Sulfuric acid leaching of South African chromite. Part 2: optimization of leaching conditions

    Sulfuric Acid Leaching of South African Chromite. Part 2: Optimization of Leaching Conditions Maofa Jiang, Qing Zhao, Chengjun Liu, Peiyang Shi, Bo Zhang, Dapeng Yang, Henrik Sax´en, Ron Zevenhoven PII: DOI: Reference:

S0301-7516(14)00078-7 doi: 10.1016/j.minpro.2014.05.009 MINPRO 2625

To appear in:

International Journal of Mineral Processing

Received date: Revised date: Accepted date:

4 January 2014 2 April 2014 28 May 2014

Please cite this article as: Jiang, Maofa, Zhao, Qing, Liu, Chengjun, Shi, Peiyang, Zhang, Bo, Yang, Dapeng, Sax´en, Henrik, Zevenhoven, Ron, Sulfuric Acid Leaching of South African Chromite. Part 2: Optimization of Leaching Conditions, International Journal of Mineral Processing (2014), doi: 10.1016/j.minpro.2014.05.009

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Sulfuric Acid Leaching of South African Chromite.

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Part 2: Optimization of Leaching Conditions Maofa Jianga, Qing Zhaoa,b*, Chengjun Liua, Peiyang Shia, Bo Zhanga, Dapeng Yanga,

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Henrik Saxénb, Ron Zevenhovenb

a

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Key Laboratory for Ecological Metallurgy of Multimetallic Ores (Ministry of Education), Northeastern University, Shenyang 110819, China

b

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Thermal and Flow Engineering Laboratory, Åbo Akademi University, Åbo/Turku 20500, Finland

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*Corresponding author: Qing Zhao, Address: Åbo Akademi University, Biskopsgatan 8, 20500 Åbo, Finland. Tel.: +358 0414928863. E-mail:

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[email protected].

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ACCEPTED MANUSCRIPT Abstract: The effects of temperature, duration, sulfuric acid concentration and oxidant on the leaching of South African chromite were investigated in this work. It was found that the extraction yield of chromium increased with temperature, limited by a chromium-rich sulfate precipitating from the solution when the

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temperature became too high, resulting in the decline of chromium recovery. The spinel phase of the ore shrank while the extraction yield of chromium increased with the duration of the leaching, leveling out after

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60 minutes. A porous amorphous silica phase was obtained as the metal elements leached and the spinel phase flaked, creating a resistance for the dissolution of the inner spinel phase. The results also showed that the sulfuric acid concentration and oxidant have a marked influence on the extraction yield of chromium

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and on the leaching reaction rate, and the best results were obtained for 80 %-wt sulfuric acid and an oxidant/chromite (mass) ratio oxidant above 1/10. The extraction yield of chromium was improved by the

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employment of agitation, and 1800 rad·s-1 was the reasonable speed.

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Keywords: Chromite; Sulfuric acid leaching; Sulfate; Optimization; Leaching conditions

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ACCEPTED MANUSCRIPT 1. Introduction

The acid leaching process is a mature method for the extraction of metal elements from ores and

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secondary resources, which is commonly used in the treatment of nickel ore (Thubakgale et al., 2013), copper ore (Li and Zhou, 2009), uranium ore (Hu et al., 2009) and iron ore (Wang et al., 2012). Because no

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chromium (Ⅵ)-bearing substances are produced, the acid leaching process has attracted a lot of attention in the chromium salt manufacturing industry for human health and environmental pollution control reasons (Ji,

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2012). The chemical activity of sulfuric acid has made it the first choice for this cleaner process. Liu et al. (2011a; 2011b; Liu and Shi, 2011) carefully investigated the effects of different conditions on the leaching

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behavior of Pakistani chromite and demonstrated that the extraction yield of chromium was significantly improved by elevating leaching temperature, pressure and acid dosage within reasonable ranges. Shi et al. (2004; 2011) (Shi and Liu, 2002) focused on an industrial application of this cleaner approach using

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Pakistani chromite, producing a basic chromic sulfate product using sulfuric acid leaching followed by

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separation of iron.

