Actual case examples - Amazon S3 · Actual case examples The following cases are given here only to the extent needed to illustrate the general principles that are dis- ... the additional

chapter 4

Actual case examples

The following cases are given here only to the extentneeded to illustrate the general principles that are dis-cussed in this book. Obviously, the detailed account ofeach of these cases could have filled a book (assumingthat such details were allowed for publication). Theparticular cases chosen here come mostly from the au-thor’s direct experience, but are “old” cases, not anywith relevance to an ongoing operating corporation.

4.1 Nature and man: the Dead Sea

The Dead Sea is one of the world’s natural wonders; the deepest and one ofthe hottest places on Earth. During milleniums, it has accumulated chloridesand bromides of magnesium, calcium, sodium, and potassium. The averagecomposition of the sea brine reached a steady state, as the average yearlyamount of fresh water that was brought by the Jordan River into the DeadSea brine was equaled by the amount of water that had evaporated fromthis brine.

In the first half of the 20th century, processes were investigated to recoverthe potassium chloride from this brine as a vendable product (potash).Chemists at the Hebrew University in Jerusalem (M. Novominski, M. Lan-goski, and others) studied the entire relevant physical–chemical solids/liq-uid saturation system. They found that when the Dead Sea brine evaporatedand gradually concentrated in a

solar pond

, salt (sodium chloride) reachedfirst its saturation point and precipitated. Then carnallite (a hydrated doublesalt of magnesium and potassium chloride) was crystallized together withsome more sodium chloride. The scientists also found that the mixture ofthe carnallite crystals and sodium chloride, obtained from solar ponds, couldbe leached at ambient temperatures with a large amount of water to leavea number of fine potash crystals with a rather low yield. Or the mixturecould be leached alternatively with a limited amount of water at a highertemperature to decompose the carnallite and dissolve all the magnesium

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chloride, allowing to separate by filtration the remaining solid sodium andpotassium chlorides. These can be hot-leached and then the hot filtrate brinecan be cooled and concentrated under vacuum in conventional equipmentto crystallize the potash. The remaining brine can be recycled back into thesolar pond to repeat the process.

1

This straightforward process was eventually developed and an indus-trial plant was built to produce potash at the southern end of the Dead Seanear the biblical site of Sodom. The plant included the following successivesections (see the excellent description by J. Epstein, Reference 2): large solarponds for salt, solar ponds for carnallite,

wet harvesting

of the crystals fromthe carnallite, solids–brine separation, decomposition of carnallite in twocountercurrent stages, hot leaching of the solids in a circulating brine, hotfiltration of the salt, vacuum cooling crystallizers, potash washing and dry-ing, and all the adjacent services required for a desert location. In this firstventure, the solar ponds with

wet-harvesting

were, indeed, the critical newelement essential to efficiently handle millions of tons per year of corrosiveslurry. This was done with floating dredges that crisscrossed the ponds,slurrying the crystals from the bottom, and pumping the slurry into a floatingpipeline to the shore.

But as we said at the beginning, this is a changing world and two

human-induced changes

, which were outside the control of the operating Dead SeaPotash Company, occurred later and required new process developments.First, starting from the 1950s,

all the fresh water

from the Jordan River wasdiverted for agriculture development in Israel and Jordan, and practically

no water

was allowed to drain anymore into the Dead Sea. The age-oldsteady-state was ended and the concentration of the Dead Sea brine startedto increase,

slowly but inexorably

with more salt precipitated at the bottom ofthe sea and, consequently, the carnallite

production

of the existing solar ponds

increased

. The trend of these changes could be followed, analyzed, and pre-dicted exactly from the 1960s, and since it was imperative that

all

crystalsproduced in the carnallite solar ponds be removed to avoid clogging thewhole system, it meant that the potash production capacity should be

increased

accordingly. Finally, this additional raw material was availablealmost for free, so why not build more potash production in the 1970s?

But when expansion plans were prepared and approved and their execu-tion was about to start, the so-called

“energy crisis”

of 1973 happened and thecost of thermal energy jumped almost overnight by a factor of four to fivetimes. With the new production costs and the uncertainty concerning thefuture situation in this regard, the additional potash production by the hotleach process could possibly lose money. So the

processes basic concepts

had tobe urgently reconsidered, including the potential use of some elements whichwere known but not considered essential in the previous economic context.

It was known, for example, that in the crystals mixture produced in thesolar ponds, the salt and the carnallite were precipitated practically in sep-arate crystals, and that the size distribution of the carnallite crystals wasrelatively coarser than that of the salt crystals. It was possible to separate



about 25 to 30% of the carnallite, in rather pure form, from the feed to thepotash plant just by coarse wet-sieving of the slurry.

It also was known for a long time that, when a controlled quantity ofwater is added to carnallite crystals at ambient temperature, all the magne-sium chloride would go into a solution with about 20 to 25% of potassiumchloride, leaving the remaining potassium chloride as fine solids. There wereno incentives to make use of this information up to that point, since it wouldhave only complicated the straightforward “hot leach” process. But whenseverly pressed by the energy crisis, its reconsideration allowed a corporatetask force to develop a “

cold crystallization”

process, which required almostno thermal energy.

From a carnallite stream with a relatively small salt content, a

cold crys-tallization

system would produce

reasonably coarse and clean

potash crystalswithout heating and cooling. This process was analyzed in detail and itsimplementation depended on the development of a

novel type of continuousindustrial reactor–crystallizer

, in which the rather pure carnallite solids werefed and dissolved in one part, while the potash was crystallized in anotherpart, solids were decanted in quiet zones and brine was circulated betweenthe different parts.

