Centrifuges: How they operate and how to select one

Mike Vastola, Chemical Centrfuae

Division, TEMA Systems, Inc.,

‘USA guides us through the centrijiige selection process. ea~~rltluges=

[email protected]#

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ssuming every mechanical

and/or settling basins were utilized to per-

application

and electrical

form most liquid/solid

tation centrifuge.

aspect of a cen-

separations.

requires

a filtering

or sedimen-

Specific final product

trifuge will be maintained in

However, these methods are very often

capacity and/or quality requirements

top-notch

time-consuming

dictate whether

condition, the

..focus of attention for the

.-.

Qmq#

proper selection of a centrifuge should cen-

therefore

and labor intensive and

and introduction

of the cen-

tre around the qualitative and quantitative

trifuge over 100 years ago, centrifuges

characteristics

now among the most commonly

of the process feed slurry. In

other words, for any type of centrifuge to

uous or batch.

not very practical. With the

development

type of process equipment

will

or not it should be contin-

are

FILTERING A filtering

selected

used to separate

CENTRIFUGES

centrifuge

is one in which a per-

forated or slotted wall basket or bowl is

produce an acceptable dewatered product

a liquid from a solid because of their relia-

covered with some type of filter media.

consistently, the feed slurry conditions must

bility, capacity and process containment

This media can be paper or woven cloth

be held as constant as possible.

capability to name a few things.

Unfortunately, cess conditions liquid/solid

varying upstream pro-

can and do affect a

centrifugation

application.

BATCH OR CONTINUOUS? Simply stated, as the liquid/solid

Because of this, it would be useful to know

processed

what feed slurry characteristics

cially induced inertial

style of centrifuge For example, concentration,

a particular

can or cannot tolerate.

characteristics

such as solids

type of solids, liquid load-

through a centrifuge,

force. In addition to the solid like the density, size,

shape and consistency,

the speed of rota-

tion for a given diameter will also influence

ing will go a long way towards a successful

moves away from the center

process.

explain that in the beginning,

courses

static filters

the g-force

basket or bowl

how fast the solid phase

The higher the rotational

separation

that is exerted

of rotation.

speed, the higher on the solid

filtering

on a

surface

or accumulate

purpose is for clearing and purifying the liquid phase and/or for purifying and lowering the moisture content of the solids.

SEDIMENTATION CENTRIFUGES A sedimentation

centrifuge

a non-perforated

wall basket or bowl is uti-

on

insoluble

is one in which or depositing of

solids that will settle in a liquid

but cannot be effectively

collected

on a fil-

ter media. The solid wall provides a surface

a sedimentation

for the solid phase to accumulate

surface. This abili-

pact. Depending

ty to accelerate

and com-

istics of the solid phase and the level of g-

on the physical character-

the liquid/solid

forces at work, the compaction

separation

be low or high. The ultimate purpose is for

rate is

lvhat revolutionized filtering

and

sedimentation technology.

effect can

the liquid phase to clarify and the solid phase to achieve some acceptable degree of dryness from the compacting

action.

slurry character-

FILTERING OR SEDIMENTINC?

istics will deter-

A simple screening procedure

mine \\-hethcr or

on a representative

not a liquid/solid

ry will yield data that will help determine

Specific process

May 2000

enable the media to restrict the passage of

lized for the accumulation

phase and thus the faster the solids will either collect

16

of open area made up

the solid phase and yet allow the liquid

the solid and liquid phases of a feed slurry,

selection

reasonable percentage

of holes of a certain size and shape that will

by which

is known as

first in the lab and then in pilot plant test-

Basic liquid/solid

the artifi-

bars.

The filter media is necessary to provide a

phase to pass through freely. The ultimate

of rotation

centrifugal

centrifuge

slurry is

from the center

phase characteristics

trifuge. Individually, testing and evaluating

holes, laser cut or

liquid phase in a straight line and away

perature

of a cen-

reaction

perforated

pierced plates or wedge-wire

the solid phase tends to move through the

ing, type of liquid, pH, viscosity and temall affect the operation

sheets, metallic or synthetic woven mesh screens,

performed

sample of the feed slur-

Filtration+Separation

what type of centrifuge will indicate whcthcr mentation

the 500ml volumet-

should be consid-

ered for pilot testing. The procedure a filtering

or continuous

results

or sedi-

ric flask. With the mouth of the flask

or batch cen-

covered by a finger-

trifuge should be tested based on the final

tip, invert the flask

liquid clarity, cake purity, dryness and/or

over the center of

final capacity requirements.

the vacuum funnel so that the mother

This procedure

consists of three simple

steps:

1’ lquor flows out and forms a pool of mother liquor

Static Settling

Test

By placing a representative

approximately sample of feed

‘A” to

9%”on top of the

slurry, in a 1OOOml beaker and allowing it

cake bed. Support

to stand for thirty 30 minutes,

this flask so that the

fercnccs between

density dif-

the liquid and solid phas-

es should be evident, the following

obscr-

distance between the cake and the rim the results and evaluate levels

properly

1.

