OA17134A - Treatment of acid mine drainage - Google Patents

Abstract

A method for treating acid mine drainage includes mixing acid mine drainage and alkaline tailings from a gold recovery process. The acid mine drainage is thereby neutralized.

Description

TREATMENT OF ACID MINE DRAINAGE

FIELD OF THE INVENTION

This invention relates to acid mine drainage. In particular, this invention relates to a process for treating acid mine drainage.

BACKGROUND OF THE INVENTION

Vast quantities of acid mine drainage (AMD) are contained within the now abandoned underground mining voids on the Witwatersrand In South Africa and other locations. AMD arises from the contact of minerai sulphides, e.g. Iron sulphide or pyrite, with water and oxygen, which chemlcally generates a dilute sulphuric acid. AMD is thus typified as a low pH, corrosive aqueous substance having a high content of dissoived métal salts.

Although a host of chemical processes contrlbute to the formation of acid mine drainage, pyrite oxidation is by far the greatest contributor. A générai équation for pyrite oxidation is:

2FeS₂(s) + 7O₂(g) + 2H₂O(I) -> 2Fe²⁺(aq) + 4SO₄ ²‘(aq) + 4H⁺(aq)... (1)

The oxidation of sulphide to sulphate solubilises iron II (ferrous Iron), which is then subsequently oxidized to iron III (ferrie Iron) according to the équation:

4Fe²⁺(aq) + O2(g) + 4H⁺(aq) -> 4Fe³⁺(aq) + 2H2O(I)..........................(2)

The oxidation of sulphide to sulphate and the oxidation of ferrous Iron to ferrie Iron can either occur chemically spontaneously or it can be catalyzed by mlcroorganlsms that dérivé energy from the oxidation reaction. The ferrie iron produced can further oxtdize additional pyrite and itself form additional ferrous iron according to the réaction:

FeSî(s) + 14Fe^(aq) + 8H₂O(I) -» l5Fe²⁺(aq) + 2SO<²'(aq) + 16H*(aq)... (3)

The nett effect of these reactions is to increase the concentration of hydrogen ions in solution, thereby lowering the pH and maintaining the solubîlity of ferrie iron.

Being an acidic medium, AMD is capable .of dissolving and mobilizing other toxic métal salts found in tailings dumps, rock and reef dumps, and the underground cavities in which the AMD is formed, e.g. salts of copper, nickel, zinc, manganèse and aluminium. It will be appreciated that AMD will thus contain a variety of dissolved métal salts which would be harmful if allowed to escape to the environment.

Water levels within underground mining basins, including those in the Witwatersrand area in South Africa, hâve accumulated AMD and hâve continued to rise over the years. Levels are now very high and AMD is overflowing in certain areas, e.g. on the West Rand in South Africa, ln the large central Witwatersrand basin, AMD Is widely expected to overflow from the mining cavities in approximately the next two to four years.

It is thus an aim of this invention to provide a means of alleviating these AMD problems.

SUMMARY OFTHE INVENTION

According to the invention, there Is provided a process for treating acid mine drainage, the method including mixing acid mine drainage and alkaline tailings from a gold recovery process, thereby to neutralize the acid mine drainage.

The acid mine drainage (AMD) may thus be as hereinbefore described, i.e. arising from contact of minerai sulphides with water and oxygen, particularly in tailings dumps, waste and reef stockpiles, and underground mlning voids. The AMD typically comprises dilute sulphuric acid, and has a pH In the range 2to 6.

' 5 The AMD that Is used in the invention will thus typically be in the form of an AMD stream, e.g. an AMD stream emanating from an underground mining . void.

The alkaline tailings will also typically be jn the form of an alkaline tailings 10 stream emanating from the gold recovery process.

The alkaline tailings stream may hâve a minimum pH of at least 9.5, preferably at least 10.5, this being the operationai range in the gold recovery process for effective gold recovery.

The alkaline tailings stream may hâve a residual dissolved oxygen concentration emanating from the gold recovery process; the residul dissolved oxygen concentration may be at least 2 mg/l, preferably at least 10 mg/l, more preferably at least 14 mg/i.

The gold recovery process may Include a carbon-in-leach (CIL) circuit or a separate leach and carbon-ln-pulp (CIP) circuit and the alkaline tailings stream may thus be a CIL or CIP circuit tailings stream.

The mixing may be effected in a mixing stage, e.g. in a mixing vessel, with the . AMD stream and the alkaline tailings stream being fed continuously into the mixing stage, and a neutralized AMD stream being withdrawn continuously from the mixing stage, e.g. for further processing or disposai.

The process may include agitating the mixing stage while the AMD stream _t and the alkaline tailings stream are fed into It and the neutralized AMD stream is withdrawn from It. Agitating the mixing stage may be by mechanical means,

e.g. by means of at least one turbine blade impeller that would maintain the

I i , mixed products as a slurry. Alternatively, the mixing may be achieved by the injection of compressed air into an appropriately designed mixing vessei.

The process may include introducing an oxidising agent into the mixing stage.

The oxidising agent may be selected from air, oxygen, oxygen enriched air or hydrogen peroxide. introducing the oxidising agent into the mixing stage may also agitate the mixture of the AMD stream and the alkaline tailings stream in the mixing stage. .

ÎO The process may Include Introducing a neutralizing agent Into the mixing stage. The neutralizing agent may be an alkali, e.g. sodium hydroxide, calcium carbonate or calcium hydroxide. Preferably the neutralizing agent is calcium hydroxide (slaked lime).

i 5 The process may include destroying a cyanide content in the neutralized AMD . stream. The neutralized AMD stream may thereafter be disposed.

