# Convective-Microwave-IR Hybrid Drying of Kaolin Clay--Kinetics of Process
## 1 Introduction  
Ceramic materials are widely used in many fields, for example in the construction industry (bricks, roof tiles), the electrical industry (insulators), as sanitary or tableware. The basic method of producing ceramics consists of the following processes: mixing raw materials with water, casting or forming the product, drying and firing [1]. The drying step is particularly important, as a material that is not sufficiently dried would be destroyed during firing. Since wet material is subject to shrinkage during drying, it can also be deformed or cracked. The basic method for drying ceramic materials is convection drying. The energy needed to heat the material and evaporate the moisture is supplied by hot air flowing past the material to be dried. This method of heat exchange is inefficient because a lot of energy is lost along with the air flowing out of the dryer. Therefore, the main disadvantages of convective drying of ceramic materials are that the process is slow, time-consuming and energy-consuming [1]. This is the reason why methods of speeding up the process or replacing the convective heat supply with other more efficient methods are sought. One possibility is to use thermal radiation [1]. The electromagnetic wave range of thermal radiation is 0.1-100 \(\upmu\)m. The infrared radiation (IR) range is 0.75-100 \(\upmu\)m. Therefore, this range is used most frequently for heat transfer, also in drying [2]. Infrared heating consists of absorbing the electromagnetic wave on the surface of the dried body. Therefore, IR radiation is used primarily to dry materials in the form of a thin layer, for example,paper, paints, coatings, adhesives, ink, textiles, board [2]. In recent years, IR drying research has been extended to food. Thermal radiation has been shown to be successfully used for carrot [3], paddy [4], sweet potato [5] and mushroom [6] drying. It is obvious that, in the case of IR drying, for the process to be efficient, the surface of the material must be characterized by high absorptivity and low reflectivity. Another possibility of supplying energy to the dried material is to use microwave heating [1]. This method of heating results from the dissipation of electromagnetic wave energy. Microwaves interact with dipole water molecules, making them vibrate. The energy of the vibrations is damped by viscous interactions and converted to heat [7]. The main advantages of microwave drying result from the direct application of heat to the moisture within the material. Thanks to this, heating can be fast and efficient with low energy losses. The operation and control of microwave drying are easy. However, microwave drying also has disadvantages. The efficient supply of energy to the inside of the dried material can cause the evaporation of moisture inside the material, causing an increase in pressure, resulting in its destruction [8]. It should also be taken into account that the microwave field generated inside the dryer is nonuniform. Industrially, microwave drying is used, among others, for lumber, textiles, paper, automobile tires and food drying [7]. Research also includes the drying of ceramics [9; 10; 11; 12]. [PERSON] and [PERSON] [13] investigated the drying of kaolin bricks numerically and compared three drying methods: convection, convection-microwave and convection-infrared. As a result of the calculations, they found that the fastest method is convection-infrared drying; however, this method can cause the greatest damage to the material. Convection-microwave drying turned out to be slightly slower than convection-infrared drying but much faster than convection drying. Since the stresses obtained as a result of convection-microwave drying were the lowest of the tested processes, this type of drying was considered the best. Investigations of hybrid drying processes involving a combination of microwave and infrared heating of the material were carried out in several works [14; 15; 16; 17; 18]. All these works concern food drying. [PERSON] et al.