As chromite sources in China are lean or scarce, and South Africa today holds about 75 % of the world’s

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known chromite reserves, South Africa is the major country from which China imports chromite. However, the understanding of how to efficiently leach this kind of chromite is still insufficient. This is a reason why the sulfuric acid leaching study presented in this paper was focused on the optimization of the process using

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South Africa chromite as raw mineral. In order to further examine the reason for the sulfate precipitation observed in the leaching experiments reported in part 1 of this investigation (Zhao et al., 2014), this study also analyzed the effect of the leaching conditions on the sulfate formation.

2. Material and Methods

2.1 Samples and Experimental Procedures

South African chromite (45.18 %-wt of Cr2O3) was used in the experimental investigation, and the chemical composition, X-ray diffraction (XRD) pattern, photograph and scanning electron microscopy (SEM) images were reported in part 1 of the study (Zhao et al., 2014). 3

ACCEPTED MANUSCRIPT Chromite powder with a size smaller than 74 μm was obtained by grinding and screening of the ore. Oxidant and 80 mL sulfuric acid was mixed in an Erlenmeyer flask on an automatic temperature-controlled electric heater (cf. Fig. 3 (b) in part 1). 10 g chromite powder was poured into the solution when the

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temperature reached the set point, after which the agitation was started and maintained during the whole leaching process. Deionized water was added to the vessel to maintain the volume of the solution at the

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original level in case the concentration of sulfuric acid would otherwise become too high due to evaporation. After a period of time, the leachate was diluted with deionized water for subsequent filtration, and the pregnant filtrate was analyzed for chromium by inductively coupled plasma (ICP) and chemical

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analysis according to the PRC national standard. The extraction yield of chromium, expressed as the mass ratio of chromium in the filtrate and chromium in the raw material, was determined. The leaching residues

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were weighted after drying for 24 hours at 80 C, after which analysis was carried out by ICP, XRD and thermogravimetric and differential scanning calorimetry (TG-DSC).

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Some leaching tests using a chromite lump for SEM analysis were also conducted in this study, and the

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2.2 Process Chemistry

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experimental procedure and apparatuses are reported in part 1 of this study (Zhao et al., 2014).

Based on the results obtained from part 1 of this study (Zhao et al., 2014), the ion reactions between

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South African chromite and sulfate acid with [(1) + (3)] and without oxidant [(2) + (3)] are given as

(Mga,Feb)(Crc,Ald,Fef)2O4 + 8H+ - be-→ aMg2+ + (b + 2f)Fe3+ + 2cCr3+ + 2dAl3+ + 4H2O

(1)

(Mga,Feb)(Crc,Ald,Fef)2O4 + 8H+ → aMg2+ + bFe2+ + 2cCr3+ + 2dAl3+ + 2fFe3+ + 4H2O

(2)

xMgSiO3·yAl2SiO5 + 2(x + 3y)H+ → xMg2+ + 2yAl3+ + (x + y)SiO2 + (x + 3y)H2O

(3)

where a + b = 1 and c + d + f = 1.

3. Results and Discussion

The extraction yields of chromium with different leaching conditions are reported in Table 1. The 4

ACCEPTED MANUSCRIPT leaching conditions investigated in the current work included varying temperature (140, 150, 160, 170, 180, 190 and 200 C), duration (10, 20, 30, 40, 50, 60, 90 and 120 minutes), concentration of sulfuric acid (50, 70, 80 and 90 %-wt), oxidant/chromite ratio (0, 1/20, 1/10 and 1/5) and agitation speed (0, 600, 1200, 1800

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and 2400 rad·s-1), respectively.

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Table 1 Extraction yields of chromium under different leaching conditions.

