4 Cyanidation

8/12/2019 4 Cyanidation

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EXTRACTIVE METALLURGY OFEXTRACTIVE METALLURGY OF GOLDGOLD

44.. CyanidationCyanidation

Fathi HabashiFathi Habashi

Laval University, Quebec City, CanadaLaval University, Quebec City, Canada

[email protected]@arul.ulaval.ca

The word cyanide is derived from GreekThe word cyanide is derived from Greekcyanoscyanosmeaning blue, becausemeaning blue, becausehydrocyanic acid (blue acid) was obtainedhydrocyanic acid (blue acid) was obtainedfor the first time from Prussian bluefor the first time from Prussian bluepigment when heated with sulfuric acid.pigment when heated with sulfuric acid.

The dissolving action of cyanide solutionThe dissolving action of cyanide solution

on metallic gold was known as early ason metallic gold was known as early as1783 by the Swedish chemist Carl Wilhelm1783 by the Swedish chemist Carl WilhelmScheele (1742Scheele (17421786).1786).


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FranzFranz ElsnerElsner (1802(1802--1874) in Germany in1874) in Germany in

1846 studied this reaction and noted that1846 studied this reaction and noted that

atmospheric oxygen played an importantatmospheric oxygen played an important

role during dissolution.role during dissolution.

John Stewart MacArthurJohn Stewart MacArthur(1856(18561920)1920)

The application of thisThe application of thisknowledge to extract goldknowledge to extract goldfrom its ores wasfrom its ores waspatented in 1887 inpatented in 1887 inEngland by MacArthur.England by MacArthur.

German chemist GuidoGerman chemist GuidoBodlBodlndernder (1855(1855--1904) in1904) in

18961896 foundfound outout thatthathydrogenhydrogen peroxideperoxide isisformedformed duringduring thethereactionreaction..


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Its impact on hydrometallurgy had beenIts impact on hydrometallurgy had beentremendoustremendous

-- Extremely large reactors known asExtremely large reactors known as DorrDorragitatorsagitatorsin which the finely ground ore wasin which the finely ground ore wasagitated with the cyanide leaching agent andagitated with the cyanide leaching agent andequipped with compressed air injection in theequipped with compressed air injection in thepulp have been designed and built by thepulp have been designed and built by themetallurgical engineer John Dorr.metallurgical engineer John Dorr.

-- Huge filtration plants designed to obtain clearHuge filtration plants designed to obtain clearleach solutions for metals recovery wereleach solutions for metals recovery weresimilarly constructed.similarly constructed.

The ancient process of cementation whichThe ancient process of cementation which

was applied for precipitating copper fromwas applied for precipitating copper from

solution by scrap iron was applied to goldsolution by scrap iron was applied to gold

solutions, iron being replaced by zinc.solutions, iron being replaced by zinc.

In spite of all these advances inIn spite of all these advances in

engineering and the wide application ofengineering and the wide application of

the process, the theory still remainedthe process, the theory still remained

lagging behind.lagging behind.


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The dissolution of gold and silver in cyanideThe dissolution of gold and silver in cyanidesolutions was one of the puzzles that facedsolutions was one of the puzzles that facedmetallurgists for many years for two reasons:metallurgists for many years for two reasons:

-- Gold, the most noble metal that dissolved onlyGold, the most noble metal that dissolved onlyin aquain aqua regiaregia, dissolved readily in a very dilute, dissolved readily in a very dilutesolution of sodium cyanide.solution of sodium cyanide.

--Although gold did not tarnish in air, air wasAlthough gold did not tarnish in air, air wasessential for its dissolution.essential for its dissolution.

[[

CyanidationCyanidation is similar to corrosionis similar to corrosion

The puzzle was solved when it was recognizedin 1947 that the dissolution of gold is similarto a corrosion phenomenon.

