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Pressure acid leaching of arid-region nickel laterite ore
Hydrometallurgy 78 (2005) 264 – 270 www.elsevier.com/locate/hydromet
Pressure acid leaching of arid-region nickel laterite ore Part IV: Effect of acid loading and additives with nontronite ores J.A. Johnson*, R.G. McDonald, D.M. Muir, J.-P. Tranne A.J. Parker CRC for Hydrometallurgy, CSIRO Minerals, P.O. Box 90, Bentley, WA, 6982, Australia Received 16 February 2005; received in revised form 5 April 2005; accepted 13 April 2005
Abstract The extraction of nickel from nontronite-rich laterite ores is enhanced by either increasing the acid loading, or the addition of a small amount of sodium to the process water. The results of this study indicate that at least 15% less acid can be used to get the same extraction of nickel from a nontronite-rich laterite ore in process water containing 5 g/L sodium ion, compared with the same ore in fresh process water. In addition, nickel extractions from a second nontronite-rich laterite ore were increased by up to 3% compared with fresh water, if process water sodium ion levels is present at 5–11 g/L at a given acid loading of 420 kg/t ore. Crown Copyright D 2005 Published by Elsevier B.V. All rights reserved. Keywords: Nickel laterite; Pressure acid leaching; Nontronite; Acid loading; Salinity
1. Introduction Laterite nickel deposits can be found in both tropical areas (Cuba, New Caledonia, Indonesia, Philippines) and dry-land areas (Albania, Greece, Western Australia) (Whittington and Muir, 2000; Krause et al., 1997). Typically, the dry-land laterites are less weathered than tropical laterites and contain more clay minerals (e.g. nontronite) and less goethite. High temperature pressure acid leaching extracts the nickel and cobalt values from both types of laterites, and this DOI of original article: 10.1016/j.hydromet.2005.04.003. * Corresponding author. Tel.: +61 8 9334 8000; fax: +61 9334 8001. E-mail address: [email protected] (J.A. Johnson).
process is in current use at Moa Bay, Cuba and at Cawse and Murrin Murrin in Western Australia (Whittington and Muir, 2000). Other projects, utilising pressure acid leaching technology are currently under construction or under consideration (Adams et al., 2004; Torres et al., 2004; Tsuchida et al., 2004). Process water quality can affect the high temperature acid leaching of nickel laterite ores. Whittington et al. (2003) summarised the effect of acid charge and process water salinity on the chemistry occurring during the high temperature acid leaching of Bulong (blended) ore. Water salinity affected the relative amounts of alunite/jarosite and hematite that formed in the leach residue which, in turn, affected the acid consumption. In particular, more alunite/jarosite formed in saline water than tap water, at the expense
0304-386X/$ - see front matter. Crown Copyright D 2005 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2005.04.002
J.A. Johnson et al. / Hydrometallurgy 78 (2005) 264–270
of hematite. This resulted in a higher acid consumption and lower concentration of residual acid in the leach slurry, and hence lower nickel extractions (Whittington et al., 2003). A general acid consumption equation, which takes into account the variation in residue mineralogy with leach conditions has also been developed (Whittington et al., 2003). A study with Cawse limonite ore (Johnson et al., 2002) found that higher nickel extractions could be obtained in sea water, relative to fresh water, at a given acid loading despite the free acidity of sea water leach liquor, being less than that of the fresh water liquor, at the end of leaching. Similar studies by Marshall and Buarzaiga (2004) and Buarzaiga and Laframboise (2004) with tropical limonite ores found that nickel extraction was improved by leaching in sea water, though the leach liquor free acidities were also higher, relative to fresh water. It therefore appears that leach chemistry is quite ore-type specific. Residue mineralogy, acid consumption and nickel extraction are all intimately linked and changes to one can be correlated with changes to the others. The current paper examines the effect of acid loading on the leaching kinetics and nickel extraction from two nontronite ores, supplied from Murrin Murrin. It also examines the effect of water salinity on leaching chemistry, especially in relation to acid consumption, and compares the results obtained to those from other ores. Finally, the effect of reducing acid loading, whilst maintaining a specific process water salinity, with a second nontronite ore is examined, with respect to leaching chemistry and consequent residue mineralogy.
2. Experimental 2.1. Ore and process water preparation and analysis Two plant pressure acid leach (PAL) slurries from Murrin Murrin were provided for this work. Where required, sub-samples of slurries were filtered, washed with water, dried at 40 8C, and then reslurried with the relevant process water. Table 1 summarises the elemental compositions of the ores used in this testwork. Tap water, sea water and hypersaline process water were used as received to make up ore slurries. In addition, diluted hypersaline pro-
265
Table 1 Composition of ores used in this study Sample
Murrin 1 Murrin 2
Ore composition (wt%) Ni
Co
Mg
Mn
Al
Fe
Si
Cr
Na
1.3 1.4
0.1 0.1
6.3 3.5
0.4 0.4
2.4 2.9
19.2 25.2
19.3 18.5
0.9 0.9
0.1 0.1
cess water (BW3) was made up from a mixture of 60% hypersaline water and 40% tap water. Table 2 summarises the chemical analyses of the process waters used. In selected reactions with tap water, Na2SO4 was used to control solution sodium levels and this was added to the slurry immediately prior to heating the autoclave. Approximate phase compositions of each ore were calculated by Rietveld analysis of the respective X-ray power diffraction (XRPD) traces. The lower magnesium content of the second Murrin Murrin ore is reflected in the lesser amount of serpentine type minerals present. The approximate phase compositions are as follows: Murrin Murrin Ore 1 Nontronite 50%, Serpentine 20%, Goethite 15%, Maghemite 10%, with 5% Quartz and Dolomite. Murrin Murrin Ore 2 Nontronite 70%, Serpentine 9%, Goethite 9%, Maghemite 9%, with 3% Quartz and Dolomite. 2.2. High pressure acid leaching and leach residue characterisation The leaching of Murrin Murrin ores were conducted in process waters of varying salinity. Reactions were conducted at 255 8C, the same temperature as that used at the Murrin Murrin plant. The equipment and leaching techniques and leach liquor and residue characterisation have previously been described in detail by Whittington et al. (2003).
