Comparing iron phosphate and hematite precipitation processes for iron removal from chloride leach solutions

Minerals Engineering 98 (2016) 14–21

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Comparing iron phosphate and hematite precipitation processes for iron removal from chloride leach solutions Saviour Masambi, Christie Dorfling ⇑, Steven Bradshaw Department of Process Engineering, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa

a r t i c l e

i n f o

Article history: Received 16 November 2015 Revised 27 June 2016 Accepted 2 July 2016

Keywords: Hematite Iron phosphate Precipitation

a b s t r a c t The presence of iron in pregnant leach solutions presents significant processing challenges. The removal of iron impurities from leach solutions by means of iron phosphate precipitation may be a feasible alternative to more conventional iron oxide/iron hydroxide precipitation processes. This study compares the performance of the iron phosphate precipitation process with that of the hematite process at different operating temperatures (40 °C–90 °C), pH conditions (pH 1–pH 3) and seeding measures for the removal of iron from a chloride leach solution. The extent of iron removal, co-precipitation of nickel and copper, and solid-liquid separation were used as performance criteria for the comparison. Seeded iron phosphate precipitation at pH 1 and 40 °C resulted in 98.8% iron removal with 0.5% nickel and 2.8% copper losses. 99.8% iron removal was achieved with the iron phosphate precipitation process at pH 1 and 80 °C, but the nickel and copper losses increased to 8.7% and 20.8%, respectively, with the increase in temperature. Seeded hematite precipitation at pH 1 and 80 °C yielded 99.6% iron removal with 3.5% nickel and 1.7% copper losses. For the hematite process, nickel and copper losses decreased with an increase in temperature. Increasing the pH yielded higher nickel and copper losses for both processes. All seeded precipitation experiments produced easily filterable precipitates. Unseeded iron phosphate precipitates produced at 40 °C and pH 1 were filterable, but increased nickel and copper losses were observed. Unseeded hematite precipitation resulted in high nickel and copper losses, with the precipitates practically impossible to filter. Iron phosphate precipitates exhibited more favourable settling characteristics than the precipitate produced with the hematite process. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction In hydrometallurgical processes, iron is commonly removed from pregnant leach solutions by precipitation as jarosite, goethite or hematite. The jarosite process is extensively used during iron removal in zinc hydrometallurgy. The co-precipitation of metal ions (Zn, Cu, Ni, Mn, Al, Ga and Ge) has been cited as one of the major weaknesses of the jarosite process (Claassen et al., 2002; Wang et al., 2011). Furthermore, jarosite compounds are universally classified as hazardous wastes because of the adverse effects to the environment and human health (Asokan et al., 2006). The essential feature of goethite precipitation is that the ferric concentration of the solution should be maintained at approximately 1 g/L. Goethite is said to be precipitated as either a-goethite in sulphate systems or b-goethite (akaganeite) in chloride systems. Akaganeite has been reported to sustain co-precipitation of cations and its filtration is practically impossible (Cohen et al., 2005; Dutrizac ⇑ Corresponding author. E-mail address: [email protected] (C. Dorfling). http://dx.doi.org/10.1016/j.mineng.2016.07.001 0892-6875/Ó 2016 Elsevier Ltd. All rights reserved.

and Riveros, 1999). Controlled precipitation of a-goethite in sulphate systems ensures lesser impurities in the precipitates and its filtration is easily achievable (Ismael and Carvalho, 2003). Production of large volumes of goethite has been reported to be an environmental concern (Dutrizac and Riveros, 1999). Hematite (Fe2O3) is the preferred iron precipitate because it is a stable, high density and more pure form of iron. These properties of hematite make it easier for disposal and offer the potential for it to qualify as a by-product; it is used as starting material in the production of iron and steel, pigments and ferrites (Riveros and Dutrizac, 1997). Twidwell et al. (1987) reported the invention of a method for recovering metal values from mixed aqueous solutions by selective phosphate precipitation. It was reported that the precipitate produced was easily separated by conventional solid-liquid filtration techniques, and that the process was applicable in most systems (sulphates, chlorides and mixtures of these lixiviants). Because of its distinctive selectivity towards trivalent metal precipitation over divalent metal precipitation, phosphate precipitation was suggested to be a suitable iron removal method during the purification of solutions containing divalent metals such as nickel and copper

