J. Cent. South Univ. (2020) 27: 3249-3258

DOI: https://doi.org/10.1007/s11771-020-4463-2

Nitric acid pressure leaching of limonitic laterite ores: Regeneration of HNO₃ and simultaneous synthesis of fibrous CaSO₄·2H₂O by-products

SHAO Shuang(邵爽)^{1, 2}, MA Bao-zhong(马保中)^{1, 2, 3}, WANG Xin(王昕)²,

ZHANG Wen-juan(张文娟)^{1, 2, 3}, CHEN Yong-qiang(陈永强)^{1, 2, 3}, WANG Cheng-yan(王成彦)^{1, 2, 3}

1. State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing,Beijing 100083, China;

2. School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing,Beijing 100083, China;

3. Beijing Key Laboratory of Green Recycling and Extraction of Metals, Beijing 100083, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020

Abstract: An innovative technology, nitric acid pressure leaching of limonitic laterite ores, was proposed by our research team. The HNO₃ regeneration is considerable significance for the improvement of the proposed technology and its commercial application, but it has not been systematically investigated. Herein, regenerating HNO₃ from Ca(NO₃)₂ solution with low-cost H₂SO₄, and simultaneous synthesis of fibrous CaSO₄·2H₂O by-products were studied. As a theoretical basis, the solubility of CaSO₄·2H₂O in HNO₃ medium is studied. It is concluded that the solubility of CaSO₄·2H₂O increases with increasing temperature or increasing HNO₃ concentration, which has considerable guiding significance for the subsequent experimental research and analysis. Then, the effects of various factors on the residual Ca²⁺ concentration of filtrate, the regenerated HNO₃ concentration and the morphology of synthesized products are investigated using ICP-AES and SEM. And the effect mechanism is also analyzed. The results indicate the regenerated HNO₃ concentration reaches 116 g/L with the residual Ca²⁺ concentration being 9.7 g/L at the optimum conditions. Moreover, fibrous CaSO₄·2H₂O by-products with high aspect ratios (length, 406.32 μm; diameter, 14.71 μm; aspect ratio, 27.62) can be simultaneously synthesized.

Key words: limonitic laterite ores; Ca(NO₃)₂ solution; HNO₃ regeneration; CaSO₄·2H₂O by-products; solubility

Cite this article as: SHAO Shuang, MA Bao-zhong, WANG Xin, ZHANG Wen-juan, CHEN Yong-qiang, WANG Cheng-yan. Nitric acid pressure leaching of limonitic laterite ores: Regeneration of HNO₃ and simultaneous synthesis of fibrous CaSO₄·2H₂O by-products [J]. Journal of Central South University, 2020, 27(11): 3249-3258. DOI: https://doi.org/10.1007/s11771-020-4463-2.

1 Introduction

At present, high pressure acid leaching and reducing roasting–atmospheric acid leaching are mainly applied to treat limonitic laterite ores [1, 2]. Ni/Co recovery is achieved by precipitation, and chelating ion exchange resins with high selectivity [3]. Nitric acid is a widely used leaching agent with strong oxidizability, which can sufficiently oxidize the divalent iron to hematite while dissolving valuable metals [4, 5]. So, nitric acid was selected and developed to treat limonitic laterite ores by our research team [4, 5]. And a series of laboratory tests and pilot-scale tests were completed and some exciting results were achieved. The technical route of the HNO₃ pressure leaching for limonitic laterite ores treatment is shown in Figure 1 [4, 5]. After selective pressure leaching limonitic laterite ores with HNO₃, the leach iron residue is separated and applied to iron making. Subsequently, Fe/Al, Ni/Co, and Mg dissolved in the leach liquor are successively precipitated in the form of hydroxides by adding CaO and controlling the pH value. As a result, a large amount of Ca(NO₃)₂ solution is produced.

Figure 1 A technical route of nitric acid pressure leaching for limonitic laterite ores treatment

Given that HNO₃ is usually expensive, regenerating HNO₃ from Ca(NO₃)₂ solution is undoubtedly the key step of the proposed technology commercial application. Herein, a method for regeneration of HNO₃ from Ca(NO₃)₂ solution with low-cost H₂SO₄ was proposed (marked in Figure 1). Our previous studies [4-6] indicated that HNO₃ is easily regenerated by mixing Ca(NO₃)₂ solution and H₂SO₄ solution (Eq. (1)), and fibrous calcium sulfate by-products can be simultaneously synthesized. Regeneration of HNO₃ with H₂SO₄ and simultaneous synthesis of fibrous calcium sulfate by-products, will undoubtedly reduce the production cost. However, the effects of reaction conditions on the regenerated HNO₃ concentration and the morphology of synthesized calcium sulfate have not been systematically studied.

