Sink Float Tanks

A Rotajet sink float tank separates various types of plastics according to its density whilst removing contaminates prior to upstream processing.  

Operation of Sink Float Tanks

Rotajet sink tanks separate various types of plastics by density whilst removing contaminates prior to upstream processing.  Water is generally used in float sink tanks as the density media. The specific gravity of water is 1000kgs / M3, which makes it ideal for the separation of polypropylene and polyethylene from PVC`s and ABS. Other contaminants such as glass, grit and dirt sink as they have specific gravities greater than 1. In some applications the specific gravity of the water can be increased by the addition of various chemicals. This then allows a series of Rotajet float sink tanks to separate a range of plastics.

Particle feed size:

We recommend that the in-feed material is a shred or granulate with a particle size less than 40mm. This allows the Rotajet float sink tank to operate effectively and produces the optimum environment in which the contamination and different plastics can be separated.

Feeding material into a Rotajet float sink tank:

Rotajet float sink tanks are typically fed by a belt conveyor, screw auger or blower. Depending on the application the material can be discharged onto the surface of the tank, or fed by a screw feeder. The screw feeders, are mounted above the surface of the liquid and are submerged into the tank, this gives a uniform flow into the tank, breaks up any lumps, and ensures that all materials are fully submerged at the start of the process. This produces a high purity product as all particles are forced to either float or sink.

Sinking Fraction Discharge:

The standard Rotajet float sink tanks are fitted with bottom screw discharge augers. To increase durability, the augers are manufactured completely from stainless steel and are fully welded. The screws are located in a trough in the bottom of the tanks. The troughs are fitted with easy to replace wear plates, decreasing production downtime. Depending on the throughput you require the number and diameter of the screws can be altered. The torque on the screws is mounted and feeds back to the PLC. Once the sinking fraction is discharged from the Float sink, we normally recommend drying the material in a Rotajet mechanical dryer, the water can then be pumped back to the float sink tank.

Floating Fraction Discharge:

The floating fraction is driven along the surface of the float sink tank by a series of rotating paddles. Each paddle completely submerges the floating fraction as it is pushed along the surface of the float sink tank. The speed of the rotating paddles can be changed by the PLC, varying the agitation present and the discharge rate. The floating fraction is transferred into an incline discharge screw via a weir at the end of the float sink tank.

The incline discharge screw body is manufactured with easy to replace screens; these screens allow the water from the discharge weir to be screened from the floating fraction. The water is then fed by gravity into a Rotajet Vibratory Sieve, normally fitted with a 300-micron screen. The filtered water is then pumped into a holding tank, ready for reuse. We recommend that the floating fraction is discharged into a Rotajet Mechanical Dryer. The water removed during this process can also be recirculated through the vibratory sieve for filtering and reuse.

For optimum performance, the level of working fluid has to be maintained in the Rotajet float sink tank. This is done by reusing the filtered solution in the storage tanks and introducing fresh solution as required.

sink float tank
Sink float tank for plastic separation

Contact Rotajet

We aim to get back to any enquiry within 12 hours, so please contact us if you are interested in a specific machine or would like to discuss an individual cleaning application. Our friendly team are happy to help via email at or by phone on +44 (0) 1924 271446.