Radial Control Head
Control Head

Original liquid or suspension from which during an extraction an extract can be made that is enriched with a certain component, often with the aid of an extraction agent.


Plow-like transport implement located at the bottom of circular thickeners, to convey the settled sludge to a sludge discharge opening in the center. The rake rotates at an extremely low speed to prevent a stirring up of settled sludge.


Phenomena in filters observed during the cake removal step, when the driving pressure difference is shut off. Some of the liquid remaining in the structure of the filter medium or in voids of the filter cell can be sucked back by capillary action in the demoistured filter cake.


of a filter cake by addition of liquid and application of stirring energy. Applications for this can be found in dilution washing where a yet contaminated filter cake is re-suspended in a pure wash liquid and filtered anew.


Commonly, a receiver is perceived as a cylindrical container downstream of a solidliquid separation process in which gases are expelled from a liquid. In vacuum or overpressure filters the liquid-gas (air) mixture evolves from the cake demoisturing zone while in centrifuges it is the gas (air) carried along with the centrate.

Rectangular Settling Basin
Gravity Thickener
Remanent Moisture

The liquid portion of a bulk that cannot be removed any further by mechanical means. Essentially, it comprises interstitial liquid, adhesive liquid, and inner liquid. Vacuum or overpressure filtration can also form hydraulically isolated liquid regions in the bulk.

Remanent Saturation
Residual Moisture

Mass-related definition of the liquid content in a bulk following a separation process. The determination of the residual moisture (RM) is simply performed by respective weighing of the moist and dry cakes. The mass of the liquid mL is then related to the total mass of the moist bulk mtot, i. e. the sum of solids mass ms and liquids mass mL. The residual moisture is stated as [mass %]. The residual moisture of materials with different densities cannot be compared with each other:

Residual Moisture Measurement

determination of the residual moisture of moist materials (e. g. filter cake, sediment, bulk) is either performed off-line or on-line. A representative sample is gravimetrically analyzed, i. e. the moist material is weighed, then dried, and weighed again, in the off-line technique, while in on-line determination the residual moisture is measured directly in the product flow. It should be mentioned, that both the microwave technique (i. e. integral residual moisture) as well as the infrared absorption process (i. e. measurement of the surface moisture) have proven reliabilities.

Residual Product Layer

In a number of filter types, such as the pan filter or the peeler centrifuge, the filter cake shall not be removed completely, because a mechanical cake discharge device like a screw or a peeler knife can destroy the filter medium. Therefore a several mm thick product layer is left on the filter medium. Over the course of a number of filtration cycles, this can clog the medium with fine particles and render it impermeable. It has to be regenerated periodically by back flushing (backflush) or removed. The existence of the residual product layer is of advantage for the extensive prevention of turbidity at the beginning of the cake formation, since it acts as an additional filter medium.

Residual Volume Filtration

In discontinuously working candle filters or leaf filters the problem arises of how to process the residual suspension volume left in the lower vessel section after the main filtration phase is completed. One possibility is to let it drain off and filter it with the next batch. Another one is to filter the heel, i. e. the residual volume, by spraying it onto the already formed filter cake via a recycle loop.

Resistance Force

The sum of pressure and friction forces exerted by a fluid flow on the surface of a particle. The component in flow direction is called resistance force and the one perpendicular to it is described as dynamic buoyancy.


Revamping of filter plants is carried out to increase the filter capacity, to reduce the residual moisture of the filter cake or to improve the handling and availability of filters. According to a special optimization program developed by the BOKELA company filters like disc, drum, pan filters etc. can be upgraded in three steps including first test trials to determine the optimization potential up to the start up of the modified filter.

Reverse Osmosis
Reynolds Number

Most important dimensionless number in viscous flow named after the English physicist Reynolds. It is the ratio of inertial to frictional forces, and contains the flow velocity v, a characteristic length l, and the kinematic viscosity ν:


is a property of some Non-Newtonian fluids. The viscosity of a rheopex solution increases with increasing duration of shear forces acting on it, i. e. the fluid becomes thicker.

Richardson & Zaki Equation

Equation describing the settling velocity u of the suspensions separation zone in hindered sedimentation where the particle size and shape lose influence on the settling rate and the concentration of particles c (volume concentration) becomes the decisive factor:

Here uSt is a fictitious settling velocity of a representative, average size particle. The actual suspension settling velocity decreases rapidly with increasing concentration.

Rip Weave
Plain Weave
Rising Channel

A vertical bore hole through the disc package of disc stack separators, used for separating immiscible liquids with different densities. The positioning of the channels depends on the volume ratio of the components to be separated, and the degree of clarity required of either component.

Roller Discharge

Special type of filter cake removal at drum filters. It is applied with sticky, dough-like, and pasty filter cakes (e. g. red mud in bauxite processing), where a scraper discharge with compressed air blowback would not be successful. The filter cake is taken up by a roller of small diameter, rotating opposite to the drum and pressing against the filter cake, which then can be cut away by a knife. Often a toothed comb is utilized instead of a knife, in order to give the cake remaining on the roller a jagged structure. This leads to an especially good connection with the cake to be newly removed.

Rotary Disc Filter
Disc Filter
Rotary Drum Filter
Drum Filter
Rotary Filter

Continuously working vacuum filter or pressure filter (see BoHiBar Filtration Hyperbaric Filter) with the filter elements rotating at a specified number of revolutions on a hollow filter shaft through which generally the filtrate pipes exit. Typical examples for this are drum filters, disc filters and pan filters. The rotary filters also include the belt filters that have a horizontal filter belt leading running around two shafts.

Rotary Pan Filter
Pan Filter
Rotary Pressure Filter
Rotary Filter
Rotary Siphon Cup

A special rotary cup in peeler centrifuges with a radius a few cm larger than the filter medium. This produces a liquid head of several cm height behind the filter medium. Its suction generates a vacuum behind the filter medium up to the vapor pressure of the liquid, and a vacuum filtration can superimpose the centrifugation. As soon as the first pores of the filter cake are demoistured, pressure compensation occurs and the siphon effect collapses.

Rotary Vacuum Filter
Rotary Filter
Rotary Valve
Star Feeder

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