapes; creating continuous hollow cores is less practical. Hand moulding lacks precision and throughput. Step-by-Step Solution: Match shape to process: long, hollow, uniform → extrusion best.Assess alternatives: pressing and hand moulding are less suitable for continuous hollow cavities.Slip casting: viable for special shapes but inefficient for standard hollow bricks. Verification / Alternative check: Refractory production lines for hollow insulating bricks typically use auger extrusion with dies producing multi-hole sections. Why Other Options Are Wrong: Slip casting: slow, mold-dependent; not economical for standard hollow bricks. Hand moulding: inconsistent quality. Pressing: optimized for solid shapes. Common Pitfalls: Overlooking the role of die design in forming internal voids; assuming pressing can easily create longitudinal hollows. Final Answer: Extrusion

Question 16

Thermal expansion behavior — Which refractories characteristically show overall expansion upon heating through service temperatures?

Accepted Answer

Introduction: Dimensional stability is critical in refractory linings. Silica bricks are known for their unusual positive permanent expansion on heating due to silica polymorphic transformations, which can tighten roof joints in service. Fireclay bricks exhibit more conventional thermal expansion behavior and may not show the same pronounced permanent expansion. Given Data / Assumptions: Silica bricks contain high SiO2; fireclay bricks are alumino-silicate with lower SiO2/higher Al2O3. Heating cycles pass through the quartz, tridymite, and cristobalite transformation ranges. Question asks which shows expansion on heating as a notable characteristic. Concept / Approach: Silica has several high-temperature polymorphs with different specific volumes. As temperature rises, transformations lead to net expansion, some of which may be irreversible (permanent set). This behavior can be advantageous for furnace roofs by tightening joints. Fireclay bricks expand thermally but do not exhibit the pronounced permanent expansion associated with high-silica compositions. Step-by-Step Solution: Recognize silica behavior: marked expansion across polymorphic transitions.Recognize fireclay behavior: typical thermal expansion without large permanent growth.Therefore, silica bricks are the clear answer. Verification / Alternative check: Refractory property charts show positive permanent expansion for silica bricks; design practices exploit this for roof construction. Why Other Options Are Wrong: Fireclay alone: lacks the same strong permanent expansion trait. Both/Neither: conflicts with well-documented silica behavior. Common Pitfalls: Confusing reversible thermal expansion with irreversible permanent expansion; ignoring polymorphism in silica. Final Answer: Silica bricks

Question 17

In blast furnace hot-blast stoves, the outer combustion chamber is typically lined with which type of refractory brick to provide insulation and structural protection at service temperatures?

Accepted Answer

Introduction / Context: Hot-blast stoves used with blast furnaces have a combustion chamber and checkerwork that must withstand sustained thermal cycling. Different zones see different temperatures and chemical environments, so refractories are selected accordingly. This question checks understanding of where fireclay (alumino-silicate) bricks are commonly applied within the stove lining scheme. Given Data / Assumptions: Component: outer combustion chamber of a blast-furnace stove. Operating environment: hot gases, but lower peak skin temperatures than the inner hot face or checker dome. Goal: adequate refractoriness, insulation, economy, and compatibility with brickwork load-bearing. Concept / Approach: Stove linings are graded. The hottest, highly silica-tolerant zones (e.g., dome and checker areas) often use silica bricks for creep resistance at very high temperatures. The outer combustion chamber and backup courses typically use fireclay bricks due to adequate temperature rating, thermal shock tolerance, availability, and cost effectiveness. Ultra-high-end refractories (e.g., zirconia, chrome–magnesite) are reserved for extreme conditions (basic slags, severe corrosion), not usually required for the outer chamber lining of stoves. Step-by-Step Solution: Identify the stove zone: outer combustion chamber (cooler than the hot-face dome).Match performance need with material: fireclay fits typical temperature and budget.Exclude higher-cost, specialty bricks not justified by service conditions. Verification / Alternative check: Typical stove-lining schedules in ironmaking practice show silica bricks at the high-temperature crown/dome and checker zones, with fireclay courses used outward for insulation and structural backing where temperatures are lower. Why Other Options Are Wrong: Silica bricks: Common at the hottest crowns/domes, not usually for the outer chamber courses. Chrome–magnesite: Basic refractories for corrosive basic slags, not needed here. Zirconia: Very high temperature specialty; over-specified and uneconomical for this zone. Common Pitfalls: Assuming the same brick is used everywhere; ignoring lining gradation and cost-performance optimization. Final Answer: Fireclay bricks

Question 18

Industrial refractory bricks are typically dried prior to firing using which production equipment or method in modern plants?