3

This new design was piloted and demonstrated in an intensive program.The crystal mixture pumped from the carnallite ponds to feed the existinghot leach potash plant was first wet-screened to separate as much as possiblethe coarse carnallite fraction. A first cold crystallization plant was success-fully build for several hundred thousands ton per year of potash. This newdevelopment allowed a few years of breathing time in the race against natureand the oil “lords.”

Then, the gist of the problem was proposed to the physical mineral sep-arations scientists. Given a crystal mixture of carnallite (specific gravity 1.6)with salt (specific gravity 2.1) in a slurry with

end brine

, a residual solar pondby-product from the potash production of Dead Sea brine (in fact, a concen-trated solution of magnesium chloride with a specific gravity 1.35), how canone

separate

a greater part

of the carnallite in a

reasonable pure form

, in millionsof tons per years, and at

very low cost

? This physical separation did not haveto be completed, since the remaining mixture could still be treated by hotleach, but the content of the

pure

carnallite fraction should be above 95%.This challenge was again solved by a novel technology: by

centrifugaljigging

on a tumbler centrifuge equipped with a conical wedge-wire screenwith a rather large aperture. This type of centrifuge was developed earlierin Germany as a large-capacity screening device to produce low moisturecoarse salt cakes. It was found that the pulsations in the expanded fluid bedof crystals, flowing on the inside of the conical wedge-wire screen, causedthe heavier salt crystals to concentrate nearer the screen and, thus have thepriority of passage through, leaving most of the lighter carnallite crystalsbehind. The large-scale application of this technology allowed anotherexpansion of the “cold crystallization” plant and more breathing time in thecontinuing race against the clogging of the solar ponds.



Finally, the separation of the salt from carnallite in the finer size frac-tions was obtained by adaptation of the conventional froth-flotation tech-nology for salt used in other lands, to the particular conditions of the DeadSea chemistry. Today, the multimillion tons per year production of potashfrom the Dead Sea is

using all of these originally developed technologies

in anoptimum combination.

4.2 Magnesium chloride-based industries

In the early 1960s, it was apparent that end brine was a raw material verysuitable for the production of magnesium oxide (MgO = periclase). Thismaterial is widely used for refractory bricks. Up to that time, a part ofsmall deposits of natural magnesium carbonate, all the existing worldproduction of this material was based on precipitation of magnesiumhydroxide from sea bitterns. Such production is done in a

very dilute system

with hydrated calcined lime, which is an energy-intensive raw material.The application of a similar technology to the Dead Sea end brine woulddeny any advantage of its higher concentration, leaving only the disad-vantages of a desert location.

On the other hand, it was known that the thermal decomposition of suchend brine can produce solid magnesium oxide and a vapor phase with awater/hydrochloric acid mixture. The detailed conditions required to conductcontinuously such thermal decomposition processes were studied by Dr. J.Aman from the Hebrew University in Jerusalem, who developed and patentedin the 1950s the

direct-contact

continuous “Aman reactor.”

4

This is a sort ofspray dryer (vertical cylindrical/conical chamber) in which the end brine issprayed at certain locations from the top while hot flue gases at 800 to 900

°

Care introduced tangentially at the middle height, creating a definite

internalflow pattern

. The solid impure MgO particles remaining from the liquid dropsare settled and removed from the lower conical outlet and the gases exitingfrom the top are directed to a direct-contact absorption column, producing a18 to 20% HCl solution (somewhat below the 22% azeothropic concentration).

This novel process was piloted in the 1960s, and its enormous industrial

potential

was then demonstrated. However, its implementation remainedcritically dependent on the economic utilization of the HCl by-product and,thus, it was delayed until a proper combination could be organized. Otherissues were connected to the presence of smaller quantities of magnesiumbromide in the brine, which would produce elementary bromine in the gases,and this needed to be dealt with. This started a solvent extraction processfor separating a stream of pure magnesium bromide from the end brine, butthis is a different story.

Note that the same Aman process technology was also licensed andapplied successfully in other countries by the Ruthner Company for thedecomposition of the

iron chloride

solution resulting from steel picklingplants, where the recovered HCl solution could be recycled and reused onsite in the pickling plant.



4.3 Economic uses for the HCl by-product solutions

4.3.1 Strategic policy

In the 1960s, the managing team of the IMI Institute for R&D, directed byDr. A. Baniel, created a corporate

strategic policy

defining the need of devel-oping

economic uses

for by-product HCl solutions. As part of the Amanprocess for magnesia, a number of promising “acid-salt” double decompo-sition processes were under consideration aimed at upgrading the value ofthe chlorides of potassium, sodium, and magnesium (available in very largequantities at low cost) into the relevant sulfate, phosphate, or nitrate salts.Implementation of any one of these potential processes would also yieldHCl as a by-product solution (as indeed, the IMI process for potassiumnitrate when it was implemented by Haifa Chemicals Co.).

5

Up to that time, HCl was mainly a traditional by-product of organicchlorination reactions and of small-scale chemical industries. In most of thesecases, the HCl was wasted, neutralized with lime and/or limestone, anddisposed of as CaCl

2

in the sea. Only in very large organic chlorinating instal-lations could the HCl by-product be collected as a solution and recycled to anelectrolysis section to regenerate elementary chlorine, or collected and recycledby the Kellog’s Kel-Chlor process.

6

This route was hardly more economical,but it was possibly less problematic than the neutralization route.

