THE FERRIC ION - GOD S GIFT TO HYDROMETALLURGISTS TO KEEP EM HUMBLE. By Chris Fleming SGS Lakefield Research Ltd.

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1 THE FERRIC ION - GOD S GIFT TO HYDROMETALLURGISTS TO KEEP EM HUMBLE By Chris Fleming SGS Lakefield Research Ltd.

2 TOPICS The first SEx war There s nothing basic about basic iron sulphate 2

3 THE FIRST SEx WAR History Great interest in solvent extraction in the 1960 s, initially for uranium and then for copper. Low grade oxide copper ores were being processed in the USA and Chile by acidic heap leaching (small scale). Copper sulphate in solution was recovered by cementation onto scrap iron, and then smelted and refined. Most leach liquors contained ferric ions, which reacted with the scrap iron wastefully, and the process was: (i) (ii) Expensive Yielded an impure copper product 3

4 THE FIRST SEx WAR History Early economic projections predicted the cost of SEx/EW would be half the cost of cementation/smelting/refining. The perfect reagent had to be able to extract copper (II) from weak acid leach solution (ph 1-2), be strippable in strong acid (50-100g/L H 2 SO 4 ) to be compatible with EW, and be very selective for copper particularly versus the ferric ion. Copper SEx reagent development was spearheaded by two US companies, General Mills, who produced the LIX reagents, and Ashland Chemical, who produced Kelex

5 COPPER SEx REAGENTS LIX R KELEX HO C = N OH R N O H HYDROXYOXIME HYDROXYQUINOLINE 5

6 THE FIRST SEx WAR History The first copper SEx plant was built at the Blue Bird Mine in Arizona in 1968 (6000 tpa Cu), soon followed by a much bigger plant at Nchanga mine in Zambia (65,000 tpa Cu). More plants followed, and 3% of world copper production was via SEx/EW by By 2007, this had grown to 22% of annual Cu production, (3.5M tons of cathode copper). This was being produced in 70 SEx/EW plants in 16 countries (60% in Chile). But who was making the SEx reagents and who was winning the reagent war? 6

7 Copper extraction with LIX and KELEX reagents as a function of ph 90 LIX LIX LIX 64N 2.9 LIX KELEX Copper Extraction (%) ph Initial Rate of Copper Extraction (g/l/min) KELEX LIX 64N

8 KINETIC AND STABILITY CONSTANTS FOR THE REACTION OF Cu(II) AND Fe(III) WITH HYDROXYQUINOLINES Metal Cu( II) Fe( III) Initial Rate of Extraction with Kelex 100 (g/l/min) Stability with 8 Hydroxy Quinoline Log β 2 = 23.0 Log β 3 =

9 RATES OF EXTRACTION OF Cu(II) AND Fe(III) BY KELEX Extraction with 10% KELEX 100 (%) Cu 2+ from ph 1 solution Cu 2+ Cu 2+ from ph 2 solution Cu 2+ Fe 3+ from ph 1 solution Fe 3+ from ph 2 solution Stirring time, min 9

10 RATES OF EXTRACTION OF Cu(II) AND Fe(III) BY LIX65N AND ITS PRECUSOR, SALICYLALDOXIME LIX 65N mol % Salicylaldoxime mol % h Cu Metal Extracted (%) Fe h Cu Fe h Cu Fe

11 THE MORAL OF THE STORY If you want to win a SExwar, it is better to be slow and selective than to be fast and flirtatious. 11

12 THERE S NOTHING BASIC ABOUT BASIC IRON SULPHATE

13 BACKGROUND Basic iron sulphate (BFS) is a solid compound that is formed under certain conditions during the oxidation of pyrite or other iron sulphide minerals with oxygen at high temperatures in an autoclave. Iron sulphide minerals are oxidized in an autoclave to produce ferric sulphate and sulphuric acid in solution. The ferric sulphate then hydrolyzes slowly, precipitating back out of solution as hematite and/or BFS. 13

14 OXIDATION 2FeS H 2 O fi 2FeSO 4 + 2H 2 SO 4 4FeSO 4 + 2H 2 SO 4 + O 2 fi 2Fe 2 (SO 4 ) 3 + 2H 2 O Overall: 4FeS H 2 O fi 2Fe 2 (SO 4 ) 3 + 2H 2 SO 4 14

15 HYDROLYSIS Ferric sulphate hydrolyzes to hematite at higher temperatures and lower acidity Fe 2 (SO 4 ) 3 + 3H 2 O fi Fe 2 O 3 + 3H 2 SO 4 and it hydrolyzes to BFS at lower temperatures and higher acidity Fe 2 (SO 4 ) 3 + 2H 2 O 2Fe(OH)SO 4 + H 2 SO 4 15

16 OXIDATION AND HYDROLYSIS Overall reaction for the oxidation of pyrite to ferric sulphate followed by hydrolysis to hematite 4FeS O 2 + 8H 2 O 2Fe 2 O 3 + 8H 2 SO 4 Overall reaction for the oxidation of pyrite to ferric sulphate followed by hydrolysis to BFS 4FeS O 2 + 6H 2 O 4Fe(OH)SO 4 + 4H 2 SO 4 16

