Innovations in Drying Technology for Kaolin Projects

(Zha Jianjun, Changzhou Wujin Mingxing Drying Equipment Co., Ltd., Changzhou 213004)

Abstract This paper provides a detailed account of technological innovations in drying for kaolin projects, specifically the advancements in the LPG centrifugal dryer, which have revolutionized traditional production processes. Through an analysis of drying principles, characteristics, and processes, the paper demonstrates the superior performance of the LPG centrifugal dryer in kaolin drying.

Keywords: LPG centrifugal dryer, kaolin, droplets, spray drying, technological innovation

Paper-grade kaolin products require strict particle size distribution and high whiteness, making the processing procedures relatively complex. Drying is the final and critical step in the production process. Improving drying equipment, optimizing the drying process, and scaling up, standardizing, and automating drying equipment have become imperative. Drawing on years of experience, Changzhou Wujin Mingxing Drying Equipment Co., Ltd. has developed a set of proven solutions to address key technical challenges in the kaolin drying process and can supply a complete system—from hot air furnaces, atomizers, drying towers, and dust collectors to electrical control systems. The company’s LPG series of high-speed centrifugal spray dryers and YPG pressure dryers have improved the heat capacity coefficient and evaporation intensity, while also addressing issues such as the recovery of particles from production exhaust gases and the recovery of SO₂ from flue gas.

1 LPG Centrifugal Spray Dryer

1.1 Working Principle The LPG spray dryer atomizes the kaolin slurry into fine droplets in a stream of hot air. As the droplets fall, the moisture evaporates, resulting in a powdered or granular product. Hot air is introduced at the top of the drying tower, while the slurry is pumped to the top of the tower and atomized into fine droplets by the atomizer. These droplets have a very large specific surface area; upon contact with the high-temperature hot air, the moisture evaporates rapidly, and the product is dried in a very short time before being discharged from the bottom of the drying tower. After the droplets come into contact with the hot air, the temperature of the hot air drops significantly and its humidity increases; it is then extracted as exhaust gas by an exhaust fan. The fine powder entrained in the exhaust gas is recovered using a separation device. The kaolin drying process consists of two phases: the isothermal phase and the deceleration phase. During the isothermal phase, moisture evaporation occurs on the surface of the kaolin droplets, and the evaporation rate is controlled by the diffusion rate of vapor through the surrounding gas film. The primary driving force is the temperature difference between the surrounding hot air and the droplets; the greater the temperature difference, the faster the evaporation rate. The diffusion rate of moisture through the particles exceeds the evaporation rate. When the diffusion rate decreases to the point where it can no longer maintain saturation on the particle surface, the evaporation rate begins to slow down, and the drying process enters the deceleration phase. At this point, the particle temperature begins to rise; by the end of drying, the temperature of the kaolin material approaches that of the surrounding air.

1.2 Features

This machine offers numerous advantages for drying kaolin products:

① Rapid drying. After centrifugal atomization, the surface area of the kaolin slurry increases significantly. In a high-temperature air stream, 95%–98% of the moisture can be evaporated instantly, with the drying process taking only a few seconds. This meets product quality requirements while also saving time and costs.

② The co-current spray drying configuration allows the droplets to flow in the same direction as the hot air. Although the hot air temperature is high, the temperature inside the drying chamber drops rapidly upon contact with the spray droplets, preventing the material from being overheated.

③ Since the drying process is completed instantaneously, the particles essentially retain the spherical shape of the droplets, resulting in a product with good dispersibility, flowability, and solubility.

④ The production process is simplified, and operational control is convenient. Spray drying is typically used for slurries with a moisture content of 40%–60%; even for special materials with a moisture content as high as 90%, a powdered product can be produced in a single step. After drying, the product requires no further grinding or screening, which simplifies the production process and improves product purity. Within certain ranges, operating conditions can be adjusted to effectively control the product’s particle size, bulk density, and moisture content.

