​Study of Wet Grinding Composite Process of Calcined Kaolin and Titanium Dioxide

Employing a wet ultrafine grinding method, a mechanical physical composite of calcined kaolin and titanium dioxide was conducted. The impact of grinding composite process conditions on the physical properties such as whiteness and covering power of the calcined kaolin/titanium dioxide composite powder material was investigated through orthogonal and single-factor experiments. The results highlight that composite ratio, grinding duration, pH, and suspension mass fraction are the primary 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 serve as a substitute material for titanium dioxide.

Introduction

Titanium dioxide is a widely used and high-performance white pigment in various industries such as coatings, plastics, papermaking, ink, rubber, chemical fibers, and cosmetics. The increasing demand for titanium dioxide has led to a prominent supply-demand imbalance. The research, development, and establishment of titanium dioxide plants require significant investments, and the production process carries a heavy environmental burden. These factors have motivated scientists and users to explore substitutes with lower production costs, reduced environmental impact, and performance levels close to titanium dioxide [3,4].

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

Sample, Method, and Coating Effect EvaluationSampleTitanium dioxide sourced 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 obtained 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 conducted using the Mi.nI_Zeta horizontal sand mill manufactured by Netch (Shanghai) Instruments Co., Ltd. Initially, a certain mass fraction of titanium dioxide suspension was ground to a specific particle size. Then, pH adjustment, 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 analyzed using the BT-1500 centrifugal sedimentation particle size analyzer produced by Dandong Best Instruments Co., Ltd. Whiteness measured using the DN-B type whiteness meter produced by Hangzhou Gaohui Automation Instrument Co., Ltd. Sample covering power measured using a black and white grid glass plate.

Results and DiscussionOrthogonal ExperimentResults of the four-factor (kaolin mass fraction, pH, grinding time, composite mass ratio) three-level orthogonal experiment and analysis 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 impact of coating time on covering power is insignificant.

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 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 indicates 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 seen 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 shown in Figure 3.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 seen 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. Orthogonal experiment results 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 align closely with small-scale experiments.