Kaolin is a kind of clay and clay rock mainly composed of clay minerals of kaolinite family. It belongs to non-metallic mineral and is named after Gaoling Village of Jingdezhen, Jiangxi Province. Kaolin is divided into non-coal-bearing kaolin and coal-bearing kaolin (rock). At present, the vast majority of kaolin produced in the world belongs to the former, while the exploitation and utilization of coal-bearing kaolin (rock) is still in its infancy.

Coal - series kaolin, also known as gangue, is associated with coal. In recent years, coal-based kaolin has been calcined, acid-base modified and other treatment methods, so that it can be widely used in the preparation of high-grade papermaking filler, pigment grade products, molecular sieve and other high value-added products, can be widely used in papermaking, rubber, wire and cable industries.

Coal measures kaolin is widely distributed and has complex types in China, but not all of them can be used as raw materials to produce double "90" (whiteness >90%, oil absorption value >90g/100g) calcined kaolin for paper coating. After decades of exploration and practice of coal gangue processing and utilization workers, it is concluded that the gangue kaolin, which occurs in the main coal seams of the carboniferous and Permian coal measures in north China, is a high quality raw material for the production of high whiteness paper coating grade calcined kaolin. However, compared with the calcined products in Britain and America, it is difficult for the calcined kaolin products in China to simultaneously meet the two evaluation indexes of whiteness >90% and oil absorption value >90g/100g.

The following is a comparative study of BASF kaolin (KA) and Shuozhou Jinkun calcined kaolin (KC) soil in the United States. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) are used to observe the microstructure and phase identification of the two calcined kaolin. Through TG TG analysis to calculate the loss of combustion, and determine the grain size oil absorption value whiteness and other performance parameters, analyze the difference between the two, so as to get some enlightenment, which is convenient for the modification of coal measures kaolin.

01.Analysis of the characteristics of kaolin ore

Basf calcined kaolin

Source: Georgia Mineral Belt, the largest kaolin deposit and producing area in the United States.

Type of deposit: secondary sedimentary deposit. The kaolin deposit is characterized by its good homogeneity. In the process of natural sedimentation, kaolin is naturally graded according to particle size. Kaolinite is the main mineral composition, with more than 60% particles <2μm and ore content >90%. The kaolin is soft kaolin

Main characteristics: The ore body has high sand content and high natural whiteness, but its harmful component iron content is slightly higher, and the whiteness is low after calcination, so it usually needs to be washed, bleached, desanding and other processing.

Jinkun kaolin

Source: Pinglu District, Shuozhou City, Shanxi Province

Type of deposit : The Gaolingyan deposit in Pinglu District, Shuozhou City, Shanxi Province is a sedimentary deposit in carboniferous strata. The ore is hard clay rock, and the kaolinite ore in the area has simple minerals, mainly flake kaolinite, with an average of more than 90%, with a diameter of 0.2~1.0μm. The impurity minerals are mainly quartz and siliceous debris, with a content of 3%~10%, followed by organic matter, with a content of about 1%. Other clay minerals such as illite, hydromica, montmorillonite, etc. account for only 0.5%~1%, and metal minerals are trace, mainly limonite and pyrite. The kaolin is a hard coal series kaolin.

Main features: The average content of Al2O3 is 36.26%, the average content of Fe2O3 is 0.65%, and most of them contain about 1% organic matter and carbon. The ore itself is mostly gray and black, with low natural whiteness. It usually needs to be processed by grinding, calcination and decarbonization, with high whiteness after burning (95% whiteness). The ore quality is good.

02.Chemical composition analysis of calcined kaolin from different producing areas

The chemical composition test results of the two calcined kaolin are shown in Table 1.

As can be seen from Table 1, there is little difference in their chemical composition. The test results show that the content of CaO, MgO, K2O and Na2O in the two samples are all less than 0.5%, while the content of Fe and Ti is higher. The calculated Content of Fe2O3 in the Calcined Kaolin of The United States is 1.74%, while the content of TiO2 is 1.39%. The content of Fe2O3 in the calcined kaolin of Shuozhou coal measures is slightly lower, with the content of Fe2O3 being 0.82%. TiO2 content was 0.95%.