3. Effect of acidity on leach chemistry 3.1. Free acidity and acid consumption Acid loadings were varied from 360 kg/t ore to 440 kg/t ore, with intermediate levels of 390 kg/t and
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Table 2 Process water analyses
Table 4 Effect of acid loading and salinity residue mineralogy
Water type
Density Liquor composition (mg/L) (g/mL) Mg Si SO2 Na Ca 4
Murrin Murrin Seawater BW3 Bulong
1.002 1.023 1.069 1.113
Run # Na Acid Phase (wt%) (g/L) loading Amorphous Alunite/ Hematite Quartz (kg/t ore) Jarositea
Cl
265 17.4 462 657 64 1500 1330 0.1 2760 11190 420 18300 3984 6.3 6747 30472 446 54003 7240 7.8 11070 50870 780 82670
420 kg/t used. It is noted that the level of 420 kg/t ore acid addition represents the level used on the plant for the particular PAL feed supplied. The effect of changing the acid loading on leach liquor acidity, leach residue mineralogy and metal extraction was examined. Not surprisingly, increasing the acid loading resulted in higher leach liquor free acidities, as shown in Table 3. The overall acid consumption, calculated from free acidities and also shown in Table 3, remained at about 330 kg/t in each reaction, except where the least acid was added. In this case, acid consumption dropped to 295 kg/t. This implies that the lowest acid loading (360 kg/t ore) was insufficient to dissolve all acid soluble components in the ore. Hydrolysis of iron and aluminium yields alunite/ jarosite with only slightly different compositions, hematite and silica, as shown in Table 4. The predominance of hematite, and the presence of a number of aluminium-rich sodium–hydronium alunite/jarosites, is consistent with results obtained by Whittington et al. (2003) in low-salinity waters. Fig. 1 indicates the generalised process chemistry of the initial dissolu-
1 2 3 4 5 6 7 8
0.6 0.6 0.6 0.6 5 11 30 50 a
360 390 420 440 420 420 420 420
61 60 59 57 54 52 51 49
9 9 10 11 17 25 31 32
27 28 29 29 27 21 14 13
1.0 1.4 0.9 1.3 1.0 1.1 1.0 1.0
Total concentration of alunite/jarosite phases.
tion, whereas Eqs. (1) and (2) (no sodium) and Eq. (3) (sodium present) summarise the precipitation reactions that occur. (FexAl1x)2(SO4)3+4H2OYFe2O3+2H2SO4
(1)
3ðFex Al1x Þ2 ðSO4 Þ3 þ 14H2 O Y2ðH3 OÞðFe; AlÞ3 ðSO4 Þ2 ðOHÞ6 þ 5H2 SO4
ð2Þ
3ðFex Al1x Þ2 ðSO4 Þ3 þ Na2 SO4 þ 12H2 O Y2NaðFe; AlÞ3 ðSO4 Þ2 ðOHÞ6 þ 6H2 SO4
ð3Þ
3.2. Metal extraction As acid loading and leach liquor free acidity increased, the overall extraction of nickel, cobalt and magnesium from the ore at 120 min increased
Table 3 Effect of acid loading and salinity on final free acidity, acid consumption and element extraction Run #
1 2 3 4 5 6 7 8
Na (g/L)
Acid loading (kg/t ore)
Final free acidity (g/L)
Final acid consumption (kg/t ore)
% Extraction (120 min) Ni
Co
Mg
Al
0.6 0.6 0.6 0.6 5 11 30 50
360 390 420 440 420 420 420 420
42.6 46.7 52.7 59.3 42.7 39.3 27.4 29.7
295 325 333 327 356 360 384 385
85.3 89.4 93.0 94.8 96.4 96.0 92.5 90.5
87.0 86.7 90.6 93.8 95.3 93.0 90.0 89.1
94.9 95.8 97.0 97.3 98.1 98.3 97.3 96.8
8.6 7.9 12.8 13.5 1.7 1.4 1.3 1.3
J.A. Johnson et al. / Hydrometallurgy 78 (2005) 264–270
267
Fig. 1. Schematic diagram indicating the generalised reactions occurring upon dissolution of the ore components [(1)–(3)] and subsequent precipitation of silica (4). Modified from Whittington et al. (2003), using additional data from Madsen et al. (2005).
(Table 3). In addition, the use of higher acid loadings resulted in faster nickel extraction kinetics, as illustrated in Fig. 2. The results are consistent with previous leaching studies where acid loadings have been varied (Papangelakis et al., 1996; Whittington et al., 2003). From a processing viewpoint, the results confirmed that a minimum leach liquor free acidity of about 50–60 g/L H2SO4 must be obtained in fresh water to achieve nickel and cobalt extractions of greater than 90%.
The higher acidity also increased the extraction of other elements—most significantly magnesium and aluminium (Table 3). This is significant in downstream processing as these elements must be separated from nickel and cobalt solutions during purification. In addition, the increased costs of neutralisation for more highly acidic solutions, compared with only marginal increases in nickel extraction, need to be considered.
4. Effect of process water salinity on leach chemistry
% Ni Extracted
100
90
80 440 kg/t 420 kg/t
70
390 kg/t 360 kg/t
60
0
30
60
90
120
Time (min) Fig. 2. Effect of acid loading on nickel extraction kinetics (0.6 g/L Na+).
