SYSTEMS

Technical Field

The present invention belongs to the area of hydrometallurgy, and in particular to the leaching of ores and refers to an improved control basis for reducing un-optimized operating conditions in liquid/solid decant unit operations and systems.

Background

The proper separation of liquids and solids belong to the key unit operations in hydrometal- lurgy in general and in leaching of crude ores with an aqueous solution of an effective leaching agent in particular. From the state of the art various solids/liquid separation processes are well known, as for example dewatering thickeners, counter-current decanters (CCD) or pinned bed clarifiers (PBC).

For example US 3,685,654 (Gordon Sherrit Mines) discloses a thickener for the concentration of a solids-liquid suspension includes a vessel or tank into which such a suspension is continuously introduced and a rotating rake assembly for exerting mechanical pressure on the solids- sediment layer settling in the bottom of the vessel. US 5,601,704 (Graver) refers to an automatic feedback control system for a water treatment apparatus, such as a recirculating solids contact clarifier, that maintains steady-state operation of the clarifier by accurately measuring the concentration of suspended solids at designated portions of the clarifier and automatically adjusting clarifier variables to maintain optimum conditions despite changes in the inlet flow rate, composition or temperature.

All these methods feature combining mixing, settling and decanting steps with or without the assistance of settling aids like for example flocculation or coagulation agents. Typically, the processes are supported by any source of agitation or movement of the phases. However, existing processes still show serious drawbacks which can have a negative impact on the efficacy of the solids/liquid separation and therefore on the yield and conversion of the overall leaching process:

(i) Dosing of the settling aids: Fluctuating solids composition in the feed slurry or fluctuations in the feed slurry volumetric flow often cause fluctuations in the overall solids/liquid separation process. Manual dosage rate estimates and adjustments and erratic fluctuations in dosing controller input reference parameters can cause erratic settling aid dosing pump output having a negative impact on the settling aid dosage mass flow. As a consequence the composition of the products leaving the solids/liquid separation is constantly changing and causing problems in the following steps of the recovery process.

(ii) Underflow discharge slurry densities:

Inaccurate or inconsistent settling aid dosage mass flow leads to fluctuating underflow discharge slurry densities. Again, inconsistency of the compositions is a major problem for processing the liquids in the following operations.

(iii) Inefficient liberation of dissolved soluble metal species

Low system underflow slurry densities, higher entrained solution content per underflow slurry unit volume, and inefficient feed solids dispersion within contact wash aqueous so- lution prior to solid settling are responsible for an unsatisfying liberation of dissolved soluble metal species from the slurry being treated into the separated aqueous phase of the liquid/solid unit operation.

(iv) Inefficient dissolution of un-dissolved soluble metal species

Incomplete conversion of the lixiviant-soluble available contained metal in leach feed and product material, often caused by unbalanced metal leaching system parameters, lead to unsatisfactory dissolution of soluble metal species, from the solids of the slurry being treated, into the aqueous phase of the liquid/solid unit operation.

Summary One aspect of the invention relates to a method for dewatering a slurry from the leaching of a valuable ore with an aqueous solution of an effective leaching agent. The method comprises: decanting the slurry via a solids/liquid decantation system to provide an overflow depleted from solids and an underflow rich in solids that is washed counter-current through one or more solids/liquid decantation stages; monitoring and steering the leach product thickener with a first controller; and monitoring and steering the counter-current decanter with a second controller. Various embodiments are listed below. It will be understood that the embodiments listed below may be combined not only as listed below, but in other suitable combinations in accordance with the scope of the invention.

In embodiment one, said first controller monitors and steers the flow and the density of the leach product thickener underflow and the dosing of the flocculants.

In embodiment two said first controller monitors the flow of the leach product thickener underflow by controlling the minimum thickener underflow discharge flow.

Embodiment three includes a modification to the system wherein said first controller activates a linear flow intervention controller that increases the wash water flow output until the minimum product thickener underflow discharge flow is above the required minimum when the slurry discharge flow falls below the calculated minimum to prevent solids settling out.

In embodiment four, said first controller monitors the density of the leach product thickener underflow by a densitometer and an in-line control valve.

In a fifth embodiment, the product thickener underflow density is less than a process specified minimum, said first controller activates a leach product thickener flocculent dosing controller that increases the output of the leach product thickener flocculent dosing pump until the product thickener underflow density increases to affect the underflow density control valve opening and the output is above the process defined minimum.