It is an oxidationreduction process in whichcyanide ion forms a strong complex with Au+

ion and the reduction of oxygen on the surface

of metal may take place partially to thehydrogen peroxide stage or completely tohydroxyl ion


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Demonstration the electrochemical nature of

dissolution of a gold particle in cyanide solution,

1947

Oxidation: Au Au+ + e

Complex formation: Au+ + 2CN [Au(CN)2]

Reduction: O2 + 2H2O + 2e H2O2 + 2OH

O2 + 2H2O + 4e 4OH

Overall reactions:

2Au + 4CN+ O2 + 2H2O

2[Au(CN)2]

+ H2O2 + 2OH

4Au + 8CN+ O2 + 2H2O 4[Au(CN)2]+ 4OH


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The electrochemical nature of this process hasThe electrochemical nature of this process has

been demonstrated by embedding a small goldbeen demonstrated by embedding a small gold

sphere in a KCN gel to which air was introducedsphere in a KCN gel to which air was introduced

from one direction.from one direction.

It was found that the gold corroded at theIt was found that the gold corroded at the

surface far away from the air flow, i.e., ansurface far away from the air flow, i.e., an

oxygen concentration cell was formed aroundoxygen concentration cell was formed around

the sphere: The surface less exposed to oxygenthe sphere: The surface less exposed to oxygen

acted as anode while the surface in directacted as anode while the surface in directcontact with oxygen acted as cathode.contact with oxygen acted as cathode.

Stoichiometry of cyanidation:A) Consumption of cyanide

Amount of gold

dissolved

Observedconsumption of

cyanideTime

[hours]

mgequivalent

(x 103

)mg

moles

(x 103

)

Molar ratio

[KCN/Au]

0.5

1.0

3.0

7.010.0

0.96

2.01

7.49

15.9118.77

4.86

10.2

38.0

80.995.1

0.62

1.26

4.94

10.5012.46

9.52

19.4

76.0

162.0192.0

1.96

1.90

2.00

2.012.01

Average 2.0


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Stoichiometry of cyanidation:B) Consumption of oxygen

Time

minutes

Ag dissolved

equivalent (x 105

)

O2consumed

moles (x 105

)[Ag]/[O2]

10

20

30

40

120

2.96

7.76

9.36

12.42

36.06

1.45

3.07

4.17

5.40

16.00

2.05

2.50

2.25

2.30

2.25

Average 2.2

Stoichiometry of cyanidation:

C) Formation of hydrogen peroxide

Metal dissolved H2O2formed

mgequivalent

(x 105

)mg

moles

(x 105

)

Metal

equiv./moles

H2O2

Gold 57.3

47.6

29.0

24.1

5.11

4.02

15.0

11.8

1.93

2.04

Silver 2.96

7.76

9.36

12.4236.06

1.43

3.09

4.00

5.5114.76

2.06

2.50

2.24

2.262.44

Average 2.3


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For every 1 equivalent of metal dissolved, 2

moles of cyanide are consumed.

For nearly every 2 equivalents of metal dissolved,

1 mole of oxygen is consumed.

For nearly every 2 equivalents of metal dissolved,

1 mole of H2O2 is produced.

According to this stoichiometry, 1 gram mole of oxygen and 4 grammoles of NaCN should be present in solution.

At room temperature and at atmospheric pressure, 8.2 mg O2 aredissolved in 1 liter of water. This corresponds to 0.27 x 103 mol/L.

Accordingly, gold dissolution should occur at a concentration ofNaCN equal to 4 x 0.27 x 103 x 49 = 0.05 g/L or 0.005%.

Thus a very dilute sodium cyanide solution would be enough fordissolving gold.

The fact that oxygen was necessary for dissolution was not readilyrecognized because as seen from the above calculations, oxygen insolution as a result of air solubility is enough to bring about thereaction.


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Effect of cyanide concentration

The rate of golddissolution increaseslinearly with increasingcyanide concentrationuntil a maximum isreached, beyond which afurther increase incyanide had no effect.

This was contrary tocommon experience sincethe rate of dissolution of ametal, for example, iron in

an acid increased withincreasing acidconcentration.

The explanation of this phenomenon will be

dealt with later.


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Effect of hydrogen ion concentration

It is essential that the cyanide solution be kept alkalineduring leaching to prevent the formation of HCN which isa poisonous gas. Hydrogen cyanide may form as a resultof absorption of atmospheric CO2:

CO2 + H2O H2CO3H2CO3 + CN

HCN + HCO3

High alkalinity, however, decreases the rate ofdissolution.

