The Role of Sand Mill in Titanium Dioxide Processing

In the production process of titanium dioxide using the chloride method and gas-phase oxidation, a significant amount of agglomerates is generated. These agglomerates cannot be fully dispersed during the pulping process. If these agglomerates are not subjected to sand milling, they will be directly encapsulated during the coating process. The agglomerates that undergo pulverization in the pulverizing machine exhibit incomplete coating and irregular particle shapes after grinding. This negatively impacts yield and is a significant factor contributing to the poor dispersion and color variation of the final product. To address this issue, experiments were conducted to screen the agglomerate quantity in the raw materials after oxidation and the slurry after sand milling, aiming to understand the role of the sand mill in the post-processing of titanium dioxide.

Working Principle of the Sand MillThe sand mill is a critical step in post-processing, required not only for the sulfate process but also for the chloride method. It is a wet, continuous production machine for dispersing ultrafine particles. The purpose of sand milling is to further break down aggregated particles, agglomerates, and flocculated particles formed in the previous steps. Due to the weak bonding between these particles, they can easily be broken apart through mechanical grinding. With the aid of dispersants, these particles re-agglomerate, resulting in some larger particles being ground to a size suitable for application. The slurry, which has undergone pulping and dispersion, is introduced into the sand mill's barrel via a slurry pump. The material and the grinding medium inside the barrel are stirred vigorously by a high-speed disperser, intensifying the collision, friction, and shear between solid particles and the grinding medium. This process aims to further grind the particles and disperse the agglomerates. The material, after grinding and dispersion, is separated by a separator, and the ground material flows out from the outlet pipe, producing a product with the desired particle size. Typically, the sand mill is used to control the particle size of titanium dioxide particles to be less than 5 micrometers.

Screening of Agglomerates Experiment2.1 Condition of Raw Material Before Sand MillingBefore entering the dispersion distribution box of the oxidized feed, the feed material was passed through 40-mesh and 60-mesh sieves for 10 minutes each. The experiment was repeated three times, and the samples were dried, weighed, and the content of neodymium dioxide was determined. The data is shown in Table 1.

In this experiment, the use of an 80-mesh sieve was eliminated due to the minimal quantity of agglomerates obtained when using a 40-mesh sieve and the blockage of the 60-mesh sieve due to excessive particle concentration. Experiments were conducted using only 40-mesh and 60-mesh sieves. Table 1 shows that the use of the 60-mesh sieve resulted in a higher quantity of agglomerates, accounting for 0.071% (on a dry basis) of the total titanium dioxide content, with titanium dioxide being the main component of the agglomerates.

2.2 High-Speed Dispersion of AgglomeratesThe oxidized feed material obtained through the 60-mesh sieve was subjected to high-speed dispersion to obtain a titanium dioxide suspension slurry. The mass of dispersed titanium dioxide was measured. The data is shown in Table 2.

Table 2 shows that high-speed dispersion alone cannot fully break the agglomerates. Approximately 99.14g of titanium dioxide (representing 42.89% of the sample) remained agglomerated in the slurry, with around 57% of the agglomerates not fully dispersed.

2.3 Screening of Agglomerates on the Outlet PipeAfter sand milling, the slurry was screened using 60-mesh and 80-mesh sieves on the outlet pipe to isolate the agglomerates. The data is presented in Table 3.

Using a 60-mesh sieve, there were almost no agglomerates, so further experiments were abandoned. When a sieve of 80 meshes was used for screening, the quantity of agglomerates was minimal, accounting for 0.00075% (on a dry basis) of the total titanium dioxide content, with 98.9% of the agglomerates being successfully milled.

ConclusionThe experimental results demonstrate that the slurry without sand milling contains a significant quantity of agglomerates. Using a 60-mesh sieve for agglomerate separation yielded wet agglomerates of 1424g/h, whereas the slurry after sand milling contained only a trace quantity of wet agglomerates, 15g/h, when using an 80-mesh sieve. The sand mill plays a crucial role in post-processing, as it can effectively break down agglomerates and agglomerated particles, almost completely opening up the agglomerates in the raw materials after oxidation. High-speed dispersion alone cannot achieve the same level of agglomerate reduction. This experiment verifies the essential role of the sand mill in the post-processing of titanium dioxide.