introduction to hydrometallurgy_ui_lecture notes

Section

4

Introduction to Hydrometallurgy

Specific Objectives

To be able to discuss and answer questions on the following:

Typical hydro processing steps (with examples):

Pretreatment

Leaching reactions and techniques

Solution purification and concentration

Metal recovery

Electrorefining

Leach regeneration and reagent recovery

Practical considerations in selecting a hydro process.

Reference

Rosenqvist T, 1983. Principles of Extractive Metallurgy, 2nd. Edition, 506 pages

(McGraw-Hill: New York), ISBN 0 07 053910 3.

Introduction to Hydrometallurgy v2013 1

PROCESSING STEPS IN HYDROMETALLURGY

Most hydrometallurgical processes can be divided into five main steps as shown in the

following diagram, followed by residue treatment and possibly byproduct recovery.

The important steps will be discussed in more detail.

Leaching

Solid/Liquid Separation

Leach Regeneration /

Reagent Recovery

Solution Purification/

Concentration

Pretreatment

Residue Treatment Tailings Disposal

Ore

Metal or Product

Recovery

Product

Makeup Reagent

Pure / Concentrated Solution

Impurities

Additives for Purification

Liquor Recyle

Figure 1. Typical Hydrometallurgical Processing Steps

1. Pretreatment

Pretreatment steps can be either (1) physical processes such as grinding,

concentration (e.g. flotation) or agglomeration or (2) chemical processes such as

roasting.

1.1 Physical Processes

1.1.1 Concentration

Hydro processes are normally most suitable for ores which cannot be concentrated.

However two commercial examples where concentration is used are in the

Roast/Leach/Electrowin process for zinc production, which involves the production of

a ZnS concentrate by flotation as the first step, and also the CLEAR process for

copper which uses flotation to produce a copper sulfide concentrate as the first step.

1.1.2 Agglomeration

In the leaching of low grade ores, the reagent is often percolated through heaps (heap

leaching). If fines, clays or slimes are present these can block channels and stop

reagent flow. The process can be greatly improved by agglomeration of fines into

porous pellets or balls by mixing with cheap binders such as lime or cement.


1.1.3 Grinding

Grinding of ores liberates the valuable mineral and makes it more accessible to the

leach solution. However, size reduction (especially fine grinding) is expensive and

becomes prohibitively so as the grade of the ore decreases.

1.2 Chemical Alteration

This can either be by gas/solid reaction (roasting) or by liquid/solid reaction.

1.2.1 Roasting

Some roasting processes have been considered in the introduction to pyrometallurgy

lecture. Some of the more important commercial applications are summarised here:

Roasting of ZnS: Air is used at 600 oC to produce ZnO, which is easier to leach than

ZnS, for subsequent leaching in sulfuric acid:

2ZnS + 3O2 2ZnO + 2SO2(g)

Roasting of Auriferous Pyrite: In many major deposits, gold is associated with iron sulfides such as pyrite (FeS2), from which it is often difficult to leach. Roasting in air at

600 oC results in an insoluble porous iron oxide calcine from which gold can be readily

leached using cyanide solution.

Au/FeS2 + O2 Au + Fe2O3 + SO2(g)

Roasting of Nickel Laterites: Nickel laterites contain around 1.5% Ni (actually as

complex oxides such as (Fe,Ni)O(OH) but unlike sulfide ores cannot be upgraded by

mineral processing. The Caron process involves an initial reducing roast at around

900 oC using coal or oil, followed by leaching of the resultant nickel metal in

ammoniacial liquor (e.g. Greenvale in Qld). The reduction reaction can be idealised

as:

2NiO + H2 + CO 2Ni + H2O + CO2

A number of other roasting processes are used on a smaller commercial scale. These

include:

Segregation roast: Low grade copper oxide ores are difficult to concentrate and are

treated hydrometallurgically, usually by leaching with sulfuric acid. One different

process involves so-called "segregation roasting" in which the ore and a small amount

of salt (NaCl) and carbon are heated at around 600 oC. A complex reaction occurs

and copper is concentrated by migrating onto carbon where it is reduced to its metallic

form. The calcine is screened to separate carbon and the copper recovered from it by

leaching. This complex reaction can be idealised by:

CuO + C NaCl

Cu(on carbon) + CO(g)

Becher Process: Ilmenite (FeTiO3) from mineral sands, can be upgraded into

synthetic rutile TiO2 by removal of iron. An important method is the Becher Process

developed in W.A. Ilmenite is firstly roasted in air to oxidize all iron to Fe3+

, and then

with coal to reduce it to metallic Fe, which is finally removed by accelerated "rusting" to Fe2O3 in aerated water. This complex process can be idealised as:

FeO.TiO2 roastair

Fe2O3.TiO2 + TiO2 roastC

Fe + TiO2


Sulfation Roast: Copper and cobalt sulfides can be converted into water soluble

sulfates by roasting at 600 oC in air under closely controlled conditions, such that the

iron impurities remain as insoluble ferric oxide. The valuable sulfates can then be

preferentially leached in dilute sulfuric acid. Sulfation roasting of chalcopyrite can be

idealised as:

CuFeS2 Air /SO2 CuSO4 + Fe2O3 + SO2(g)

1.2.2 Solid/Liquid Reaction

A commercial example is "pug-roasting" in which oxides are heated with concentrated

sulfuric acid to produce water soluble sulfates in a highly exothermic reaction. For example, in the sulfate process for the production of TiO2 from ilmenite, ilmenite is first

pug-roasted at 100-200 oC with concentrated H2SO4 to yield a honeycombed mass

of iron and titanium sulfates which are then dissolved in water:

FeTiO3 + 2H2SO4 TiO.SO4 + FeSO4 + 2H2O

Other examples which have not yet been used commercially include the roasting of CuFeS2 with S to produce Cu2S which is more easily leached (than CuFeS2) and the

treatment of complex Cu/Pb sulfides with sulfuric acid to produce sulfates.