The natural pH of a 0.1% KCN solution is 10.5 due tohydrolysis:

CN+ H2O HCN + OH


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The alkalinity of cyanide solutions should,

therefore, be carefully controlled to achieve

high dissolution rates. In practice, the pH

usually ranges from 11 to 12.


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Solutions kept alkaline by Ca(OH)2, when

compared to others at the same pH kept alkaline

with KOH, show a remarkable retarding effect in

the case of lime.


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Effect of agitation

The rate of dissolution increases with

increased agitation,

Effect of foreign ions

Most gold occurs as native metal, nearly all alloyed with variousamounts of silver.

Certain minerals are characteristically associated with gold, andthe most important are pyrite, galena, zinc blend, arsenopyrite,stibnite, pyrrhotite, and chalcopyrite.

Various selenium minerals and magnetite may also be present.

In Witwatersrand, South Africa, uraninite, and to a lesser extent,thucholite are associated with the gold ore; uranium is recoveredas a by-product of gold milling.

Carbonaceous matter is associated with some gold ores. The mostcommon gangue minerals are quartz, feldspar, micas, garnet, and

calcite. Although the gangue minerals are insoluble in cyanide solution,

some metallic minerals are soluble to some extent.

Carbonaceous matter in gold ore is a source of trouble, because itadsorbs gold cyanide complex.


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With the exception of a few ions such as Na+, K+, Cl,NO3

- and SO42- which have no effect on the rate of

dissolution of gold and silver in cyanide solution, ionsmay have an accelerating or a retarding effect.

Lead(II) ions may have either an accelerating or aretarding effect, depending on their concentration insolution.

The study of the effect of these foreign ions oncyanidation is complicated because the dissolutionprocess is composed of two simultaneous reactions: The

oxidation reaction involving the formation of the auro- orthe argento-cyanide ion and the reduction of oxygen asmentioned above.

Which of these reactions is affected by the foreign

ion is usually difficult to say, and many

contradictory statements are found in the

literature.

Data available were mainly obtained by studying

the effect of foreign ion in the gold leaf test.


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An approach to this problem is to study the effectof the foreign ion on the cathodic and the anodicreactions separately. This can be achieved bystudying the change in potential-current densitycharacteristics of the gold (or silver) electrode intwo separate experiments:

Gold electrode + [Au(CN)2]+ CN+ foreign ion,

oxygen being excluded from the system.

Gold electrode + O2 + H2O + foreign ion, in theabsence of cyanide ion.

In the first experiment, the effect of the foreign ion on thereduction of oxygen on the gold surface can be studied,and in the second experiment, the effect of the same ionon the anodic reaction of gold dissolution in cyanide.

The results of such tests should be interpreted with care,however, as the application of external emf to causedissolution does not correspond to actual cyanidationpractice.

Thus, for example, under these conditions oxygen isreduced to OHand not to H2O2.

Another approach involves the use of radioactiveindicators of the ions being studied


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Accelerating effect

It has been reported that small amounts of lead, bismuth, thallium, andmercuric salts accelerate the dissolution. This was confirmed later bymany researchers.

From calculations of electrode potentials in cyanide solutions, it wasconcluded that gold metal can actually displace the ions of only thesefour metals.

It was, therefore, suggested that the rapid dissolution of gold in thepresence of these ions might be due to alteration in the surfacecharacter of gold by alloying with the displaced metals.

Early researchers detected the presence of lead on the surface of goldwhen the cyanide solution contained Pb2+ ions, but did not identify its

nature.

It was recently confirmed

by electron microscopic

studies that lead alloys

with gold on the surface.

The alloy then dissolves

rapidly, hence lead acts as

a catalyst.


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The presence of metallic cations such as Fe2+, Cu2+, Zn2+, Ni2+,Mn2+, Ca2+, and Ba2+ (the latter two only at high alkalinity) has aretarding effect.

Lead(II) plays a unique role in cyanidation, and there has beenmuch confusion regarding its effect on the rate of dissolution ofgold.

Besides the accelerating action already described, some reportsdescribe a retarding effect.

It can be concluded from those studies that when Pb2+ is present invery small amounts as compared to the CN ion, an acceleratingeffect is observed, whereas when the [Pb2+]/[CN] ratio exceeds a

certain value there is a retarding effect.