Ca(NO₃)₂+H₂SO₄+2H₂O=2HNO₃+CaSO₄·2H₂O(↓) (1)

Calcium sulfate whiskers have been widely applied as reinforcing materials in papermaking and building due to its excellent performance (high strength, non-toxicity, superior workability, good compatibility, etc.) and overwhelming economic advantage [7-15]. The performance of calcium sulfate whiskers is mainly affected by their morphology and size (especially the aspect ratio) [11]. The aspect ratio is one of the most important evaluation criteria of calcium sulfate whiskers [8]. And the calcium sulfate whiskers with high aspect ratio in particular are marketable.

Generally, the synthesis processes of calcium sulfate whiskers include hydrothermal method [16, 17], salt-solution method [18-21] and atmospheric acidification method [22-25]. Calcium sulfate whiskers are synthesized in mixed medium of regenerated HNO₃ and residual H₂SO₄ in this study, which belongs to the scope of atmospheric acidification method. Atmospheric acidification method has milder reaction condition (atmospheric pressure and the temperature of lower than 90 °C) than hydrothermal method and takes less time than salt-solution method [22, 26]. Thus, it costs less and is more efficient. MAO et al [24, 25] prepared calcium sulfate whiskers in aqueous HCl solutions and investigated the effects of metal ions and additives on the morphology and size of products. EL MOUSSAOUITI et al [23] explored the crystallization of calcium sulfate in concentrated H₃PO₄ solutions. It is noted that there is a paucity of information available on the synthesis of calcium sulfate whiskers in mixed acid medium of HNO₃ and H₂SO₄.

The low sulfur content of regenerated HNO₃ used for cyclic leaching will facilitate the production of high-grade leach iron residue. The residual sulfur content in regenerated HNO₃ can be indirectly monitored by investigating the residual Ca²⁺ concentration. So, low residual Ca²⁺ concentration is an important indicator in regenerated HNO₃. The residual Ca²⁺ concentration is essentially the solubility of calcium sulfate in mixed acid medium. At present, the solubilities of calcium sulfate in H₂O and H₂SO₄ have been widely measured [27, 28], but its solubility in HNO₃ medium is rarely conducted. The solubility of calcium sulfate in different media is mainly affected by temperature and solution composition [29–32]. In this study, the solubility measurement of CaSO₄·2H₂O in HNO₃ medium was first carried out to provide a theoretical basis for the subsequent experimental research and analysis. Subsequently, the influences of several factors, such as reaction temperature, solution addition sequence, and dilution volume ratio of H₂SO₄, on the residual Ca²⁺ concentration of filtrate, the regenerated HNO₃ concentration and the morphology of calcium sulfate whiskers were systematically investigated to determine the optimum conditions. And the effect mechanism was also analyzed.

2 Experimental

2.1 Materials

Analytically grade Ca(NO₃)₂·4H₂O (purity≥ 99.0%), CaSO₄·2H₂O (purity≥99.0%), H₂SO₄ (purity, 95.0%-98.0%) and HNO₃ (purity, 65.0%- 68.0%) were purchased from Sinopharm Chemical Reagent Co., Ltd. In this study, all chemicals were used directly without any further purification. And deionized water was used to prepare solution.

2.2 Procedures

2.2.1 Solubility measurement

First, HNO₃ solutions with predetermined concentrations (50, 100, 150 g/L) were prepared and placed in a thermostatic shaking bath (THZ-82A, Jiangsu Changzhou Ronghua Instrument Manufacture Co., Ltd) with a speed of 200 r/min at corresponding temperatures (30, 50, 70, 80, 90 °C). Then a small amount of CaSO₄·2H₂O was added to the HNO₃ solutions per 12 h until the solid phase was no longer dissolved. The mixed products were then continually shaken for 24 h to ensure that the equilibrium state was achieved. Finally, the samples were placed for 48 h so that the supernatant became clear. Sampling of the supernatant using a glass pipette preheated to the sampling temperature and sampling of the equilibrating solid phase were for further analysis.

