Diluents SX Bishop

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Your Source or Success

4th Annual

CONFERENCE & EXHIBITION

November 17-20, 1998

Hotel Vancouver

Vancouver, British Columbia

Randol International Ltd.

21578 Mountsfield Drive

Golden, Colorado 80401 USA

Phone:

303)

526-1626; ax

303) 526-1650

Web site: www.randol.com

E-mail: [email protected]

1



SOLVENT EXTRACTION DILUENTS

WHAT ARE THEY AND How DO THEY

AFFECT SX PLANT COSTS?

M.D. Bishop

Phillips Mining Chemicals

1768 Highway 123

Bartlesville, OK 74004

Phone: (918) 661-9380; Fax: (918) 662-5201

E-mail: [email protected]

The regulation was designed o decrease round evel ozone

and is basedon studies, which have shown that hydrocarbon

vapors can contribute o the fonnation of ground level ozone.

The compounds,which are excluded by legislation, have been

studied and detennined o not conb'ibute o ozone fonnation.

High molecularweight hydrocarbons regenerallyconsideredo

haveminimal impact due o their low volatil ity.

The California Air ResourcesBoard (CARB) recognizes he

low impact of higher molecular weight hydrocarbons. CARB

defineshydrocarbons f greater han C'2 or with vapor pressures

less than 0.1 mm of Hg at 200 C as LVP-VOC (Low Vapor

Pressure - VOC) substances. The LVP-VOC designation

includes hosechemicalcompounds nd he weight percentof a

mixture with a boiling point greater than 2160 C (4210 F).'

Thesecompoundsare excluded rom some regulatory require-

ments when used in consumergoods. However, there are no

exemptions or industrial point sources.

There are wo generalb'ends n distillate physical properties

that help define the boundaries for practical diluents.

Although theseb'endsare well known, it is important o review

them in consideration of solvent extraction operations.

Consideration f these actorsgenerally imits commercialdilu-

ents to those compounds with carbon numbers primarily

betweenCIO nd C2o.

.

The vapor pressureof the hydrocarbondecreases s the

molecularweight (carbonnumber) ncreases.

. The viscosity of the hydrocarbon ncreases s he molecu-

lar weight increases.

The vapor pressure f a hydrocarbon s related o its volatili-

ty. Lower molecular weight hydrocarbons ave higher vapor

pressure.This leads o increased olatilit).; i.e. they evaporate

more rapidly. The vapor pressureof typical diluents is in the

rangeof I to 3 mm Hg at 380C. Sincevapor pressures related

to flash point, lower molecularweight hydrocarbonswill havea

lower flash point. However lash point is, as discussedater,not

definitive.

Solvent extraction diluent and extractantare blended n the

atiosas determinedby the plant operational equire-

The resultantsolution is the plant organic.The diluent is

he majority fraction of the plant organic and, as such,

an essential oll in efficient operationof the solventextrac-

This paper discusses ome of the terms associated

or mini-

costs.

Modem solvent extraction diluents are constantcomposition

distillates that have been developedspecifically for

extractioncircuits. They containa rangeof hydro-

of aliphatic, aromatic, and naphtheniccom-

The terms are used n the petroleum ndustry o denote

of the chemical compound. Aliphatic compounds

molecular structures; alkanes, alkenes, and

and their cyclic analogs'. Aromatics are those com-

one or more unsaturated ing structures)

n the molecule.Although this term includesclassi-

aromatics uchas benzene nd oluene, hesecompounds re

present n any appreciable oncentrationn modem diluents.

modem diluents are heaviercompoundsmore

called alkylaromatics or arenes.The distinguishing

of thesearomatics s an unsaturated ing structurewith

or more aliphatic chains attached o the ring. Cycloalkanes

n the petroleum ndustry. This term

not be confusedwith naphthalenecamphor). The origi-

meanings f the term "aliphatic" (fatty) and "aromatic" (fra-

are no longer significant.'