17 260 Basic Iron Sulphate Fe(OH)SO Temp (ºC) 140 Hematite Fe 2 O Fe 3+ Fe (OH) 3 Goethite FeO.OH ph Stability domains of compounds of the ferric ion in water as a function of temperature and ph 17

18 WHY IS BFS BAD NEWS IN A CYANIDATION CIRCUIT? BFS is not basic, it is actually acidic..and it must be neutralized before cyanidation Fe(OH)SO 4 + Ca(OH) 2 ph>7 Fe(OH) 3 + CaSO 4 The rate of release of acid by BFS is extremely slow in weakly acidic solution (ph <7). This means BFS cannot be neutralized with a cheap alkali such as limestone. ph3.5 Fe(OH)SO 4 + CaCO 3 + H 2 O Fe(OH) 3 + CaSO 4 + CO 2 very slow 18

19 WHY IS BFS BAD NEWS IN A CYANIDATION CIRCUIT? Even under alkaline conditions, the rate of release of acid by BFS is quite slow but it is persistent at the ph needed for cyanide leaching (ph ~10) Consequently, the ph constantly drifts downwards under normal cyanide leach operating conditions, into the ph region where cyanide is converted to HCN gas. 19

20 WHY IS BASIC IRON SULPHATE BAD NEWS IN CYANIDATION CIRCUITS For health and safety reasons related to HCN formation, the BSF must be fully neutralized prior to cyanidation. This will take hours and add significantly to plant capital cost. As a result, most of the sulphate generated in the autoclave has to be neutralized with hydrated lime, rather than limestone. Lime can be at least 10 times the price of limestone. If not dealt with appropriately, the increased capex and opex associated with BFS formation could eliminate POX from consideration for many refractory gold projects 20

21 WHAT IS THE BEST SOLUTION? THE HOT CURE PROCESS 21

22 THE HOT CURE PROCESS The basis of the hot cure process is the fact that the hydrolysis reaction that produces BFS in the autoclave is reversible at lower temperatures: BFS Formation Fe 2 (SO 4 ) 3 + 2H 2 O T>150ºCFe(OH)SO 4 + H 2 SO 4 BFS Decomposition Fe(OH)SO 4 + H 2 SO ºC fast RT very slow Fe 2 (SO 4 ) 3 + 2H 2 O 22

23 Fe 3+ Fe (OH) 3 THE HOT CURE PROCESS BFS Temp (ºC) 140 Hematite Fe 2 O Goethite FeO.OH ph 2Fe(OH)SO 4 + H 2 SO 4 Fe 2 (SO 4 ) 3 + 2H 2 O 23

24 THE HOT CURE PROCESS Once the basic iron sulphate has decomposed to ferric sulphate, it can be separated from the solids by CCD or filtration, and neutralized with limestone Fe 2 (SO 4 ) 3 + 3CaCO 3 + 3H 2 O 2Fe(OH) 3 + 3CaSO 4 +3CO 2 24

25 NEUTRALIZATION OF THE ACID AND SULPHATE WITH LIMESTONE BFS Temp (ºC) 140 Hematite Fe 2 O (1) Fe 3+ (2) Fe(OH) 3 Goethite FeO.OH ph (1) Fe 2 (SO 4 ) 3 + 3CaCO 3 + H 2 O 2FeO.OH + 3CaSO 4 + 3CO 2 (2) Fe 2 (SO 4 ) 3 + 3CaCO 3 + 3H 2 O 2Fe(OH) 3 + 3CaSO 4 + 3CO 2 25

26 A REFRACTORY GOLD POX FLOWSHEET INCORPORATING HOT CURING Oxygen Pressure Oxidation Concentrate Hot Cure C 4 to 12 hours Steam CO 2 Solid/Liquid Separation CaCO 3 Neutralisation Liquid Solid NaCN Ca(OH) 2 Liquid Base Metal Recovery? Solid CaSO 4 Fe(OH) 3 Solid/Liquid Separation Gold Recovery Cyanide Leach Tailings Cyanide Destruction 26

27 QUIMSACOCHA PROJECT, ECUADOR (IAMGOLD CORPORATION) 40 Conc. In Autoclave Discharge Solids (%) SO 4 10 Fe 0-3 ACD Time at 90ºC (hr) Fe (mass/mass) = 18.1% SO 4 (mass/mass) = 32.0% Fe/SO 4 =

28 QUIMSACOCHA PROJECT H 2 SO 4 Autoclave Discharge Solution (g/l) Fe ACD Time (hr) 28

29 QUIMSACOCHA PROJECT Concentrate head grade: 24 g/t Au, 104 g/t Ag PRODUCT RECOVERY ALKALI CONSUMED APPROX. COST Au % Ag % CaCO 3 kg/t Ca(OH) 2 kg/t $/t Autoclave Discharge Hot Cure Discharge

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THE CESL PROCESS: SUCCESSFUL REFINING OF A COMPLEX COPPER SULPHIDE CONCENTRATE

THE CESL PROCESS: SUCCESSFUL REFINING OF A COMPLEX COPPER SULPHIDE CONCENTRATE GLENN BARR AND WILLY GRIEVE 1 INTRODUCTION Cominco Engineering Services Ltd. (CESL) is a wholly - owned subsidiary of Cominco