MIRAMAR MINING CORPORATION

Transcription

1 MIRAMAR MINING CORPORATION INTERIM REPORT DORIS NORTH GOLD PROJECT: INVESTIGATION INTO THE USE OF CARO S ACID FOR CYANIDE DESTRUCTION Submitted by: Bateman Minerals Limited Bartlett Road, Boksburg Republic of South Africa P.O. Box 25937, East Rand, h September 2003

2 MIRAMAR MINING CORPORATION BATEMAN MINERALS (PTY) LIMITED DORIS NORTH GOLD PROJECT REPORT NO. M TABLE OF CONTENTS 1 EXECUTIVE SUMMARY 4 2 INTRODUCTION 6 3 CYANIDE DESTRUCTION PROCESSES FERROUS SULPHATE ALKALINE CHLORINATION HYDROGEN PEROXIDE SULPHUR DIOXIDE INCO PROCESS NORANDA CARO S ACID (EFFLOX PROCESS) SUMMARY OF DESTRUCTION OPTIONS PROCESS SELECTION 14 4 REVIEW OF CYANIDE DESTRUCTION TESTWORK INTRODUCTION TEST PROCEDURES TESTWORK RESULTS CARO S ACID SIGHTER TESTS BULK SAMPLE CARO S ACID TEST CYANIDE AND CHEMICAL SPECIATION RESULTS CONCLUDING COMMENTS ON TESTWORK 21 5 CYANIDE DETOXIFICATION CIRCUIT PROCESS DESIGN PROCESS DESIGN CRITERIA PROJECTED REAGENT CONSUMPTIONS PROPOSED DORIS NORTH PROCESS DESCRIPTION OPERATING PHILOSOPHY 25 APPENDICES Appendix A BLOCK PROCESS FLOWSHEET Appendix B DRAFT AMMTEC REPORT NO. (CYANIDE DETOXIFICATION SECTION) Revision A 26 September of 28

3 MIRAMAR MINING CORPORATION BATEMAN MINERALS (PTY) LIMITED DORIS NORTH GOLD PROJECT REPORT NO. M DISCLAIMER This report (Investigation into the Use of Caro s Acid for Cyanide Destruction) has been prepared for Miramar Mining Corporation upon written instruction by their environmental consultant, AMEC Vancouver, to assist with the submission of the Environmental Impact Statement for the Doris North Gold Project. Bateman followed standard professional procedures in preparing the report, the contents of which is based in part on data, information and assumptions provided by Miramar Mining Corporation. Save as expressly set out in the report, Bateman did not attempt to verify the accuracy or sufficiency of such data, information and assumptions and Bateman does not warrant or guarantee the correctness of such data, information assumptions nor any findings, observations and conclusions based upon such information data and assumptions. This report has been prepared for the sole and exclusive use of Miramar Mining Corporation and Bateman accepts no liability whatsoever, to any other organisation or person to whom this report is presented for any loss or damage arising from the use, reliance upon or the interpretation of this report or for any design, engineering or other work performed by others using this report. Revision A 26 September of 28

4 MIRAMAR MINING CORPORATION BATEMAN MINERALS (PTY) LIMITED DORIS NORTH GOLD PROJECT REPORT NO. M EXECUTIVE SUMMARY Bateman Minerals (Pty) Limited (Bateman) was commissioned by Miramar Mining Corporation (Miramar) to devise and co-ordinate an appropriate testwork programme for Doris North ore, and to interpret the results for incorporation into a process plant re-design exercise to follow later in An important part of the programme was aimed at validating the use of Caro s Acid as the preferred cyanide detoxification method at Doris North in light of difficulties encountered during the prefeasibility study. One area of process design that was not finalised to an acceptable level in the 2002 testwork programme was cyanide detoxification, due to complications arising from the use of intensive cyanide leaching conditions which generated byproducts that were difficult to remove in cyanide deoxification. The process design was changed from one featuring intensive leaching of gravity/flotation concentrate followed by filtration and direct electrowinning, to one of CIL treatment of the concentrate at more conventional cyanide levels. However, there was insufficient time to undertake cyanide detoxification testwork on the lower cyanide leach product, hence the initiation of the new test programme in The 2003 programme incorporated the carbon-in-leach (CIL) cyanide treatment of gravity/flotation concentrates at more conventional cyanide levels (750 to 1500 ppm NaCN) followed by Caro s Acid detoxification and recombination with flotation tailings for final discharge. Testwork was completed satisfactorily with very low WAD cyanide, thiocyanate and total cyanide levels being achieved in final effluent. In particular, the target of less than 0.5 ppm WAD cyanide was achieved comfortably with only modest use of Caro s Acid addition and without the need for copper sulphate to precipitate iron cyanides. Total cyanide (WAD plus iron cyanides) was also well within the desired 1 ppm upper limit. The table below summarises cyanide speciation for cyanide leach solution (detox feed), cyanide detoxified solution, blended tails solution (after mixing with flotation tailings in prescribed proportions), and blended tails after one month of aging. ANALYSIS Cyanide Leach Soln Cyanide Detoxified Soln Blended Effluent Blended Effluent [HS9467] [MH3051] (aged 1 month) CN(free), ppm 120 <5 <5 <5 CN(wad), ppm CN(total), ppm CNO, ppm SCN, ppm < Worthy of note is the 70% drop in cyanate levels in only one month, suggesting that long term buildup should not be an issue. Comment on levels of metal cations and other potentially deleterious components of the blended talings lies outside Bateman s scope of work and will be addressed in a separate report by Miramar s environmental advisor AMEC Vancouver. Revision A 26 September of 28

5 MIRAMAR MINING CORPORATION BATEMAN MINERALS (PTY) LIMITED DORIS NORTH GOLD PROJECT REPORT NO. M This interim report will be incorporated into a larger report to follow early next year, which will encompass the complete optimisation testwork programme and the Doris North process plant revalidation exercise. To assist with preparation of the final EIS submission, the current report has met the following objectives: an interpretation of the Caros Acid cyanide destruction testwork performed on cyanide leach residue pulps produced from a bulk sample of Doris North ore; a description of the Caro s Acid process, including chemistry, past experience and performance at other operations where Caro s Acid has been used in a similar application; the rationale and history behind the decision to select Caro s Acid as the preferred cyanide detoxifcation process for the Doris North Gold, and a discussion on other alternatives considered and rejected; a description of how the Caro s Acid detoxification process will be incorporated into the Doris North flowsheet, including operating philosophy, expected reagent consumptions and key process criteria. Overall, Bateman consider the use of Caro s Acid for cyanide detoxification at Doris North as safe from an occupational health and safety point of view and the process is considered robust due to its inherent design simplicity and very high turn up/turn down ability for reagent dosage. It has also been demonstrated that it easily achieves the required cyanide discharge limits for minimal impact on the environment. Revision A 26 September of 28