All diluents in current commercial use are volatile organic

VOCs) as defined by regulation. US regulations

a VOC as any compoundof carbonand hydrogenunless

by legislation. For example, methane CH4) is com-

of only carbonand hydrogenbut is not a VOC. It is exclud-

by legislation.

121

COPPER HVDROMET ROUNDTABLE 98



~.:Q:..~ ~.~.9~

A common mIsconception is that aromatic compounds

evaporate faster than aliphatic compounds. Evaporation rate

is related to th( vaJ"'f pressure of the product. Table I pro-

vides the tempcrawrts ~uired for the selected compounds to

achieve the spC"-'itW vapor pressure. The data demonstrates

the aromatic c(\It\I"\Undsshown require a higher temperature

than the corresl"lftJing aliphatic compounds to achieve a spec-

ified vapor prtSSUf(' The~fore, for a given carbon number and

temperature, an at\'n1stic compound may have a lower vapor

pressure than th( ",~Sr<'nding aliphatic compound. It would

have lower volatill~

High molecu1a.- ~i~ht compounds are preferred from a

volatility standf'~": H..'I~'ever.he other characteristic of longer

chains, viscosi~, "'~ antoplay. Increasing the carbon chains

tends o increa.~

\

~cosi~' f the diluent. Higherviscosity

oils will requirt' ..").~.L~ po~'er.and higher shear rates to fonn

the emulsion. Th~ ,SrI result an ncreased phase disengagement

times and potentoS )..'rt'ase5n entrainment losses. Commercial

diluents will ha\-: 1 \~,,'Si~ between 2 to 5 cSt at 250 C.

It is impon~~ -;.' nNe that commercial diluents are not

.'kerosene." Kt:~.~ ~J'I,3\1~has a significantly lower distilla-

tion range than ",~i,tl diluents. The distillatiOn range of

diluents usual" :".'«Ii- the maximum limits for kerosene.

The flash poin: ~'\' ..t':\'S~e ~~ically. ranges between 125 and

1350F (51 to 5- ,,~'. \:l~ts "'111ypIcally exhibit a flash point

'It least 300 F t ::-.:-" ..~\ igher than the flash point of kerosene.

~hcse diffcrc~~ ,r- ,,"~uibute to a better public and regula-

Jry perception ~'\. ~,'\-:nt ~xD'actionoperations.

What do dlluents do?

A diluent's primary function is to adjust the extractantcon-

centration o the desiredoperating ange. The desiredoperating

range s determinedby the type of extractant, he metal content

of the pregnant each solution (PLS), and other site-specific

requirements.

The diluent's secondary ffects can significantly affect plant

operation. Secondary ffects nclude:

. Promoting apid transferkinetics

.

Optimizing phasedisengagement

. Enhancing electivity

.

Minimizing entrainment > both organic n aqueous nd

aqueousn organic

Transferkinetics s the rate at which the metal s loadedon or

eluted rom the organicphase.The diluent enhanceshis by con-

tributing to the formation of uniformly sized droplets n the dis-

continuousphase. The dropletsmust form rapidly in the mixer

without excessive shear and remain distributed throughout

the continuousphaseduring mixing and initial discharge o the

senler. Droplet size must be small enough o allow rapid metal

exchange ut largeenough o coalesce apidly and completely n

the senler.A viscousdiluent will requireadditionalshearor mix-

ing energy o form and distribute the droplets and will tend to

haveextended hasedisengagementime. Slow transferkinetics

leads o increased esidence ime in the mixer. Extendedshear

can ead o smallerdropletswhich, in turn, can ead o increased

entrainment.

COPPERHvQR::*- ~~~TA8lE '98

122



M.D. BtSHOP

Phase isengagementime is the time required for the aque-

ousand organic phases o separate.Phasebreak time is easily

measuredn a laboratorybatch cell and correlates o the disper-

sionband n the plant. Large dropletscoalesce asterbut require

longer imes for metal transfer. Small droplets ransfer metals

rapidly but may not coalesce apidly or completely.Optimized

phase isengagementontributes o maximizing plant flow rates

andproduction.