The process may include subjecting the neutralized AMD stream to lîquid/solid séparation or seulement, in a séparation or seulement stage, whereby a 20 predpitate formed when neutralizing the AMD stream Is co-separated or coseUled with a solids component of the alkaline tailings stream, thereby to produce a predpitate- and tailings-containing slurry and a substantially predpitate- and solids-free stream. The liquid/solid séparation stage may be a gravlty separator, e.g. a conlcal boUom thickener or a clarifier.

, The predpitate and tailings slurry may hâve a solids content of at least 40 wt%, preferably at least 50 wt%, more preferably at least 60 wt%.

The substantially precipitate-free stream may hâve a solids content of about 30 50 mg/1, preferably about 20 mg/l, more preferably about 10 mg/l and a pH of at least 8.5, preferably at least 8.0, more preferably at least 7.5.

The predpitate and tailings slurry may be disposed of to a tailings disposa! facility or sûmes dam. .

The substantially precipitate-free stream may be disposed of, e.g. to the environment or to a tailings disposai facllity or sûmes dam, or it may be reused in the goid recovery process or it may be directed to a water treatment 5 operation for further upgrading to qualities acceptable for domestic or agricultural water use. Preferably the precipitate-free stream is reused In the gold recovery process or for further upgrading for domestic or agricultural reuse.

The South African Government, led by the Department of Water Affairs t (DWA), is instituting emergency measures to neutralize AMD produced ln underground mining voids in South Africa. In the medium term it is envisaged that neutralized AMD, although saline, could be used in industry. In the short term, however, neutralized AMD Is likely simply to be discharged to the r environment. In the long term, the DWA plans to extend the AMD treatment process to generate potable water for domestic and/or agricultural use.

The conventional process for neutralizing AMD involves adding calcium . carbonate and calcium hydroxide (slaked lime) or calcium hydroxide only into

AMD and agitating the mixture in the presence of air or oxygen, thereby to precipitate the harmful dissolved métal salts as insoluble métal hydroxides or carbonates, e.g. Fe2(CO3)3· The precipitate is collected as a slurry or sludge and typlcally deposited onto existing gold plant tailings disposai facllities or slime dams. The AMD neutralization process is costly, mostiy due to the large 25 volumes involved, the mass of the neutralizing chemicals required and the ' energy cost of supplying large volumes of air. Furthermore, a relatively high percentage of the water in the treated AMD is unavailable for re-use after treatment as it is associated with the precipitate slurry and discarded therewith.

• There are a large number of gold recovery process plants operating on the

Witwatersrand. In ; carbon-in-leach (CIL) and carbon-in-pulp (CIP) gold recovery processes, cyanide Is used to leach gold from a gold-bearing slurry into solution, whereafter a gold-cyanide complex is adsorbed onto activated carbon, ln order for the gold effectively to be leached Into solution by the cyanlde, it Is necessary to increase the pH of the gold bearing slurry to approximately 10.5, e.g. with lime or sodium hydroxide, and to introduce air or oxygen into the system thereby to oxidise the reactive iron and cyanide 5 species which enable the leaching of gold into solution as well enable as the adsorption of the gold-cyanide complex onto the activated carbon.

Gold Is recovered from the loaded activated carbon by subjecting the loaded activated to an elution process wherein the gold-cyanide complex Is desorbed .10 from the activated carbon by means of a sodium hydroxide solution. The gold-bearing solution or eluate then reports to an electrowinning circuit where gold is recovered onto electrowinning cathodes, stripped from the cathodes, calcined and smelted to produce gold bullion.

.15 The gold-depleted slurry or tailings of the gold recovery process is routed to a residue or tallings section from where It Is pumped to a tailings disposai faciiity or sûmes dam. The CIL tailings hâve a high pH and a comparatively high dissolved oxygen content owlng to the process by which gold is extracted from the gold-bearing slurry. .

BRIEF DESCRIPTION OF FIGURES

The invention will now be described, by way example, with reference to the accompanying diagrammatic drawings and graphs.

ln the drawings:

FIGURE 1 is a diagram of a conventional AMD neutralization process; FIGURE 2 is a diagram of a CIL circuit of a conventional carbon-lnleach gold recovery process;

FIGURE 3 is a diagram of an AMD treatment process in accordance with a first embodiment of the invention;

FIGURE 4 is a diagram of an AMD treatment process In accordance with a second embodiment of the invention, being that of the Example, for neutralizing AMD using Mogale Gold Plant tailings;

t ' FIGURE 5 is, for the Example, a graph or plot of thickener (i.e. the modified No 3 sludge clarifier) underflow densifies ('RD') achieved during the trial;

FIGURE 6 Is, for the Example, a graph or plot of pH levels of the volumétrie ratios of Mogale Gold 1 tailings to AMD recorded during the trial;

FIGURE 7 Is, for the Example, a graph or plot of pH levels of gold plant tailings and thickener process water after tailings/AMD blending;

FIGURE 8 is, for the Example, a graph or plot of cyanide concentrations during AMD neutralization; and •10 FIGURE 9 is, for the Example, a graph or plot of gold monitored on the process water at sections of the plant.

DETAILED DESCRIPTION OF THE INVENTION

Referring to Figure 1 of the drawings, reference numéral 10 generally indicates a conventional AMD neutralization process. The process 10 comprises an aération tank or mixing tank 12, one or more thickeners or clarifiers 14, a sludge collecter 16 and a neutralized AMD water tank 18.

An AMD stream 20 having a pH of between 2 and 6 is fed into the aération tank 12 along with calcium carbonate and slaked lime (calcium hydroxide) or slaked lime only 22 as a neutrallzing agent and air or oxygen 24 as an oxidlsing agent. The lime 22 functions to neutralize the AMD stream 20 by raising the pH to between 7 and 8, while the air or oxygen 24 serves to oxidise the dissolved métal species thereby to enable subséquent précipitation reactions to take place.