2. Types of Leaching Reactions

In general, commercial leaching reactions can be classified into the following main

types: water salvation, acid attack, alkali attack, complexation and oxidation (often with

complexation).

2.1 Water solvation

Water solvation refers to the dissolution of naturally soluble salts in water, such as

CuSO4 or CoSO4 from sulfation roasting:

CuSO4(s) Cu2+

(aq) + SO4

2

(aq)

These ions are actually solvated (=complexed) with water molecules and the usual

convention (not followed here for convenience) is to indicate this by Cu2+

(aq) and

SO42

(aq).

2.2 Acid attack

Most important examples are the dissolution of ZnO calcine (with subsequent

electrowinning) with sulfuric acid and the leaching of low grade copper oxide ores also

with sulfuric acid (the latter accounts for the majority of copper extracted by

hydrometallurgy). Reactions are simple:

ZnO + 2H+ Zn2+

+ 2H2O

CuO + 2H+ Cu2+

+ 2H2O

where the spectator ion is SO4

2 in both cases.

2.3 Alkali attack

Most important example is the Bayer Process which begins with the dissolution of

bauxite using NaOH:


Al(OH)3 + OH

Al(OH)4

where the spectator ion is Na+.

2.4 Complexation

Most commercial leaching reactions which involve complex ion formation also involve

oxidation and are included below.

2.5 Oxidation

Solids can be brought into solution by oxidation (=anodic reaction) which involves the

loss of electrons. This must be accompanied by reduction (= cathodic reaction) within

the leach solution to absorb the electrons. Anodic reactions of industrial importance

involve either the oxidation of metals (=corrosion) or the oxidation of sulfur contained

in sulfides. The most important cathodic reactions are those using either dissolved

oxygen or ferric ions as oxidants (= electron absorbers) in leach solutions.

Industrial Anodic Reactions (Oxidation)

Oxidation of Metals

Au Au+ + 1e (Leaching gold in NaCN) (1)

U4+ U6+ + 2e

(Leaching of uranium oxide) (2)

Ni Ni2+

+ 2e

(Caron Process for low grade Ni ores) (3)

Oxidation of Sulfur

S S0 + 2e (CuFeS2

for Heap Leaching & CLEAR process) (4)

(Direct acid pressure leaching of ZnS) (5)

S S+ + 8e (Pressure oxidation NiS & ZnS) (5)

Industrial Cathodic Reactions (Reduction)

(a) Oxygen Gas in Water (O0 + 2e O)

O2 (g) + 4H+ + 4e 2H2O (6a)

or, in less acidic solution,

O2(g) + 2H2O + 4e 4OH (6b)

(b) Ferric Ions

Fe3+

+ e Fe2+

(7)

Oxygen is the most powerful oxidant and is in plentiful supply in the atmosphere but

this reactant must still be supplied and dissolved in the solution to maintain the

necessary oxygen levels in the reactor. An alternative is to use ferric ions to absorb

electrons but only in acid solutions since ferric hydroxide will precipitate otherwise.

Ferric ions often act as intermediates during acidic oxidation, that is, electrons are

absorbed through (7), but the ferrous ions are reoxidised to ferric by oxygen through

reaction (6a). The end result is equivalent to aerial oxidation but via ferric ions.


Complete Leaching Reactions (oxidation plus reduction)

1. Leaching of gold

Gold occurs mainly in the metallic form and is leached via reaction (1) which is carried

out in the presence of CN

(added as NaCN or KCN) since this forms a stable

complex and thus improves gold solubility. Complex formation may be written as:

Au + 2CN Au(CN)2

+ e (8)

Air is used as the oxidising agent. Reactions (1) and (8) are combined to eliminate the

electrons and give the process reaction as:

4Au + O2 + 2H2O + 8CN 4Au(CN)2 + 4OH (9)

where sodium is a spectator ion.

2. Leaching of uranium oxide

Uranium mainly occurs in low grade oxide ores as mixtures of UO3 (U as U

6+

) and UO2

(U as U4+

). UO3 can be directly dissolved as the complex ion UO2

2+ (e.g. as

UO2(SO4)

2

2-

) but the tetravalent ion in UO2 must firstly be oxidised before it will

dissolve. Reaction (2) is usually carried out in the presence of H2SO

4, which allows the

formation of the stable UO2(SO4)

22- complex although Na2CO3 is sometimes used

with the formation of the UO2(CO

3)3

4- complex. The reaction for sulfuric acid is:

UO2 + 2SO42 UO2(SO4)2

2 + 2e (10)

Air is commonly used as an oxidant (although Fe3+/Fe2+ acts as an intermediate).

