Thickening Static

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Static thickener as prethickening in front of pusher centrifuges Nik Keller

February 2003 1/5

IntroductionSlurries from crystallisation plants have normally a too low solids content for a proper operation of the pusher centrifuge. This results in worse cake formation with flooding(liquid is flowing over the cake forming canyons) resulting in high vibrations, which doesnot allow a further operation of the machine. It is necessary to increase the solids con-tent by a useful unit. For crystalline products a normal value should be 50 %-w/w. Abetter indication is the static sedimentation volume. To measure this value, a slurrysample is taken into a cylinder to settle down the solids. The solids level should godown to 70 % or more for a proper operation of the centrifuge.

Static thickener An often used unit for the prethickening of slurries in front of pusher centrifuges is thestatic thickener. A conic tank with overflow is continuously fed with slurry. During theretention time the solids settled down, the liquid is clarified and leaves the thickener via

the overflow.

Because of the conic design the solids may go down only by and leaves the thickener at the lowest point. Depending on the product characteristics there may be solids de-posits, this needs the installation of a turning scraper. An important value is the openingangle of the cone, normally this is in the range of 30 35 .

The design parameter of a static thickener is the clarify area. This means, that theheight of the thickener has no influence to the separation efficiency. Products with lowsedimentation speed needs a large clarify area when handling large quantities in oneunit. Here it might be an advantage when using a round settler with a turning scraper. Inthis case the turning scraper is responsible for the transport of the solids to the dis-charge.

PreconditionsBecause of the above mentioned working principle the following preconditions are nec-essary for thickening of a slurry in a static thickener:

Generally the solids needs a higher density than the liquid, this means that evenunder normal gravity the solids has to decant.

The viscosity of the liquid may not be too high. The static sedimentation speed has to be known (to be tested in a cylinder).

From the static sedimentation test is also known the solids concentration after sedi-mentation. This value has to be high enough for pusher centrifuge operation be-cause of the fact, that in a continuous operated static thickener for sure no higher solids content in the discharged slurry may be found.

The slurry has to be stable over longer time, the relative long retention time mayhave no influence to the slurry.


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Parameter For designing and operation of a static thickener the following parameters are available:

Sedimentation speed: This value may be determined in a cylinder with graduation.

At the beginning of the sedimentation test, the sedimentation speed may be more or less constant and only in the area of compression, the speed will be lower andlower. This first range is called the linear sedimentation speed. The speed calcu-lated with the time, when the level of the settled down solids is constant, is calledmean sedimentation speed, withthis value normally the sizing of the thickener is made. Speciallywhen testing fine products, nosettled down solids level may beseen, but it is possible to see asolids level going up. In such a

case the clarification of the liquidis observed. The sedimentationspeed is calculated as the ratioof settled down height and thecorresponding time.

Slurry quantity: Together with the settling speed the slurry quantity determine theoverflow diameter of the static thickener. On an installed thickener it is possible toadjust the quality of the overflow via the slurry quantity.

Cone angle: The cone angle influences, how easy the solids may be discharged (assmaller the angle, as easier). On the other side it determine also the height of thethickener. Normally an angle of approx. 30 35 is necessary for a safe operationof the thickener without solids deposits on the wall, but also the product propertieshave to be taken into account. On certain products it is necessary to us a turningscraper (for example pure sodiumhydrogencarbonate or pure adipic acid).

Area of operationCompared with a hydro cyclone the static thickener has the advantage, that it might beoperated in a much large range of feed slurry. The static thickener itself might be oper-ated between 0 and 100 % of nominal load, but at very low slurry quantities it might be

not so easy to discharge the slurry.

Also the solids content in the slurry to the thickener might be varied in a very widerange with the same restriction as mentioned above.

Further on, the static thickener is working as a buffer because of the volume. If for in-stance the centrifuge has the be washed, normally a large enough volume is availableso, that the thickener might be fed during stopping of the slurry discharge.

Normally the filtrate of the pusher centrifuge is not solids free. Thats why the filtrate ismostly going back into the static thickener. The hydraulic load of the thickener is higher

(during designing this has to be taken into consideration), but on the other side onlysolids free liquid is leaving the thickener, the solids recovery is maximal.

0

20

40

6080

100

120

140

160

180

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220

240

260

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95

Time (s)

H e

i g h t ( m m

)

Height

Lin. settling speed, 18 m/h

Mean settling speed, 9.2 m/h


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The discharge of the slurry out of the thickener is sometimes problematic. On one sidethere may be lumps which blocks the discharge pipe, on the other side the solids con-tent may be so high, that the flowing properties are very bad. Here the Ferrum mixingunit or the dosing device DAU may help. Both units are equipped with a vertically ar-ranged mixer (the motor is on the lowest point of the unit, so no long shaft is necessary)rotating with approx. 10 20 min-1. This avoids solids deposits near the dischargeopenings. On the DAU there are dosing opening direct near the mixer, this minimisesthe danger for blockages of the dosing opening.