At the pH of between 7 and 8, the métal cations in the AMD stream 20 react with the carbonate and/or hydroxide anions resulting from the dissolution of the lime 22 added to the AMD stream 20, and precipitate out of solution as . insoluble métal carbonates and/or hydroxides. The suspension of neutralized

AMD 21 and insoluble métal carbonates and/or hydroxides produced in the aération tank 12 is fed into one or more clarifiers 14. The clarifiers 14 are typically conical bottom liquid/solid gravity séparation vessels or thickeners.

Typically a number of clarifiers 14 are installed in parailel in order to provide sufficient résidence time in each clarifier 14 for the insoluble métal carbonates and/or hydroxides to settle out of the neutralized AMD stream 21. It will be appreciated that any number of clarifiers 14 may be arranged in parailel or sériés, depending on the volume of AMD 20 required to be treated. As the precipitate settles in the clarifiers 14, a thickened precipitate slurry of insoluble métal carbonates and/or hydroxides is produced and is removed from the bottom of the clarifiers 14 along flow line 26 to a sludge collecter 16. From the sludge collecter 16, the slurry is pumped te a tailings disposai facility or slimes dam (not shown) along flow line 28. The substantially metal-free and precipitate-free supematant liquid or overflow from the clarifiers 14 flows along flow line 30 to the neutralized AMD water tank 18. The overflow Is disposed of along flow line 32, e.g. to the environment or Is reused or is subjected te further downstream processing

Referring to Figure 2 of the drawings, reference numéral 100 generally indicates a carbon-ln-leach section of a conventional carbon-in-leach (CIL) gold recovery process. The section 100 comprises a preconditioning stage 102, a carbon-in-leach circuit 104, a cyanide destruction stage 106 and a tailings tank 110.

Gold bearing slurry 112 from a slurry préparation facility (not shown) is fed to the preconditioning stage 102. Typically two mechanically agitated preconditioning tanks (not shown) are provided in the preconditioning stage 102 and lime 114 is added to the first of those two tanks to raise the pH of the gold bearing slurry to a pH of approximately 10.5, being the optimal pH for gold dissolution. Oxygen 116 is added te both the preconditioning tanks in the preconditioning stage 102 by injecting oxygen 116 into the tanks or the slurry streams, thereby te oxidise the reactive sulphide species in the slurry 112. Pre-oxidation Is an important step in reduclng cyanide consumption and increasing gold recovery. Pre-oxidation oxidizes the reactive iron sulphide species te ferrie hydroxide species which are stable in cyanide solutions, according to the following set of chemical équations:

2FeS + 0₂ + 2H₂O «- 2Fe²⁺ +2S + 40H' (4)

4Fe²⁺ + 0₂ + 2H₂O ~ 4Fe³⁺ + 40H* (5)

Fe³⁺+ 30H* <-»> Fe(OH)₃ (6) ' 5 Pre-oxidation also transforme the sulphur species into sulphate species:

2S²' + 2O₂ + H₂O ~ S₂O₃ ²‘ + 2OH‘(7)

S₂O₃ ²' + 2OH‘ + 2O2 ~ 2SO4²' + H2O(8)

S² —> S2² —> S° —> S2O₃ ² —> SO3² —> SO4²'(9)

The pre-oxidised slurry is fed along flow line 118 to the CIL circuit 104. The CIL circuit conslsts of five or more mechanically agitated tanks (not shown). Cyanlde 120, activated granular carbon 122 and air or oxygen 124 are introduced into the CIL tanks to facilitate the leaching of gold from the gold15 bearing slurry 112 into solution, and also to facilitate the subséquent adsorption of the dissolved gold onto the activated carbon 122. Gold is leached Into solution according to the foliowing chemical réaction:

4Au + 8NaCN + O₂ + 2H₂O - 4Na[Au(CN)₂] + 4NaOH .... (10)

The CIL tanks are operated such that the gold-bearing slurry 118 passes under force of gravlty or by pumping successively from the first tank ln the circuit to the last tank in the circuit, whereas the activated carbon is retained within each Individual tank by Inter-stage screens (not shown). As the slurry 25 passes through the CIL circuit 104, gold is progressively leached from the slurry and adsorbed onto the activated carbon.

The activated carbon within the CIL circuit Is carefully managed. As the gold loading on the activated carbon Increases, the activated carbon Is pumped upstream within the CIL circuit 104, i.e. counter-current to the downward flow . of gold-bearing slurry. Loaded activated carbon Is batch pumped from the first tank of the CIL circuit 104 along flow line 126 to an elution circuit (not shown).

ln the elution circuit, a sodium hydroxide solution Is used to desorb or elute the gold from the activated carbon. Elution takes place in an elution column at elevated température and pressure, thereby completely stripplng the activated carbon of adsorbed gold. The stripped carbon Is chemically and thermally regenerated In a separate process (not shown) and retumed to the CIL circuit 104 as activated carbon 122. The gold bearing solution or eluate from the elution column is sent to an electrowinning circuit (not shown). In the electrowinning circuit, gold is recovered onto electrowinning cathodes, then subsequently stripped from the cathodes, calcined and smelted to produce gold bullion.

The now barren slurry exiting the CIL circuit 104 passes along flow line 128 to the cyanide destruction stage 106. The cyanide destruction stage 106 comprises a tank Into which the barren slurry flows and into which a cyanide destruction product 130 is Introduced, thereby to reduce the cyanide concentration of the barren slurry. Typical examples of cyanide destruction products 130 are sodium metabisulphite, hydrogen peroxide, or ferrous sulphate. The cyanide-reduced barren slurry flows along flow line 132 to the tailings tank 110, from where it is pumped along flow line 138 to a tallings disposai facility (not shown). The cyanide-reduced barren slurry has a high pH, e.g. between 9.5 and 10.5, and a dissolved oxygen concentration of, e.g. between 2 mg/l and 10 mg/l, owing to the use of lime 114 and the introduction of air 124 In the CIL circuit 104 of the process 100.