Acid leaching can thus be described by a combination of reaction (6a) with (10):

UO2 + ½O2 + 2SO42 + 2H

+ UO2(SO4)2

2 + H2O (11)

3. Leaching of Low Grade NiO Ores in Caron Process

After reduction roast of NiO to Ni, the metal is dissolved in an oxidative leach in the

presence of ammonia in order to form the stable complex Ni(NH3)62+. Reaction (3)

then becomes:

Ni + 6NH3 Ni(NH3)62+

+ 2e (12)

Air is used as the oxidant and the process in this basic solution is best described by a

combination of reactions (12) and (6b):

Ni + 6NH3 + ½O2 + H2O Ni(NH3)62+

+ 2OH (13)

4. Leaching of Chalcopyrite (CuFeS2)

Chalcopyrite is leached industrially either in the form of low grade ores or as

concentrates where reaction (4) can more completely be written as:

CuFeS2 Cu2+

+ Fe2+

+ 2So + 4e (14)

Some further oxidation of So to S6+

(as SO42) often takes place.


Low Grade Ores: These are "heap" leached on site using sulfuric acid (which will

also leach any oxide present) with ferric ions as oxidant. A combination of (14) and (7)

describes the process.

CuFeS2 + 4Fe3+

Cu2+

+ 6Fe2+

+ 2So (15)

where SO42

is a spectator ion.

This process is known as bacterial leaching since bacteria (in particular Thiobacillus

ferroxidans) play an important role by reoxidising Fe2+

back to Fe3+

much faster than

normal aerobic oxidation (= in presence of air). Since this reoxidation process

replenishes the supply of ferric ions, reaction (15) is driven forward. These bacteria

can also oxidise So to S6+

resulting in the formation of sulfate SO42.

Chalcopyrite Concentrates: Most chalcopyrite concentrates are processed by

pyrometallurgy despite many attempts to develop leaching processes. The socalled

CLEAR process (for Copper, Leach, Electrolysis and Regeneration) is a recent

commercial attempt at a hydro process in which chalcopyrite is essentially leached

with a solution of ferric chloride through reaction (7). Dissolution of the sulfide involves

both the oxidation of S2 through reaction(4) and the reduction of Cu2+ to Cu+ in the

presence of excess Cl ions (from NaCl saturation) to form the stable complex CuCl2

since CuCl alone has poor solubility in water:

CuFeS2 + 2Cl

CuCl2

+ Fe2+

+ 2So + 3e

(16)

The oxidation with ferric ion as oxidant can be described by a combination of (16) with

(7):

CuFeS2 + 2Cl

+ 3Fe3+

CuCl2

+ 4Fe2+

+ 2So (17)

where Cl

is a spectator ion. (The process is actually a little more complicated because

the reduction of Cu2+ to Cu+ is necessarily coupled with an oxidation of S2

to So,

and in fact extra cupric chloride is added to act as an oxidant in addition to ferric

chloride.)

5. Oxidation Leaching of Zinc Sulfide

At present, the vast majority of zinc sulfide is firstly roasted to ZnO and then leached

with sulfuric acid. However, a recent commercial process involves a one step leach

using pressure oxidation with air and dissolution in H2SO4 all at 200 oC. Zinc is

dissolved as the sulfate and elemental sulfur is formed. Reaction (4) thus becomes:

ZnS Zn2+

+ So + 2e

(18)

With air as the oxidant, the process can be described by a combination of (18) and

(6a).

ZnS + 1/2O2 + 2H+ Zn

2+ + S

o (19)

where SO42

is a spectator ion.

6. Leaching of Nickel Sulfide (Sherritt Gordon Process)


Nickel sulfide is oxidised with air under pressure in the presence of ammonia to form

the stable complex Ni(NH3)62+

. Sulfur in NiS is oxidised from S2 to S

6+ which exists in

the solution as SO4

2. Reaction (5) for this process is thus more completely

represented by:

NiS + 4OH

+ 6NH3 Ni(NH3)62+

+ SO42 + 4H

+ + 8e

(20)

Since oxidation is by air, the process can be described by a combination of (20) with

(6a):

NiS + 2O2 + 6NH3 Ni(NH3)62+

+ SO42 (21)

This description idealizes the process a little since the concentrate contains iron as

well as nickel and not all sulfur is initially oxidised through to SO42.

3. Leaching Techniques

A range of techniques and a variety of equipment are employed for leaching, namely:

in-situ leaching, dump and heap leaching, percolation leaching, pulp (slurry) leaching.

3.1 In-situ leaching refers to either the leaching of ore left in a mine after it has been

worked out or to the direct leaching of an ore body in the ground, usually after it has

been shattered by explosives. The leach solution is introduced above the ore body

and allowed to percolate by gravity before being collected at lower levels by a network

of sumps. It is then pumped to the surface and processed for metal recovery. The

method has been applied to low grade copper and uranium ores, and although the

cost of mining is cheap, these savings may be offset by low extraction rates and

efficiencies and the loss of leach liquor into the ground water.

3.2 In dump or heap leaching, fractured rock (0.11 m) is removed from the mine and

piled, without further size reduction, onto a prepared impervious terrain. The leaching

agent is applied at the top of the pile and then collected at the bottom for metal

recovery giving typical extraction efficiencies of 60%. This is an important method for

the treatment of low grade copper and uranium ores and the flow sheet used will be

discussed in more detail later.