A slow phase break can lead to reduced production. This

would be a result of decreasedlow rates due to failure of the

phaseso disengagen the settler. Slow phasebreakscan result

in increased ntrainmentand ncrease n the associated osts.

Conversely, rapid phasebreak can contribute o increased

copperproduction.This is provided additional PLS is available

and he tank househas he capacity. Extremely rapid phasedis-

engagements not always beneficial as metal transfer s occur-

ring as he phases oalesce. This could result in reducedmetal

transfer. n somecases, xtremely ast phasebreakscan rend o

poor coalescence, otentially leading o higher entrainment.

Studieshave demonstratedhat the diluent composition will

affect copper iron selectivity in oxime based systems. An

alkylaromatic content of 10 to 30 volume percent appears o

enhanceCu: Fe selectivity. The optimum alkylaromatic con-

tent appears o be in the 22 to 25% range.6Reduced ron trans-

fer will result in reducedelectrolyte bleed stream volume and

reducedcobalt consumption provided iron is the controlling

factor in bleed stream volume.

Entrainment s a measure f incompletecoalescing f the dis-

continuous hase.Organic n aqueous ntrainment n the oading

stage s one of the major sourcesof organic oss. Organic exit-

ing the circuit in raffinate is normally recovered rom the raffi-

nate pond. Organic in the electrolyte is absorbed n the elec-

trolyte filters. Aqueous entrainmentcontributes o transfer of

undesirableons sucha CI- through he circuit.

Each of theseelements s also influenced by and interrelat-

ed with the other plant variables ncluding extractant ype and

concentration,pH, mixer design, circuit temperature,and PLS

quality.

immediatelyafter the ignition source s withdrawn. The prim-

ary difference between he methods s that the PMCC USES a

stirring mechanismwhile the TCC doesnot.

The COC method, as the name implies, does not incorpo-

rate a cover over the sample. The vapors are free to the atmos-

phere. The ignition source s periodically introduced into the

vapor at a prescribeddistanceabove he liquid. The sample s

not stirred. For diluents, the open cup flash point will typical-

ly be 20 - 25° F (11 - 14° C) higher than the closed cup value

for the samematerial.

The Setaflashmethod (ASTM D 3828) may be used as an

alternative o the TCC or PMCC methods.The Setaflashmakes

useof a smallersampleand utilizes a timed interval n approach-

ing the estimated lash point.

Flash point is not linear with composition and can not be

relied on to predict overall volatility. Since he closedcup flash

point apparatUSontains the vapors in the headspace nd the

lighter components re volatilized preferentially, he light com-

ponentswill have a disproportionateeffect on the flash point.

Blending a 1:1 mixture of a low flash material suchas hexane a

C6 hydrocarbon)with a high flash point material such as hexa-

decane CI6) would not result in a flash point half way between

the two products. It would actually be very close o that of pure

hexane. A relative small amount of a light componentcan

reduce he flash point by severaldegrees.

Barometric pressurehas a significant effect on flash point.

The lower the barometricpressure he lower the measuredlash

point. ASTM methodsspecify that the reported lash point be

corrected or variations n barometricpressure. ll values epon-

ed are corrected o standardpressure sea evel), 760 mm Hg.

Since barometricpressuredecreases y approximately2.5 mm

Hg for each30.5 meters n increased levation', we can predict

the flash point at various altitudes. The flash point at a given

altitude can be estimatedby decreasinghe reported lash point

(corrected o sea evel) by 0.228° C per 100 meters elevation

above sea level. For example, an operation at 4,000 meters

would expect o observea 9° C decreasen flash point (4,000 +

100x 0.228).

The distillation rangeand carbonnumberare directly related.

The higher he distillation range, he higher the average arbon

number.Table 2 provides typical distillation ranges or some

commondiluents.

FLASH POINT

The flash point is defined as "the minimum temperatureat

which a liquid gives off vapor n sufficient concenb'ationo form

an ignitable mixture with air near the surface of the liquid.'"