Referring to Figure 3 of the drawings, reference numéral 150 generally refers to a process for the treatment of AMD according to a first embodiment of the invention. The process 150 Is in some respects similar to the process 10 and the process 100, and unless otherwise Indicated the same reference numérale used in Figures 1 and 2 are used in Figure 3 to indicate the same or similar features.

The process 150 Is divided into two main sections: a carbon-in-ieach gold recovery section 155 and an AMD treatment section 160. The gold recovery section 155 of the process 150 comprises a preconditioning stage 102, a carbon-ln-leach circuit 104, a cyanide destruction stage 106 and a tailings tank 110, as hereinbefore described with reference to Figure 2.

The section 155 is operated as hereinbefore described with reference to the process 100 (Figure 2). The cyanide-reduced barren slurry from the cyanide destruction stage 106 flows along the flow line 132 to the tailings tank 110, from where it is Introduced to the acid mine drainage treatment section 160 along a flow line 140. The cyanide-reduced barren slurry has a pH of ; approximately 10.5 and a dissolved oxygen content of approximately 5 mg/l. 10

The AMD treatment section 160 of the process 150 comprises an aération or mixing tank 12, one or more thlckeners 14, a thickened slurry collecter 16 and a neutralized AMD water tank 18, as hereinbefore described with reference to _t Figure 1. The clarifiers 14 of the process'10 are not suitable for the heavy duty liquid/solid séparation of gold recovery process tailings, and thus the clarifiers 14 in the process 150 are converted to, or replaced by, more robust slurry thickeners 14.

The tailings stream 140 from the section 155, initially at a pH of approximately 20 10.5, is added along with AMD water 20 at a pH of about 2 te 6, to the mixing tank 12. By mixing the streams 140 and 20, the AMD stream 20 is neutralized by the tailings stream 140 to a pH of approximately 7.5. By mixing the tailings stream 140 and the AMD stream 20, the residual cyanide in the tailings stream 140 is also reduced.

The stream 140 has an inherently sufficiently high pH and a corresponding dissolved oxygen content to neutralize arid to oxidise the AMD stream 20 thereby te precipitate out the harmful metailic salts in the AMD stream 20 as Insoluble métal carbonates and/or hydroxides, without the need for additional 30 lime or oxygen (as is required in the case of the process 10 hereinbefore ^: described). It will however be appreciated that should additional lime 22 and/or air or oxygen 24 be required to neutralize and/or oxidise the AMD stream 20 in order to aid in the précipitation of the métal species therefrom, the mass or volume of the lime 22 and/or air or oxygen 24 will be greatly reduced owing to the favourable chemical properties of the tailings stream 140 derived from the section 155.

The mixture of gold recovery process tailings 140, neutralized AMD and insoluble métal carbonates and/or hydroxldes produced in the mixing tank 12 Is fed Into one or more thickeners 14 along flow line 21. The thickeners 14 are conlcal bottom gravity séparation vessels suitable for settling solids from a gold recovery process slurry. Typically the thickeners 14 are Instalied In paraliei in order to provide sufficient résidence time for the insoluble métal carbonates and/or hydroxides to settle out of the neutralized AMD stream 21. It will be appreciated that any number of thickeners 14 may be arranged in paraliei, depending on the volume of AMD 20 requlred to be treated. As the solids settle In the thickeners 14, a thickened precipitate slurry of barren gold recovery process tailings and Insoluble métal carbonates and/or hydroxldes Is produced and is removed from the bottom of the thickeners 14 along flow line 26 to a thickened slurry collector 16. From the thickened slurry collector 16, the slurry Is pumped to a tailings disposai facillty or slimes dam (not shown) along flow line 28. The substantially metal-free and solids-free supernatant liquid or overflow from the thickeners 14 flows along flow line 30 to the neutralized AMD water tank 18. As the overflow Is suitable for general industrial use, e.g. for use in a metallurgicai process such as the section 155, the overflow Is recycled as utility water along the flow line 32. It may be feasible to discharge the overflow to the environment provided that water effluent quality régulations are complied with, e.g. if the residual cyanide content therein is sufficiently low for safe disposai.

In another embodiment of the invention (not shown), air or oxygen 24 and/or a cyanide destroying product 130 may be added to the mixing tank 12 of the section 160, thereby to destroy the residual cyanide In the neutralized AMD stream. It is to be appreciated that the low pH of the AMD stream and its ferrous sulphate would hâve already significantly reduced the cyanide content of the stream as both the lower pH and ferrous sulphate content of the AMD stream serve as effective methods for cyanide destruction. The implication here Is that the need for upstream cyanide destruction products 130 to vessel

106 or to stage 160 are significantly reduced by the effective cyanide destruction capability of the AMD.

The Inventors believe that integrating the processes 10 and 100 to create the process 150, as illustrated, has the following synergies and will yield the following surprising benefits over the conventional AMD treatment processes of the art:

1. Reagent cost, particulariy lime cost, associated with neutralizing AMD will be drastically reduced or avoided by virtue of the properties of the tailings stream which renders it suitable to be used as a neutralizing agent in the treatment of AMD;

2. The mass or volume of oxygen/air required to be injected to promote the oxidation of métal species in the AMD will be drastically reduced or avoided by virtue of the properties of the tailings stream which render It suitable to be used as an oxidizing agent in the treatment of AMD;

3. Intimate mixing can be achieved between the tailings stream and acid mine drainage stream, leading to Improved pH control of the neutralized AMD stream, with a consequential réduction In pipeline scale formation;

4. Precipitating the métal carbonate and/or hydroxide precipitate from the AMD stream with the solids from the gold recovery process tailings stream, results In one slurry stream being generated for disposai, thereby avolding duplicate process steps and maximizing the availability of neutralized water;

5. Mixing of AMD with the gold plant tailings stream will contribute to the destruction of residual cyanide, thereby either mitigating the need for or reducing the reagent requirement of a separate cyanide destruction stage In the gold recovery process, with associated cost benefits;

6. The additional of air/oxygen to the neutralized AMD would virtually destroy ail residual cyanide to levels which would permit the discharge of the neutralized AMD stream directly to the environment or other disposai facility without the need to further downstream processing.

il

EXAMPLE

Introduction . 5 The neutralisation of acid mine drainage (AMD) water by mixing with Mogale Gold 1 plant tailings slurry (also known as residues), in accordance with the invention, was Investigated in a pilot plant triai. The pilot plant trial was carried out in a process 200 (see Figure 4) for the treatment of AMD, according to a second embodiment of the invention.