3.3 In percolation leaching, ore is coarsely crushed (to 520 mm) and charged into

large vats having false bottoms. Countercurrent leaching is usually employed to

ensure extraction efficiencies of around 80%. Once the ore is spent, the vats are

emptied mechanically and the process repeated. Applications are for copper, gold and

uranium.

3.4 Pulp or slurry leaching involves the leaching of finely milled ores (200 mm) in

agitated and usually aerated vessels, at either atmospheric pressure in open vats or at

higher pressures in sealed autoclaves. The additional costs of these techniques limit

their application to high value metals (e.g. gold cyanidation in open vats) or to

concentrated minerals (e.g. NiS and ZnS oxidation in autoclaves). Open vats may be

agitated by injection of compressed air at the bottom (pachuca tanks) or mechanically

using paddles or rakes (low turbulence) or propellors and turbines (high turbulence).

Autoclaves may also be agitated by compressed air or mechanically.

4. Solution Purification and Concentration

Leach solutions may be treated directly to recover their metal values. Or they may first

have to be purified and concentrated before the metal values can be recovered


efficiently. Many impurities are themselves valuable and are often recovered in

separate byproduct stream(s).

4.1 Impurity removal

Two important methods are used, namely (i) cementation, which is based on metal

displacement using the electrochemical series and (ii) precipitation of insoluble salts

such as sulfides, hydroxides, sulfates and carbonates, which is based on the classical

table of group separation of metal ions (recall solubility products of metals) developed

by inorganic and analytical chemists. The processes of solvent extraction and ion

exchange can also be used and continue to offer new purification possibilities, but

these are still mainly used for concentration and will be discussed below.

4.1.1 Cementation

e.g. Removal of Cu, Cd and Ni from zinc leach solutions by the addition of zinc dust:

Cu2+

+ Zn Cu + Zn2+

Cd2+

+ Zn Cd + Zn2+

Ni2+

+ Zn Ni + Zn2+

4.1.2 Precipitation of insoluble compounds

Sulfide Precipitation: e.g. Removal of Co and Cu by the addition of H2S gas:

Cu2+

+ H2S CuS(s) + 2H+

Co2+

+ H2S CoS(s) + 2H+

Hydroxide Precipitation: e.g. Removal of iron and tin by pH adjustment:

Fe3+ + 3H2O Fe(OH)3 + 3H+ pH = 3

Sn4+ + 4H2O Sn(OH)4 + 4H+ pH = 1

Note: The reactions can also be written in the form Fe3+ + 3OH

Fe(OH)3 by

combining these equations with the associated water equilibrium, H+ + OH = H2O,

but it is more descriptive to write equations including H+ when dealing with acid

solutions (i.e. pH < 7).

Sulfate Precipitation: e.g. neutralisation of acidified sulfate solution using lime, prior to

effluent disposal:

Ca(OH)2 + SO42 + 2H+ CaSO4.2H2O(s)

Carbonate Precipitation: e.g removal of lead by addition of a soluble carbonate:

Pb2+

+ CO32 PbCO3(s)

4.2 Concentration

This is achieved by selective extraction of the valuable metal ion from low grade leach

solutions by "loading" into an ion specific organic solvent (solvent extraction) or onto

an ion specific resin (ion exchange). Once the exchange media is fully loaded, it is

separated from the leach solution and the valuable ion chemically stripped from it in a

process called "elution". The exchange media is regenerated for use in another

process cycle and the valuable ion collected in a more concentrated solution, ready for

subsequent recovery. Ion exchange and especially solvent extraction have become


well established since their first large scale application for uranium recovery during

World War 2 and are now used not only for costly metals, such as uranium, vanadium,

niobium, tantalum and zirconium but also for less costly metals such as copper and

zinc.

4.2.1 Ion Exchange (IX)

For example, concentration of uranium from low grade leach solutions. Uranium is

present in acid leach solutions mainly as UO2(SO4)22

. If an aqueous solution

containing this anion is brought into contact with an anion exchange resin (i.e. a resin

on which the anion is loosely bound), the following anion exchange equilibrium will be

established:

2R+X

+ UO2(SO4)22

[R+]2 UO2(SO4)22 + 2X

Here R+ stands for the resin cation and X

for anions like Cl

. Since uranium is one of

the few metals which can form anions in sulfuric acid solutions the exchange is very

selective, other metals such as Ca, Fe, etc., being retained in the aqueous phase as

cations.

The contact between the leach solution and the ion-exchanger may be accomplished

in different ways. Most common are perhaps fixed beds of granulated ionexchanger

through which the solution is percolated in a countercurrent system. After a column

has been "loaded" it is first washed with water and then "stripped" by means of a

strong acidified chloride (or nitrate) solution. This brings the uranium back into a

relatively pure aqueous phase which is around 20 times more concentrated than the

leach solution, and regenerates the resin for a further cycle. Uranium oxide is finally

precipitated from the strip solution by neutralization.

A new concentration process similar to ion exchange has recently revolutionized the

gold industry. In this so-called carbon-in-pulp process, gold as Au(CN)2

in low grade

leach solutions is loaded onto carbon particles, which are then separated from the

solution and stripped using a hot solution of concentrated NaOH/NaCN. Gold is

recovered from the strip solution by electrowinning. This method allows the economic

recovery of gold from solutions of much lower concentration than is possible with the

traditional approach where gold is recovered directly from solution by cementation with

zinc powder.

