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One Step Dewatering and Drying

By

John A. Trautmann, Jr.Vice President Technical Service

U S Filter/JWI

ABSTRACT

Many chemical manufacturers separate solids from liquids when producing theirproducts. This step typically concentrates the solids and/or clarifies the liquidfraction. After the initial solid/liquid separation is completed, it is not uncommonto incorporate a drying step into the production of the concentrated solids.

A new method of dewatering and drying is presented. This patented technologyutilizes a modified chamber filter press, hot water heat source, and a vacuumsystem. With this system, solids can be dried mechanically to any desired levelby the thermal/vacuum drying process. After the desired dryness is achieved,the dried material can be discharged. Pilot and full-scale test results indicatemoisture levels of less than 5% can be achieved repeatedly.

Applications discussed include pigments and dyestuff, pharmaceuticals,

starches, yeasts, catalysts, chemical intermediates and precious metalprecipitates. The potential for waste stream applications are also considered.

Advantages to a chemical processor include lower labor costs, a reduction incapital expenditures together with improved product quality and production rates.For waste applications, reduction in disposal costs and potential for beneficialreuse provide an incentive.


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INTRODUCTION

Historically, liquid/solid separations have been accomplished in the chemicalprocess and waste treatment industries by the utilization of a number of differenttypes of filtration equipment. Among these various types are centrifuges, rotarydrum vacuums, belt filters (in both squeeze and vacuum configurations), tubefilters, sand filters, rotary screw presses and chamber filter presses.

Once the initial gross separation of the liquids and solids is achieved, there maybe additional process steps involving either the solids or the liquid andsometimes both. If the filtered solid is the product it can, among other steps, bereslurried, sent on to the next process step or sold as is. In the case of wastetreatment the resultant solids are usually sent to land fills and the liquids areeither sent back to the head of the treatment plant, recycled back into the plant ordischarged to a POTW.

Within the chemical process industry, most filtered solids are in fact the productand usually require further reduction of the liquid content still retained in the cake.This liquid content usually averages between 75 and 50 percent by weight.Some materials exhibit either poor or excellent dewatering characteristics andmay be above or below that range. To reduce the liquid content further, varioustypes of drying equipment may be used including drying beds, vacuum dryers,

tray dryers, spray dryers, and belt dryers. The drying step may require furthermaterial handling from the discharge of the filter to the inlet of the dryer.

Widely varying levels of labor may be used to accomplish this material transferstep. They range on the low end with the use of shovels, up to sophisticatedautomatic conveying systems. No matter how little labor is required, it is still anecessary extra step, which may add cost and the possibility of materialdegradation due to mechanical transfer.

The patented process technology we are presenting utilizes one piece of processequipment to accomplish both the filtration and drying process steps. Theequipment has the ability to separate the solids from a slurry, with concentrationsas low as 1% by weight, wash the solids if desired, and to finally dry them tosolids concentrations approaching 100% by weight or just about any incremental


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METHODS

The main equipment components utilized in the patented J-VAP

process systemare:

1. Chamber filter press (J-Press)

, specifically modified, see Figure 1.2. Diaphragm squeeze type recessed chamber plates, specially

modified for use with the energy conversion module, see Figure 2.3. Energy conversion module to supply heated water and vacuum to

the plates, see Figure 3.

1. The J-Press

filter press is a widely accepted type of filtration-separationequipment that has been in use for almost a century and is commonlyreferred to as a plate and frame filter press (or chamber filter press). It isused extensively in both process and waste treatment industries whenhigh solids content cake solids and high degrees of filtrate clarity arerequired. They are also used in processes where the slurry liquid phasestill retained in the filter cake may contain a contaminant, such aschlorides or out of spec pH, and must be washed out of the cake to allowthe solid to be further processed. This is particularly true in thepigment/dyestuff industry where the slurry is usually a saturated chloridesolution of a low pH and wash water must be pumped through the press tobring the filter cakes to both a chloride free and low conductivity level.

The filter press name comes from the steel or cast iron skeleton structure

of the unit and the hydraulic closure system that actually presses the stackof filter plates together within the frame. The filtration-separation isactually accomplished with the pumping of the slurry, at pressures usuallyup to 100 psi but in special cases approaching 225 psi, into the press.The filter cloths, installed over the drainage surfaces of the plates, act as abarrier to the flow of the solids and retain these solids within the cavitiesformed by the processes within the plates, see Figure 4. The constructionand porosity level chosen for the cloths is an extremely important part of

the application process and has a great bearing on each installationssuccess. If a too open high porosity level cloth is chosen, it will allow thebypassing of solids through the filter and if too tight a porosity level ischosen, it can actually blind off prematurely and not allow cake solids tobuild and fill the filter press. Laboratory test work on new processesand/or field experience on similar or same type processes is required to


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flexible drainage surfaces allow for the introduction of squeeze media,usually water, into the area behind the drainage surface and out of contact

with the filter cake. The resultant pressurization of the media up to 225 psiafter the initial formation of cake solids yields a mechanical squeezingaction on the cakes, leading to further dewatering. These type plates givethe highest dry solids content of any currently available plate type.