6 MIRAMAR MINING CORPORATION BATEMAN MINERALS (PTY) LIMITED DORIS NORTH GOLD PROJECT REPORT NO. M INTRODUCTION Miramar Mining Corporation procured the services of Bateman Minerals (Pty) Ltd to devise and coordinate a metallurgical testwork programme for a bulk sample of ore from the Doris North Gold Project. The 2003 optimisation testwork programme followed on from the 2002 feasibility study programme which left a number of issues unresolved, and hence aimed at establishing optimal process conditions for major design parameters in the process plant. All testwork was undertaken at Ammtec Limited s laboratories in Perth, Western Australia. One area in particular area of process design that was not finalised to an acceptable level in the 2002 testwork programme was cyanide detoxification. Throughout the course of the 2002 feasibility study, the intent was to generate a gravity/flotation concentrate consisting of around 10% of new feed mass, and to treat this concentrate with cyanide solution under intense concentration conditions of 2% NaCN. After recovering soluble gold from the leach pulp using filtration and electrowinning, the leach residue and a bleed solution stream would be detoxified with Caro s Acid then the discharge blended with benign flotation tailings pulp for final discharge to the tails storage pond. Target cyanide discharge limits imposed were 1 ppm total cyanide and 0.5 ppm WAD (weak acid dissociable) cyanide, which includes free cyanide. Acceptable leach recoveries were achieved within comfortable residence times. Unfortunately, cyanide detoxification using Caro s Acid (a stoichiometric mix of sulphuric acid and hydrogen peroxide) proved onerous due to excessive amounts of deleterious byproducts such as iron cyanides and thiosulphate being generated. Despite dosing large quantities of Caro s Acid to the tailings pulp, it was not possible to achieve acceptably low cyanide discharge levels in tailings. The 2003 testwork programme incorporated a revised design featuring carbon-in-leach (CIL) cyanide treatment of gravity/flotation concentrates at more conventional cyanide levels (750 to 1500 ppm NaCN) followed by Caro s Acid detoxification and recombination with flotation tailings for final discharge. This interim report will be incorporated into a larger report to follow early next year, which will encompass the complete optimisation testwork programme and the Doris North process plant revalidation exercise. To assist with preparation of the final EIS submission, the current report has the following objectives: provide an interpretation of the Caros Acid cyanide destruction testwork performed on cyanide leach residue pulps produced from a bulk sample of Doris North ore; provide a description of the Caro s Acid process, including chemistry, past experience and performance at other operations where Caro s Acid has been used in a similar application; cyanide detoxifcation process for the Doris North Gold, and a discussion on other alternatives considered and rejected; provide a description of how the Caro s Acid detoxification process will be incorporated into the Doris North flowsheet, including operating philosophy, expected reagent consumptions and key process criteria. Revision A 26 September of 28

7 MIRAMAR MINING CORPORATION BATEMAN MINERALS (PTY) LIMITED DORIS NORTH GOLD PROJECT REPORT NO. M CYANIDE DESTRUCTION PROCESSES A number of available detoxification options are described below together with advantages and disadvantages of each option. Preliminary capital and operating costs are presented for the detoxification requirements detailed above for the favoured options. 3.1 FERROUS SULPHATE Addition of ferrous sulphate is one method used to immobilise cyanide within a tailings impoundment and is a method currently in use at a number of mines. The method is based upon adding an excess of iron (II) sulphate to form the insoluble precipitate Prussian Blue, ferric hexa cyanoferrate (II) or Fe 4 [Fe(CN) 6 ] 3. The reactions leading to the formation of this precipitate are shown in the equations below. Oxygen is needed to oxidise some iron (II) to iron (III). FeSO 4 + 6NaCN == Na 4 [Fe(CN)] 6 + Na 2 SO 4 2FeSO 4 + 2Fe(HS0 4 ) 2 + 3Na 4 Fe(CN)] 6 + O 2 = Fe 4 [Fe(CN) 6 ] 3 + 2Na 2 SO 4 + H 2 O The removal of free cyanide is reported in the literature to be generally about 90% in 60 minutes. The extent of removal does not appear to improve with increasing iron to cyanide molar ratios between 5:1 and 20:1. A drop in ph commensurate with the precipitation of iron hydroxide occurs during the cyanide removal. Testwork for an Australian client carried out by Ammtec for iron to cyanide ratio up to 5:1 showed only a small reduction in WAD cyanide for the higher addition rates. It is unlikely that higher addition levels of ferrous sulphate will be cost competitive with competing processes. This option has fallen largely into disuse due to the instability of the iron cyanide complexes which have low but measurable dissociation constants and slowly release cyanide to the environment. The process is still in use at Salsigne in France where it is primarily used to precipitate arsenic and residual cyanide following an INCO detox. 3.2 ALKALINE CHLORINATION Alkaline oxidation of cyanide by chlorine or hypochlorite has been widely applied in the plating industry and is a well-developed technique in that application. It is reportedly a favoured approach in the former CIS counties such as Kazakhstan and Uzbekistan where sulphur dioxide-based technologies are not highly regarded. The technique has also been applied in the Canadian mining industry at the sites shown in Table 3.2. The method relies on the reactions shown in the equations below. Cyanide is oxidised first to cyanogen chloride, then to cyanate and, provided sufficient hypochlorite is available, to nitrogen and carbon dioxide. NaCN + NaOCl = CNCl + 2NaOH CNCl + 2 NaOH = NaCNO + NaCl + H 2 O 2NaCNO + 3NaOCl + H 2 O = N 2 + CO 2 + 3NaCl + 2NaOH The alkaline chlorination method is effective and very fast. Cyanide removal is almost Revision A 26 September of 28