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3.1 Effect of Temperature

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As is seen from the experimental results given in Fig. 1, the extraction yield of chromium and the mass of leaching residues exhibit opposite trends with increasing temperature. The former value increased from

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about 57 % to about 93 % when the leaching temperature rose from 140 C to 160 C, while the latter value decreased from about 3 g to 1 g, indicating the significant influence of temperature. Elevation of temperature higher than 180 C led to a decrease in the chromium recovery while more solid residues were

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obtained. When the temperature reached to 200 C more than 18 g of residues was obtained after 60 minutes of leaching. The color of the residues also changed with temperature from gray to white as temperature rose from 140 C to 180 C, and became green when temperature went beyond 180 C. The phase composition of residues leached at 200 C for 60 minutes was determined on the basis of the results of ICP, energy-dispersive X-ray spectroscopy (EDS) and TG-DSC, giving the results shown in Fig. 2. Sulfate accounts for more than 90 % of the leaching residues, containing primarily chromium sulfate, iron sulfate, aluminum sulfate and magnesium sulfate. This precipitation of sulfate resulted in a decline of the extraction yield of chromium, with sulfate eventually covering the unreacted chromite powder, preventing it from further leaching.

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Fig. 1 Extraction yield of chromium and mass of leaching residues as a function of temperature.

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Fig. 2 Phase composition of leaching residues leached at 200 C for 60 minutes.

Fig. 3 shows SEM images of the original surface of the chromite lump and the surface after leaching at

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190 C for 90 minutes. The results indicated that many sulfate particles smaller than 5 μm had been precipitated in the sample, while there were hardly any such particles in the samples leached below 180 C.

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This phenomenon can be attributed to the significant effect of temperature on the leaching reaction rate,

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resulting in a sharp elevation of the amount of metal present in the solid-liquid layer. With a constant agitation speed, the mass transfer rate of metal ions remained unchanged. Much sulfate precipitated from

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the solid-liquid layer when the solution became locally saturated. Therefore, for achieving acceptable leaching results and to avoid strong precipitation, 160 C was chosen as the optimal leaching temperature

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for the investigations.

Fig. 3 SEM images of original surface of chromite lump and the surface leached at 190 C for 90 minutes

3.2 Effect of Leaching Duration

The relationship between the extraction yield of chromium and duration at 160 C with 80 % sulfuric acid and 1/10 oxidant/chromite ratio is shown in Fig. 4. The extraction yield increased with the leaching duration but did not experience any notable changes after 60 minutes, reaching 96.4 % after 90 minutes. 6

ACCEPTED MANUSCRIPT The phase transformation during 120 minutes of leaching for both the silicate phase and the spinel phase on the surface of the chromite lump were investigated by SEM-EDS. It was found that silicate transformed into porous amorphous silica within the first 15 minutes, while it took more than 60 minutes for complete

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decomposition of the spinel phase, leaving many holes in the matrix (cf. Fig. 5). The obstruction effect of the silicon-rich phase for inner spinel is the major cause for the decline of the leaching reaction rate in the

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later period, resulting in incomplete extraction of chromium.

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Fig. 4 Extraction yield of chromium as a function of leaching duration in 80 % sulfuric acid at 160 C.

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Fig. 5 SEM images of the chromite lump surface during and after leaching at 160 C in 80 % sulfuric acid.