2.2.2 HNO₃ regeneration and fibrous CaSO₄·2H₂O by-products synthesis

In a typical synthesis procedure, 100 mL Ca(NO₃)₂ solution with a Ca²⁺ concentration of 60 g/L was prepared and added into a beaker. Theoretical amount (calculated according to the molar ratio of SO₄^2- and Ca²⁺) of H₂SO₄ solution was diluted through mixing H₂SO₄ and deionized water with volume ratios of 1:4-1:12 mL/mL and was added into another beaker. The two prepared solutions were first preheated to 15-90 °C, and subsequently one solution was dropwise added into another solution at a rate of 10 mL/min using a constant flow pump. The mixture was treated at a stirring rate of 200 r/min for 30 min. The products were filtered and rinsed with 50 mL deionized water. Finally, solid products were dried at 70 °C in a vacuum oven for 8 h.

2.3 Characterizations

The concentration of residual Ca²⁺ was analyzed by inductively-coupled plasma–atomic emission spectroscopy (ICP-AES, Optima 5300DV, Perkin Elmer). The concentration of regenerated HNO₃ was calculated using the concentration of residual Ca²⁺ according to Eq. (2). The solid phases were characterized by X-ray powder diffraction (XRD, Rigaku D/MAX-rA diffractometer) using Cu K_α radiation. The morphology of products was characterized by scanning electron microscopy (SEM, HITACHI S-3500 N/INCA Oxford, Japan).

                       (2)

where c (g/L) is the concentration of regenerated HNO₃; c₀ (g/L) and c₁ (g/L) are the Ca²⁺ concentrations before and after the reaction, respectively; V₀ (L) and V₁ (L) are the solution volumes before and after the reaction, respectively; M₁ is the relative molecular mass of HNO₃; M₂ is the relative atomic mass of Ca.

3 Results and discussions

3.1 Solubility of CaSO₄·2H₂O in HNO₃ medium

As a theoretical basis, the solubility of CaSO₄·2H₂O in HNO₃ medium is first studied. In the process of regenerating HNO₃ with H₂SO₄, it is inevitable that a small amount of H₂SO₄ remains in solution without reaction. Assuming that all of the added H₂SO₄ (theoretical amount) does not convert to HNO₃, the maximum concentration of unreacted H₂SO₄ is less than 1.5 mol/L. The solubilities of calcium sulfate in H₂O and 1.5 mol/L H₂SO₄ have been measured [27, 28], and they are also analyzed for comparison with the solubility of CaSO₄·2H₂O in HNO₃ medium.

In H₂O medium [28], the solubility of CaSO₄ first has a slight increase with increasing temperature, and then gradually decreases when the temperature is above 50 °C. In general, the solubility of CaSO₄ is very small in H₂O medium. In 1.5 mol/L H₂SO₄ medium [28], the solubility of CaSO₄ increases with increasing temperature, and its value is less than 11 g/L CaSO₄ (equivalent to 14 g/L CaSO₄·2H₂O) when the temperature increases to 90 °C. As shown in Figure 2, the solubility of CaSO₄·2H₂O in HNO₃ medium increases with increasing temperature, and the increase is more pronounced when the temperature exceeds 70 °C, especially. Moreover, the solubility of CaSO₄·2H₂O increases not only with increasing temperature but also with increasing HNO₃ concentration. Based on the above discussion, lowering the reaction temperature is preferable to ensure a lower residual Ca²⁺ concentration when the HNO₃ concentration is fixed, which may have considerable guiding significance for the subsequent experimental research and analysis.

Figure 2 Solubility curves of CaSO₄·2H₂O in HNO₃ medium

Through comparing the solubility data between 1.5 mol/L H₂SO₄ and different concentration HNO₃, it clearly indicates that the solubility of CaSO₄·2H₂O in HNO₃ medium is much higher than that in H₂SO₄ medium. In the studies, at least 60% of H₂SO₄ can convert to HNO₃. Thus, the residual H₂SO₄ concentration in the solution will actually be less than 0.6 mol/L, which will have a negligible effect on the solubility of CaSO₄·2H₂O compared with regenerated HNO₃. In other words, regenerated HNO₃ has much more dominant effect on the solubility of CaSO₄·2H₂O compared with residual H₂SO₄ in multi-component acid medium. Consequently, the effect of regenerated HNO₃ on CaSO₄·2H₂O solubility will be emphatically discussed in this article.