Parts of the 200 which are the same or similar to those of the process 150 of Figure 3, are indicated with the same reference numerals.

The process 200 Indudes a milling stage or plant 202, with a milled ore transfer line 204 leading from the stage 202 to the preconditioning stage 102.

The process 200 thus indudes the preconditioning stage 102, and also indudes the CIL circuit or cascade 104.

The process 200 further indudes an elution circuit 206, with a ioaded activated carbon line 208 leading from the CIL cascade 104 to the elution circuit 206, and a regenerated activated carbon line 210 leading from the elution circuit 206 to the CiL cascade 104.

The process 200 aiso Indudes the cyanide destruction stage 106, and an AMD neutrailzation stage 212, with the flow line 132 from the cyanide destruction stage leading directly to the AMD neutralization stage 212.

A raw AMD feed line 214 ieads Into the stage 212, whiie a neutralized water 30 withdrawai line 216 Ieads from the stage 212. A barren slurry withdrawai line . 218 Ieads from the stage 212.

A 560 m³ redundant tank, convenlently located adjacent to the Mogale Water Treatment piant, was converted to serve as a mix tank for the blending of

Mogale Gold 1 tailings with incomlng AMD water. The mix tank thus forms part of the AMD neutralization stage 212.

The Mogale Gold 1 residue line 132 was engineered to divert tailings feed, 5 when required, to the mix tank. A pipeline 214 containing AMD extracted from the southern compartiment of the West Wits Pit was modified to supply AMD to the mix tank when needed.

The mix tank was equipped with an agitator (not shown) to affect the blend of 10 tailings and AMD. The tank or vessel was fitted with a pump (not shown) to deliver the mixed slurry to a modified Mogale Water Plant No 3 clarifier (not shown) to serve as a slurry thlckener and to effect the required liquid/solid séparation.

The No 3 clarifier was modified to serve as a thlckener to accept the slurry and provide a means of Iiquid solid séparation. The drive of the clarifier was upgraded to maximum capability to affect the raking of settled solids to the underflow discharge port. The supematant overfiowed and joined the downstream process water system of the Mogale Gold 1 operation.

The existing Water Treatment Plant No 3 clarifier was thus modified for the purpose of the trial. Portions of Mogale Gold 1 taliings were diverted to a mix tank and blended with Incoming South Pit AMD water before directing the mixed stream to the modified No 3 clarifier, now serving as a slurry thickener, 25 for Iiquid solid séparation.

' The trial was run over the period 21 August 2012 to 4 September 2012. Mechanical failures periodically interrupted the trial.

Procedure

- During the trial, certain parameters were measured to assist with assessing the overall effectiveness of the neutralisation of AMD with tailings. These are described in Table 1 below:

Table 1 : AMD Neutralisation with Tailings: Sampling Points

SAMPLING POINT	PARAMETERS MEASURED
Relative Density	PH	Free Cyanide PPm	WAD Cyanide PPm	Gold PPm	Iron PPm
CIL No.5 Overflow	Tank	J
Tailings Tank	Transfer		λ/ ·	λ/		V
Launder Thickener	Feeding			•	•	•
Thickener Underflow port	λ|		•	•	•	•
Thickener Launder	Overflow		λ/	J	V	J	λ/

Further explanation of the Table 1 sampling points is as follows:

• Carbon-ln-Leach (CIL 5) Tank Overflow: CIL 5 represents the final stage of the Mogale Gold 1 plant leach circuit prior to discharge to the residue (tailings) disposai section.

• Tailings Transfer Tank: This sample was extracted from the residue transfer pump suction valve. This sampling point is therefore after the pulp has been contacted with 17 Winze AMD water used to detoxify cyanide in the slurry.

• Launder feeding Thickener (i.e. the modified No 3 clarifier): This sampling point is after the AMD mix tank at the water treatment plant and is therefore after AMD has been introduced from the West Wits Pit.

• Thickener Underflow port: Taken from the valve transferring underflow solîds into the chamber undemeath the thickener.

• Thickener Overflow: Taken from the thickener (i.e. modified No 3 clarifier) overflow launder transferring process water to the process water circuit of the Mogale Gold 1 operation.

Results and Discussion

Tables 2 and 3 record al! relevant information pertaining to the pilot trial of blending AMD with Mogale Gold 1 plant tailings.

Table 2 - Summary of ali the parameters measured, monitored and analysed during the trial.

