Evaluating Ceramic Granules Produced by a Spray Drying Process

Ceramics are found in a diverse range of industries and applications, right from a simple cereal bowl to sophisticated components of a mobile phone, to body armor.

Although the applications of ceramics vary widely, their production processes are similar. All ceramics are produced by pressing and then firing powders produced
using a spray drying technique. In the spray drying process, ceramic slurries are transformed into free flowing spherical powders that have a narrow particle size distribution. Fine material is desirable in the manufacture of ceramics as this leads to a high packing density. A high packing density ensures minimal shrinkage when the ceramic component is subjected to baking despite the high cohesiveness and poor flowability.

When raw ceramic material and additives are dry blended, undesirable separation of various components may occur. In order to overcome these limitations, the mixture is blended in slurry to achieve a homogenous product prior to the spray drying process. The feed slurry is atomized by using a nozzle or a rotary atomizer in the spray drying process to form tiny droplets. Next, the atomized spray is blended with hot drying gas for evaporating the droplets to form powder. The dry powder is then used for making various ceramic components.

Although a lot of investigation has been conducted on the requirements of making a good slurry, the granular product has not been explored fully. The mechanical properties of the ‘green body’ and fired ceramic depend largely on the particle size, and the malleability and fluidity of the end product depend on the proportion of fine material and the presence of irregular particles. There is an increase in the inter-particle attraction forces as the size of particle reduces, and a simultaneous lowering of the influence of gravitational forces.

As a result, the flow of finer particles is restricted under the influence of gravity compared to coarser material. The frictional forces are greater in particles that are irregularly shaped, which in turn reduces their flowability. Therefore, in order to optimize the manufacturing conditions and the corresponding properties of the particles formed, it is important to understand the relationship between the size and shape of the granular material and the characteristics of the final product.

This article investigates the use of Morphologi G3 in determining the size and shape of granular particles that are obtained from a spray drying method. This experiment was conducted in collaboration with the Preci Corporation. The spray dryer used is shown in Figure 1.

Figure 1. Photographs of spray drying equipment at Preci Corporation.

Experimental

For the experiment, two test samples of W/Ni/Fe-based and Al₂O₃ particles were prepared by the spray drying method to almost the same particle size using the equipment shown above. Next, these samples were dispersed in the automated Sample Dispersion Unit (SDU) used on the Morphologi G3. The dispersion pressure was maintained at 1.0bar. A 5X objective was used to analyze these dispersions and the size distributions were compared. Based on the circularity index, the shape of the particles was also analyzed. To ensure a uniform comparison of particles of the same size, only those particles having a circular equivalent diameter (CED) greater than 10µm were considered.

Results

Size Measurement

The number and volume based distributions for both the samples are represented in Figure 2. It can be seen that the volume-based distribution was the same for both samples, while the number-based distribution showed a higher number of fines in the W/Ni/Fe sample.

Figure 2. Top: volume-based PSD, Bottom: number-based PSD.

Particles with a CED less than 10µm may be considered as fines in this case. Based on this consideration, the proportion of fine material may be determined (Figure 3). The proportion was therefore more in the W/Ni/Fe sample.

Figure 3. Chart showing the proportion of fine particles for alumina and W/Ni/Fe.

Shape Measurements

For the circularity measurements, particles with a high sensitivity (HS) value of one were considered as spherical particles, while those with a HS circularity value less than one were considered as irregular particles (Figure 4).

Figure 4. High sensitivity (HS) circularity measurements of alumina and W/Ni/Fe.

It was found that the Al₂O₃ sample contained more circular particles when compared with the W/Ni/Fe sample. The alumina particles were found to be more rounded than W/Ni/Fe particles, irrespective of any differences in size. The images of the particles around the mean CED for both samples (Figure 5) demonstrate the size and shape differences between the two samples. It is evident from the figure that the W/Ni/Fe sample particles are smaller and show irregularity in shape.

Figure 5. Particle images around the CED mean of each sample.

Conclusions

Based on the results obtained, the W/Ni/Fe sample was found to contain a greater proportion of fine material and irregular particles when compared with the alumina sample. This alumina sample is expected to exhibit greater malleability and flowability during the manufacture of ceramics. The manufacturing processes can be optimized for obtaining powders with optimal characteristics by monitoring the shape and size of granules produced by the spray drying slurries.

This information has been sourced, reviewed and adapted from materials provided by Malvern Panalytical.

For more information on this source, please visit Malvern Panalytical.