high pressure acid leaching of a refractory lateritic nickel ore

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High pressure acid leaching of a refractory lateritic nickel ore S ß. Kaya, Y.A. Topkaya Department of Metallurgical and Materials Engineering, Middle East Technical University (METU), 06531 Ankara, Turkey article info Article history: Available online 2 June 2011 Keywords: Hydrometallurgy Laterite Leaching Nickel Cobalt abstract This paper describes the experimental findings of the extraction of nickel and cobalt by high pressure acid leaching (HPAL) of a refractory limonitic nickel laterite ore from the Gördes region of Manisa in Turkey. By optimizing the basic HPAL process parameters: leaching at 255 °C with 0.30 sulfuric acid to ore weight ratio with a particle size of 100% 850l for 1 h of leaching, it was found that 87.3% of nickel and 88.8% of cobalt present in the ore could be extracted into the pregnant leach solution (PLS). However, these extrac- tion results were found to be relatively low compared with other similar studies. In order to understand the possible reasons for this relatively lower extraction, further investigations have shown that together with a problem related to the kinetics of the dissolution reactions, a persistent acid resistant refractory mineral present in this sample also limited the leaching process. Attempts were made with different additives to solve this problem. The effects of chemical additives such as HCl, Na 2 SO 4 , FeSO 4 , Cu + and sul- fur were tested and the effect of each addition on the degree of extraction of nickel and cobalt was determined. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Ferrous and non-ferrous alloying industries, petro-chemical works, aerospace applications, nickel based catalysts and battery production, military applications, coinage and coating practices are some of the most common areas of nickel utilization in indus- try which lead to a high demand for this metal throughout the world. When the trends in this demand are investigated from 1950 to 2009, it is clearly seen that the need for nickel has risen from below 200 ktpa to over 1300 ktpa, and this dramatic increase is growing at an average rate of 4% per annum (INSG, 2010; McDonald and Whittington, 2008). Until recently this high demand for nickel has been supplied mostly from the sulfide based nickel reserves; however, this trend is changing towards the more abundant lateritic type nickel re- sources, which account for about 70% of the world nickel reserves. This is due to technical, economic and environmental reasons (Moskalyk and Alfantazi, 2002). To illustrate, apart from the pre- dominance of lateritic deposits over sulfides, their easier mining by simple open-pit mining methods, unlike the extremely expen- sive underground mining of sulfides, makes their utilization more preferable. Finally, significant amounts of cobalt contained in late- rites makes them more competitive in the nickel industry. According to annual data of nickel production from sulfides and laterites, less than 10% of nickel was produced from lateritic sources in 1950. However, this ratio increased up to 42% in 2003, and is expected to rise to 51% by 2012 (Dalvi et al., 2004; Sudol, 2005). In the light of this historical data and the changing trend to- wards the lateritic type nickel deposits; technically, economically and environmentally feasible processing techniques are steadily gaining more and more importance. Understanding the history of formation of a mineral deposit is critical in establishing a possible processing route for that value. Typically, the lateritic nickel ores vary widely in their chemical composition and mineralogical structure when their history of for- mation is considered. Depending on their lateritization history, dif- ferent lateritic profiles may form on a single deposit having different mineralogical formations and distribution. The zones in a typical lateritic ore are classified and named as limonitic, transi- tion (nontronitic) or saprolitic according to their mineral content (Canterford, 1978). Naturally, the response of a specific ore to met- allurgical treatment depends on the chemical composition and mineralogical characteristics of that ore. Three metallurgical extraction processes are currently being applied; pyrometallurgi- cal, hydrometallurgical and the ‘Caron Process’. Roughly speaking, the energy intensive pyrometallurgical method generally uses the ferro-nickel production route or matte smelting process in order to recover the nickel mostly contained in the saprolitic part of the lateritic ore. However, the hydrometallurgical methods gener- ally exploit the highly selective dissolution features of both nickel and cobalt in the limonitic and nontronitic parts of the laterite ore in sulfuric acid media. The Caron Process uses the pyrometallurgi- cal and hydrometallurgical methods together in order to obtain the metallic values (Moyes, 2010). 0892-6875/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2011.05.004 Corresponding author. Tel.: +90 312 2102538; fax: +90 312 2102518. E-mail address: [email protected] (Y.A. Topkaya). Minerals Engineering 24 (2011) 1188–1197 Contents lists available at ScienceDirect Minerals Engineering journal homepage: www.elsevier.com/locate/mineng

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Page 1: High Pressure Acid Leaching of a Refractory Lateritic Nickel Ore

Minerals Engineering 24 (2011) 1188–1197

Contents lists available at ScienceDirect

Minerals Engineering

journal homepage: www.elsevier .com/ locate/mineng

High pressure acid leaching of a refractory lateritic nickel ore

S�. Kaya, Y.A. Topkaya ⇑Department of Metallurgical and Materials Engineering, Middle East Technical University (METU), 06531 Ankara, Turkey

a r t i c l e i n f o a b s t r a c t

Article history:Available online 2 June 2011

Keywords:HydrometallurgyLateriteLeachingNickelCobalt

0892-6875/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.mineng.2011.05.004

⇑ Corresponding author. Tel.: +90 312 2102538; faxE-mail address: [email protected] (Y.A. Topkay