8 MIRAMAR MINING CORPORATION BATEMAN MINERALS (PTY) LIMITED DORIS NORTH GOLD PROJECT REPORT NO. M instantaneous in clear solution at an OCl:CN molar ratio of 3:1 with the secondary reactions requiring approximately 15 minutes for completion. No detectable cyanide remains and about 90% of cyanate is oxidised. A severe disadvantage of this technique is that a toxic intermediate, cyanogen chloride, is produced. Further disadvantages are the inability to remove iron cyanide complexes, which are one of the main components of gold mill effluents. Hypochlorite consumption can be high when other oxidisable compounds are present, such as thiocyanate and reactive sulphide mineral. These concerns are relevant for Doris North as it is proposed to treat a flotation concentrate. Giant Yellowknife Mines used a two-stage reactor circuit to remove both arsenic and cyanide. The plant treated 9,000m 3 per day with a 25-minute retention time. The chlorine-cyanide and ironarsenic ratios were 11.4:1 and 11.6:1 respectively. The first stage operated at ph 11.5 and the second stage at ph 8.5. The effluent treatment plant achieved 98% removal of cyanide and arsenic, 99.6% removal of copper but did not remove iron, much of the nickel or any zinc. Plant costs were C$2 million (1981) for capital and C$0.47/m 3 (1986). Operating cost was equally divided between cyanide and arsenic removal. This option is not recommended because of the formation of the toxic intermediates which are believed to be carcinogenic and the inability to treat iron cyanides. 3.3 HYDROGEN PEROXIDE The hydrogen peroxide treatment process oxidises free cyanide and metal cyanides less stable than iron cyanide. A copper catalyst can be used in some cases. The process operates at a ph of 9 to 9.5. The process chemistry can be summarised by the equations below. CN - + H 2 O 2 = CNO - + H 2 O (in the presence of Cu 2+ ions) M(CN) H 2 O 2 + H - = M(OH) 2 + 4CNO - + 4H 2 0 Fe(CN) Cu 2+ = Cu 2 Fe(CN) 6 CNO - + 2H 2 O 2 = CO NH 4 + 2H 2 O 2 = H 2 O + O 2 The metal M in the second equation above may be copper, nickel or zinc which are removed as hydroxides. The iron cyanide complex is not oxidised but removed by precipitation as copper ferrocyanide. The decomposition reaction of hydrogen peroxide should be noted because it shows that hydrogen peroxide may be decomposed to oxygen and water and the oxygen lost from the system without performing any useful oxidation. The molar ratio of hydrogen peroxide to cyanide to oxidise the cyanide to cyanate is 3:1 to 8:1. The efficiency of the peroxide destruction process decreases at high copper cyanide to free cyanide ratios and the rate of reaction slows. This is less of a problem for Doris North where copper values in the leach effluent are relatively low. It is important to note that the addition rate of peroxide relative to cyanide rises as the cyanide concentration falls. The process struggles at low cyanide concentrations, and proportionately higher peroxide addition rates are required. A catalyst, TMT 15, would be required to remove copper cyanide complexes but no published Revision A 26 September of 28

9 MIRAMAR MINING CORPORATION BATEMAN MINERALS (PTY) LIMITED DORIS NORTH GOLD PROJECT REPORT NO. M addition rates could be found at the time this report was prepared. Hydrogen peroxide has been applied to all forms of gold milling waste but the most cost effective application is on clear water solution. In the 2002 feasibility study for Doris North, peroxide was shown to be highly effective in mopping up excess free cyanide in spent electrolyte (belt filter filtrate after gold removal). However, consumption escalated when attempting to use only hydrogen peroxide to achieve the required very low cyanide discharge limits for Doris North. In at least one publication Degussa advocated a thickener/filtration unit before treatment of CIP and CIL slurries. Reduced efficiency of detoxification has been observed when treatment of slurry is compared with clear water treatment. The loss of efficiency is due to decomposition of the peroxide by other components of the slurry before reaction with cyanide. Trials involving phosphate addition have been reported as a means of reducing this decomposition in other projects but no results are available. In addition, the peroxide process has difficulty reducing copper concentrations to environmental discharge limits without addition of a special complexing reagent TMT 15 which is marketed by Degussa. Nickel removal may also be incomplete without addition of TMT15. The advantages of the hydrogen peroxide process are: The ability to remove total cyanide and toxic metals. Reaction products and intermediates are non-toxic and do not release a salt such as sodium chloride or sodium sulphate to the receiving water. Excess hydrogen peroxide decomposes to water and oxygen. The oxidation reaction operates at the ph of gold mill effluents. Aeration of the waste stream is not needed. No toxic gases are used. Plant data has shown that the hydrogen peroxide process operating at a hydrogen peroxidecyanide ratio of 4.6:1 has achieved 90 to 97% cyanide removal, 55% copper removal without TMT 15 and 95% copper removal with TMT 15, and iron removal of 57% (Teck Corona and Con Mines). The disadvantages of the process are: The excessive reagent consumptions when sulphides are present The increase in reagent consumptions required to reach low cyanide levels Design of the process must take into consideration the peroxide dosage, mixing requirements, catalyst requirements, retention time, ph and percent solids. In order to be most cost effective, an automated dosing system and ph control is of primary importance. The variation in process requirements and the leasing of dosing equipment in some cases makes any generalised cost data of limited value. However, the cost of hydrogen peroxide is reported to represent 90% of the operating cost of a hydrogen peroxide based system. Revision A 26 September of 28

10 MIRAMAR MINING CORPORATION BATEMAN MINERALS (PTY) LIMITED DORIS NORTH GOLD PROJECT REPORT NO. M The process has been successfully applied at a number of mines since first installed at OK Tedi in The mines using the hydrogen peroxide process in Canada in 1987 are listed in Table 3.2. It is worthy of note that none of the Canadian plants were treating slurry for cyanide removal although plants elsewhere do treat mill slurry with hydrogen peroxide (OK Tedi where Degussa is a shareholder). No typical operating data is readily available in recent literature for this process. 3.4 SULPHUR DIOXIDE There are two processes used commercially for cyanide destruction based on sulphur dioxide, the INCO SO 2 :Air Process and the Noranda SO 2 Process. As suggested by the names, the former process uses an air - sulphur dioxide mixture and the latter uses pure sulphur dioxide. Both use copper (II) as a catalyst INCO PROCESS The INCO Process uses a mixture of sulphur dioxide and air to destroy both free cyanide and metal cyanide complexes less stable than iron cyanides. The metals complexed with cyanide are precipitated as metal hydroxides and iron is removed by precipitation as zinc or copper ferrocyanides. The overall reaction for cyanide oxidation is reported to be that shown in the equation below. CN t - + SO 2 + H 2 O = CNO - + H 2 SO 4 The stoichiometric sulphur dioxide requirement is 2.46 g/g CN -. The sulphur dioxide is normally injected at 1 to 3 volume % in air but can also be added as sodium sulphite or sodium metabisulphite. The copper catalyst must be present at about 50 mg Cu/litre. The preferred ph range is 9 to 10 but the process will operate over the range 6 to 11. Plant operating data for some existing commercial operations for slurry detoxification is summarised in Table 3.1. Table 3.1 INCO PROCESS PLANT OPERATING DATA Company Stream ph CN t, ppm* Cu, ppm Fe, ppm SO 2, g/g CN t Cu2+ Lime, g/g CN t Equity Silver Feed Effluent Mount Skukum Feed Effluent Colosseum Feed Effluent Ketza River Feed < Effluent <0.1 Skyline Feed Effluent 8.1 < * Note: denotes monthly average total cyanide figures Revision A 26 September of 28

