Solid/Liquid Separation (under gravity)
Thickeners
Why?
For the purposes of separating solids from liquids en masse rather than of solids classification, particularly in the colloidal region (<2µm), thickeners have several advantages over centrifuges.
What?
A thickener consists of a large (cylindrical) tank (often) with a broad conical section at the base, at the centre of which is an outlet pipe/valve assembly. The stream from this outlet is termed the underflow. Around the rim of the tank is a tray, rather like a gutter, into which clarified liquor (the overflow) flows (as a result of overspill from the rim). A set of slowly rotating mechanical rakes gently scour sediment and direct it to the underflow. Feed is introduced at the centre of the tank (and distributed across it) at some distance below the surface.
How?
Overview
Cursory consideration of the mass balance for such a device indicates that all solids must leave via the underflow and must not therefore be presented with any opportunity to reach the (overflow at the) rim of the tank. The design must ensure this.
At a given feed rate, the vertical flow velocity of liquid (both up and down) in the device will decrease as the area is increased and vice versa.
In the upper part of the tank net downward motion of solids is reduced, but in the lower section it is increased - relative to a freely settling particle.
What area of thickener will ensure a clarified overflow and how can it be reduced (to save space)?
Feed Conditioning
The surface properties of colloids may be exploited to assist separation, by adjusting conditions so that particle-particle interactions are net attractive.
Many particles carry electrostatic charge in aqueous media in general the effects of this may be:
1. Suppressed/reduced by addition of electrolyte
2. Eliminated/exploited by adjustment of pH (or pX, of some other surface reactive ion X).
3. Overwhelmed by addition of polymers/polyelectrolytes.
(usually termed flocculants)
The size density and strength of the aggregate particles which form are strongly dependent on the conditioning process. For it is necessary to ensure that conditioning agents are dispersed in the feed, and have sufficient time to interact with particle surfaces in appropriate quantities to ensure optimum effect.
Thickeners
Why?
For the purposes of separating solids from liquids en masse rather than of solids classification, particularly in the colloidal region (<2µm), thickeners have several advantages over centrifuges.
What?
A thickener consists of a large (cylindrical) tank (often) with a broad conical section at the base, at the centre of which is an outlet pipe/valve assembly. The stream from this outlet is termed the underflow. Around the rim of the tank is a tray, rather like a gutter, into which clarified liquor (the overflow) flows (as a result of overspill from the rim). A set of slowly rotating mechanical rakes gently scour sediment and direct it to the underflow. Feed is introduced at the centre of the tank (and distributed across it) at some distance below the surface.
How?
Overview
Cursory consideration of the mass balance for such a device indicates that all solids must leave via the underflow and must not therefore be presented with any opportunity to reach the (overflow at the) rim of the tank. The design must ensure this.
At a given feed rate, the vertical flow velocity of liquid (both up and down) in the device will decrease as the area is increased and vice versa.
In the upper part of the tank net downward motion of solids is reduced, but in the lower section it is increased - relative to a freely settling particle.
What area of thickener will ensure a clarified overflow and how can it be reduced (to save space)?
Feed Conditioning
The surface properties of colloids may be exploited to assist separation, by adjusting conditions so that particle-particle interactions are net attractive.
Many particles carry electrostatic charge in aqueous media in general the effects of this may be:
1. Suppressed/reduced by addition of electrolyte
2. Eliminated/exploited by adjustment of pH (or pX, of some other surface reactive ion X).
3. Overwhelmed by addition of polymers/polyelectrolytes.
(usually termed flocculants)
The size density and strength of the aggregate particles which form are strongly dependent on the conditioning process. For it is necessary to ensure that conditioning agents are dispersed in the feed, and have sufficient time to interact with particle surfaces in appropriate quantities to ensure optimum effect.
