稀有金属(英文版) 2015,34(01),64-70

收稿日期：20 December 2013

基金：financially supported by the National Natural Science Foundation of China (Nos. 51274044, 51304023, and U1302274);

Pressure nitric acid leaching of alkali-pretreated low-grade limonitic laterite

Yong-Lu Zhang Cheng-Yan Wang Yong-Qiang Yang Fei Yin Bao-Zhong Ma

Institute of Metallurgy, Beijing General Research Institute of Mining and Metallurgy

Abstract：

The pressure nitric acid leaching of alkali-pretreated low-grade limonitic laterite, as well as removing impurity Al(III) and preparing intermediate product of nickel/cobalt sulphide from leaching liquor were investigated. After pretreatment, iron exists in the form of amorphous iron oxides, while nickel is adsorbed on the surface of iron oxides in the form of nickel oxide. The preferable pressure leaching conditions are determined as follows: leaching temperature of 458 K, leaching duration of 60 min, initial acidity of nitric acid of 1.90 mol L-1and liquid to solid ratio of 3:1(volume to mass ratio). Under these conditions, the leaching efficiencies of Ni, Co and Al are 95 %, 88 % and 55 %, respectively, and that of Fe is less than 1 %. The loss rates of Ni and Co are 1.8 % and1.5 %, respectively, during the step of removing impurity Al(III). The sulphide precipitation process produces the interim production of nickel/cobalt sulphides, recovering greater than 99 % of Ni and Co in the purified solution.The iron-rich([60 %) pressure leaching residue with low Cr, S can be further reclaimed as the raw materials for iron making.

Keyword：

Limonitic laterite; Alkali-pretreated; Pressure-leaching; Nitric acid; Nickel;

Author： Cheng-Yan Wang,e-mail: chywang@yeah.net;

Received： 20 December 2013

1 Introduction

Among the world’s land-based nickel resources, lateriteores account for 60 %–70 % and the others are mainlysulphide nickel ores. But about 70 % of current worldwideproduction of nickel is derived from sulphide nickel ores,and the laterite ores are not used sufficiently [1–6]. There isan increasing interest in the economically utilizing of lowgrade laterites as the continuous depletion of sulphidenickel ores and high-grade nickel laterite ores [2, 3, 7–10].

The processes applied commercially to recover nickeland cobalt from laterite include pyrometallurgical andhydrometallurgical methods [11, 12]. Compared withpyrometallurgical processes suited to treat saprolitic laterite, hydrometallurgical processes are more applicable tolimonitic laterite ores and mainly include reduction-roasting ammonia–ammonium carbonate leaching (RRAL) andhigh pressure sulphuric acid leaching (HPAL) [13, 14]. Atpresent, HPAL process for treating limonitic lateritebecomes the hotspot of research and industrial application[15–18].

When the HPAL process was used to treat Cr-containinglimonitic laterite ores from Indonesia, the leaching efficiency of Ni was less than 80 % neither in sulphuric acidnor in nitric acid medium. The phase composition analysisindicates that the un-leaching Ni embeds in the chromite,silicate mineral or other indissolvable minerals in the laterite ores. Moreover, the residues of the HPAL containinghigher Cr and S are difficult to utilize in iron making. Inview of the above problems, the pretreatment of complexlaterite was investigated with a desire to achieve highleaching efficiency and great economic benefits.

In the studies of Guo et al. [19], the Cr-containinglimonitic laterite ores from Indonesia were pretreated byalkali roasting using sodium carbonate (Na₂CO₃), and thenleached with water to remove Cr and Al. Based on thesestudies, the pressure nitric acid leaching process of thepretreated limonitic laterite ore was investigated in detail.Contrast to sulphuric acid leaching, this process has lowleaching temperature, and no scaling of calcium sulphatecould not only extract the nickel and cobalt efficiently butalso produce the iron-riched residue. The residue containing low Cr and S can be utilized for the iron making. Forthe iron-riched limonitic laterite ores, the sufficient utilization of iron will bring considerable economic benefits.