The process water at Murrin Murrin has a low salt content (~ 1 g/L Na+) and therefore salinity effects are insignificant compared with other Western Australian nickel laterite processing plants. However, given that nickel extraction can be improved by leaching laterite ores in mildly saline process water, compared with fresh water (Johnson et al., 2002; Whittington et al., 2003), the leaching of a Murrin Murrin nontronitetype laterite ore in four different process waters was carried out as part of an overall evaluation of salinity and acid loading effects. The results obtained were similar to those previously observed in the leaching of Bulong nontronite-rich ore blend (Whittington et al., 2003), and therefore will not be discussed in great
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J.A. Johnson et al. / Hydrometallurgy 78 (2005) 264–270
% Ni Extracted
100
5. Effect of acid loading with added sodium-nontronite ore
90
80
1 g/L Na 5 g/L Na 10 g/L Na
70
30 g/L Na 50 g/L Na
60 0
30
60
90
120
Time (min) Fig. 3. Effect of process water salinity on nickel extraction kinetics (420 kg/t ore acid loading).
detail. Rather, a general overview of process water salinity effects will be given. In mildly saline process water, both the overall extraction of nickel (Table 4, Runs 3 and 5–8) and extraction kinetics (Fig. 3) from Murrin Murrin ore were enhanced, relative to fresh water, at the same acid loading. Final leach liquor free acidity decreased as salinity increased, but above 30 g/L Na+ there was little effect of salinity upon final acidity. The highest nickel extraction occurred in mildly saline process water (5 g/L Na+), but decreased as water salinity increased to 51 g/L Na+. As expected, aluminium extraction reduced dramatically once sufficient sodium was present, due to the enhanced formation of sodium alunite/jarosite species (Tables 3 and 4). These results are consistent with previous work by Johnson and Whittington (2004), which enhanced the nickel extraction through addition of an optimum concentration of sulfate salts (Na+, NH4+, K+). In both these studies, faster extraction kinetics occurred when small amounts of additive were present. This is despite a drop in free acidity, when compared with low sodium process water; Papangelakis et al. (1996) noted that as free acid decreased, nickel extraction also decreased. The exact reason for the enhancement is still unclear, although recent work by Whittington and Johnson (in press) suggests that greater amounts of nickel adsorb onto the amorphous silica in the leach residue when sodium is not present in the process water.
The results from this and previous work (Johnson et al., 2002; Johnson and Whittington, 2004) have shown that the mildly saline waters (5–10 g/L Na+) are beneficial in promoting faster leaching kinetics during the pressure acid leaching of nickel laterite ores. It is therefore reasonable to assume that less acid is required to leach a similar amount of nickel, where a small amount of sodium is present in process water, compared with leaching in fresh water. To test this hypothesis, comparative leaches were undertaken in das receivedT process water and process water containing 5 g/L Na+ as Na2SO4, where the acid loading was decreased in increments of 30 kg/t ore. The aim of the tests was to compare the overall effect of acid loading on nickel extraction at a given process water sodium level, and as such, no kinetics data was collected. The use of a second, richer nontronite, laterite ore (see Section 2.1) was not considered as problematic, as no direct comparison between these and previous tests were made. Similarly, the use of Na2SO4 as the source of Na+, rather than NaCl, was not considered an issue, as previous work (Johnson and Whittington, 2004) showed that the two salts gave similar overall nickel extraction, without major changes in leach chemistry. As expected, final leach liquor free acidities decreased as the acid loading decreased, and this is shown in Table 5. At the highest acid loading of 380 kg/t, there was little difference in the free acidities of the leach liquors with and without sodium addition. However, the nickel extraction where sodium was present was about 2% greater, compared with the reaction without sodium present, at the same acid loading. A similar observation was observed during Table 5 Effect of acid loading and salinity on overall extraction of selected metals from Murrin Murrin ore 2 Run # Na Acid Free % Extraction (90 min) (g/L) loading acidity Ni Co Mn Al (kg/t ore) (g/L) 9 10 11 12
0 5 5 5
380 380 350 320
53.8 52.9 38.3 26.6
93.3 95.1 94.4 93.6
91.6 95.4 93.5 95.2
Mg
83.0 15.3 94.5 79.8 2.9 95.7 77.9 1.8 96.8 78.8 1.6 95.4
J.A. Johnson et al. / Hydrometallurgy 78 (2005) 264–270
tests with either Bulong nontronitic ore or Cawse limonitic ore, when acid loadings were varied at a given process water sodium level (Johnson et al., submitted for publication). Reducing the acid loading to either 350 kg/t ore or 320 kg/t ore, at the same sodium loading in the slurry, decreased the final free acidity to 38 and 27 g/L respectively and the nickel extraction to about 94% in both reactions. These extractions, however, were still higher than that achieved with 380 kg/t acid added, without any sodium present in the process water. Thus in the presence of 5 g/L Na+, the acid loading could be reduced by at least 15%, without greatly affecting overall nickel extraction. This is illustrated in Table 5, which shows overall extraction (90 min) of selected metals from the leaching of Murrin Murrin nontronite ore 2 with acid loading ranging from 380 kg/t ore down to 320 kg/t. It should be noted that, as this sample contained about half the concentration of magnesium compared with the Murrin Murrin nontronite ore sample 1, the base acid loading was reduced accordingly. Where no sodium was added, extraction of aluminium, and to some extent manganese, were higher at the same acid loading. This is consistent with results discussed earlier in this paper and in previous work (Johnson et al., 2002; Whittington et al., 2003; Johnson and Whittington, 2004). Surprisingly, there was little difference in the extraction of either cobalt or magnesium when the acid loading, and subsequently, the free acidity decreased. This result may be a consequence of the nontronite ore used and the low level of serpentine minerals present.