This paper describes the experimental findings of the extraction of nickel and cobalt by high pressure acidleaching (HPAL) of a refractory limonitic nickel laterite ore from the Gördes region of Manisa in Turkey.By optimizing the basic HPAL process parameters: leaching at 255 �C with 0.30 sulfuric acid to ore weightratio with a particle size of 100% �850l for 1 h of leaching, it was found that 87.3% of nickel and 88.8% ofcobalt present in the ore could be extracted into the pregnant leach solution (PLS). However, these extrac-tion results were found to be relatively low compared with other similar studies. In order to understandthe possible reasons for this relatively lower extraction, further investigations have shown that togetherwith a problem related to the kinetics of the dissolution reactions, a persistent acid resistant refractorymineral present in this sample also limited the leaching process. Attempts were made with differentadditives to solve this problem. The effects of chemical additives such as HCl, Na2SO4, FeSO4, Cu+ and sul-fur were tested and the effect of each addition on the degree of extraction of nickel and cobalt wasdetermined.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Ferrous and non-ferrous alloying industries, petro-chemicalworks, aerospace applications, nickel based catalysts and batteryproduction, military applications, coinage and coating practicesare some of the most common areas of nickel utilization in indus-try which lead to a high demand for this metal throughout theworld. When the trends in this demand are investigated from1950 to 2009, it is clearly seen that the need for nickel has risenfrom below 200 ktpa to over 1300 ktpa, and this dramatic increaseis growing at an average rate of 4% per annum (INSG, 2010;McDonald and Whittington, 2008).

Until recently this high demand for nickel has been suppliedmostly from the sulfide based nickel reserves; however, this trendis changing towards the more abundant lateritic type nickel re-sources, which account for about 70% of the world nickel reserves.This is due to technical, economic and environmental reasons(Moskalyk and Alfantazi, 2002). To illustrate, apart from the pre-dominance of lateritic deposits over sulfides, their easier miningby simple open-pit mining methods, unlike the extremely expen-sive underground mining of sulfides, makes their utilization morepreferable. Finally, significant amounts of cobalt contained in late-rites makes them more competitive in the nickel industry.

According to annual data of nickel production from sulfides andlaterites, less than 10% of nickel was produced from lateritic

ll rights reserved.

: +90 312 2102518.a).

sources in 1950. However, this ratio increased up to 42% in 2003,and is expected to rise to 51% by 2012 (Dalvi et al., 2004; Sudol,2005). In the light of this historical data and the changing trend to-wards the lateritic type nickel deposits; technically, economicallyand environmentally feasible processing techniques are steadilygaining more and more importance.

Understanding the history of formation of a mineral deposit iscritical in establishing a possible processing route for that value.Typically, the lateritic nickel ores vary widely in their chemicalcomposition and mineralogical structure when their history of for-mation is considered. Depending on their lateritization history, dif-ferent lateritic profiles may form on a single deposit havingdifferent mineralogical formations and distribution. The zones ina typical lateritic ore are classified and named as limonitic, transi-tion (nontronitic) or saprolitic according to their mineral content(Canterford, 1978). Naturally, the response of a specific ore to met-allurgical treatment depends on the chemical composition andmineralogical characteristics of that ore. Three metallurgicalextraction processes are currently being applied; pyrometallurgi-cal, hydrometallurgical and the ‘Caron Process’. Roughly speaking,the energy intensive pyrometallurgical method generally uses theferro-nickel production route or matte smelting process in orderto recover the nickel mostly contained in the saprolitic part ofthe lateritic ore. However, the hydrometallurgical methods gener-ally exploit the highly selective dissolution features of both nickeland cobalt in the limonitic and nontronitic parts of the laterite orein sulfuric acid media. The Caron Process uses the pyrometallurgi-cal and hydrometallurgical methods together in order to obtain themetallic values (Moyes, 2010).

Page 2: High Pressure Acid Leaching of a Refractory Lateritic Nickel Ore

S�. Kaya, Y.A. Topkaya / Minerals Engineering 24 (2011) 1188–1197 1189

In this study, the response of limonitic nickel laterite ore fromthe Manisa/Gördes region of Turkey to sulfuric acid was investi-gated under high temperature and high pressure hydrometallurgi-cal techniques, and the most cost effective process parameterssuch as leaching temperature and duration, sulfuric acid/ore ratioand particle size were examined in order to add value to this de-posit. Moreover, attempts to solve the problems experienced dueto low nickel and cobalt extraction were made by using suitablechemical additives.

2. Experimental

2.1. Material characterization

In order to determine the general characteristics of the lateriticsample, physical, chemical and mineralogical characterizationsteps were performed. For this purpose, the ore was sampled byconing and quartering. Initially bulk and solid density measure-ments were conducted on the representative limonitic nickel later-ite ore sample. Bulk density measurements were done according tothe ratio of ore weight to ore volume, without joggle. On the otherhand, the solid density of the sample was determined by a bottlepycnometer. After the density measurements, the moisture con-tent determination of the as-received ore was determined byweighing and drying a representative limonitic sample in a dryingoven at 105 �C until a constant dry mass was attained.

