Extraction equipment, also known as extractor, is a type of mass transfer equipment used for extraction operations. It can make good contact between the extraction agent and the feed liquid and achieve perfect separation of the components contained in the feed liquid. There are two types: graded contact and differential contact. In extraction equipment, one phase is usually dispersed in the state of droplets in another phase, and dispersion in the state of liquid film is rarely used. There are many types of extraction equipment, which are divided into three categories according to the equipment structure:
Mixer Settler
It consists of two parts: a mixing chamber and a clarification chamber, and is a graded contact mass transfer equipment. A stirrer is installed in the mixing chamber to promote droplet breakup and uniform mixing. Some agitators can pump the heavy phase from below, thereby ensuring that the heavy phase flows between stages. The clarification chamber is an empty chamber with a large horizontal cross-sectional area, sometimes equipped with guide plates and wire mesh to accelerate the agglomeration and stratification of droplets. Depending on the separation requirements, the mixer-clarifier can be used in a single stage or in a cascade. When the cascade operates in countercurrent, the feed liquid and extraction agent are added to the stages at both ends of the cascade, and the raffinate and extraction liquid are exported from the stages at opposite positions. The working volume of the mixing chamber can be calculated from the total flow rate of the feed liquid and extraction agent multiplied by the time required for the extraction process. The horizontal cross-sectional area of the clarification chamber can be calculated from the flow rate of the dispersed phase liquid divided by the cohesion and stratification speed of the droplets. These operating parameters must be determined experimentally. It is generally believed that when the mixing chamber per unit volume consumes the same stirring power, the stage efficiency is roughly the same. Therefore, when scaling up the design, the production equipment can be designed according to the measured extraction time and layering speed. The mixing clarifier has a simple structure, high stage efficiency, and small amplification effect, and can be adapted to various production scales, but the investment and operating costs are relatively large.
 
Extraction tower
 
Tower equipment used for extraction includes packed towers, sieve plate towers, rotating disk towers, pulsating towers, vibrating plate towers, etc. The tower bodies are all upright cylinders. The light phase enters from the bottom of the tower and overflows from the top of the tower; the heavy phase joins from the top of the tower and is derived from the bottom of the tower; both flow in opposite directions in the tower. Except for the sieve plate tower, most of the various extraction towers are differential contact mass transfer equipment. The middle part of the tower is the working section, and the two ends are separation sections, which are respectively used for the cohesion and stratification of the dispersed phase liquid and the settlement and separation of the fine droplets entrained by the continuous phase. In the packed tower and sieve plate tower used for extraction, the liquid relies on its own energy to disperse and mix, so the equipment efficiency is low and is only used in sites where extraction is easy or where the requirements are not high.
 
Centrifugal contactor
 
Centrifugal contactor/ centrifugal extractor use centrifugal force to accelerate the settlement and stratification of droplets, so it allows intensified stirring to break up the droplets, thus strengthening the extraction operation. There are two types of centrifugal extraction machines: graded contact and differential contact. The former is added to the centrifugal separator and equipped with a stirring device to form a single-stage or multi-stage centrifugal extraction machine, including Vista type and cylindrical centrifugal extraction machines. The latter's drum is equipped with a multi-layer concentric cylinder with holes in the cylinder wall, so that the liquid can be dispersed in both film and drop shapes, such as the Podbielniak centrifugal extraction machine. The centrifugal extractor is particularly suitable for systems with a small density difference between the two phases or systems that are easily emulsified. Since the residence time of the material in the machine is very short, it is also suitable for the extraction of substances with unstable chemical and physical properties.