If only a mixing unit is installed a suitable valve has to be installed on the dischargeopening. Ferrum recommend Vee ball valves. This is a modified ball valve, thedosing opening is triangle shaped. With this form there are even with low openingsrelative large dimensions, and this improves the regulation properties of this valve for slurries (low tendency for blockages).

Filling degreeBecause of the closed form it is practically not possible to say, how much solids is in thestatic thickener. But this is important: On one side the filling degree is an indicationabout the solids content of the discharged slurry, on the other side more solids in thethickener reduces the danger for too low concentrated slurry to the centrifuge.

To improve the operation of the static thickener, it is placed on load cells. The weight of the thickener changes in function of the filling degree. Important is, that the thickener isoperated with overflow, only then the volume is constant, increased weight means in-creased solids quantity in the thickener.

Normally not the real weight is important but the change of it. Because of this mostlyonly one load cell is used. The calibration of the load cell is made as follows: 0 %should be indicated when the thickener is filled to the overflow only with mother liquid,100 % should be indicated, when the thickener is filled with thickened slurry nearly tothe overflow. During start up of the plant it has to be found empirically at what minimalindication the solids content in the discharged slurry is high enough for a proper opera-tion of the pusher centrifuge. During normal operation the weight should be approx. onthe found value, so there is buffer volume available. When the feed to the centrifugehas to be stopped (cleaning of the centrifuge or other reasons), the feed to the thick-ener may be in operation for some time, the weight of it will increase. The operator isresponsible, that the thickener is not overfilled.

Regulation (see also example on the following page)To allow in wide ranges an automatic operation of thickener and pusher centrifuge, aregulation according to the following one may be used. The measuring of the slurry or solids quantity to the centrifuge is not as easy because of the high solids content in theslurry, normally the pipe to the centrifuge is not filled completely, so a normal flow meter will not work. Thats why the current of the main motor is used as an indication for theslurry quantity.

In a certain weight range of the thickener there is preset a constant set point for the cur-

rent of the main motor. The slurry regulation valve is now opened and closed by a con-troller so, that the current of the main motor is according to the set point (this value ischosen so, that the solids quantity is approx. at nominal of the plant).


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When the solids quantity out of the crystallisation plant to the thickener is higher thannominal, the thickener weight will go up. Above a certain value (in the example 40 %),the set point for the current of the main motor is increased, and so more solids is dis-charged from the thickener to the centrifuge. The set point will go up to the maximalone, this value may correspond to the maximal load of the centrifuge or the maximalcapacity of the dryer. Is the solids quantity to the thickener even higher, the thickener weight will go up, now the operator has to take corresponding actions (slurry quantityout of the crystallisation to be reduced or an additional centrifuge to take in operation).

Is the solids quantity out of the crystallisation lower than the actual quantity to the cen-trifuge, the weight of the thickener will go down. When the weight goes below a certainvalue (in the example 35 %), the set point for the current regulation will be reduced ac-

cording to the diagram, this reduces the solids quantity out of the thickener. The setpoint should not be reduced too much because of the fact, that too low openings of the

feed valve will have the tendency for blockages. Thats why the minimal valve openingshould be limited. Now it is the responsibility of the operator to take corresponding deci-sions (increase of the discharge quantity out of the crystallizer, feed valve to the centri-fuge to be closed completely to increase the thickener weight or others).

Relative to the slurry discharge regulation it has to take into consideration, that a slurrywith high solids content does not have the same regulation behaviour than a pure liquid.There is the danger for solids deposits in front of the dosing opening which reduces thefree opening. Thats why it is not sure, that the same opening of the valve results in thesame slurry quantity. Further on there is the danger for a not constant flow because of too high concentrations. In such a case it might be senseful to add some liquid (mother

liquid or water) direct in front of the dosing opening. The above mentioned regulationonly works proper, when the slurry flows more or less regular through the valve (seediluting liquid in the schema below).

Regulation pusher centrifuge

25

30

35

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50

28 30 32 34 36 38 40 42 44 46 48 50

Thickener weight (%)

S e

t p o

i n t r o

t o r m o

t o r

( A )

Setpoint dependent on thethickener weight

Maximal current, closes feedvalve


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Schema

Real application Sodium chloride (sedimentation speed approx. 20 30 m/h, depending on the parti-

cle size)

Pure sodiumhydrogencarbonate (with turning scraper, approx. 10 15 m/h sedi-mentation speed)

Glaubersalt

M

WIC

EIC

controler

overflow thickener

slurry fromcrystallisation

diluting liquid

solids out of centrifuge

filtrate centrifuge

pusher centrifuge

static thickener

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Thickening Static