China's Refractories

Research on the effects of the amount of fillers in the form of fused alumina, mullite, and tabular alumina, zirconium-containing and silica-containing components of the batches in the form of zircon and silica sand on the properties of alumina-zirconia-silicate (AZS) refractories was conducted. It was found that the introduction of 16% zircon into the composition of samples with fillers such as fused alumina and mullite, as well as tabular alumina, results in an increase in their density, strength, and thermal shock resistance. Petrographic studies have revealed the formation of a multiphase, microcracked structure in these samples, which accounts for the improvement in their properties. The optimal composition of a batch containing a filler of tabular alumina, a finely dispersed matrix of α-alumina, 16% zircon, and 7% silica sand has been determined, which ensures the production of AZS refractories with high bulk density, cold crushing strength, and thermal shock resistance. Research on the plastic strength of the structure of optimal composition AZS mass, depending on its storage time, was conducted. It was found that up to 24 h of storage, the mass is characterized by low plastic strength. It indicates its suitability for use in the vibration casting technology of products. Storage of the mass for more than 24 h leads to an increase in the plastic strength of its structure, which reaches a maximum after 56 h of storage. The developed composition of the batch is recommended for the manufacture of AZS crucibles for induction melting of heat-resistant alloys.

This paper introduces several new test methods for refractory properties, including the square crucible slag resistance test method, induction furnace lining slag resistance test method, and standard pressure-vibration ramming method. Square crucibles offer more cutting options and can be used for quantitative analysis. At most four different materials can be tested in the same crucible at the same time to improve the efficiency. The induction furnace lining slag resistance test method has the advantage of testing multiple samples simultaneously, and the slag can be continuously added for long-term experiments. The standard pressure vibration ramming method ensures the consistency of ramming strength and vibration time during moulding, which ensures that the testing results are reliable and comparable.

In order to determine the bulk density of refractory raw materials, the so-called water method following the Archimedes principle is normally used. This is where the effect of water displacement on the mass of the sample is used to determine the bulk volume of the sample grains. During this test procedure, the surface of water infiltrated sample grains must be dried with a wet towel. Experience shows, that this drying step is the main root cause for variation in reproducibility of results and even repeatability of tests. A new spin dryer (centrifuge) was developed and introduced to automate this surface drying step, and is now included as a new method in ISO 8840:2021. The paper discusses the improvement of measurement with the new approach and industrial experiences from two big industrial players in the raw material business.

The pyrometallurgy combined with hydrometallurgy process is commonly used for recovering valuable elements from spent lithium-ion batteries. To improve the corrosion resistance of heat treatment furnace lining materials in this process, SiC-Si₃N₄-C composite refractories were prepared by heat-treating at 800-1 400 °C for 3 h, using fine particles of spent iron ladle bricks (1-0.5 and <0.5 mm), silicon carbide (2-1, 1-0.5, and <0.5 mm), silicon nitride (<0.045 mm), and graphite as raw materials. The fine particles of spent iron ladle bricks were used to replace silicon nitride with the same particle size. The effects of the spent iron ladle brick fine particles additions (0, 12.12%, 26.25%, 36.38% and 48.5%, by mass) and the heat treatment temperatures (800, 1 000, 1 200 and 1 400 °C) on the properties of the composite refractories were studied. The results show that: (1) with the <1 mm spent iron ladle brick fine particles addition increasing, the bulk density of the samples changes slightly, the apparent porosity gradually decreases, the cold modulus of rupture (CMOR) increases, and the cold compressive strength (CCS) first decreases, then increases and finally decreases slightly; (2) with the heat treatment temperature rising, the bulk density of the samples first increases and then decreases, the apparent porosity gradually decreases, and the CCS and the CMOR increase; (3) when the temperature is 1 400 °C and the spent iron ladle brick fine particles completely replace the silicon nitride fine particles with the same particle size, the sample exhibits the best comprehensive performance, with the bulk density of 2.37 g · cm^-3, apparent porosity of 14.3%, CCS of 31.6 MPa, and CMOR of 9.0 MPa, and it has a good resistance to the corrosion of crushed spent lithium-ion battery materials at 1 000 °C.

The development of refractories with low reactivity to rare earth inclusions is an important direction to solve the problem of the nozzle clogging of rare earth steel. La₂Ce₂O₇, La₂Zr₂O₇, and LaAlO₃ powders were synthesized using the high-temperature solid-state method with La₂O₃, CeO₂, ZrO₂, and Al₂O₃ (particle sizes of 5-10 μm) as raw materials, firing at 1 400 °C for 2 h. Subsequently, La₂Ce₂O₇, La₂Zr₂O₇, and Y₂O₃ powders were pressed into φ30 mm×7 mm substrate samples with PVA as a binder; and equal amounts of La₂O₃, La₂S₃, and LaAlO₃ powders were placed on their surfaces. The samples were then fired at 1 550 °C for 3 h with carbon embedded. The interfacial reaction of the three rare earth oxide refractories (La₂Ce₂O₇, La₂Zr₂O₇, and Y₂O₃) with the main rare earth inclusions (La₂O₃, La₂S₃, and LaAlO₃) in molten rare earth steel was studied. The results show that the La₂Ce₂O₇ sample has poor structural stability and readily reacts with La₂S₃, leading to cracking. The La₂Zr₂O₇ sample reacts with La₂O₃ and LaAlO₃ weakly, but performs poor La₂S₃ corrosion resistance. The Y₂O₃ sample demonstrates the weakest interaction with the three rare earth inclusions as well as the most stable structure, indicating significant potential as a specialized anti-clogging lining material for rare earth steel.

