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Column leaching of nickel laterite agglomerates: Effect of feed size
Hydrometallurgy 134–135 (2013) 144–149
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Column leaching of nickel laterite agglomerates: Effect of feed size Keith Quast a,⁎, Danfeng Xu a, William Skinner a, Ataollah Nosrati a, Tom Hilder b, David J. Robinson c, Jonas Addai-Mensah a a b c
Ian Wark Research Institute, University of South Australia, Mawson Lakes, South Australia 5095, Australia Worley Parsons Services Pty. Ltd, Level 1, 145 South Terrace, Adelaide, South Australia 5000, Australia CSIRO Minerals Down Under National Research Flagship, Australian Minerals Research Centre, PO Box 7229, Karawara, Western Australia 6152, Australia
a r t i c l e
i n f o
Article history: Received 13 September 2012 Received in revised form 5 February 2013 Accepted 7 February 2013 Available online 13 February 2013 Keywords: Nickel laterite Column leaching Agglomerates Particle size Leaching kinetics
a b s t r a c t Heap leaching is a promising, less costly, alternative technology for processing low grade nickel (Ni) laterite ores compared with traditional, energy intensive processes (e.g. autoclave/tank leaching). However, significant technical challenges remain with Ni laterite heap leaching, preventing its general adoption. This paper presents some highlights of laboratory column leaching studies undertaken to characterise, evaluate and optimise sulphuric acid leaching behaviour of Ni laterite agglomerates. The main focus of the paper is to assess the effect of the initial feed ore particle size to the agglomeration stage on the leaching behaviour of the resulting agglomerates. This type of investigation provides basic but valuable information regarding Ni laterite agglomerate robustness and leaching performance under industrially-relevant, continuous acid irrigation conditions. In particular, Ni, cobalt (Co) and other key metals' (e.g. Fe, Mg, Al and Mn) extraction rates, acid consumption and bed slump were determined at a given acid percolation rate as a function of time >100 days. The findings show that the particle size of the agglomerate feed ore has a significant impact on the subsequent column leaching performance. Ni and Co recoveries of 90% and 80%, respectively, were achieved over 100 days for −38 μm size feed, 5–40 mm agglomerates, but these decreased by 10% for the agglomerates made from coarser feed particles (i.e., 2–15 mm). Potential implications of the findings for devising strategies for improved Ni laterite plant heap leach operations are discussed. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The majority (e.g. 70%) of the world's nickel (Ni) resources occur as laterite ores which are exceptionally complex, lower grade (b1.5% Ni) and expensive to treat using conventional smelting and hydrometallurgical tank leaching methods. Typical capex values for High Pressure Acid Leach (HPAL) and Atmospheric Tank Leaching (ATL) are approximately three and two times that of heap leaching respectively (Wedderburn, 2009). Heap leaching is a promising, less costly, alternative technology for processing low grade nickel laterite ores. However, significant geotechnical and hydrometallurgical challenges persist with Ni laterite heap leaching, preventing its adoption. This paper presents recent laboratory column leaching studies undertaken to characterise and evaluate the sulphuric acid leaching behaviour of Ni laterite agglomerates. The work focussed on the effect of the initial particle size of the feed ore to the agglomeration stage on the leaching behaviour of the resulting agglomerates. Emphases are laid on establishing the initial leaching
⁎ Corresponding author. Tel.: +61 8 8302 3816; fax: +61 8 8302 3755. E-mail addresses: [email protected] (K. Quast), [email protected] (D. Xu), [email protected] (W. Skinner), [email protected] (A. Nosrati), [email protected] (T. Hilder), [email protected] (D.J. Robinson), [email protected] (J. Addai-Mensah). 0304-386X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.hydromet.2013.02.001
behaviour and analysis of performance as a major complementary exercise to our recent studies of Ni laterite agglomerates (Xu et al., 2012). A description of the effect of process variables on the drum agglomeration kinetic behaviour of this same nickel laterite ore has been reported by Nosrati et al. (2012). Robertson and van Staden (2009) have reported the steps involved in determining the amenability of ores to heap leaching. These involve bottle roll tests to determine the effects of mineralogy, crush size, acid consumption etc., followed by leaching behaviour testing in small (1 m) columns, then pilot columns and finally test heaps. This paper therefore represents the next stage of the overall process assessing and establishing the robustness of the agglomerates and reports how the effect of the initial particle size of the agglomerate feed material influences its long term leaching characteristics. 2. Previous studies Research into heap leaching of nickel laterites began at the National Technical University of Athens in the early 1990s (Stamboliadis et al., 2004) involving crushing the ore to b 10 mm, pelletising and heap leaching by 2 N sulphuric acid. Column leaching was used to simulate the heap leaching process. According to work conducted at the National Technical University of Athens, column reactors have been proven to be excellent simulators of heaps of similar height, as shown by the long
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history of industrial operation of this technique in copper, gold and uranium mines (Stamboliadis et al., 2004). Leaching experiments are typically conducted in columns of 100 mm diameter and 1 m in height. Before loading, the ore is pelletised to give a permeable bed and allow percolation of the leach solution through the column. Leach solution volume/ore weight ratio of 0.8 L/kg and a percolation rate of 45.8 L/h.m 2 have been used as an example. According to Kyle (2010), these laterites were very amenable to heap leaching extracting up to 80% of the nickel and 60% of the cobalt at a minimal acid consumption of about 120 kg/t. In fact, heap leaching was originally thought to be only applicable to certain laterite ores such as the Greek laterites or highly saprolitic ores. The process is now being investigated for limonitic ores as well using the process of agglomeration with sulphuric acid as a binder to improve the permeability of the ores (Kyle, 2010). Elliot et al. (2009) tested the leaching response of 50 arid-region nickel laterite ores from several deposits in Western Australia in 150 mm diameter columns after they had been agglomerated using 20 kg sulphuric acid/t ore in a rotating drum. Typical bed heights were 1 m and were irrigated with a 200 g/L sulphuric acid solution at 10 L/h.m2 without recycle. Leach solutions were sampled daily and the solution parameters including selected elemental concentrations and acid consumption were measured. The height (slumping) of the ore bed was recorded periodically. Leach times were typically 120–150 days. The wide variety of nickel laterite ores tested resulted in a wide variety of nickel and cobalt extractions (~10–98%) as expected. Watling et al. (2010) investigated the leaching characteristics of a particularly refractory Ni laterite ore from the Yilgarn province of Western Australia. The dominant Ni-bearing mineral phase was goethite, containing almost 80% of the Ni in the sample, with sub-dominant chromite (12%) and minor chlorite, smectite and serpentine. This sample contained ~ 45% goethite, 8% chromite, 25% quartz with ~ 8% of both serpentine and chlorite. The goethite present in this ore was relatively densely packed and thus offered reduced surface area for Ni extraction. A recent paper relating the ore mineralogy to processing Ni laterites under heap leaching conditions and published by Watling et al. (2011) provides additional information. The paper by Elliot et al. (2009) formed the basis for the column leach testing reported herein. The sample selected was one of a number being studied in a larger CSIRO Minerals Down Under (MDU) supported nickel laterite beneficiation research projects. It is a typical Western Australian arid nickel laterite sample designated “siliceous goethitic”. The run-of-mine ore sample was agglomerated using sulphuric acid prior to loading into the column. The main objective of the present work was to use column leaching tests to characterise and evaluate some aspects of the sulphuric acid leaching behaviour of a sample of agglomerates prepared from a characterised nickel laterite ore (siliceous goethitic). The main focus of the paper is the effect of the initial particle size of the feed to the agglomeration stage on the leaching behaviour of the resulting agglomerates. Details of the method of preparation of the agglomerates used in the current leaching test have been reported by Nosrati et al. (2012). These authors investigated the effects of binder type/composition and dosage, drum speed, temperature and batch time on drum agglomeration behaviour of a siliceous goethitic Ni laterite ore. The starting material for the paper by Nosrati et al. (2012) was all passing 2 mm. The starting material for the current study was the same ore, but crushed to all passing 15 mm, 2 mm, 38 μm and a 60:40 blend of b15 mm and b 38 μm materials. 3. Material examined A sample of low grade Ni laterite ore (~ 1 wt.% Ni) from Western Australia, characterised as siliceous goethite (SG), was used in this study. Quantitative, powder X-ray diffraction and QEMSCAN analyses showed complex mineral associations where the hydrophilic quartz, goethite, nontronite and Mg-bearing silicates (e.g., serpentines,