03. Oil absorption value and whiteness test

Oil absorption value and whiteness are the most important properties of kaolin for paper coating. There is still a big gap in oil absorption value between Chinese calcined kaolin and foreign calcined kaolin. According to GB/ T3780.2-2007 "Carbon black -- Part 2: Determination of oil absorption value", test the oil absorption value of the sample. See Table 2 for the whiteness and oil absorption value data of the two. The oil absorption value of American calcined kaolin is 30g/100g higher than that of Shuozhou calcined kaolin, and the latter is slightly better than the former in whiteness.

       04.X-ray diffraction analysis

The X-ray diffraction pattern of the sample is shown in Figure 1, where K represents kaolinite and M represents mullite. As can be seen from Figure 1, the raw kaolin of coal measures presents typical characteristic maps of kaolinite, while the calcined samples have basically become metaolin of amorphous form, with a small amount of mullite. It can be inferred that the calcination temperatures of both are around 1000℃.

05. Microscopic morphology analysis of samples

The microscopic pictures of American calcined kaolin and Shuozhou calcined coal series kaolin under scanning electron microscope are shown in Figure 2. Figure (b) shows the calcined kaolin from the United States, with obvious lamellar structure and nearly hexagonal structure. Figure (a) shows the calcined kaolin from Shuozhou coal measures, with increased particle cohesiency and obvious lamellar accumulation. It may be due to the higher calcination temperature, the coal measure kaolin dehydroxyl more thoroughly, resulting in more obvious sintering phenomenon, more structural collapse, and then more massive structure. This has also become a reason for the low oil absorption value of kaolin calcined coal series in China.

       06 . Grain size distribution of samples

The particle size interval distribution of the three different samples is shown in Figure 3. As can be seen from Figure 3, the variation trend of calcined kaolin and uncalcined coal measures kaolin in the large particle size range is basically similar, and small particles <0.5μm have a tiny protrusion. Calcined kaolin is relatively small in size, but the size distribution of American calcined kaolin is narrow. According to the particle accumulation theory, the particle size distribution is wide, and the small particles fill the gap of the large particle accumulation, and then the accumulation density increases, resulting in the gap that can store oil becomes smaller, so the oil absorption value decreases.

By comparing calcined kaolin from the two producing areas, it can be found that:

American calcined kaolin with higher oil absorption value is basically the same as Shuozhou calcined kaolin in chemical composition, but its Al2O3 content is higher, TiO2 and Fe2O3 content is also slightly higher than shuozhou calcined kaolin. It is a soft kaolin with high natural whiteness, but the whiteness decreases after calcination. On the micro level, the stacked lamellar structure is obvious, and a small amount of Mullite phase is generated. It is estimated that the calcination temperature is about 1000℃, and the particle size distribution is narrow. And Shuozhou coal measures kaolin is hard kaolin, pure texture, low content of harmful components, mostly contains about 1% of organic matter, carbon, the ore itself is mostly gray black, low natural whiteness, usually need to grind, calcination decarbonization and other processing, after burning high whiteness (95% whiteness), the ore quality is good.

Through comparison, it is found that the ultrafine calcined kaolin produced by Jinkun Company from Louzigou coal measure kaolin in Pinglu district has reached (and partly exceeded) the quality standards of American calcined soil in whiteness, particle size, coating resistance to washing, sieve residue and dispersion, but there is still a certain gap in oil absorption value.

In order to improve the oil absorption value of calcined kaolin, it can be achieved by reducing particle size, narrowing particle size distribution and avoiding sintering, or by making pores and increasing specific surface area.

Source: Li Yajun et al., Comparison of characteristics of Coal Measures Kaolin from Shuozhou and BASF Kaolin from USA