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(Twidwell et al., 1987). Garole et al. (2012) furthermore reported that the iron phosphate precipitation process is more efficient and economical than high temperature hydroxide precipitation processes characterised by precipitates that are difficult to separate. To date, very little information about the phosphate precipitation process for iron removal has been published, and no quantitative comparison of the technical performance of this process and conventional iron oxide/iron hydroxide processes has been done. The objective of this study was hence to investigate the iron phosphate precipitation process as an alternative iron removal process for purification of a nickel-copper chloride leach solution. More specifically, this study compares the performance of the iron phosphate precipitation process with that of the commonly used hematite precipitation process in terms of iron removal, the extent of nickel and copper co-precipitation, and solid-liquid separation. The work formed part of a project aimed at the development of a process for copper and nickel recovery from a pregnant leach solution, with iron removal by precipitation envisaged to be the first processing step. The pregnant leach solution originates from a metal production facility that utilises hydrochloric acid leaching for the dissolution of base metals from a specific feed material. Given the operational requirements of the upstream leaching process, the leach solution produced contains approximately 4.6 M HCl, 45 g/L Fe, 3 g/L Cu and 3 g/L Ni.

more common type of iron(III) phosphate, ranged between 4.57  103 and 2.24  108. The dimorphic nature of iron(III) phosphate (orthorhombic or monoclinic structure) was the proposed reason for the differences in solubility constants reported in literature. Twidwell et al. (1986) reported that conversion of ferric phosphate to ferric hydroxide, and therefore regeneration of phosphate, is possible at moderate temperature (25–50 °C) and high pH (pH 11–pH 12) conditions. Complete conversion with caustic soda was achieved after two hours reaction time at pH 12 and 50 °C. Unlike the precipitates produced in a conventional hydroxide precipitation process, the ferric hydroxide precipitates produced by the conversion of ferric phosphate filter as easily as ferric phosphate and do not contain heavy metals (which presents environmental challenges) because of the purity and morphology inherited from the ferric phosphate precipitates. The conversion reaction is shown in Eq. (2).

1.1. Iron phosphate precipitation

2Fe3þ þ 3H2 O $ Fe2 O3 þ 6Hþ

Jenkins et al. (1971) presented an overview of research work performed to develop a process for phosphate removal from wastewater by means of precipitation with iron added as iron chloride salts. The same principle has been applied in hydrometallurgy for iron removal from leach solutions by means of precipitation with phosphate addition (Cruz et al., 1980; Garole et al., 2012; Twidwell et al., 1987). Cruz et al. (1980) investigated the removal of iron from electrowinning solutions. It was reported that iron(III) phosphate was precipitated at pH 2 and a temperature of 50 °C. The morphology of the iron phosphate was spherites and agglomerates of spherites. Redissolution of the phosphate precipitate was, however, observed at a ferric to phosphate ratio higher than 3.15. Twidwell et al. (1987) reported that the phosphate precipitation process selectively recovers trivalent metal cations as easily separable crystals, making it suitable for purification of sulphate or chloride solutions containing divalent metal cations such as nickel and copper. Twidwell et al. (1987) suggested a similar precipitation temperature (60 °C) and pH (pH 2) to those proposed by Cruz et al. (1980). The findings of Cruz et al. (1980) and Twidwell et al. (1987) were confirmed by Garole et al. (2012). Garole et al. (2012) reported that the low co-precipitation of divalent metallic cations can in part be ascribed to poor adsorption on the precipitated iron phosphate, which exhibited spherical shapes and a low specific surface area. Typical reaction times recorded ranged from 30 min to 1 h, achieving more than 99% iron removal. Iron phosphate precipitation was proposed to proceed according to Eq. (1).

Dutrizac and Riveros (1999) investigated the precipitation of hematite from chloride solutions at temperatures below 100 °C and ambient pressure mainly by employing seeding. In the absence of seeding, a reaction time of almost 100 h was required for akaganeite to precipitate and transform to hematite. Tests conducted at 100 °C yielded hematite after only 2 h of reaction time when seeded with 15 g/L Fe2O3. It was reported that hematite precipitation occurred via two pathways: the precipitation of akaganeite (a metastable phase) which then gradually transforms to hematite, and the direct precipitation of hematite. A combination of both pathways was suggested to be possible, but the direct formation of hematite was identified as the more likely pathway in seeded reactions. Decreasing the temperature to 60 °C did not have an adverse effect on the iron removal achieved by hematite precipitation. More than 99% of the iron initially in solution was reported to be in the hematite precipitate. Cohen et al. (2005) also investigated iron removal by hematite precipitation. It was reported that pH values below 1 did not significantly affect filtration rates and ensured almost complete removal of iron, provided seeding levels were between 10 and 20 g/L. Seeding had a significant impact on the rate of filtration. Typical filtration rates averaged 83 mL/min and increased with an increase in operating temperature and reaction time. Seeding beyond 20 g/L did not have any further impact on filtration rates. Unseeded experiments performed at temperatures below 100 °C did, however, produce precipitates that were not practically possible to filter; this was attributed to the formation akaganeite instead of hematite.

