How does ceramic membrane achieve oil-water separation?

China is one of the countries with the most scarce water resources in the world. Water pollution has further exacerbated the shortage of water resources, forming a vicious cycle that endangers the ecological environment, affects people's health, and restricts the development of industry and agriculture. One important source of water pollution is oily wastewater. The treatment of oily wastewater is a global challenge. If effective oil-water separation technology is not adopted, it not only causes economic losses but also serious environmental pollution.

The Classification of "oil":

The oil in oily wastewater is mainly composed of alkanes, aromatics, and cycloalkanes. The dispersed state varies depending on the oil content, properties of the water body, and the influence of substances such as surfactants and electrolytes contained in the water. According to the size of oil droplets, the oil in oily wastewater can be divided into four categories: suspended oil, dispersed oil, emulsified oil, and dissolved oil.

Suspended oil: Oil droplet size ≥ 100 μ m. Oil exists in the form of a continuous phase in water.

Disperse oil: oil droplets with a particle size of 10-100 μ Between m, oil exists in the form of droplets in water, which easily aggregate into suspended oil and float on the water surface.

Emulsified oil: Oil droplets with a particle size between 0.1 and 10 μ Between m, it has high stability, and emulsifiers can make oil more evenly dispersed in continuous phase water, thereby achieving a certain thermodynamic stability of the system.

Dissolved oil: Oil droplet size ≤ 0.1 μ m. Oil combines with other substances in the form of chemical bonds, making it difficult to separate.

Main methods for oil-water separation

At present, the technologies for oil-water separation mainly include chemical, biological, and physical methods. Chemical methods include coagulation, oxidation, acidification, and salt precipitation; Biological methods include activated sludge method and biofilm method; Physical methods include coarsening, flotation, adsorption, and membrane separation. However, these methods all have their own drawbacks, such as the introduction of new impurities by coagulation and oxidation methods, the corrosion of equipment by acidification methods, low efficiency of oil and water treatment by salt precipitation methods, high requirements for sewage types by activated sludge and biofilm methods, and high cost and energy consumption by flotation and adsorption methods. These methods cannot effectively separate emulsified oil-water mixtures, especially when the particle size of emulsified oil droplets is less than 20 μ When m is reached, an electric field needs to be applied or chemical substances need to be added to de emulsify, resulting in energy consumption and secondary pollution.

Compared with the above methods, membrane separation technology can adjust the pore size as needed to achieve precise separation. Moreover, membrane separation method does not require the introduction of a third phase, has mild operating conditions, and high separation efficiency, making it an effective way to achieve oil-water separation.

Among them, ceramic films have good chemical stability, thermal stability, and mechanical stability, and can work under some harsh conditions such as corrosive and high-temperature environments. At the same time, ceramic membranes often have high strength, which is conducive to high-strength backwashing. More importantly, most inorganic ceramic membranes have hydrophilic properties, which can reduce the aggregation of oil droplets at the membrane pores and to some extent reduce membrane fouling.

The oil-water separation mechanism of ceramic membranes

Ceramic membrane is a porous ceramic thin membrane material with selective separation function, which is formed by high-temperature sintering of alumina (Al2O3), titanium oxide (TiO2), zirconia (ZrO2) and other materials. Traditional ceramic membranes have an asymmetric and pore gradient microstructure, consisting of a porous support layer, a transition layer, and an active separation layer.

Ceramic membrane filtration is a fluid separation process in the form of cross flow filtration. The membrane separation mechanism of microfiltration membranes and ultrafiltration membranes commonly used for oil-water separation is generally based on the screening principle. The separation of oil particles mainly depends on the size of the membrane pore size, that is, the raw material liquid is in the membrane tube

High speed flow, driven by pressure, allows molecular substances (water molecules) to pass through the membrane, while macromolecules (oil droplets) are trapped by the membrane to achieve solid-liquid separation, concentration, and purification.

Low cost preparation methods for ceramic membranes

Compared to the cost of raw materials, the complex preparation process of ceramic films poses a more challenging issue of high cost. Research has shown that material cost, sintering energy consumption, and manufacturing process cost account for 20%, 60%, and 20% of the total cost of membrane preparation, respectively. Therefore, reducing sintering energy consumption can significantly reduce the preparation cost of ceramic membranes.

The main ways to reduce energy consumption in membrane sintering can be roughly divided into three types:

(1) Reduce sintering temperature;

(2) Reduce sintering time;

(3) Reduce sintering frequency.

One effective strategy to reduce the number of sintering cycles is to perform one-step sintering on multi-layer asymmetric membranes, which can be further divided into layer coating co sintering method and one-step forming sintering method.

1. Layer by layer coating co sintering method

Layer by layer coating co sintering refers to the gradual coating of transition layer and functional layer on a macroporous substrate, followed by one-step co sintering to prepare a film. On the one hand, this method can omit the layer by layer sintering steps of the transition layer or functional layer (separation layer), greatly reducing the energy consumption and preparation cycle required for multiple sintering, and lowering the preparation cost. On the other hand, the size of oil droplets in most oil in water lotion varies from tens of nanometers to tens of microns, while the existing research mainly focuses on the treatment of micron sized oil droplets. It is difficult to remove a large number of 2-200nm oil particles in industrial processes, because it is difficult to prepare small pore ceramic membranes through a one-step or two-step simple coating process. Therefore, the ceramic ultrafiltration membrane prepared by layer by layer coating co sintering method has significant advantages in removing small oil particles, and is currently one of the most effective ways to prepare low-cost and high-precision oil-water separation ceramic membranes.

2. One step molding sintering method

Centrifugal casting is a technique that uses centrifugal force to settle ceramic particles with different sizes and densities in a suspension at different speeds, ultimately forming a gradient asymmetric structure. Freeze casting, also known as ice template method, involves freezing and sublimating the solvent in a frozen suspension under reduced pressure and temperature, during which crystallization grows to construct a gradient distributed pore structure. Both of these methods can achieve one-step sintering to prepare asymmetric oil-water separation ceramic membranes, but their industrial applications are limited due to the involvement of precision equipment or the consumption of additional energy.

3. Phase conversion membrane technology

The phase conversion membrane technology was first applied in the preparation of polymer membranes, among which the non solvent induced phase conversion method (NIPS) is the most commonly used phase conversion membrane technology. Compared with the above various methods, the simple membrane preparation process makes it more potential for large-scale application. At present, phase conversion assisted sintering technology (phase conversion/sintering) has become one of the important low-cost ceramic membrane preparation techniques and is widely used in the preparation of asymmetric ceramic membranes. In the process of preparing oil-water separation ceramic membranes using phase conversion/sintering, various factors such as changing the casting solution system and phase conversion conditions can be used to effectively regulate the pore structure (finger shaped or sponge shaped pores, etc.) and oil-water separation performance of the membrane.