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smectites) comprised the dominant minerals, with hematite, asbolane and kaolinite as the minor mineral phases (Table 1). Detailed mineralogy of this ore has been provided in a recently completed study accepted for publication (Swierczek et al., 2012). Furthermore, Ni and cobalt (Co) mineralizations were broadly disseminated in goethite, layered silicates (i.e., serpentines, smectites) and hydrous Ni-silicates, whilst a significant proportion of the oxides and layered silicates contained a small but noticeable amount of Ni. The “as received” ore (ROM) contained 12 wt.% moisture with particle size b15 mm (80% passing 2.4 mm, 50% passing 0.6 mm, and 20% passing 38 μm). Finer feed (b2 mm) ore was obtained by crushing the b15 mm ROM ore after semidrying and passing the product through a 2 mm sieve (28% passing 0.6 mm, 26% passing 38 μm). Stirred mill material was generated by grinding batches of the b2 mm laterite in a 1.7 L Netzsch mill at a pulp density of 40% solids by weight using a media charge of 50% of 3 mm grinding beads. This produced a product with 80% passing 38 μm. The b38 μm Ni laterite material was produced by wet sieving a stirred-mill product through a 38 μm sieve, bone-drying at a temperature of 50 °C for 72 h and then dry-brushing through a 2 mm sieve. The Ni grade was slightly increased (Table 2) compared to b38 μm Ni laterite feed due to the rejection of the >38 μm materials which contained less Ni. 4. Procedure 4.1. Preparation of agglomerates Agglomeration tests were conducted in a batch, laboratory scale, 316 stainless steel agglomerator (0.3 m in diameter and 0.2 m in length) at a fixed rotational speed of 60 rpm corresponding to 77% critical speed. To investigate the effect of feed particle size on column leaching behaviour of N laterite agglomerates, four types of SG feed ore material (5 kg, air equilibrated weight): (i) b 15 mm ROM, (ii) b 2 mm crushed from the b 15 mm material, (iii) b38 μm stirred mill product and (iv) a b15 mm/b38 μm blend (60/40 by weight) were used to produce agglomerates. Dilute sulphuric acid solution (200 g/L) was used as a binder. The use of more concentrated sulphuric acid is not appropriate for the current process of agglomeration. Where concentrated (e.g. 98%) sulphuric acid is employed in industrial applications, water is added for dilution, hence the use of 200 g/L sulphuric acid is consistent with the industry norm. Ore feed and binder charges were: 1250 g ore and 25 wt.%, 23 wt.%, or 33 wt.% charges of binder for b15 mm ROM, b 2 mm crushed, b 38 μm stirred mill product and b15 mm/b38 μm blend, respectively, per batch experimental run. Before charging, the ore and binder were first pre-mixed over 2 min and the mixture was transferred into the drum and agglomeration commenced. The different amounts of binder content used in this work were pre-determined as the optimum required to generate agglomerates in the size range 5–40 mm at 14 min of agglomeration time. The differences in binder content required are due to the variation in the initial moisture content, ore mineralogy and size/surface areas of the feed particles. By changing the binder dosage, the agglomeration time can be reduced to b 3 min, however the optimum conditions were chosen for laboratory research to cover a wide range of feed
Table 1 Mineralogical composition of ROM SG Ni laterite ore. Mineral phase
Mass %
Quartz Kaolinite Mg-bearing silicates Nontronite (smectite group) Goethite Hematite Asbolane Other
36.06 0.21 8.71 18.77 27.43 2.81 0.40 5.64
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Table 2 Chemical composition of different size SG Ni laterite ore. Element
b15 mm
b2 mm
b38 μm
Ni, % Co, % Mg, % Fe, % Mn, ppm Zn, ppm Cu, ppm Al, % Cr, ppm As, ppm Ca, % Si, % Moisture, %
1.01 0.071 2.81 19.8 3030 260 5 1.75 11,400 10 0.02 23.8 12
1.01 0.067 2.98 20.1 2620 270 20 1.84 11,900 10 0.03 22.9 4.9
1.25 0.077 3.81 23.1 2700 380 75 2.47 13,900 20 0.12 19.9 4.4
sizes. Pre-mixing was essential because we were starting with bone dry powder in the laboratory test work. The agglomeration process followed exactly the procedures described by Nosrati et al. (2012). All the agglomerates were cured for 48 h at room temperature (~25 °C and 30% relative humidity), prior to loading in the columns. 4.2. Column leaching tests The column leaching test work was based on the methods reported by Agatzini-Leonardou and Dimaki (1994) and Elliot et al. (2009). Leaching tests were conducted using Perspex columns, 125 mm in diameter and 2 m long. A layer of coarse (~ 20 mm size) quartz chips approximately 140 mm deep was placed in the bottom of each column to prevent the agglomerates from plugging the pregnant solution outlet. A known mass (5 kg) of cured agglomerates was gently placed in the column to a depth of about 450 mm. This was followed by another layer of coarse silica chips which facilitated the distribution of the sulphuric acid lixiviant over the bed of agglomerates. Sulphuric acid strength of 200 g/L was metered over ~100 days without recycle, and checked periodically to allow calculation of the acid consumption. The irrigation rate for each column was 96 mL/h corresponding to
8.5 L/m 2.h. Pregnant leach solutions were sampled daily for the first 30 days, and then every four days for the remainder of the test. The elemental concentrations measured were for Ni, Co, Al, Fe, Mg and Mn by standard ICP. The height and slumping of the ore bed was recorded periodically by % slump = change in height / initial column height ∗ 100. In order to simulate the effect of hydrostatic load and hence increasing the bed height up to 3.5 m on the strength and slumping of the agglomerates during heap leaching, weights up to 30 kg were added after the columns had been irrigated for approximately 100 days and the slump was recorded. 5. Results 5.1. Column leaching test results In order to establish the need for feed ore particle agglomeration prior to leaching, a 5 kg sample of b15 mm unagglomerated ROM ore was also tested in one of the columns. When this sample was irrigated it blocked within 24 h, showing that agglomeration prior to column (or heap) leaching of this material is essential. A photograph of the column leaching setup is shown in Fig. 1, also providing details of the 5–40 mm agglomerates investigated. Fig. 2 shows the cumulative Ni extraction over 100 days of leaching for the agglomerates made from four different initial feed types. In all the tests it must be remembered that fresh leach solution was continuously metered through the bed of agglomerates (i.e. there is no solution recycle). Evidently Ni extraction rates were highest for the b38 μm and b38 μm/b15 mm mix feed agglomerate samples. Fig. 3 shows the cumulative mass recovery of Ni as a function of cumulative acid consumption. Depending on moisture content and feed assay, there is 30–50 g of Ni available for dissolution (see Table 2). Again, on equivalent acid consumption basis, the b38 μm and b38 μm/b15 mm ore feed agglomerates showed greater Ni recoveries. Fig. 4 clearly demonstrates the above feed particle size dependency when the data are plotted as acid consumption vs. Ni recovery. These data are also plotted as Ni extraction as a function of litres of solution applied/kg of ore (see Fig. 5). The leaching kinetics for the cobalt are
Fig. 1. Photograph of a section and packed columns of Ni laterite agglomerates used in the column leach tests.