4.2.2 Solvent Extraction (SX)

For example, concentration of uranium and copper from low grade leach solutions.

Copper is now the most extensive application. For this purpose, organic cation

exchange liquids know under trade names like LIX 64 (LIX = Liquid Ion Exchange),

are dissolved in a carrier such as kerosene and mixed with the aqueous leach

solution. The organic liquid is immiscible with water and copper is selectively

transferred into it by cationic exchange:

2RH + Cu2+

R2Cu + 2H+

Here R represents the anionic part of the organic molecule, and Cu2+ and H+

represent ions in the aqueous phase. The R2Cu organic compound is called a

chelate. Similar exchange reactions may be written for other metal ions in the aqueous

solution such as Fe2+, but the equilibrium constant for the exchange is very much

lower making possible a selective separation of copper.


Once loaded with copper, the organic liquid is separated from the aqueous solution

and treated with a much smaller volume of acid to reverse the above equilibrium and

thus strip Cu2+ into a pure and concentrated solution suitable for electrowinning.

5. Metal or Product Recovery

In this final extraction step the object is to obtain as high a purity product as possibly

while maintaining a high recovery. The product can either be a metal or some

intermediate compound which can subsequently be reduced to metal by

pyrometallurgical methods such as fused salt electrolysis. Metals are reduced from

solution by either cementation, gaseous reduction, or electrowinning while

intermediates are often recovered by hydrolysis.

5.1 Cementation

Examples are the recovery of copper from low grade leach solutions using scrap iron

and the recovery of gold from cyanide leach solutions with zinc powder:

Cu2+ + Fe Cu + Fe2+ (in presence of SO42

)

2Au(CN)2

+ Zn 2Au + Zn(CN)42 (in presence of Na+)

The product in both cases is relatively impure and must be refined by pyrometallurgy

in the case of gold and by aqueous electrorefining in the case of copper. For larger

scale copper operations, the trend is to eliminate cementation. Instead, solvent

extraction is used to give a solution which is concentrated and pure enough for

successful electrowinning. In this way it is possible to economically recover pure

copper from exceptionally low grade ores.

5.2 Gaseous Reduction

In the Sherritt Gordon Process, nickel is recovered by H2 pressure reduction (30 atm)

at 200oC in an autoclave:

Ni(NH3)22+ + H2 Ni + 2NH4

+ (in presence of SO42

)

In one commercial operation, copper is recovered from warmed sulfate leach solutions

by hydrogen reduction:

Cu2+ + H2 Cu + 2H+

5.3 Electrowinning

Recovery of copper and zinc is widely practiced. The metal is deposited at the

cathode with the evolution of oxygen at the inert (e.g. lead, graphite) anode. This

method is the most expensive, but gives the highest purity product.

Cu2+ + H2O Cu + 1/2O2 + 2H+

Zn2+ + H2O Zn + 1/2O2 + 2H+

Both are in acidic SO42 solutions.


5.4 Hydrolysis/Precipitation

Al(OH)3 is recovered in the Bayer process by cooling the leach liquor from 150 oC to

100 oC (precipitation):

Al(OH)4

(aq) + Na+(aq) Al(OH)3(s) + Na

+(aq) + OH

(aq)

In this case, cooling increases K for this precipitation reaction and dilution lowers the

concentration (= activity) of OH

in the basic solution (which lowers the pH). Both effects favour precipitation of Al(OH)3.

TiO(OH)2 is crystallized by hydrolysis of titanium sulfate (TiOSO4) solution in the

sulfate route for TiO2 production from ilmenite. To enable the reaction to proceed,

steam is blown through the concentrated liquor. The resultant heating increases K for

the precipitation reaction and dilution by H2O lowers the H+ concentration in the acid

solution (raises the pH). Both effects favour the formation of TiO(OH)2 by the following

reaction:

TiO2+ + 2H2O TiO(OH)2(s) + 2H+ (in presence of SO42

)

The TiO(OH)2 product is then calcined to yield TiO2:

The recovery stage of the Caron Process for low grade nickel ores involves the boiling off of volatile ammonia with a resultant precipitation of Ni(OH)2 according to:

Ni(NH3)62+ + 2H2O Ni(OH)2(s) + 2NH4

+ + 4NH3(g)

Since this process is done in the presence of CO32

(ammonium carbonate is used in

the leach) the final product also contains nickel carbonate. However the mixed

precipitate is readily calcined to NiO.

6. Leach Regeneration and Reagent Recovery

Water in large quantities is a valuable commodity (especially in arid regions) and most

hydrometallurgical processes operate with recycled water. Thus, rather than dispose

of "spent liquor", it is passed through a leach regeneration step which aims (1) to

remove any impurities (either by precipitation or bleeding off of liquor) that build up in

the circuit and adversely affect the operation (2), to regenerate the leaching reagent if

possible (e.g.. by oxidation of Fe2+ back to Fe3+ when this is the oxidising agent) and

finally (3) to adjust the water balance by either making up plant losses or by removal

(distillation) of additions (e.g. when hydrolysis is used as a recovery step or wash

water is added to the circuit).