11 MIRAMAR MINING CORPORATION BATEMAN MINERALS (PTY) LIMITED DORIS NORTH GOLD PROJECT REPORT NO. M The range of sulphur dioxide additions is from 4 to 6 g/g total cyanide. The sulphur dioxide usage is approximately double the calculated stoichiometric amount. Lime addition was often not necessary but had a maximum value of 4 g/g total cyanide. A license fee would normally apply, based on a unit cost per kg of sulphur dioxide required, with actual charges per tonne being therefore highly variable between plants. However, the INCO patent has now lapsed and it is now possible to install SO2:Air type treatment circuits without involving INCO, as has been the case for many recent Australian and international gold plant installations. It should be noted that sulphuric acid will form and cause a reduction in the slurry ph if lime is not added for ph control. Thiocyanate is also oxidised but only slowly. Approximately 20% thiocyanate will be oxidised before complete cyanide oxidation occurs. Nickel can accelerate thiocyanate decomposition. The oxidation is not temperature sensitive between 4 and 60 deg C and is completed in less than 1 hour. Advantages of this process over hydrogen peroxide are presented below: Thiocyanate does not consume large quantities of oxidant and is left largely unreacted; The reaction reduces the ph of a gold mill slurry towards the optimum ph of the process without additional acid; The source of the sulphur dioxide can be chosen from gaseous sulphur dioxide, sodium sulphite or sodium metabisulphite. Total cyanide levels are typically reduced to less than 1 ppm. Metal ions are normally removed to less than 1 ppm and low levels of arsenic are also reported to be removed. The reagent cost is reported to be less than 40% of that for hydrogen peroxide in a similar location, and is slightly higher than Caro s Acid plants. The main disadvantages of the process are: If excess amounts of thiocyanate are present, achieving low total cyanide discharge will be difficult (if it is included in the total cyanide accounting). Typically LC 50 toxicity in a 96 hr test for rainbow trout is 144 ppm thiocyanate. Capital cost required is considerably greater than peroxide or Caro s Acid installations NORANDA The Noranda Process is similar to the INCO Process except that pure sulphur dioxide is used instead of a sulphur dioxide air mixture. The process was trialed at the Hemlo mine and is reported to be the only application. The optimum ph range was reported to be 7 to 9. The sulphur dioxide to cyanide ratio was 6:1 and the copper to cyanide ratio was 0.5:1. The process achieved 99.8% cyanide removal down to 0.1 ppm and 99.3% copper removal down to 0.04 ppm. Revision A 26 September of 28

12 MIRAMAR MINING CORPORATION BATEMAN MINERALS (PTY) LIMITED DORIS NORTH GOLD PROJECT REPORT NO. M Testwork undertaken by Ammtec for an Australian operation in the mid 1990s reported residual WAD cyanide levels in the range 0.15 to 0.30 ppm and soluble copper levels in the range 0.14 to 0.23 ppm. The main advantage of the process is effective removal of WAD and iron cyanide species down to low levels. The main disadvantage is the need to either generate sulphur dioxide on site through sulphur burning, or to transport liquid sulphur dioxide to site. Either option significantly increases the level of hazardous chemical protection requirement for the operation. 3.5 CARO S ACID (EFFLOX PROCESS) Caro s acid is an oxidant prepared by reacting hydrogen peroxide with sulphuric acid. It is a more powerful oxidant than hydrogen peroxide and does not require a catalyst to remove copper cyanide complexes. WAD cyanide is quoted in the literature as being removed at near stoichiometric addition rates for Caro s Acid, amounts which are approximately half the addition required by hydrogen peroxide. Further, the molar ratio of acid to cyanide remains constant over a range of cyanide concentrations down to 0.l ppm. The reagent is prepared as follows: H 2 O 2 + H 2 SO 4 = H 2 SO 5 + H 2 O The following reactions are involved: SO CN - = SO CNO - 4SO SCN - + H 2 O = 5SO CNO - + 2H + The reaction times are fast (seconds) to the extent that pipe reactors can be used reducing the capital cost requirements of the process. Residual cyanide levels in the range 1-2ppm can readily be achieved, and the precipitation of the copper after the reaction is effective. An advantage of the process is the reduction in the ph level of the solution that is a result of the reaction. Testwork on Doris North ore produced a terminal ph range of 8.2 to 8.5, which is close to the ore s natural alkalinity in fresh water. The reagent preparation station is normally supplied on lease or alternatively can be built up as a capital item. Isotainers or bulka boxes of sulphuric acid and hydrogen peroxide are used for small operations such as Doris North as feed stock to the Caro s Acid plant. Reagents are pumped via small dosing pumps on a continuous basis via stainless steel braided Teflon lines into a Teflon mixing block. The ratio of addition of the two reagents is dictated by the Caro s Acid control module. The in-line mixer is configured with a number of high shear chambers to promote rapid reaction between acid and peroxide to form Caro s Acid. The resultant Caro s Acid is injected via stainless steel braided Teflon lines directly into the pulp, either into a pump suctions or into an agitated reaction vessel. The capital cost is very small as reaction time is fast, air injection is not required, and no up front mixing and storage of feed stock and detox reagents is required. Commercial applications of Caro s Acid technology for cyanide destruction are found in Nevada USA where the process is used by Newmont (Lone Tree Mine), Barrick (Gold Strike Mine), and Santa Fe Gold. In addition, the process is being used successfully at Beaconsfield Gold Mine in Tasmania, Australia. This plant was designed and constructed by Bateman through its joint venture Revision A 26 September of 28

13 MIRAMAR MINING CORPORATION BATEMAN MINERALS (PTY) LIMITED DORIS NORTH GOLD PROJECT REPORT NO. M with Kinhill Engineers, Bateman Kinhill, in the late 1990s. Miramar has also recently used Caro s Acid for seasonal cyanide detoxification of excess tails pond solution after spring thaw at the Con Mine in Canada and has reported very favourable results. 3.6 SUMMARY OF DESTRUCTION OPTIONS A list of plants in Canada using cyanide destruction processes was published in 1987 and has been included as Table 3.2. The use of the processes appears to follow a chronological trend with alkaline chlorination being replaced by sulphur dioxide:air, and sulphur dioxide:air being replaced by hydrogen peroxide. Caro s Acid has seen some use in North America and Australia, but is not as widespread as sulphur dioxide:air. The reference in which the table is published concluded that the selection of process must be individually considered based on the nature of the effluent to be treated, the capabilities and costs of the processes (reagents, power etc.) and the regulatory effluent quality limits that must be met. Table 3.2 CANADIAN PLANTS USING CYANIDE DESTRUCTION Mine Start Date Process Effluent Giant 1981 Alk. Chlor. TPO Mosquito Creek 1981 Alk. Chlor. WBS Equity Silver 1981 SO 2 / Air MS McBean 1984 SO 2 / Air WBS McLelland 1987 SO 2 / Air TPO Mount Skukum 1986 SO 2 / Air MS Golden Giant 1986 SO2 TPO Con 1987 H 2 O 2 TPO David Bell 1987 H 2 O 2 TPO Detour Lake 1987 H 2 O 2 TPO Gordex (heap) 1987 H 2 O 2 WBS Hope Brook 1987 H 2 O 2 WBS Mascot Gold 1987 H 2 O 2 WBS Page Williams 1987 H 2 O 2 TPO Tartan Lake 1987 H 2 O 2 WBS Legend TPO = = tailings pond overflow WBS == waste barren solution MS = = mill slurry A summary of capabilities of the INCO process, the Caro s acid process, and the hydrogen peroxide process has been included as Table 3.3 as these are the destruction processes which are generally favoured by regulatory authorities. Revision A 26 September of 28