2 Experimental

2.1 Materials and reagents

The limonitic laterite ores used in the present work werecollected from Indonesia and then pretreated by alkaliroasting and water leaching before pressure nitric acidleaching [19]. The leaching liquor was purified byremoving Al(III) and then was used to produce the interimproduction of nickel/cobalt sulphides by sulphide precipitating process. Figure 1 shows the brief flow sheet of thisprocess. Figures 2 and 3 show the XRD patterns of thelimonitic laterite ores and the pretreated ores, respectively.Figure 2 indicates that the major mineral phase is goethitein the laterite ores, and magnetite, hematite, aluminium–chromite and serpentine are the minor ones. It can be foundfrom Fig. 3 that the intensity of characteristic peaks ofgoethite is obviously weakened and the peak of hematitecannot be observed, but a new phase of bunsenite is discerned. During alkali-pretreated process, the main nickeland cobalt-bearing minerals are mostly transformed toamorphous phase that can be easier leached by nitric acidto recover Ni and Co. As shown in Fig. 4, the fine granulanickel oxides embed in the amorphous iron oxides. Theelemental composition of pretreated laterite ore is presented in Table 1.

Fig. 1 Brief flow sheet of recovering Ni/Co from laterite ores

In this study, the analytical grade reagents, includingnitric acid, sulphuric acid, calcium carbonate, sodiumsulphide, etc., were used, and all aqueous solutions wereprepared using distilled water.

Fig. 2 XRD pattern of limonitic laterite ore

Fig. 3 XRD pattern of pretreated laterite ore

Fig. 4 Reflection macro image of fine granular nickel oxides embedin amorphous iron oxides

Table 1 Chemical compositions of pretreated laterite ore (wt%)  下载原图

Table 1 Chemical compositions of pretreated laterite ore (wt%)

2.2 Pressure nitric acid leaching

The leaching experiments were performed in 2-verticaltitanium autoclave equipped with magnetic stirrer with adigital controller unit, an internal water-cooling titaniumspiral tube and an electrical heating mantle which controlled the temperature of the reaction medium with anaccuracy of ±1 °C.

The reactor was charged with slurry which was preparedby mixing leaching solvent of nitric acid and pretreated oreand then agitating continuously with a constant speed andheating up to a required temperature. After the leachingreaction was sustained for a predetermined time, theautoclave was cooled to the temperature of 353 K and thenthe gas was exhausted till the pressure was zero. Theleaching slurry was withdrawn from reactor, and wasvacuum filtered. The filter cake was dried in oven at 353 Kfor 8 h for sample preparation. The residue and leachedliquor samples were chemically analysed for determiningthe elements of Ni, Co, Fe and Al.

2.3 Sulphide precipitation of nickel and cobalt

The removal of Al(III) from leaching liquor was carried outin the glass reactor before sulphide precipitation. Leachingliquor was adjusted to the appropriate p H value of 4.0 byadding 30 wt% Ca CO₃slurry for 60 min to precipitate themajor portion of Al(III) as aluminium hydroxides. Thepurified solution and precipitation were separated by filtration and then analyzed, respectively, to determine theimpurity removal efficiency and loss of nickel and cobalt.To the purified solution, sodium sulphide was added toprecipitate the Ni and Co as nickel and cobalt sulphideswhich were filtrated from solution and then analyzed.

2.4 Analysis methods

The metal concentrations in solution were determined byinductively coupled plasma-atomic emission spectrometry(ICP-AES, ICPE-9000), and the p H value was measuredwith a p H/m V-metre. The phase composition and micromorphology of solid samples were investigated by X-raydiffraction (XRD, Ultima IV), reflection macroscope andscanning electron microscopy (SEM, HITACHI S-3500N).

3 Results and discussion

3.1 Pressure nitric acid leaching

The metal oxides like Ni O, Co O and Fe₂O₃in pretreatedlaterite ore are soluble under autoclave conditions and reactwith acid according to the following stoichiometric reactions [1, 15]:

Ferric cations hydrolyse rapidly after the dissolution ofiron oxide, forming directly hematite according to Reaction(4) [15]:

The effects of leaching temperature, leaching time,initial acid concentration and the liquor to solid ratio(volume to mass ratio) on the Ni and Co leaching efficiencywere investigated to obtain the optimal reaction conditions.

3.1.1 Effect of leaching temperature

The experiments were performed in the temperature rangeof 433–478 K under the conditions: initial nitric acidconcentration of 1.60 mol L^-1, liquid to solid ratio of 3:1and agitation speed of 500 r min^-1for 60 min, to study theeffects of temperature on the leaching efficiencies of Ni,Co, Fe and Al. The results are shown in Fig. 5. It can beobserved that the Ni and Co leaching efficiencies increaserapidly with the increase of the temperature from 433 to458 K, but rise slowly with the temperature above 458 K.The leaching efficiency of Fe intends to decrease during thewhole temperature range because the ferric hydrolyticreaction, as shown in Reaction (4), processes increasinglyreadily with the temperature elevating. The leaching efficiency of Al shows a pronounced decline with the temperature of below 443 K, while maintains about 50 %–55 % when the temperature exceeds 443 K. Therefore, thetemperature is chosen as 458 K.