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3.3 Effect of Sulfuric Acid Concentration

E-pH diagrams of different types of chromium-bearing spinel in acid solution at 160 C were analyzed to understand the effect of sulfuric acid concentration and oxidant on the leaching conditions. As examples, two diagrams for the Mg-Fe-Cr-H2O and Mg-Al-Cr-H2O system at 160 C calculated by FactSage 6.4 software are shown in Fig. 6. For ordinate E values below zero, the system is able to supply the electrons to the species found in the solution. Some reducing agent (or cathode electrode) could enable this. By contrast, the system tends to remove electrons from the species if E > 0. This condition holds true if oxidant is present (or close to an anode). In this study, an oxidizing environment was maintained by the presence of oxidant and the employment of concentrated sulfuric acid. As seen in the diagram, the chromium-bearing spinel is stable in weak acid solutions with a low oxidation potential even at 160 C. To ensure a complete chromium recovery, leaching conditions were chosen within the area delimited by dotted box in Fig. 6 by controlling the sulfuric acid concentration and oxidant dosage so that all of the metal (primarily Cr, Fe, Mg 7

ACCEPTED MANUSCRIPT and Al) could theoretically be leached from the spinel phase. Sulfuric acid with a concentration higher than 50 % was used to dissolve chromite and some results are presented in Fig. 7. The extraction yield of chromium is seen to improve from about 40 % to about 93 %

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with an increase in concentration of the sulfuric acid from 50 % to 80 %, but the recovery of chromium decreased above the upper concentration in the presence of sulfate in the leaching residues. The leaching

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reaction rate and the viscosity of the solution both increased with rising acidity, generating more metal ions.

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As a result of this, the metal concentration in the solid-liquid layer increased until precipitation occurred.

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Fig. 6 E-pH diagrams of Mg-Fe-Cr-H2O and Mg-Al-Cr-H2O at 160 C

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Fig. 7 Extraction yield of chromium and mass of leaching residues as a function of sulfuric acid concentration at 160 C.

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3.4 Effect of Oxidant

As seen from Fig. 6, the spinel phase is prone to decompose in an acidic solution with high oxidation potential at 160 C. This is most probably due to of the reducibility of chromium (Ⅲ) and iron (Ⅱ) in the spinel phase, which can influence the leaching behavior of chromite in the presence of an oxidant. The effect of the oxidant on the extraction yield of chromium was investigated and is illustrated by the results presented in Fig. 8, showing that the oxidant had a remarkable effect on the leaching efficiency: when the chromite was leached in 80 % sulfuric acid for 60 minutes at 160 C the extraction yield of chromium was only about 32 % without oxidant, but exceeded 93 % when the oxidant/chromite ratio was above 1/10. Moreover, no chromium (Ⅵ), not even in trace amounts (＜1ppm), was detected in any of the samples. Clearly, the oxidant used in the current study has the ability to oxidize chromium (Ⅲ) to chromium (Ⅳ) and 8

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iron (Ⅱ) to iron (Ⅲ) but cannot oxidize chromium (Ⅳ) to a higher valence.

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Fig. 8 Extraction yield of chromium as a function of oxidant/chromite ratio at 160 C in 80 % sulfuric acid.

The morphology of the spinel phase in the chromite lump leached without and with (1/10) oxidant for 30 minutes and 60 minutes was analyzed using SEM, some images are provided in Fig. 9. The findings

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indicated that similar “groove” corrosion on the surface of spinel phase occurred and developed independent of whether oxidant was used or not, although the corrosion rate was significantly enhanced by

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the use of oxidant both in the horizontal and vertical directions. The high stability of the spinel structure

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makes it difficult to decompose the chromite by acid solutions with a low oxidation potential. However, the

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ionic radius of chromium (Ⅲ) and iron (Ⅱ) in octahedral sites and tetrahedral sites could be decreased by the redox reaction when oxidant is employed, resulting in a decline of lattice stability and an enhancement of the leaching rate. The effects of sulfuric acid concentration and oxidant on the leaching behavior show

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general agreement with the E-pH diagram (cf. Fig. 6).

Fig. 9 SEM images of chromite lump with and without oxidant leached for 30 minutes and 60 minutes at 160 C in 80 % sulfuric acid.