It is also known that the sulfide ion and

certain flotation agents such as xanthate ion,

which is sometimes used to concentrate the

sulfides with which gold is associated have a

retarding effect on the rate.


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Retarding effect may be due to one or more

of the following reasons:

Consumption of oxygen from solution

Consumption of free cyanide from solution

Film formation on the surface of the metal

Consumption of oxygen from solution

Because oxygen is necessary for gold dissolution,

any side reactions in which the cyanide solution is

deprived of its oxygen content will lead to a

decrease in the rate.

Pyrrhotite accompanying gold in some ores

decomposes in alkaline medium forming ferrous

hydroxide and sulfide ion:FeS + 2OH Fe(OH)2 + S

2


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In presence of oxygen, ferrous hydroxide is easilyoxidized to ferric hydroxide:

2Fe(OH)2 +1/2O2 + H2O 2Fe(OH)3

while sulfide ion is partly oxidized to thiosulfateand polysulfide:

2S2+ 2O2 + H2O S2O32+ 2OH

S2+ H2O HS+ OH

2HS+ 1/2O2 S22+ H2O

thus contributing to the depletion of oxygen fromsolution.

Consumption of free cyanide from solution

Formation of complex cyanides: Some copper, zinc, and ironminerals that may be associated with gold ore dissolve in cyanidesolution and therefore deplete the solution of its cyanide content,e.g.:

ZnS + 4CN [Zn(CN)4]2 + S2

Formation of thiocyanate: Sulfide ion liberated, when the orecontains sulfide minerals, reacts with cyanide and oxygen to formthiocyanate ion, which has no action on gold:

S2 + CN + 1/2O2 + H2O CNS + 2OH

Adsorption on gangue material: Auriferous ores and concentratesmay contain quartz, aluminosilicates, or other silicates which, iffinely divided in an aqueous alkaline medium, form colloidal silicaand alumina; if iron sulfides are present in the ore, ferrichydroxide is also formed. These gangue materials have a strongadsorptive capacity for potassium cyanide.


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Film formation on the surface of the

metal

Sulfides: The retarding effect of the sulfide ion in the cyanide solution is well-known. As little as 0.5 ppm of the sulfide ion retards the dissolution. This cannot beaccounted for by the depletion of the solution of its cyanide or oxygen contents, asthe leaching solution usually contains excess oxygen and excess cyanide. It isbelieved that an insoluble aurous sulfide film is formed on the gold, which protectsit from dissolution.

The effect of sulfide ion on the electrode potential of gold in KCN solution in theabsence of oxygen is negligible, while its effect on the electrode potential of gold inthe absence of KCN but in the presence of oxygen is great. It appears that the tracesulfide poisons the gold surface toward the cathodic reduction of oxygen but doesnot affect the anodic reaction.

Peroxides: Calcium ion has no effect on gold dissolution. At pH > 11.5, however, therate of dissolution is greatly reduced. At the same pH, solutions kept alkaline byCa(OH)2 show a remarkable decrease in the rate of gold and silver dissolutionwhen compared with others kept alkaline with KOH. It was suggested that thedecrease may be due to the formation of calcium peroxide on the metal surface,which prevents the reaction with cyanide. Calcium peroxide was thought to beformed by the reaction of lime with H2O2 accumulating in solution according to:

Ca(OH)2 + H2O2 CaO2 + 2H2O

It was possible by chemical analysis to identify the precipitate formed ascalcium peroxide. This was confirmed later by means of X-ray diffractionanalysis; Ba(OH)2 behaves similarly. Lime is one of the reagentscommonly used in cyanide mills to adjust the pH of the pulp and to helpsettling. Its use must therefore be carefully considered.

Oxides: Ozone when added to cyanide solution decreased the rate ofdissolution of gold. Apparently a layer of gold oxide, which caused a visualchange of the gold to brick red, produced the retarding effect. However,potassium cyanide is also oxidized by ozone according to:

3KCN + O3 3KCNO The rate of dissolution decreases with the addition of as little as 0.4 ppm of

potassium ethyl xanthate. When flotation is used to concentrate the

sulfides with which gold is associated, prior to cyanidation, difficulties arefrequently faced. The gold surface was reported to turn reddish. A goldxanthate film was probably formed. This was confirmed by usingpotassium xanthate marked with S35.