6. Conclusions/process considerations Improvements in the rate of extraction of nickel from Murrin Murrin nontronite ore were made by either increasing the acid loading in the pressure leaching reaction or by adding a small amount of Na2SO4 to the reaction, so that the total process water sodium levels did not exceed 11 g/L. The costs of an extra 60 kg/t acid addition, or 27 kg/t Na2SO4 addition, must be balanced against a 1–3% increase in the extraction of nickel and changes in the nature of reaction products. In particular, the stability of sodium alunite/jarosite products, produced when
269
sodium is added that is discharged to the tailings dam, needs to be considered. Similarly, the cost of downstream neutralisation when using a higher acid loading must be included. In separate work, it was shown that acid loading could be reduced by at least 15%, without affecting extraction of nickel, when 5 g/L sodium is added to the slurry before leaching. In this case, neutralisation costs will be less due to the lower final free acidity of the autoclave discharge. However, the effect of the added salt upon downstream processing (e.g. thickening, precipitation, solvent extraction) needs to be considered.
Acknowledgements The authors gratefully acknowledge Barry Whittington (CSIRO Minerals) for constructive comments and the work of Brett Cashmore and Rachel TillerJeffery of CSIRO Minerals in assisting with autoclave runs. Milan Chovancek, Greta Puddey and Darren Atheis, of CSIRO Minerals, are thanked for providing analytical support. Michael Rodriguez of Minara Resources Ltd. is acknowledged for supply of PAL slurries from Murrin Murrin. References Adams, M., van der Meulen, D., Czerny, C., Adamini, P., Turner, J., Jayasekera, S., Amaranti, J., Mosher, J., Miller, M., White, D., Miller, G., 2004. Piloting of the beneficiation and EPALR circuits for Ravensthorpe Nickel Operations. In: Imrie, W.P., Lane, D.M. (Eds.), International Laterite Nickel Symposium. TMS, Warrendale, pp. 193 – 202. Buarzaiga, M., Laframboise, M., 2004. Effect of process water and leach temperature on the leaching of behaviour of New Caledonian nickel laterite ores. In: Collins, M.J., Papangelakis, V.G. (Eds.), Pressure Hydrometallurgy. Canadian Institute of Mining, Metallurgy and Petroleum, Montreal, pp. 181 – 197. Johnson, J.A., Whittington, B.I., 2004. Effect of ammonium, sodium and potassium sulphates and chlorides in the pressure acid leaching of Western Australian nickel laterite ore. In: Collins, M.J., Papangelakis, V.G. (Eds.), Pressure Hydrometallurgy. Canadian Institute of Mining, Metallurgy and Petroleum, Montreal, pp. 199 – 214. Johnson, J.A., McDonald, R.G., Whittington, B.I., Quan, L.P., Muir, D.M., 2002. Process water salinity effects in the pressure acid leaching of Cawse nickel laterite ore. In: Peek, E., van Weert, G. (Eds.), Chloride Metallurgy, vol. 1. Canadian Institute of Mining, Metallurgy and Petroleum, Montreal, pp. 339 – 354.
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Johnson, J.A., Cashmore, B.C., Hockridge, R.J., submitted for publication. Optimisation of nickel extraction from laterite ores by high pressure acid leaching with addition of sodium sulphate. Minerals Engineering. Krause, E., Singhal, A., Blakey, B.C., Papangelakis, V.G., Georgiou, D., 1997. Sulphuric acid leaching of nickeliferous laterite ores. In: Cooper, W.C., Mihaylov, I. (Eds.), Proceedings of the Nickel Cobalt 97 International Symposium, vol. 1. Canadian Institute of Mining Metallurgy and Petroleum, Montreal, pp. 441 – 458. Madsen, I.C., Scarlett, N.Y., Whittington, B.I., 2005. Pressure acid leaching of nickel laterite ores: an in-situ diffraction study of the mechanism and rate of reaction. Unpublished data. Marshall, D., Buarzaiga, M., 2004. Effect of process water on high pressure sulphuric acid leaching of laterite ores. In: Imrie, W.P., Lane, D.M. (Eds.), International Laterite Nickel Symposium. TMS, Warrendale, pp. 263 – 271. Papangelakis, V.G., Georgiou, D., Rubisov, D.H., 1996. Control of iron during the sulphuric acid pressure leaching of limonitic laterites. In: Dutrizac, J.E., Harris, G.B. (Eds.), Iron Control and
Disposal. Canadian Institute of Mining, Metallurgy and Petroleum, Montreal, pp. 263 – 274. Torres, V.M., Carmo, O.A., Evelin, S.S., Rodriguez, R.L., Costa, M.L., 2004. Niquel De Vermhelo Project—prefeasibility study. In: Imrie, W.P., Lane, D.M. (Eds.), International Laterite Nickel Symposium. TMS, Warrendale, pp. 203 – 218. Tsuchida, N., Ozaki, Y., Nakai, O., Kobayshi, H., 2004. Development of process design for Coral Bay Nickel Project. In: Imrie, W.P., Lane, D.M. (Eds.), International Laterite Nickel Symposium. TMS, Warrendale, pp. 151 – 160. Whittington, B.I., Muir, D.M., 2000. Pressure acid leaching of nickel laterites: a review. Mineral Processing and Extractive Metallurgy Reviews 21, 527 – 600. Whittington, B.I., McDonald, R.G., Johnson, J.A., Muir, D.M., 2003. Pressure acid leaching of arid region nickel laterite ore: Part I. Effect of water quality. Hydrometallurgy 70, 31 – 46. Whittington, B.I., Johnson, J.A., in press. Pressure acid leaching of arid region nickel laterite ore: Part III. Nickel losses in the residue. Hydrometallurgy.