Fe3þ þ PO34 þ 2H2 O $ FePO4  2H2 O

ð1Þ

The variation in iron phosphate solubility at 50 °C with pH was determined by Huang (2001) and reported by Twidwell and Dahnke (2001). The solubility was reported to decrease linearly with increasing pH. At pH 0, the solubility is approximately 1050 mg/L; above pH 2 iron phosphate is virtually insoluble. Robins et al. (1991) reviewed literature on the solubility and speciation of iron(III) phosphate and concluded that the reported solubility constants of ferric phosphate dihydrate, which is the

FePO4 þ 3NaOH $ FeðOHÞ3 þ Na3 PO4

ð2Þ

1.2. Hematite precipitation Traditionally, hematite precipitation has been reported to be possible at a temperature greater than 100 °C and under pressure oxidation of greater than 5 bar (Dutrizac and Monhemius, 1986). Hematite precipitation proceeds according to Eq. (3).

ð3Þ

2. Experimental 2.1. Materials Unless stated otherwise, all chemicals were reagent grade chemicals supplied by Sigma-Aldrich. Synthetic leach solutions containing 45 g/L iron, 3 g/L nickel and 3 g/L copper were prepared using iron(II) chloride tetrahydrate, copper chloride, nickel

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chloride hexahydrate and hydrochloric acid. Oxidation of ferrous to ferric was achieved by sparging oxygen through the synthetic leach solution. Neutralisation of the leach solution was achieved through the addition of sodium hydroxide solution prepared using ACS reagent grade sodium hydroxide pellets and demineralised water, while phosphate was introduced in the form of sodium phosphate. Iron(III) phosphate and iron(III) oxide (purified grade) were used for seeding of the phosphate precipitation process and the hematite process, respectively. 2.2. Equipment All precipitation tests were performed in a closed 1.6 L glass vessel. The reactor was fitted with an overhead mechanical agitation system to drive a PTFE coated dual (axial and radial) impeller system. For oxidation, oxygen was sparged through a PTFE tube positioned below the bottom impeller. The temperature of the solution was maintained within 2 °C of the setpoint temperature by means of a heating plate with feedback control. pH control was achieved by a Thermo Scientific Alpha pH 560 controller; the controller output was a solenoid valve connected to the outlet of a burette filled with sodium hydroxide, which was gravity fed into the reactor as required. Sampling was done using syringes connected to sampling tubes inserted through the reactor lid. 2.3. Experimental design The hematite process and the iron phosphate process were compared at different levels of pH and temperature with and without seeding. Experiments were conducted to validate the effect of the solution pH, the solution temperature and seeding on the precipitation of iron(III) and the resulting losses of nickel and copper. A summary of the independent variable levels investigated for both precipitation processes is presented in Table 1. 2.4. Experimental procedure and sample analysis Oxidation of ferrous in the synthetic leach solution was achieved by sparging oxygen through the solution in the agitated reactor (impeller speed of 600 rpm) at 80 °C until 98% oxidation had been achieved; the extent of oxidation was verified by means of titration with potassium dichromate using sodium diphenylamine as indicator. After oxidation, the solution was allowed to reach the setpoint temperature for the precipitation experiment. The oxidized solution, containing 4.6 M HCl, was neutralized to pH 0 using a 15.5 M NaOH solution; subsequently, a 1 M NaOH standard solution was used to control the pH to the setpoint value. In the case of iron phosphate precipitation experiments, stoichiometric amounts of sodium phosphate were added before attaining pH 0. For seeded precipitation, seeds were added at pH 0 or at pH 0.5 for tests performed at pH 1 and above. For both hematite and iron phosphate experiments, the starting time for seeded and unseeded precipitation experiments was noted upon attaining the desired pH. Solution samples were taken at the sampling times specified in Table 1; the duration of the experiment was 2 h. The solution

Table 1 Experimental planning and variables investigated for the respective precipitation processes. Variable

Phosphate

Hematite

Solution pH Solution temperature (°C) Seeding (g) Sampling times (min)