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Gold leaf test

The accumulation of impurities in solution retards

dissolution. The effectiveness of the cyanide

solution is tested frequently by measuring the

time required for a gold leaf to dissolve under

prescribed conditions of shaking.

The faster the gold leaf dissolves, the more

effective is the cyanide solution. One way of

minimizing such difficulties is by adding a leadsalt such as lead oxide, nitrate, or acetate.

This precipitates the sulfide ion as soon as it is

formed in the form of insoluble lead sulfide. The

addition of small amounts of potassium

permanganate also eliminates this difficulty by

oxidizing the sulfide ion to sulfate.

Another way is to agitate the ore pulp in an

alkaline medium using Ca(OH)2 to decomposesulfide minerals, and form harmless precipitates:


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FeS + 2OH Fe(OH)2 + S2

2Fe(OH)2 +1/2O2 + H2O 2Fe(OH)3

S2+ 2O2 SO42

SO42+ Ca2+ CaSO4

The sulfide-free pulp is then subjected to

cyanidation.

Recent advances


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Thiourea process

The solubilizing action of gold and silver in thiourea maybe represented by the oxidationreduction couple asfollows:

Au Au+ + e

Au+ + 2(NH2)2CS[(H2N)2CSAuSC(NH2)2]

+

Fe3+ + e Fe2+

This takes place in acid medium, is faster thancyanidation, and less toxic.

However, thiourea undergoes appreciable decompositionduring leaching

Heap leaching

The increased price of gold in the 1970s madepossible the leaching of old tailings containing aslittle as 1 ppm gold thanks to the newly developedheap leaching technology that was developed forthe leaching of low grade copper and uranium ores.

To improve the percolation of the leach solution inthe bed, the fine tailings were sometimesagglomerated by slurrying with water and a smallamount of cement which was allowed to set.

The heaps were then constructed from the hardagglomerates and leached with the alkaline cyanidesolution.


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Treatment of refractory ores

A problem in gold metallurgy was the treatment of gold locked upin pyrite or arsenopyrite crystals and therefore unresponsive tocyanidation.

Roasting followed by leaching is an expensive and pollutingproposition.

A hydrometallurgical approach proved to be a successful solutionfor this type of ores.

Barrick Goldstrike now treats a pyrite orewater slurry inautoclaves at high temperature and oxygen pressure. Horizontalautoclaves are used, each being 30 m long and 5 m in diameter,

operating at 160180 C and 2000 kPa, with a retention time of 20minutes (Figures and ). The autoclaves are made of carbon steel 8cm thick, lined with as 6 mm lead membrane and two layers ofacid-resisting brick 22.5 cm total thickness.

After this treatment, the ore is then suitable for cyanidation.


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Pressure leaching operation for treating refractory

gold ores prior to cyanidation at Elko, Nevada

Inside Barrick

Goldstrike plant


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Start up Plant location Owner Feed Medium Capacity, t/d

Number

of

autoclaves

1985 McLaughlin,

USA

Homestake,

USA

ore acid 2700 3

1986 San Bento,

Brazil

Genmin,

South Africa

concentrate acid 240 2

1988 Mercur,

Utah, USA

American Barrick,

Canada

ore alkaline 680 1

1989 Getchell, USA First Miss Gold ore acid 2730 3

1990 Goldstrike,

Nevada, USA

American Barrick,

Canada

ore acid 1360 1

1991 Goldstrike,

Nevada, USA

American Barrick,

Canada

ore acid 5450 3

1991 Pargera, Papua

New Guinea

Placer Dome,

Canada


1991 Campbell,

Canada

Placer Dome,

Canada


1992 Con, Lihir Nerco Minerals concentrate 90 1

1993 Goldstrike, USA American Barrick,

Canada

ore acid 11580 6

1994 Pargera,

PapuaNew Guinea

Placer Dome,

Canada


Understudy

Lihir, PapuaNewGuinea

Gold ores containing carbonaceous

material

Gold ores containing carbonaceous material are also known asrefractory ores and are difficult to treat, not only because part ofthe gold is tied up with the organic matter but also becausedissolved gold is adsorbed on the carbon present in the ore andtherefore reports in the taillings.

