Review on Rotary Tube Furnace Modification Technology of Zeolite and Ceramsite Sand

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Review on Rotary Tube Furnace Modification Technology of Zeolite and Ceramsite Sand

1. Research Background and Significance of the Technology
Zeolite is a natural porous aluminosilicate mineral. Due to its unique pore structure and capabilities of ion exchange and adsorption, it is widely used in water treatment, soil improvement and other fields. However, natural zeolite has defects such as uneven pore size, limited adsorption capacity and poor thermal stability, which restrict its high-end applications. Ceramsite sand is made from clay, shale and fly ash through high-temperature sintering, and is commonly used as water treatment filter media and building insulation materials. Nevertheless, its small specific surface area, low adsorption selectivity and insufficient mechanical strength make it difficult to adapt to complex working conditions.

With the development of the environmental protection and new material industries, higher requirements have been put forward for their performance. The rotary tube furnace, featuring precise temperature control (±5℃), uniform material heating, high treatment efficiency and continuous production, provides an ideal platform for modification. Through this technology, the pore structure, surface chemical properties and mechanical properties of zeolite and ceramsite sand can be optimized in a targeted manner, improving their adsorption capacity, ion exchange efficiency and stability, and expanding their applications in water treatment, solid waste disposal and other fields. It conforms to the "green and low-carbon" concept, promotes the high-value utilization of mineral materials and industrial by-products, and is of great significance to industrial upgrading and ecological protection.

2. Core Principles of Rotary Tube Furnace Modification Treatment

2.1 Zeolite Modification Principle
It is mainly realized through thermal activation, doping modification and pore regulation:
Thermal activation: Under an inert/oxidizing atmosphere at 200-800℃, crystal water and impurity organics in zeolite are removed, reducing pore blockage (average pore size increased from 2-5nm to 5-15nm), promoting crystal structure rearrangement, exposing more ion exchange sites, and increasing adsorption capacity by 30%-50%.
Doping modification: Zeolite is mixed with metal oxides (Fe₂O₃, TiO₂, etc.) or non-metallic compounds (SiO₂, etc.) in a certain proportion and fed into the furnace. At 600-1000℃, the doped substances bond with the zeolite framework to form new active sites. For example, Fe₂O₃ doping makes the adsorption efficiency of zeolite for Pb²⁺ and Cd²⁺ exceed 95%, and TiO₂ doping endows it with photocatalytic performance, realizing simultaneous adsorption and degradation of pollutants.
Pore regulation: By controlling the furnace's heating rate (5-15℃/min), holding time (1-4h) and atmosphere (CO₂, water vapor), the pore structure can be adjusted. For instance, high-temperature treatment in a steam atmosphere promotes pore expansion and cross-linking, forming a hierarchical structure of micropores-mesopores-macropores, and the specific surface area increases from 300-500m²/g to 600-800m²/g, enhancing the adsorption capacity for macromolecular pollutants.

2.2 Ceramsite Sand Modification Principle
It focuses on pore optimization, surface modification and strength enhancement:
Pore optimization: During sintering, the heating curve (preheating 200-400℃, sintering 1000-1200℃, cooling 500-200℃) is precisely controlled, and inert gas (N₂) is introduced or foaming agents (such as carbonates) are added. For example, adding 5%-10% CaCO₃, which decomposes to produce CO₂ at high temperature, increases the porosity from 30%-40% to 50%-60% with uniform pore distribution, avoiding strength reduction.
Surface modification: Ceramsite sand is mixed with modifiers (silane coupling agent, hydroxyapatite) at high temperature, and the modifier forms a functional coating on the surface. For example, KH-550 modification hydroxylates the surface, enhancing the adsorption affinity for organic pollutants; the hydroxyapatite coating enables the phosphate removal rate to exceed 90%.
Strength enhancement: Adjusting the sintering temperature and holding time optimizes the crystal structure. For example, holding at 1100-1150℃ for 2-3h promotes the melting and recrystallization of quartz and feldspar inside, forming a dense glassy structure. The compressive strength increases from 5-10MPa to 15-20MPa, and the bulk density is ≤1.2g/cm³, meeting the lightweight and high-strength requirements of filter media.

3. Rotary Tube Furnace Modification Process Methods

3.1 Zeolite Modification Process Flow
Raw material pretreatment: Natural zeolite is crushed and ground to 100-200 mesh, washed with water to remove soluble impurities, soaked in 10%-15% hydrochloric acid for 2-4h to remove metal ions, and dried at 105℃ for 4-6h (moisture content ≤5%). For doping modification, zeolite is mixed with dopants (such as Fe₂O₃) at a ratio of 9:1-7:3, and ground in a ball mill (300-500r/min) for 30-60min until uniform.
In-furnace heating treatment: The pretreated materials are fed into the furnace through a screw feeder, with the furnace rotation speed of 5-15r/min. Thermal activation (inert atmosphere, 500-700℃, heating rate 10℃/min, holding time 2h); doping modification (oxidizing atmosphere, 700-900℃, heating rate 8℃/min, holding time 3h); pore regulation (steam atmosphere, 600-800℃, heating rate 12℃/min, holding time 2.5h).
Post-treatment and characterization: After being discharged, the materials are cooled to room temperature by water cooling or natural cooling, and screened to remove agglomerated particles. XRD is used to analyze the crystal structure, SEM to observe the pore morphology, BET to test the specific surface area and pore size distribution, and ion exchange and adsorption experiments are combined to evaluate the modification effect.

3.2 Ceramsite Sand Modification Process Flow
Raw material preparation: Fly ash, clay and shale are mixed at a ratio of 4:3:3, added with water (moisture content 15%-20%) and stirred into a paste. It is extruded into 5-10mm cylindrical green bodies by an extruder, cut, aged at room temperature for 24h, and dried at 105℃ for 8-12h. For modification, foaming agents (such as CaCO₃) or surface modifiers are added during the raw material mixing stage.
In-furnace sintering modification: The dried green bodies are fed into the furnace, with stepwise heating: preheating at 200-400℃ for 1h (to remove moisture); heating from 400-1000℃ at 5℃/min (to prevent cracking); holding at 1000-1200℃ for 2-3h (N₂ atmosphere to promote foaming); cooling from 1200-200℃ at 3℃/min (to prevent thermal stress cracking). The furnace rotation speed is 3-8r/min to ensure uniform heating.
Finished product treatment and testing: After sintering, the product is screened (5-10mm), and its bulk density, porosity and compressive strength are tested; SEM is used to observe the pore structure, FTIR to analyze surface functional groups; static adsorption experiments are conducted to test the pollutant removal capacity and evaluate the modification effect.

4. Conclusion
The rotary tube furnace modification technology provides an efficient and controllable way to improve the performance of zeolite and ceramsite sand. Through mechanisms such as thermal activation and doping modification, it significantly improves their adsorption capacity, ion exchange efficiency and mechanical properties, and has broad application prospects in water treatment, soil improvement and other fields. Although the current technology faces challenges such as high cost, pending process optimization and insufficient long-term stability, with the development of low-cost technologies, the design of multi-functional processes and the breakthrough in stability, this technology will gradually realize large-scale industrial application, provide support for the development of the environmental protection and new material industries, and promote the coordinated development of "resource utilization-ecological protection-industrial upgrading".

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