The limonitic sample was dried and ground to 100% �38l andanalyzed chemically by the Inductively Coupled Plasma (ICP)method. Chemical analyses of the pregnant leach solutions afterthe leaching experiments were conducted using AAS, while thedried and ground solid leach residues were mainly analyzed byusing a Niton X-Met 820 X-ray Fluorescence (XRF) analyzer andby Atomic Absorption Spectroscopy (AAS) together. In order todetermine the types of minerals present in the ore samples, a Rig-aku D/MAX2200/PC model X-ray Diffractometer with a Cu-Ka X-ray tube working under 40 kV and 40 mA was used during the min-eralogical analysis of the limonitic nickel laterite sample. Also themineralogical analysis results were verified by the aid of a NovaNanosem 430 Scanning Electron Microscope (SEM). Investigationof the thermal behavior of the minerals present in the limoniticnickel laterite sample was the last step in the mineralogical char-acterization study. For this purpose, the ore samples were driedand ground to a size of 100% �38l and subjected to DifferentialThermal Analysis (DTA) and Thermo Gravimetric Analysis (TGA)by a Simultaneous Setaram DTA and TGA device. These analyseswere conducted at a heating rate of 10 �C/min within the temper-ature range of 35–1000 �C in air in an alumina crucible.

2.2. Experimental set-up and procedure

High pressure acid leaching experiments were conducted in aParr-4532 model, 2 l, titanium grade-4 autoclave which wasequipped with automatic heating and cooling units, and a magnet-ically driven stirring system. To test the ore sample, a slurry of150 g of limonitic nickel laterite ore was prepared with deionizedwater according to a predetermined solid/liquid ratio of 30 wt.%solids. After the slurry was prepared in the autoclave, technicalgrade sulfuric acid (96–98 wt.%) was added and the lid of the auto-clave was closed carefully in order to prevent any leakage duringthe high pressure leaching operation. At this point, any analyticalreagent (AR) of high purity, to test its effect on leaching, was alsoadded before sealing the autoclave, and the system was allowedto heat up to the desired set-point temperature. The start of thereaction, defined as zero time, was defined as when the reactortemperature reached the set-point and the reaction was left to pro-

ceed until the target leaching duration was attained. After the re-quired leaching duration was completed, the system was allowedto cool to room temperature for approximately 1 h, by means ofwater flowing through a titanium cooling coil. In order to performsolid/liquid separation after leaching, the resultant slurry was fil-tered by the aid of a vacuum pump and Whatman grade-40 filterpaper on a Buchner funnel. After loading the pregnant leach solu-tion with leached metal cations, the solid that remained in theBuchner funnel was washed well in order to completely removethe pregnant leach solution. The resultant leach residue was thendried overnight at 105 �C ready for chemical analysis. Leach resi-dues and pregnant leach solutions obtained were analyzed chemi-cally as explained previously. Meanwhile, the resultant pregnantleach solutions were also analyzed for residual acid. Oxidation–reduction potential (ORP) and density measurements were alsoperformed.

During the residual free acid determinations, 0.2 M sodiumhydroxide (NaOH) solution was used to neutralize the free acidwhich remained in the pregnant leach solution. In order to sup-press the interfering effect of some ions during titration, 280 g/Ldi-potassium oxalate monohydrate (K2C2O4�H2O) solution wasused as a chelating agent during the titration. At the same timethe pH meter was calibrated to pH 7.0 by a special buffer solution.The titration procedure was as follows; 20 cc of 280 g/L di-potas-sium oxalate monohydrate solution was diluted with 5 cc of deion-ized water and the pH of this mixture was measured with the pre-calibrated pH meter. Then, 5 cc of pregnant leach solution wasadded into this mixture and the solution was homogenized bythe help of a magnetic stirrer. Finally, this solution was titratedwith 0.2 M sodium hydroxide solution to the initial pH level. Then,from the amount of sodium hydroxide consumed, the residual acidin the pregnant leach solution was determined. For the oxidation–reduction potential measurements, a Pt-Ag/AgCl electrode (satu-rated with KCl) was used and the measured values were reportedaccording to the Pt-Ag/AgCl reference electrode.

2.3. Experimental parameters studied

Throughout the experiments various process parameters werestudied. Initially, the experiments began with the investigation ofthe effect of leaching temperature on nickel and cobalt extractions.Then the effects of leaching duration, sulfuric acid/ore ratio (w/w),and particle size were examined. In addition to these basic processparameters, the effect of prior heat treatment and the effects ofvarious additives were studied and the list of process parametersinvestigated is summarized in Table 1.

3. Results and discussion

3.1. Characterization results

The bulk and solid densities of the representative Gördes limo-nitic nickel laterite ore were found to be 1.04 and 3.26 g/cm3,respectively. According to the moisture content measurement, itwas calculated that the sample contained 23.47% of physically heldwater. This suggested that it would be a very energy intensive pro-cess just in terms of moisture content if a pyrometallurgical pro-cessing route was selected for the treatment of this ore. Chemicalanalysis of the limonitic nickel laterite sample is given in Table 2.By combining the chemical analysis results together with XRDand DTA/TGA findings, it was found that the limonitic sample con-tained high amounts of quartz (SiO2), hematite (Fe2O3), and goe-thite (FeOOH) minerals as well as the mineral smectite, havingthe idealized mineral formula of (Na0.3Fe2(Si,Al)4O10(OH)2�nH2O).In order to verify the previous analyses, SEM studies on the laterite

Page 3: High Pressure Acid Leaching of a Refractory Lateritic Nickel Ore

Table 1List of process parameters studied during HPAL experiments.