4.3.2 Coupling of HCl-producing and consuming plants

Some industrial uses with economic justification were developed within thisstrategy (see discussion below), but the

basic problem

that remained for sev-eral decades was the

critical coupling

in the implementation between the plant

producing

the HCl and the plant

consuming

it, in their geographical location,in quantity, and in timing. It should be remembered that the HCl–watersystem is dominated by an

azeotrope

at 20 to 22% HCl, so that every ton ofHCl generated below the azeothrope is accompanied over the fence by 4 to5 tons of water, and the transportation of such solutions would be impracticalover any significant distance.

“Breaking the azeothrope” (i.e., obtaining more concentrated solutionsor even 100% dry HCl) is possible, but complicated and expensive, both ininvestment and in energy consumption; for example, by using a cycle ofCaCl

2

brine. This was a wide field of creative process design, aiming at abetter use of the energy and expensive heat exchangers, and for possiblesynergetic utilization of sources of low-temperature waste heat.

7

(See alsoChapter 6, Section 4.)

4.3.3 Timing of implementation

As an acid reagent, HCl could be used to replace sulfuric acid in severalmineral industries. Some new processes in hydrometallurgy and mineralrefining were studied and a few of these could have been developed and



used if a reliable HCl source could have been made

available at the right time

;for example, the cleaning of sand for the glass industry, the purification ofdifferent sorts of clays, the reprocessing of nonferrous scraps, etc.

4.3.4 Production of pure phosphoric acid

The main novel process that was actually developed and used on a largescale in several plants was the production of

phosphoric acid

by hydrochloricacid leaching of (calcium) phosphate rock. The conventional process with

sulfuric acid

gives a

solid gypsum

residue, which is separated by filtration fromthe

impure

“wet”

phosphoric acid (WPA) solution. There exists also processesbased on nitric acid.

When using a hydrochloric acid solution to dissolve the phosphate rock,the water-soluble residual CaCl

2

remains in the same aqueous solution withthe phosphoric acid. A new separation process, therefore, was required toisolate the phosphoric acid from the CaCl

2

(and all the other soluble impu-rities). This result was provided in a pioneering breakthrough by A. Banieland R. Blumberg, by way of solvent extraction.

The first IMI “standard” phosphoric acid process was quite complexwith six different multistage, countercurrent batteries. A comprehensivedescription of all the issues related to its development and implementationwas presented at an international scientific conference,

5

as an

IMI staff report

prepared by a dozen senior staff members, each one in his/her specialty.This

unique

approach started in this book, but unfortunately it was not wellunderstood and was not pursued in further scientific publications.

The new process also accomplished a thorough

purification

of the phos-phoric acid product, which could aim at the higher value markets. Suchmarkets were traditionally supplied by “thermal” phosphoric acid, obtainedvia elementary phosphorus (see below). This novel process was actuallylicensed and implemented first in Japan, Brazil, and Spain, where someexisting sources of by-product HCl already existed, before it was used inIsrael in large plants fed with HCl by-product from potassium nitrate andpericlase productions.

4.3.5 Technological difficulties

After the basic chemical research and the bench-scale demonstration of thenew process, the developing team at the IMI Institute for R&D had to facesome difficult

technological

issues on the way to implementation.

4.3.5.1 Materials of construction

HCl is a well-known, “nasty” component to work with, as it attacks practi-cally any metal. Previously, it could be handled in industry only in smallglass equipment (i.e., Pyrex), or small glass-lined steel (i.e., Pfaudler), or insome cases, in rubber-lined steel (limited to the lower temperature range ofless than 60

°

C). As all of these options for materials of construction were



very expensive, sensitive, and quite limiting in large volumes, it was obviousfrom the start that plants for the large-scale production of relatively cheapmaterials could not be built exclusively from expensive materials.

Fortunately, during the same period, the technology for the design anderection of large equipment and piping made from

plastic

was being devel-oped in several advanced industrial countries. This technology used sheetsand tubes of thermoplastic PVC and polyethylene (and, later, polypropylene)with possibly the external reinforcement of layers of glass-fiber/polyestersetting mixtures and of steel members. Later, the fabrication of equipmentmade exclusively from reinforced polyester or epoxy setting mixtures wasestablished. This new technology was the

critical engineering basis

for anyHCl-based industry at that

time. Thus, the process development group hadto take a very active role in locating the best know-how available worldwide,in establishing specialized companies and workshops in Israel, in creatingdesign standards and testing procedures, and in merging all of these into apractical working system. Another limitation was that the use of

plasticizer

materials in the thermoplastic material was strictly prohibited for all vesselscontaining solvents, as these plastisizers would be leached out by the solvent.(Today, this fabrication technology is essentially available widely as a stan-dard engineering choice, but there are continuing new improvements inmaterials and in design techniques that have to be evaluated.)

At the same time, it was obvious that the use of these thermoplasticmaterials would limit the process temperature to below 60

°

C (at most). Thus,any solvent stripping operating at higher temperatures would remain mostlywith conventional glass-lined equipment, although thermo-setting resinscould sometimes be used for limited functions, and should be minimized aspossible. Another prosaic but important limitation was that, for structuralstrength design considerations, all plastic vessels needed to be

round

(verticalcylinders) and this affected both the

internal functional

design and the plant’sgeneral layout considerations.

4.3.5.2 Safe, stable conditions for solvent extraction in large mineral plants

At the beginning of the project, the “explosion-proof” conditions associatedwith the handling of relatively large quantities of organic solvents withrather low ignition points (i.e., butanol, pentanol, and the like) were wellknown in petroleum refineries and petrochemical installations, but ratherunfamiliar in the mineral/chemical industry. The process developmentgroup had to recruit experienced consultants in this area and make a specialeffort to study, assimilate, and adapt the explosion-proof codes to theseparticular projects, even for such simple items as the venting of excess gases.