14 MIRAMAR MINING CORPORATION BATEMAN MINERALS (PTY) LIMITED DORIS NORTH GOLD PROJECT REPORT NO. M Table 3.3 SUMMARY OF CAPABILITIES Species Removal (%) Parameter INCO SO2/Air Caro s Acid Hydrogen Peroxide total >99 >99 >99 copper >95 >99 55 to 95 * iron >95 >99 50 to 90 * zinc >99 >99 50 to thiocyanate 20 # > cyanate natural 100 slurries Yes Yes Limited Note: * requires addition of TMT 15 for high removal values # can achieve total removal if required by addition of nickel 3.7 PROCESS SELECTION The main reasons for selecting the Caro s Acid over alternative processes for the Doris North flowsheet are as follows: Low capital outlay no holding/mixing tanks, blowers or exotic materials of construction required Elimination of reaction catalyst requirements Significant reduction in reaction times Simple non-proprietary control system Ability to completely automate the detoxification process with little operator intervention required, aside from routine WAD and iron cyanide analyses Use of reagent is efficient with little wastage from gaseous decomposition, plus power cost is minimal since air blowers are not required Caro s Acid is the only process that enables almost complete removal of deleterious cyanide species from plant effluents. Cyanate is a by-product of the process but its breakdown is quite rapid, plus biological tolerance for cyanate is quite high compared with other cyanide species (reported as 46 ppm for rainbow trout in a 96 hr LC 50 test regime). Bateman have experience with the process from other operations where it has proved to be robust in handling process upsets Revision A 26 September of 28

15 MIRAMAR MINING CORPORATION BATEMAN MINERALS (PTY) LIMITED DORIS NORTH GOLD PROJECT REPORT NO. M REVIEW OF CYANIDE DESTRUCTION TESTWORK 4.1 INTRODUCTION Caro s Acid cyanide detoxification testwork has been undertaken on leach slurries from the current Doris North metallurgical testwork programme. This programme has the primary objectives of confirming achievable gold recoveries to gravity/flotation concentrates and subsequent cyanide leach recoveries in carbon-in-leach circuit configuration. The detoxification work was aimed at producing a final tailing, after blending with benign flotation tailings, which contains WAD and total cyanide levels of 0.5 and 1.0 ppm respectively using Caro s Acid as the cyanide oxidant. The entire testwork was undertaken at Ammtec Limited s laboratories in Perth, Western Australia. The test programme was undertaken in two phases. The first phase consisted of a series of sighter tests to evaluate various Caro s Acid salt dosing molar ratios, the effect of copper sulphate addition on iron precipitation, and ph effects. The second phase was the bulk run used to generate sample for environmental testing. In addition to the Caro s Acid detox work, sub-samples of leach liquors from before detox, after detox, and after combining with flotation tailings solution, were analysed at Ammtec for cyanide and selected chemical speciation. Additional samples of solution and solids were sent to BC Research for analysis under the direction of AMEC in Vancouver. Furthermore, a sample of combined tailings was sent to AMEC in Perth for tailings sedimentation testwork, again under the direction of AMEC Vancouver. Revision A 26 September of 28

16 MIRAMAR MINING CORPORATION BATEMAN MINERALS (PTY) LIMITED DORIS NORTH GOLD PROJECT REPORT NO. M TEST PROCEDURES Ammtec to provide details. 4.3 TESTWORK RESULTS CARO S ACID SIGHTER TESTS Initial Tests Mass balance calculations based on laboratory concentrate mass pulls indicated that final blended tailings will have the following makeup: Stream Pulp Density % of total solids % of total solution (% solids w/w) Leach residue Flotation tailings Final tailings This means that the cyanide content remaining in the cyanide leach residue after detoxification will be reduced in strength through dilution with flotation tailings solution by a factor of 16.2, (reciprocal of 6.2%). Since the target WAD and total cyanide levels targeted are 0.5 and 1 ppm respectively, allowable cyanide levels in detox discharge before dilution with flotation tailings are 16.2 times higher, or 8.1 ppm and 16.2 ppm for WAD and total cyanide respectively. Therefore, initial levels aimed for in the test programme were approximately 8 ppm or less for WAD cyanide and 16 ppm or less for total cyanide (which includes WAD and iron cyanides). The initial round of sighter tests was undertaken on sub-samples of bulk leach residue slurry of pulp density 40% solids w/w. Preliminary sighter test results are summarised below as Table 4.1. Table 4.1 SIGHTER DETOX TEST RESULTS - PRELIMINARY Test No. Molar Ratio Final Solution Assays (mg/l) CN(wad):KHSO 5 [actual assays, not corrected for dilution] Cu Ni Zn Fe CN(free) CN(wad) CN(tot) ph ph calc Start Final Feed MH3034 1: < MH3035 1: < MH3036 1: < MH3037 1: < The sighter test results indicated that copper, nickel and zinc cyanide complexes can be broken down to low levels with relatively low dosage rates of Caro s Acid (molar ratios of 3 to 4 per mole of WAD cyanide), as did free cyanide (see Tests MH3034 and MH3035). The titration limit for free cyanide is 1 ppm, so actual cyanide levels were most likely much lower. WAD cyanide levels attained were very acceptable at less than 3 ppm, however, total cyanide at 16.6 to 17.7 ppm (versus Revision A 26 September of 28

17 MIRAMAR MINING CORPORATION BATEMAN MINERALS (PTY) LIMITED DORIS NORTH GOLD PROJECT REPORT NO. M the target of 16 ppm) was still higher than desired high due to the persistence of iron cyanide species. Increasing Caro s Acid dosage ratio to 5:1 (Test MH3036) only produced a marginal drop in iron cyanides and halved nickel cyanide. The addition of 20 ppm copper sulphate in Test MH3037 to precipitate iron had some effect, dropping iron from 4.8 ppm to 4 ppm, but free cyanide was seen to increase due to the release of cyanide from the iron cyanide complexes. Total cyanide was within the 16 ppm calculated limit, but additional tests were commissioned to see if lower levels could be achieved, the results of which are discussed in the next section. Further Sighter Tests - Optimisation Following the initial series of sighter tests, Ammtec were instructed to dilute the feed pulp to 30.2% solids w/w with Perth tap water to emulate the effect of loaded carbon and safety screens spray water additions in the plant (3 m 3 /h each), a significant factor in the plant design that was excluded in the initial tests. This had the following effect on tailings makeup: Stream Pulp Density % of total solids % of total solution (% solids w/w) Leach residue Flotation tailings Final tailings The dilution factor now dropped to 7.14 (reciprocal of 14%) from 16.2 in the initial sighter tests. In real terms, however, the amount of cyanide present in the detox feed was the same as the cyanide concentration dropped from 220 ppm (see Table 4.1) to 132 ppm (Table 4.2) as a result of the dilution. With the drop in dilution factor, target WAD and total cyanide levels became 3.5 ppm and 7 ppm respectively. Test MH3038 was undertaken with 4:1 Caro s Acid molar ratio without any copper sulphate addition, but the ph was allowed to drop down to 7.2 by the end of the contact period rather than maintain ph above 9 with lime. Tests MH3039 and 40 investigated different molar dosing ratios. Optimisation sighter test results are presented as Table 4.2. Table 4.2 SIGHTER DETOX TEST RESULTS - OPTIMISATION Test No. Molar Ratio Final Solution Assays (mg/l) CN(wad):KHSO5 [actual assays, not corrected for dilution] Cu Ni Zn Fe CN (free) CN (wad) Feed (diluted) CN (tot) ph ph calc Start Final MH3038 1: < MH3039 1: < MH3040 1: < The combination of these process conditions resulted in target levels of 3.5 ppm WAD and 7.0 ppm total cyanide being easily met in all cases. Revision A 26 September of 28