Experiments Fixed parameters Studied parameters Additive

Leaching temp. (�C) 60 min, �850l, 0.3 acid/ore ratio 245, 250, 255, 260, 265, 270 No additionLeaching duration (min) 255 �C, �850l, 0.3 acid/ore ratio 0, 60, 90, 180, 360 No additionSulfuric acid/ore ratio (w/w) 255 �C, 60 min, �850l 0.275, 0.300, 0.325, 0.350 No additionParticle size (l) 255 �C, 60 min, 0.3 acid/ore ratio �38, �425, �850, �1400, �2000 No additionPrior heat treatment 255 �C, 60 min, �850l, 0.3 acid/ore ratio Limonite heat treated at 350 �C No additionHCl addition (kg/ton dry ore) 255 �C, 60 min, �850l, 0.3 acid/ore ratio 33.33, 66.67 HCl (37 wt.%)(AR)Na2SO4 addition (gpl) 255 �C, 60 min, �850l, 0.3 acid/ore ratio 12.5, 25.0 Na2SO4 (AR)FeSO4 addition (gpl) 255 �C, 60 min, �850l, 0.3 acid/ore ratio 14.3, 28.6 FeSO4 (AR)Cu+ addition (gpl) 255 �C, 60 min, �850l, 0.3 acid/ore ratio 1, 2, 3 Cu2O (AR)Sulfur addition (kg/ton dry ore) 255 �C, 60 min, �850l, 0.3 acid/ore ratio 1, 2, 3 S (AR)

Table 2Chemical analysis of limonitic nickel laterite sample.

Constituent Fe Ni Co Cr2O3 MnO As Al2O3

Content (wt.%) 28.70 1.28 0.083 1.99 0.59 0.68 5.83

Constituent SiO2 MgO CaO K2O TiO2 CuO SContent (wt.%) 28.8 2.26 1.27 0.12 0.13 0.04 0.43

1190 S�. Kaya, Y.A. Topkaya / Minerals Engineering 24 (2011) 1188–1197

sample indicated that the nickel was mainly present in the crystallattices of the minerals goethite (FeOOH), hematite (Fe2O3), ser-pentine ((Mg, Fe)3Si2O5(OH)4) and smectite (Na0.3Fe2(Si,Al)4O10(OH)2�nH2O). Cobalt, however, was mainly present in thecrystal structure of the mineral asbolane having the idealized for-mula of (Co, Ni)1�y(Mn4+O2)2�x(OH)2�2y+2x�nH2O. Similar resultswere reported by Büyükakinci for Gördes limonites in a previousstudy (Büyükakinci and Topkaya, 2009).

3.2. Extraction results

The effect of temperature on the degree of nickel and cobaltextractions was initially investigated and then the effects of leach-ing duration, sulfuric acid/ore ratio and particle size were testedseparately in order to obtain the optimum nickel and cobalt extrac-tions with the most cost effective process parameters. The overallresults of these experiments are discussed in the followingsections.

3.2.1. Effect of temperatureAccording to the extraction behavior of nickel with increasing

temperature, it was noticed that there was an increasing trend inthe extraction of nickel with the process parameters given in Ta-ble 1. However, beyond 265 �C this increasing trend seemed to dis-appear with further increase in temperature. Georgiou andPapangelakis in 1988 reported similar behavior between the tem-peratures of 230 and 270 �C. A study conducted by Chou et al. in1977 showed that leaching up to 275 �C increases nickel yield,but leaching conducted at 300 �C leads to a decrease in the degreeof nickel extraction. The cause of this behavior was reported in theliterature by indicating that in excess of 270 �C, the possibility ofnickel–magnesium sulfate (Mg, Ni)SO4�H2O co-precipitation in-creases severely due to the lower solubility of magnesium at highertemperatures (Whittington and Muir, 2000). This precipitation isgenerally accompanied with nickel and cobalt losses to the precip-itate. It is also reported that as the temperature of pressure leach-ing goes beyond 280 �C, the possibility of Fe(OH)SO4 (basic ironsulfate) and Al(OH)SO4 (basic aluminum sulfate) formations in-crease, with accompanied acid losses which are not desirable dur-ing the pressure acid leaching process.

When the extraction behavior of cobalt is considered, it is seenfrom Fig. 1 that cobalt is not as sensitive to increasing process tem-peratures as nickel and its intensity decreases beyond 265 �C.

When this result is compared with the reported data in the litera-ture, it is evident that almost 80% of the cobalt is rapidly taken intothe leach solution, possibly due to the presence of cobalt in thereadily-leached manganese mineral ((Co, Ni)1�y(MnO2)2�x(OH)2�2y+2x�n(H2O)), asbolane (Georgiou and Papangelakis, 1998).Georgiou and Papangelakis (2009) showed that treating lateriticore between the temperatures of 230 and 270 �C had essentiallyno effect upon the rate of cobalt extraction. Cobalt was leachedreadily in the first 10 to 20 min of leaching, and its dissolution rateslowed down thereafter. Chou et al. also emphasized the high ini-tial rate of cobalt extraction. Therefore, the extraction behavior ofcobalt is generally reported to be less affected by increasing tem-perature (Chou et al., 1977). Other than nickel and cobalt extrac-tions, it is interesting to note that as a result of increasing leachtemperatures, remarkable changes in the extractions of iron, alu-minum and, to a limited extent, chromium were observed. This isas a result of temperature rise in the pressure leach process, theprecipitation rates of iron and aluminum increasing in parallelwith temperature with inverse solubility of hematite and aluniteprecipitates. This has also been supported by Georgiou and Papan-gelakis (1998) who report that the increasing leaching tempera-tures favor increasing nickel and cobalt concentration ratios ‘[(Nior Co)/(Fe + Al)]’ as a result of the inverse solubility of hematiteand alunite in the pregnant leach solution.

