1.4 General methods for the preparation of ceramic membrane
1.4.1 Support fabrication
and 10 years for polymeric and ceramic membranes, respectively (Mulder 1991). Therefore, though ceramic membranes involve higher initial costs, their ability to prove higher flux and applicability to wide range of temperature, chemical processing conditions could favor them to be the choice in contrary to the polymeric membranes. Though the separation characteristics of ceramic membrane processes are similar to polymeric membranes, they are not yet widely applied in industrial scale applications owing to its higher cost. Under these circumstances, the development and usage of comparatively low cost membranes with longer life span is anticipated to drive the economic competitiveness of ceramic membranes in the industry.
c. Fabrication of ceramic support
d. Consolidating or firing (heat treatment)
Figure 1.5: Typical cross-section of ceramic membrane structure
a. Choice of appropriate inorganic raw materials:
The morphological property of various raw materials influences the porosity (or structural density) and pore size of the ceramic support and is therefore an important parameter to achieve membranes with desired attributes. The structural density of the support increases with decreasing grain size of the raw materials. Typically, the ratio of grain to pore size is strongly dependent on shape of the particles and is about 2.5. In order to facilitate the coating of a homogeneous thin layer on the support, the pore size of the thin layer must be adaptable with the grain size of the layer on which it is to be deposited. The presence of large pores on the internal surface of the channels cloud leads to penetration of the skin layer grains into the support which will give raise to defects such as pinholes and cracks in the membrane
morphology. The structural density of the membrane must be sufficient enough to ensure excellent mechanical resistance. On the other hand, lower structural density offers higher transport resistance for fluid through the support. Therefore, tradeoffs exits for the optimization of grain size to achieve acceptable combinations of raw materials can be used to target the variation in grain size. Thereby, additional variations in pore size distributions can be achieved after sintering.
b. Preparation of ceramic powder/paste
During the paste preparation step, the ceramic powder is usually mixed with the solvent (water) and organic additives (Burggraaf and Cot 1996) to induce plastic properties to the prepared paste. The plasticity of the paste enables flexibility to shape the ceramic structure without cohesion. Another desired effect of the organic additives is to increase the unfired material strength during the shaping and dying steps which enables the elimination of defects such as cracks. Typical organic additives used during the paste preparation include binder, plasticizers, lubricants and deflocculants. Plasticizers include plastic properties to the paste.
Generally, viscous and wetting polymers are used as plasticizers. Lubricants such as glycerin assist the paste to slide in the extrusion apparatus. By controlling the surface charge of the particles, deflocculants avoid powder agglomeration by steric effect. Moreover, mixing and pugging are also important steps in paste preparation. Mixing is an essential to obtain good dispersion and perfect homogeneity by the even distribution of constituents. Similarly, pugging is necessary to obtain paste. During pugging, the progressive addition of water to the powder mixture leads to achieving a high viscous paste. The powder preparation procedure is very simple in comparison to that of the paste preparation. During the powder preparation step, the
powder is usually mixed in a mixer or ball mill with suitable binders. Subsequently, the powders are sieved with the suitable screen (30 - 40 mesh).
c. Ceramic support fabrication
There are several methods are adopted to fabricate the ceramic support by researchers. Most of the ceramic supports are prepared by powder pressing, colloidal processing and paste processing routes. The detailed classification methods for fabrication of ceramic membrane supports are presented in Figure 1.6.
Figure 1.6: Classification of membrane supports production methods based on the precursor aggregate stage
(i) Powder pressing method
The powder pressing method has been used for the preparation of ceramic tiles and high- density products in the earlier stages. Later on, because of the characteristics of the ceramic materials, there were attempts to use this method for the fabrication of membrane supports.
Typically, two types of pressing methods, axial pressing and isostatic pressing, have been used Method of fabrication of membrane supports
Paste processing
Isostatic pressing Uniaxial
pressing
Colloidal processing Powder pressing
Centrifugal casting Slip
casting
Gel casting
Extrusion Manual Paste casting
for the fabrication of membrane supports. The axial pressing (uniaxial and biaxial) technique is economical and appropriate for high volume creation of simple geometrical forms like flat and circular supports. Axial pressing is classified into two types, namely, dry pressing and wet pressing. In order to shape the ceramic raw materials, the wet pressing requires the addition of water, whereas dry pressing can be performed without addition of water to the raw materials.
In isostatic pressing, pressure is applied from various directions to achieve better homogeny of compaction and enhance shape capability compared with uni-axial pressing. Isostatic pressing methods are classified into cold and hot isostatic pressing. In cold isostatic pressing (CIP), the isostatic pressure is created by applying external pressure onto the fluid, normally water or oil.