This problem was solved by aqueous oxidation using chlorine.Some of the gold may be solubilized by the chlorine water but themajor function of the controlled chlorination is to oxidize organicmatter before cyanidation.

A plant at Carlin, Nevada, uses this technology. Another solution to this problem was found by using the carbon-in-

leach process


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Cyanidation under pressure

Cyanidation under high oxygen pressure has been usedcommercially at the Consolidated Murchison Mine near Gravellotein South Africa.

The process has been developed by Lurgi in Germany using tubeautoclaves 1.5 km long and 5 cm inner diameter.

Leaching is conducted at ambient temperature but at about 5 MPaoxygen pressure.

As a result, residence time is only 15 minutes at 85% recovery. Itshould be noted that at high oxygen pressure, a high cyanideconcentration must be used to achieve high reaction rates.

In practice the leach solution is 0.2% to 0.5% NaCN. Although

cyanide solutions are susceptible to oxidation, the short residencetime renders this drawback negligible.

MECHANISM OF CYANIDATION

Cyanidation reaction is a heterogeneous processinvolving a solid [the ore], a liquid [the aqueouscyanide solution], and a gas [air or oxygen].

A homogeneous process takes place in one phaseonly; it can be in a gas phase for example, thereaction of hydrogen and chlorine to form HCl, or aliquid phase for example, the neutralization of anacid by a base.

Heterogeneous reactions take place between morethan one phase; the surface separating the phasesis called interface at which the reacting species haveto be transferred.


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The interface

The interface is the boundary between twophases.

For example, in a solidgas reaction theinterface is the outside surface of the solid incontact with the gas.

For two immiscible liquids, the interface is thesurface of contact between the two liquids.

For a solid-liquid reaction the interface is thesurface of the solid.

Lattice defects, non-stoichiometry, and

impurities at the interface influence the rates

of chemical reactions.

When a product layer is formed on the

reacting solid the kinetics of these reactions

will evidentally be governed by the character

of this coating whether it is porous ornonporous.


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Only two phases are actually taking part in anyheterogeneous process, although more may bepresent in the reaction mixture.

The reason is that the overall rate of reaction isdetermined by the rate of a single step the rate-determining step, which is slower than all others.

For example, the dissolution of gold (solid) insodium cyanide solution (liquid) in presence ofoxygen (gas), the process is reduced to a solidliquid reaction since oxygen can be transferred from

the gas phase to the liquid phase at a faster ratethan the other reactions taking place.

The boundary layer

A solid in contact with a liquid is covered by a stagnant

film of liquid about 0.03 mm in thickness called the

Nernst boundary layer.

The existence of this layer is manifested when a liquid is

flowing in a pipe in a streamline flow, the velocity of the

liquid will be maximum at the center and gradually

decreases to zero at the inside walls.

This concept was first applied to explain the dissolution

of solids and was later extended to other heterogeneous

reactions such as solidgas and liquidliquid reactions.


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When a solid C is agitated in water and the

solution is analyzed at intervals, it will be found

that the rate of increase of the solute

concentration follows the equation

where [C] is the concentration of the solute at time t, [C]s is its solubility in

water at the experimental temperature, i.e., its concentration at saturation,

and kis the velocity constant.

C dtd

----------- k s C =


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C d s C -------------------------

0

C

k tdt

=

C s C lnd

0

C

k tdt

=

2.303C s

C s C -------------------------log kt=

The plot of log [C]s/([C]s [C]) against tgives a

straight line.

It was therefore suggested that a saturated layer is

rapidly formed at the interface and that the

observed velocity is the rate at which the solvate

molecules diffuse through this layer into the bulk

of the solution.


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Diffusion

Diffusion is the tendency of any substance to

spread uniformly throughout the space available

to it.

It is a result of molecular movement; no other

outside force is influencing the mixing action.

It is exhibited by gases, liquids, and solids, but it

is most rapid in gases, and most slow in solids.

Diffusion in gases, liquids, and solids in governedby Ficks law.