0, 1, 2, 3 40, 60, 80 0, 30 0, 10, 20, 30, 60, 120

0, 1, 2, 3 60, 80 0, 30 0, 10, 20, 30, 60, 120

samples were filtered using 0.45 lm syringe filters. The samples were then sealed and stored in 15 mL centrifuge tubes for analysis using inductively coupled plasma optical emission spectrometry (ICP-OES). The concentrations of nickel, copper, iron, sodium and phosphorous were analysed by this method. For the purpose of precipitate characterisation, 200 mL of the slurry were collected at the end of the experiment and filtered using a Büchner funnel with quantitative filter paper. The filtration rate was determined by measuring the time required for filtration to be completed. The precipitates collected in the filter cake were washed with deionised water to ensure that nickel and copper were not mechanically trapped in the precipitate, where after the precipitate was left to dry for 20 h at 40 °C. Solid samples were prepared by means of the coning and quartering method. The chemical composition of the precipitates was determined using X-ray fluorescence spectrometry (XRF). X-ray diffraction (XRD) was used to identify phases formed and to obtain an indication of the degree of amorphousness of some of the precipitates produced. The samples were prepared for analysis using a back loading preparation method. A PANalytical Empyrean diffractometer with PIXcel detector and Fe filtered Co Ka radiation was used to perform the analyses. X’Pert Highscore Plus software was used for identification of phases, and phase quantification was done using the Rietveld method. Amorphous phases were not taken into account in the quantification. A ZEISS EVO MA 15 Variable Pressure scanning electron microscopy (SEM) with an energy dispersive X-ray spectrometer (EDS) was used for analysis of particle characteristics, identification of possible reaction products, and evaluation of nickel and copper distribution in the precipitate. Particle size distribution of the precipitate was determined using a Saturn DigiSizer 5200 instrument. Settling tests were performed to characterise the settling behaviour of precipitates produced with the different processes and at different operating conditions. Suspensions of precipitate in water were placed in a 1 L glass cylinder, mixed to obtain a homogenous suspensions, and then placed on a flat surface. The settling of particles was captured using a video camera and the change in the solid-liquid interface with time was calculated using a MatlabTM interface tracking tool.

3. Results and discussion 3.1. Iron phosphate precipitation 3.1.1. Effect of pH and temperature Fig. 1 shows results of seeded precipitation experiments conducted at varying pH and temperature conditions. Iron was almost completely precipitated out of solution after 20 min of reaction time at all pH values (1, 2 and 3) and temperatures investigated (40 °C, 60 °C and 80 °C). However, nickel and copper losses varied with varying pH and temperature. Copper losses can be ascribed to a combination of copper occlusion and copper precipitation. At pH 3, in particular, no conclusions regarding the contribution of occlusion to overall copper losses can be made given the significant extent of copper precipitation expected at this pH. It was not possible to identify the copper compounds present in the precipitate during this investigation due to the low copper and high iron content of the precipitate. However, atacamite (CuCl23Cu(OH)2) has been reported to generally form as the dominant copper precipitate during precipitation from chloride solutions (Lundström et al., 2012; McDonald and Muir, 2007; Valkama et al., 2014). In the case of nickel, losses are believed to have occurred primarily because of occlusion since no potential nickel containing solid phases are thermodynamically stable in the range of operating conditions investigated in this study.

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50%

20%

25%

10%

Fe precipitation

30%

Ni, Cu precipitation

Fe precipitation

75%

0%

0% 0

20

40 60 80 Time (min)

Fe removal (%)

40%

100% 75%

30%

50%

20%

25%

10% 0%

0%

100 120

0

Ni loss (%)

20

40 60 80 Time (min)

Fe removal (%)

Cu loss (%)

100 120 Ni loss (%)

Cu loss (%)

(a)

(b) 100%

Fe, Ni, Cu precipitation

100%

Fe, Ni, Cu precipitation

Ni, Cu precipitation

40%

100%

75% 50% 25% 0% 0

20

40 60 80 Time (min)

Fe removal (%)

100 120 Ni loss (%)

75% 50% 25% 0% 0

20

40 60 80 Time (min)

Fe removal (%)

100 120 Ni loss (%)

Cu loss (%)

Cu loss (%)

(c)

(d)

Fig. 1. Seeded precipitation of iron phosphate depicting losses of nickel and copper at varying pH and temperatures: (a) pH 1, 40 °C, (b) pH 1, 60 °C, (c) pH 2, 40 °C, (d) pH 3, 40 °C.