3.5 Effect of Agitation Speed

To study the relationship between the extraction yield of chromite and agitation speed, tests were carried 9

ACCEPTED MANUSCRIPT out using five different agitation speeds in the range of 0 ~ 2400 rad·s-1 at 160 C in 80 % sulfuric acid. With reference to the results in Fig. 10, it was found that less than 60 % of chromium was leached out when

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no agitation was employed, and much chromium-bearing spinel and sulfate were detected in the leaching

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residues. Without the agitation, the mass transfer of metal ions from the solid-liquid interface to the bulk

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solution was very slow, and the temperature of the boundary layer rose to a high level by the exothermal leaching reaction, so precipitation of sulfate occurred. The leaching reaction between the covered chromite and the acid solution was retarded by the sulfate and porous amorphous silica transformed from silicate (cf.

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part 1). The extraction yield of chromium increased with the elevation of the agitation speed from 0.1 to 1800 rad·s-1 but varied little when the agitation speed was greater than 1800 rad·s-1, indicating that mass

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transfer was not limiting the leaching above this speed. Therefore, 1800 rad·s-1 was selected in this study to

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ensure a high chromium recovery while avoiding a strong sulfate precipitation.

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Fig. 10 Extraction yield of chromium as a function of agitation speed at 160 C in 80 % sulfuric acid.

4. Conclusions

The effects of temperature, duration, sulfuric acid concentration and oxidant on sulfuric acid leaching of South African chromite have been studied using both a chromite lump and a chromite powder fraction. The initial results of the investigation of sulfuric acid leaching of South African chromite are reported in part 1 of this study (Zhao et al., 2014). Results given in the present part showed that the extraction yield of chromium exhibited an increasing trend with temperature until 180 C was reached. Above this temperature sulfates, including chromium sulfate, iron sulfate, magnesium sulfate and aluminum sulfate, precipitated from the solution, resulting in a decrease of the chromium recovery. More than 93 % chromium was leached into the solution after 60 minutes of leaching at 160 C as the spinel phase shrank. A porous amorphous silica phase was obtained as the metal elements leached and the spinel phase flaked, which 10

ACCEPTED MANUSCRIPT obstructed further dissolution of the inner spinel phase. In agreement with findings from an analysis of E-pH diagrams, the sulfuric acid concentration and oxidant were experimentally found to have a marked influence on the extraction yield of chromium. The best leaching results were obtained with 80 %-wt

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sulfuric acid and an oxidant/chromite ratio of 1/10. The extraction yield of chromium was improved by the

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employment of proper agitation, and 1800 rad·s-1 was found to be a sufficient speed. Future work will focus on clarifying the kinetics of the leaching process in more detail. Another line of future work would be to apply mathematical modeling to explain the evolution of the extraction yield,

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paying attention to the structural changes of the grains during the leaching process.

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Acknowledgements

The authors gratefully acknowledge supports by China Scholarship Council (CSC) for the visit of Qing

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Zhao to Åbo Akademi University, Finland. The National Key Basic Research Program of China (No.

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2012CB626812), National Natural Science Foundation of China (No. 51104039), Program for New Century Excellent Talents in University of Ministry of Education of China (No. NCET-11-0077), China

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Postdoctoral Science Foundation (No. 20100481208), Natural Science Foundation of Liaoning Province of China (No. 201102062), Programs for Science and Technology Development of Liaoning Province of China (No. 2012221013) are also acknowledged, as well as the language corrections suggested by Lei Shao

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of Åbo Akademi University.

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ACCEPTED MANUSCRIPT References

Hu, K.G., Li, J.P., Tan, K.X., Shi, W., 2009. Investigation on the correlative factors affecting acid leaching

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rate of a uranium deposit ore. Mineral Engineering Research 24 (4), 51-54.

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Ji, Z., 2012. Preparation of trivalent chromium compounds from chromite by acid-leaching technique. Inorganic Chemicals industry 44 (12), 1-5.

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Li, Q., Zhou, P., 2009. Research on acid leaching of Yunnan low-grade oxidized copper Ore. Yunnan Metallurgy 38(4), 15-17.