Material of construction as used in J-VAP

is polypropylene. There areother polymers, co-polymers and elastomers available, but we have notfound anything currently available that is superior. However, other

materials and construction methods are being investigated for their abilityto yield higher efficiency in regards to heat transfer.

3. The energy conversion module is the part of the system that produces theenergy forms, which accomplishes the final drying cycle. The module, seeFigure 3, consists of the necessary components to heat the squeeze waterto approximately 180F and feed it, at approximately 50 psi, to thediaphragm squeeze plates. Along with the heating system, is a vacuumgeneration system, which is connected to the discharge manifold of thepress. Currently we are using liquid ring vacuum pumps, but vacuumgenerating eductors can also be used if higher energy consumption is notof primary importance.

OPERATION

The sequence of operation is:! Press close.! Slurry feed to form cake, at 50 to 75 psi.! Air blow press to evacuate free liquid, at 40 to 50 psi.! Drying cycle, hot water at 180F and 50 psi plus vacuum at 26-28

inches of mercury.! Open press.! Discharge dried solids.

Extensive testing and field experience has shown that the total cycle time canrange from 4 to 8 hours. This time is dependent on the filterability of the slurry,the compressibility of the resultant filter cakes and the amount of retained liquidin the cake that must be removed to attain the desired dry content results.


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DESCRIPTION

The system is able to dry the solids within the press by basically lowering thevapor pressure of the liquid and boiling it off at a lower temperature than 212F.

Figure 5 is a vapor pressure chart of common liquids, which shows that the waterphase may be changed from liquid to vapor at temperatures as low as 100F if 28inches of vacuum can be maintained. It is fairly easy to attain the 28 inches orgreater when operating a pilot size, 470mm (18.5 inch), square filter press in our

lab environment but a may be more difficult in the real industrial operating world.We have performed R & D work in sealing the plates interfaces with theobjective of improved vacuum performances. Most of the work was targetingspecial cloth edge sealing preparations. Our R & D work has proven that we cannormally maintain the vacuum at greater than 27 inches to keep the drying timeswithin reason. As vacuum is lost, the vapor pressure/temperature relationshipincreases and the removal rate of the filter cakes water is reduced, therebyextending the drying time. We therefore are very thorough in our filter cloth

selection process and must choose not only a type, which suits the filtrationcharacteristics of the slurry but also has a construction type, which yields gooddiaphragm plate edge sealing.

During the drying step we monitor several pressures and temperatures. Themost important and the one that indicates both how efficient and how the unit isoperating, is the temperature in the discharge manifold. We monitor the exhausttemperature at a point between the energy conversion modules vacuumconnection but close to the press. During the drying, this temperature will usuallyrun at approximately 120F. This temperature is created by a combination of the180F squeeze water being constantly circulated through the press and the offsetting cooling effect of the vaporization of the moisture within the filter cakes. Ifwe did not have this vaporization cooling effect, the temperature of the cakewould eventually reach 180F and, in fact, is the measure of the end of the dryingcycle. Testing and field experience has shown that when the exhaust

temperature starts to spike and reaches approximately 150F we have reachedan acceptable level of drying and the cycle can be shut down, the press openedand solids discharged.

If the temperature is allowed to approach 180F, the dry solids content of thecake can and will approach a completely dry 100%, or 0% moisture. In some


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of repeatability and can be coupled with a PLC to offer complete systemautomation from beginning to end of cycle.

APPLICATION EXPERIENCE

The installations, which are now operating or scheduled for start-up this quarter,cover a broad spectrum of both process and waste treatment processes. Theyinclude:

1. Metal hydroxide sludges from plating waste streams, two (2) installations.

2. Precious metals recovery from a metals refining plant.3. Biological and industrial waste; combined streams at a Naval Base.4. Chemical production; tungstic acid in solid form.5. Metals refining; zinc leach residue.6. Primary and secondary waste.

We have also run successful pilot scale tests on other processes:

! Organic pigments production! Kaolin clay production! Silicon dioxide production! Paper mill waste sludge! Municipal waste sludge! Calcium carbonate production! Starch production! Glass grinding waste sludge! Germanium production

While we have had success not only in the field but also with pilot plant results,not all pilot plant tests, however good the results are, result in orders forequipment. Economics is usually the governing factor while the adage leavewell enough aloneis running a close second.

CONCLUSION

We know that this technology will find its place in the process and wastetreatment fields. It has shown itself to be able to offer:

! Dewatering and drying in one piece of equipment.


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FIG. 1

J-PRESS


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FIG 2A


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FIG. 2B

J-VAP PLATE CROSS SECTION


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FIG. 4

STD PLATE PACK CROSS SECTION


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