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800
100%
<15 mm
Acid Consumption, kg/t
Ni Extraction
80%
60% <15 mm
40% < 2 mm <38 um
20%
<2 mm
600
<38 um <38 um/<15 mm
400
200
<38 um/<15 mm
0% 0
50
100
0 0%
150
Leaching Time, days
shown in Fig. 6 for different sizes of agglomerator ore feed. Fig. 7 shows the recovery of different gangue mineral elements for periods up to 18 days of leaching. This figure demonstrates the leaching characteristics of the major host gangue minerals. The leaching trends shown are consistent with the mineralogical composition of the laterite ore (Table 1) and its chemical analysis as a function of initial particle size (Table 2). The values of slumping for each column are given in Table 3. Progressively adding a further 30 kg to each column, still under acid irrigation, resulted in an additional slump of ≤ 1% for all the agglomerate beds. Cracking of the agglomerates throughout the column leach tests was very minor with only b5 agglomerates showing cracks visible from the outside of the column.
6. Discussion Fig. 1 shows the setup of the four leaching columns in operation. The layers of quartz at the top and the bottom of the agglomerates facilitated sulphuric acid lixiviant distribution with no channelling and prevented any blockage of the column or flooding, thus simulating successful solution distribution and percolation desirable in a heap leach. This gives us confidence that the type of agglomerate produced and the test results could be applicable to those generated in a heap leach. Although the top size of the agglomerates was 40 mm, most of the agglomerates were much smaller than this (see Fig. 1). Stamboliadis et al. (2004) and Agatzini-Leonardou and Dimaki (1994) use 100 mm diameter columns, and CSIRO use 150 mm diameter columns for column leaching.
80%
100%
From Table 2, this sample of low grade Ni laterite is typical of those found in Western Australia (Elliot et al., 2009; Watling et al., 2010, 2011) containing about 1% Ni. Mineralogical analysis using QEMSCAN shows the major mineral species as quartz, goethite and nontronite, also in keeping with those typically found in Western Australia (Table 1). Whilst the dominant Ni-bearing mineral phase is substantially goethite, Ni is distributed throughout most of the other gangue mineral phases, including, smectite, serpentine, talc, chlorite and manganese oxides, where it is often associated with Co (Watling et al., 2011). There is also an upgrade in Ni content of the b38 μm feed due to the rejection of approximately 20% of the coarse (>38 μm) material from the Netzsch mill product which contained less Ni than the ROM feed. This concept is used in industrial practice to preconcentrate and upgrade the feed ore to acid leaching and was incorporated in the design of the Ravensthorpe nickel laterite plant proposed to treat a Western Australian Ni laterite deposit (Adams et al., 2004; Miller, 2000; Miller et al., 2004). Low grade scats are also produced at Bulong and Murrin Murrin sites. These results provide useful information on the effect of different agglomerate feed sizes on the kinetics of column leaching of Ni laterite agglomerates. Allowing the agglomerates to air dry for 48 h leads to the partial evaporation of water and hence curing of the agglomerates, so when they are irrigated, the lixiviant will initially wet and saturate the agglomerate, causing initial solution consumption. From Fig. 2 it can be seen that the Ni extraction for the agglomerates made from the finer (b38 μm) particles leach more rapidly than those made of the coarser particles. This is understandable and anticipated since the higher surface area of the initial fine feed particles will give a
0.06
100%
0.05
80%
0.04 0.03 <15 mm
60%
40% <15 mm
<2 mm
0.01
60%
Fig. 4. Cumulative acid consumption vs. cumulative Ni recovery.
Ni Extraction
Cumulative Ni Mass Recovery, kg
40%
Ni Recovery
Fig. 2. Cumulative Ni extractions vs. time for different agglomerate feed sizes.
0.02
20%
< 2 mm
20%
<38 um
<38 um
<38 um/<15 mm
0
<38 um/<15 mm
0% 0
1
2
3
Cumulative Acid Consumption, kg Fig. 3. Cumulative nickel mass recovery vs. cumulative acid consumption.
0
10
20
30
40
50
Litres Solution Applied/kg of Ore, L/kg Fig. 5. Cumulative nickel extraction vs. litres of applied solution/kg of ore.
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100%
80
(Mn)
Co Extraction
80%
60
60%
40
40% 20
<15 mm <2 mm
20%
<38 um
0
<38 um/<15 mm
(Fe)
0% 0
50
100
150 60
Leaching Time, days Fig. 6. Cumulative Co extraction vs. time.
greater area of contact with the lixiviant. These agglomerates have been shown to be porous (Nosrati et al., 2012), providing good contacts between acid and soluble species in the agglomerate. The column leaching behaviour observed over more than 100 days is typical (Watling et al., 2010) and consistent with industrial heap leaching (e.g. at Murrin Murrin, Readett and Fox, 2009). The solution application data shown in Fig. 5 (e.g. 20 L/kg at 60% nickel recovery) are in good agreement with the Murrin Murrin data reported by Readett and Fox (2009) for a 3 m high heap which indicated a value of 80% nickel recovery at the same application rate. According to Robertson and van Staden (2009), low grade Ni laterite processing is normally associated with very high sulphuric acid consumption (typically 500 kg/t). The acid consumptions shown in Figs. 3 and 4 are therefore in line with operating practice. Robertson and van Staden also reported an almost linear increase of acid consumption with increasing Ni recovery, with acid consumptions of ~400 g/t for Ni recoveries of 70%. Kyle (2010) reported 65–85% Ni recoveries in heap leaching after 120–150 days with acid consumption in the range 200–600 g/t, whilst Elliot et al. (2009) reported 200–800 g/t acid consumption for similar Ni recovery rates. Fig. 3 shows that the acid consumption rate was highest for the b38 μm agglomerate feed, followed by the blend, then the b2 mm and least for the b15 mm feed. This is in line with the greater surface area of the initial finer material, exposing more of all the acid-consuming minerals and leading to a greater acid consumption and concomitantly higher Ni recovery. This is also aided by the higher initial Ni feed grade of the fine material. This is also reflected in trends displayed by the graphs in Fig. 4, where for a given Ni recovery (e.g., 60%), the acid consumption is higher for the lower feed grade material. Cobalt extraction behaviour is shown in Fig. 6 and this indicates that the leaching kinetics are faster for the finer than the coarser feed material-based agglomerates. The fact that the agglomerates made from the b 2 mm feed leach faster than the b38 μm/b 15 mm blend, cognizance of the ratio of the individual size fractions, suggests that some heterogeneity in the sampling from the feed material may be a factor. The short leach time curves for Mn species leached from the agglomerates mirror the curves for the leached Co ions. This confirms that most of the cobalt is predominantly associated with the mineral asbolane, ((Ni, Co) Mn hydroxide), whereas the nickel is distributed in several mineral phases. From Fig. 7 it can be seen that the shapes of the leaching rate curves for all the elements shown here follow those for Ni, indicating that this value metal is dispersed in most of the ore's mineral phases, as intimated by Watling et al. (2010, 2011). Only the initial leach data for the “impurity” elements were recorded just to give an indication of the association of Ni and Co with other metal species. The data also show that the Mg minerals (predominantly
Element Extraction, %
40
20
0
(Mg) 60
40
20
0
(Al) <15 mm <2 mm
60
<38 um <38 um/<15 mm 40
20
0
5
10
15
20
Leaching Time, Days Fig. 7. Cumulative extractions for Mn, Fe, Mg and Al vs. time for the first 18 days. Table 3 Summary of final metal recovery, acid consumption and slump for SG laterite agglomerates subjected to sulphuric acid column leaching. Feed
Running days
Ni rec
Co rec
Cum. kg/t acid consumed
SG b15 mm ROM SG b15 mm SG b2 mm SG b38 μm SG b38 μm/ b15 mm
1 (blocked) 102 104 101 101
– 71% 81% 89% 89%
– 45% 67% 80% 52%
– 684 569 541 510
30 kg load Load/slump over # of days 11.8%/110 16.4%/112 9%/106 13%/109
days days days days
K. Quast et al. / Hydrometallurgy 134–135 (2013) 144–149