The pressure acid leach experiments conducted at differentleaching temperatures showed that more nickel and cobalt couldbe extracted into the pregnant leach liquor in just 1 h of leachingat higher leaching temperatures in the autoclave. However, sincehigher leaching temperatures mean higher initial investment andoperational costs, the optimal process temperature should be se-lected whether or not the extra nickel and cobalt credit compen-sates for the former production expenses. Therefore, for the restof the tests a leaching temperature of 255 �C was selected, andhigher nickel and cobalt extractions were obtained by optimizingthe other process parameters effectively.

3.2.2. Effect of leaching durationThe data obtained from leaching duration experiments showed

that the nickel and cobalt behaved similarly when the leaching dura-tion was prolonged. It is shown in Fig. 2 that from the starting point,up to 180 min of leaching duration, the nickel and cobalt extractionsincreased as intended. However, although increasing nickel extrac-tion continued from 180 min up to 360 min of leaching duration, this

Page 4: High Pressure Acid Leaching of a Refractory Lateritic Nickel Ore

Fig. 1. Effect of temperature upon degree of nickel and cobalt extractions.

Fig. 2. Effect of leaching duration upon degree of nickel and cobalt extractions.

S�. Kaya, Y.A. Topkaya / Minerals Engineering 24 (2011) 1188–1197 1191

trend resulted in an extraction value of around �95%, which sug-gested that further increase in leaching duration would be of limiteduse in increasing the degree of nickel and cobalt extractions. Thus, itwas thought that, together with a problem related with the kineticsof the chemical reactions, a persistent leaching behavior limited theprocess. In other words, these results suggested that there may bevery acid resistant refractory minerals present in this limonitic nick-el laterite sample which prevented the leaching of nickel and cobaltfrom the lattice of these minerals. When the mineralogical charac-terization step is referred to again, the presence of hematite withinthe limonitic sample appeared to be the possible reason for thisextraction behavior. This difficulty in the leaching behavior of hema-tite was also reported by Kui Lui et al. who stated that leaching be-comes more difficult among the nickel containing minerals in thefollowing order; Lizardite > goethite > maghemite > magnetite >hematite > chromite � ringwoodite (Liu et al., 2009, 2010). There-fore, this incomplete extraction of nickel stemming from the primaryhematite was examined throughout this study to see whether it ispossible to overcome this problem.

Apart from nickel and cobalt, marked decreases in the extrac-tion of iron, aluminum and chromium were observed after pro-longed leaching, due to increased precipitation reactions favoredby extended leaching.

The pressure acid leach experiments conducted at differentleaching durations showed that more nickel and cobalt could beextracted into the pregnant leach liquor at 255 �C by increasingthe duration of leaching in the autoclave. However, as in the caseof temperature, there was a limit to this increase, possibly due tothe difficulty in leaching refractory minerals suspected to bemainly hematite. Therefore, prolonged leaching may be helpful inincreasing the desired level of nickel and cobalt extractions. How-ever, since prolonged leaching decreases the overall capacity of theplant and thus increases the costs, the optimal leaching durationshould be selected whether the extra nickel and cobalt creditscompensate for the former production expenses. Therefore, forthe remaining tests just 60 min of leaching was selected, and high-er nickel and cobalt extractions were obtained by suitable addi-tives and optimizing other process parameters more effectively.

3.2.3. Effect of sulfuric acid/ore ratioWhile examining the results of the effect of sulfuric acid con-

centration on nickel and cobalt extractions, the amount of residualacid in the pregnant leach liquor after leaching is also of impor-tance. It has been reported from other research that for high nickeland cobalt extractions sufficient free acid should remain in thepregnant liquor in order to maintain the stability of dissolved spe-cies and prevent undesired nickel and cobalt losses (Whittington

Page 5: High Pressure Acid Leaching of a Refractory Lateritic Nickel Ore

Table 3Free acidity with respect to sulfuric acid concentration.

Acid amount (kg/ton dry ore) 275 300 325 350Free acidity (gpl) 39.2 41.2 44.5 44.9

1192 S�. Kaya, Y.A. Topkaya / Minerals Engineering 24 (2011) 1188–1197

and Muir, 2000). However, while various researchers have reportedthe positive effect of residual acidity in the pregnant leach liquor,there is also a limiting value to the amount of this residual acidpresent in the solution. During the solution purification and metalrecovery steps this residual acid should be neutralized somehow,CaCO3 often being the most suitable reactant. Therefore, theamount of residual acid becomes directly related with the con-sumption of basic reagent and, in turn, it determines the viabilityof the process economics in terms of the rising cost ofconsumables.

Practical data from various processing plants help to clarify theabove. Testwork on Ramu ore at 250 �C suggests that 30 g/L ofresidual acid was optimal for sufficient nickel and cobalt extrac-tion. Similarly, 30–40 g/L and 40 g/L of free acid were necessaryfor Bulong and Murrin Murrin ores respectively (Whittington andMuir, 2000). Therefore, for Gördes limonitic nickel laterite ore40 g/L residual sulfuric acid was targeted for satisfactory results.The actually measured residual acid values are given in Table 3.