It should be ensured that no phase transformation occurs during the solubility measurement. Thus, XRD analysis of the equilibrating solid phase was performed as shown in Figure 3. The XRD results show that all equilibrating solid phases in different HNO₃ concentrations at 90 °C are still single-phase CaSO₄·2H₂O. This indicates that 90 °C has not reached the phase transition temperature of CaSO₄·2H₂O to CaSO₄·0.5H₂O, and the equilibrating solid phase is CaSO₄·2H₂O below 90 °C. In other words, it can be concluded that CaSO₄·2H₂O is the most stable phase when the temperature is below 90 °C in HNO₃ medium.

Figure 3 XRD patterns of equilibrating solid phase at 90 °C

3.2 HNO₃ regeneration and fibrous CaSO₄·2H₂O by-products synthesis

3.2.1 Effect of reaction temperature

According to Figure 2, temperature is a key effect factor for residual Ca²⁺ concentration. Therefore, under the conditions of adding H₂SO₄ solution into Ca(NO₃)₂ solution and 1:4 mL/mL H₂SO₄ dilution volume ratio, the variations in the residual Ca²⁺ concentration and regenerated HNO₃ concentration with reaction temperature were investigated as shown in Figure 4(a). It is observed the concentration of regenerated HNO₃ decreases gradually with increasing reaction temperature, which will lead to a lower solubility of CaSO₄·2H₂O according to Figure 2. However, the residual Ca²⁺ concentration remains basically unchanged as the temperature increases from 15 to 50 °C, and then increases obviously when the temperature is over 50 °C. This can be attributed to the fact that temperature has more dominant effect on the solubility of CaSO₄·2H₂O compared with the regenerated HNO₃ concentration.

Figure 4 Effects of parameters on residual Ca²⁺ concentration and regenerated HNO₃ concentration:

Figure 5 shows the morphology of calcium sulfate synthesized at different reaction temperatures under the condition of adding H₂SO₄ solution into Ca(NO₃)₂ solution. The calcium sulfate synthesized at 50 °C is composed of tiny particles and irregular sheets. Further increasing temperature to 70 °C, the flaky products decrease obviously and the needle-like products increase significantly. With the increase of reaction temperature, the crystallinity and aspect ratio of calcium sulfate whiskers increase obviously. These findings suggest that reaction temperature has a significant effect on the crystallization of calcium sulfate, which is consistent with the results of obtained by SONG et al [15]. It is noted that the solubility of calcium sulfate increasing results in the supersaturation decreasing. Crystal growth rate is faster than nucleation rate at low supersaturation during crystallization process. This is why the crystal morphology of calcium sulfate becomes better with increasing temperature.

In general, in order to ensure a lower residual Ca²⁺ concentration and a higher regenerated HNO₃ concentration, 50 °C was selected as the optimum reaction temperature under the condition of adding H₂SO₄ solution into Ca(NO₃)₂ solution.

The effect of reaction temperature on the residual Ca²⁺ concentration and regenerated HNO₃ concentration is shown in Figure 4(b) under the conditions of adding Ca(NO₃)₂ solution into H₂SO₄ solution and 1:4 mL/mL H₂SO₄ dilution volume ratio. The residual Ca²⁺ concentration increases obviously with increasing reaction temperature, and the increase is more pronounced when the reaction temperature exceeds 70 °C, which is completely matched with the variation trend of CaSO₄·2H₂O solubility with temperature (Figure 2). The concentration of regenerated HNO₃ drops sharply when the temperature exceeds 50 °C. As shown in Figure 4(b), the concentration of regenerated HNO₃ reaches 116 g/L at 50 °C, while only 90 g/L at 80 °C.

Under the condition of adding Ca(NO₃)₂ solution into H₂SO₄ solution, the SEM images of calcium sulfate synthesized at different reaction temperatures are shown in Figure 6. It can be observed that the length of calcium sulfate whiskers synthesized at 50 °C reaches to 406.32 μm. Further increasing temperature leads to the synthesis of good crystallinity and high aspect ratios whiskers. And the products with the best crystal morphology and uniform size are synthesized at 80 °C. The viscosity of solution decreases with the increase of temperature, which contributes to the formation of whiskers with uniform size [15]. When the reaction temperature continues increasing to 90 °C, the length of whiskers is obviously shorter than 80 °C. This might be attributed to the vigorous movement of Ca²⁺ and SO₄^2- in the solution at high temperature. The vigorous movement of ions may interrupt the ordered arrangement of atoms, which is not conducive to the axial growth of crystals.