Thereare hree commonmethods or determining lash point:

.

PenskyMartin ClosedCup [PMCC] ASTM D 93

.

TagliabeuClosedCup [TCC] ASTM D 56

.

ClevelandOpen Cup [COC] ASTM D 92

All threemethods nvolve placing a sampleof the material o

be ested n a brasscup, heating he material n a controtledman-

ner and ate,and periodicatly ntroducingan gnition source nto

the vapor spaceabove he liquid. The flash point is the point at

which the vapors gnite when he ignition source s introduced.

80th the PMCC and TCC incorporatea cover over the sam-

ple to contain the vapors in a fixed heads ace. The ignition

source s introduced hrough a pon in the cover. The port is

opened ust before introducing the ignition source and closed

123

COPPER HVDROMET ROUNDTABLE 98



There are many sourcesof diluent loss. They include but are

not limited to the following.

.

Entrainment

.

Evaporation

.

Filter Loss

.

Crud Loss

.

Solubility

.

arryover to Electrolyte

Carbonnumberdisnibutions for SX II and SX 12 are shown

I. ComparingTable2 with Figure I shows he correla-

between the distillation range and the carbon number.

II hasan average arbonnumberof 14.7while SX 12 hasan

carbon number of 13.3. Although the averagecarbon

is only 1.4 units more han he average or SX

s viscosity s about 1.5 cSt higher at 25° C. Higher distilla-

are availablebut will exhibit higher viscosity as dis-

The primary areas where distillation range and flash point

costsare electrical areaclassificationand safety

The National Fire ProtectionAgency classifies lamma-

and combustible products based on their closed cup flash

The classificationsare:

Flashpoint < 100°F (37.8° C) ~ Class FlammableLiquid

Flashpoint at or> 100°F (37.8° C) and < 140°F (60.00C)

~ Class I CombustibleLiquid

lashpoint at or> 140°F (60.0° C) and < 2000F (93.3°C)

~ Class lIA CombustibleLiquid

Flashpoint at or > 200° F (93.3° C) ~ Class IIB

CombustibleLiquid'

The flash point of most commercialdiluents s such hat they

classified as Class IlIA combustible liquids. NFPA

497M states hat Class lIA combustible iquids do not

mixtures with air at ambient temperatures

above heir flash points. The vaporscool

on to state hat the

requiring electrical classification for Class IlIA liquids is

non-existent.'"

Each operation must conduct a site-specific evaluation to

reas.

However,considerationof theseguide-

proper classificationof the areamay result n reduced

equirements or explosion proof

equipment.

Entrainment

Organic entrainment s recognizedas one of the largesthid-

den costsof a diluent. Entrainment s the ncompletecoalescence

of the dispersedphaseprior to discharge rom the settler. Plant

organic s the exttactant diluent mixture. Although both organ-

ic in aqueousand aqueous n organic enttainment occur, only

organic enttainment n the aqueous hase esults n diluent loss.

(Aqueousentrainment n organiccontributes ts own setofprob-

lems.) Since organic entrainment esults n the loss of both the

low cost diluent as well as he higher cost exttactant ts costsare

significantly greater han the diluent componentalone. The cost

of the organic can be easily calculated.

Enttainment ossesare difficult to reliably evaluate n labora-

tory and pilot plant trials. This is due to the difficulties encoun-

tered n trying to duplicate plant shear ates, emperatures, nd

residenceimes. t is difficult to obtain representative amples n

any circuit due o the natureof the system. The entrainedorgan-

ic continues o coalesce eading o inhomogeneous amples. In

our opinion, enttainment osses houldbe evaluatedduring actu-

al plant operations.

Many factorscontribute o enttainment osses.PLS quality is

extremely important. The presenceand amount of polar com-

pounds n the PLS can significantly affect enttainment evels.

Sourcesof polar compounds nclude vegetation,old mine tim-

bers, oils and greases rom mining equipment,etc. High levels

of suspended olids are well known to contribute to increased

entrainment. We have seen evidence of microorganismscon-

tributing to entrainment.

