terral: review of copper minerals heap leaching; cognis; lix user conference; la serena, june 2004...

Terral: Review of Copper Minerals Heap Leaching; COGNIS; LIX User Conference; La Serena, June 2004

REVIEW ON

HEAP LEACHING OF COPPER ORES

Carlos Avendaño V.Sociedad Terral S.A.


* The common approach observes the ore behavior, which has been detected through test

work and experience, with minimum manipulation.

* Terral’s Approach:

- Recognizes the characteristics and behavior of the ore.

- Separates the conditions to be observed and the available degrees of freedom.

- Conciliates both aspects, establishing and reaching an objective based on the economical criteria of the operation.


AN IDEAL LEACHING

* Must maximize metallurgical recoveries.

* Must minimize consumption of acid, water and energy.

* Must consider that the resulting PLS complies with the copper and acid conditions, which will be further required by the SX plant in order to maintain its maximum efficiency.

* Must generate a clean PLS, without suspended solids,colloids, iron, chlorine, manganese or total sulfates, so that transfers to the electrolyte and risks of crud formation are minimized.


AN IDEAL LEACHING

* Must eliminate the effects of eventual reducers and clays with ion exchange capacity and complex silicates in the ore.

* Curing doses - when used - must be calculated in a way that they are consistent with: - Maximum benefit of kinetics, - Minimum enhancement of the acid consumption and contribution to excess in the PLS- No inclusion, or minimum amount, of soluble impurities in the PLS.

* Must avoid formation of phreatic layers and local flooding that might cause channeling, slipping or heap erosion.


AN IDEAL LEACHING

* Ideally, it must allow a joint treatment of ores with diverse mineralogy, including oxide and sulfide species.

* Must allow managing the iron behavior, in order to:

- ensure that the required quantities and ratios of “ferric- ferrous” are present, allowing to regenerate acid, to create oxidizing conditions and to co-precipitate impurities;- avoid formation of ferric colloidal precipitates, which promote channeling of solutions;- allow water removal from the precipitates when these formations cannot be prevented; - ensure the appropriate contents for bacterial activity in sulfide leaching.


- In Curing- In Irrigation- By internal generation

1. THE BASIC EQUATION:

ACID CONTRIBUTION = CONSUMPTION + ACID EXCESS

- By copper and by gangue- The acid excess only appears when consumption is finished.

The equity problem deals with the fact that gangue acid consumption depends on the acid availability (at least within a range); therefore, the consumption also depends on the contribution and the terms of the equation are mutually dependent.


If: ACID CONTRIBUTION < ACID CONSUMPTIONThen: - Metallurgical recovery is affected

If: ACID CONTRIBUTION > ACID CONSUMPTION

Then:

- A maximum metallurgical recovery is almost achieved- A good kinetics is obtained- The “net consumption" of acid increases- Impurities are incorporated to the solutions- Acid excess in PLS may occur


If: ACID CONTRIBUTION = ACID CONSUMPTION

UNDER LOW ACID CONCENTRATION IN A PLS AND WITHIN AN ENVIRONMENT THAT PROMOTES REDOX REACTIONS.

Then we can expect:

Positive effects:* Iron and copper oxidation are triggered* Interfering agents are inactivated: reducers are oxidized and clays with ion exchange capacity remain passive* A maximum metallurgical recovery is achieved, due to the contribution of both Fe+3 in sulfide leaching, and Cu0 , which was precipitated by reducers. * Decreased consumption of net acid.* Solutions are cleaned and fewer impurities are added to the system.

Negative effects:* Kinetics is affected


2. The “irrigation ratio”

The “irrigation ratio” provides irrigation solutions that activate the Physical-Chemistry of the system.

It is associated to the conditions that determine:

- the required acid to obtain the copper concentrations, acid and impurities that were targeted in the PLS,

- the interactions among the kinetics of copper recovery, of acid consumption and impurity dissolution, and

- the definition of a feasible recovery target, under economical criteria based on both the conditions studied in metallurgical tests and in the operation.


* The way to use the “irrigation ratio” leads to “leaching techniques”, which contain parameters and dependent variables.

