classification and dehydration of dyes and pigments

A decanter centrifuge separates solids from one or two liquid phases in one single continuous process.

This is done using centrifugal forces that can be well beyond 3000 times greater than gravity.

It consists of a solid cylindirical bowl rotating at high speed, a scroll rotating at the same axis with

a slightly different speed, a drive group adjusting the speed difference and the body which carries

all the rotating elements.

Liquid and suspended solids are fed along the centre line to a distribution room within the bowl and then

accelerated into the bowl by centrifugal force through the feeding point. This centrifugal force then causes

the suspended solids to settle and accumulate at the bowl wall.

Sludge cake is discharged by scroll from the conical part meanwhile clarified liquid flows back along the

bowl and is discharged through the cylindirical end of the bowl by plates which can adjust the

level of clarified liquid.

A decanter centrifuge is a device, which employs a high rotational speed to separate components of different

densities. This becomes relevant in the majority of industrial jobs where solids, liquids and gases are

merged into a single mixture and the separation of these different phases is necessary. A decanter

centrifuge separates solid materials from liquids in slurry and therefore plays an important role in

wastewater treatment, chemical, oil and food processing industries. There are several factors that

affect the performance of a decanter centrifuge and some design heuristics to be followed which

are dependent upon given applications.

The operating principle of a decanter centrifuge is based on gravitational separation. Naturally, a

component with a higher density would fall to the bottom of a mixture, while the less dense component

would be suspended above it. A decanter centrifuge increases the rate of settling through the use of

continuous rotation, producing a gravitational force between 1000 to 4000 times that of normal

gravitational force. This reduces the settling time of the components by a large magnitude, whereby

mixtures previously having to take hours to settle can be settled in a matter of seconds using a decanter

centrifuge. This form of separation enables more rapid and controllable results

The main application of decanter centrifuges is to separate large amounts of solids from liquids on a

continuous basis. They are also used to wash and dry various solids in industry, such as polystyrene

beads, clarify liquids and concentrate solids. Table 1.0 displays various examples of the utilisation of

decanter centrifuges in various industries.

Generally the decanter centrifuge has more advantages than disadvantages; however there are some

limitations when compared to other processes.

Advantages:

1.Decanter centrifuges have a clean appearance and have little to no odour problems.

2.Not only is the device easy to install and fast at starting up and shutting down, but also only requires a

small area for operation compared to other competitive processes.

3.The decanter centrifuge is versatile as different lengths of the cylindrical bowl section and the cone angle

can be selected for different applications. Also the system can be pre-programmed with various design

curves to predict the sludge type, while some competitive processes, such as a belt filter press, cannot

change the belt type to operate for different sludge types.Its versatility allows the machine to have various

functions such as operating for thickening or dewatering.

The machine can operate with a higher throughput capacity than smaller machines.This also reduces

the number of units required.

4.The device is simple to optimise and operate as it has few major variables and reliable feedback

information.

5.The decanter centrifuge has reduced labour costs compared to other processes, as it requires low

continuous maintenance and operator attention.

6.Compared to some competitive process such as the belt filter process, the decanter centrifuge has

more process flexibility and higher levels of performance.

Limitations:

1.The decanter centrifuge cannot separate biological solids with very small density differences, such as

cells and viruses.A competitive process that is capable of separating these difficult-to-separate solids is

the tubular-bowl centrifuge.

2.The machine can be very noisy and can cause vibration.

3.The device has a high-energy consumption due to high G-forces.

4.The decanter centrifuge has high equipment capital costs. Hard surfacing and abrasion protection

materials are required for the scroll to reduce wear and therefore reduce the maintenance of the scroll wear.

The main types of decanter centrifuges are the vertical orientation, horizontal orientation and Conveyor

or Scroll.

In vertical decanter centrifuges the rotating assembly is mounted vertically with its weight supported by a

single bearing at the bottom or suspended from the top.The gearbox and bowl are suspended from

the drive head, which is connected to the frame.The vertical decanter allows for high temperature

or high-pressure operation due to the orientation and the rotational seals provided at one end. However,

this makes the device more expensive than the horizontal decanter centrifuge, which is non-pressurised

and open. The advantage of the vertical machine over the horizontal machine is that the noise emitted

during production is much lower due to less vibration.

In horizontal decanter centrifuges, as shown in figure 1, the rotating assembly is mounted horizontally

with bearings on each end to a rigid frame, which provides a good sealing surface for high-pressure

applications.The feed enters through one end of the bearings, while the gearbox is attached to the other

end and is operated below critical speed.Capacities range up to 40,000 pounds (18,000 kg) of solids per

hour with liquid feed rates of up to 300 US gallons (1.1 m3) per minute. The horizontal machine is

arranged in a way that slurry can be introduced at the centre of a rotating horizontal cylindrical bowl.

The scroll discharge screw forces the solids to one end of the bowl as it is collected on the walls. This

orientation is the most common design implemented in industry.