The Wet Grinding Composite Process of Calcined Kaolin and Titanium Dioxide

The mechanical physical composite of calcined kaolin and titanium dioxide was conducted using a wet ultrafine grinding method. This study investigated the effects of the grinding composite process conditions on the physical properties such as whiteness and covering power of the calcined kaolin/titanium dioxide composite powder material through orthogonal and single-factor experiments. The results indicate that the composite ratio, grinding time, pH, and suspension mass fraction are the main factors influencing the coating effect. Under conditions of a 50:50 ratio of calcined kaolin/titanium dioxide, 10 minutes of grinding time, pH of 5, and 50% mass fraction of kaolin in suspension, the produced composite powder closely matches titanium dioxide in major physical performance indicators and can be used as a substitute material for titanium dioxide.

IntroductionTitanium dioxide is a widely used and high-performance white pigment in various industries such as coatings, plastics, papermaking, ink, rubber, chemical fibers, and cosmetics. With increasing demand for titanium dioxide, its supply-demand imbalance has become prominent. Due to the significant investment required for research, development, and production of titanium dioxide, coupled with the heavy environmental burden in the production process, there's a push among scientists and users to develop alternative materials with lower production costs and environmental impact while having performance levels close to titanium dioxide.

This study utilized the wet grinding method to mechanically physically composite ultrafine calcined kaolin with titanium dioxide, preparing composite powder materials with particle size, whiteness, covering power, and other main physical performance indicators similar to titanium dioxide.

Sample, Method, and Coating Effect EvaluationSampleTitanium dioxide was obtained from a titanium dioxide company in Liaoning, with primary physical indicators including whiteness of 96.5, particle size D10=0.14μm, D50=0.70μm, D97=2.40μm, and covering power of 23.35g/m².Ultrafine calcined kaolin was sourced from a kaolin company in Inner Mongolia, with primary physical indicators including whiteness of 96.3, particle size D10=0.18μm, D50=0.9μm, D97=2.96μm, and covering power of 181.88g/m².Grinding Media: Yttria-stabilized zirconia beads with a density of 6.0.MethodExperiments were conducted using the Mi.nI_Zeta horizontal sand mill manufactured by Netch (Shanghai) Instruments Co., Ltd. Initially, a titanium dioxide suspension with a certain mass fraction was ground to a specific particle size. Then, adjusting the pH, a certain amount of dispersant and a certain mass fraction of ultrafine calcined kaolin suspension were added for composite grinding, followed by drying and dispersing the composite product.Characterization MethodParticle size distribution was analyzed using the BT-1500 centrifugal sedimentation particle size analyzer produced by Dandong Best Instruments Co., Ltd. Whiteness was measured using the DN-B type whiteness meter produced by Hangzhou Gaohui Automation Instrument Co., Ltd. Covering power of the samples was measured using a black and white grid glass plate.

Results and DiscussionOrthogonal ExperimentThe results of the four-factor (kaolin mass fraction, pH, grinding time, composite mass ratio) three-level orthogonal experiment and analysis are presented in Table 1. It's evident that the significance sequence of each factor affecting the experimental results (covering power) is composite mass ratio > pH > kaolin mass fraction > grinding time. The covering power of the coated product is better at a slurry mass fraction of 50%, pH between 5 and 6, and a 50:50 composite mass ratio. The grinding time has an insignificant impact on covering power.Wet Grinding Composite Process of Calcined Kaolin and Titanium Dioxide

Single-Factor ExperimentpHAt a kaolin slurry mass fraction of 50%, a composite mass ratio of 50/50, and a grinding time of 10 minutes, the impact of pH on the whiteness and covering power of the composite powder is shown in Figure 1.From Figure 1, it's observed that sample whiteness gradually increases with pH from 3 to 7, while covering power is optimal at pH=5. Checking the Zeta potential of kaolin and titanium dioxide at different pH levels, it's noted that at this pH, the Zeta potentials of both minerals are opposite and have a large difference, indicating strong electrostatic adsorption. Hence, pH=5 is preferable.

Kaolin Slurry Mass FractionAt pH=5, with a grinding time of 10 minutes and a composite mass ratio of 50/50, the impact of kaolin slurry mass fraction on the particle size and covering power of the composite powder is shown in Figure 2.

From Figure 2, it's observed that the mass fraction has a minimal impact on whiteness, while covering power shows a gentle change between 30% and 50%. Considering the issue of water usage, 50% seems more suitable.Grinding TimeAt a kaolin slurry mass fraction of 50%, pH=5, and a composite mass ratio of 50/50, the impact of grinding time on the whiteness and covering power of the composite powder is shown in Figure 3.Wet Grinding Composite Process of Calcined Kaolin and Titanium Dioxide

From Figure 3, it's seen that grinding time has little effect on whiteness, while covering power is optimal at a grinding time of 10 minutes.Composite Mass RatioAt pH=5, a grinding time of 10 minutes, and a kaolin slurry mass fraction of 50%, the experimental results for composite mass ratios (calcined kaolin g/titanium dioxide g) of 80/20, 70/30, 60/40, 50/50, and 40/60 are shown in Figure 4. From Figure 4, it's observed that the composite mass ratio has minimal impact on whiteness. Covering power shows little variation between 70/30 and 40/60, with 50/50 showing the best performance. Under a 50/50 ratio, the composite product's particle size is D50=0.59μm, D97=2.10μm, significantly improving covering power compared to kaolin alone and nearly equaling the covering power of 100% titanium dioxide.

Figure 5 illustrates that the covering power of the expanded test samples from 70/30 to 50/50 compared to titanium dioxide follows a similar trend to the small test samples, reaching optimum at 50/50. Additionally, the covering power of the 70/30 product significantly improves when compared to the original kaolin sample.

Conclusiona. The results of the orthogonal experiment indicate that the main factors affecting the physical properties of the calcined kaolin/titanium dioxide composite powder material are the composite mass ratio, grinding time, pH, and suspension mass fraction.b. Optimal grinding composite process conditions are: composite mass ratio=50/50, grinding time=10 minutes, pH=5, suspension mass fraction=50%. Under these conditions, the calcined kaolin/titanium dioxide composite powder material closely matches the main physical properties of titanium dioxide and can be used as a substitute in certain application fields.c. The results of laboratory scale-up tests are consistent with small-scale experiments.