Pressure acid leaching of arid-region nickel laterite ore Part IV: Effect of acid loading and additives with nontronite ores J.A. Johnson*, R.G. McDonald, D.M. Muir, J.-P. Tranne A.J. Parker CRC for Hydrometallurgy, CSIRO Minerals, P.O. Box 90, Bentley, WA, 6982, Australia Received 16 February 2005; received in revised form 5 April 2005; accepted 13 April 2005
Abstract The extraction of nickel from nontronite-rich laterite ores is enhanced by either increasing the acid loading, or the addition of a small amount of sodium to the process water. The results of this study indicate that at least 15% less acid can be used to get the same extraction of nickel from a nontronite-rich laterite ore in process water containing 5 g/L sodium ion, compared with the same ore in fresh process water. In addition, nickel extractions from a second nontronite-rich laterite ore were increased by up to 3% compared with fresh water, if process water sodium ion levels is present at 5–11 g/L at a given acid loading of 420 kg/t ore. Crown Copyright D 2005 Published by Elsevier B.V. All rights reserved. Keywords: Nickel laterite; Pressure acid leaching; Nontronite; Acid loading; Salinity
1. Introduction Laterite nickel deposits can be found in both tropical areas (Cuba, New Caledonia, Indonesia, Philippines) and dry-land areas (Albania, Greece, Western Australia) (Whittington and Muir, 2000; Krause et al., 1997). Typically, the dry-land laterites are less weathered than tropical laterites and contain more clay minerals (e.g. nontronite) and less goethite. High temperature pressure acid leaching extracts the nickel and cobalt values from both types of laterites, and this DOI of original article: 10.1016/j.hydromet.2005.04.003. * Corresponding author. Tel.: +61 8 9334 8000; fax: +61 9334 8001. E-mail address: [email protected] (J.A. Johnson).
process is in current use at Moa Bay, Cuba and at Cawse and Murrin Murrin in Western Australia (Whittington and Muir, 2000). Other projects, utilising pressure acid leaching technology are currently under construction or under consideration (Adams et al., 2004; Torres et al., 2004; Tsuchida et al., 2004). Process water quality can affect the high temperature acid leaching of nickel laterite ores. Whittington et al. (2003) summarised the effect of acid charge and process water salinity on the chemistry occurring during the high temperature acid leaching of Bulong (blended) ore. Water salinity affected the relative amounts of alunite/jarosite and hematite that formed in the leach residue which, in turn, affected the acid consumption. In particular, more alunite/jarosite formed in saline water than tap water, at the expense
0304-386X/$ - see front matter. Crown Copyright D 2005 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2005.04.002
J.A. Johnson et al. / Hydrometallurgy 78 (2005) 264–270
of hematite. This resulted in a higher acid consumption and lower concentration of residual acid in the leach slurry, and hence lower nickel extractions (Whittington et al., 2003). A general acid consumption equation, which takes into account the variation in residue mineralogy with leach conditions has also been developed (Whittington et al., 2003). A study with Cawse limonite ore (Johnson et al., 2002) found that higher nickel extractions could be obtained in sea water, relative to fresh water, at a given acid loading despite the free acidity of sea water leach liquor, being less than that of the fresh water liquor, at the end of leaching. Similar studies by Marshall and Buarzaiga (2004) and Buarzaiga and Laframboise (2004) with tropical limonite ores found that nickel extraction was improved by leaching in sea water, though the leach liquor free acidities were also higher, relative to fresh water. It therefore appears that leach chemistry is quite ore-type specific. Residue mineralogy, acid consumption and nickel extraction are all intimately linked and changes to one can be correlated with changes to the others. The current paper examines the effect of acid loading on the leaching kinetics and nickel extraction from two nontronite ores, supplied from Murrin Murrin. It also examines the effect of water salinity on leaching chemistry, especially in relation to acid consumption, and compares the results obtained to those from other ores. Finally, the effect of reducing acid loading, whilst maintaining a specific process water salinity, with a second nontronite ore is examined, with respect to leaching chemistry and consequent residue mineralogy.
2. Experimental 2.1. Ore and process water preparation and analysis Two plant pressure acid leach (PAL) slurries from Murrin Murrin were provided for this work. Where required, sub-samples of slurries were filtered, washed with water, dried at 40 8C, and then reslurried with the relevant process water. Table 1 summarises the elemental compositions of the ores used in this testwork. Tap water, sea water and hypersaline process water were used as received to make up ore slurries. In addition, diluted hypersaline pro-
265
Table 1 Composition of ores used in this study Sample
Murrin 1 Murrin 2
Ore composition (wt%) Ni
Co
Mg
Mn
Al
Fe
Si
Cr
Na
1.3 1.4
0.1 0.1
6.3 3.5
0.4 0.4
2.4 2.9
19.2 25.2
19.3 18.5
0.9 0.9
0.1 0.1
cess water (BW3) was made up from a mixture of 60% hypersaline water and 40% tap water. Table 2 summarises the chemical analyses of the process waters used. In selected reactions with tap water, Na2SO4 was used to control solution sodium levels and this was added to the slurry immediately prior to heating the autoclave. Approximate phase compositions of each ore were calculated by Rietveld analysis of the respective X-ray power diffraction (XRPD) traces. The lower magnesium content of the second Murrin Murrin ore is reflected in the lesser amount of serpentine type minerals present. The approximate phase compositions are as follows: Murrin Murrin Ore 1 Nontronite 50%, Serpentine 20%, Goethite 15%, Maghemite 10%, with 5% Quartz and Dolomite. Murrin Murrin Ore 2 Nontronite 70%, Serpentine 9%, Goethite 9%, Maghemite 9%, with 3% Quartz and Dolomite. 2.2. High pressure acid leaching and leach residue characterisation The leaching of Murrin Murrin ores were conducted in process waters of varying salinity. Reactions were conducted at 255 8C, the same temperature as that used at the Murrin Murrin plant. The equipment and leaching techniques and leach liquor and residue characterisation have previously been described in detail by Whittington et al. (2003).