magnesium silicates) leach more rapidly. Fe leaches slightly slower, since goethite is more refractory than the silicate minerals, but this may be complicated by the substitution of other elements in the goethite matrix (Watling et al., 2011). The extraction curves for Ni and Fe are noted to be very similar, suggesting that Fe-based minerals are significant Ni hosts. Al shows the lowest leaching rate for all the elements reported here, although it is present in a number of the minerals, some of which will be refractory to acid dissolution. The slump data shown in Table 3 give maximum values of approximately 16% after 100 days. This is within the range noted by Elliot et al. (2009) where they compared the mineralogy of the low slump laterites (b 3%) to that of the high slump laterites (>25%). The columns that showed high slump during prolonged leaching were high in goethite and/or quartz, but low in smectite. Since the ore being tested here contained moderate amounts of goethite, quartz and nontronite (see Table 1); a moderate to high slump is anticipated, as was observed. The highest slump was observed for the agglomerates formed from the b 2 mm feed material, with the lowest slump displayed by the agglomerates formed from the b38 μm material. The slump shown by the blend is in between the two. The small slump showed by the b15 mm agglomerates is probably due to the presence of coarse particles in the agglomerates causing a network structure to be developed where the competent particles support the agglomerate material made from the finer particles. This would simulate the effect of the addition of scats to the Murrin Murrin heap leach to reinforce its geotechnical strength or stability (Readett and Fox, 2009). The addition of 30 kg of load, corresponding to raising the height of Ni laterite heap to ~3 m, showed little increase in slump, even though irrigation was continued throughout the time of additional loading. This would indicate that constructing and operating Ni laterite heap heights of ~3.0–3.5 m should be possible with this material, even in the absence of scats which would increase the bulk density of the bed of agglomerates in many commercial operations. The whole idea was to mimic a 3 m high heap and see if the agglomerates were able to withstand this equivalent load. The results of the complementary column compression/loading test run under similar acid irrigation rate strongly support the inference to a 3 m high heap (CSIRO unpublished report, 2012). Column slumping varies as a consequence of the relative degree of several factors including (but not exclusively) collapse of the agglomerates due to leaching of soluble inter-granular bonds or bridges formed and the inherent loss of significant mass fraction of the column material, off-set by the wall effects of the column and possible swelling of clays within the agglomerates. Small amounts of slumping are generally acceptable as long as they are not associated with significant fines generation and thus blockage and pooling on columns/heaps. Surprisingly though, large degrees of slumping have been observed with no loss of column integrity (Elliot et al., 2009). Post column leach slumping (under additional stress) was designed to simulate the conditions under which the leached agglomerates near the bottom of the heap find themselves under and reassuringly the evidence suggests that additional stress has a marginal effect on the total slumping on most agglomerates. 7. Conclusions The present results provide some initial information on the column leaching behaviour of agglomerated siliceous goethitic Ni laterite ore ahead of the next stage in the development of the process where the construction of a pilot heap is recommended. The effects of different feed ore particle size on the agglomerate behaviour show that the
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increase in primary particles' surface area leads to more rapid sulphuric acid leaching kinetics, as would be expected. It may also be contended that the Ni laterite agglomerates formed from the siliceous goethitic sample are sufficiently robust to withstand a heap of reasonable height (up to 3.5 m) without collapsing as demonstrated by the loaded column slump data. Percolation of acid lixiviant through beds of agglomerates within the 5–40 mm size range used here is excellent, with no reduction in lixiviant flow observed over the 100 days of continuous operation of the columns. Acid consumption rate is within the typical range for low grade lateritic ores (~500 kg/t). Conducting simple column leach tests over similar leach times on other agglomerated ores should therefore provide suitable data for incorporation in plant design, optimization and improved Ni laterite heap leaching operations. On a cost-benefit analysis, wet grinding and drying may only be justified for cases where they may lead to a significant improvement in leach kinetics and Ni/Co recovery/yield. Acknowledgements The authors wish to thank CSIRO through their Minerals Down Under National Research Flagship for their in-kind and generous financial support for the Nickel Laterite Cluster project. 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 EPAL circuits for Ravensthorpe Nickel Operations. In: Imrie, W.P., et al. (Ed.), Proceedings, International Laterite Nickel Symposium. The Minerals, Metals and Materials Society, pp. 193–202. Agatzini-Leonardou, S., Dimaki, D., 1994. Heap leaching of poor nickel laterites by sulphuric acid at ambient temperature. Proceedings, “Hydrometallurgy ‘94” Published for the Institution of Mining and Metallurgy and the Society of Chemical Industry. Chapman and Hall, pp. 193–208. Elliot, A., Fletcher, H., Li, J., Watling, H., Robinson, D., 2009. Heap leaching of nickel laterites — a challenge and an opportunity. In: Budac, J.J., Frazer, R., Mihaylov, I., Papangelakis, V.P., Robinson, D.J. (Eds.), Proceedings, Hydrometallurgy of Nickel and Cobalt 2009. CIM, Montreal, Canada, pp. 537–552. Kyle, J., 2010. Nickel laterite processing technologies—where to next? ALTA Nickel/Cobalt/ Copper Conference, 24–27 May, Perth, Western Australia (36 pp.). Miller, G., 2000. Pilot plant testwork and design of the Ravensthorpe beneficiation plant. Proceedings, Ni/Co ALTA conference (17 pp.). Miller, G.W., Sampson, D., Fleay, J., Conway-Mortimer, J., Roche, E., 2004. Ravensthorpe Nickel Project beneficiation prediction MLR and interpretation of results. In: Imrie, W.P. (Ed.), Proceedings, International Laterite Nickel Symposium. The Minerals, Metals and Materials Society, pp. 121–136. Nosrati, A., Addai-Mensah, J., Robinson, D.J., 2012. Drum agglomeration behaviour of nickel laterite ore: effect of process variables. Hydrometallurgy 125–126, 90–99. Readett, D.J., Fox, J., 2009. Development of heap leaching and its integration into the Murrin Murrin operations. Paper Presented to ALTA Nickel/Cobalt Conference, Perth, May 25–27, 2009. Robertson, S.W., van Staden, P.J., 2009. The progression of metallurgical testwork during heap leach design. Hydrometallurgical Conference 2009. Southern African Institute of Mining and Metallurgy, pp. 31–42. Stamboliadis, E., Alevizos, G., Zafiratos, J., 2004. Leaching residue of nickeliferous laterites as a source of iron concentrate. Miner. Eng. 17, 245–252. Swierczek, Z., Quast, K., Addai-Mensah, J., Connor, J.N., Robinson, D.J., Li, J., 2012. Challenges in the mineralogical characterisation of low-grade nickel laterites, paper # 339. Proceedings, International Mineral Processing Congress, New Delhi, India 5352–5364. Watling, H., Das, G., Elliot, A., Li, J., McDonald, R., Robinson, D., 2010. Process options for difficult arid-region nickel laterites. Proceedings, XXV International Mineral Processing Congress, Brisbane, Australia, September, pp. 417–427. Watling, H., Elliot, A.D., Fletcher, H.M., Robinson, D.J., Sully, D.M., 2011. Ore mineralogy of nickel laterites: controls on processing characteristics under simulated heap-leach conditions. Aust. J. Earth Sci. 58 (7), 725–744. Wedderburn, B., 2009. Nickel laterite processing—a shift towards heap leaching. Proceedings, ALTA Nickel/Cobalt Conference, Perth, May 25–27, 2009. Xu, D., Hilder, T., Quast, K., Skinner, W., Addai-Mensah, J., Robinson, D.J., 2012. Column leaching behaviour of nickel laterite agglomerates, paper # 188. Proceedings, International Mineral Processing Congress, New Delhi, India, 5883–5891.