In addition, the composition of the solvent stock circulating in the plantcould hardly be taken as a constant, as it undertook various chemicaldegradations and additional reactions, mostly with the unavoidable impu-rities flowing through the plant streams. For example, most phosphate orescontain some organic matter soluble in acidic leach solutions, which are



partly extracted and accumulated in the solvent, and require specific clean-ing procedures.. Such reactions can produce contamination in the productor even change some of the solvent’s properties. Thus, a surprising amountof sophisticated R&D in

organic chemistry

was needed for such mineralprocess development.

4.3.5.3 Clean starting solution for solvent extraction

One of the main enemies of industrial solvent extraction is the

crud

consistingof fine solid precipitates, which accumulates at the interface between thetwo liquid phases and may prevent their separation and cause emulsions.This crud may also clog lines and build up in equipment.

When dissolving, for instance, a typical phosphate ore into a hydrochlo-ric acid solution (with minimum acid excess), most of it goes into a solution.The resulting slurry is degassed under vacuum and the solid residue (con-sisting of sand, clays, dirt, etc.) is then flocculated and separated by coun-tercurrent decantation, and the overflow is polished by filtration. This is theeasy conventional part. However, when the

clear filtrate solution

comes incontact with the organic solvent, the solubility conditions change, and it wasoften found that

crud would precipitate

.For example, part of this crud can be

organic colloidal material

originatingfrom the natural phosphate ore, which was maintained completely in solu-tion in the strongly acid solution, but can be precipitated when part of theacid is extracted and flocculated by the organic reagent. Other parts of thiscrud can be mineral or

metallic salts,

which were initially kept in solution bythe strong acidity. As these elements were defined, the process developmentteam had to devise ways to avoid this crud, or at least reduce it to proportionsthat could be handled with periodical cleaning schedules. This aim could beachieved by either changing the properties of the starting phosphate ore, ifthere was an affordable choice (e.g., by using a more expensive calcinedphosphate concentrate with no organic matter), or by adding pretreatments(such as ion exchange) to the solution before its transfer to the solventextraction section.

4.3.5.4 Recovery of the residual solvent from different exit streams

All the effective solvents were partially water-soluble, and their saturationsolubility in the exit aqueous streams was of the order of a few percents,depending on the temperature and on the other solutes presents. In principle,these residual solvents could be stripped down to the allowed and affordablelevel of, say, less than 100 ppm in a distillation column with sufficient numberof stages and reflux. Again, this may seem a trivial question of engineering,but it was rapidly apparent to the process development team that the invest-ment on the glass-lined equipment, the possible attack of fluoride anions onthe glass lining, the possible deposition of calcium fluoride from wastesolutions whenever heated, and the associated thermal energy and coolingwater would be a

critical load

on the economics.



Thus, every possible way to decrease these costs had to be consideredin the process development. The overall technical–economical optimizationcould recommend a different solvent, which possibly may have been lesseffective in the separation, but cheaper to recover. Other practical questionsalso needed to be addressed, such as the possible fouling of the highertemperature stripping equipment with solid incrustations and, in particular,on the heat exchanger surfaces.

4.3.5.5 Large-capacity liquid–liquid contacting equipment

The implementation of the new processes required

large-capacity

liquid–liq-uid contacting equipment

8–23

for multiple-stage countercurrent batteriesmade of suitable materials, i.e., plastic (see above). The concept of the mixers-settlers was already established on bench scale and used in pilots and rela-tively small industrial installations (i.e., for uranium extraction). However,the design of

efficient large-scale equipment

was not established at the timeand some issues had to be solved.

First of all, hydraulic heads for the flow of liquid streams from stage-to-stage in both directions had to be worked out. The simplest solution formaintaining such hydraulic heads is to install two interstage pumps for eachstage (with the associated sumps and level controls, but with very lowheads), which, in addition to the mixer, amounts to three explosion-proofmotors per stage. No problem. One has only to multiply the number of stagesby three, but the result is not a simple number, but more likely a snowball.The cost of an explosion-proof motor is 2 to 3 times that of an ordinary one,but its installation can cost 10 times more, and the level control loop willdouble that total. One alternative can be to design with only one transferpump, plus a difference in height, but this difference would accumulate andcertainly complicate the vertical layout for large multistage batteries. There-fore, to keep all the mixers–settlers on the same level with only one motorper stage, there was a clear and imperative need to use each motor for morethan one task. This prompted from the beginning a

hydraulic

research anddevelopment program as an integral part of the new chemical process imple-mentation on a large scale.

Figure 4.1 illustrates the principle of the patented IMI “pump-mix” con-cept, with a vertical pump (between two static baffles) on the same shaftand above the mixing axial propeller. This pump design has a very steep {Q@ H} curve, so that the level in the mixer is self-regulated without any levelcontrol hardware. A manual weir for each ratio of liquid densities fixes thelevel of the apparent interface in the settler. This design was successfullyinstalled in a large number of industrial installations for those cases wheretwo liquid phases had relatively close densities and low viscosity and couldbe easily dispersed and circulated in the liquid–liquid mixer. A reasonablemass transfer was obtained with the large range of droplet sizes.