3. Effect of acidity on leach chemistry 3.1. Free acidity and acid consumption Acid loadings were varied from 360 kg/t ore to 440 kg/t ore, with intermediate levels of 390 kg/t and
266
J.A. Johnson et al. / Hydrometallurgy 78 (2005) 264–270
Table 2 Process water analyses
Table 4 Effect of acid loading and salinity residue mineralogy
Water type
Density Liquor composition (mg/L) (g/mL) Mg Si SO2 Na Ca 4
Murrin Murrin Seawater BW3 Bulong
1.002 1.023 1.069 1.113
Run # Na Acid Phase (wt%) (g/L) loading Amorphous Alunite/ Hematite Quartz (kg/t ore) Jarositea
Cl
265 17.4 462 657 64 1500 1330 0.1 2760 11190 420 18300 3984 6.3 6747 30472 446 54003 7240 7.8 11070 50870 780 82670
420 kg/t used. It is noted that the level of 420 kg/t ore acid addition represents the level used on the plant for the particular PAL feed supplied. The effect of changing the acid loading on leach liquor acidity, leach residue mineralogy and metal extraction was examined. Not surprisingly, increasing the acid loading resulted in higher leach liquor free acidities, as shown in Table 3. The overall acid consumption, calculated from free acidities and also shown in Table 3, remained at about 330 kg/t in each reaction, except where the least acid was added. In this case, acid consumption dropped to 295 kg/t. This implies that the lowest acid loading (360 kg/t ore) was insufficient to dissolve all acid soluble components in the ore. Hydrolysis of iron and aluminium yields alunite/ jarosite with only slightly different compositions, hematite and silica, as shown in Table 4. The predominance of hematite, and the presence of a number of aluminium-rich sodium–hydronium alunite/jarosites, is consistent with results obtained by Whittington et al. (2003) in low-salinity waters. Fig. 1 indicates the generalised process chemistry of the initial dissolu-
1 2 3 4 5 6 7 8
0.6 0.6 0.6 0.6 5 11 30 50 a
360 390 420 440 420 420 420 420
61 60 59 57 54 52 51 49
9 9 10 11 17 25 31 32
27 28 29 29 27 21 14 13
1.0 1.4 0.9 1.3 1.0 1.1 1.0 1.0
Total concentration of alunite/jarosite phases.
tion, whereas Eqs. (1) and (2) (no sodium) and Eq. (3) (sodium present) summarise the precipitation reactions that occur. (FexAl1x)2(SO4)3+4H2OYFe2O3+2H2SO4
(1)
3ðFex Al1x Þ2 ðSO4 Þ3 þ 14H2 O Y2ðH3 OÞðFe; AlÞ3 ðSO4 Þ2 ðOHÞ6 þ 5H2 SO4
ð2Þ
3ðFex Al1x Þ2 ðSO4 Þ3 þ Na2 SO4 þ 12H2 O Y2NaðFe; AlÞ3 ðSO4 Þ2 ðOHÞ6 þ 6H2 SO4
ð3Þ
3.2. Metal extraction As acid loading and leach liquor free acidity increased, the overall extraction of nickel, cobalt and magnesium from the ore at 120 min increased
Table 3 Effect of acid loading and salinity on final free acidity, acid consumption and element extraction Run #
1 2 3 4 5 6 7 8
Na (g/L)
Acid loading (kg/t ore)
Final free acidity (g/L)
Final acid consumption (kg/t ore)
% Extraction (120 min) Ni
Co
Mg
Al
0.6 0.6 0.6 0.6 5 11 30 50
360 390 420 440 420 420 420 420
42.6 46.7 52.7 59.3 42.7 39.3 27.4 29.7
295 325 333 327 356 360 384 385
85.3 89.4 93.0 94.8 96.4 96.0 92.5 90.5
87.0 86.7 90.6 93.8 95.3 93.0 90.0 89.1
94.9 95.8 97.0 97.3 98.1 98.3 97.3 96.8
8.6 7.9 12.8 13.5 1.7 1.4 1.3 1.3
J.A. Johnson et al. / Hydrometallurgy 78 (2005) 264–270
267
Fig. 1. Schematic diagram indicating the generalised reactions occurring upon dissolution of the ore components [(1)–(3)] and subsequent precipitation of silica (4). Modified from Whittington et al. (2003), using additional data from Madsen et al. (2005).
(Table 3). In addition, the use of higher acid loadings resulted in faster nickel extraction kinetics, as illustrated in Fig. 2. The results are consistent with previous leaching studies where acid loadings have been varied (Papangelakis et al., 1996; Whittington et al., 2003). From a processing viewpoint, the results confirmed that a minimum leach liquor free acidity of about 50–60 g/L H2SO4 must be obtained in fresh water to achieve nickel and cobalt extractions of greater than 90%.
The higher acidity also increased the extraction of other elements—most significantly magnesium and aluminium (Table 3). This is significant in downstream processing as these elements must be separated from nickel and cobalt solutions during purification. In addition, the increased costs of neutralisation for more highly acidic solutions, compared with only marginal increases in nickel extraction, need to be considered.
4. Effect of process water salinity on leach chemistry
% Ni Extracted
100
90
80 440 kg/t 420 kg/t
70
390 kg/t 360 kg/t
60
0
30
60
90
120
Time (min) Fig. 2. Effect of acid loading on nickel extraction kinetics (0.6 g/L Na+).