Settled solids volume

er liquor will stay in the flask until the vac-

Outstanding,

8-12 gpmper

2.

Floating solids volume (not always

uum is re-started.

Excellent,

6-lOgpmpersq./ft

Good,

4-8 gpm per sq./ft

Fair,

2-6 gpm per sq. /ft

5. good or poor

and quickly, the remaining

Simultaneously

If done

Record

of the volumetric

present)

flask is constant.

8,

vations may or may not occur and should bc noted:

moth-

turn on the vacuum and

start the stopwatch. The mother liquor*

in

of permeability

based on gpm per /sq.ft: sq./ft

3.

Liquid clarity - excellent,

4.

Number of phases: 1, 2, or 3

the inverted flask should begin to drain out

Marginal,

l-3 gpm per sq./ft

5.

Separation rate/quality

into the pool and through the cake bed as

Poor,

0.5-lgpm

9.

rates above the 2-

excellent,

per sq./ft

the vacuum draws the liquid phase through

good, poor

the capillaries and inter-particle

If there arc distinct liquid and solids

matrix of

If permeability

phases present in the beaker, then a density

the cake bed and eventually into the vacuum

6gpm per sq. /ft range are achieved,

difference is confirmed

flask. *Note:

continuous

ing for a centrifuge

and further screen-

is indicated.

F‘iltra tion Rate Test

If the mother liquor quickly

centrifugal

drains through the cake without a vacuum,

should be investigated.

the filtration test can be terminated.

fixed angle, tumbling,

6.

screen or pushers.

Record the total time, in seconds, that

filtering

would be

These

vibratory,

Using a 6” vacuum funnel, a 6” # I fast fil-

it takes for the mother liquor to vacate the

10. If l-3gpmpersq./ftorbelowis

tcr disk, a vacuum flask, a sink aspirator

volumetric

achieved, then batch centrifuge

a vacuum pump, a 500ml volumetric a stop-watch,

and representative

or

flask,

samples

of feed and mother liquor, a determination can be made with regard to the pcrmeability of the settled solid phase accumulated

in

the beaker test above. This permeability rate will he converted

to gallons per

minute per square foot and will indicate what class of centrifuge

should bc investi-

gated fur&r.

I

Setup the vacuum funnel and filter

ElaSk on a ring stand 2

Place the filter disk in the funnel and

seal it down to the filter bottom

phase cake bed and stop flowing freely into

units should bc investigated.

the vacuum flask.

be vertical

7.

inverting

In general: The area of the 6” filter disk is approxi-

mately 28 square inches or -0.20

per sq./ft,

Convert

the final filtration

fluffy or sticky solids that arc susceptible

rat-c through the

compaction

and thus restrict

cake bed to gallons per minute per square

drainage and do not make centrifugal tration applicable

For example, through the

if it took 10 seconds for the

of mother liquc>r to pass 0.20 square feet of cakebed

indicate that anvi further testing for a ccn

trif’uge applical ion mentation

would

centrifuge.To

investigated,

with the selection.

of 1” to

tributed evenly and that there arc no cracks in the cake bed surface due to over drying. m:

the vacuum should be turned off

fil-

or practical. This would

final rate in gallons per minute per square

1.5”. Note: Make sure that the cake is dis-

involve a sedi detcrminc

which

unit needs to br

the following test will help

1 square foot divided by 0.20 square foot equals an area multiplier

of 5

0.13 gallons per 0.20 square feet times the arca multiplier

of 5 equals 0.65 gal-

lons/sqft

see what effect g-force

60 seconds per minute divided by 10 seconds equals a volume multiplier 0.65 gallons/+

ty rate, then perform

of 6

times 6 equals 3.9

the feed slurry’s distinct

the following

test LO

and time has on

ability lo separate into

phases.

1. Fill two (2) test tubes with samples of

before cracking can occur.

gallons per minute per square foot perme

the feed slurry and place them in a bench

4.

ability

top test tube spinner, 1000 g’s is typical.

Put 5OOml of clean mother Liquor in

to

mother liquor,

foot.

foot would be:

solids to build up to a bed-depth

rate is less than

0.13 gallons.

the surface with a small amount of mother

surface of the filter disk and allow the

and

that the solid phase consists of very fine or

500 ml is approximately

liquor. With very slight vacuum, begin to pour

peelers

this usually indicates

type of sedimentation

the thoroughly mixed feed slurry onto the

filtering

These would

basket or horizontal

11. If the pcrmcability 0.5gpm

worm/

bag units.

surface area and into the vacuum flask, the

3.

by rinsing

square

feet.