Organic recovered rom the raffinate ponds often exhibits

slow phasebreak, high entrainment,and other signs of degra-

dation. These detrimental effects are diluted when the materi-

al is returned o the circuit but may still contribute to a small

decrease n performance. The recovered organic should be

tested and treated, if necessary,before being returned to the

circuit.

Entrainment ossesare minimized by standardplant practices

such as controlling suspended olids to the extent possible,and

keeping he circuit clean. The mixers should be operated t the

minimum speed equired o provide sufficient mixing to achieve

copper ransfer.Monitoring the organiccondition and reatingas

necessaryo removecontamination s beneficial.

Diluent solubility provides a measure of the minimum

entrainment oss. Solubility for Orfom& SX 7 and SX 12, two

widely used diluents, was determined o be 4.5 and 3.8 ppm

respectively.

ROUNDTABLE '98

124



M.D. BISHOP

Evaporation

Evaporative ossesare a concern o operations or both cost

and environmental easons.Factors affecting evaporative oss

include circuit temperature,altitude, and plant design. Higher

circuit temperatures ill tend to increaseevaporative oss.High

altitudeoperationsare expected o have higher evaporative oss-

esdue o reducedbarometricpressure.Plant design also affects

evaporative ossesas covered mixer-settlerswill have less air

movement ver the organic surface han uncovered ircuits.

It is well acceptedhat diluent is preferentially ost from plant

organic. The amount of diluent required o maintain the plant

organic'sextractant:diluent ratio is greater han the amount of

diluent required basedon the quantity of extractantadded or

make-up. Conventionalwisdom" has contributed his differen-

tialloss to evaporation. Logic suppons his concept.Sincedilu-

ents are lower molecular weight than extractants he diluent

would evaporatewhile the higher molecular weight extractant

would not evaporate.

Phillips Mining Chemicalshas nvestigatedevaporative oss

in an effon to quantify the contribution of evaporation o diluent

loss. We have adapted ASTM method D 972 StandardTest

Method or Evanoration ,oss ofLubricatin~ Greases;;;;dO:;l;.o

measured iluent evaporative osses.We have also measured

evaporationrom beakers.

Figure 2 presentsa summary of a long-term comparisonof

three diluents using data obtained from a simple beaker est.

In this test, approximately 300 grams of test diluent was dis-

pensednto a 600 ml beaker. The average nternal diameterof

the beakerwas determinedand the exposedsurfaceareacalcu-

lated. The beakerwas periodically weighed and he loss calcu-

latedas kg/m: of surfacearea. Using this method. t was deter-

mined hat he cumulative ossafter 60 days s approximately7.4

kg/m:. The average oss over this period is 0.123 kg/m2 day.

Restatinghe osses asedon volume and using a densityof 0.82

glml yields a cumulative loss of 9.02 Um1over the 60 day period

with an average loss of 0.150 Um1. day.

Incremental evaporative losses change with time as the

lighter components are preferentially lost from the liquid.

Overall, evaporative losses are relatively low. The cumulative

losses after one year range from 22 to 25 kglm1 for the three dilu-

ents. The average loss ranges from 0.062 to 0.069 kg/m1

.

ay.

Again, restating the results as volume results in losses of

0.076 to 0.084 Um1 a day. The difference between SX 7 and SX

12 provides some indication of the variability of the test as well

as some variation between lots.

Figure 3 presents the test data as percent loss. Referring back

to the distillation ranges shown in Table 2, we can see the corre-

lation between distillation range and evaporation rate. The IBP

for SX 12 is lower than C I and we observe a higher evaporation

rate for SX 12. The temperature differential at the 10% distilla-

tion point for the two products is much less and the cumulative

evaporative losses for the two products are merging. The 200/0

distillation point is higher for SX 12 and we observe the cumu-

lative evaporative loss for SX 12 is now lower than for Cl.

Evaporative losses are highly related to the distillation range of

the diluent. The entire distillation range should be considered

when evaluating evaporative losses.