Therefore, when the nickel and cobalt extraction results in Fig. 3and the residual acid measurement values in Table 3 are evaluatedtogether, 300 kg of sulfuric acid per ton of dry limonitic nickel lat-erite ore seemed to be the most appropriate selection among theother alternatives, since the extra acid addition did not satisfythe required increase in extraction values, in fact increasing theacid and base consumptions pointlessly. In addition to the above,the leach residue samples were analyzed by XRF and the sulfurcontents of the leach residue samples were also determined.According to the obtained results, the sample leached with300 kg of sulfuric acid was found to contain 1.89% sulfur, whereasthe sample leached with 350 kg of sulfuric acid contained 2.25%sulfur, which showed that the increased sulfuric acid addition alsoincreased the sulfur losses in the leach residue needlessly. There-fore, this finding also showed that the addition of 300 kg of sulfuricacid would give the best result when the overall considerationswere taken into account.

3.2.4. Effect of particle sizeThe results of particle size experiments given in Fig. 4 show that

the responses of nickel and cobalt extractions were positive whenfiner particles were fed into the autoclave. As seen from Fig. 4, the

Fig. 3. Effect of sulfuric acid concentration upo

particle size became more important in terms of nickel extractionwhen the feed size was decreased below 425 lm, and furtherreduction in the particle size seemed to give more effective results.Similarly, the extraction of cobalt was increased in such a way thatas the particle size was reduced from �2000 lm to �425 lm, therewas an almost linear increase in the cobalt extraction. However,this trend became steeper and led to an exponential increase inthe extraction of cobalt into the pregnant leach solution whenthe size was reduced from �425 lm to �38 lm. This was due tothe effect of increased specific surface area of refractory minerals,such as hematite and quartz, containing small fractions of nickeland cobalt, which was verified by scanning electron microscopyof the limonitic nickel laterite sample.

In addition to changes in nickel and cobalt extractions, Chouet al. also showed that excessive grinding of the ore created extrasurface area, which in turn provided additional nucleation sitesfor iron and aluminum precipitation. Therefore, the impurity con-centration of pregnant leach solution decreased as a result of overgrinding. Similar trends in the extractions of iron and aluminumwere observed during the leaching experiments. Extractions of ironand aluminum tended to decrease as a result of reduction in thefeed size which was in agreement with the statement made byChou et al. in 1977.

When the overall results of the particle size experiments areconsidered, it can be said that grinding was beneficial in terms ofhigher nickel and cobalt extractions and cleaner PLS characteristicsfor further downstream processing. However, this is limited bytechnical and economical reasons, as grinding becomes more diffi-cult and energy consuming when the particle size is reduced exces-sively. The rheology of the pulp fed into the autoclave is also a veryimportant consideration which should be kept in mind during thefinal decision making process. Since the rheological properties andease of stirring in the autoclave are a function of the feed size it be-comes very difficult to provide effective stirring when very coarseparticle sized feed is fed into the system. Therefore, in the light ofthese considerations, the limonitic samples ground to 100%�850 lm were used in the remaining experiments. By using theabove mentioned process parameters, 87.3% of nickel and 88.8%of cobalt present in the ore were extracted into the pregnant leachsolution and the solution contained 4677 ppm Ni, 266 ppm Co,2620 ppm Al, 1364 ppm Mn, 4754 ppm Mg, 98 ppm As, 123 ppmCr, and 2296 ppm Fe.

3.2.5. Effect of prior heat treatmentPreviously the effects of temperature, leaching duration, acid/

ore ratio, and particle size on nickel and cobalt extraction results

n degree of nickel and cobalt extractions.

Page 6: High Pressure Acid Leaching of a Refractory Lateritic Nickel Ore

Fig. 4. Effect of particle size upon degree of nickel and cobalt extractions.

S�. Kaya, Y.A. Topkaya / Minerals Engineering 24 (2011) 1188–1197 1193

have been described and further tests were then performed at255 �C with 0.30 acid to ore weight ratio, with a particle size of100% �850 lm for 1 h duration. When these optimum parameterswere used and the leach residue sample was analyzed by XRD, itwas found (Fig. 5) that there were no considerable changes inthe quartz peaks after leaching, showing that quartz had remainedalmost intact during the leaching operation. However, the charac-teristic peaks of smectite and goethite within the limonitic nickellaterite ore had disappeared, indicating that these minerals wereleached out from the original ore sample. After leaching, the inten-sity of the characteristic hematite peaks increased in the XRD pat-tern of the leach residue, verifying that iron had tended toprecipitate in the form of secondary hematite. Therefore, it is likelythat nickel present within the primary hematite mineral may bethe main reason for the low extraction results experienced withthese limonitic nickel laterite samples. In order to verify this, the

Fig. 5. XRD result comparison of lim

limonitic sample was heated to 350 �C in order to transform allthe goethite to hematite. The selection of heat treatment tempera-ture was based on the thermal analysis result for the limoniticsample given in Fig. 6. As clearly seen in Fig. 6, at about 270 �C goe-thite present in the limonitic sample has transformed into hema-tite according to the dehydroxylation reaction of goethite.However, in order to be on the safe side, the heat treatment tem-perature was chosen as 350 �C. The nickel and cobalt extraction re-sults obtained after pressure leaching for the heat treated limoniticore are given in Fig. 7 together with those of the original ore ob-tained under the same experimental conditions, for comparisonpurposes. It is obvious from Fig. 7 that on transforming the goe-thite to hematite, there was a tremendous decrease in the extrac-tion of nickel and cobalt which means that it is more difficult toleach and obtain nickel from hematite compared with goethite.Moreover, a scanning electron microscopy study on the leach res-

onite ore and its leach residue.