The process water at Murrin Murrin has a low salt content (~ 1 g/L Na+) and therefore salinity effects are insignificant compared with other Western Australian nickel laterite processing plants. However, given that nickel extraction can be improved by leaching laterite ores in mildly saline process water, compared with fresh water (Johnson et al., 2002; Whittington et al., 2003), the leaching of a Murrin Murrin nontronitetype laterite ore in four different process waters was carried out as part of an overall evaluation of salinity and acid loading effects. The results obtained were similar to those previously observed in the leaching of Bulong nontronite-rich ore blend (Whittington et al., 2003), and therefore will not be discussed in great
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J.A. Johnson et al. / Hydrometallurgy 78 (2005) 264–270
% Ni Extracted
100
5. Effect of acid loading with added sodium-nontronite ore
90
80
1 g/L Na 5 g/L Na 10 g/L Na
70
30 g/L Na 50 g/L Na
60 0
30
60
90
120
Time (min) Fig. 3. Effect of process water salinity on nickel extraction kinetics (420 kg/t ore acid loading).
detail. Rather, a general overview of process water salinity effects will be given. In mildly saline process water, both the overall extraction of nickel (Table 4, Runs 3 and 5–8) and extraction kinetics (Fig. 3) from Murrin Murrin ore were enhanced, relative to fresh water, at the same acid loading. Final leach liquor free acidity decreased as salinity increased, but above 30 g/L Na+ there was little effect of salinity upon final acidity. The highest nickel extraction occurred in mildly saline process water (5 g/L Na+), but decreased as water salinity increased to 51 g/L Na+. As expected, aluminium extraction reduced dramatically once sufficient sodium was present, due to the enhanced formation of sodium alunite/jarosite species (Tables 3 and 4). These results are consistent with previous work by Johnson and Whittington (2004), which enhanced the nickel extraction through addition of an optimum concentration of sulfate salts (Na+, NH4+, K+). In both these studies, faster extraction kinetics occurred when small amounts of additive were present. This is despite a drop in free acidity, when compared with low sodium process water; Papangelakis et al. (1996) noted that as free acid decreased, nickel extraction also decreased. The exact reason for the enhancement is still unclear, although recent work by Whittington and Johnson (in press) suggests that greater amounts of nickel adsorb onto the amorphous silica in the leach residue when sodium is not present in the process water.
The results from this and previous work (Johnson et al., 2002; Johnson and Whittington, 2004) have shown that the mildly saline waters (5–10 g/L Na+) are beneficial in promoting faster leaching kinetics during the pressure acid leaching of nickel laterite ores. It is therefore reasonable to assume that less acid is required to leach a similar amount of nickel, where a small amount of sodium is present in process water, compared with leaching in fresh water. To test this hypothesis, comparative leaches were undertaken in das receivedT process water and process water containing 5 g/L Na+ as Na2SO4, where the acid loading was decreased in increments of 30 kg/t ore. The aim of the tests was to compare the overall effect of acid loading on nickel extraction at a given process water sodium level, and as such, no kinetics data was collected. The use of a second, richer nontronite, laterite ore (see Section 2.1) was not considered as problematic, as no direct comparison between these and previous tests were made. Similarly, the use of Na2SO4 as the source of Na+, rather than NaCl, was not considered an issue, as previous work (Johnson and Whittington, 2004) showed that the two salts gave similar overall nickel extraction, without major changes in leach chemistry. As expected, final leach liquor free acidities decreased as the acid loading decreased, and this is shown in Table 5. At the highest acid loading of 380 kg/t, there was little difference in the free acidities of the leach liquors with and without sodium addition. However, the nickel extraction where sodium was present was about 2% greater, compared with the reaction without sodium present, at the same acid loading. A similar observation was observed during Table 5 Effect of acid loading and salinity on overall extraction of selected metals from Murrin Murrin ore 2 Run # Na Acid Free % Extraction (90 min) (g/L) loading acidity Ni Co Mn Al (kg/t ore) (g/L) 9 10 11 12
0 5 5 5
380 380 350 320
53.8 52.9 38.3 26.6
93.3 95.1 94.4 93.6
91.6 95.4 93.5 95.2
Mg
83.0 15.3 94.5 79.8 2.9 95.7 77.9 1.8 96.8 78.8 1.6 95.4
J.A. Johnson et al. / Hydrometallurgy 78 (2005) 264–270
tests with either Bulong nontronitic ore or Cawse limonitic ore, when acid loadings were varied at a given process water sodium level (Johnson et al., submitted for publication). Reducing the acid loading to either 350 kg/t ore or 320 kg/t ore, at the same sodium loading in the slurry, decreased the final free acidity to 38 and 27 g/L respectively and the nickel extraction to about 94% in both reactions. These extractions, however, were still higher than that achieved with 380 kg/t acid added, without any sodium present in the process water. Thus in the presence of 5 g/L Na+, the acid loading could be reduced by at least 15%, without greatly affecting overall nickel extraction. This is illustrated in Table 5, which shows overall extraction (90 min) of selected metals from the leaching of Murrin Murrin nontronite ore 2 with acid loading ranging from 380 kg/t ore down to 320 kg/t. It should be noted that, as this sample contained about half the concentration of magnesium compared with the Murrin Murrin nontronite ore sample 1, the base acid loading was reduced accordingly. Where no sodium was added, extraction of aluminium, and to some extent manganese, were higher at the same acid loading. This is consistent with results discussed earlier in this paper and in previous work (Johnson et al., 2002; Whittington et al., 2003; Johnson and Whittington, 2004). Surprisingly, there was little difference in the extraction of either cobalt or magnesium when the acid loading, and subsequently, the free acidity decreased. This result may be a consequence of the nontronite ore used and the low level of serpentine minerals present.