Chemical vapor deposition is the predominant method to prepare MgAl₂O₄ fibers. However, it faces several challenges, including exorbitantly high reaction temperatures, substantial production costs, and relatively low yields. In this study, porous MgAl₂O₄ fibers were fabricated through a solid-state reaction method, utilizing MgSO₄·5Mg(OH)₂·3H₂O whiskers as templates, mixed with either aluminum sol or α-Al₂O₃ micropowder. The impact of various parameters on the synthesis of porous MgAl₂O₄ fibres was systematically investigated, including the heat treatment temperature (1 000, 1 100 and 1 300 °C), the holding time (3 and 10 h) and the aluminum source (aluminum sol or α-Al₂O₃ micropowder). The results reveal that: (1) in comparison with fibers synthesized using α-Al₂O₃ as the aluminum source, those prepared with aluminum sol exhibit a significantly higher generation amount of MgAl₂O₄; (2) as the heat treatment temperature increases, Al₂O₃ gradually reacts with MgO, continuously increasing the formation amount of porous MgAl₂O₄ with small and uniformly distributed nanopores, and the synthesized porous MgAl₂O₄ fibres have small and uniform nanopores; (3) the optimal synthesis process involves using aluminum sol as the aluminum source and firing at 1 300 °C for 3 h.

To enhance the performance, reactive MgO-bonded corundum-spinel castables were prepared using tabular corundum, reactive alumina, reactive MgO, and aluminum lactate as raw materials. The influence of the reactive alumina addition (0, 8%, 13%, and 18%, by mass) on the microstructure, phase composition, physical properties, as well as the resistance to slag corrosion and penetration of the castables was investigated. The results show that: (1) the reactive alumina promotes the sintering of the reactive MgO-bonded corundum-spinel castables, enhancing the bonding between aggregates and matrix, thus densifying the microstructure; (2) with the increase of the reactive alumina, the volume stability of the samples increases, the apparent porosity decreases, the bulk density increases, and the cold modulus of rupture and cold compressive strength as well as the slag resistance are improved; (3) the reactive MgO-bonded corundum-spinel castable with 18% reactive alumina shows optimal comprehensive performance, with the bulk density of 3.05 g·cm^-3, apparent porosity of 18.5%, permanent linear change rate after heating of 0.31%, cold modulus of rupture of 13.5 MPa and cold compressive strengths of 73.8 MPa, as well as the best slag resistance. However, considering particle size distribution and raw material costs, there is no need to further increase the reactive alumina addition.

To improve the utilization rate and application value of magnesite tailings, magnesia composites were prepared using light-calcined magnesite tailing powder as the raw material, silicon powder, and 95-graphite as additives, and phenolic resin as the binder by the solid-phase reaction pressureless sintering method. The effects of Si powder and graphite powder additions on the cold modulus of rupture, density, linear change rate, thermal shock resistance, microstructure, and phase composition of the composites were investigated in a carbon embedded atmosphere. The results show that with increasing additions of silicon powder and graphite powder, the forsterite and silicon carbide contents in the materials increase, finally forming an M₂S-SiC-C multiphase material with M₂S as the main crystal phase. The carbon monoxide and silicon monoxide gases produced during the reaction are detrimental to the sintering of the material, resulting in the decrease of the cold modulus of rupture, bulk density, and linear shrinkage, and the increase of the porosity. After thermal shock, the strength retention ratio of the materials increases significantly compared with that of the sample without additives, because both the increased forsterite content and the generation of silicon carbide whiskers in the materials contribute to improving the thermal shock resistance.

To improve the thermal insulation performance of SiO₂ aerogels at high temperatures, SiO₂ precursor solutions were prepared via a sol-gel two-step method. Fe₂O₃ powder was extra added as an opacifier to the SiO₂ precursor solutions with mass fractions of 0, 0.2%, 0.5%, 1.0%, and 3.0%; and Fe₂O₃-SiO₂ composite aerogels were fabricated using CO₂ supercritical drying technology. The effects of the Fe₂O₃ extra addition on the aerogels were investigated. The results show that: (1) Fe₂O₃ doping does not alter the aerogel morphology; Fe₂O₃ suppresses the mass loss at high temperatures and enhances the high-temperature stability of the composite; (2) below 800 °C, the aerogel with 0.5% Fe₂O₃ exhibits the lowest thermal conductivity and the best thermal insulation performance; at 800-1 000 °C, the aerogel with 1% Fe₂O₃ exhibits the lowest thermal conductivity and a good nanoporous structure; (3) by adjusting the Fe₂O₃ extra addition, composite aerogels suitable for different temperature ranges can be tailored.