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 ferricferrous 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.
1. THE BASIC EQUATION: ACID CONTRIBUTION = CONSUMPTION + ACID EXCESS - In Curing - In Irrigation - By internal generation - 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 CONSUMPTION Then: - 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 Cu 0, 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: 1 st Series of Reactions: 4 FeSO 4(impregn) + O 2(gas) + 10 H 2 O 4 Fe(OH) 3(solid) + 4 H 2 SO 4(solut.) 4 Fe(OH) 3(solid) + 6 H 2 SO 4(solut.) 2 Fe 2 (SO 4 ) 3(solut.) + 12 H 2 O 4 FeSO 4(impregn.) + O 2(gas) + 2 H 2 SO 4(solut.) 2 Fe 2 (SO 4 ) 3(solut.) + 2 H 2 O - 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.
2 nd Series of Reactions: Combining the previous reaction with: 12 FeSO 4(impregn.) + 3 O 2(gas) + 6 H 2 O 4 Fe(OH) 3(solid) + 4 Fe 2 (SO 4 ) 3(soluc.) 4Fe(OH) 3(solid) +4Fe 2 (SO 4 ) 3(solut.) +2Na 2 SO 4 +12H 2 O 4NaFe 3 (SO 4 ) 2 (OH) 6(solid) + 6H 2 SO 4(solut.) 12FeSO 4(impregn.) +3O 2(gas) +6H 2 O +2Na 2 SO 4 4NaFe 3 (SO 4 ) 2 (OH) 6(solid) + 6H 2 SO 4(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 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 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
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.
O re A c id ( eventual) W a te r ( eventual) C u r in g W a te r * P articles A cid u latio n. * P a rtia l S u lfa ta tio n o f S p ec ies. * D estruction of Interfering A gents. R aff. to PLS ILS to PLS Rich Sol. ILS to ILS Raff. to IL S W a te r to IL S ADVANCE ILS ILS RECIRCULATION RAFFINATE AND W ATER * Adjusts high copper concentrations * Provides "irrigation ratio" to achieve * Com pletes ore bed exhausting to and reduces acid s in PLS, even target recoveries, through recirculation achieve high m etallurgical at th e ex p en se o f a slig h tly lo w er o f o n e p art o f th e h eap to its p o n d, reco v ery reco v ery k in etics w ith in th e p erio d. w h ile th e o th er p art ad v an ces to th e * R e-in co rp o rates h yd ro lyzed ferric io n s * Precipitates iron and other ions, in a irrigation of fresh ore in order to ions to ILS, but solubilizable, to controlled m anner, into com pounds of generate PLS. support leaching of fresh m ineral and th e jaro site typ e, to o b tain clean P L S. th en to p recip itate it, d efin itely, * S lig h t raffin ate ad d itio n to co m p lete to th e fresh o re zo n e. acidulation of the particles. * Incorporates the required acid to * C o m p lete d estru ctio n o f in terferin g ag en ts. ex h au st th e o re. * W ater w ash recovers im pregnation. PLS IL S A c id U n lo a d e d O rg. SX Extraction Stages SX Strip. Stages SX O rg. W ash Stage R affinate O r g. T k. Loaded Org. A c id W a te r EW plant W a te r Electroly te B leed C O N F I G U R A T I O N O F A B A L A N C E D L X -S X - E W S Y S T E M (B Y I L S R E C I R C U L A T I O N )
Ore Acid (eventual) Water (eventual) Curing Water Raff. to PLS PLS to PLS Raff. to PLS Water to PLS Acid PLS Acid SX Extraction Stages Unloaded Org. SX Strip. Stages SX Org. Wash Stage Raffinate Org. Tk. Loaded Org. Acid Water EW Plant Water Electrolyte Bleed CONFIGURATION OF A BALANCED LX-SX-EW SYSTEM Terral: Review of Copper (BY Minerals RECIRCULATION Heap Leaching; OF ACIDULATED COGNIS; LIX PLS) User Conference; La Serena, June 2004
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.