Date	RD	CL 5 pH	Free CM	RD	Residue pH	Free CM	Feed-to Clarifier	Underflow U/FRD	Overftow	DD Science Analysis
RD	PH	PH	Plant Free CN ppm	FreeCN PPm	WADCM	Fe PPm	Au ppm
21-Aug	1.410	11.1	194	1.340	10.0	33	1.160	7.0	1.380	7.0	4.0
22-Aug	1.390	10.6	238	1.310	9.5	23	1.170	8.3	1.420	8.7	4.0	1.0	4.6	1.9	0.013
23-Aug	1.406	10.3	177	1.340	8.5	17	1250	7.0	1.640	7.9	6.0
24-Aug	1.380	10.1	225	1.310	9.5	33	1.130	8.6	1.420	9.0	4.0	<0.5	4.3	4	0.011
30-Aug	1.400	10.0	207	1.330	9.6	30	1.180	7.1	1.370	7.6	4.5
31-Aug	1.415	10.0	227	1280	8.7	15	1.120	7.9	1.360	7.6	4.0
1-Sep	1.410	10.3	221	1.280	9.1	16	1.070	7.9	1.300	8.0	5.0
4-Sep	1.400	10.1	212	1.300	9.6	21	1.108	7.3	1.300	6.8	5.0
Avg	1.401	10.3	213	1-311	9.3	24	1.150	7.6	1.400	7£	4.6	1Ό	4.5	3.0	0.012

Table 3 - Tailings and AMD flow rates based on total tonnage and ratios achieved

Date	CIL No. 5	TailingsfResidue	17Winze AMD	AMD Mix Tank Feed	Feed to clarifier	South Pit	AMD
Tons tfday	RD	V»luny m3/day	RD	V*lurry m3/day	Vad m3/day	RD	Vdurry m3/day	RD	V«lurfy m3/day	Vnd m3/day	Vmd Total	Ratio
21-Aug	3410	1.410	5237	1.340	6315	1078	1.340	6315	1.160	13419	7104	8182	1.6
22-Aug	3360	1.387	5467	1.310	6824	1358	1.310	6824	1.170	12444	5620	6978	1.3
23-Aug	2340	1.406	3629	1.340	4333	704	1.340	4333	1.250	5893	1560	2264	0.6
24-Aug	1620	1.381	2677	1.300	3400	723	1.3Ô0	3400	1.130	7846	4446	5169 '	1.9
30-Aug	1410	1.406	2187	1.340	2611	424	1.340	2611	1.180	4932	2321	2745	1.3
31-Aug	1660	1.415	2519	1.290	3604	1086	1.290	3604	1.120	8710	5106	6191	2.5
1-Sep	1860	1.410	2856	1.300	3904	1047	1.300	3904	1.070	16730	12826	13874	4.9
4-Sep	1410	1.410	2165	1.280	3171	1005	1.280	3171	1.108	8220	5050	6055	2.8

With reference to these Tables, the following comments are made:

Operatîonal Performance

The modifications made to the No 3 water. treatment plant sludge clarifier to serve as a fully on line thickener handling a tailings/AMD slurry mix were sufficiently successful for purposes of the pilot trial.

The trial ran initially for a four day period (21 August - 24 August 2012). At that point the thickener rakes became fully bogged down and the clarifier underflow chamber flooded. The trial recommenced six days later on 30 August 2012 and suffered a further two day stoppage before the trial was finally discontinued on 4 September 2012.

Despite the troubiesome operation, the trial still generated information on several key operating parameters as described hereunder.

The trial unfortunately also revealed that the existing water plant sludge clarifiers (three units) could not effectively be converted to serve as thickeners to affect liquid/solid séparation on a feed of run-of-mine tailings. As an alternative, the upstream cycloning of tailings prior to the overflow feeding the modified clarifiers Is being considered.

For the record, Figure 5 plots the average thickener underflow densities of the converted No 3 clarifier. Here it can be seen that the preferred higher underflow densities (which minimise the discharge of water in tailings) could not be achieved. This was pureiy a mechanical constraint attributed to the modified clarifier and not caused by any inhérent settling difficulty within the tailings/AMD blend.

Importantly, the clarities of the thickener-overflow were consistently good throughout the trial. This supports the benefit of settling neutralised AMD sludge with plant tailings.

AMD/Talllngs Mixing Ratios

An important parameter required from the pilot trial was the extent of the neutralising capability of Mogale Gold 1 plant tailings and, more specifically, the quantifies of neutralised AMD that can be generated from blending AMD with the plant tailings. The AMD considered In this Instance was the AMD extracted from both the 17 Winze and that AMD extracted from the southem section of the West Wits Pit. Both these water sources are directly connected to the Western Basin underground mining void and both hâve similar 10 characteristics Including a comparatively high pH (approx pH5 - pH6) arlslng from the ongoing déposition of plant tailings into the West Wits Pit.

No volume measuring instrumentation was available for the pilot trial. As a conséquence, the data was Inferred from the relative densifies of slurry 15 measured at the following points:

> Density of slurry exitlng the last stage of CIL 5 and prior to the addition of 17 Winze water for cyanide destruction > Density of slurry exiting the plant residue section, i.e. after 17 Winze water addition but preceding South West Wits Pit water addition > Density of slurry feeding the thickener (converted No 3 clarifier), i.e. after the addition of South Pit water in the mix tank.

The determined information from the trial Is summarised in Figure 6. The recorded ratio is based on the volumétrie quantity of plant tailings that 25 neutralises AMD arislng from 17 Winze and the West Wits South Pit.