In a sixth embodiment, said first controller monitors the dosing of the leach product flocculent by the leach solids mass flow.

In embodiment seven, when the flocculent dosing rate per unit mass of feed solids and the input concentration of made-up flocculents determining the required flow rate of flocculent per mass flow of solids fed into the leach product thickener deviates from the process specified conditions, said first controller activates said leach product thickener flocculent dosing controller that increases or decreases the output of the leach product thickener flocculent dosing pump until the set point of the feed solids mass flow is reached again. In an eighth embodiment, said second controller monitors and steers the flow and the density of the counter-current decanter underflow, the dosing of the flocculants and the flow of the counter-current decanter overflow.

In embodiment nine, a second controller monitors the flow of the counter-current decanter un- derflow by controlling the minimum counter-current decanter underflow discharge flow.

In a tenth embodiment, when the slurry discharge flow falls below the calculated minimum to prevent solids settling out, said second controller activates a linear flow intervention controller that increases the wash water flow output until the minimum counter-current decanter underflow discharge flow is above the required minimum. In embodiment eleven, said second controller monitors the density of the leach product thickener underflow by a densitometer and an in-line control valve.

In a twelfth embodiment, when the counter-current decanter underflow density is less than a process specified minimum, said second controller activates a counter-current decanter floccu- lent dosing controller that increases the output of the counter-current decanter flocculent dosing pump until the counter-current decanter underflow density increases to affect the underflow density control valve opening and the output is above the process defined minimum.

In a thirteenth embodiment, said second controller monitors the dosing of counter-current decanter flocculent by the leach thickener underflow solids mass flow.

In embodiment fourteen, when the flocculent dosing rate per unit mass of feed solids and the input concentration of made-up flocculents determining the required flow rate of flocculent per mass flow of solids fed into the counter-current decanter deviates from the process specified conditions, said second controller activates said counter-current decanter flocculent dosing controller that increases or decreases the output of the counter-current decanter flocculent dosing pump until the set point of the leach product thickener underflow solids mass flow is reached again.

In a fifteenth embodiment, said second controller monitors the flow of the counter-current decanter overflow by a flow meter. In embodiment sixteen, when the level of aqueous overflow in the counter-current decanter tank moves outside process-defined battery conditions, the output of the tank discharge pump is adjusted accordingly.

In a seventeenth embodiment, the control system is part of a leaching process for the recovery of valuable metals from ores using split-circuit technology.

Brief Description of the Drawings

Figure 1 depicts the key process control parameters for decantation solids/liquid separation systems according to the prior art; Figure 2 depicts a thickener and counter-current decanter underflow and flocculent dosing control according to one or more embodiments of the invention; and

Figure 3 depicts the leaching of copper ores using a control system in a split-circuit process according to one or more embodiments of the invention.

Detailed Description

Aspects of the present invention refer to a method for dewatering a slurry from the leaching of a valuable ore with an aqueous solution of an effective leaching agent via a solids/liquid decantation system, providing an overflow depleted from solids and an underflow rich in solids that is washed counter-current through a number of solids/liquid decantation stages, the improve- ment implementing into the process line

(a) a first Controller (CI) for monitoring and steering the leach product thickener; and

(b) a second Controller (C2) for monitoring and steering the counter-current decanter.

The method according to the present invention provides a reliable control basis that can significantly reduce un-optimised operating conditions in liquid/solid decant unit operations and sys- terns.

This contrasts what is encountered in the prior art. Figure 1 depicts a simplified flow sheet of mechanized solids/liquid separation decantation system using a settling aid 145 to facilitate solids/liquid separation of feed slurry 100 as it is known from the prior art. The dotted lines represent external system operation feedback, and the solid lines represent internal unit operation feedback. The decantation liquid/solid separation system 130 receives volume and mass information on feed slurry 100, settling aid 145 and aqueous wash solution 150 from volume/mass flow flowmeters 105, 140 and 155, respectively and density information on feed slurry 100 from the densitometer 1 10. The system 130 also controls the system aqueous overflow 120 and the underflow slurry 160 based on rake torque, rake height, bed mass and bed level. The internal unit operation feedback line can also feature a volume/mass flow flowmeter 125 for the system aqueous overflowl20, and a volume/mass flow flowmeter 170 and densitometer 165 for system underflow slurry 160. Typical indicating transmitters that can be used by operations as controller systems and circuit inputs to affect favourable control are shown on the various feed and discharge systems. Indicating transmitters associated directly with the safe mechanical operation of the solid liquid separation decantation unit operation are also shown.