Fig. 5 Effect of temperature on leaching of pretreated laterite ore

3.1.2 Effect of initial nitric acid concentration

Figure 6 shows the effects of initial nitric acid concentration on the leaching efficiencies of Ni, Co, Fe and Al. TheFe leaching efficiency is less than 1 % and that of Al alterslittle, maintaining about 50 % with the acid concentrationvarying from 1.00 to 2.70 mol L^-1. By contrast, the Ni andCo leaching efficiencies increase significantly with theinitial nitric acid increasing and the acid level of1.90 mol L^-1is sufficient to leach most of the Ni and Co.

3.1.3 Effect of leaching time

Leaching experiments were conducted to investigate theeffect of leaching time under the conditions: temperature of458 K, initial acid concentration of 1.90 mol L^-1, liquor tosolid ratio of 3:1 and agitation speed of 500 r min^-1.Results are plotted in Fig. 7, which show that leaching timehas no remarkable effect on the Fe leaching efficiencywhich remains less than 1 %, but evidently affects theleaching efficiency of Ni and Co in the initial 60 min. It isseen that Al leaching efficiency increases slowly with anincrease in leaching time and remains about 50 % after80 min. Thus, leaching time for higher Ni and Co recoveryis determined to be 60 min.

Fig. 6 Effect of nitric acid concentration on leaching efficiency ofpretreated laterite ore

Fig. 7 Effect of leaching time on leaching efficiency of pretreatedlaterite ore

3.1.4 Effect of liquid to solid ratio

The pretreated ore was leached in solution containing1.90 mol L^-1nitric acid for 60 min at 458 K with liquid tosolid ratios changing from 3:2 to 5:1. As shown in Fig. 8,with the liquid to solid ratio changing from 3:2 to 3:1, theleaching efficiencies of Ni and Co increase evidently, butmaintain steadily with the liquid to solid ratio increasingcontinually. The leaching efficiencies of Fe and Al remainconstant within the whole liquid to solid ratio range. Theoptimal liquid to solid ratio is therefore considered as 3.

Based on the above experiments, the optimal conditions ofpressure acid leaching pretreated laterite ore are determinedas leaching temperature of 458 K, leaching time of 60 min,initial nitric acid of 1.90 mol L^-1and liquid to solid ratio of3:1. Under the optimal conditions, the leaching efficiencies ofNi, Co, Al and Fe are about 95 %, 88 %, 55 % and less than1 %, respectively. During the process, Ni and Co areselectively leached and the leaching residue contains about0.028 % of Ni, 0.005 % of Co and more than 60.000 % of Fe.

Fig. 8 Effect of liquid to solid ratio on leaching efficiency ofpretreated laterite ore

Fig. 9 XRD pattern of pressure leaching residue of pretreated lateriteore

Fig. 10 SEM image of pressure leaching residue of pretreated lateriteore

The XRD pattern of the leaching residue is shown inFig. 9, which indicates that the main diffraction peaks ofthe leaching residue match with the standard diffractionpeaks of hematite and talcum. The result illustrates that theamorphous iron phase in pretreated laterite ore transformsto hematite phase during the pressure acid leaching. TheSEM image of the leaching residue is shown in Fig. 10,and the further SEM–EDS analysis indicates that the mainelements contained in hematite phase are Fe (70.63 %), O(26.66 %) and Si (2.62 %).

3.1.5 Contrast of leaching residues produced by nitric/sulphuric acid leaching of pretreated ores and nitricacid direct leaching of raw ores

The alkali pretreatment destroys the mineral lattices beaing Ni of the laterite, which makes more Ni be exposed anthen easily leached during the subsequent pressure acleaching process.

The subsequent contrast experiments of pressure nitracid leaching for raw laterite ore and pretreated laterite orand pressure sulphuric acid leaching for pretreated lateriore were carried out to evaluate the effects of pretreatmeand leaching media on the leaching of Ni and Co and tiron grade of acid leaching residues.