Once the water balance is correct, fresh makeup reagent is added to completely

regenerate the liquor for leaching. In some processes, the reagent is recovered separately during the product recovery step (e.g. the distillation of NH3) and recycled

to make up the leach solution. The ability to recover or regenerate the leaching

reagent is an important economic aspect of process design especially when

expensive reagents such as ammonia are used. Some examples for commercial

processes are:

6.1 Bayer Process

NaOH is regenerated when Al(OH)3 is recovered by hydrolysis. This involves cooling

and dilution of the liquor. Extra water from residue washing is also added to the circuit.


Excess water is removed by evaporation and make-up caustic added before the liquor

is recycled to the leach.

6.2 Roast/Leach/Electrowinning of Zinc

Zinc recovered by electrowinning which regenerates sulfuric acid at the anode (see

eq. for electrowinning). The sulfuric acid is recycled to the zinc oxide leach tanks.

6.3 Sherritt Gordon and Caron Processes

Nickel is stabilized in solution through complex formation with NH3. This volatile

reagent is removed by boiling and recycled to the leaching step.

7. Heap Leaching of Low Grade Ores

Heap leaching is an important method for treating low grade ores and can be used for,

copper sulfides (especially chalcopyrite) using Fe3+ as oxidant in the presence of

bacteria, copper and uranium oxides using dilute H2SO4 (and Fe3+/air in the case of

uranium) and gold ores using NaCN and air as source of oxidant.

A significant proportion of the world's copper is produced by heap leaching of low

grade oxide and sulfide ores, with the latter being bacterially assisted. Copper is

selectively extracted from the low grade solution by solvent extraction, and is then

electrowon from the concentrated strip solution, where acid is regenerated. This can

be recycled to the leaching step if electrowinning is carried out on-site. Figure 2 shows

a typical flowsheet for heap leaching of a low grade copper ore.

The leaching of copper oxide and chalcopyrite may be represented by the following

equations:

CuO + H2SO4 Cu+ + H2O + SO4

CuFeS + 4Fe bacteria

Cu2+ + 6Fe + 2S0

Bacterial leaching is optimised by ensuring the presence of nutrients like PO43

,

NO3

, Cl

, trace elements and air. The optimum conditions for bacterial growth are pH

2 and 35 oC but they grow and survive under a remarkably wide range of conditions.

The solvent extraction step may be represented by the following general equation:

Solvent Extraction:

2RH + Cu2+ extract

strip

R2Cu + 2H+

where R represents an organic extractant.

The electrowinning reactions are:

Cu2+ + 2e

Cu cathode

H2O + 2e

1/2O2 + 2H+ anode


Impervious pad

Graded

Heap

Acid Spray / Fe3+

STORAGE POND:

Oxidise Fe2+ -> Fe3+

Precipitate - basic iron

sulfate

MAKEUP ACID from

electrowinning

SOLVENT EXTRACT

Cu2+

Drainage channels

Cu2+

Fe2+

Fe2+

Fe3+

HEAP LEACH

Concentrated & pure

Cu2+ solution to

ELECTROWINNING Cu plus recycle acid

Figure 2 Heap Leaching of Low Grade Ores

Leach Regeneration is accomplished through aerial oxidation combined with

hydrolysis in the storage pond removing excess iron and sulfate through the precipitation of a basic iron sulfate (Fe3(SO4)2(OH)6).

8. ELECTROREFINING

Although electrorefining is by far the most commercially important hydrometallurgical

refining process, its large scale use is only for the refining of copper and nickel

produced from pyrometallurgy. Copper, zinc and nickel recovered from hydro

processes by electrowinning are relatively pure and only cementation products such

as crude copper need to be electrolytically refined on a regular basis.

The principles of electrorefining have already been briefly reviewed. The process is

carried out in an electrolytic cell in which the impure metal forms the anode. This is

dissolved with the passage of an electric current and is redeposited as a pure metal on

the cathode. Impurities either enter the electrolyte or precipitate as insoluble

compounds.

These insoluble impurities or anode slimes, represent a significant source of precious

metals such as Au, Ag, Pt and Pd, since these occur along with copper and nickel in

sulfide deposits (check the periodic table!) and always report as impurities in Cu and

Ni produced by pyrometallurgy. This ability to readily concentrate virtually all of the

precious metals is a major advantage in the pyrometallurgical treatment of Ni and Cu

and is one of the reasons why pyro continues to dominate over hydro processes for

the extraction of these metals from sulfide concentrates.

9. PRACTICAL CONSIDERATIONS

From a chemical point of view, the best hydro process for a particular mineral has the

most selective leach and provides for the most selective removal of impurities.

However, a number of other practical and economic considerations also have to be

taken into account when selecting the most suitable process.


In common with pyrometallurgical processes, a hydrometallurgical flow sheet must be

designed to minimize (1) energy input (coal, oil or gas), (2) the number of process

steps to carry out extraction and recovery, removal of impurities, reagent regeneration,

and to produce a product of marketable purity and (3) the labour necessary to run the

plant. Considerations specific to hydrometallurgical processes include:

1. Ore grade and the leaching method to be employed: The grade of ore determines

whether insitu (very low grade), heap leaching (low grade), vat (higher grade) or

pressure leaching (concentrates) can economically be used.

2. Mineralogy and type of leach system: The reactivity of the mineral and gangue

towards common leaching reagents determines the leach system that can be used.

The object is to leach the mineral but not the gangue using a flow sheet that provides

for removal of impurities and regeneration of the leach solution. If significant precious

metals are present, then their recovery must be provided for.