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Hydrometallurgy journal homepage: www.elsevier.com/locate/hydromet
Column leaching of nickel laterite agglomerates: Effect of feed size Keith Quast a,⁎, Danfeng Xu a, William Skinner a, Ataollah Nosrati a, Tom Hilder b, David J. Robinson c, Jonas Addai-Mensah a a b c
Ian Wark Research Institute, University of South Australia, Mawson Lakes, South Australia 5095, Australia Worley Parsons Services Pty. Ltd, Level 1, 145 South Terrace, Adelaide, South Australia 5000, Australia CSIRO Minerals Down Under National Research Flagship, Australian Minerals Research Centre, PO Box 7229, Karawara, Western Australia 6152, Australia
a r t i c l e
i n f o
Article history: Received 13 September 2012 Received in revised form 5 February 2013 Accepted 7 February 2013 Available online 13 February 2013 Keywords: Nickel laterite Column leaching Agglomerates Particle size Leaching kinetics
a b s t r a c t Heap leaching is a promising, less costly, alternative technology for processing low grade nickel (Ni) laterite ores compared with traditional, energy intensive processes (e.g. autoclave/tank leaching). However, significant technical challenges remain with Ni laterite heap leaching, preventing its general adoption. This paper presents some highlights of laboratory column leaching studies undertaken to characterise, evaluate and optimise sulphuric acid leaching behaviour of Ni laterite agglomerates. The main focus of the paper is to assess the effect of the initial feed ore particle size to the agglomeration stage on the leaching behaviour of the resulting agglomerates. This type of investigation provides basic but valuable information regarding Ni laterite agglomerate robustness and leaching performance under industrially-relevant, continuous acid irrigation conditions. In particular, Ni, cobalt (Co) and other key metals' (e.g. Fe, Mg, Al and Mn) extraction rates, acid consumption and bed slump were determined at a given acid percolation rate as a function of time >100 days. The findings show that the particle size of the agglomerate feed ore has a significant impact on the subsequent column leaching performance. Ni and Co recoveries of 90% and 80%, respectively, were achieved over 100 days for −38 μm size feed, 5–40 mm agglomerates, but these decreased by 10% for the agglomerates made from coarser feed particles (i.e., 2–15 mm). Potential implications of the findings for devising strategies for improved Ni laterite plant heap leach operations are discussed. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The majority (e.g. 70%) of the world's nickel (Ni) resources occur as laterite ores which are exceptionally complex, lower grade (b1.5% Ni) and expensive to treat using conventional smelting and hydrometallurgical tank leaching methods. Typical capex values for High Pressure Acid Leach (HPAL) and Atmospheric Tank Leaching (ATL) are approximately three and two times that of heap leaching respectively (Wedderburn, 2009). Heap leaching is a promising, less costly, alternative technology for processing low grade nickel laterite ores. However, significant geotechnical and hydrometallurgical challenges persist with Ni laterite heap leaching, preventing its adoption. This paper presents recent laboratory column leaching studies undertaken to characterise and evaluate the sulphuric acid leaching behaviour of Ni laterite agglomerates. The work focussed on the effect of the initial particle size of the feed ore to the agglomeration stage on the leaching behaviour of the resulting agglomerates. Emphases are laid on establishing the initial leaching
⁎ Corresponding author. Tel.: +61 8 8302 3816; fax: +61 8 8302 3755. E-mail addresses: [email protected] (K. Quast), [email protected] (D. Xu), [email protected] (W. Skinner), [email protected] (A. Nosrati), [email protected] (T. Hilder), [email protected] (D.J. Robinson), [email protected] (J. Addai-Mensah). 0304-386X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.hydromet.2013.02.001
behaviour and analysis of performance as a major complementary exercise to our recent studies of Ni laterite agglomerates (Xu et al., 2012). A description of the effect of process variables on the drum agglomeration kinetic behaviour of this same nickel laterite ore has been reported by Nosrati et al. (2012). Robertson and van Staden (2009) have reported the steps involved in determining the amenability of ores to heap leaching. These involve bottle roll tests to determine the effects of mineralogy, crush size, acid consumption etc., followed by leaching behaviour testing in small (1 m) columns, then pilot columns and finally test heaps. This paper therefore represents the next stage of the overall process assessing and establishing the robustness of the agglomerates and reports how the effect of the initial particle size of the agglomerate feed material influences its long term leaching characteristics. 2. Previous studies Research into heap leaching of nickel laterites began at the National Technical University of Athens in the early 1990s (Stamboliadis et al., 2004) involving crushing the ore to b 10 mm, pelletising and heap leaching by 2 N sulphuric acid. Column leaching was used to simulate the heap leaching process. According to work conducted at the National Technical University of Athens, column reactors have been proven to be excellent simulators of heaps of similar height, as shown by the long
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history of industrial operation of this technique in copper, gold and uranium mines (Stamboliadis et al., 2004). Leaching experiments are typically conducted in columns of 100 mm diameter and 1 m in height. Before loading, the ore is pelletised to give a permeable bed and allow percolation of the leach solution through the column. Leach solution volume/ore weight ratio of 0.8 L/kg and a percolation rate of 45.8 L/h.m 2 have been used as an example. According to Kyle (2010), these laterites were very amenable to heap leaching extracting up to 80% of the nickel and 60% of the cobalt at a minimal acid consumption of about 120 kg/t. In fact, heap leaching was originally thought to be only applicable to certain laterite ores such as the Greek laterites or highly saprolitic ores. The process is now being investigated for limonitic ores as well using the process of agglomeration with sulphuric acid as a binder to improve the permeability of the ores (Kyle, 2010). Elliot et al. (2009) tested the leaching response of 50 arid-region nickel laterite ores from several deposits in Western Australia in 150 mm diameter columns after they had been agglomerated using 20 kg sulphuric acid/t ore in a rotating drum. Typical bed heights were 1 m and were irrigated with a 200 g/L sulphuric acid solution at 10 L/h.m2 without recycle. Leach solutions were sampled daily and the solution parameters including selected elemental concentrations and acid consumption were measured. The height (slumping) of the ore bed was recorded periodically. Leach times were typically 120–150 days. The wide variety of nickel laterite ores tested resulted in a wide variety of nickel and cobalt extractions (~10–98%) as expected. Watling et al. (2010) investigated the leaching characteristics of a particularly refractory Ni laterite ore from the Yilgarn province of Western Australia. The dominant Ni-bearing mineral phase was goethite, containing almost 80% of the Ni in the sample, with sub-dominant chromite (12%) and minor chlorite, smectite and serpentine. This sample contained ~ 45% goethite, 8% chromite, 25% quartz with ~ 8% of both serpentine and chlorite. The goethite present in this ore was relatively densely packed and thus offered reduced surface area for Ni extraction. A recent paper relating the ore mineralogy to processing Ni laterites under heap leaching conditions and published by Watling et al. (2011) provides additional information. The paper by Elliot et al. (2009) formed the basis for the column leach testing reported herein. The sample selected was one of a number being studied in a larger CSIRO Minerals Down Under (MDU) supported nickel laterite beneficiation research projects. It is a typical Western Australian arid nickel laterite sample designated “siliceous goethitic”. The run-of-mine ore sample was agglomerated using sulphuric acid prior to loading into the column. The main objective of the present work was to use column leaching tests to characterise and evaluate some aspects of the sulphuric acid leaching behaviour of a sample of agglomerates prepared from a characterised nickel laterite ore (siliceous goethitic). The main focus of the paper is the effect of the initial particle size of the feed to the agglomeration stage on the leaching behaviour of the resulting agglomerates. Details of the method of preparation of the agglomerates used in the current leaching test have been reported by Nosrati et al. (2012). These authors investigated the effects of binder type/composition and dosage, drum speed, temperature and batch time on drum agglomeration behaviour of a siliceous goethitic Ni laterite ore. The starting material for the paper by Nosrati et al. (2012) was all passing 2 mm. The starting material for the current study was the same ore, but crushed to all passing 15 mm, 2 mm, 38 μm and a 60:40 blend of b15 mm and b 38 μm materials. 3. Material examined A sample of low grade Ni laterite ore (~ 1 wt.% Ni) from Western Australia, characterised as siliceous goethite (SG), was used in this study. Quantitative, powder X-ray diffraction and QEMSCAN analyses showed complex mineral associations where the hydrophilic quartz, goethite, nontronite and Mg-bearing silicates (e.g., serpentines,