0.13 ga!lcns

SLIi3I;O:t.

flask, pass through the solid

then

units

2. Spin for thirty seconds,

stop unit and

observe the phases for separation,

the next step in the selection

liquid

clarity, and cake compaction. 3, If necessary, repeat # 2 above up to If a satisfactory

two-phase

separation

is

Once feed is terminated,

trifuges are available would be helpful.

phase from the rotating bowl and expose

filtering

centrifuge

will

accept a continuous

flow of process slurry

and simultaneously

remove mother liquor

achieved within 90 seconds, then the sam-

through the cake bed and screen as solids

ple should be further

collect

evaluated with a

decanter sedimentation Decanters

type centrifuge.

typically have a 2000-3000

g-

force range, maximum. If more than 90 seconds are required achieve a definite two-phase

to

separation

or

if the liquid phase is cloudy and not completely void of insoluble

solids, then the

sample should be further

evaluated with

the disk sedimentation

on the screening

the solids. The unit slows down and the solids are plowed out automatically can be stopped completely

or it

and the solids

can be removed manually.

surface, wash if

- a batch solids retaining

disk cen-

necessary, remove wash liquor, lower final

trifuge,

moisture

and discharge the solids to a

of one or two liquid phases by overflow

allows the continuous

removal

receiver

or dryer. The conveying mecha-

and skimming. When the solids holding

nism for the solids can be a simple fixed

capacity is exceeded,

angle, a changing angle, a vibratory

off and the bowl must be cleaned out

motion,

manually.

a scroll, or a pusher plate.

- a continuous

sedimentation

cen-

trifuge will accept a continuous

type centrifuge.

a skimming

device will remove the mother liquor

- a continuous

three times. (90 seconds total)

process,

knowing more about what types of cen-

- a batch tubular centrifuge

flow of

process slurry and simultaneously

the unit is turned

same general principle,

remove

tle solids holding capacity and are used pri-

Disk units have the capability to attain

mother liquor as solids accumulate

4000.5OOOg

bowl wall, apply wash in the decanter if

very small particles.

necessary,

level is reached, it must be shut down and

Often times, this higher g-

force range is required

to effectively

clarify

lower final moisture

on the

utilizes the

they have very lit-

and dis-

the liquid phase or achieve a good three-

charge solids to a re-slurry

phase, liquid/liquid/solid

dryer. The solids removal mechanism

Finally, in extreme mentation

separation.

cases, a tubular sedi-

type centrifuge

unit may need

be a scroll/conveyor,

receiver

or

- in a decanter

60,OOOg’s. This high g-force may be

also cleaned out manually.

PILOT vs REAL-LIFE EXPERIENCE

nozzle or ejection

centrifuge,

Once a high solids

can

system.

to be evaluated, they can produce up to

marily for polishing liquids by removing

the mother

Although knowing the characteristics

the process feed slurry and the limitations

required when the solids are less than 0.5

plate and the settled solids are mechanical-

of the different centrifuges

microns and low in density and low in con-

ly conveyed,

fairly accurate prediction

centration.

beach area for drying and eventually exit

Remember,

being comfortable

filtering or sedimentation or a continuous

with a

centrifuge

or batch centrifuge

just be based on the interpretation data from the screening

via a scroll, up an angled

in a disk centrifuge,

cannot of the

procedure.

the mother liquor

representative

operation

continuously

solids concentra-

tion dropping or increasing, rates fluctuating, temperature

or viscosity

feed or wash

show up during a simple screening ry upsets in a liquid/solid

separation pro-

a specific separation

centrifuge

will 1.

Is it affordable?

2.

Is it repeatable?

3.

IS it reliable?

4.

Is it applicable?

trifuge,

metered

in to the cen-

the feed device is turned off and to rotate and allows the

mother liquor to drain through the collectis

questions with regards to a centrifuge’s

cess slurry. Once the flow or volume has

the unit continues

cess, the final type and style of centrifuge selected to perform

process stream.

Pilot plant testing will answer several

accept a measured flow or volume of probeen completely

proce-

for feed slur-

data from an

ultimate desirability:

ejection.

- a batch-filtering

changes do not

dure. Because of the potential

via a fixed orifice nozzle or

via intermittent

screen or basket blinding,

should be made without

and knowledge of having

extended run with a pilot plant unit on a

or not there are one or two liquids pre-

such as crystal agglomeration,

the comfort

operated and collected

tube depending on whether

It is very important

that the economics

ed solids, through the screen and out

of using a centrifuge

still a question mark until some long term

through the openings in the basket. The

aration project be established as soon as

data and experience

next steps would be to wash to improve

possible so that time and money is not

is gathered.