The following Table 1 lists the various unit operation controllers, systems and circuits depicted in Figure 1 , and typical application of these components in operating philosophies employed by hydrometallurgical processing plants for solids/liquid separation decantation unit operations and circuits.

Table 1

Operation Controllers

System Controller comTvpical IndicatTvpical operatDescription of ponent ing Transmitter ing philosophies data manipula¬

Feedback tion

Feed Slurry Volume/Mass Field and conManual control Operator- flow meter trol room PLC of feed slurry adjusted slurry

(Process Logic flow feed pump or

Controller) control valve feedback from output to reinstrument quired slurry

mass/ volume throughput from a PLC

Manual control Operator- of settling aid adjusted settling flow aid pump / control valve output to accommodate adjustment on feed slurry volume/ mass flow

Feed slurry Densitometer Field and conManual control Operator-

(Cont.) trol room PLC of settling aid adjusted settling feedback from flow aid pump / coninstrument trol valve output to accommodate increase or decrease in feed slurry density and flow (mass flow)

Settling Aid Volume/ Mass Field and conManual interface Operator- flow meter trol room PLC of settling aid adjusted settling feedback from flow control aid pump / coninstrument adjustment trol valve output to accommodate increase or decrease in feed slurry density

Aqueous OverVolume/ Mass Field and conManual interface Operator- flow /Wash soflow meter trol room PLC of overflow adjusted overlution for up- feedback from aqueous flow flow aqueous stream liqinstrument control adjustpump / control uid/solid separament valve output to tion decanter balance wash throughput through entire solids/liquid separation circuit. This is done primarily to maintain process water balance.

System UnderVolume/ Mass Field and conManual interface Operator-adjust flow Slurry flow meter trol room PLC of underflow underflow slurry feedback from slurry flow conpump output to instrument trol adjustment affect underflow flow control.

Densitometer Field and conManual interface Operator-adjust trol room PLC of underflow underflow slurry feedback from slurry flow conpump output to instrument trol adjustment affect underflow density control.

Unit Operation: Rake torque Field and conManual interface Operator-adjust

Rake Torque indicator trol room PLC of underflow underflow slurry feedback from slurry flow conpump output to instrument trol adjustment affect a reduction in rake torque.

Rake torque Field and conManual interface Operator-adjust indicator trol room PLC of settling aid settling aid dosfeedback from dosing control ing pump output instrument adjustment to affect a reduction in rake torque.

Rake torque Field, control Automated rake Unit operation indicator room PLC and raise-lower adlocal field PLC unit operation justment initiates rake- local field PLC raise-lower confeedback from trol

instrument

Unit Operation: Bed Mass PresField and PLC Manual interface Operator-

Bed Mass sure Transmitter feedback from of underflow adjusted underinstrument slurry flow conflow slurry

trol adjustment pump output to affect an increase or decrease in unit operation bed mass.

Unit operation: Bed Level Manual interface Operator ad¬

Bed Level Transmitter of underflow justed settling slurry flow conaid dosing pump trol adjustment output to affect an increase or decrease in bed level.

The level of operator intervention for the above control scenario is high. This often results in metal extraction operations that utilize liquid/solid decant unit operations or systems, running at un-optimized states for high percentages of total operational run time. In addition to this, the potential for the total liquid/solid decant system to reach unsustainable operating conditions, where either a unit operation or a total system needs to be taken off the line, is high. Therefore, an effective control of these criteria is key for running an efficient lixiviation process. In particular the problem underlying the present invention has been to ensure a target liquid-solid separation process underflow slurry composition and an overflow solution composition that are achieved consistently with associated maximum mass transfer of the target phases, while not impacting negatively on any of the associated liquid/solid separation process unit operations, i. e. maximizing plant availability by reducing unplanned downtime due to mechanical failure of equipment. More particularly, the present invention aims to provide a reliable control basis that can significantly reduce un-optimised operating conditions in liquid/solid decant unit operations and systems. In contrast, Figure 2 shows a simplified diagram highlighting the various controllers around a solids/liquid separator decanter control basis according to one or more embodiments of the invention. The identified unit operations being discussed are sequentially for the leach product thickener, and then for the Counter-Current Decanter 1 (CCD1), with system interaction control from the leach circuit and Counter-Current Decanter 2 (CCD2). Figure 2 shows product leach thickener underflow density control 310, which may function by controlled low grade pregnant leach solution (LG PLS) addition- primary control. The control 310 receives density information from densitometer 340 and operates the control valve 350. Product leach thickener underflow flow control 320 (initially by minimum required "linear velocity" flow to the pump VSD) intervention control by increased rake torque control (secondary) and increased bed height (tertiary) control. CCD1 flocculant dosing control 330 (leach product thickener (previous thickener underflow density gives input to the controller) flocculent pump VSD is the controller output. Flocculent pump intervention control on high rake torque and increased bed height (secondary control to underflow pump control).

