Counter-Current Liquid-Liquid Extraction

Christopher Barnes


 

The Counter-Current-Liquid-Liquid extraction unit is used to validate the extraction of precious materials from wastewater of industrial processes. The unit’s main components are the frame, the electrical housing, and the mixer/settler boxes. 

 

The frame consists of two “A” shaped side panels, two panels that form the base and support the mixer/settler boxes, and the electrical housing is located at the top of frame. Inside the electrical is the wiring for the motors and the rheostat to control the motors. Each motor has its own rheostat and power switch. There is also a main power switch to control the power to the entire system.

 

The mixer/settler boxes come in pairs attached to a single base plate. There are three compartments within the mixer/settler boxes. The mixer compartment is where the two liquid phases come into contact. The two phases are fed simultaneously via a tee junction below the mixer compartment. An impeller, powered by a motor, is placed just above the port that is also connected to the tee mentioned earlier. The impeller creates a suction, pulling the two liquid phases into the mixer box. The impeller also mixes the two liquids, maximizing the interaction between the two liquids. There is a wall inside the mixer/settler box that is shorter than the walls of the entire box. This allows the binary mixture to overflow into the next compartment, the settler section. The settler section is longer than the mixer portion. Its function is to allow the two phases to separate. On the other end of the settler section is another wall that is shorter than the mixer box wall. It allows the upper layer liquid to overflow into the third section, the overflow section. Near this wall on the settler section side is another port at the base. This is for the lower layer liquid to leave the settler and move to the next mixer/settler box. The overflow section is the smallest part of a mixer/settler box and has a port on the bottom for the upper layer liquid to flow to its next destination.

 

Plastic tubing is used to connect the mixer/settler boxes in terms of liquid flow. During construction, a flow diagram is used to ensure that the sections of one mixer/settler box is connected correctly to another box. “Y” overflow weirs are used to control the height of the lower layer in each box. This in turn effects the amount of upper layer in each box. The tubing connected to the settler port, where the lower layer exits the current settler box, is connected to a an inverted “Y” intersection. The “Y” is attached to a fixed bolt via small plastic plate. Turning the screw will cause the plate to move up/down the threads of the bolt. This in turn affects the position of “Y” overflow weir. The lower layer leaving the current settler box will not flow over the inverted “Y” until the level in the settler box is high enough. Ideally, the settle portion will have a 1:1 ratio of upper and lower layer such to allow the unstable emulsion break prior to the exiting of the two fluids. This is controlled via the “Y” overflows.

 

There are two main processes that occur within the liquid-liquid extraction unit: the extraction process and the stripping process. The extraction process takes an aqueous feed and bringing into contact with an extractant. The stripping process takes the loaded extractant and strips/cleans it of the elements it removed from the aqueous feed. Different compounds can be used for the stripping agent. Flow ratio of the fluids allow the stripped product to become more concentrated too.

 

The units are flexible on how many mixer/settler boxes are needed for different extraction processes. This is usually based on contact time needed between aqueous feed and extractant. To explain the flow a feed points of the liquid-liquid extraction unit, we will say that there are four extraction boxes and four stripping boxes. These will be denoted as E4,E3,E2,E1,S4,S3,S2 and S1. The “1” boxes are the extracting and stripping boxes that are right next to each other. The numbers increase the further away from the middle of the unit. The extractant enters the extracting box that is on the opposite side of the stripping section, or the extracting box labeled with the largest number. This is E4 in this demonstration. The path of the extractant,in terms of mixer/settler boxes is: 

 

E4 >> E3 >> E2 >> E1 >> S1 >> S2 >> S3 >> S4

 

Extractant will be pumped into the system into the E4 mixer. It will mix with the aqueous feed there, separate from the aqueous feed in the settler box, and then overflow into the overflow compartment. The extractant will enter the tubing connecting the port to the tee at the next box, E3. This process is repeated at E3 and the extractant will flow to E2. This continues all the way down to S4. The overflow section at S4 has a line that returns the extractant to its feed tank. The extractant is recycled in this process. The Extractant is always the upper layer liquid in this process unless halogenated solvents are used in the extractant phase formulations. 

 

For counter-current liquid-liquid extraction (CCLLX),the aqueous feed is fed into E1 and travels the opposite direction of the extractant. The feed will meet extractant at a tee underneath the E1 mixer box and sucked up via the impeller in E1. The feed will then settle as the lower layer, flow to the inverted “Y” intersection, and to the next extractant box, E2. The path of aqueous feed in this demonstration is:

 

 E1 >> E2 >> E3 >> E4

 

After the feed reaches the extractant box at the end of the unit, it leaves the system and is collected as Raffinate in the process. 

 

The stripping agent typically follows the same direction as the aqueous feed(opposite direction of extractant) and is controlled by similar inverted “Y” overflows. It would enter the system at S4 and travel to S1, where it would leave the system as the product. In this demonstration, we are using cross-current stripping. Our stripping agent is pumped to each individual stripping box and controlled with valves. Once a desired amount of stripping agent is added to the stripping boxes, the pump is turned off and valves shut. This gives us a fixed amount of lower layer and not a continuous flow of stripping agent. The stripping boxes have internal recycling piping so the stripping agent will contact the extractant multiple times, allowing for a higher concentration of desired product. Once the stripping agent is nearly spent, the lower layer is drained with a control valve and then fresh strip is pumped back into the stripping boxes. 

 

There are additional components  that can be incorporated into the CCLLX unit. In this demonstration we are using pH controller in our extractant boxes and a weak acid stripping.