But at a later stage, when implementing such processes involving thecontact and equilibration of a

heavy aqueous brine

with a

light organic solvent

,the above design could no longer give the adequate hydraulic heads, mass



transfer, and phase separation rate. So, as an integral part of these newprocess developments, a new liquid–liquid mixer had to be designed andtested, resulting in the IMI “turbine pump mix” design (see Figure 4.2),which produced a

controlled

droplet distribution when operated with a

vari-able speed

drive.Finally, the use of plastic materials of construction also necessitated

the functional design of a relatively

new round settler

(instead of the con-ventional “shoebox” design), with a central inlet of the two liquid-sus-pension from the mixer, and radial flow of all separated phases. Thisdesign required a fundamental study of the basic hydraulics connected tothe settling coalescence process and of quantitative design procedures forsuch a settler. This study resulted in a later stage to the invention of thepatented “compact settler” design, making use of racks of inclined parti-tions to save the largest part of the area and of the internal volume, andreduce the expensive solvent inventory. (See also Chapter 6, Section 6.4.1and Figures 6.5 and 6.6.)

Figure 4.1

Mixer-settler with pump mix.

Figure 4.2

Mixer-settler with turbine pump mix.

apparentinterface

heavy phase

heavyphase

lightphase

vent

lightin

heavyin

mixed phase

weir

apparentinterface

heavy phase

heavyphase

lightphase

vent

light in

heavy in

mixedphase

tangentialconnection

light phase

turbine

stator

weir



4.4 Phosphoric acid diversification processes

4.4.1 Different quality specifications

The different users of phosphoric acid require

different quality specs

, which arelisted below in order of decreasing purity and purchase cost per unit of P

2

O

5

:

1. Chemically pure/pharmaceutical grade (CP or PG)2. Food grade — FGPA3. Technical grade — for different phosphate salts4. Animal feed grade — for cattle and poultry feed supplements5. Detergent grade — mostly for sodium tripolyphosphate (STPP and

similar)6. Liquid fertilizer grade — giving a clear aqueous solution after neu-

tralization7. Solid fertilizer grade — lowest acceptable grade (almost anything goes)

4.4.2 Solvent extraction opening

Up to the introduction of the solvent extraction processes in several coun-tries,

24,25

there were only two grades of phosphoric acid available:

• The wet process acid (WPA) that was adequate only for the solidfertilizer grade, as it contained a few percents of sulfuric acid, someF, Ca, Mg, Fe, Al, etc.

• Expensive “thermal acid,” which should be used for any of theother grades.

The production of thermal acid (and these markets) was limited not onlyby its cost (2 to 3 times more than WPA), but also by the serious ecologicalhazards related to the elementary phosphorus. The

unfulfilled potential

and

the needs

were clear to the whole industry. Various chemical treatments werestarted in various places in connection with specific partial neutralizationprocesses of WPA.

As soon as the solvent extraction technology got established worldwideand the phosphoric acid extraction and purification was demonstrated,

26–27

there was a worldwide rush by R&D units in this industry to establish newprocesses, to patent different related issues, and to build producing plants.28

The solvent extraction technology allowed for producing different qualitygrades of phosphoric acid at varying production costs, starting with the mer-chant qualities of WPA, which could be produced on site or be purchased. Butone should also note that any one of these processes would leave a more impureresidual stream containing between 30 and 70% of the starting phosphatevalues. This should be downgraded and compounded into a solid fertilizer-grade by-product or mixed, if possible, with merchant WPA. This meant thattheir implementation could only be in proximity to a large solid fertilizer plant.



There was a very intensive worldwide struggle in the 1960s and 1970suntil the novelty appeared to be more or less exhausted and the worldwidemarket saturated; this despite a very fast increase in the demand for the prod-ucts, mostly for the detergent and liquid fertilizer uses. (The Tennessee ValleyAuthority [TVA] point of view is summarized in Reference 29.)

Following are some of the processes developed by the IMI team in thisparticular field.

4.4.3 IMI “cleaning” process

The IMI “cleaning” process30 was implemented in 1974 in a large plant thatis still functioning in the port town of Coatzacoalcas in southern Mexico,near a large WPA producer. The solvent extraction process is extremelysimple and flexible and is based on the extraction of phosphoric acid from a53 to 54% P2O5 WPA feed with a diisopropylether (IPE) organic stream at alower temperature and its back extraction at a higher temperature as cleanproduct. This is at a concentration of 50% P2O5, which is used either fordetergent grade or for liquid fertilizer grade.

The novelty of this process, which is quite unique, is that each operationis conducted in a temperature-controlled invariant system, in which three liquidphases with fixed compositions coexist in the zone delimited by the points:

A. Light phase, almost only IPEB. Intermediate zone with a relatively high solubility of phosphoric acidC. Heavy aqueous solution with very little IPE, as shown in Figure 4.3

on a triangular equilibrium diagram for the tertiary system phospho-ric acid-water-IPE

Figure 4.3 Triangular equilibrium diagram H3PO4-water-IPE.

H2O H3PO4

IPE

WPA

A

B

C



Without this particular feature, IPE would be a quite inefficient solvent. But theWPA is mixed with the recycled solvent in a weight ratio such that thetotal mixture composition fell into the three liquids zone, as close as possibleto the line BC. An extract B is separated and most of the impurities remainwith the residual C. Some water is then added to the extract to separate italong the line AC, which will give the Clean Acid(c) and the recycledsolvent (A). All the mass transfer and the final results are obtained in asingle equilibrium stage for each operation (for a fixed number of components,more phases at equilibrium = less degrees of freedom = simpler process, as Gibbswould have said), although a second mixer-settler was provided in the plantas backup and energy optimization. However, small amounts of the cat-ionic impurities and sulfuric acid entering with the feed WPA (more com-ponents) are co-extracted and can be reduced to the extent needed by acountercurrent, backwash reflux battery.