This pressure is uniformly applied to ceramic powder to form the required shape of the membrane support. In the case of hot isostatic pressing, the isostatic pressure is created by heating the encapsulated fluid (mostly argon gas) to the working temperature. This isostatic pressure forms the ceramic support of desired shape.
(ii) Colloidal processing method
In general, colloidal processing involves a sequence of steps such as powder synthesis and purification, colloidal/suspension preparation, consolidation into the chosen component form, elimination of the solvent phase, and heat treatment (sintering temperature) to produce the membrane support for optimal performance. Based on the consolidation mechanism, the colloidal processing methods are classified in to three major classifications, namely slip, centrifugal, and gel casting. In slip casting, ceramic membranes have been fabricated by pouring a stable slip (suspension of clay or ceramic material in water) in a porous mold, especially gypsum mold, or in nonporous surfaces, such as Petri dishes or glass plates. The centrifugal casting method is used to prepare tubular membrane supports from the slip solution subjected
to a higher centrifugal force. The biggest particles present in the suspension move first to the mold wall followed by the smaller particles. The quality of the outer surface of the tubular support depends on the surface quality of the mold, whereas the inner surface of the support depends on the suspension quality, especially the quantity of the smallest particles present in the suspension (Burggraaf and Cot 1996). The speed of the centrifuge and the particle size distribution of the slip solution are the main parameters that influence the mean pore size of the membrane supports. Gel casting forms a strong and cross-linked polymer-solvent gel after being poured into a mold. According to this process, ceramic powders are dissolved in water based monomer solution to form consistent slurry. Subsequently, the catalyst and initiator are added to the slurry and poured in the mold of desired shape. Before filling the mold, the slurry must be deaerated, and it must be poured carefully to the mold to avoid the introduction of defects, which may affect the characteristics of the membrane support. The chemical cross- linking reactions form a strong hydrogel that permanently immobilizes the ceramic particle.
The support is demolded, dried, and sintered to get the required membrane support.
(iii) Paste processing method
One of the most classical method for the preparation of ceramic membrane is paste processing.
Extrusion and manual pasting techniques are used for the fabrication of membrane supports, however; extruded supports are mostly suitable for industrial applications. Moreover, the majority of the ceramic supports are prepared by extrusion using clay materials. This is accredited to the plastic properties of the clay materials. With clay materials, the paste can be formed instantly and extruded simply to the desirable shape with lower pressure in extrusion process. The mixing of inorganic materials with additives (binders/plasticizers/lubricants) provides the necessary plasticity to the materials that gives exceptional shape forming abilities
without lost of its cohesion. Generally, cellulose derivatives (hydroxyethyl cellulose, carboxymethyl cellulose, methyl cellulose, etc.) are used as binders, organic polymers (polyvinyl alcohol, polyacrylic acid, polyethylene glycol, etc.) are utilized as plasticizers or lubricants, and a starch derivative, principally corn starch, is used as the pore-forming agent.
In the extrusion method, the homogeneous paste is forced through the opening of a die with the help of an endless screw, especially auger or extruder (in industry) or a piston (in the laboratory) (see Figure 1.7). Ceramic membranes are fabricated with various specifications (e.g., inner and outer diameter of the tubes and number of channels) by changing the geometry of the die. Tubular and multichannel membrane supports have been widely prepared by this method.
The high surface-to-volume ratio of the modules provides good opportunity to process large feed rates. Due to this reason, the membranes have enhanced implementation in industries. The most important parameters that determine the membrane properties (mean pore size and porosity) are particle size of the ceramic or clay powder, nature and proportion of organic additives, pugging and ageing of the paste, extrusion pressure, and velocity.
Figure 1.7: Schematic of an extrusion apparatus: (1) paste, (2) piston, (3) endless screw (4) die
1
2
4 3
In the manual pasting method, the membrane supports are fabricated manually on a flat porous (gypsum) or nonporous surface. The pressure is applied manually to form the required shape. It is the simplest and the oldest technique compared with other fabrication methods and does not require any instrument for fabrication. However, controlling the microstructure (to produce optimum reproducible results) is a hard challenge and requires skills to achieve membrane supports without defects.
d. Consolidation or firing (heat treatment)
The firing treatment strengthens the green membrane support. Heat treatment enables the elimination of bonded water and organic additives to finally achieve the temperature of allotropic transitions. Therefore, to visualize upon the completion of the firing step, thermogravimetric analysis is necessary. The firing treatment can be described in two stages in which the first corresponds to the densification and grain growth. During sintering, membrane properties such as pore diameters, density and mechanical resistance depend on the temperature and time of sintering.