It was noted that the ratios developed were somewhat erratic. These relate more to the methods of estimating volumétrie flows. Table 4 below summarises the average quantifies of tailings and AMD recorded during the trial. This table reflects that, on average, one m³ of tailings will neutralise about 2 m³ of a combination of 17 Winze and West Wits Pit water. Should the entire Mogale Gold 1 tailings stream (estimated at 7800 m³ per day based on 5 000 t/day) be used, then an estimated 14-16 megaliters/day of the AMD would be neutralised. Over 40 megaliters/day of neutralised AMD is potentially likely to be generated when both Mogale Gold 1 and Mogale Gold 2 plants are on line. These amounts will vary though depending on the levels of acidity of AMD extracted from the void. '

Table 4: Average quantifies of 17 Wlnze and West Wits Pit AMD neutralised during the trial period

	Date	CIL No. 5	AMD	Ratio
		V slurry m’/day A	Vnmo Total m3/day B	B/A
	21-Aug	5237	8182	1.6
	22-Aug	5467	6978	1.3
	23-Aug	3629	2264	0.6
	24-Aug	2677	5169	1.9
	30-Aug	21B7	2745	1.3
	31-Aug	2519	6191	2.5
z i	1-Sep	2656	13874	4.9
	4-Sep	2165	6055	2.6
	Average	3342	6432 '	2.1

pH levels

The pH levels recorded during the various stages of neutralisation of AMD with Mogale Gold 1 tailings are reported in Table 2. Figure 7 reflects the initial pH of tailings, the intermediate pH after the addition of 17 Winze AMD and 15 then the final pH of the thickener overflow, the latter being représentative of the final tailings/AMD blend. Not recorded In the graph is the initial pH of the AMD from the 17 Winze and the West Wits Pit. These values remained - consistently between pH 5 - pH6.

The important parameter In this Instance is the final pH of the thickener overflow. The target pH was pH 7.5 - pH 8.0, that being the idéal range for process water génération and for discharge to the environment if so required.

, At these pH ieveis, heavy métal salts would hâve precipitated and most of the residual cyanide destroyed. Lower pH levels would resuit in Incomplète précipitation, higher levels would resuit in higher residual cyanide levels and cause unnecessary scaling in process water tanks and piping.

Figure 7 indicates that pH 7 - 9 was achieved. This Is regarded as satisfactory for the limited controls that were présent during the trial and indicative that improved control would be possible with more careful design of the circuit.

The mixing of AMD with gold plant tailings will resuit in a loss of acid neutralislng capability of the tailings stream that finally reports to the West Wits Plt tailings disposai facllity (TDF). The extent of this réduction and the potential Impact from the TDF to the environment was not assessed for the plant trial. This work, known as a waste characterisation assessment, wifi be necessary to complété the overail assessment of the tallings/AMD process. This latter program should however be extended to include a leach test on the settled tailings such as that method undertaken by the Council of Geoscience. The presence of thoroughly mixed settled sludge within the tailings Is likely to retard the transmissivity of solutions and also reduce unwanted oxidation due to the very fine gelatinous sludge occupying the voids of the tailings particles, both of which could reduce impacts to the environment from a TDF.

Free and WAD Cyanide

Cyanide is a key reagent within the goid recovery operation. Ali practices involvlng cyanide are governed by a Code of Practice. The Mogale Gold code is based on both International and South African Chamber of Mines Guidelines for Cyanide Management.

Fortunately, AMD was found to serve as a detoxifying agent for cyanide solutions and slurries, the more AMD used, the greater the cyanide decay within the solution or slurry. Prior to givlng considération to the current pilot trial, this practice was always undertaken on Mogale Gold. A target level of below 20ppm Weak Acid Dissoclate (WAD) cyanide, measured as NaCN (100%), Is required within Mogale Gold tailings, that being the standard for any backfill operation. This level has always been achieved by the addition of 17 Winze AMD at the residue section and then with the subséquent discharge onto a tailings disposai faciiity, i.e. the West Wits Pit.

Figure 8 tracks the levels of free cyanide throughout the trial. The final measured free cyanide levels were measured at the plant to be in the région of 4 - 6 ppm free cyanide measured as NaCN (100%). These free cyanide ' levels were generally confirmed by separate samples submltted to an accredited laboratory, DD Science Laboratories cc Environmental Monitoring.

Whereas the measurement of free cyanide is determined by simple titration, the measurement of WAD cyanide represents the standard by which cyanide levels must be measured. WAD cyanide considers both free cyanide and cyanide salts that hâve the potential to reiease cyanide toxins. As a 15 conséquence, the WAD cyanide content of thickener overflow water was . measured on two occasions during the trial and was determined by DD Science Laboratories cc Environmental Monitoring to be 4.3 and 4.6 ppm NaCN (100%) WAD.

Whereas 20ppm WAD cyanide would be permitted for backfill operations, the standard for reiease to the environment is currently considerably less than 1 ppm WAD. This level was not achieved during the trial. Oxygénation of the slurry is expected to lower the cyanide levels appreciably. Alternatively, other cyanide destruction products such as sulphur dioxide (SO₂) which exlsts in various reagent forms, e.g. sodium meta bisulphite (Na₂S₂0s), can be added, but noting that reduced quantifies would be required as the majority of cyanide destruction has already been achieved by the addition of AMD.

Gold Content f

‘ Two samples of thickener overflow were analysed for gold content during the trial. The gold content was determined to be 0.011 and 0.013 gAu/t. These values are similar to the daily recorded gold content of soluble gold in the gold : plant tailings stream.

However, before and during the trial, the gold content of overall Mogale Gold process water was measured after blending of the thickener water with the general process water stream. The sampling point chosen was the discharge from the Rock Mill silos which are located 6km from the blending point. The data is presented ln Figure 9.

It is apparent that there was an increase in the quantity of dissolved gold during the trial. It cannot be conclusively stated that this was a resuit of the trial. However any effort to further reduce the cyanide content should asslst with preventing unwanted gold dissolution outside of the CIL leach circuit.

Iron Content

The Iron (total Fe) content of 17 Winze and West Wits Pit extracted AMD is currently 390ppm total Fe. Two thickener overfiow samples were taken during the pilot trial after blending tailings with AMD. The Fe contents were measured to be 1.9ppm and 4.0ppm thereby reflecting almost complété précipitation of the métal.

Conclusions

The following was determined from the pilot plant trial of neutralising AMD with Mogale Gold 1 plant tailings:

(i) The trial revealed that the Mogale Plant water treatment clarifiers cannot be effectively modified to serve as a thickener for the seulement of run-of-mine tailings blended with AMD. Alternative liquid/solid séparation equipment/methods are required.