18 MIRAMAR MINING CORPORATION BATEMAN MINERALS (PTY) LIMITED DORIS NORTH GOLD PROJECT REPORT NO. M BULK SAMPLE CARO S ACID TEST For the bulk run, the conditions for test MH3040 were adopted as it minimized the amount of free cyanide present, this species being more toxic to aquatic life than iron cyanides. Results of the bulk run are presented as Table 4.3. Table 4.3 BULK RUN DETOX TEST RESULTS Test No. Molar Ratio Final Solution Assays (mg/l) CN(wad):KHSO5 [actual assays, not corrected for dilution] Cu Ni Zn Fe CN(free) CN(wad) CN(tot) ph ph Feed (diluted) calc Start Final MH3051 1: <0.1 < Overall WAD cyanide at 1.8 ppm was higher than the equivalent sighter test (MH3040), despite returning similar soluble metal species. However, the total cyanide value at 1.9 ppm is still well inside the maximum desirable value of 7 ppm, whilst WAD is well inside the 3.5 ppm limit CYANIDE AND CHEMICAL SPECIATION RESULTS On completion of the cyanide detoxification tests, the bulk run detox slurry was combined in the correct proportions with flotation tailing slurry and mixed thoroughly, then allowed to settle. Subsamples of solution were taken for analysis at Ammtec, and for additional analyses at BC Research in Canada. In addition to these analyses, flotation tailings solution, cyanide leach solution and detoxified leach solution analyses were undertaken. A sub-sample of blended tails solution was left aside to be aged for 1 month, and was then re-analysed to determine the extent of change in solution chemistry. Table 4.4 summarises cyanide speciation results for the four samples. Table 4.4 CYANIDE SPECIATION RESULTS ANALYSIS Float Tails Soln Cyanide Leach Soln Cyanide Detoxified Soln Blended Effluent Blended Effluent [HS9312] [HS9467] [MH3051] (aged 1 month) CN(free), ppm <5 120 <5 <5 <5 CN(wad), ppm < CN(total), ppm CNO, ppm < SCN, ppm < NOTE: CN(total) includes cyanide as thiocyanate (SCN) Revision A 26 September of 28

19 MIRAMAR MINING CORPORATION BATEMAN MINERALS (PTY) LIMITED DORIS NORTH GOLD PROJECT REPORT NO. M Table 4.5 summarises chemical speciation results for the four samples. Table 4.5 CHEMICAL SPECIATION RESULTS Float Tails Soln Cyanide Leach Sol.n Cyanide Detoxified Sol.n Blended Effluent Blended Effluent ANALYSIS [HS9312] [HS9467] [MH3051] (aged 1 month) Nitrate (NO - 3 ) as N 2 ND Ammonia (NH 3 ) as N < NO 3 - & NO 2 - as N NA 8 NA NA NA Nitrite (NO 2 - ) as N <1 ND ph Free acid, g/l Sulphate (SO 4 ), g/l < Hardness, mg/l CaCO Conductivity, ms/cm 955 ms/cm Alkalinity, mg/l CaCO Acidity NA NA NA NA NA NOTES: NA = Not Applicable ND = Not Determined [due to interference, possibly organic matter or surfactants] All results in ppm unless otherwise stated Levels of all species present in the blended effluent are considered to be very low, though their effect on aquatic life requires assessment by Miramar s environmental consultant, AMEC. It can be seen that the blended effluent WAD cyanide levels at 0.04 ppm is well inside the target limits of 0.5 ppm required for safe discharge to the tails storage pond. Similarly, the total cyanide value of 0.29 ppm is comfortably inside the 1 ppm limit. An additional favourable outcome is a thiocyanate level of less than 0.1 ppm in the blended effluent. Worth noting is that the total cyanide figure includes thiocyanate as well as WAD, free and iron cyanide values. After one month s aging time, the blended effluent cyanate content drops from 32 ppm to 9.1 ppm, with a slight increase in ammonia from 1 to 4.4 ppm. However, these results suggest that the majority of the cyanate converted to nitrogen and carbon dioxide, as the resultant ammonia increase does not account for the almost fourfold drop in cyanate content in blended effluent. This result suggests that the life cycle of cyanate generated by the Caro's Acid detox process is relatively shortlived, and that a long term buildup in ammonia content in Tails Lake is unlikely, especially given the high dilution of the existing water stocks in the lake and the resultant ph drop caused by mixing of blended effluent with this water supply. Total cyanide is also seen to drop from 0.29 ppm to 0.20 ppm, signifying a further degradation in soluble cyanide species to volatile or precipitated chemical species. Table 4.6 summarises ICP cation and sulphate analyses of the four solution samples. AMEC will comment on the effect of levels of these species on the aquatic environment. Revision A 26 September of 28

20 MIRAMAR MINING CORPORATION BATEMAN MINERALS (PTY) LIMITED DORIS NORTH GOLD PROJECT REPORT NO. M Table 4.6 ICP SOLUTION RESULTS ANALYSIS Float Tails Soln [HS9312] Cyanide Leach Soln [HS9467] Cyanide Detoxified Soln [MH3051] Blended Effluent Blended Effluent (Aged 1 month) Dissolved Total Dissolved Total Dissolved Total Dissolved Total Dissolved Total Al < As B Ba < < Ca Cd <0.005 <0.005 <0.005 <0.005 <0.005 <0.005 <0.005 <0.005 <0.005 <0.005 Co <0.005 < Cr < Cu Fe < Hg < < < < < < < < < < K Mg Mn Mo Na Ni < < Pb <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.02 <0.05 SO V <0.005 < < <0.005 <0.005 Zn < < Revision A 26 September of 28