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smectites) comprised the dominant minerals, with hematite, asbolane and kaolinite as the minor mineral phases (Table 1). Detailed mineralogy of this ore has been provided in a recently completed study accepted for publication (Swierczek et al., 2012). Furthermore, Ni and cobalt (Co) mineralizations were broadly disseminated in goethite, layered silicates (i.e., serpentines, smectites) and hydrous Ni-silicates, whilst a significant proportion of the oxides and layered silicates contained a small but noticeable amount of Ni. The “as received” ore (ROM) contained 12 wt.% moisture with particle size b15 mm (80% passing 2.4 mm, 50% passing 0.6 mm, and 20% passing 38 μm). Finer feed (b2 mm) ore was obtained by crushing the b15 mm ROM ore after semidrying and passing the product through a 2 mm sieve (28% passing 0.6 mm, 26% passing 38 μm). Stirred mill material was generated by grinding batches of the b2 mm laterite in a 1.7 L Netzsch mill at a pulp density of 40% solids by weight using a media charge of 50% of 3 mm grinding beads. This produced a product with 80% passing 38 μm. The b38 μm Ni laterite material was produced by wet sieving a stirred-mill product through a 38 μm sieve, bone-drying at a temperature of 50 °C for 72 h and then dry-brushing through a 2 mm sieve. The Ni grade was slightly increased (Table 2) compared to b38 μm Ni laterite feed due to the rejection of the >38 μm materials which contained less Ni. 4. Procedure 4.1. Preparation of agglomerates Agglomeration tests were conducted in a batch, laboratory scale, 316 stainless steel agglomerator (0.3 m in diameter and 0.2 m in length) at a fixed rotational speed of 60 rpm corresponding to 77% critical speed. To investigate the effect of feed particle size on column leaching behaviour of N laterite agglomerates, four types of SG feed ore material (5 kg, air equilibrated weight): (i) b 15 mm ROM, (ii) b 2 mm crushed from the b 15 mm material, (iii) b38 μm stirred mill product and (iv) a b15 mm/b38 μm blend (60/40 by weight) were used to produce agglomerates. Dilute sulphuric acid solution (200 g/L) was used as a binder. The use of more concentrated sulphuric acid is not appropriate for the current process of agglomeration. Where concentrated (e.g. 98%) sulphuric acid is employed in industrial applications, water is added for dilution, hence the use of 200 g/L sulphuric acid is consistent with the industry norm. Ore feed and binder charges were: 1250 g ore and 25 wt.%, 23 wt.%, or 33 wt.% charges of binder for b15 mm ROM, b 2 mm crushed, b 38 μm stirred mill product and b15 mm/b38 μm blend, respectively, per batch experimental run. Before charging, the ore and binder were first pre-mixed over 2 min and the mixture was transferred into the drum and agglomeration commenced. The different amounts of binder content used in this work were pre-determined as the optimum required to generate agglomerates in the size range 5–40 mm at 14 min of agglomeration time. The differences in binder content required are due to the variation in the initial moisture content, ore mineralogy and size/surface areas of the feed particles. By changing the binder dosage, the agglomeration time can be reduced to b 3 min, however the optimum conditions were chosen for laboratory research to cover a wide range of feed
Table 1 Mineralogical composition of ROM SG Ni laterite ore. Mineral phase
Mass %
Quartz Kaolinite Mg-bearing silicates Nontronite (smectite group) Goethite Hematite Asbolane Other
36.06 0.21 8.71 18.77 27.43 2.81 0.40 5.64
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Table 2 Chemical composition of different size SG Ni laterite ore. Element
b15 mm
b2 mm
b38 μm
Ni, % Co, % Mg, % Fe, % Mn, ppm Zn, ppm Cu, ppm Al, % Cr, ppm As, ppm Ca, % Si, % Moisture, %
1.01 0.071 2.81 19.8 3030 260 5 1.75 11,400 10 0.02 23.8 12
1.01 0.067 2.98 20.1 2620 270 20 1.84 11,900 10 0.03 22.9 4.9
1.25 0.077 3.81 23.1 2700 380 75 2.47 13,900 20 0.12 19.9 4.4
sizes. Pre-mixing was essential because we were starting with bone dry powder in the laboratory test work. The agglomeration process followed exactly the procedures described by Nosrati et al. (2012). All the agglomerates were cured for 48 h at room temperature (~25 °C and 30% relative humidity), prior to loading in the columns. 4.2. Column leaching tests The column leaching test work was based on the methods reported by Agatzini-Leonardou and Dimaki (1994) and Elliot et al. (2009). Leaching tests were conducted using Perspex columns, 125 mm in diameter and 2 m long. A layer of coarse (~ 20 mm size) quartz chips approximately 140 mm deep was placed in the bottom of each column to prevent the agglomerates from plugging the pregnant solution outlet. A known mass (5 kg) of cured agglomerates was gently placed in the column to a depth of about 450 mm. This was followed by another layer of coarse silica chips which facilitated the distribution of the sulphuric acid lixiviant over the bed of agglomerates. Sulphuric acid strength of 200 g/L was metered over ~100 days without recycle, and checked periodically to allow calculation of the acid consumption. The irrigation rate for each column was 96 mL/h corresponding to
8.5 L/m 2.h. Pregnant leach solutions were sampled daily for the first 30 days, and then every four days for the remainder of the test. The elemental concentrations measured were for Ni, Co, Al, Fe, Mg and Mn by standard ICP. The height and slumping of the ore bed was recorded periodically by % slump = change in height / initial column height ∗ 100. In order to simulate the effect of hydrostatic load and hence increasing the bed height up to 3.5 m on the strength and slumping of the agglomerates during heap leaching, weights up to 30 kg were added after the columns had been irrigated for approximately 100 days and the slump was recorded. 5. Results 5.1. Column leaching test results In order to establish the need for feed ore particle agglomeration prior to leaching, a 5 kg sample of b15 mm unagglomerated ROM ore was also tested in one of the columns. When this sample was irrigated it blocked within 24 h, showing that agglomeration prior to column (or heap) leaching of this material is essential. A photograph of the column leaching setup is shown in Fig. 1, also providing details of the 5–40 mm agglomerates investigated. Fig. 2 shows the cumulative Ni extraction over 100 days of leaching for the agglomerates made from four different initial feed types. In all the tests it must be remembered that fresh leach solution was continuously metered through the bed of agglomerates (i.e. there is no solution recycle). Evidently Ni extraction rates were highest for the b38 μm and b38 μm/b15 mm mix feed agglomerate samples. Fig. 3 shows the cumulative mass recovery of Ni as a function of cumulative acid consumption. Depending on moisture content and feed assay, there is 30–50 g of Ni available for dissolution (see Table 2). Again, on equivalent acid consumption basis, the b38 μm and b38 μm/b15 mm ore feed agglomerates showed greater Ni recoveries. Fig. 4 clearly demonstrates the above feed particle size dependency when the data are plotted as acid consumption vs. Ni recovery. These data are also plotted as Ni extraction as a function of litres of solution applied/kg of ore (see Fig. 5). The leaching kinetics for the cobalt are
Fig. 1. Photograph of a section and packed columns of Ni laterite agglomerates used in the column leach tests.