(ii) Despite the erratic operation of the thickener, relevant information on selected operating parameters was still achieved.

(iii) Good seulement of the mixture of mixed tailings and AMD was achieved using the existing plant flocculant. The clarities of the supernatant (thickener overfiow) were good.

(iv) The trial revealed that one part volume of gold plant tailings Is sufficient to neutralise two part volumes of AMD extracted either from the

Western Basin Underground mining void either at the 17 Winze or West Wits Pit extraction points. This implies that Mogale Gold tailings has the potential to neutralise 14 - 16megaliters per day of AMD when Mogale Gold 1 only is on line and over 40megaliters per day of AMD 5 when both Mogale Gold 1 and Mogale Gold 2 plants are on line.

These amounts will vary depending on the levels of acidity of AMD extracted from the void.

(v) The pH of the resulting slurries, after blending tailings with AMD, varied between pH7 - pH9. This is comparable to existing high density 10 sludge AMD treatment plants using lime as a neutralising agent.

i (vi) The mixing of AMD with gold plant tailings will resuit In a loss of neutralising capability of the tailings. The presence of thoroughly mixed settled sludge within the tailings is likeiy to retard the transmissivity of solutions and also reduce unwanted oxidation due to 15 the very fine gelatinous sludge occupying the voids of the tailings ; particles, both of which could reduce impacts to the environment from a

TDF.

(vii) WAD cyanide was reduced to approxlmately 4ppm NaCN in the mixing progress. This is well within the permissible level for tailings 20 déposition. However this level must be reduced further before discharge of solutions to the environment. Oxygénation or sulpbur dioxide (e.g. using sodium metabisulphite) tests are proposed to achieve the required levels.

(viii) The dissoived gold content of Mogale Gold process water appeared to 25 increase during the trial. This parameter should continue to be monitored during subséquent trials. The value is likeiy to be reduced with Improved cyanide destruction.

Summary

The trial revealed that effective seulement (and hence liquid/solid séparation) of the mixed slurry can be achieved. Clarities of the supernatant were good.

It was determined that one part by volume of gold plant tailings can neutralise two parts by volume of current AMD extracted from the Western Basin , underground mining void at the 17 Winze and West Wits Pit extraction points.

This extrapolâtes to an AMD neutralising capability of roughly 15megaliters per day If the Mogaie Gold 1 plant opérâtes at full capacity. The Intended recommissloning of the Mogale Goid 2 operation will provide the potentlal for

Mogale Gold to neutralise over 40 megaliters per day of AMD from the void.

c

The pH of the AMD/tailings slurry mix was maintained at satisfactory leveis of between pH7-pH9.

Weak Acid Dissociable (WAD) cyanide levels of the résultant solutions were reduced to below 4ppm in the process of mixing. This level Is well within Tailings Disposai Faciiity déposition requirements. However the cyanide level will need to be further reduced for any discharge of solutions to the environment. This Is likely to be achieved by oxygen Injection say to the mix 15 tank. This aspect was not explored during the trial.

i· *

The resuiting loss in the neutralising capacity of tailings following the contact with AMD must still be pursued. Spécifie waste characterlsation testwork Is recommended ln this regard.

The overall trial has provided an improved level of confidence in the proposed AMD neutralisation with gold plant tailings, sufficientiy to allow for the commercial aspects of the process to be more thoroughly pursued, albeit with due regard to the outstanding requirements mentioned in this report.

.

The Inventors hâve thus surprisingly discovered a method or process for treating acid mine drainage that makes use of the bénéficiai chemical . properties of a gold recovery process tailings stream.

Claims (12)

1. A method for treating acid mine drainage, the method Including mixing acid mine drainage and alkaline tailings from a gold recovery process, thereby to neutralize the acid mine drainage.

2. The method according to Claim 1, wherein the acid mine drainage ('AMD') Is In the form of an AMD stream, while the alkaline tailings are In the form of an alkaline tailings stream emanating from the gold recovery process. '

3. The method according to Claim 2, wherein the alkaline tailings stream has a pH of at least 9.5, and a résiduel dissolved oxygen concentration emanating from the gold recovery process.

4. The method according to Claim 2 or Claim 3, wherein the mixing Is effected in a mixing stage, with the AMD stream and the alkaline tailings stream being fed continuously Into the mixing stage, and a neutralized AMD stream being withdrawn continuously from the mixing stage.

5. The method according to Claim 4, which Includes agitating the mixing stage while the AMD stream and the alkaline tailings stream are fed into it and the neutralized AMD stream is withdrawn from It.

6. The method according to Claim 3 or Claim 4, which Includes introducing an oxidislng agent Into the mixing stage.

7. The method according to any one of Clalms 4 to 6 inclusive, which Includes Introducing a neutralizing agent Into the mixing stage.

8. The method according to Claim 7, wherein the neutralizing agent Is selected from the group consisting in sodium hydroxide, calcium carbonate or calcium hydroxide.

9. The method according to any one of Claims 4 to 8 inclusive, which Includes destroying a cyanide content in the neutralized AMD stream, and thereafter disposing the neutralized AMD stream.

10. The method according to any one of Claims 4 to 9 inclusive, which Includes subjecting the neutralized AMD stream to liquid/solid séparation, in a séparation stage, whereby a precipitate formed when neutralizing the AMD stream is co-separated with a solids component of the

10 alkaiine tailings stream, thereby to produce a precipitate- and tailingscontalning slurry and a substantially precipitate- and solids-free stream.

11. The method according to Claim 10, wherein the precipitate- and tailings-containing slurry has a solids content of at least 40 wt%.

12. The method according to Claim 10 or Claim 11, wherein the substantially precipitate- and solids-free stream has a solids content of about 50 mg/l, and a pH of at least 8.5.