In general, mineral reactivity depends on the class of compound and whether the

cation can form a stable complex in the leach solution. As a guide, carbonates are

more reactive than hydroxides, then oxides, and sulfides are the least reactive.

However, even minerals of the same chemical type vary widely in their reactivity. An oxide like ilmenite (FeTiO3) is relatively inert towards even concentrated H2SO4 (and

is pugroasted at 200oC) whereas copper oxide such as tenorite (CuO) will dissolve

readily in dilute acid. In some cases the gangue may be more reactive than the mineral, e.g. dilute sulfuric acid cannot be used when MgCO3 and CaCO3 are present

along with CuO and NH3/(NH4)2CO3 is the preferred leach.

The mineral and gangue reactivity is a useful guide, but selectivity or separation may

be achieved by controlling the thermodynamics through adjustment of temperature

and by controlling the kinetics through adjustment of temperature and time.

3. Reagent Costs: As a rule of thumb, the cost of the various sections of a mining and

processing operation can be apportioned as follows:

Mining/Processing Operation % overall cost

Mining 20

Mineral Processing 20

Extraction – fixed cost Plant depreciation, infrastructure, inventory

30

Extraction – running costs Chemicals, energy, labour, waste treatment, maintenance, sales

30

Clearly, the amount that can be spent on leaching reagents is relatively small and is

limited by the value of the metal produced and the grade of the ore. That is, more

expensive chemicals can be used for more valuable metals or for higher grade ores.

Even then, expensive chemicals can only be used if they can be recycled efficiently

with little overall loss. Within the above framework, the cost of chemicals used to

produce one tonne of metal should not exceed 10% of the metal value.

4. Equipment costs: One must also consider the cost of equipment in terms of (1) the

complexity (pressure vessels cost a lot more than open vats), (2) the materials of

construction (corrosive chemicals require expensive alloys and plastics) and (3) the

maintenance costs (more complex equipment and those handling corrosive chemicals

have higher maintenance costs).

5. Flow sheet considerations: Solid/liquid separations must be kept to a minimum

(these can be slow and expensive) and the water balance must be compatible with the


region where the plant is located (processes with large water usage are not suitable

for arid regions).

6. Waste Treatment and Tailings Disposal: Whilst hydro processes do not in general

pollute the atmosphere, great care must be taken not to pollute ground water and

effluent streams and to control discharge from tailings ponds and dumps. For

example, sulfuric acid is a relatively cheap leaching agent, but when used with

minerals containing iron, ferrous sulfate is formed and this creates a disposal problem.

It cannot be discharged into streams and therefore must either be neutralized, or

crystalized and decomposed; both are costly operations.

Basic Chemical Principles in Hydrometallurgy

Hydrometallurgical reactions occur mainly in aqueous solutions. From a

thermodynamic point of view, reactions involving ions are handled in the same way as

the solid-gas reactions already considered previously in this unit. That is, equations

can be added and subtracted as before, equilibrium constants can be calculated from

Go values, and, if equilibrium is achieved, the amount of substances present can be

obtained from activity values using appropriate activity scales. We have yet to define

the activity scale for ions in solutions.

Thermodynamics can only predict what should happen and not how fast it will happen.

If the approach to equilibrium is very slow, “metastable” products can result rather than

those predicted from equilibrium calculations. Metastable substances are

intermediates with a prolonged life because of a slow approach to equilibrium. This

can be the case for some hydro processes since they are carried out at relatively low

temperatures. In contrast, pure processes have relatively fast reactions rates and thus,

thermodynamic calculations give a more reliable prediction of the likely outcome.

However, in both cases, thermodynamic predictions are of fundamental importance

since they establish the framework of possible reactions. Kinetic effects can rarely be

predicted and have to be established by observations.

A number of important chemical considerations applicable to an understanding of

hydrometallurgy will now be reviewed, namely: strong and weak electrolytes, activities

of ions in solution, pH of a solution, solubility products, hydrolysis, formation of

complexes, redox reactions, and electrochemical potential.

Strong and Weak Electrolytes

The large majority of substances (acids, bases, and salts) behave as so-called

electrolytes in aqueous solutions. That is, they dissociate into ions and are classified

as either strong electrolytes, when completely dissociated, or weak electrolytes, when

only partly dissociated.

A strong electrolyte, say NaCl is represented by the following equation.

NaCl Na + Cl

The single arrow indicates complete dissociation.

A weak electrolyte on the other hand, say CH3COOH, is represented by the following

equation:

CH3COOH H + CH3COO

The double arrow indicates partial dissociation. The extent of the dissociation in this

case is described by a constant, which depends only on the temperature of the

system.


In order to use this equation to calculate the concentrations of H+ and CH3COO, it is

necessary to relate the activities to concentration using an activity scale based on

standard or reference state. A knowledge of standard states is also essential for the

calculation of K since this is determined from the standard free energy, i.e. the free

energy difference between products and reactants of the dissociation when all are in

their defined standard states, where Go = RT ln K.

The definition of a standard state and the resulting relationship between activity and

concentration sets a scale for activity of ions in solution. A knowledge of this scale is

essential not only for the above calculation of concentrations for weak electrolytes but

indeed for any thermodynamic calculations involving ions in solution.

Activities of Ions in Solution

The standard state for ions in solution is defines as 1 molal (m) solution of the ions in a

so-called ideal ionic solution, where 1 molal is 1 mole in 1000 g of solvent (equivalent

to 1 mole per liter or 1 molar for aqueous solutions at 25 oC).