By the end of the screening empirical

data collected

procedure,

particle

consistency,

the

and the known

process data such as the percent centration,

moisture,

feed con-

size distribution,

and wash requirements

shape, will

pretty much indicate the types of centrifuge that could be acceptable application.

for an

However, pilot testing with

representative

feed slurry over an extend-

and discharge to a receiver

for a liquid/solid

expended on the investigation

purity, drain again, spin to lower final

WHICH CENTRIFUGE?

or

equipment

a batch sedimentation

centrifuge

will

flow or volume of pro-

restrictions.

confirm

the applicability

of the centrifuge

selected

for the pilot

testing,

the mother

must be monitored

liquor and the accumulated

the operation

and data collection

and recorded

Once the solids holding capacity of the unit

assure that the centrifuge ble of repeatedly

observing

indicated by

a break point where what goes

over

several shifts for several days. This will

has been reached,

generally

of a piece of

To accurately

cess slurry. The unit will rotate and allow solids to separate into two distinct phases.

sep-

that cannot be purchased later

on due to economic

dryer. accept a measured

of

No final selection of a pro-

or a skimming

sent. The settled solids are removed either

size reduction,

allows for a in the selection

will exit either via a center overflow tube

Changes that can impact a centrifuge’s particle

a centrifuge.

duction centrifuge

through discharge ports.

and

of

liquor will exit over an adjustable weir

ble variations

selected

is capa-

handling all of the possi-

that could occur in the pro-

ed period of time will add more credence

in comes out or by timing, the feed device

cess. Finally, the unit tested should exhibit

to the selection.

is turned off.

good reliability

18

With pilot testing being

May 2000

for the use for which it is

Filtration+Separation

intended

as well as yield consistently

good

Production

rate in #‘s/br

or gpm

In summary, between the feed slurry

Specific gravity of the slurry

process parameters

Percent concentration

by weight

centrifuge

should involve a factory service/process

Percent concentration

by volume

collected from the screening procedure

engineer. An experienced

Solids, crystalline,

quality results. Performing

the initial pilot test work

with establishing parameters

person can help

Solids size in microns

the proper operating

for the centrifuge

granular, fibrous

selected

or mesh

once all the data

selection of the proper type centrifuge should be relatively simple and accom-

testing. After all of the pilot testing is

Desired Phase, liquid or solid

plished with great confidence.

completed,

Desired clarity or purity

can

then be analyzed and evaluated to finally

Temperature

&ntacc. .d4~kePbstola, Chemical Centrfige

trifuges will be needed to meet produc-

PH Wash requirements

TEMA Systems Inc, Cincinnati,

tion requirements.

Wash liquor

determine

operating

which and how many cenNote: Pilot plant test

parameters

should not exceed

those that are the limitation

S134897811;Fax+l

anything with the test unit that cannot

design factors that will

influence the final selection

could include:

Miami University, with a Bachelors

on the production

unit.

g-force

based

Filtering

or Sedimentation

Centrifuge

and solids

density.

Application

Basket or Bowl diameter

Basket or bowl volumetric

capacity

Degree

Centrifuges

In review, the feed slurry process factors

Liquid loading sensitivity

theTechnical

that will influence the final selection

Solids loading sensitivity

Centrifuge

Screen type and size selection Continuous

options

or Batch

engineer

8 years, and then on to The Western Machine

include:

He then went on to

finishing and environmental

G-force could

Ohio, USA

in

work for Square D Co, USA as a metal

Basket or bowl RPM

CONCLUSIONS

in 1969 from

Oxford,

Chemistry/Biology.

be duplicated For example,

limitations

5134894817

Mike Vastola graduated The centrifuge

Co working

on Batch

Manager, Division

Chemical

at TEMA

CS-Program:

temperature filtration up to 850°C pleated filter-elements performance

for high

Centrifuges

for nine years.

in a limited space

chemical

protection

treatment

for

3 variety of ap~l~cst~~~s

‘, ~P~~-~ro~~arn~

Systems,

USA, where he has worked on Continuous

ceramic filter elements for high

for States

for 10 years. He is currently

WF filter media: The perfect solution for your filtration applications.

filter media made of needle felt

Division,

Ohio, USA. Tel: + I

AB~~~~~~~~TH~~

range

of the pro-

duction unit. In other words, do not do

on basket or bowl diameter

Final moisture

as

well as the pilot testing is complied, the fmal

Viscosity

the data that is collected

for

and the different

design functions,

filter media for extremely fine dusts