Ficks law

td

dnDA

Xd

dC= A

dx

C1

>


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Where dn is the quantity of a dissolved substance whichdiffuses in time dt through the cross-sectional area A,from the direction of high concentration to that of lowconcentration, and is proportional to the concentrationgradient in the direction of diffusion dC/dXand the crosssectionA.

D is the diffusion coefficient of the substance anddefines the quantity (grams or moles) diffusing in unittime through a cross section of 1 cm2, when theconcentration gradient is unity.

The concentration must be expressed in the same units.The diffusion coefficient is usually expressed in cm2sec1.

Effect of temperature on the

diffusion

The effect of temperature on

the diffusion in aqueous

solution follows the Stokes

Einstein equation.

Where R is the gas

constant, N is Avogadros

number, the viscosity of

the medium, and r is the

radius of the diffusingmolecule (supposedly in

the form of a sphere).

D

RT

6rN-----------------=


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Thus, the rate constant kcan be identified by A/dV. This

equation explained the following observations:

- The rate of dissolution of a solid in water increased with

increased agitation because the thickness of the boundary

layer decreased.

- The rate of dissolution was not influenced by increased

temperature; an activation energy < 5 kcal/mole was

usually observed. This is in agreement with a physical

process such as diffusion.

Types of heterogeneous reactions

It was first thought that all dissolution processes

were controlled by diffusion.

But, when many processes were studied and it was

found that in some cases the rates were

independent of stirring and the activation energy

was higher than 5 kcal/mole, it was concluded that,

only physical processes are diffusion-controlled, but

chemical and electrochemical reactions may bediffusion-controlled, chemically controlled, or

intermediate-controlled reactions.


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Physical processes

The dissolution of an ionic solid such as NaCl

in water is a typical example.

A crystal of the salt immersed in a certain

volume of water will be immediately covered

by a thin layer of a saturated solution of NaCl.

The ions will then diffuse spontaneously in

water following Ficks law until the whole

volume of water becomes a saturatedsolution.

Chemical processes

Consider an ionic or acovalent solid in form of aplate of surface area A incontact with an aqueoussolution containing areagent R that reacts withthe species C dissolvingfrom the solid.

At the interface, the

concentration of thesolute will be that of asaturated solution, [C]s,and in the bulk of solutionwill equal [C].


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Three cases may be considered:

When the rate of reaction of the reagent with the dissolvingspecies in the bulk of the solution is very fast, then theconcentration of C will equal zero and the process will becontrolled by the rate of diffusion of C through the boundarylayer.

When the rate of reaction of the reagent with the dissolving speciesin the bulk of the solution is very slow, then the diffusion does notplay any role, and there will be accumulation of C in solution, i.e.,[C] [C]s. The rate of reaction will, therefore, depend on theconcentration of the reagent R since [C]s is constant, i.e., theprocess is chemically controlled.

When the rate of reaction of the reagent with the dissolvingspecies in the bulk of the solution equals that of the rate of

diffusion, then the process is known as intermediatecontrol.

Electrochemical processes

This is the case of a metal or a semiconducting

solid which reacts through an oxidationreduction

process, i.e., transfer of electrons.

The reagent in solution of concentration [D]

diffuses through the boundary layer to pick up

electrons from the interface.

The symbol D stands for depolarizer, i.e., an

oxidant.


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V1 = k1A1[D] and V2 = k2A2[C]

where V1 and V2 are the velocity of the cathodic

and the anodic reactions, respectively.

At the steady state, the rate of the cathodic

reaction equals that of the anodic reaction, i.e.:

k1A1[D] = k2A2[C]

A1

2

------k2 C

1 D ---------------=


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1

1 2+------------------

k2 C

1 D 2 C +------------------------------------=

A2

1 2+

------------------k1 D

1 D 2 C +

------------------------------------=

Therefore:

A1

k2 C

1 D 2 C +------------------------------------=

A2

k1 D

1 D 2 C +------------------------------------=


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where A is the total surface area of the solid in

contact with the solution:

A =A1 +A2

Substituting the value of A1 (or A2) in any of the

rate equations, gives the following:

Rate of dissolution

k1 2A D C

1 D 2 C +------------------------------------=

[D] = the concentration of the depolarizer

[C] = the concentration of the complexing agent A1 = thesurface area of the cathodic zone