About 60% of the phosphoric acid is recovered as “clean acid,” whilethe “residual acid” containing 40% P2O5 (with most of the impurities) isreturned and back-mixed into the fertilizer plant. The traces of the volatilesolvent are removed from the two exit streams in two steam-stripping dis-tillation columns.

4.4.4 “Close-cycle” purification process

The IMI “close-cycle” purification process31,32 to produce a quite pure phos-phoric acid from WPA was a modification of the “standard” process in whichthe CaCl2-rejected solution was concentrated, roughly cleaned, and recycledto be mixed with the WPA feed. The rest of the process was similar. Thisallowed by-passing the situation where HCl was not available. The processworked well and the product was very pure, but the process was quitecomplicated. Several studies by large corporations showed that it could bejustified economically only if it was implemented on a very large scale. Suchscale exceeded the demand for such pure product in most geographical areas.

4.4.5 Mixed process

A mixed process is practiced in Israel by mixing a certain portion of WPA(at a 28 to 30% P2O5 solution before its final concentration) into the HClleach operation in which the phosphate concentrate is dissolved. Suchaddition increase the average concentration of P2O5 in the leach solutionand makes use of the acidity of the sulfuric acid in the WPA. The sulfateprecipitates as gypsum with other impurities before the filtration of thesolid residue. Straight sulfuric acid can also be used, but this wouldincreases the load of gypsum on the filter, which needs to be HCl-resistantin such cases. The “mixed” process is then operated in the same way asthe “standard” process, but in a more concentrated and cost-efficient way.It also avoids the restrictions caused by the limited supply of HCl and thelow concentration of its solution.



4.4.6 New proposals

New processes for phosphoric acid published since the 1990s included oenfor obtaining phosphoric acid from phosphate rock and hydrochloric acid viaferric phosphate, which was patented in 1997 and published at the Interna-tional Solvent Extraction Conference (ISEC) in 1999.33 It aimed at cutting dras-tically the volumes and number of contact stages involved in the standard IMIprocess, at the cost of a couple of solid–liquid separations by using the verylow solubility of ferric phosphate (see also Chapter 5, Figure 5.1).

4.5 Citric acid by fermentation and solvent extraction4.5.1 Conventional lime sulfuric acid process for citric acid

Citric acid is an expensive but widely used food additive, giving acidity andlemon flavor to industrial food and soft drink products. Sodium citrate isalso much used as a detergent component for domestic laundry and in manypharmaceutic and fine chemical products. Citric acid is produced by aerobicfermentation in deep tanks, starting with a carbohydrate solution containingdifferent additives and seeded microorganisms. After the practical comple-tion of the fermentation and the filtration of the suspended material, thecitric acid contained in the fermentation “broth” needs to be separated fromany residual carbohydrates and from all the various impurities and by-products, in a form suitable to produce pure citric acid crystals.

The chemical separation route used by most producers consisted, in theaddition of hydrated lime (“liming”), the precipitation of calcium citrate,the filtration and washing of the solids, then the decomposition of the filtercake in a sulfuric acid solution in a strictly controlled ratio to liberate thecitric acid and precipitate the calcium as gypsum. After filtering and wash-ing the gypsum, the solution is concentrated and the citric acid crystallized.The product crystals are washed and the wash solution returned to theconcentrator. The remaining mother liquor is bled and recycled back to theliming. While all producers are keeping confidential the details of theiroperating procedure, it is probable that they are also using additionalpurification steps on different streams, such as active carbon, adsorbingfilter aids, ion exchange, etc. This chemical separation route was used formany years, but is rather delicate to operate, as it had three solid–liquidseparations with washing, resulting in a relatively low citric recovery yield,a high consumption of many reagents, a costly waste disposal, and theoccasional dumping of contaminated batches. This was an obvious placefor a better separation process.

4.5.2 IMI-Miles solvent extraction process for citric acid

With the increasing understanding of the temperature effect on the revers-ible extraction/separation of mildly strong acids with tertiary amine



extracting agents, the IMI team, under Dr. A. Baniel, proposed in 197034 anew process to replace the chemical route described above. This was rap-idly developed, demonstrated, patented, and licensed to one of the majorproducers in those days, Miles Laboratories, Inc. The new process was thenpiloted and implemented in close cooperation with the Miles technicalteam, under Dr. Toby Wegrich, in an existing large plant in the U.S., affect-ing only those sections that were to be replaced. The satisfactory operationresulted in a much increased citric recovery (that relates to the plant pro-duction capacity) and in lower production costs. This process was thenused in other plants of the company, giving them a strong advantage overany competition worldwide.

4.5.3 Newer solvent extraction process for citric acid

About 20 years later, another large American corporation (Cargill, Inc.)decided to get into the citric acid business from the start. The company haddeveloped its own fermentation technology and knew, of course, about theIMI patent licensed to Miles (which was still in force), and were looking fora similar solution. Dr. A. Baniel and David Gonen provided this result in avery creative but simple way35 (which should be very instructive for futuresimilar cases).

The first process intended to use the fermentation broth as it wasproduced then in the existing Miles plants, so the attention of the devel-opers and the original patent claims were referred to this particular rangeof citric concentration. But the citric concentration can be increased 2 to 3times by evaporation, if needed, before the separation process. Such changeallowed not only to avoid the formal wording of the claims in the originalpatent, but also to take advantage of the higher concentrations to get amore compact and efficient separation process with relatively smallerequipment and less solvent inventory. This novel concept was rapidlydeveloped and demonstrated in close cooperation with the designatedcorporate task force. A large plant was built and operated very successfullyon this basis.