6. Conclusions/process considerations Improvements in the rate of extraction of nickel from Murrin Murrin nontronite ore were made by either increasing the acid loading in the pressure leaching reaction or by adding a small amount of Na2SO4 to the reaction, so that the total process water sodium levels did not exceed 11 g/L. The costs of an extra 60 kg/t acid addition, or 27 kg/t Na2SO4 addition, must be balanced against a 1–3% increase in the extraction of nickel and changes in the nature of reaction products. In particular, the stability of sodium alunite/jarosite products, produced when
269
sodium is added that is discharged to the tailings dam, needs to be considered. Similarly, the cost of downstream neutralisation when using a higher acid loading must be included. In separate work, it was shown that acid loading could be reduced by at least 15%, without affecting extraction of nickel, when 5 g/L sodium is added to the slurry before leaching. In this case, neutralisation costs will be less due to the lower final free acidity of the autoclave discharge. However, the effect of the added salt upon downstream processing (e.g. thickening, precipitation, solvent extraction) needs to be considered.
Acknowledgements The authors gratefully acknowledge Barry Whittington (CSIRO Minerals) for constructive comments and the work of Brett Cashmore and Rachel TillerJeffery of CSIRO Minerals in assisting with autoclave runs. Milan Chovancek, Greta Puddey and Darren Atheis, of CSIRO Minerals, are thanked for providing analytical support. Michael Rodriguez of Minara Resources Ltd. is acknowledged for supply of PAL slurries from Murrin Murrin. References Adams, M., van der Meulen, D., Czerny, C., Adamini, P., Turner, J., Jayasekera, S., Amaranti, J., Mosher, J., Miller, M., White, D., Miller, G., 2004. Piloting of the beneficiation and EPALR circuits for Ravensthorpe Nickel Operations. In: Imrie, W.P., Lane, D.M. (Eds.), International Laterite Nickel Symposium. TMS, Warrendale, pp. 193 – 202. Buarzaiga, M., Laframboise, M., 2004. Effect of process water and leach temperature on the leaching of behaviour of New Caledonian nickel laterite ores. In: Collins, M.J., Papangelakis, V.G. (Eds.), Pressure Hydrometallurgy. Canadian Institute of Mining, Metallurgy and Petroleum, Montreal, pp. 181 – 197. Johnson, J.A., Whittington, B.I., 2004. Effect of ammonium, sodium and potassium sulphates and chlorides in the pressure acid leaching of Western Australian nickel laterite ore. In: Collins, M.J., Papangelakis, V.G. (Eds.), Pressure Hydrometallurgy. Canadian Institute of Mining, Metallurgy and Petroleum, Montreal, pp. 199 – 214. Johnson, J.A., McDonald, R.G., Whittington, B.I., Quan, L.P., Muir, D.M., 2002. Process water salinity effects in the pressure acid leaching of Cawse nickel laterite ore. In: Peek, E., van Weert, G. (Eds.), Chloride Metallurgy, vol. 1. Canadian Institute of Mining, Metallurgy and Petroleum, Montreal, pp. 339 – 354.
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Johnson, J.A., Cashmore, B.C., Hockridge, R.J., submitted for publication. Optimisation of nickel extraction from laterite ores by high pressure acid leaching with addition of sodium sulphate. Minerals Engineering. Krause, E., Singhal, A., Blakey, B.C., Papangelakis, V.G., Georgiou, D., 1997. Sulphuric acid leaching of nickeliferous laterite ores. In: Cooper, W.C., Mihaylov, I. (Eds.), Proceedings of the Nickel Cobalt 97 International Symposium, vol. 1. Canadian Institute of Mining Metallurgy and Petroleum, Montreal, pp. 441 – 458. Madsen, I.C., Scarlett, N.Y., Whittington, B.I., 2005. Pressure acid leaching of nickel laterite ores: an in-situ diffraction study of the mechanism and rate of reaction. Unpublished data. Marshall, D., Buarzaiga, M., 2004. Effect of process water on high pressure sulphuric acid leaching of laterite ores. In: Imrie, W.P., Lane, D.M. (Eds.), International Laterite Nickel Symposium. TMS, Warrendale, pp. 263 – 271. Papangelakis, V.G., Georgiou, D., Rubisov, D.H., 1996. Control of iron during the sulphuric acid pressure leaching of limonitic laterites. In: Dutrizac, J.E., Harris, G.B. (Eds.), Iron Control and
Disposal. Canadian Institute of Mining, Metallurgy and Petroleum, Montreal, pp. 263 – 274. Torres, V.M., Carmo, O.A., Evelin, S.S., Rodriguez, R.L., Costa, M.L., 2004. Niquel De Vermhelo Project—prefeasibility study. In: Imrie, W.P., Lane, D.M. (Eds.), International Laterite Nickel Symposium. TMS, Warrendale, pp. 203 – 218. Tsuchida, N., Ozaki, Y., Nakai, O., Kobayshi, H., 2004. Development of process design for Coral Bay Nickel Project. In: Imrie, W.P., Lane, D.M. (Eds.), International Laterite Nickel Symposium. TMS, Warrendale, pp. 151 – 160. Whittington, B.I., Muir, D.M., 2000. Pressure acid leaching of nickel laterites: a review. Mineral Processing and Extractive Metallurgy Reviews 21, 527 – 600. Whittington, B.I., McDonald, R.G., Johnson, J.A., Muir, D.M., 2003. Pressure acid leaching of arid region nickel laterite ore: Part I. Effect of water quality. Hydrometallurgy 70, 31 – 46. Whittington, B.I., Johnson, J.A., in press. Pressure acid leaching of arid region nickel laterite ore: Part III. Nickel losses in the residue. Hydrometallurgy.