Page 7: High Pressure Acid Leaching of a Refractory Lateritic Nickel Ore

Fig. 6. DTA/TGA analysis results of limonitic sample.

Fig. 7. Effect of prior heat treatment upon degree of nickel and cobalt extractions.

1194 S�. Kaya, Y.A. Topkaya / Minerals Engineering 24 (2011) 1188–1197

idue has also shown that the losses of nickel were mainly related tothe iron minerals. Therefore, in the following stages, it was at-tempted to overcome this difficulty by making various changesto the leaching environment.

3.2.6. Effect of HCl additionOn evaluation of the results of hydrochloric acid addition shown

in Fig. 8, the findings were satisfactory only in terms of increasingthe nickel and cobalt extractions, as the extremely corrosivebehavior of the acidic mixture resulted in damage to the leachingequipment. As a result of severe acidic attack, some rupture plateson the autoclave have exploded during acid mixture testing whichled to the termination of further tests. Thus, the overall conclusionwas that HCl together with H2SO4 might be an alternative in orderto enhance the extraction behavior of nickel and cobalt from therefractory minerals of the limonitic nickel laterite ore. However,

the autoclave should be designed to endure the severe corrosiveleaching conditions.

3.2.7. Effect of Na2SO4 additionAlthough some studies report the beneficial effects of Na2SO4

addition, the experimental results of Gördes limonitic nickel later-ite ore given in Fig. 9 were unsatisfactory due to the observed de-crease in the degree of nickel and cobalt extractions. However, thecontribution of Na2SO4 in changing the solution impurity concen-trations and residue mineralogy could not be ignored, as whenthe extractions of Fe, Al, Mn, Mg, and Cr in the pregnant leach solu-tion were considered, a remarkable decrease in their amounts wasobserved. The reason for the decrease of these ions is explained bythe increased stability of natroalunite and/or natrojarosite andtheir precipitation as a result of sodium ions present in the solution(Johnson et al., 2005). In addition to the observed decrease in theconcentration of impurity elements, nickel and cobalt may alsosubstitute into the alunite/jarosite structure during precipitation,which may be the reason for the observed low nickel and cobaltextraction values (Whittington and Muir, 2000). Parallel to the for-mation of natroalunite and/or natrojarosite, the amount of residualacid was measured and it was seen that the free acid in the solu-tion continuously decreased from 41.2 gpl to 37.8 and 33.9, respec-tively as a result of increases in the Na2SO4 concentration. Thedecrease in residual acid and the formation of natroalunite and/or natrojarosite were explained and formulated according to thechemical reactions given below (Johnson et al., 2005):

3Fe2ðSO4Þ3 þ Na2SO4 þ 12H2O! 2NaðFeÞ3ðSO4Þ2ðOHÞ6 þ 6H2SO4

ð1Þ

3Al2ðSO4Þ3 þ Na2SO4 þ 12H2O! 2NaðAlÞ3ðSO4Þ2ðOHÞ6 þ 6H2SO4

ð2Þ

Page 8: High Pressure Acid Leaching of a Refractory Lateritic Nickel Ore

Fig. 8. Effect of HCl addition upon degree of nickel and cobalt extractions.

Fig. 9. Effect of Na2SO4 addition upon degree of nickel and cobalt extractions.

S�. Kaya, Y.A. Topkaya / Minerals Engineering 24 (2011) 1188–1197 1195

According to these reactions and from the residual acid mea-surements, the presence of sodium ions in the solution affectsthe iron and aluminum chemistry and inhibits the regenerationof sulfuric acid to the solution.

The changes observed in the reaction chemistry and residuemineralogy also affected the filtration behavior of the leach residueafter pressure leaching. As a result of changes in the precipitationreactions, the leach residue showed colloidal behavior during fil-tration which could be explained by the reduction in free acidityhaving hydrolyzed the silicic acid to colloidal silica, which in turncomplicated the solid–liquid separation process (Whittington andMuir, 2000). Therefore, although the addition of Na2SO4 was veryeffective for solution purification purposes, it seemed that thisadditive would not solve the problem of low nickel and cobaltextraction results.

3.2.8. Effect of FeSO4 additionVarious researchers have reported the positive effect of the

presence of reducing species in the pregnant leach solution duringpressure acid leaching of lateritic nickel resources. Therefore, inthis part of the study, the effect of divalent iron addition to theleach solution was investigated in order to enhance the nickeland cobalt extraction values. As stated in the literature, the addi-tion of divalent iron led to a drastic fall in the reduction potentialof the leach solution. To be more specific, the reduction potential of

the pregnant leach solution decreased from 489 mV to 366 and344 mV, respectively after the FeSO4 additions, and this reducingsolution led to the desired increase in the nickel and cobalt extrac-tion values, as shown in Fig. 10. This behavior was explained byTindall and Muir (1997) as the presence of divalent iron speciesfacilitates bond breakage and aids iron oxide dissolution via elec-tron transfer in the leach solution, thus increasing the extractionof valuable metals from the goethite and hematite matrix. Also re-lated to the FeSO4 addition is that the amount of iron present in theleach solution increased due to iron sulfate addition as expected.Before ferrous ion addition, most of the iron in the PLS was inthe form of ferric ion. However after this addition the amount offerrous ion was naturally increased, which is a disadvantage fordownstream processes, as iron present in the ferrous state shouldbe oxidized to the ferric state during downstream processes, mak-ing the process more complicated. So apart from the negative effectof ferrous ions during the downstream solution purification pro-cesses, this addition was effective in enhancing the extractionbehaviors of nickel and cobalt on changing the solution character-istics to a more reducing character.