The contrast experiments of pressure acid leaching weperformed under their respective optimal conditions. Tmain compositions of the pressure acid leaching residuare presented in Table 2. It can be seen that the residuedirect pressure nitric acid leaching for the raw laterite ocontains more Cr, Ni and Co with low grade of iron; tresidue of pressure sulphuric acid leaching for the prtreated ore also with lower grade of iron contains relativeplenty of S which is detrimental to the utilization of tresidue in iron making. By contrast, the residue of pressunitric acid leaching for pretreated ore has lower contentNi, S, Co and Cr and higher grade of iron, thus it canutilized as iron-making material.

3.2 Removal of aluminium

As shown in Table 3, the pressure acid leaching solutiocontains impurity Al and Fe which are required to remofor the subsequent step of sulphide precipitation. Tseparation was performed in a glass reactor. The leachinsolution was adjusted to the appropriate p H value of 4.0 badding 30 wt% Ca CO₃slurry for 60 min to the neutraization and precipitation of the major portion of alumiium. The pressure nitric acid leaching is an oxidatioprocess; therefore, the iron in leaching solution is ferrcation which can also be removed in the process of netralization and precipitation.

The experiments of removing Al(III) were carried out at298 K, because the former experiments indicate that theeffect of temperature on the Al(III) precipitating efficiencyis less when the terminal p H value is selected. Figure 11presents the effect of terminal p H values on theprecipitating efficiency of Al and the loss of Ni and Co. Itcan be seen that the Al(III) precipitating efficiencyincreases significantly to about 99 % with terminal p Hvalue increasing in the range of 3.5–3.8 and the loss of Niand Co are about 1.8 % and 1.5 %, respectively. Noremarkable Al is precipitated with the terminal p H valueexceeding 3.8, but the loss of Ni and Co increases, correspondingly. Therefore, the feasible p H value ranges from3.8 to 4.0. After this process, the Fe(III) and Al(III) insolution are less than 0.005 and 0.010 g L^-1, respectively.The SEM image of depositing aluminium residue is shownin Fig. 12. It indicates that the residue exists in the form of

Table 2 Main compositions of pressure leaching residues (wt%)  下载原图

Table 2 Main compositions of pressure leaching residues (wt%)

Table 3 Concentrations of elements in leaching solution (g L^-1)  下载原图

Table 3 Concentrations of elements in leaching solution (g L^-1)

Fig. 11 Effect of p H values on deposition of Al and loss of Ni andCo

Fig. 12 SEM image of depositing aluminium residueaggregate granules and the particle diameters are abou20 lm.

3.3 Sulphide precipitating of nickel and cobalt

Metal sulphide precipitation is a frequently used andimportant process in hydrometallurgical treatment of oresand effluents with some advantages, including lower solubility of metal sulphide precipitates, potential for selectivemetal removal, fast reaction rates, etc. In this research,sodium sulphide was used as sulphide agent for collectingNi and Co from leaching liquor. Previous work related tothe sulphide precipitation of metals was well documented[20]. It is also found that the process of sulphide precipitation is sensitive to the dose of sulphide agent added in theliquor [20, 21]; hence, the effect of mole ratio (n) of Na₂Sto (Ni²?? Co^2?) on Ni precipitating efficiency was studied. The results are shown in Fig. 13, which indicate thatthe optimum mole ratio is 1.2:1.0 with the Ni and Coprecipitating efficiency of greater than 99 %. The Ni andCo in the sulphide are 42.72 % and 2.07 %, respectively.The SEM image of Ni/Co sulphide powder is shown inFig. 14. As shown in Fig. 14, the Ni/Co sulphide powder isamorphous aggregation.

Fig. 13 Effect of sulphide agent quantity on Ni precipitatingefficiency

Fig. 14 SEM image of Ni/Co sulphide powder

4 Conclusion

The process of alkali-pretreated pressure nitric acidleaching can selectively leach Ni and Co efficiently andproduce iron-riched residue which can be utilized for ironmaking. The leaching liquor was purified by removingAl(III) and then was used to produce the interim productionof nickel/cobalt sulphides. The optimal pressure nitric acidleaching conditions are determined as leaching temperatureof 458 K, leaching time of 60 min, initial concentration ofnitric acid of 1.90 mol L^-1and liquid to solid ratio of 3:1.Under these conditions, the leaching efficiencies of Ni, Coand Al are about 95 %, 88 % and 55 %, respectively, andthat of Fe is less than 1 %. More than 99 % Al(III) andFe(III) are removed from leaching solution by neutralization precipitating with less than 2 % loss of Ni and Co,respectively. Greater than 99 % of the Ni and Co in thepurified solution are precipitated as Ni/Co sulphide whichcan be further utilized to refine Ni and Co.