21 MIRAMAR MINING CORPORATION BATEMAN MINERALS (PTY) LIMITED DORIS NORTH GOLD PROJECT REPORT NO. M CONCLUDING COMMENTS ON TESTWORK The preceding narrative serves to provide a summary of Caro s Acid detoxification testwork undertaken at Ammtec on Doris North leach slurry. The testwork results demonstrate that it is possible to achieve WAD and total cyanide discharge levels well within the project environmental guidelines supplied to Bateman. Caro s Acid dosage required is quite modest, with a ratio of 3:1 being selected for design purposes (Caro s Acid salt to WAD cyanide content). The addition of copper sulphate for iron precipitation appears not to be as effective as allowing the solution ph to drop without adding lime as a buffer. Copper sulphate will be available in the plant as a sulphide activator in the flotation circuit, and if required after startup it will be a simple matter to retrofit an addition line to the detox circuit should iron cyanide levels become problematic. For design purposes, Bateman have allowed for a higher mass pull to concentrate of 10%, which will change the dilution factor to Based on the optimisation sighter tests and bulk run test results, the desired discharge limits of 0.5 WAD and 1.0 ppm total cyanide will still be met comfortably using quite a flexible range of molar ratios from 2 to 4:1 Caro s Acid to cyanide. Therefore, it can be concluded that the use of Caro s Acid has been demonstrated at the laboratory level to be an acceptable and robust method for cyanide destruction at Doris North. Revision A 26 September of 28

22 MIRAMAR MINING CORPORATION BATEMAN MINERALS (PTY) LIMITED DORIS NORTH GOLD PROJECT REPORT NO. M CYANIDE DETOXIFICATION CIRCUIT PROCESS DESIGN This section of the report provides a preliminary process design package for the cyanide detoxification section of the Doris North process plant. The full design exercise is planned for commencement in November 2003, but the essential criteria and control philosophy should be largely unchanged from what has been presented in this report since all detox testwork has been completed to the satisfaction of both Bateman and Miramar. 5.1 PROCESS DESIGN CRITERIA Table 5.1 presents principal process design criteria for the cyanide detoxification circuit at Doris North. Table 5.1 PROCESS DESIGN CRITERIA CYANIDE DETOX AND TAILINGS Parameter Unit Nominal Plant Duty dry tonnes/day new feed Peak Plant Duty dry tonnes/day new feed CYANIDE DETOXIFICATION Detoxification Method Caro's Acid Caro's Acid Feed Source concentrate leach residue pulp concentrate leach residue pulp Operating Pulp Density % Feed Solution Flow Rate m3/h Solids in Feed tph Volumetric Flow Rate - maximum m3/h Operating ph Detox Feed 10 to to 10.5 Detox Discharge 7.5 to to 8.0 Free and WAD Cyanide in Detox Feed Average ppm Design ppm Free and WAD Cyanide in Detox Discharge Average ppm Design ppm Cyanide Destroyed Average kg/h Design kg/h Caro's Acid Requirement Molar Ratio (Caro's Acid:Cyanide) Stoichiometric Dose Rate kg/kg CN Design kg/h Reaction Time mins Number of Reactor Stages 2 2 Minimum Live Reactor Volume m Live Volume Required for Contact m Revision A 26 September of 28

23 MIRAMAR MINING CORPORATION BATEMAN MINERALS (PTY) LIMITED DORIS NORTH GOLD PROJECT REPORT NO. M Nominal Plant Duty dry tonnes/day new feed Peak Plant Duty dry tonnes/day new feed Parameter Unit Hydrogen Peroxide (makeup for Caro s Acid) Concentration %w/w concentration noted kg/h Sulphuric Acid (makeup for Caro s Acid) Concentration %w/w concentration noted kg/h Stoichiometric Factor TAILINGS DISPOSAL Feed Source Flotation tails + detox d/c Flotation tails + detox d/c Solids Rate t/h Slurry Pulp Density %w/w Solution Flow Rate m3/h Volumetric Flow Rate m3/h Final Tailings : Detox Discharge Ratio Solids Solution Sump Residence Time - minimum mins Sump Minimum Live Volume m Tailings Disposal Method pump to storage dam pump to storage dam Operating ph in Final Discharge 8.0 to to 8.5 Target Cyanide in Discharge to Tails Dam WAD ppm Total ppm PROJECTED REAGENT CONSUMPTIONS The annual consumption of reagents used in the cyanide detoxification circuit for nominal (690 tpd new plant feed) and peak (800 tpd new plant feed) process plant duties are provided as Table 5.2. Projected makeup reagents for Caro s Acid production are based on a molar ratio of 3:1 Caro s Acid to cyanide content in detox feed. Although a 2:1 molar ratio has been demonstrated in the Ammtec testwork to be adequate to achieve desired discharge levels, the use of 50% strength hydrogen peroxide will result in a degree of inefficiency in terms of conversion to Caro s Acid, around 70%, versus much higher conversion for 70% strength peroxide. However, special permitting and storage requirements are need for the higher strength peroxide, so 50% strength has been adopted for design purposes. Conversion to 70% strength should be further investigated as the savings in operating costs could be significant. There is also scope to optimise the residual leach residue cyanide level and molar dosing ratio in the plant once operations commence, further reducing operating costs. Table 5.2 PROJECTED DETOX AREA ANNUAL REAGENT CONSUMPTION Parameter Unit Nominal Plant Duty dry tonnes/day new feed Peak Plant Duty dry tonnes/day new feed Hydrogen Peroxide, 50% w/w strength kg/t tpa Sulphuric Acid, 93% w/w strength kg/t tpa Revision A 26 September of 28

24 MIRAMAR MINING CORPORATION BATEMAN MINERALS (PTY) LIMITED DORIS NORTH GOLD PROJECT REPORT NO. M Hydrogen peroxide and sulphuric acid will be delivered to Doris North site on an annual basis by cargo ship then offloaded to a barge for transport to the site s landing jetty. The actual containers will either be 1 m 3 bulk boxes in protective cages or isotainers of capacity up to 15 tonnes, the limit of the off-loading crane. 5.3 PROPOSED DORIS NORTH PROCESS DESCRIPTION A detailed block flowsheet of the proposed Doris North flowsheet operating at peak feed rate of 800 tonnes per day new feed is included as Appendix A. Bateman have designed a modularised treatment plant to treat the high grade Doris North resource. It has the advantage of being easily broken down into sections that are comparatively easy to relocate to another deposit for rapidly reassembly. The process description of the plant is provided here. Run-of-mine ore is crushed in a single stage crushing plant to -200 mm size and fed directly to a SAG mill. SAG mill discharge slurry launders into a cyclone sump and is pumped to a pack of hydrocyclones which allow control over grind size. Cyclone underflow gravitates to the SAG mill feed, with a split being diverted via a distribution box to flash flotation, described further in this section. Two stage gravity concentration is included within the milling circuit circulating load to remove gravity recoverable gold. This is dispatched continuously to the goldroom where it is further upgraded on a shaking table and smelted in a diesel fired furnace to produce gold doré. Gravity table tails is returned via a dedicated pump to the SAG mill discharge pump sump. A flash flotation cell is also included within the SAG mill circuit circulating load to recover coarse liberated sulphides and dispatch these continuously to the concentrate thickener. SAG mill product ex-cyclones is 80% -106 um. Cyclone overflow is subjected to scavenger flotation in a series of tank flotation cells to recover remaining free and sulphide associated gold. Reagents used in flotation are potassium amyl xanthate for sulphide collection, methyl isobutyl carbinol (MIBC) as a frother and copper sulphate for pyrite activation. Flash and scavenger flotation are combined in a small thickener to dewater the feed ahead of regrinding to liberate entrained gold. The regrind mill is of the vertical type, operating in open circuit, with mill discharge being pumped to the head of a bank of carbon-in-leach tanks. Lime slurry is added to the regrind mill feed to condition the pulp ph to the region of 10.5 ahead of cyanide leaching. Sodium cyanide solution is added to the feed launder to the first and second leach tanks to maintain an initial cyanide concentration of 1000 ppm. Activated carbon is used to adsorb solubilised gold from solution, and is moved counter-current to the slurry process flow though the leach tanks using airlifts or recessed impellor pumps. Fully loaded carbon is recovered from the first leach tank onto a dewatering screen where residual slurry is washed from the carbon. Screen underflow returns o the leach tanks. The loaded carbon is then acid washed with dilute hydrochloric acid to remove inorganic compounds that may be fouling the carbon. Acid washed carbon is then transferred to an elution column where loaded gold is removed under pressurised conditions at elevated temperatures in a strongly alkaline environment. Pregnant liquor is passed through an electrowinning cell to recover gold values, with cell tails being recirculated to the elution column to recover additional loaded gold. After a pre-determined period of time, the elution is terminated and the remaining solution is pumped back to the leach circuit. Stripped carbon is pumped up to the feed hopper of a regeneration kiln, which serves to re-activate carbon by re-opening pores and increasing contact surface area. The kiln operates continuously but carbon is returned batch wise after quenching to the rear end of the CIL circuit for advancement up Revision A 26 September of 28