It is apparent from the data that the evaporation rate

decreases over time. This is probably due to the lower mole-

cular weight components evaporating first. This data would

be representative of a circuit at the time of the initial fill. It

must also be recognized that this data does not account for the

presence of extractant.

SX circuits are dynamic and are operated for extended peri-

ods. The circuit organic will reach a quasi steady state compo-

sition dependent on the amount of organic lost through all

mechanisms and the corresponding make-up volumes.

Although it would be difficult to detern1ine the point in time

125

COPPER HVDROMET ROUNDTABLE '98



M.D. BISHOP

"""."",.",..,.".""",

This paperhas presented n overview of someof the physical

characteristics f solvent extraction diluents ncluding viscosity,

flash point, distillation range,and carbonnumber.The electrical

areaclassification or diluents s considered. The primary func-

tions of diluent are discussed. nformation egardingevaporation

losses nd possibleways o minimize those osses re presented.

REFERENCES

I. Morrison and Boyd, Organic Chemistry3rd Edition, 1973,

when a circuit has reached a steady state, the composition at

that time will give a more reliable indication of evaporative

loss and emissions.

Evaporation s related to the amount of exposed surface.

Evaporative ossescan be significantly overstated f the sample

depthdoesnot accurately eflect he depthof the organic ayer n

plant operations.A high area:volwne ratio leads o an increased

evaporationate. This factor becomes specially ignificant f the

resultsare calculated s percentageoss.Tests houldbe designed

to approximatehe depthof the organic ayer n the settler.

Evaporative osses an be minimized by following a few sim-

ple steps.

.

Locate he plant to minimize wind exposure

.

Cover he mixer settler

.

Promptly recover organic rom the raffmate pond

Think about a closed bottle containing a highly volatile sub-

stance. As long as the bottle is closed and the saturated apors

are not removed, here is no evaporative oss. Every time you

open the bottle and allow some of the vapor saturatedair to

escapesome material is lost. The same thing happens n SX

operations. Minimizing the air exchangeseduces he evapora-

tive loss. Diluent vaporsareheavyand end o condense ear he

surfaceof the organic. Obviously,wind will have ess mpact on

covered mixer-settlers. Considerationof site location with an

effon to minimize wind around he mixer-settlerswill help min-

imize evaporative oss.

Raffinatepondsare ypically uncoveredand have a largesur-

facearea. Organic on the raffmate pond s subject o solar heat-

ing accentuated y the dark color of the organic and o more air

exchanges.These actors ncreaseevaporative osses. Organic

on the raffmatepond is subject o more biological and chemical

degradation.Prompt recovery rom the raffinate pond will help

minimize his sourceof loss.

p.40.

2. Sax, N.I.; Lewis, R.J. Hawley's CondensedChemical

Dictionary II th Edition, 1987,p. 98.

3. Morrison and Boyd, Organic Chemistry3rd Edition, 1973,

p.40.

4. California Code of Regulations, Consumer Products,

ProposedRegulation Order, Regulation for Reducing Volatile

Organic Compound Emissions from Consumer Products,

Subchapter 8.5, Article 2. Consumer Products, 94508.

Defmitions, Item (a) (78).

5. CRC Handbook of Chemistry and Physics 59th Edition

p. D241 - D255.

6. Bishop, M.D., et ai, Technical DevelopmentsLeading o

Modem Solvent Extraction DiJuents, SME Annual Meeting,

1996,Preprint # 96-49.

7. NFPA 321, Basic Classification of Flammable and

CombustibleLiquids, 1991Edition, p. 321-5.

8. Dean, J.A. Lange's Handbook of Chemistry and Physics

13th Edition. 1985,p. 2-67.

9. NFPA 321, Basic Classification of Flammable and

CombustibleLiquids, 1991Edition, p. 321-5.

10. NFPA 497A, Classification of Class 1 Hazardous

(Classified) Locations for Electrical InstaJlations n Chemical

ProcessAreas, 1992Edition, p. 497A-7.

HVDROMETROUNDTABLE98

126

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