A2 = the surface area of the anodic zone

k1 and k2 are the velocity constants at the cathodic and theanodic zones, respectively, then:

V1 = k1A1[D] and V2 = k2A2[C]

where V1 and V2 are the velocity of the cathodic and the

anodic reactions, respectively.At the steady state, the rate of the cathodic reaction equalsthat of the anodic reaction, i.e.:


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k1A

1[D] =

2 2[C]

A1

2------

k2 C

1 D ---------------= A1

1 2+------------------

k2 C

1 D 2 C +------------------------------------=

2

1 2+------------------

1 D

1 D 2 C +------------------------------------=

Therefore:

A1

k2 C

1 D 2 C +------------------------------------=

2

1 D

1 D 2 C +------------------------------------=

whereA is the total surface area of the solid in contact withthe solution:

A =A1 +A2 Substituting the value of A1 (or A2) in any of the rate

equations, gives the following:

Rate of dissolution

k1 2A D C

1 D 2 C +------------------------------------=


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This equation has the following characteristics:

At low concentration of C, the second term in the denominator may beneglected in comparison with the first, and the rate equation simplifies to:Rate = k2A[C]

i.e., the rate of dissolution in this case is only a function of the hydrogen ionor the complexing agent concentration.

At high concentration of C, the first term in the denominator may beneglected in comparison with the second, and the velocity equation simplifiesto

Rate = k1A[D]

i.e., the rate of dissolution under these conditions depends only on theconcentration of the depolarizer.

When the first and second terms in the denominator are of equal magnitude,i.e., when

k1[D] = k2[C]

then the rate of dissolution reaches its limiting value, i.e., when the rate curve

changes its direction. This change takes place at a certain ratio of [C]/[D], ascan be deduced from the equation:

C D

---------k1

2

----- Constant= =

The process was found to be strongly dependent on the speed of agitation,

and the activation energy to be < 5 kcal/mole which are the characteristics

of a diffusion-controlled process.

Cathodic reaction: O2+ 2H

2O + 2e

OH

+ H

2O

2

Anodic reaction: Au Au++ e

Au++ 2CN

[Au(CN)2]

Overall reaction:

2Au + 4CN+ O

2+ 2H

2O2[Au(CN)

2]

+ 2OH

+ H

2O

2


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According to Ficks law,

O2 d

td--------------

DO2

----------A1 O2 O2 i =

CN

dtd

-------------------D

CN

-------------A2 CN

CN i =

Since the process is diffusion-controlled,

[O2]i = 0 and [CN]i = 0

O2 d

td--------------

DO2

----------A1 O2 =

CN d

td-------------------

DCN

-------------A2 CN =


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Since the amount of metal dissolved is twice that of oxygen consumed

and half that of cyanide consumed as can be seen from the overall

equation, therefore:

Rate of dissolution 2O2 d

td--------------

DO2

----------A1 O2 = =

12---

CN d

td-------------------= ---

DCN

-------------A2 CN

=

It follows from these equations that, at the steady state:

2DO2

----------A1 O2

12---D

CN

------------- 2 CN

=

But, since A, the total surface area of metal incontact with the aqueous phase =A1 +A2, therefore:

Rate2AD

CNDO2

CN

O2

D

CN CN

4D

O2 O2 +

-----------------------------------------------------------------------=


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At low cyanide concentration, the first term in

the denominator may be neglected in

comparison with the second, so that the

equation simplifies to:

Rate12---

ADCN

----------------- CN

=

= k1[CN

]

At high cyanide concentration, the second

term in the denominator may be neglected in

comparison with the first, and the equation

simplifies to:

Rate 2ADO2

-------------- O2 =

= k

2[O

2]


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When DCN

CN 4DO2 O2 =

CN

O2

---------------- 4DO2

DCN

-------------=

the rate of dissolution reaches its limiting value.


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4.22 109

2 1 1.835

10 2.765

10 516

10 0.276

10

1.835

10 516

10 4 2.765

10 0.276

10 + ----------------------------------------------------------------------------------------------------------------------------------=

13922

10

93 11

10 3 11

10+ --------------------------------------------------------=

= 3.4 10

3cm

T NKS

4 Cyanidation

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