Nomenclature

In order to avoid ambiguities the following control system nomenclature is defined and applicable to terminology associated with this patent application.

As used herein, "indicating transmitter" refers to any instrument or device that gives feedback either as a local display in the field or to a central Process Logic Controller (PLC) (e.g. level transmitter, flow meter, densitometer, pH meter, pressure transmitter, temperature transmitter, turbidity meter).

As used herein, "controller" refers to any device that receives an input signal either directly or indirectly from an indicating transmitter or PLC and responds accordingly by either increasing or decreasing the output of that device (e.g. pump variable speed controllers and drives, proportional output control valves).

As used herein, "unit operation" refers to an installed item of equipment with associated drives and discharge system.

As used herein, "unit operation control (UOC) " refers to a control integrally associated with any one unit operation. For example, the following list of indicating transmitters and controllers are associated with an agitated tank at unit operation level:

• Tank Level Transmitter

• Tank Temperature transmitter

• Tank Agitator Ammeter (Current transmitter)

• Tank Discharge Slurry Flow Transmitter

• Tank Discharge Slurry Densitometer

• Tank Discharge Slurry pump Variable Speed Drive

As used herein, "system" refers to a group of unit operations that are integrally associated (both from a process and control perspective).

As used herein, "system control" refers to control integrally associated with a group of associated unit operations. For example, the following list of indicating transmitters will affect a control response from the tank unit operation:

Tank Feed Slurry Flow Transmitter (better referred to as Adjacent System Tank Discharge Slurry Flow Transmitter)

• Adjacent Unit Operation Tank Feed Slurry Pump Variable Speed Drive

• Adjacent Unit Operation Tank Discharge Slurry Flow Transmitter

All the above indicating transmitters and controllers are either feedback indicators (supply information about the status of the process unit operation at any given time) or are controllers responding in a certain way to the feedback status of the process unit operation as provided by the feedback indicators.

As used herein, "circuit" refers to a collection of systems that respond to feedback indicators from associated systems. For example, the indicating level transmitter of a tank (unit operation) could be linked to an upstream feed tank discharge pump variable speed drive, causing the drive either to speed up or slow down, depending on a corresponding decrease or increase in tank (unit operation) level. Collectively the control of the two unit operations are referencing feedback indicators and implementing control outside the individual systems control. Figure 1 shows typical indicating transmitters and controllers associated with a mechanical liquid/solid decantation unit operation or system.

In one or more embodiments, as used herein, "primary control" refers to a default control for any target controllable process parameter. Primary control is generally applicable when the unit operation, system or circuit control is within defined process limit parameters, (i.e. at a state of acceptable equilibrium). Rate of change (step changes) in control parameters for primary con- trailers is generally gentle (small).

As used herein, "Intervention control: These are controllers that are only activated when the unit operation, system or circuit parameters move outside the defined process limit parameters. Intervention controllers rate of change steps are large and are designed to bring the unit operation, system or circuit control back within defined process limit parameters with the minimum delay.

Leach product thickener control

In one or more embodiments, the present invention refers to a method according to which said first Controller (CI) monitors and steers the flow and the density of the leach product thickener underflow and the dosing of the flocculants.

