Ceramic membranes are produced in a multi-step process, and each step plays a crucial role in achieving durable, high-quality ceramic membranes. Basically, ceramic membranes are produced in four overall steps:
- A silicon carbide mix is made.
- The silicon carbide mix is extruded into the ceramic membrane substrate, and the coating is added to the membrane substrate.
- The membrane is sintered, and now this is a ceramic membrane ready for application.
However, the four steps have many more details, so let us dive into the details.
1) Silicon Carbide Mix
In the first phase, a paste is made with a mixture of multiple raw materials, which contains silicon carbide powder, dispersant, and solvent. It is crucial to use the correct raw materials and quantities to obtain consistent, high-quality membranes. The mix is homogenized thoroughly before a binder is added to strengthen the mechanical stability of the membrane.
2) Extrusion
The silicon carbide mix is extruded to the correct shape and cut at the right length in the second phase. It is imperative to extrude the coarse membrane support to produce even the most complex geometries when the mixture is wet. The membrane substrate can be extruded in customized geometries, beneficial for various filtration applications. Likewise, the membrane support must be smooth and homogeneous in order to obtain high flux and mechanical strength. Once the correct geometry is obtained, the membrane substrate should dry. If it is not dried sufficiently, the membrane shape might be damaged, which will lead to a malfunctioning membrane. Thus, it is essential to obtain complete dryness to have a stable and firm ceramic membrane substrate.
3) Coating
In the third step, a coating layer is added to the membrane substrate. The coating controls the membrane pore size, and thus, the selectivity. Furthermore, the coating provides ruggedness and durability. Learn more about what the ceramic membrane coating layer does here. The coating can be added by utilizing three methods, which are 1) spray coating, 2) dip coating, and 3) slip coating. However, dip-coating is the most preferred method due to its simplicity. Still, the selected method should be chosen with care as it affects the thickness of the layer. For instance, the dip-coating technique delivers layers within the range of 0.16-100 microns, while the spray coating technique delivers layers within 60-200 microns.
Thus, the method should be chosen based on its simplicity, the membrane geometry, and the filtration range the membrane operates within. Moreover, more layers can be added to produce upper layers with higher selectivity. Typically, up to four coating layers can be added to the membrane substrate.
Afterward, the membrane should dry again to obtain a consistent coating layer. It is essential to get an even coating layer because an uneven layer will make different parts of one membrane perform differently.
4) Sintering
In the fourth step, the ceramic membrane is burned in a high-temperature furnace with an inert atmosphere of up to 2100 ˚C for 2-3 days. This process provides durable physical and chemical properties.
In comparison, oxide-based membranes are merely sintered in a furnace of 1200-1600°C. It is the high sintering temperature of up to 2100 ˚C as well as the duration of 2-3 days that heavily add to the production costs of ceramic membranes and make the industrial-scale production of SiC membranes costly. Still, the high sintering temperature is necessary to achieve the desired physical and chemical properties of a SiC membrane.