* The combination of both will lead to process configurations that, simultaneously, solve the operational problems, which are usually related to :

Mineral handling: Crushing, agglomeration, loading and heap configuration.

Solutions handling: Irrigation configuration, sequence of solution application, irrigation media and rates, forms of collection of solutions, water consumption.

Operational aspects: Drag of fines, flooding and channeling of solutions, heap erosion, air injection and others.


ACID & IMPURITIES BALANCE:

1st Series of Reactions:

4 FeSO4(impregn) + O2(gas) + 10 H2O 4 Fe(OH)3(solid) + 4 H2SO4(solut.)

4 Fe(OH)3(solid) + 6 H2SO4(solut.) 2 Fe2(SO4)3(solut.) + 12 H2O

4 FeSO4(impregn.) + O2(gas) + 2 H2SO4(solut.) 2 Fe2(SO4)3(solut.) + 2 H2O

- Iron species dissolve mainly to Fe+2 state.- Fe+2 impregnates the rock when irrigation is stopped.- Fe+3 is oxidized upon contact with gasified air or by bacterial

action, and- Fe+3 changes to hydrolyzed state, either simple or complex.- The hydrolyzed Fe+3 is re-dissolved in the following irrigation

acid, consuming 0,86 Kg of fresh acid/Kg of oxidized ferric ion. - Fe+3 eventually participates in redox reactions and returns to

ferrous state Fe+2.


2nd Series of Reactions:Combining the previous reaction with:

12 FeSO4(impregn.) + 3 O2(gas) + 6 H2O 4 Fe(OH)3(solid) + 4 Fe2(SO4)3(soluc.)

4Fe(OH)3(solid)+4Fe2(SO4)3(solut.)+2Na2SO4+12H2O 4NaFe3(SO4)2(OH)6(solid) + 6H2SO4(solut.)

12FeSO4(impregn.) +3O2(gas) +6H2O +2Na2SO4 4NaFe3(SO4)2(OH)6(solid) + 6H2SO4(solut.)

- Consequent oxidizations from the 1st Series are repeated, or- the previous hydrolyzing Fe+3 participates re-dissolved, according to

reactions.- When pH increases due to mineral acid consumption and the presence of

other ions, formation of jarosites occurs.- Thus, solutions lose: iron (Fe+3), sulfates (1,15 Kg/KgFe) and other ions

that are present (e.g.: Na+ = 0,13 Kg/KgFe) by precipitation, which is virtually permanent, and 0,86 Kg of sulfuric acid/Kg of precipitated iron.

- Consequently, jarosites contribute to impurity elimination from the solutions and to the return of part of the consumed acid.

- The various jarosites incorporate other co-abated elements (Mg, Al and others) .

- Jarosites are re-dissolved in environments with higher acidity than the one in which they were formed.


The concentration of Fe+3 ions available in the solution depends on the balance between:

FERRIC CONTRIBUTIONS = FERRIC CONSUMPTION

The causes of the balance cannot be possibly analyzed in leaching:

- Accounting of the especies is external: it measures outgoing and incoming ions. Only the consequent overall abatement or dissolution is observed.

- Meanwhile, many dissolutions and ion abatements take place at the same time inside the test, which are recorded from outside.

Thus, the quantity of precipitates can only be detected through the

gravel mineralogy, but not through metallurgical accounting

By:- Initial presence of Fe+3

- Presence of Fe+2 oxidization- Re-dissolution of hydrolyzed Fe+3

By:- Fe+3 consumption from the ore and reduction to Fe+2

- Precipitation of jarosites


INTERMITTENT OR PULSE IRRIGATION

- Leaching takes place over a sequence of short irrigation and resting periods

- Resting periods allow solution drainage and squeezing; the particles remain humid but with no liquid that may isolate them from the air in the ore bed


Metallurgical Effects:

* A decrease in the required “irrigation ratio” to achieve target recovery, since the mineral particles absorb the reagents of each of the new irrigation solutions more easily and efficiently. Likewise, products are also poured off during squeezing in each resting stage.

* This effect makes it unnecessary to urge on irrigation rates for improving kinetics and recoveries and, at the same time, avoids impairment of the copper concentration of the PLS.