⑤ To prevent contamination of the material and extend equipment service life, all parts in contact with the material are made of 1Cr18Ni9Ti stainless steel; for ease of operation, the control system employs an integrated control approach, with indicator devices and on/off controls for each component installed in the control cabinet. Our company has now successfully developed a large-scale dryer with an evaporation capacity of 6,000 kg/h.

1.3 Technical parameters are shown in Table 1.

2 Drying Characteristics and Process Analysis of Kaolin Droplets

The drying process in kaolin production can be classified as evaporation drying of soluble solid droplets. In the drying process of an LPG spray dryer, the relative velocity of droplets moving in the gas stream is generally very low for most of the time. Even if the initial velocity is high, the Reynolds number (Re) is typically very low—ranging from approximately 10⁻¹ to 10²—due to the small size of the droplets. For droplets smaller than 50 µm, even when moving at a relative velocity of 100–150 m/s, the Reynolds number (Re) does not exceed 200. When the droplet velocity rapidly decays to match the airflow velocity, the droplet moves within the range described by Stokes, where the Reynolds number is less than 2. Therefore, the theory of droplet evaporation in still air, based on boundary layer theory, can be applied to spray-drying conditions. According to available data, when the droplet diameter is 500 µm, the time required for complete drying in hot air at 300°C is approximately 4.6 s; whereas for droplets with a diameter of 100 µm and a hot air temperature of 150°C or higher, the time required for complete drying is less than 1 s. Kaolin droplets contain solutes, making the drying process more complex. During the isochronous stage, the internal moisture in the droplets rapidly diffuses to the droplet surface, where it evaporates and vaporizes. When the droplet surface solidifies, the evaporation rate slows down, and the drying process enters a deceleration stage. After a hardened outer shell forms on the droplet surface, the residual moisture inside generates a certain amount of internal pressure. When this pressure increases to the point where the outer shell ruptures and perforations form, the product takes on a porous, loose structure. Therefore, this process takes much longer than the evaporation drying of pure droplets, lasting approximately 10–15 s. From the above analysis, it can be seen that the drying time for kaolin droplets containing soluble solutes is equal to the sum of the drying times for the constant-rate and deceleration phases, and is primarily related to the initial droplet diameter and critical moisture content. The extent to which the vapor pressure is reduced due to the presence of dissolved salts in the kaolin slurry varies with the size of each droplet. Furthermore, the formation of the solid phase occurs in different sequences depending on droplet size, resulting in varying resistance to moisture transfer. During the spray drying process of slurries, their characteristics vary due to differences in the original slurry concentration. The LPG spray dryer utilizes highly efficient and advanced atomizer and air distribution technologies, automatically adjusting to accommodate slurries of different concentrations. The operating condition curves for various models of LPG spray dryers are shown in Figure 1.

Figure 1: Operating Curve of an LPG Spray Dryer

  3. Conclusion    

  Kaolin products have broad application prospects; therefore, large-scale spray dryers are bound to find widespread use in the processing of kaolin products.

  Typical Users:

  1. Yankuang Group Guangxi Beihai Kaolin Co., Ltd. 3 sets of spray drying equipment with an annual production capacity of 100,000 metric tons of kaolin (evaporation rates: 4,500 kg, 3,000 kg, and 2,000 kg), equipped with 3 sets of in-house hot air furnaces, with heat output of 4 million kcal, 3 million kcal, and 2 million kcal

  2. Huaibei Jinyan Kaolin Co., Ltd. (a subsidiary of the Huaibei Mining Bureau) — Annual production capacity of 10,000 metric tons of kaolin; LPG2000-type spray drying equipment

  3. Maoming Xinli Kaolin Co., Ltd. — Annual production capacity of 10,000 metric tons of kaolin; spray dryer

  4. Lincang Boshang Kaolin Co., Ltd., a subsidiary of Yunnan Yuntianhua Group: Annual production capacity of 30,000 metric tons of kaolin spray drying equipment, equipped with direct-fired coal-fired hot air furnaces