The high-temperature performance of a furnace lining depends primarily on the physical and chemical properties as well as the mineralogical composition of the refractory materials used. Given the selection of raw materials, the sintering process is the critical step for achieving an optimal microstructure, thereby fully realizing the lining’s high-temperature capabilities. The degree of densification during sintering is influenced by factors such as the chemical composition of the refractory materials, particle size distribution, the sintering process itself, and the sintering temperature.
Today, a specialized manufacturer of medium-frequency furnace lining materials will explain in detail how to enhance the heat resistance of these linings by examining the key factors that affect their performance.

Remove the mica paper during furnace lining construction.
Process the crystalline quartz sand used for the lining as follows:
Manual sorting: Primarily to remove lumps and other impurities;
Magnetic separation: Magnetic impurities must be completely removed;
Dry ramming mix: Must undergo a slow drying process at 200°C–300°C, maintaining the temperature for at least 4 hours.
Selection of binder for medium-frequency electric furnaces: Use boric anhydride (B2O3) instead of boric acid (H3BO3) as the binder, with an addition rate of 1.1%–1.5%.
Selection and proportioning of lining materials:
Material selection: Note that not all quartz sand with SiO2 ≥ 99% is suitable for induction furnace linings; the quartz grain size is critical. Coarser grains with fewer lattice defects are preferred (e.g., crystalline quartz sand, which has high SiO2 purity and a white, transparent appearance). Larger furnace capacities require higher-quality grains.
Proportioning: Quartz sand mix ratio for the lining: 6–8 mesh (10%–15%), 10–20 mesh (25%–30%), 20–40 mesh (25%–30%), and 270 mesh (25%–30%).
Ramming the lining: The quality of the ramming process directly affects the sintering quality. Uniform grain size distribution prevents segregation; a high-density rammed sand layer reduces the likelihood of post-sintering cracks, thereby extending the service life of the induction furnace lining.
Ramming the furnace bottom: The bottom is approximately 280 mm thick; fill the sand in four stages. During manual ramming, ensure uniform density to prevent a loose or porous lining structure after baking and sintering. Therefore, the thickness of the added material must be strictly controlled; generally, the sand filling thickness should not exceed 100 mm per layer, and the furnace wall layer thickness should be kept within 60 mm. Operations are carried out by teams working in shifts (4–6 people per shift), with personnel rotating every 30 minutes of ramming. Operators move slowly around the furnace, applying force evenly to avoid uneven density.
Ramming the furnace wall: The lining thickness is 110–120 mm. Dry ramming material is added in batches and distributed evenly, with a filling thickness of no more than 60 mm. Ramming is performed for 15 minutes (manually) until the level reaches the top edge of the induction coil. The crucible former is not removed after ramming; it serves to facilitate induction heating during the drying and sintering processes.
Drying and sintering specifications: To achieve the required three-layer structure of the furnace lining, the drying and sintering process is generally divided into three stages:
Drying stage: The crucible former is heated to 600°C at rates of 25°C/h and then 50°C/h, followed by a 4-hour holding period; the objective is to completely remove moisture from the furnace lining.
Semi-sintering stage: The temperature is raised to 900°C at 50°C/h (held for 3 hours) and then to 1200°C at 100°C/h (held for 3 hours). The heating rate must be strictly controlled to prevent cracking.
Complete sintering stage: During high-temperature sintering, the sintered structure of the medium-frequency electric furnace crucible is fundamental to its service life. Variations in sintering temperature or insufficient thickness of the sintered layer will significantly reduce the service life.