A scale of ion activities is defined by assigning the value 1 to the activity of the

standard or reference state. Thus, a = 1 when m = 1. A straight line joining these

points has the equation a = 0 which establishes the following activity scale for ions in

solution.

(i) For an ideal ionic solution:

a = m

This is known as Henry’s Law.

(ii) For a non-ideal solution:

a = m

where is the ionic activity coefficient.

Real ionic equation solutions only obey Henry’s Law for very dilute solutions. That is,

1 as m 0. In fact few solutions behave ideally at concentrations as high as m = 1,

and the standard state, which requires the solution to be ideal at m = 1, is said to be

hypothetical. Further, ionic activity coefficients, although simple in concept, are difficult

to experimentally define and measure. We need not be concerned with these

complications.

The pH of a Solution

The hydrogen ion activity of an aqueous solution is one of its most important

properties. It is conveniently expressed by its negative logarithm (base 10), which is

known as pH.

The activity coefficient for H+ is normally taken as one (this is an approximation!) to

give the familiar expression:

pH = log [H+]

where [H+] represents the molarity of H

+ for aqueous solution.

Note that pH + pOH = 14. Therefore,

[H+] = 10pH

and [OH-] = 10pOH

and [H+] x [OH] = 1 x 1014


Solubility Product (Ksp)

Since hydrometallurgy deals with the dissolution of solids and their precipitation from

solution, one equilibrium of great importance is that which establishes the maximum

solubility of ionic solids in water. For example, the maximum solubility of ferric

hydroxide in water can be represented by the following equation:

Fe(OH)3(s) Fe3+

(aq) + 3OH(aq)

The solubility product may be represented by the following equilibrium expression:

33 OHFe spK

The solubility product (Ksp) is an equilibrium constant and has only one value for any

ionic solid at a given temperature. The solubility on the other hand is an equilibrium

position, which changes if there are common ions in the solutions. In all cases,

however, the product of the concentrations of the ions in solution must satisfy the

solubility product.

Example

The solubility product (Ksp) for Fe(OH)3 is 4 10-38. Estimate the solubility (in molL

-1 &

mg/L) of Fe3+

in (a) sulfuric acid leach solution at pH 1.5 and (b) after partial

neutralisation of the solution at pH = 3.5.

Solution

(a) At pH = 1.5, pOH = 12.5 [OH-] = 1012.5 molL

-1. Therefore,

3833 104OHFe spK

3835.123 104101Fe

35.12

383

101

104Fe

M1

31

1

3

1

FeLmg7000

Lg70

Femol

g55.85molL1

(b) At pH = 3.5, pOH = 10.5 [OH-] = 1 1010.5. Therefore,

3833 104OHFe spK

3835.103 104101Fe


35.10

383

101

104Fe

M6101

312

315

3

16

FeLmg107

FeLg107

Femol

85.55molL101

g

Formation of Complex Ions in Solution

Complex ions are soluble chemical species comprising of a metal ion surrounded by

ligands, which act as electron pair donors as shown in the following general equation:

M + nL MLn

This allows the dissolution of an otherwise insoluble solid by reducing the

concentration of the hydrated metal ion. The equilibrium constant, known as Kstab, is

the ratio between the activity of the complex and the activities of the reacting species

may be represented by the following general equilibrium expression:

n

nstabK

LM

ML

For example, cobalt(II) forms a solid hydroxide but adding ammonia results in the

formation of the soluble hexaamine cobalt (II) complex as shown in the following

equations:

Co2+

(aq) + 2OH(aq) Co(OH)2(s)

Co2+

(aq) + 6NH3(aq) Co(NH3)63+

(aq)

the Kstab expression for the formation of the hexaamine cobalt(II) complex may be

written as

63

3

3

63

NHCo

)NH(Co

stabK

Example 1

Calculate the solubility (in molL-1 and gL

-1) of AgCl in (1) a hypersaline water

containing 4.0 M Cl and (b) in the same hypersaline water but containing 2.0 M NH3.

The Ksp for AgCl(s) is 1.6 10-10 and Kstab for Ag(NH3)2

+ is 1 107.

Solution

(a) The dissolution of AgCl is

AgCl(s) Ag+ (aq) + Cl(aq)


AgLg108.3

Agmol

gL87.107Lmol105.3

Lmol105.3

0.4

106.1Ag

0.4Ag106.1

ClAg

19

1111

111

10

10

spK

(b) In the same hypersaline water containing 2.0 M NH3

AgCl(s) + 2NH3(aq) Ag(NH3)2+(aq) + Cl(aq)

Let x = [Ag(NH3)2+], 4x = [Cl] and 2.0 2x = [NH3]

Agmolg86.107M100.8

AgM100.8

2x-2.0

0.4100.1106.1

NH

Cl)Ag(NH100.1106.1

14

4

2

710

2

3

23710'

x

xx

KKK stabsp

Exercise 1

Calculate the solubility of Co3+

in a NH3/(NH4+) buffer containing 0.01 M NH3 and 0.1 M

NH4+. The Kstab for Co(NH3)6

3+ is 1 x 10

34 and the Ksp for Co(OH)3 is 1 x 10

-43.

introduction to hydrometallurgy_ui_lecture notes

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