Membrane fabrication methods .1 Fabrication of membrane support

Different configurations of membranes can be fabricated using a variety of membrane fabrication techniques. The detailed discussion about various methods used for the fabrication of ceramic membranes along with their pictorial representation is presented in the following section.

1.5.1.1 Extrusion method

Fig. 1.7 Schematic of extrusion process

Extrusion is the process of fabricating inorganic membranes, which involve feeding of raw materials in paste form and it is used to fabricate membranes of tubular configuration. The

ceramic paste is forced through the opening of a die with the help of an endless screw or piston.

The membranes can be produced with various diameters as well as different numbers of channels depending upon the structure of the die used in fabrication (Fig. 1.7). The high surface to volume ratio of the fabricated tubular membranes helps in handling large feed rates, which is the prime cause of its enhanced application in various industries (Monash et al., 2013). In certain cases, the use of binders helps in improving the plasticity and binding ability of the ceramic paste (Boussemghoune et al., 2020).

1.5.1.2 Slip casting method

Slip casting is one of the most essential techniques implemented for the production of ceramic products. A slip is prepared by dispersing the raw material for membrane fabrication in a medium such as water or any other organic solvent. Use of deflocculating agents and binders in preparing the slip is known to enhance stability of the slip and enable easy handling of the green body (Adams, 1971). The slip is then poured in a predesigned mold and allowed to set for a certain duration. The wall of the mold absorbs the liquid medium, thus making the slip solidify very quickly. Once the slip is dried, the mold is removed and the dried slip subsequently takes the shape of the mold (Trunec and Maca, 2014) (Fig. 1.8).

This method is greatly being used for the fabrication of ceramic composite membranes, wherein one or more number of active layers are deposited over the already prepared ceramic support. This method is quite inexpensive as it does not involve the use of any sophisticated equipment. The fabricated green body is known to possess higher green density than the ones fabricated using pressing technique (Adams, 1971).

1.5.1.3 Pressing method

For the past few decades, pressing method is widely used for fabricating ceramics and tiles in ceramic manufacturing industries. This method of pressing can be broadly classified into axial and isostatic pressing. In both these methods, the raw materials are pressed in a mold to get the desired flat shape. The axial pressing method involves applying pressure axially in one or two directions, depending upon whether uniaxial or biaxial pressing is preferred (Monash et al., 2013). On the contrary, isostatic pressing is hydrostatic in nature, which involves the application of uniform pressure in all directions (Francis, 2015). However, so far as the literature is concerned, the membranes fabricated using fly ash as one of the key precursors are mainly manufactured using uniaxial pressing (or uniaxial compaction) method. The process of uniaxial pressing can be summarized into three main steps as shown in Fig. 1.9: filling the die

or mold, compaction at high pressure and ejection of the green compact body (Lemoisson et al., 2005). However, uniaxial pressing can be again categorized into dry as well as wet pressing.

In dry pressing, the raw materials are used in dry form, whereas in wet pressing, binder solution is added to raw materials to form a paste (Monash et al., 2013). The paste is then pressed in a similar way as dry pressing under high pressure in a mold of desired shape. The green compact body, thus obtained after compaction, needs to be dried and sintered to get membranes with desired grain density as well as mechanical strength (Francis, 2015). This is a very simple method for fabricating symmetric flat ceramic membranes with a very low fabrication cost.

The method is preferred for bulk production of flat ceramic membranes (Lemmoison et al., 2005).

Fig. 1.9 Schematic for uniaxial pressing method

1.5.1.4 Centrifugal casting method

Centrifugal casting is a process specifically used for fabricating tubular ceramic membranes.

In this method, ceramic paste is poured into a launder projecting into the end of a horizontal or vertical cylinder and centrifugal force is used to hold the ceramic paste in the cylinder wall, thus forming tubular ceramic membrane (Richardson et al., 1994). Rotational speed of the mold

is the source of centrifugal force in this process. During this process, the larger particles will move to the mold wall first, followed by the smaller particles. The quality of inner surface of the fabricated membrane primarily depends on the quantity of smaller particles present in the raw material suspension, while the quality of the outer surface is mostly dependent on the quality of the mold (Monash et al., 2013). The fabricated ceramic tubes are then dried and sintered for further use in separation processes. Fig. 1.10 is the schematic representation of centrifugal casting process used for fabrication of tubular membrane.