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800
100%
<15 mm
Acid Consumption, kg/t
Ni Extraction
80%
60% <15 mm
40% < 2 mm <38 um
20%
<2 mm
600
<38 um <38 um/<15 mm
400
200
<38 um/<15 mm
0% 0
50
100
0 0%
150
Leaching Time, days
shown in Fig. 6 for different sizes of agglomerator ore feed. Fig. 7 shows the recovery of different gangue mineral elements for periods up to 18 days of leaching. This figure demonstrates the leaching characteristics of the major host gangue minerals. The leaching trends shown are consistent with the mineralogical composition of the laterite ore (Table 1) and its chemical analysis as a function of initial particle size (Table 2). The values of slumping for each column are given in Table 3. Progressively adding a further 30 kg to each column, still under acid irrigation, resulted in an additional slump of ≤ 1% for all the agglomerate beds. Cracking of the agglomerates throughout the column leach tests was very minor with only b5 agglomerates showing cracks visible from the outside of the column.
6. Discussion Fig. 1 shows the setup of the four leaching columns in operation. The layers of quartz at the top and the bottom of the agglomerates facilitated sulphuric acid lixiviant distribution with no channelling and prevented any blockage of the column or flooding, thus simulating successful solution distribution and percolation desirable in a heap leach. This gives us confidence that the type of agglomerate produced and the test results could be applicable to those generated in a heap leach. Although the top size of the agglomerates was 40 mm, most of the agglomerates were much smaller than this (see Fig. 1). Stamboliadis et al. (2004) and Agatzini-Leonardou and Dimaki (1994) use 100 mm diameter columns, and CSIRO use 150 mm diameter columns for column leaching.
80%
100%
From Table 2, this sample of low grade Ni laterite is typical of those found in Western Australia (Elliot et al., 2009; Watling et al., 2010, 2011) containing about 1% Ni. Mineralogical analysis using QEMSCAN shows the major mineral species as quartz, goethite and nontronite, also in keeping with those typically found in Western Australia (Table 1). Whilst the dominant Ni-bearing mineral phase is substantially goethite, Ni is distributed throughout most of the other gangue mineral phases, including, smectite, serpentine, talc, chlorite and manganese oxides, where it is often associated with Co (Watling et al., 2011). There is also an upgrade in Ni content of the b38 μm feed due to the rejection of approximately 20% of the coarse (>38 μm) material from the Netzsch mill product which contained less Ni than the ROM feed. This concept is used in industrial practice to preconcentrate and upgrade the feed ore to acid leaching and was incorporated in the design of the Ravensthorpe nickel laterite plant proposed to treat a Western Australian Ni laterite deposit (Adams et al., 2004; Miller, 2000; Miller et al., 2004). Low grade scats are also produced at Bulong and Murrin Murrin sites. These results provide useful information on the effect of different agglomerate feed sizes on the kinetics of column leaching of Ni laterite agglomerates. Allowing the agglomerates to air dry for 48 h leads to the partial evaporation of water and hence curing of the agglomerates, so when they are irrigated, the lixiviant will initially wet and saturate the agglomerate, causing initial solution consumption. From Fig. 2 it can be seen that the Ni extraction for the agglomerates made from the finer (b38 μm) particles leach more rapidly than those made of the coarser particles. This is understandable and anticipated since the higher surface area of the initial fine feed particles will give a
0.06
100%
0.05
80%
0.04 0.03 <15 mm
60%
40% <15 mm
<2 mm
0.01
60%
Fig. 4. Cumulative acid consumption vs. cumulative Ni recovery.
Ni Extraction
Cumulative Ni Mass Recovery, kg
40%
Ni Recovery
Fig. 2. Cumulative Ni extractions vs. time for different agglomerate feed sizes.
0.02
20%
< 2 mm
20%
<38 um
<38 um
<38 um/<15 mm
0
<38 um/<15 mm
0% 0
1
2
3
Cumulative Acid Consumption, kg Fig. 3. Cumulative nickel mass recovery vs. cumulative acid consumption.
0
10
20
30
40
50
Litres Solution Applied/kg of Ore, L/kg Fig. 5. Cumulative nickel extraction vs. litres of applied solution/kg of ore.
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100%
80
(Mn)
Co Extraction
80%
60
60%
40
40% 20
<15 mm <2 mm
20%
<38 um
0
<38 um/<15 mm
(Fe)
0% 0
50
100
150 60
Leaching Time, days Fig. 6. Cumulative Co extraction vs. time.
greater area of contact with the lixiviant. These agglomerates have been shown to be porous (Nosrati et al., 2012), providing good contacts between acid and soluble species in the agglomerate. The column leaching behaviour observed over more than 100 days is typical (Watling et al., 2010) and consistent with industrial heap leaching (e.g. at Murrin Murrin, Readett and Fox, 2009). The solution application data shown in Fig. 5 (e.g. 20 L/kg at 60% nickel recovery) are in good agreement with the Murrin Murrin data reported by Readett and Fox (2009) for a 3 m high heap which indicated a value of 80% nickel recovery at the same application rate. According to Robertson and van Staden (2009), low grade Ni laterite processing is normally associated with very high sulphuric acid consumption (typically 500 kg/t). The acid consumptions shown in Figs. 3 and 4 are therefore in line with operating practice. Robertson and van Staden also reported an almost linear increase of acid consumption with increasing Ni recovery, with acid consumptions of ~400 g/t for Ni recoveries of 70%. Kyle (2010) reported 65–85% Ni recoveries in heap leaching after 120–150 days with acid consumption in the range 200–600 g/t, whilst Elliot et al. (2009) reported 200–800 g/t acid consumption for similar Ni recovery rates. Fig. 3 shows that the acid consumption rate was highest for the b38 μm agglomerate feed, followed by the blend, then the b2 mm and least for the b15 mm feed. This is in line with the greater surface area of the initial finer material, exposing more of all the acid-consuming minerals and leading to a greater acid consumption and concomitantly higher Ni recovery. This is also aided by the higher initial Ni feed grade of the fine material. This is also reflected in trends displayed by the graphs in Fig. 4, where for a given Ni recovery (e.g., 60%), the acid consumption is higher for the lower feed grade material. Cobalt extraction behaviour is shown in Fig. 6 and this indicates that the leaching kinetics are faster for the finer than the coarser feed material-based agglomerates. The fact that the agglomerates made from the b 2 mm feed leach faster than the b38 μm/b 15 mm blend, cognizance of the ratio of the individual size fractions, suggests that some heterogeneity in the sampling from the feed material may be a factor. The short leach time curves for Mn species leached from the agglomerates mirror the curves for the leached Co ions. This confirms that most of the cobalt is predominantly associated with the mineral asbolane, ((Ni, Co) Mn hydroxide), whereas the nickel is distributed in several mineral phases. From Fig. 7 it can be seen that the shapes of the leaching rate curves for all the elements shown here follow those for Ni, indicating that this value metal is dispersed in most of the ore's mineral phases, as intimated by Watling et al. (2010, 2011). Only the initial leach data for the “impurity” elements were recorded just to give an indication of the association of Ni and Co with other metal species. The data also show that the Mg minerals (predominantly
Element Extraction, %
40
20
0
(Mg) 60
40
20
0
(Al) <15 mm <2 mm
60
<38 um <38 um/<15 mm 40
20
0
5
10
15
20
Leaching Time, Days Fig. 7. Cumulative extractions for Mn, Fe, Mg and Al vs. time for the first 18 days. Table 3 Summary of final metal recovery, acid consumption and slump for SG laterite agglomerates subjected to sulphuric acid column leaching. Feed
Running days
Ni rec
Co rec
Cum. kg/t acid consumed
SG b15 mm ROM SG b15 mm SG b2 mm SG b38 μm SG b38 μm/ b15 mm
1 (blocked) 102 104 101 101
– 71% 81% 89% 89%
– 45% 67% 80% 52%
– 684 569 541 510
30 kg load Load/slump over # of days 11.8%/110 16.4%/112 9%/106 13%/109
days days days days
K. Quast et al. / Hydrometallurgy 134–135 (2013) 144–149
