The ceramic membrane separation process is a "staggered flow filtration" form of fluid separation process: the raw material liquid flows at high speed in the membrane tube, the clarified permeate containing small molecules is driven by pressure to pass outwards through the membrane in the vertical direction, and the turbid concentrate containing large molecules is retained by the membrane, so that the fluid can achieve the purpose of separation, concentration and purification.
Ceramic membranes are asymmetric composite membranes made from ceramic carriers with a porosity of 30% to 50% and a pore size of 50nm to 15μm, using the sol-gel method or other processes. The structure of ceramic membranes used for separation is usually sandwiched: support layer (also known as carrier layer), transition layer (also known as intermediate layer), membrane layer (also known as separation layer). The pore size of the support layer is generally 1~20μm, with a porosity of 30%~65%, and its role is to increase the mechanical strength of the membrane; the pore size of the intermediate layer is smaller than that of the support layer, and its role is to prevent the permeation of particles to the porous support layer during the preparation of the membrane layer, with a thickness of about 20~60μm and a porosity of 30%~40%; the membrane layer has a separation function, with a pore size ranging from 0.8nm~1μm , with a thickness of about 3~10μm and a porosity of 40%~55%. The pore size distribution of the entire membrane decreases from the support layer to the membrane layer, forming an asymmetrical structural distribution.
Ceramic membranes can be divided into microfiltration (pore size greater than 50nm), ultrafiltration (pore size 2~50nm), nanofiltration (pore size less than 2nm) and other types according to pore size. When separation is carried out, small molecules pass through the membrane under the action of external forces, while large molecules are retained by the membrane, thus achieving the purpose of separation, concentration, purification, de-hybridisation and de-bacterisation.
Research on ceramic membranes began in the 1940s and its development can be divided into three phases: the nuclear industry period for the isotopic separation of uranium, the liquid separation period with inorganic microfiltration and ultrafiltration membranes, and the period of comprehensive development with membrane catalytic reactions as the core. In 1998, there were 452 membrane and membrane equipment manufacturers and operating companies, including 50 metal membrane plants and 94 ceramic membrane plants.
Due to the late development period and high cost, the market share of the inorganic separation membrane field is still relatively small, in 1997 the US inorganic membrane market sales of 100 million dollars, of which ceramic membranes accounted for about 80%, accounting for only 9% of the membrane market. It is also estimated that in 2004, the world market sales of ceramic membranes exceeded US$10 billion, and the market share of inorganic membranes accounted for 12%. Due to the successful application of ceramic membranes in precision filtration and separation, their market sales are developing at an annual growth rate of 30%.