It can be seen from Fig. 1(a) that during the first 10 min, the losses of nickel and copper increased with time and after that nickel and copper losses appeared to start reducing and attained equilibrium after about 30 min. It is proposed that this was due to Ostwald ripening. It is suggested that nickel and copper losses, during the first 10 min, were due to occlusion of nickel and copper liquid within the iron phosphate structure. As the smaller crystals of iron phosphate re-dissolved to form larger crystals, the trapped liquid was released back into solution. Copper precipitation was unlikely at pH 1 since formation of copper precipitates such as atacamite have been reported to be significant only at pH values of approximately 2.6 and above (Lundström et al., 2012; McDonald and Muir, 2007; Valkama et al., 2014). The purity of these precipitates can thus be improved by lowering supersaturation; low supersaturation ensures heterogeneous nucleation and subsequently well-defined crystal growth as well as the formation of larger particles, which limit co-precipitation by occlusion. pH and temperature are process variables typically recommended for supersaturation control in iron precipitation applications (Claassen and Sandenbergh, 2007; Demopoulos, 2009; Dirksen and Ring, 1991). In general, lower supersaturation can be attained by decreasing the pH and increasing the temperature. The decreasing nickel and copper co-precipitation with a decrease in pH is evident from Fig. 1. At pH 1 and 40 °C, the nickel loss was 0.5% for a reaction time of 30 min. At this pH, temperature and reaction time, the copper loss was 2.8%. Increasing the pH to 2 resulted in nickel and copper losses of 14% and 77%, respectively, for the same reaction time and temperature. The nickel and copper losses were even higher for tests conducted at pH 3, regardless of the temperature and reaction time. XRF analysis furthermore

showed that the nickel and copper content of the precipitate increased from 0.2% at pH 1 to 0.7% and 1.1%, respectively, at pH 3 and 40 °C. The significant increase in especially copper losses with an increase in pH was also due to the fact that the loss of copper through precipitation as species such as atacamite became more significant compared to losses through occlusion. The precipitation of copper species made a larger contribution to the overall copper loss at the higher pH values, which could also explain why the local maximum in metal losses at 10 min was less evident for the tests performed at the higher pH values. Even though increasing the temperature from 40 °C to 60 °C and 80 °C did not have as large an impact as increasing the pH, it can be seen from Fig. 1(a) and (b) that lower temperature precipitation resulted in lower losses of nickel and copper for the same pH and retention time. In this study, the increase in nickel and copper co-precipitation with an increase in temperature can be ascribed to the formation of iron compounds such as goethite or jarosite in addition to the iron phosphate precipitate at the higher temperatures. This was confirmed by XRF analysis, which indicated that the iron to phosphorous molar ratio in the precipitate increased from 1.06 at pH 1 and 40 °C to 1.33 at pH 1 and 80 °C. 3.1.2. Effect of seeding As was the case for iron precipitation in seeded tests (Fig. 1), iron precipitation attained equilibrium with virtually complete iron removal after 20 min when no iron phosphate seeding was employed. Seeding did, however, have an effect on the co-precipitation behaviour of nickel and copper, as shown in Fig. 2. Hove et al. (2009) reported that seeding favours formation of larger, more pure and more stable particles at the expense of the smaller, impure particles typical of homogeneous nucleation. In

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30%

Cu precipitation

Ni precipitation

30%

20%

10%

0%

20%

10%

0% 0

20

40

60

Time (min)

Unseeded

0

20

40

60

Time (min)

Seeded

(a)

Unseeded

Seeded

(b)

Fig. 2. Co-precipitation of (a) nickel and (b) copper during seeded and unseeded iron phosphate precipitation at 40 °C and pH 1.

the absence of seeding, homogenous nucleation results in high nucleation rates and the rapid formation of a large number of small particles, with growth typically being diffusion limited. Seeded precipitation provides nucleation sites that allow well defined particle growth governed by the surface reaction rather than diffusion. As a result, the possibility of nickel and copper occlusion is reduced by the addition of seeding material, which explains the data shown in Fig. 2. The dissolution of smaller and impure precipitates is furthermore enhanced in seeded precipitation compared to unseeded precipitation.