(al) Leach product underflow flow

In one or more embodiments, monitoring of the flow of the leach product underflow is conducted by controlling the minimum thickener underflow discharge flow. In case the slurry dis- charge flow falls below the calculated minimum to prevent solids settling out said first Controller (CI) activates a Linear Flow Intervention Controller 1 (LFIC1) that increases the wash water flow output until the minimum product thickener underflow discharge flow is above the required minimum. More precise, the control philosophy for the leach product thickener underflow flow control uses the minimum thickener underflow discharge flow, required to ensure slurry discharge linear (line) velocity is maintained above the minimum solids settling velocity (predetermined for process-specific slurry). In the event that the slurry discharge flow falls below the calculated minimum to prevent solids settling out (< minimum linear velocity), and the pump output is at maximum, or the underflow pump is stopped or tripped, a minimum linear flow intervention controller is activated. The Linear Flow Intervention Controller activates the wash water control valve to increase wash water flow output until the minimum product thickener underflow discharge flow is above the required minimum. The intervention controller remains active for as long as the pump is not available or pump output is at maximum. This ensures that in the event of the possibility of a partial or complete choke developing on the leach product thickener underflow line, there is a flush backup system to minimize the frequency of this occurring. The underflow discharge pump will increase or decrease back down to the minimum discharge flow in response to any increase in rake torque. In one embodiment, rake torque is the underflow flow intervention control activator, but two additional parameters could also be consid- ered as alternative intervention controller for underflow flow control:

• Rake height level transmitter (an increase in rake height will cause a proportional increase in underflow pump output).

• Bed mass pressure transmitter (an increase in bed mass pressure will cause a proportional increase in underflow pump output). Either of these alternatives can be used, however in a specific embodiment, a rake height level transmitter is used.

(a2) Leach product underflow density In one or more embodiments, monitoring of the density of the leach product underflow is conducted by a densitometer and an in-line control valve. In case the product thickener underflow density is less than a process specified minimum said first Controller (CI) activates a Leach Product Thickener Flocculent Dosing Controller (LPT-FDC1) that increases the output of the Leach Product Thickener Flocculent Dosing Pump (LPT-FDPl) until the product thickener underflow density increases to affect the underflow density control valve opening and the output is above the process defined minimum.

More precise, the leach product thickener underflow density is automatically controlled around a process-defined set point, by controlled addition of wash solution into the suction line of the Leach Product Thickener Underflow Discharge Pump. The mechanism of control is by way of an in-line control valve. By controlling the underflow density by wash water dilution, fluctuations in thickener underflow density can be reduced, which in turn affords opportunity for tighter flocculent dosing master control. In order to maintain target underflow pump discharge densities and to prevent unnecessary dilution of the advance slurry to the next unit operation, the underflow density controller requires leach product thickener underflow densities exiting the thickener, prior to wash solution addition, to be slightly higher than the "dilution-addition" underflow density control set point. This is achieved by including a fall-back default status for the product thickener flocculent dosing controller. In the event that the product thickener underflow density control valve output is less than a process specified minimum (either deter- mined by valve actuator position or by underflow wash flow meter), the Product Thickener Flocculent Dosing Controller will incrementally ramp up the output of the product thickener flocculent dosing pump, until the product thickener underflow density increases to affect the underflow density control valve (wash water addition valve) opening. Once the density control valve output is above the process-defined minimum, flocculent dosing control reverts back to the product thickener flocculent dosing master controller. The flocculent dosing intervention controller will also ensure that thickener upset conditions promoting sliming (ineffective solid bed formation), is also addressed, as, under these conditions, target underflow densities will not be achieved, resulting in the underflow density control valve (wash valve) remaining closed.

(a3) Dosing of the leach product flocculent In one or more embodiments, dosing of the leach product flocculent is monitored by the leach solids mass flow. In case the flocculent dosing rate per unit mass of feed solids and the input concentration of made-up flocculants determining the required flow rate of flocculent per mass flow of solids fed into the leach product thickener, deviates from the process specified condi- tions, said Controller (CI) activates said Leach Product Thickener Flocculent Dosing Controller (LPT-FDC1) that increases or decreases the output of the Leach Product Thickener Flocculent Dosing Pump (LPT-FDP1) until the set point of the feed solids mass flow is reached again.

More precise, the leach product thickener flocculent dosing master controller uses the leach feed solids mass flow as the reference for product thickener flocculent dosing pump output. The process-specific flocculent dosing rate per unit mass of feed solids and the input concentration of made-up flocculent, determine the required flow rate of flocculent per mass flow of solids fed into the leach product thickener. In the event that the leach product thickener wash water control valve output deviates from the process specified conditions, the leach product thickener flocculent dosing controller will increase or decrease the output of the flocculent dos- ing pump around the feed solids mass flow feedback set point. In the case of the leach product thickener, the reference mass flow is calculated from the leach feed volumetric flow meter and leach feed densitometer. In response to any increase in rake torque that will affect an increase in underflow pump speed a decrease of flocculent addition flow rate is included in the controller for this process condition. Due to the leach product thickener receiving all leach product slurry and no counter current-washing occurring for this unit operation, no thickener overflow control is necessary.

