Advantages of multistage centrifugal extractors:

Compared with traditional liquid-liquid extraction equipment, Robatel centrifugal extractors offer the following advantages:

- short retention time and low product hold-up

- high centrifugal force for efficient phase separation

- rapid operational equilibrium facilitating batch operation when frequent stops are required

- high extraction capacity in compact equipment

- extremely rugged construction with a large number of stages assembled in a single rotor: up to 5

stages for the LX 320 and LX 360 up to 7 stages for the LX 520 and up to 6 stages for the LX 570

- extraction efficiency (measured as the number of theoretical stages) is independent of the total

and relative flowrates of the two phases. each mechanical stage is nearly equivalent to a

theoretical stage

- hydraulic stability simplifies operation and eliminates the need for delicate adjustement of the

liquids inlet and outlet pressures

- low installation cost (no special foundation; one drive motor for several extraction stages)

- the auxiliary apparatus is simplified

Operation principle:

The feed solution initially containing one or more solutes (heavy phase in the schematic) and an immiscible solvent having a different density (light phase in the schematic) flow counter-currently through the mechanical stages which are stacked in the extractor's rotor. The succesive mixing and separation operations performed in each mechanical stage permit the mass transfer of the solutes from the feed to the solvent.

Each mechanical stage is nearly equivalent to a theoretical stage and consists of:

- a mixing chamber where where the two phases are mixed by means of a stationary agitation disk

(item A) mounted on a central drum which is attached to the casing's cover

The high relative speed between the stationary agitation disk and the rotating walls of the mixing chamber creates an extremely fine emulsion with a high interfacial area which facilitates mass transfer such that equilibrium solute concentrations are rapidly obtained, even if the phase flow ratio is high. The agitation disk and the mixing chamber's inlet and outlet channels form a pump which draws the two phases from the adjacent stages and transfers the emulsion to:

- a setting chamber where the two previously-mixed liquids are thoroughly separated by centrifugal

force. A pair of overflow weirs (items B and C) stabilize the interphase position such that it is

independent of the liquids flowrates.

The heavy phase overflow weirs (item B) are interchangeable disks whose internal diameter is selected as a function of the phase density ratio to optimize the extractor's performance.

Moreover, each set of overflow weirs is suitable for a wide range of phase density ratios (e.g. from 1.20 to 1.35) to minimize the number of adjustments required.

The two phases are fed into the extractor through two pipes mounted on the casing's cover. |Therefore there are no mechanical seals required.

The enriched solvent (extract) and the impoverished phase (raffinate) are removed from the extractor either by gravity discharge, or by centripelal turbines which impart a discharge pressure, permitting the connection of several extractors in series for applications requiring a large number of stages.

When the bowl comes to a stop the liquids are drained through a pipe on the bottom of the casing.

Construction characteristics:

- These extractors are mounted on a cast-iron frame with anti-vibration supports. Therefore they do

not require a special foundation.

- The rotor is supported by a solid stainless steel hub mounted on a shaft made of high strength

stainless steel that is equipped with roller bearings

- The casing forms a vapor-tight enclosure around the rotor, permitting operation with inert gas

blanketing. The shaft is equipped with lip seals.

- The LX 320 and LX 520 models may be equipped with gravity outlets or with one or two turbines for

pressurized discharge

- For the LX 360 and LX 570 models the light phase is always discharged by a turbine or by gravity

- All parts in contact with the liquids to be treated are 316 L stainless steel 18 Cr, 10 Ni, 2,5 Mo

(other materials on request)

- All models are delivered with an electric motor connected to the rotor by a V-belt drive with

interchangeable pulleys for speed adjustment

- The LX 520 and LX 570 models may be supplied with a hydraulic coupling to reduce the motor inrush

current and permit the use of a standing motor

If the hydraulic coupling is not supplied, a standard motor can be used in conjunction with:

- an electronic soft-starter

- or an A.C. variable frequency drive which provides variable speed operation