3.1.3. Precipitate characterisation Scanning electron microscopy indicated that the precipitate consisted of an amorphous iron phosphate precipitate with flakes of sodium chloride. The iron phosphate precipitate consisted primarily of spherical particles with a dense morphology. This observation agrees with the results reported by Cruz et al. (1980) and Garole et al. (2012), who ascribed the low co-precipitation of other metallic cations to this specific precipitate morphology. The colour of the precipitates formed in seeded and unseeded experiments at all pH values at 40 °C was bright yellow, which is characteristic of iron phosphate. At 60 °C, the precipitates formed were still yellow but not as bright as the precipitates produced at 40 °C. As the temperature was increased to 80 °C, the precipitates were mostly light brown, especially in the case of unseeded runs and runs conducted at pH 2 and pH 3. These observations indicate that iron phosphate was formed in both seeded and unseeded runs. The iron phosphate content of the precipitates decreased with an increase in the temperature and pH as iron compounds other than iron phosphate as well as copper precipitates started to form. SEM EDS results confirmed that the iron precipitation was primarily due to iron phosphate formation given the clear similarity between the distribution of iron and the distribution of phosphorous in the precipitate. XRF analysis also indicated an iron to phosphorous molar ratio of between 0.94 and 1.34; the values significantly above the stoichiometric iron to phosphorous molar ratio of 1 were observed for precipitates formed at higher temperatures and pH values, where precipitation of iron oxides contributed to the overall iron removal (as discussed in Section 3.1.1). In terms of XRD analysis, the iron phosphate precipitates produced at all conditions tested were found to be largely amorphous. Apart from trace amounts of iron oxides detected in the precipitates formed at higher temperature and pH conditions and trace amounts of sodium chloride, no other crystalline phases were identified. Beck et al. (2011) reported similar observations, namely that amorphous iron phosphate precipitated from the reaction of ferric

iron and sodium phosphate unless deliberate additional processes were involved (Beck et al., 2011). 3.2. Hematite precipitation 3.2.1. Effect of pH and temperature The effects of pH variations on the precipitation of hematite and the resulting nickel and copper losses were similar to those observed during the precipitation of iron phosphate. From Fig. 3 (a)–(d), it can be shown that iron precipitation was almost complete (above 99% removal) after 20 min at pH values of 1, 2 and 3. Nickel and copper losses generally increased with an increase in pH. From Fig. 3(a) and (b), it can be seen that precipitation at 80 °C recorded a similar iron removal profile to that observed at 60 °C. Attempts to precipitate hematite at temperatures lower than 80 °C at pH 1 did, however, result in higher nickel and copper losses; the nickel and copper losses observed at 30 min increased from 3.5% and 1.7% to 13.2% and 11.9%, respectively, when the temperature was decreased to 60 °C. Similar observations were made for tests performed at pH 2. As was the case for iron phosphate precipitation, combined precipitation of iron and copper occurred at the higher pH values, and no conclusions regarding the contribution of occlusion to overall copper losses can be made. At pH 1, the losses of nickel and copper were attributed primarily to occlusion of liquid nickel and copper within the akaganeite structure. Hematite is traditionally precipitated at temperatures above 100 °C. Reducing the temperature favours the precipitation of akaganeite, as shown by the results presented in Section 3.2.3. Akaganeite is characterised by square molecular channels formed through the connection of 4 by 2 rows of octahedral structures. These channels in the akaganeite structure are sufficiently large to permit the passage of anions and other impurities. Hematite, on the other hand, exhibits a compact crystal structure. Akaganeite therefore exhibits a larger possibility of trapping nickel and copper in its crystal lattice compared to hematite, which explains why larger nickel and copper losses were observed for the lower temperature precipitation tests. 3.2.2. Effect of seeding Iron removal was rapid, with concentration profiles similar to those shown in Fig. 3, in unseeded experiments. As was the case with the iron phosphate precipitation process, seeding did result in reduced nickel and copper losses, as shown in Fig. 4. Dutrizac and Riveros (1999) demonstrated that hematite can be precipitated at temperatures as low as 60 °C when precipitation is seeded. It was however reported that even in seeded experiments, akaganeite precipitation was not entirely avoided but seeding

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30%

50%

20%

25%

10%

0%

Fe precipitation

75%

100%

0% 20

40 60 80 Time (min)

Fe removal (%)

75%

30%

50%

20%

25%

10%

0%

100 120

0% 0

Ni loss (%)

20

40 60 80 Time (min)

Fe removal (%)

Cu loss (%)

(b) 40% 30%

50%

20%

25%

10%

0%

Fe precipitation

75%

0% 40 60 80 Time (min)

Fe removal (%)

40%

100% Ni, Cu precipitation

Fe precipitation

100%

20

Ni loss (%)

Cu loss (%)

(a)

0

100 120

75%

30%

50%

20%

25%

10% 0%

0%

100 120

Ni, Cu precipitation

0

40% Ni, Cu precipitation

40% Ni, Cu precipitation

Fe precipitation

100%

0

Ni loss (%)

20

40 60 80 Time (min)

Fe removal (%)

Cu loss (%)

100 120 Ni loss (%)

Cu loss (%)

(c)

(d)

Fig. 3. Seeded precipitation of hematite and loss of nickel and copper at varying pH and temperatures: (a) pH 1, 80 °C, (b) pH 1, 60 °C, (c) pH 2, 80 °C, (d) pH 3, 80 °C.

