Applying Taguchi method for optimization of the synthesis condition of nano-porous alumina membrane by slip casting method
Introduction
Ceramic membranes have been used in a number of industries including water purification or filtration, clarification and sterilization of beverages, concentration of proteins in the food and dairy industries, purification and recovery of lignin in paper industries. In addition they can be used in the petrochemical industry to separate high molecular hydrocarbons at elevated temperatures [1].
Some outstanding properties of ceramic membranes such as long lifetime, very good separation ability, constant filter quality and catalytic properties which are all due to their intrinsic nature have given considerable advantages to this group of membrane materials. These advantages along with some others such as high chemical, mechanical, thermal and microbiological stability and improvement in their fabrication techniques have caused a rapid growth in their use in recent years. Alumina, silica, titania and zirconia are different ceramic materials used for the synthesis of the porous membranes which among them alumina is the most popular [1], [2].
From the most important properties of alumina membranes can point to high pressure resistance, back flushable, permanent resistance to concentrated hot acidic and caustic solutions, high temperature resistance, sterilisable by steam, high permeation flux and their resistance to oxidation and solvents.
Long operating experience of users with ceramic membranes based on aluminum oxide, even under extreme conditions, has caused this type of membrane to be considered as a dependable filter medium with an excellent chemical, thermal and mechanical strength that is matched by almost no other materials [2].
High permeability property of ceramic membranes can only be achieved in an asymmetric configuration. The advantage of creating an asymmetric membrane is to produce a smaller pore size over a thinner region, thereby giving superior permeation properties. There are some different methods to synthesis asymmetric ceramic membrane known as: slip casting, sol–gel, gel casting, sedimentation, dip coating, etc. Slip casting is one of common techniques used to fabricate ceramics with simple to complex shapes from particle suspensions. In this method a ceramic powder suspension is poured into a porous plaster mould of the desired shape. The liquid of the initial slip is withdrawn into the pores of the mould by the capillary action. As the liquid penetrates through the mould, a cast or cake is simultaneously shaped on the plaster surface. The depth of liquid penetration into the mould and cake thickness both increase with time [2], [3].
Two driving forces due to the gravity force and the capillary suction of the plaster mould are responsible for particle segregation in the slip casting process. Both of these depend on other factors such as particle density, particle size distribution, particle interaction forces and solid content. In the slip casting process, the separation of particles by size (or density) under influence of these two driving forces can occur through two different segregation mechanisms: (i) settling of the coarser (denser) particles; (ii) clogging the cake by the finer (lighter) ones. In contrast with sedimentation that occurs only in one direction (vertical), the clogging effect can occur in any direction parallel to liquid flow, in processes like slip casting and filtration. Fine particles have a lower inertia momentum and are more easily transported by the fluid. Thus, they will be the first to be deposited at the mould wall. Some can even penetrate and clog the larger pores within the mould [4].
The key factor to gain asymmetric structure is to prepare colloidally unstable or met stable suspensions. This can be achieved by using powder with broad particle size distribution and dispersant, causing segregation based on particles diameter. This produces a functionally gradient or continuously enlarging mean pore diameter profile from bottom to top over the cross section of the structure [5], [6].
Binders are used in casting processes to provide sufficient strength to the body so that the green bodies can be molded and retained in the desired shape without breaking or damage, before and during sintering process. Also they cause to achieve higher porosity after they are burned out during sintering process [3], [6].
The objective of this study was to estimate the best synthesis conditions of alumina ultra filtration membrane based on slip casting route by using Taguchi experimental design. The main parameters were sintering temperature, solid content, and the binder content. Moreover, effect of different ball milling times on sintered body properties was probed. In Taguchi method, the results of experiments were analyzed to achieve the following objectives: (1) to find the best or optimal synthesis condition for the product or process, (2) to identify the contribution of individual factors and (3) to estimate the value of the defined response under optimal condition. Analysis of variance (ANOVA), was also used to analyze the results of experiments. By studying the main effects of each factor, the general trends of the influencing factors, can be characterized.
Section snippets
Selection of effecting factors and their levels
Three factors with three levels were chosen and studied in this work.
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Sintering temperature: 1400 °C, 1500 °C and 1600 °C.
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Solid content: 15%, 20% and 30%.
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Amount of PVA solution per 100 g of alumina powder: 100 cc, 200 cc and 300 cc.
Selection of orthogonal array (OA) and assignment of factors
Standard table known as L9 OA with three levels and three factors (shown in Table 1) was used for the design of the experiments. Each row in the table represents a trial condition with the factor levels. The columns correspond to the factors specified in this study and each
Sedimentation tests
As it is shown in Fig. 1, the optimum amount of Tiron tended to increase with an increase in solid content. The same trend was observed before by other researchers with another kind of dispersant [7]. Sedimentation height decreased till optimum amount of Tiron was reached and then remained constant or even increased which was in agreement with the findings of others [7], [8], [9], [10], [11]. When 15 wt% alumina suspensions was used, the proper amount of Tiron that led to the lowest sediment
Conclusions
In this work Taguchi method was implemented to find the optimum condition for synthesizing an alumina ceramic from porosity point of view. The results obtained from experiments revealed that:
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The optimum amount of dispersant was dependent on the solid content and it became larger for bigger value of the solid content.
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Ball milling had important effect on porosity and pore size but high milling times caused defects appeared in the surface of samples.
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Sintering temperature was the most effective
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