3.2.9. Effect of Cu+ additionAfter obtaining somewhat encouraging results from the FeSO4

addition experiments, further tests to evaluate the positive effectsof reducing solution characteristics were attempted with the

Page 9: High Pressure Acid Leaching of a Refractory Lateritic Nickel Ore

Fig. 10. Effect of FeSO4 addition upon degree of nickel and cobalt extractions.

1196 S�. Kaya, Y.A. Topkaya / Minerals Engineering 24 (2011) 1188–1197

addition of cuprous ions (Cu+) into the leach solution. The catalyticmechanism of cuprous ions is correlated with the ease of electrontransfer between Cu+ and iron species. According to the proposedmechanism in the literature, the dissolution of iron species in thepresence of cuprous ions is catalyzed by the following electro-chemical reaction, while nickel present within the crystal latticeof iron minerals is predicted to be liberated into the solution (Byer-ley et al., 1979; Parker and Espenson, 1969);

Fe3þ þ Cuþ ! Fe2þ þ Cu2þ ð3Þ

According to the results shown in Fig. 11, the positive effectof cuprous ions in extracting the nickel present in the hematitecrystal lattice was similar to that of previous studies performedby other researchers. In the case of cobalt, previous work hasshown that the extraction of cobalt can be enhanced when thepotential of the solution is adjusted to a more reducing character(Tindall and Muir, 1997). This was verified by the addition ofreducing ferrous ions into the leach solution. On addition of cu-prous ions in increasing amounts, the reduction potential of thesolution was lowered from 489 mV to 449, 440, and 410 mV,respectively. However, the expected enhancement in extractionbehavior of cobalt was not as good as that obtained by ferrous

Fig. 11. Effect of Cu+ addition upon degr

ion addition. According to Fig. 11, only a slight increase in theextraction of cobalt was observed when compared to nickel. Thiswas due to higher ORP values obtained in the case of Cu(I) addi-tions as a result of the significant variation of FeSO4 to Cu(I) con-centrations in the liquor.

3.2.10. Effect of S additionExperimental evidence up to this stage has shown that there is a

close relationship between the electrochemical nature of the leach-ing solution and the observed nickel and cobalt extractions. Toconfirm this, the same constant test parameters were used as inthe case of ferrous and cuprous ions additions, and the additionof sulfur to change the reduction potential of the leach solutionwas planned in order to facilitate electron transfer in the solution.For the no-sulfur added experiment the reduction potential was489 mV. However, on addition of sulfur the reduction potentialswere measured as 373, 349, and 334 mV, respectively withincreasing sulfur content. Thus sulfur seemed to be very effectivein controlling the reduction potential of the leach solution. Accord-ing to the results given in Fig. 12, almost the same extractions ofnickel and cobalt were observed as in the case of cuprous ion addi-tion. Slight increases in the extractions of nickel and cobalt werebelieved to be the result of more reducing solution characteristics.

ee of nickel and cobalt extractions.

Page 10: High Pressure Acid Leaching of a Refractory Lateritic Nickel Ore

Fig. 12. Effect of S addition upon degree of nickel and cobalt extractions.

S�. Kaya, Y.A. Topkaya / Minerals Engineering 24 (2011) 1188–1197 1197

4. Conclusions

In this experimental study the basic HPAL process parameters,such as leaching temperature, leaching duration, sulfuric acid toore weight ratio and particle size of a limonitic nickel laterite orewere studied. The observed enhancement in the degree of nickeland cobalt extractions with increase in leaching temperature andduration, together with decreased particle size, have shown that,together with a problem related to the kinetics of the chemicalreactions, a persistent leaching behavior also limited the process.On optimizing the basic HPAL process parameters – leaching at255 �C with 0.30 sulfuric acid to ore weight ratio with a particlesize of 100% �850l for 1 h, it was found that 87.3% of nickel and88.8% of cobalt present in the ore could be extracted into the preg-nant leach solution. However, these results were found to be belowthe desired values. Therefore, the possible reasons for this behaviorwere investigated and the presence of difficult to leach hematitemineral was found to be the most probable one. Heat treatmentof the limonitic nickel laterite ore prior to leaching has shown thatgoethite present in the ore had fully transformed to hematite andafter this transformation a tremendous decrease in the nickel andcobalt extractions was observed. Therefore, in order to dissolve thenickel and cobalt present in the difficult to leach hematite mineralpresent in the limonitic nickel laterite ore, additions of HCl, ferrousions, cuprous ions and sulfur were tried, respectively. The data ob-tained for the overall extraction with the various additives showedthat the improvement of nickel and cobalt extractions was �3%.

Acknowledgements

The authors would like to express special thanks to META Nikel-Kobalt A.S�. both for supplying the limonitic nickel laterite ore sam-ple of Gördes and for doing the chemical analyses by XRF and AAS.Also, METU Center Laboratory for DTA-TGA analyses and METUMetallurgical and Materials as well as Chemical EngineeringDepartments for performing XRD and AAS analyses are gratefullyacknowledged.

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