40%

Cu precipitation

Ni precipitation

40% 30% 20% 10% 0%

30% 20% 10% 0%

0

20

40

60

Time (min)

Unseeded

Seeded

(a)

0

20

40

60

Time (min)

Unseeded

Seeded

(b)

Fig. 4. Co-precipitation of (a) nickel and (b) copper during seeded and unseeded hematite precipitation at 80 °C and pH 1.

accelerated its transformation to hematite. It is proposed that the lower nickel and copper losses in seeded precipitation compared to unseeded precipitation is attributed to the formation of hematite in seeded precipitation as opposed to the formation of akaganeite in unseeded experiments. 3.2.3. Precipitate characterisation Scanning electron microscope images of the hematite precipitate indicated that the morphology of the particles were compact with a number of agglomerates and aggregates. Elemental maps obtained using SEM EDS indicated a uniform distribution of nickel and copper in the precipitate, which is similar to what was found for iron phosphate precipitation.

The colour of precipitates produced in hematite runs depended mostly on seeding. All unseeded experiments produced precipitates with various shades of brown. The seeded experiments produced red precipitates, with the precipitate formed at pH 1 and 80 °C having the deepest red colour characteristic of hematite. As the pH was increased and the temperature decreased, the red colour of the precipitates softened and, hence, the hematite content of the precipitate decreased. These observations were confirmed by the XRD data presented in Table 2, which illustrate the effect of temperature and pH on the composition of the crystalline portion of the precipitate formed during seeded hematite precipitation. The ratio of crystalline precipitate to amorphous precipitate (e.g., ferrihydrite) was not

S. Masambi et al. / Minerals Engineering 98 (2016) 14–21

Table 2 Mineralogical composition of crystalline precipitates formed at different conditions of seeded hematite precipitation. Conditions pH pH pH pH

1, 1, 2, 3,

60 °C 80 °C 80 °C 80 °C

Hematite

Akaganeite

Other

63 96 59 56

37 – 41 44

– 2% magnetite, 2% halite – –

quantified. The percentage hematite in the crystalline precipitate increased with a temperature increase from 60 °C to 80 °C as well as with a decrease in pH. These results are in agreement with the explanation regarding increased nickel and copper losses at high pH and low temperature conditions, which was ascribed to the formation of increasing amounts of akaganeite and decreasing amounts of hematite. Although copper precipitates such as atacamite were not detected using XRD analysis, this was likely due to initial solution composition which resulted in a very small amount of copper precipitate forming compared to the amount of iron precipitate formation. 3.3. Solid-liquid separation 3.3.1. Filtration From Fig. 5, it can be seen that iron phosphate precipitates produced at all conditions tested were filterable with filtration rates between 50 mL/min and 130 mL/min; these rates are around the average filtration rate of 83 mL/min reported by Cohen et al. (2005). The effect of temperature and pH on filtration rates can be explained in terms of the particle size distribution of the precipitate. As was discussed in Section 3.1.1, decreasing the pH and increasing temperature resulted in lower supersaturation; this enhanced heterogeneous nucleation, formation of larger particles, higher filter cake porosity, and hence better filterability. It was practically impossible to filter precipitates produced during hematite precipitation performed without seeding. Seeded hematite precipitation experiments were, however, easily filtered. As illustrated in Fig. 6, the filtration rate was in the ranges 35–55 mL/min at pH 1, 65–105 mL/min at pH 2 and 110–130 mL/ min at pH 3. Cohen et al. (2005) attributed better filterability of precipitate produced in seeded experiments to the formation of hematite in seeded precipitation as opposed to akaganeite being the main precipitate formed in unseeded precipitation. The increase in filtration rate with an increasing pH can be attributed to the formation of aggregates and agglomerates, which was more significant for the hematite process than for the iron phosphate precipitation process due to smaller particles formed in the hematite process. The formation of aggregates and agglomerates is enhanced by reducing the zeta potential, which decreases with increase in solution pH.