Counter-Current decanter control

In another embodiment of the present invention said second Controller (C2) monitors and steers the flow and the density of the counter-current decanter underflow, the dosing of the flocculants and the flow of the counter-current decanter overflow. In the following the abbreviation CCD1 is used for the counter-current decanter which is the first in a line of up to 10 separation units.

(bl) Counter-current decanter underflow flow In one or more embodiments, monitoring of the flow of the counter-current decanter underflow may be conducted by controlling the minimum counter-current decanter underflow discharge flow. In case the slurry discharge flow falls below the calculated minimum to prevent solids settling out said second Controller (C2) activates a Linear Flow Intervention Controller 1 (LFIC2) that increases the wash water flow output until the minimum counter-current decanter underflow discharge flow is above the required minimum.

More precise, the control philosophy for CCDl underflow flow control uses the minimum CCDl underflow discharge flow, required to ensure slurry discharge linear (line) velocity is maintained above the minimum solids settling velocity (predetermined for process-specific slurry). Minimum Linear Flow Intervention Control: In the event that the slurry discharge flow falls below the calculated minimum to prevent solids settling out (< minimum linear velocity), and the pump output is at maximum, or the CCDl underflow pump is stopped or tripped, a minimum linear flow intervention controller is activated. The linear flow intervention controller activates the wash water control valve to increase wash water flow output until the minimum CCDl underflow discharge flow is above the required minimum. The intervention controller remains active for as long as the pump is not available or pump output is at maximum. This ensures that in the event of a partial or complete choke developing on the CCDl underflow line, there is a flush backup system to minimize the frequency of this occurring. This could increase plant availability considerably. The CCDl underflow discharge pump will increase or decrease back to the minimum discharge flow in response to any increase in CCDl rake torque. In one or more embodiments, rake torque is the underflow flow intervention control activator, but two additional parameters can also be considered as alternative intervention controller for underflow flow control:

• Rake height level transmitter (an increase in rake height will cause a proportional increase in underflow pump output).

• Bed mass pressure transmitter (an increase in bed mass pressure will cause a proportional increase in underflow pump output).

Either of these alternatives can be used, however in a specific embodiment, a rake height level transmitter is used. (b2) Counter-current decanter underflow density

In one or more embodiments, monitoring of the density of the counter-current decanter underflow is conducted by a densitometer and an in-line control valve. In case the counter-current decanter underflow density is less than a process specified minimum said second Controller (C2) activates a Counter-Current Decanter Flocculent Dosing Controller (CCD-FDC2) that increases the output of the Counter-Current Decanter Flocculent Dosing Pump (CCD-FDP2) until the counter-current decanter underflow density increases to affect the underflow density control valve opening and the output is above the process defined minimum.

For the example illustrated in Figure 2, the CCD1 underflow density is automatically con- trolled around a process-defined set point, by controlled addition of a bleed stream of CCD2 - the second decanter in line - overflow solution (which forms the wash solution in this case), into the suction line of the CCD1 underflow discharge pump. The mechanism of control is by way of an in-line control valve. By controlling the underflow density by wash water dilution, fluctuations in thickener underflow density can be reduced, which in turn affords opportunity for tighter flocculent dosing master control. In order to maintain target underflow pump discharge densities and to prevent unnecessary dilution of the advance slurry to the next unit operation, the underflow density controller requires CCD1 underflow densities exiting the CCD, prior to wash solution addition, to be slightly higher than the "dilution addition" underflow density control set point. This is achieved by including a fall-back default status for the CCD1 flocculent dosing controller. In the event that the CCD 1 underflow density control valve output is less than a process-specified minimum (either determined by valve actuator position or by underflow wash flow meter), the CCD1 flocculent dosing controller will incrementally ramp up the output of the CCD1 flocculent dosing pump, until the CCD1 underflow density increases to affect the underflow density control valve (wash water addition valve) opening. Once the den- sity control valve output is above the process defined minimum, flocculent dosing control reverts back to the CCD1 flocculent dosing master controller. The flocculent dosing intervention controller will also ensure that counter-current decanter upset conditions promoting sliming (ineffective solid bed formation), is also addressed, as, under these conditions, target underflow densities will not be achieved, resulting in the underflow density control valve (wash valve) remaining closed. (b3) Dosing of the leach product flocculent