Fig. 1.10 Centrifugal casting process

1.5.1.5 Tape casting method

In this process, as observed in Fig. 1.11, the ceramic suspension is spread over a moving plastic conveyor belt using a doctor’s blade. The ceramic slurry used in tape casting is mainly a non- aqueous slurry, consisting of binders, plasticizers and other additives along with the raw materials used for membrane fabrication. The obtained green flat sheet membrane is then heat treated, during which the solvent vaporizes and the membrane with desired properties is formed (Trunec and Maca, 2014; Gadow and Kern, 2014). Prior to sintering, the obtained green membranes can be cut into desired shape and dimensions.

Fig. 1.11 Tape casting process

1.5.1.6 Phase inversion method

Fig. 1.12 Phase inversion process

In this process, suspension used for membrane fabrication is transformed from liquid to solid state in a controlled manner to obtain a desired membrane structure. The suspension is first cast on a suitable support surface using a Doctor’s blade and then immersed into a coagulation bath.

In case of hollow fiber membrane, the suspension needs to be extruded through a spinneret to get the desired membrane shape. The coagulation bath contains non-solvent, which gets

exchanged with the solvent in the casted membrane during immersion and, as a result, precipitation of polymer film takes place (Brown, 2001; Abdulhameed et al., 2017). The membrane body, thus obtained, is further subjected to sintering. The schematic representation of the whole process is represented in Fig. 1.12.

1.5.2 Fabrication of composite membrane

Several researchers have focused on membrane coating on pre-synthesized support to reduce the membrane pore size for an efficient filtration. The fabricated composite membranes thus have an asymmetric structure with a thin layer of permselective material deposited over highly porous support. The coating can be carried out in many ways, including dip coating, spray coating, chemical vapor deposition, hydrothermal synthesis, etc. (Fig 1.13 (a)-(d)). Dip coating is one of the cheapest coating techniques practiced by many researchers. In this process, the support membrane is dipped in a slurry solution containing the ceramic powders, binder and solvent. When the support is withdrawn from the slurry solution, a film of slurry is deposited over its surface, which is then dried and sintered for use in filtration applications (Mohammadzadeh et al., 2020). The thickness of the film deposited can be maintained by controlling the various parameters associated with the dip coating process such as withdrawal speed, immersion time, cycles of coating, loading of ceramics in the slurry, etc. (Lončarević and Čupić, 2019).

Similar to dip coating, membrane supports are also being used for fabricating composite ceramic membranes through the process of spray coating. Suspension with desired composition is sprayed over the membrane surface from a certain distance using a spray gun. The coated surface is then dried and sintered to obtain membranes with desired physical and mechanical properties (Zou et al., 2019a).

Fig. 1.13 Various methods used for fabrication of composite ceramic membranes [(a) Dip coating (b) Spray coating (c) Chemical vapor deposition (d) Hydrothermal synthesis]

Chemical vapor deposition is also gaining importance in recent times for coating purposes. In this process, the support is exposed to one or more volatile precursors, usually under vacuum.

When a precursor is in contact with the support matrix, it reacts and/or decomposes to form a coating over it (Bunshah, 1994; Behera et al., 2020). Chemical vapor deposition is known for its exceptionally high deposition rates and, hence, it is preferred over other conventional deposition processes (Creighton and Ho, 2001). In addition to these methods, hydrothermal synthesis can be used for coating fly ash supports. In this method, heating of gels or flocculates is done in the presence of water in a high-pressure autoclave within a temperature range of 373- 573 K (Avci and Önsan, 2018). Crystal formation and deposition on the support matrix as well as the surface take place during the process, thus reducing the pore size of the support membrane.