Why wasn’t this increase in concentration thought of and introduced inthe IMI-Miles process from the beginning? Only because the exact frameworkof allowed changes (in the existing and producing plant) were defined fromthe start as a precondition for the novel process design, to limit the risks thatthe implementing corporation would be ready to accept. Why wasn’t thisincrease in broth concentration studied and patented later by the operatingcompany after they had a working plant and complete control of the tech-nology? Because then, the common rule in industry was applied: “If it worksand makes a nice profit, don’t touch it.”

Twenty years later, starting with a confident new team and a blank sheet,this preconcentration was a perfectly normal option for consideration. Thislesson can be applied to many other processes.



4.6 Preparation of paper filler by ultra-fine wet grinding of white carbonate

White paper, made with “neutral/alkaline sizing,” contains between 20 and35% by weight of white filler powder, which is mostly precipitated calciumcarbonate in the form of crystalline needles. This filler gives to the finishedpaper its whiteness, opacity, and weight. Such precipitated calcium carbon-ate is not difficult to make, but it is energy-intensive and has to be producedon a relatively large scale. Thus it is relatively expensive, particularly if ithas to be dried, packaged, and transported for long distances. So, the eco-nomic need prompted the question: Why can’t it be replaced by finelyground, white calcium carbonate?

A new process technology was developed and implemented on a mod-erate scale near a large paper mill. This dedicated exclusive user wasreceiving the slurry in accordance with his own specification, ready to mixinto the feed going to the paper machine, naturally flocculated with aconsistent size distribution, characterized by an average size of 1 micron,with most smaller than 2 microns, and with a minimum content of minushalf a micron.36

The novelty and the particular features of the process technology, whichwas needed to obtain such final particles, were in the operating conditionof a regular iron ball mill, such as the pulp density, the residence time, thetemperature and certain chemical additives. However, since the final sizespecification cannot be obtained in a single pass, an extensive external circuitwas needed for fine-size classification, separating the product’s fine particlesfrom the recycled coarser particles. This circuit represented the main processchallenge, considering the requirement that the product particles should retaintheir natural flocculation. Generally, in the technology of “fine particles,”dispersing agents would be used to achieve such size classification, but theywere not allowed in this case.

The process solution was derived from the previous research work doneby one of the developers in the mechanism of hydrocyclones,37–40 whichallowed the design of batteries of microhydrocyclones in a countercurrentarrangement, handling large flows of diluted slurry (Figure 4.4). In addition,an original automatic control scheme was designed to handle the naturalfluctuations in the raw material and mechanical system, based on the con-tinuous measurement of the size distribution in the product slurry, which oper-ated a number of flow splitters affecting the recycle cycles. This lesson canbe applied to other similar microparticle systems.

These filler particles were more or less round, whereas the usual precip-itated calcium carbonate generally consists of elongated needles and thisaffected somewhat the “usual appearance” of the finished paper, althoughmost users were unable to perceive the difference. This ultra-fine grindingplant was happily operated by the Polichrom Company for about 12 years,but then the trading conditions in the area were changed, forcing the papermill to modify its line of products and its operating procedure.



4.7 Worth another thought• The use of the by-product from one process in one plant as a raw material

for another process in another plant creates a critical coupling betweenthe two plants, in geographical location, in quantities, and in timing.

• The implementation of a new process can also require new solutionsas regards materials of construction, design standards, and new func-tional equipment.

• The introduction of a new process to replace part of an existing plantis generally preconditioned into the existing conditions of the remain-ing sections. However, once it is well integrated into the production,an overall optimization should be studied for further improvementor future plants.

References1. Kenat, J., The production of potash from the Dead Sea, Second Symposium on

Salt, Cleveland, OH, 1965.2. Epstein, J.A., The recovery of potash from the Dead Sea, Chem. Ind., 572–576,

July 1977.

Figure 4.4 Principle of an ultra-fine wet grinding and classification process.

Ball Mill

solidfeed

centrifugefine

productslurry

waterrecycle

split

split

water



3. Tzisner, T., The Maklef plant for cold crystallization of potash, Comm. IsraelSoc. Chem. Eng., personal communication, October 1989.

4. Aman, J., British Patent 793.700, 1950; Israel Patent 8722, 1956.5. IMI Corporation, Development and Implementation of Solvent Extraction

Processes in the Chemical Industries, staff report at the Int. Solvent ExtractionConference, The Hague, 1386–1408, 1971.

6. Van Dijk, C.P. and Schreiner, W.C., Hydrogen chloride to chlorine via the Kel-Chlor process, Chem. Eng. Prog., 69, 57–61, 1973.

7. Mizrahi, J., Barnea, E., and Gottesman, E., Production of Concentrated HClfrom Aqueous Solutions Thereof, Israel Patent 36,304, 1972.

8. Mizrahi, J. and Barnea, E., A Gravitational Settler Vessel, Israel Patent 30,304, 1968.9. Mizrahi, J. and Barnea, E., A Liquid–Liquid Mixer, Israel Patent, 43,692, 1973.

10. Mizrahi, J. and Barnea, E., A Gravitational Settler, Israel Patent, 43,692, 1973.11. Barnea, E. and Mizrahi, J., Compact settler gives efficient separation of liq-

uid–liquid dispersions, Proc. Eng., 60–63, 1973.12. Barnea, E. and Mizrahi, J., A generalized approach to the fluid dynamics of

particulate systems, I: General correlation for fluidisation and sedimentationin solid multiparticle systems, J. Chem. Eng., 5, 171–189, 1973.