Figure 5 SEM images of calcium sulfate synthesized at different reaction temperatures (adding H₂SO₄ solution into Ca(NO₃)₂ solution):

Figure 6 SEM images of calcium sulfate synthesized at different reaction temperatures (adding Ca(NO₃)₂ solution to H₂SO₄ solution):

For comprehensive consideration of the regenerated HNO₃ concentration and products morphology, 50 °C was selected as the optimum reaction temperature under the condition of adding Ca(NO₃)₂ solution into H₂SO₄ solution.

3.2.2 Effect of solution addition sequence

As shown in Figure 5, the calcium sulfate crystals mainly are platelet-like at 50 °C and good crystallinity whiskers can be synthesized by increasing temperature to 80 °C under the condition of adding H₂SO₄ solution into Ca(NO₃)₂ solution. While under the condition of adding Ca(NO₃)₂ solution into H₂SO₄ solution (Figure 6), needle-like whiskers have been successfully prepared at 50 °C and the length of calcium sulfate whiskers reaches to 406.32 μm. The above discussions suggest that calcium sulfate whiskers can be synthesized at lower temperature under the condition of adding Ca(NO₃)₂ solution into H₂SO₄ solution.

At the identical reaction temperature, a comparison between Figures 5 and 6 clearly shows that the products synthesized under the condition of adding Ca(NO₃)₂ solution into H₂SO₄ solution has a fewer flake structure, more uniform morphology, larger aspect ratios and better crystallinity. Obviously, the crystallization of calcium sulfate whiskers can occur in high acid environment under the condition of adding Ca(NO₃)₂ solution into H₂SO₄ solution. In the crystallization process, the solubility of CaSO₄·2H₂O is greater due to the high acid environment and high temperature resulting from the exothermic phenomenon of H₂SO₄. It can be inferred that the supersaturation is smaller at this time. And lower supersaturation is conducive to crystal growth. In addition, a comparison between Figure 4(a) and Figure 4(b) clearly shows that the solution addition sequence has little effect on the concentration of residual Ca²⁺ and regenerated HNO₃.

Therefore, on the premise that the concentration of regenerated HNO₃ is not affected, adding Ca(NO₃)₂ solution into H₂SO₄ solution was selected as the optimum solution addition sequence to obtain higher quality calcium sulfate by- products.

3.2.3 Effect of H₂SO₄ dilution volume ratio

The experiments were carried out by adding Ca(NO₃)₂ solution into H₂SO₄ solution at 50 °C, and the effect of H₂SO₄ dilution volume ratio (1:4-1:12 mL/mL) on the residual Ca²⁺ concentration and regenerated HNO₃ concentration is shown in Figure 4(c). It can be seen from Figure 4(c) that the regenerated HNO₃ concentration decreases linearly with increasing the dilution volume ratio of H₂SO₄, which is caused by a large increase in the solution volume. The residual Ca²⁺ concentration decreases with increasing the dilution volume ratio of H₂SO₄. This is partly due to the reduced CaSO₄·2H₂O solubility in low concentration of regenerated HNO₃ solution, and partly due to the increased solution volume. In short, lower H₂SO₄ dilution volume ratio should be promoted to ensure a higher concentration of regenerated HNO₃.

The influence of H₂SO₄ dilution volume ratio on the morphology of calcium sulfate is shown in Figures 6(a) and Figure 7. Calcium sulfate whiskers can be synthesized at different H₂SO₄ dilution volume ratios. However, a comparison between Figures 6(a) and Figure 7 clearly showed that the diameter of whiskers becomes wider and the aspect ratio decreases obviously with increasing the dilution volume ratio of H₂SO₄. Thus, lower H₂SO₄ dilution volume ratio is more beneficial to the synthesis of whiskers. Hence, 1:4 mL/mL was selected as the optimum dilution volume ratio of H₂SO₄.

Figure 7 SEM images of calcium sulfate synthesized at different dilution volume ratios of H₂SO₄:

According to the single-factor experimental results, the optimum conditions for regenerating HNO₃ and synthesizing calcium sulfate whiskers are 50 °C, solution addition sequence of adding Ca(NO₃)₂ solution into H₂SO₄ solution, H₂SO₄ dilution volume ratio of 1:4 mL/mL. The morphology of the products synthesized at the optimum conditions is shown in Figure 6(a) and fibrous calcium sulfate (length, 406.32 μm; diameter, 14.71 μm; aspect ratio, 27.62) is synthesized. The corresponding XRD result is shown in Figure 8, which indicates that the synthesized product is CaSO₄·2H₂O. In summary, HNO₃ with a concentration of 116 g/L (the residual Ca²⁺ being 9.7 g/L) can be regenerated and CaSO₄·2H₂O whiskers with good crystallinity and high aspect ratios can be efficiently synthesized, simultaneously.