magnesium silicates) leach more rapidly. Fe leaches slightly slower, since goethite is more refractory than the silicate minerals, but this may be complicated by the substitution of other elements in the goethite matrix (Watling et al., 2011). The extraction curves for Ni and Fe are noted to be very similar, suggesting that Fe-based minerals are significant Ni hosts. Al shows the lowest leaching rate for all the elements reported here, although it is present in a number of the minerals, some of which will be refractory to acid dissolution. The slump data shown in Table 3 give maximum values of approximately 16% after 100 days. This is within the range noted by Elliot et al. (2009) where they compared the mineralogy of the low slump laterites (b 3%) to that of the high slump laterites (>25%). The columns that showed high slump during prolonged leaching were high in goethite and/or quartz, but low in smectite. Since the ore being tested here contained moderate amounts of goethite, quartz and nontronite (see Table 1); a moderate to high slump is anticipated, as was observed. The highest slump was observed for the agglomerates formed from the b 2 mm feed material, with the lowest slump displayed by the agglomerates formed from the b38 μm material. The slump shown by the blend is in between the two. The small slump showed by the b15 mm agglomerates is probably due to the presence of coarse particles in the agglomerates causing a network structure to be developed where the competent particles support the agglomerate material made from the finer particles. This would simulate the effect of the addition of scats to the Murrin Murrin heap leach to reinforce its geotechnical strength or stability (Readett and Fox, 2009). The addition of 30 kg of load, corresponding to raising the height of Ni laterite heap to ~3 m, showed little increase in slump, even though irrigation was continued throughout the time of additional loading. This would indicate that constructing and operating Ni laterite heap heights of ~3.0–3.5 m should be possible with this material, even in the absence of scats which would increase the bulk density of the bed of agglomerates in many commercial operations. The whole idea was to mimic a 3 m high heap and see if the agglomerates were able to withstand this equivalent load. The results of the complementary column compression/loading test run under similar acid irrigation rate strongly support the inference to a 3 m high heap (CSIRO unpublished report, 2012). Column slumping varies as a consequence of the relative degree of several factors including (but not exclusively) collapse of the agglomerates due to leaching of soluble inter-granular bonds or bridges formed and the inherent loss of significant mass fraction of the column material, off-set by the wall effects of the column and possible swelling of clays within the agglomerates. Small amounts of slumping are generally acceptable as long as they are not associated with significant fines generation and thus blockage and pooling on columns/heaps. Surprisingly though, large degrees of slumping have been observed with no loss of column integrity (Elliot et al., 2009). Post column leach slumping (under additional stress) was designed to simulate the conditions under which the leached agglomerates near the bottom of the heap find themselves under and reassuringly the evidence suggests that additional stress has a marginal effect on the total slumping on most agglomerates. 7. Conclusions The present results provide some initial information on the column leaching behaviour of agglomerated siliceous goethitic Ni laterite ore ahead of the next stage in the development of the process where the construction of a pilot heap is recommended. The effects of different feed ore particle size on the agglomerate behaviour show that the
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increase in primary particles' surface area leads to more rapid sulphuric acid leaching kinetics, as would be expected. It may also be contended that the Ni laterite agglomerates formed from the siliceous goethitic sample are sufficiently robust to withstand a heap of reasonable height (up to 3.5 m) without collapsing as demonstrated by the loaded column slump data. Percolation of acid lixiviant through beds of agglomerates within the 5–40 mm size range used here is excellent, with no reduction in lixiviant flow observed over the 100 days of continuous operation of the columns. Acid consumption rate is within the typical range for low grade lateritic ores (~500 kg/t). Conducting simple column leach tests over similar leach times on other agglomerated ores should therefore provide suitable data for incorporation in plant design, optimization and improved Ni laterite heap leaching operations. On a cost-benefit analysis, wet grinding and drying may only be justified for cases where they may lead to a significant improvement in leach kinetics and Ni/Co recovery/yield. Acknowledgements The authors wish to thank CSIRO through their Minerals Down Under National Research Flagship for their in-kind and generous financial support for the Nickel Laterite Cluster project. 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 EPAL circuits for Ravensthorpe Nickel Operations. In: Imrie, W.P., et al. (Ed.), Proceedings, International Laterite Nickel Symposium. The Minerals, Metals and Materials Society, pp. 193–202. Agatzini-Leonardou, S., Dimaki, D., 1994. Heap leaching of poor nickel laterites by sulphuric acid at ambient temperature. Proceedings, “Hydrometallurgy ‘94” Published for the Institution of Mining and Metallurgy and the Society of Chemical Industry. Chapman and Hall, pp. 193–208. Elliot, A., Fletcher, H., Li, J., Watling, H., Robinson, D., 2009. Heap leaching of nickel laterites — a challenge and an opportunity. In: Budac, J.J., Frazer, R., Mihaylov, I., Papangelakis, V.P., Robinson, D.J. (Eds.), Proceedings, Hydrometallurgy of Nickel and Cobalt 2009. CIM, Montreal, Canada, pp. 537–552. Kyle, J., 2010. Nickel laterite processing technologies—where to next? ALTA Nickel/Cobalt/ Copper Conference, 24–27 May, Perth, Western Australia (36 pp.). Miller, G., 2000. Pilot plant testwork and design of the Ravensthorpe beneficiation plant. Proceedings, Ni/Co ALTA conference (17 pp.). Miller, G.W., Sampson, D., Fleay, J., Conway-Mortimer, J., Roche, E., 2004. Ravensthorpe Nickel Project beneficiation prediction MLR and interpretation of results. In: Imrie, W.P. (Ed.), Proceedings, International Laterite Nickel Symposium. The Minerals, Metals and Materials Society, pp. 121–136. Nosrati, A., Addai-Mensah, J., Robinson, D.J., 2012. Drum agglomeration behaviour of nickel laterite ore: effect of process variables. Hydrometallurgy 125–126, 90–99. Readett, D.J., Fox, J., 2009. Development of heap leaching and its integration into the Murrin Murrin operations. Paper Presented to ALTA Nickel/Cobalt Conference, Perth, May 25–27, 2009. Robertson, S.W., van Staden, P.J., 2009. The progression of metallurgical testwork during heap leach design. Hydrometallurgical Conference 2009. Southern African Institute of Mining and Metallurgy, pp. 31–42. Stamboliadis, E., Alevizos, G., Zafiratos, J., 2004. Leaching residue of nickeliferous laterites as a source of iron concentrate. Miner. Eng. 17, 245–252. Swierczek, Z., Quast, K., Addai-Mensah, J., Connor, J.N., Robinson, D.J., Li, J., 2012. Challenges in the mineralogical characterisation of low-grade nickel laterites, paper # 339. Proceedings, International Mineral Processing Congress, New Delhi, India 5352–5364. Watling, H., Das, G., Elliot, A., Li, J., McDonald, R., Robinson, D., 2010. Process options for difficult arid-region nickel laterites. Proceedings, XXV International Mineral Processing Congress, Brisbane, Australia, September, pp. 417–427. Watling, H., Elliot, A.D., Fletcher, H.M., Robinson, D.J., Sully, D.M., 2011. Ore mineralogy of nickel laterites: controls on processing characteristics under simulated heap-leach conditions. Aust. J. Earth Sci. 58 (7), 725–744. Wedderburn, B., 2009. Nickel laterite processing—a shift towards heap leaching. Proceedings, ALTA Nickel/Cobalt Conference, Perth, May 25–27, 2009. Xu, D., Hilder, T., Quast, K., Skinner, W., Addai-Mensah, J., Robinson, D.J., 2012. Column leaching behaviour of nickel laterite agglomerates, paper # 188. Proceedings, International Mineral Processing Congress, New Delhi, India, 5883–5891.