13. Mizrahi, J., Barnea, E., and Meyer, D., The Development of Efficient IndustrialMixer-Settlers, paper presented at the Int. Solvent Extraction Conference,Lyon, France, 1, 14l-168, l974,

14. Barnea, E. and Mizrahi, J., A generalized approach to the fluid dynamics ofparticulate systems, II: Sedimentation and fluidisation of clouds of sphericalliquid drops, Can. J. Chem. Eng., 53, 461–468, 1975.

15. Barnea, E. and Mizrahi, J., Separation mechanism of liquid-liquid dispersionsin a deep-layer gravity settler (four-parts series), I: The structure of the dis-persion band, II: Flow patterns of the dispersed and continuous phases withinthe dispersion band, III: Hindered settling and drop-to-drop coalescence inthe dispersion band, IV: Continuous settler characteristics, Trans. Inst. Chem.Eng., 53, 61–69, 70–74, 75–80, 83–93, 1975.

16. Barnea, E. and Mizrahi, J., A generalized approach to the fluid dynamics ofparticulate systems, II: Sedimentation and fluidisation of clouds of sphericalliquid drops, Can. J. Chem. Eng., 53, 461–468, 1975.

17. Glasser, D., Arnold, D.R., Bryson, A.W., and Vieler, A., Aspects of mixerssettlers design, Min. Sci. Eng., 8, 23–31, 1976.

18. Barnea, E. and Mizrahi, J., On the “effective” viscosity of liquid-liquid dis-persions, I&EC Fundam., 120, 1976.

19. Barnea, E. and Mizrahi, J., The Effects of a Packed-Bed Diffuser Precoalesceron the Capacity of Simple Gravity Settlers and on Compact Settlers, paperpresented at the Int. Solvent Extraction Conference, Toronto, 374–384, 1977.

20. Barnea, E., The Application of Basic Principles and Models for Liquid Mixingand Separation to Some Special and Complex Mixer-Settler Design, paperpresented at the Int. Solvent Extraction Conference, Toronto, 347–355, 1977.

21. Barnea, E., Meyer, D., and Wahrman, D., Logical Design of Mixers, paperpresented at the Int. Solvent Extraction Conference, Liege, France, 6–12, 1980.

22. Harel, G., Kogan, M., Meyer, D., and Semiat, R., Mass Transfer Characteristicsof the IMI Turbine Pump-Mix, paper presented at the Int. Solvent ExtractionConference, Denver, 26–27, 1983.

23. Cusack, R. and Karr, A., Extractor Design and Specification, Chem. Eng.,113–118, 1991.



24. Toyo Soda Manufacturing Co., Japanese Patent 7,753, 1964.25. Albright and Wilson Ltd., German Patent Application, 2,320,877, 1973.26. Baniel, A. and Blumberg, R., in Phosphoric Acid, Slack, A.F., Ed., Vol.1, Part II,

Marcel Dekker, New York, 1968.27. Blumberg, R., Industrial extraction of phosphoric acid, Solv. Extrac. Rev., 1,

93–104, 1971.28. Blumberg, R., Meyer, D., and Mizrahi, J., Development and implementation

of solvent extraction processes in the chemical industries, paper presented atthe Int. Solvent Extraction Conference, The Hague, 1386–1408, 1971.

29. McCullough, J.F., Phosphoric acid purification: comparing the process choic-es, Chem. Eng., 101–103, 1976.

30. Mizrahi, J., IMI Technology for Cleaning Wet Process Phosphoric Acid bySolvent Extraction, paper presented at the Symp. Am. Chem. Soc., 1973. (Thedata in Figure 4.1 was included by Blumberg, R. in a communication to Isr.Chem. Eng. J., September 1973.)

31. Blumberg, R., Miscellaneous Inorganic Processes, in Handbook of Solvent Ex-traction, Lo, T.C., Baird, M.H.I., and Hanson, C., Eds., John Wiley & Sons, 827,1983.

32. Slack, A.V., Phosphoric Acid, Part 2, Marcel Dekker, New York, 721, 1968.33. Mizrahi, J., New Process for Phosphoric Acid from Phosphate Rock and

Hydrochloric Acid Via Ferric Phosphate, paper presented at the Int. SolventExtraction Conference, Barcelona, 1999; also Israel Patent Application,120,963, 1997.

34. Baniel, A., Bkumberg, R., Haidu, K., U.S. Patent 4,275,234, 1971.35. Baniel, A. and Gonen, D., European Patent 91304805, 28.5.91.36. Hirsch, M., Hirsch, I., and Mizrahi, J., Production of white carbonate paper-

fillers by a new ultra-fine wet grinding technology, Ind. Miner., 67–69, 1985.37. Mizrahi, J., Separation mechanisms in hydro-cyclone classifiers, Brit. Chem.

Eng., 10, 686–692, 1965.38. Cohen, E., Beaven, C.H.J., and Mizrahi, J., The residence time of mineral

particles in hydro-cyclones, Trans. Inst. Miner. Met. (London), 129–138, 1966.39. Mizrahi, J. and Cohen, J., Studies of factors influencing the action of hydro-

cyclones, Trans. Inst. Miner. Met. (London), 318–330, 1966.40. Mizrahi, J. and Goldberg, M., Computer simulation of unflocculated hindered

settling, Isr. J. Tech., 318–392, 1969.



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