In one or more embodiments, dosing of the leach product flocculent is monitored by the leach thickener underflow solids mass flow. In case the flocculent dosing rate per unit mass of feed solids and the input concentration of made-up flocculents determining the required flow rate of flocculent per mass flow of solids fed into the counter-current decanter, deviates from the process specified conditions, said second Controller (C2) activates said Counter-Current Decanter Flocculent Dosing Controller (CCD-FDC1) that increases or decreases the output of the Counter-Current Decanter Flocculent Dosing Pump (CCD-FDP1) until the set point of the leach product thickener underflow solids mass flow is reached again.

More precise, the CCD 1 flocculent dosing master controller uses the leach thickener underflow solids mass flow as the reference for CCD1 flocculent dosing pump output. The process- specific flocculent dosing rate per unit mass of feed solids and the input concentration of made- up flocculent determine the required flow rate of flocculent per mass flow of solids into CCD I . In the event that the CCD1 wash water control valve output is above the process- specified minimum (ref 3.1.2), the CCD1 flocculent dosing controller will increase or decrease the output of the flocculent dosing pump around the leach product thickener underflow solids mass flow feedback. In response to any increase in rake torque, that will affect an increase in underflow pump speed a decrease of flocculent addition flow rate is included in the controller for this process condition.

(b4) Counter-current decanter overflow flow

In one or more embodiments, monitoring of the counter-current decanter overflow flow is conducted by a flow meter. In case the level of aqueous overflow in the counter-current decanter tank moves outside process-defined battery conditions, the output of the tank discharge pump (TDP) is adjusted accordingly.

Split-Circuit Technology

In another embodiment, the method is incorporated into a leaching process for the recovery of valuable metals, in particular but not limited to copper, nickel, manganese, iron and gold, using the so-called split-circuit technology that is subject to US patent US 7,799,294 (Cognis) which is incorporated by reference. The process encompasses

(a) providing a first aqueous leach pulp which comprises a mixture of leached solids and an aqueous leach solution comprising a metal, a leaching agent and water;

(b) subjecting the first aqueous leach pulp to a first solid-liquid separation to provide a first clarified aqueous leach solution and a second aqueous leach pulp, wherein the second aqueous leach pulp comprises the leached solids at a percent solids level greater than the first pulp;

(c) subjecting the first clarified aqueous leach solution to a first solvent extraction prior to any significant dilution, whereby a first aqueous raffinate is obtained;

(d) subjecting the second aqueous leach pulp to a second solid-liquid separation with dilution via an aqueous stream to obtain a second clarified aqueous leach solution; and

(e) subjecting the second clarified aqueous leach solution to a second solvent extraction whereby a second aqueous raffinate is obtained.

Figure 3 shows a flow sheet for the recovery of copper by leach product slurry solids/liquid separation. In this case the overflow reports to a high grade (HG) copper solvent extraction circuit, while the underflow is washed counter-current through a number of solids/liquid separation decantation stages, using raffinate from the low grade copper solvent extraction circuit. The number of washing stages may vary from one process circuit to the next, however the control philosophy described for each unit operation is the same for any number of washing stages.

The figure is also useful illustrating the overflow control (step b4): The CCD1 overflow flow is controlled around the low grade (LG) raffinate flow input at CCD 7. The output of the CCD 1 needle tank discharge pump is increased or decreased around the moving average flow from the CCD7 LG raffinate flow meter (wash water flow meter). The moving average ensures smoother controller step changes around the flow controller output. The piping configuration for any CCD needle tank pump discharge is such that total discharge flow is measured prior to the off-take line for the underflow bleed wash line. The CCD1 overflow includes an intervention controller linked to the CCD1 level transmitter. In the event that the level of aqueous overflow in the CCD1 needle tank moves outside process-defined battery limit high and low levels, the CCDl needle tank discharge pump output is increased or decreased accordingly. Once the CCDl needle tank level returns to within the process-defined battery limits, control reverts back to the primary CCDl overflow flow control. Similar control philosophies are prescribed for all additional downstream CCDs with the exception of the final CCD (in this example, CCD 7). The LG raffinate feed flow is rationed according to the total mass flow of leach product underflow solids feeding into CCDl .