The calcination temperature profile of calcined zinc oxide is a key factor in determining the quality of the final product. Its design must precisely match the physicochemical transformation characteristics of zinc oxide, directly impacting the product's crystal structure, impurity control, particle morphology, and industrial performance. The rationality of the temperature profile lies in the coordinated effects of the three stages: heating rate, constant temperature range, and cooling control. Adjustment of parameters in each stage is closely related to the conversion efficiency, purity, and stability of the calcined zinc oxide.
The heating stage requires a balance between pyrolysis efficiency and structural stability. Raw zinc raw materials often contain water of crystallization, organic impurities, and unreacted zinc compounds, all of which decompose within different temperature ranges. If the temperature is raised too quickly, the raw material surface will harden rapidly, hindering the escape of internal gases, leading to localized stress concentrations and the formation of raw material inclusions or porosity defects. If the temperature is raised too slowly, the pyrolysis time will be prolonged, energy consumption will increase, and abnormal grain growth may occur. A reasonable heating profile requires setting step-by-step rates based on the raw material characteristics. For example, slow heating in the low-temperature range promotes the removal of crystalline water and avoids raw material explosion caused by rapid evaporation; accelerated heating in the high-temperature range completes the carbonization of organic matter and reduces the impact of residual carbon on product purity.
The constant temperature stage is a critical window for crystal transformation. The preparation of calcined zinc oxide requires high-temperature constant temperature to achieve complete transformation from an amorphous or low-temperature phase to a stable hexagonal system. The constant temperature must be precisely controlled above the critical point for complete crystal transformation, typically between 1200°C and 1250°C, to ensure sufficient repair of lattice defects while avoiding excessive temperatures that could lead to excessive grain fusion. The constant temperature duration is also critical. Insufficient time will result in the retention of unconverted low-temperature phases, reducing the thermal stability of the product. Excessive time may cause grain coarsening, reduce specific surface area, and weaken the activity of calcined zinc oxide in catalytic reactions or glaze fluxing. For example, in ceramic glaze applications, insufficiently constant temperature in calcined zinc oxide can lead to defects such as pinholes and bubbles in the glaze, while excessively constant temperature can lead to grain coarsening, resulting in a loss of glaze gloss.
Control of the cooling stage directly impacts product morphology and impurity distribution. Rapid cooling can induce lattice stress, increasing brittleness in the calcined zinc oxide; slow cooling can promote segregation of impurity elements at grain boundaries, reducing purity. A staged cooling strategy is often employed in industry, such as natural cooling to below 900°C in a high-temperature zone, followed by accelerated cooling through forced ventilation or water cooling, to balance lattice stability and impurity control. Furthermore, the cooling rate must be tailored to the type of calcining equipment. Continuous production in rotary kilns requires controlling material residence time by adjusting the kiln speed, while batch kilns require optimized airflow distribution within the furnace to achieve uniform cooling.
The relevance of the calcining temperature profile to the performance of calcined zinc oxide products is reflected in multiple indicators. Fully calcined zinc oxide has a specific gravity of 2.0 to 2.5, significantly improving its fluidity compared to uncalcined products and reducing shrinkage cracking in glazes. Calcined zinc oxide with a complete crystal structure offers a stronger fluxing effect, reducing the glaze's coefficient of expansion and improving thermal stability. Products with properly controlled impurities exhibit higher selectivity in catalytic reactions, such as in rubber vulcanization, reducing side reactions and extending the material's service life. Conversely, an uncontrolled temperature profile can lead to defects in the calcined zinc oxide, such as inclusions, large balls, coarse grinding, and secondary iron contamination, directly impacting the quality of downstream products.
In practical applications, the optimization of the calcination temperature profile requires dynamic adjustments based on raw material characteristics, equipment performance, and product standards. For example, calcined zinc oxide produced by the wet process, due to its low impurity content, can be appropriately maintained at a shorter hold time. However, the higher iron content of raw materials produced by the direct process requires a longer residence time in the high-temperature zone to promote the volatilization of the iron oxide. In addition, modern calcining equipment uses intelligent control systems to achieve precise control of the temperature curve. For example, an infrared thermometer is used to monitor the temperature field in the kiln in real time, and the fuel supply is dynamically adjusted in combination with a feedback algorithm to ensure the stability of the temperature curve, thereby significantly improving the consistency of calcined zinc oxide products.