Figure 8 XRD pattern of product synthesized at optimum conditions

It is worth noting that HNO₃ can be regenerated and recycled for leaching in this study. Meanwhile, fibrous CaSO₄·2H₂O by-products can be simultaneously synthesized, and the synthesis process is simple and easy to operate. Through the content of this study, the process of nitric acid pressure leaching limonitic laterite ores is further improved. HNO₃ recycling and the synthesis of fibrous CaSO₄·2H₂O by-products undoubtedly reduce the production cost. These make this technology beneficial to realize commercial application.

4 Conclusions

This study aimed to improve the HNO₃ regeneration step in the process of nitric acid pressure leaching of limonitic laterite ores. Conclusions can be drawn as follows:

1) The solubility of CaSO₄·2H₂O in HNO₃ medium increases with increasing temperature or increasing HNO₃ concentration.

2) The optimum conditions for the HNO₃ regeneration and whiskers synthesis are 50 °C, adding Ca(NO₃)₂ solution into H₂SO₄ solution and 1:4 mL/mL H₂SO₄ dilution volume ratio. Under these conditions, HNO₃ with a concentration of 116 g/L (the residual Ca²⁺ is 9.7 g/L) can be regenerated, and fibrous CaSO₄·2H₂O by-products with high aspect ratios (length, 406.32 μm; diameter, 14.71 μm; aspect ratio, 27.62) can be simultaneously synthesized.

3) The regenerated HNO₃ concentration decreases gradually and the residual Ca²⁺ concentration increases obviously with increasing reaction temperature. Therefore, low temperature is preferable for the HNO₃ regeneration.

Contributors

The overarching research goals were developed by WANG Cheng-yan and MA Bao-zhong. SHAO Shuang performed the experiments and collected the data. WANG Xin and ZHANG Wen-juan analyzed the measured data and the calculated results. The research activity planning and execution were managed by CHEN Yong-qiang. The initial draft of the manuscript was written by SHAO Shuang. MA Bao-zhong and WANG Cheng-yan reviewed and edited the manuscript.

Conflict of interest

SHAO Shuang, MA Bao-zhong, WANG Xin, ZHANG Wen-juan, CHEN Yong-qiang, and WANG Cheng-yan declare that they have no conflict of interest.

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(Edited by YANG Hua)

中文导读

褐铁型红土镍矿硝酸加压浸出：再生HNO₃并同步制备CaSO₄·2H₂O副产物

摘要：HNO₃再生对于褐铁型红土镍矿硝酸加压浸出技术的完善和工业应用意义重大。本文采用低成本的H₂SO₄从Ca(NO₃)₂溶液中再生HNO₃，并同步制备CaSO₄·2H₂O副产物。作为理论基础，测定了CaSO₄·2H₂O在HNO₃介质中的溶解度。结果表明，CaSO₄·2H₂O的溶解度随着温度的升高或HNO₃浓度的增大而不断增大，这对后续的实验研究和结果分析具有指导意义。借助电感耦合等离子体原子发射光谱分析和扫描电子显微镜等检测手段考察了各因素对滤液残留Ca²⁺浓度、再生HNO₃浓度和产物形貌的影响，并分析了其影响机理。结果表明，在最优条件下，再生HNO₃浓度达116 g/L，滤液残留Ca²⁺浓度为9.7 g/L，同步制备的CaSO₄·2H₂O副产物具备高纵横比(长406.32 μm；直径14.71 μm；纵横比27.62)。

关键词：褐铁型红土镍矿；Ca(NO₃)₂溶液；HNO₃再生；CaSO₄·2H₂O副产物；溶解度

Foundation item: Project(2182040) supported by the Beijing Natural Science Foundation, China; Projects(51674026, 51974025, U1802253) supported by the National Natural Science Foundation of China; Project(FRF-TT-19-001) supported by the Fundamental Research Funds for the Central Universities, China

Received date: 2020-02-24; Accepted date: 2020-08-02

Corresponding author: MA Bao-zhong, PhD, Professor; Tel: +86-10-62333170; E-mail: bzhma_ustb@yeah.net; ORCID: https://orcid. org/0000-0003-3907-9097; WANG Cheng-yan, PhD, Professor; Tel: +86-10-62332271; E-mail: chywang@yeah. net; ORCID: https://orcid.org/0000-0003-3982-6208