Filtration rate (mL/min)

140

140

Filtration rate (mL/min)

20

120 100 80 60 40 20 0 pH 1

3.4. Process comparison Based on the experimental results obtained, a comparison of the key operating conditions and performance criteria between iron phosphate precipitation and hematite precipitate is summarised in Table 3. All iron removal and nickel and copper losses are reported for the samples taken at 30 min, since this was typically the time required for the iron precipitation to reach equilibrium and at which point the nickel and copper losses were close to a minimum. Given the high nickel and copper losses at pH values above pH 1, the fact that iron removal in excess of 98% was achieved at pH 1, and the acidity of typical pregnant leach solutions, iron removal by seeded precipitation would not be performed at pH values

Table 3 Summarised comparison of key parameters for seeded iron phosphate and seeded hematite precipitation processes. Parameter

Iron phosphate

Hematite

Seeding

Unseeded precipitation resulted in increased nickel and copper losses Neutralizing reagent 3 (NaOH) and PO3 4 . PO4 can be regenerated 98.8% (40 °C) 99.9% (60 °C) 99.8% (80 °C) 0.5% (40 °C) 9.2% (60 °C) 8.7% (80 °C) 2.8% (40 °C) 33.1% (60 °C) 20.8% (80 °C) 50–130 mL/min Settling complete after 26 min Bright yellow, easy to clean

Unseeded precipitation resulted in increased nickel and copper losses as well as poor filtration Neutralizing reagent (NaOH)

Iron removal at pH 1 Nickel losses at pH 1

60 40

Copper losses at pH 1

20 0 pH 1

pH 2

40°C

60°C

pH 3

80°C

Fig. 5. Iron phosphate filtration rates as a function of pH at various temperatures.

pH 3

90°C

3.3.2. Sedimentation Analysing the settling profile of seeded iron phosphate precipitated at pH 1 and 40 °C indicated that iron phosphate precipitate settled completely after approximately 26 min, forming a clear solid-liquid interface. The clarity of the settling interface reduced with an increase in pH, and settling required approximately 36 min to be completed for the precipitate formed at pH 2 and 40 °C. The solids produced during hematite precipitation experiments could not be settled by gravity, with no solid-liquid interface forming.

Reagents

80

80°C

Fig. 6. Hematite filtration rates as a function of pH at various temperatures.

120 100

pH 2

60°C

Filtration Sedimentation Precipitate colour

98.2% (60 °C) 99.6% (80 °C) 13.2% (60 °C) 3.5% (80 °C) 11.9% (60 °C) 1.7% (80 °C) 35–130 mL/min No settling observed Deep red, difficulty to clean

S. Masambi et al. / Minerals Engineering 98 (2016) 14–21

above pH 1. The most suitable precipitation process for a specific application will depend on a number of factors such as the operating temperature of the upstream and downstream unit operations, main operational objectives, and operating costs in specific local markets. At 40 °C, the iron phosphate precipitation process can remove 98.8% of iron in solution with minimal nickel and copper losses (0.5% and 2.8%, respectively) after 30 min. The iron removal increased to 99.9% when the temperature was increased to 60 °C, but the nickel and copper losses increased to 9.2% and 33.1%, respectively. At the same temperature, the hematite precipitation process achieved 98.2% iron removal with higher nickel losses (13.2%) but significantly lower copper losses (11.9%) than the iron phosphate precipitation process. At 80 °C, the hematite and iron phosphate precipitation processes achieve comparable iron removal (99.6% and 99.8%, respectively). Based on the nickel and copper losses, however, it is clear that the hematite precipitation process would be the preferred option at the higher temperature. 4. Conclusion Iron phosphate precipitation performed at a temperature of 40 °C and pH 1 achieved 98.8% iron removal after 30 min reaction time with 0.5% nickel and 2.8% copper losses through co-precipitation. Although iron precipitation increased with an increase in temperature, the nickel and copper losses also increased to 8.7% and 20.8%, respectively, at 80 °C. These high nickel and copper losses render the iron phosphate precipitation process less desirable compared to the hematite process at higher operation temperatures. The performance of the hematite precipitation processes generally improved with an increase in temperature. At 80 °C and pH 1, 99.6% iron removal with 3.5% nickel and 1.7% copper losses were achieved after 30 min. For both processes, the nickel and copper losses increased significantly when the pH was increased or when seeding was not used. The precipitate formed during both the seeded iron phosphate precipitation and the seeded hematite precipitation was easily filterable. The solids produced in the iron phosphate precipitate process showed good settling behaviour, with settling tests yielding a clear solid–liquid interface after 26 min of gravity settling; in the case of hematite precipitate, no settling was however observed after 8 h. The iron phosphate precipitation process has been shown to be a suitable iron removal process for consideration in certain applications, for example where the upstream or downstream unit operations are operated at lower temperatures than what would typically be required for efficient seeded hematite precipitation.

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