25 MIRAMAR MINING CORPORATION BATEMAN MINERALS (PTY) LIMITED DORIS NORTH GOLD PROJECT REPORT NO. M through the circuit. Electowon gold loaded onto steel wool cathodes is periodically removed, calcined in an oven and smelted with fluxes to produce gold dore. Cyanide leach residue slurry exits the final leach tank and gravitates over a carbon safety screen to recover fine carbon. Screen underflow discharges into the first of two agitated covered reactors. Caro s Acid is injected into the first contact tank at an initial ph of 10.5, with discharge from the second contact tank being of the order of 8 to 8.5. Lime may be added for ph control as required. Cyanide detoxified slurry exits the second reactor and gravitates into the final tailings hopper, where it is combined with scavenger flotation tailing slurry and is pumped to Tails Lake, the selected tails storage facility. After dilution with the lake s water, cyanide level is expected to be well inside world s best practice limits. Water supply for the plant will be drawn from the opposite end of Tails Lake, but in winter months water will be drawn from Doris Lake. 5.4 OPERATING PHILOSOPHY Process control of the cyanide detoxification circuit is relatively straightforward, requiring little operator intervention on a routine basis. Cyanide leach residue pulp gravitates into the first of two cyanide detoxification reactors. Caro s Acid is produced by pumping sulphuric acid and hydrogen peroxide from separate storage vessels in set ratio into a Teflon mixing block via stainless steel braided Teflon lines. The control module is supplied by the vendor, and insures that the required stoichiometric mix is maintained regardless of the final Caro s Acid demand rate. The individual dosing pump outputs are controlled by speed controllers according to the preset mixing ratio. Caro s Acid dosing rate into the first reaction tank is set by the operator based on cyanide titration of the feed slurry and Caro s Acid Reactor (150-TK-01) discharge slurry. The actual discharge cyanide level desired is dictated by the dilution ratio of flotation tails solution to detox discharge solution, which is determined by flow meters and density gauges on individual process streams. This is purely a supervisory value that allows the operator to determine what level the detox discharge WAD cyanide level needs to be to maintain a maximum of 0.5 ppm in final tails discharge. Cyanide levels are monitored on a routine basis using the following suggested methods: Free cyanide silver nitrate titration of detox feed and discharge on a two hourly frequency WAD cyanide picric acid colorimetric comparison of detox feed and discharge on a four hourly basis (visual comparison against a colour chart for a set solution makeup) Iron cyanide AAS analysis using iron specific tube on a composited shift basis. Iron cyanides in the specific ph range of 8 to 11 are usually a factor of 2.8 x iron concentration. ORP probe thee potentiometric probes can be left in contact with slurry on a continuous basis to monitor changes in redox potential. A site-specific relationship between potential and cyanide levels needs to be developed, but generally, negative values indicate a surplus of cyanide species present. This can be input directly to the DCS for continuous logging, with excessive values triggering operator intervention and/or feed stoppage. Revision A 26 September of 28

26 MIRAMAR MINING CORPORATION BATEMAN MINERALS (PTY) LIMITED DORIS NORTH GOLD PROJECT REPORT NO. M Prevention of excessive cyanide discharge if the feed to the detox circuit is stopped either by ORP alarm or operator intervention due to excessive WAD cyanide levels (typically in excess of 4 ppm), new feed to the SAG mill is stopped, followed by dumping of pump sumps. The tails line is automatically flushed with process water to avoid bogging and to dilute cyanide values to safe limits until the process upset is resolved. Final tailings sump level is controlled by a loop between level controller and the variable speed drive of the final tailings pumps. Revision A 26 September of 28

27 MIRAMAR MINING CORPORATION BATEMAN MINERALS (PTY) LIMITED DORIS NORTH GOLD PROJECT REPORT NO. M Appendix A BLOCK PROCESS FLOWSHEET Revision A 26 September of 28

28 MIRAMAR MINING CORPORATION BATEMAN MINERALS (PTY) LIMITED DORIS NORTH GOLD PROJECT REPORT NO. M Appendix B DRAFT AMMTEC REPORT NO. (CYANIDE DETOXIFICATION SECTION) Revision A 26 September of 28

29 MODULAR PLANT BLOCK MASS BALANCE CLIENT: PROJECT: JOB NUMBER: DATE: REVISION: Miramar Mining Corporation Doris North Process Optimisation (post-feasibility study) M3952 September 19, 2003 B New Mill Feed LEGEND solids, mtph sol.n, mtph % w/w vol, m3/h Total Mill Feed Flash Float Tails Dilution CUF Diversion 11.1 SAG MILL Flash Float Cons SAG Mill d/c FLASH FLOTATION Sprays 5.0 D/C TROMMEL Spray Water 3.55 Trommel u/s Total Gravity Tails (avg) Dilution 5.2 SAG MILL D/C SUMP Gravity Feed Flash Float Feed Ro. Grav. Cons Hutch Water ROUGHER GRAVITY CONCENTRATOR Dilution 12.7 Cyclone Feed CYCLONES Cyclone o/f SCAVENGER FLOTATION CONDITIONER Scav. Float Feed Scav. Float Cons Launder Water SCAVENGER FLOTATION CELLS Dilution Hutch Water 3.7 CLEANER GRAVITY 0.2 CONCENTRATION Cl. Grav. Cons Cl. Grav. Tails Cyclone u/f GRAVITY TABLE CONCENTRATION (batch) Table Tails (24 hr basis) Scav. Float Tails Combined Cons Au to smelting (batch)

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