Accepted Answer

Introduction / Context: Refractory manufacture involves shaping, drying, and firing. Proper drying prevents cracking, warping, and explosive spalling during subsequent high-temperature firing. Understanding where drying occurs in a continuous plant layout is part of basic refractory processing knowledge. Given Data / Assumptions: Modern, continuous industrial operation rather than artisanal practice. Presence of continuous kilns with controlled thermal profiles. Need to remove mechanically held and capillary moisture uniformly. Concept / Approach: In many modern plants, tunnel kilns incorporate controlled zones: a drying or preheating section, a firing zone, and a cooling section. Green bricks pass continuously through, so drying is integrated. Rotary kilns are typically used for calcination or firing granular feeds (e.g., clinker, dead-burned magnesia), not for drying shaped bricks. Sun drying is inconsistent and unsuitable for quality-controlled production. Step-by-Step Solution: Identify typical equipment for shaped refractories → tunnel kilns with drying/preheat.Eliminate rotary kilns (not used for brick drying) and sun (uncontrolled).Select tunnel kilns as the standard industrial choice. Verification / Alternative check: Standard refractory plant flow diagrams show green bricks loaded onto cars entering the tunnel kiln where moisture is first driven off before high-temperature firing. Why Other Options Are Wrong: Rotary kilns: Suited to powders/aggregates and firing, not uniform drying of shaped bricks. Sun drying: Non-uniform, weather-dependent, and industrially unreliable. None of these: Incorrect because tunnel kilns are widely used. Common Pitfalls: Confusing stand-alone driers with the integrated drying zones of tunnel kilns; assuming artisanal methods apply to industrial production. Final Answer: Tunnel kilns (with drying/preheat zones)

Question 19

Magnesite (basic) refractories are preferred for furnace construction under which combined conditions of service?

Accepted Answer

Introduction / Context: Magnesite refractories are basic (MgO-rich) materials valued for high refractoriness and resistance to basic slags. However, they have limitations in thermal shock resistance. Choosing them correctly requires balancing chemical and thermal service demands. Given Data / Assumptions: Chemical environment: basic slags are possible. Thermal environment: high operating temperatures are expected. Operational pattern: prefer steady temperature with limited cycling. Concept / Approach: Magnesite bricks excel in high-temperature, basic environments but generally have poorer thermal shock resistance than some alumino-silicate refractories. Hence, they are best in applications aimed at attaining and maintaining high temperatures with minimal rapid temperature swings. Avoid using them where severe quenching or frequent temperature cycling occurs. Step-by-Step Solution: Match material strength (high refractoriness/basicity) with service → high-temperature/basic furnace.Account for thermal shock limitation → prefer low fluctuation in temperature.Therefore, select both conditions: (b) and (c). Verification / Alternative check: Material datasheets commonly warn of thermal shock concerns with basic bricks while emphasizing their high-temperature chemical resistance. Why Other Options Are Wrong: (a): Opposite of where magnesite shines; it is chosen precisely for basic environments. (b) only: Ignores their primary value at high temperature. (c) only: Ignores thermal shock limitation consideration. Common Pitfalls: Assuming a single property (temperature rating) determines selection; overlooking thermal shock behavior in service scheduling. Final Answer: Both (b) and (c)

Question 20

Cermets (ceramic–metal composites) combine high strength with high-temperature resistance. In which applications are cermets widely used due to these properties?

Accepted Answer

Introduction / Context: Cermets are engineered composites of ceramic and metallic phases. They leverage ceramic hardness and thermal stability with metallic toughness and thermal shock tolerance, enabling service in extreme environments. Recognizing their typical application areas is foundational in materials selection. Given Data / Assumptions: Properties: high-temperature capability, wear resistance, and sometimes oxidation resistance. Applications: high-performance, high-stress, or high-temperature environments. Comparison: conventional refractories vs advanced structural components. Concept / Approach: Cermets (e.g., TiC–Ni, WC–Co) are commonly used in aerospace (missile/spacecraft hot structures or guidance components), nuclear applications (control rod followers, some wear parts), and cutting tools. These are structural/functional components rather than mass thermal linings of large industrial furnaces, for which conventional refractories are more economical. Step-by-Step Solution: Identify property–application mapping: high-temperature strength → aerospace, nuclear.Exclude bulk furnace linings where castables/bricks are more economical.Select aerospace/nuclear as correct typical use case. Verification / Alternative check: Materials handbooks list cermets in cutting tools, rocket nozzles, and certain nuclear hardware, not as primary large-area refractory linings. Why Other Options Are Wrong: Hearth/roof/insulation of large furnaces: Typically lined with refractory bricks, castables, or fiber modules; cermets are unnecessary and uneconomical at scale. Common Pitfalls: Assuming superior properties imply universal usage; overlooking cost and fabrication constraints for large surfaces. Final Answer: Nuclear reactors, missiles, and spacecraft components

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