Physical-Chemical Effects:

* Particles exposure to air during pulses, oxidizes the reducers as well as the metallic copper precipitated by them; it alters and inhibits the action of clays with ion exchange capacity and the formation of colloidal and silicates during the process.

* Particles exposure to air allows oxidation of Fe+2 impregnated ions, which easily precipitate by hydrolysis, but in a disseminated way and with minimum amounts of water, which avoids formation of colloidal gels that cause channelling.

* When association with other ions is possible, part of the hydrolyzed Fe+3 will dissolve when acid is made available in the subsequent irrigation, and part of the iron will precipitate to the state of disseminated jarosites almost permanently.

* In bacterial leaching, it provides oxygen to bacteria.


Physical-Chemical Effects:

* Re-dissolved Fe+3 ions support copper recovery and return to the Fe+2 state, and then repeat the cycle in the following irrigation pulse, according to acid availability in the next irrigation solutions.

* In the permanent precipitation of jarosites, ions from impurities are markedly dragged through diverse mechanisms; among them: a considerable portion of chlorides and sulfates.

* Depending on the pH environment around the Fe+2 ion, when oxidizing to Fe+3 occurs during the resting period, the acid is partially regenerated, which joins the new solution as an active agent, being then deducted from external consumption.


Hydraulic effects:

* Decreased channeling, because during the resting periods of each pulse, the previous irrigation channeling is erased, which compels formation of new access routes in the following irrigation cycle.

* Flooding and heap slipping risks decrease, since it is possible to perform ore bed drainage before saturation.

* During resting periods, the huge colloidal jarosite gels are dehydrated and change into the most dehydrated “sol” state - - smaller in size - which maintains the ore bed porosity.


* Particles Acidulation. * Partial Sulfatation of Species. * Destruction of Interfering Agents.

ADVANCE ILS ILS RECIRCULATION RAFFINATE AND WATER * Adjusts high copper concentrations * Provides "irrigation ratio" to achieve * Completes ore bed exhausting to and reduces acid’s in PLS, even target recoveries, through recirculation achieve high metallurgical at the expense of a slightly lower of one part of the heap to its pond, recovery recovery kinetics within the period. while the other part advances to the * Re-incorporates hydrolyzed ferric ions * Precipitates iron and other ions, in a irrigation of fresh ore in order to ions to ILS, but solubilizable, to controlled manner, into compounds of generate PLS. support leaching of fresh mineral and the jarosite type, to obtain clean PLS. then to precipitate it, definitely, * Slight raffinate addition to complete to the fresh ore zone. acidulation of the particles. * Incorporates the required acid to * Complete destruction of interfering agents. exhaust the ore.

* Water wash recovers impregnation.

CONFIGURATION OF A BALANCED LX-SX-EW SYSTEM

(BY ILS RECIRCULATION)

Curing

Ore

SX Extraction Stages

Acid (eventual)

Electrolyte Bleed

PLS

Raffinate

ILS

Water

Raff. to PLS

ILS to PLS Rich Sol. Sola

ILS to ILS Raff. to ILS

Water to ILS

SX Strip. Stages

SX Org. Wash Stage

EW plant

Org. Tk.

Water

Acid

Loaded Org.

Acid

Water

Unloaded Org.

Water (eventual)


CONFIGURATION OF A BALANCED LX-SX-EW SYSTEM (BY RECIRCULATION OF ACIDULATED PLS)

Curing

Ore Acid (eventual)

PLS

Raffinate

Water

Raff. to PLS

PLS to PLS

Raff. to PLS

Water to PLS

Acid

Water (eventual)

Acid

Electrolyte Bleed

EW Plant Water

Acid

Water

SX Extraction Stages

Unloaded Org.

Org. Tk. Loaded Org.

SX Strip. Stages SX Org.

Wash Stage


CONCLUSIONS:

- Metallurgical recovery can be a target goal - and not necessarily a result or a variable depending on the ore -within a range that is usually bigger than expected.

- This target recovery goal is limited by economical factors that are associated to the leaching techniques.

- Different leaching techniques involve different kinetics and costs.

- Consequently, the main independent variable is the most convenient “leaching technique” to achieve the target recovery.

terral: review of copper minerals heap leaching; cognis; lix user conference; la serena, june 2004...

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