Liu, C.J., Qi, J., Jiang, M.F., 2011. Experimental study on sulfuric acid leaching behavior of chromite with different temperature. Advanced Materials Research 30 (1), 51-56.

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Liu, C.J., Shi, P.Y., Jiang, M.F., 2011. A method of leaching process of chromite. CN, 101979679A. Liu, C.J., Shi, P.Y., 2011. The effect of sulfuric acid addition on the leaching behavior of chromite. Industrial Heating 40 (3), 59-76.

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Shi, P.Y., Jiang, M.F., Liu, C.J., 2004. A method of preparation of basic chromic sulfate. CN, 1526646A. Shi, P.Y., Liu, C.J., Jiang, M.F., 2011. Separation of chromium and iron ions from multi-component solution. CN, 101974688A.

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Shi, P.Y., Liu, S.L., 2002. Study on sulfuric acid leaching of chromite. Journal of Rare Earths 20 (9),

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472-474.

Thubakgale, C.K., Mbaya, R.K.K., Kabongo, K., 2013. A study of atmospheric acid leaching of a South

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African nickel laterite. Miner. Eng. 54, 79-81. Wang, B.Q., Guo, Q., Wei, G.Y., Zhang, P.Y., Qu, J.K., Qi, T., 2012. Characterization and atmospheric hydrochloric acid leaching of a limonitic laterite from Indonesia. Hydrometallurgy 129-130, 7-13. Zhao, Q., Liu, C.J., Shi, P.Y., Zhang, B., Jiang, M.F., Zhang, Q.S., Saxén, H., Zevenhoven, R., 2014.

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Sulfuric acid leaching of South African chromite. Part 1: study on leaching behavior. Submited to Int. J. Miner. Process.

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Fig. 1 Extraction yield of chromium and mass of leaching residues as a function of temperature.

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Fig. 2 Phase composition of leaching residues leached at 200 C for 60 minutes.

Fig. 3 SEM images of original surface of chromite lump and the surface leached at 190 C for 90 minutes

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Fig. 4 Extraction yield of chromium as a function of leaching duration in 80 % sulfuric acid at 160 C.

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Fig. 5 SEM images of the chromite lump surface during and after leaching at 160 C in 80 % sulfuric acid.

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Fig. 6 E-pH diagrams of Mg-Fe-Cr-H2O and Mg-Al-Cr-H2O at 160 C

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Fig. 7 Extraction yield of chromium and mass of leaching residues as a function of sulfuric acid concentration at 160 C.

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Fig. 8 Extraction yield of chromium as a function of oxidant/chromite ratio at 160 C in 80 % sulfuric acid.

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sulfuric acid.

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Fig. 9 SEM images of chromite lump with and without oxidant leached for 30 minutes and 60 minutes at 160 C in 80 %

Fig. 10 Extraction yield of chromium as a function of agitation speed at 160 C in 80 % sulfuric acid.

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ACCEPTED MANUSCRIPT Table 1 Extraction yields of chromium under different leaching conditions. T

t

Sulfuric acid

Oxidant/chromite

Agitation speed

Extraction yield of

(C)

(minutes)

concentration (%-wt)

(mass) ratio

(rad·s-1)

chromium (%)

1

140

60

80

1/10

1800

57.3

2

150

60

80

1/10

3

160

60

80

1/10

4

170

60

80

1/10

5

180

60

80

1/10

6

190

60

80

7

200

60

80

8

160

10

80

9

160

20

80

10

160

30

80

11

160

40

80

12

160

50

13

160

90

14

160

120

15

160

60

16

160

60

17

160

18

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93.1

1800

93.3

1800

93.9

1/10

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1800

68.6

1/10

1800

32.5

1/10

1800

28.8

1/10

1800

52.1

1/10

1800

63.8

1/10

1800

76.3

80

1/10

1800

88.6

80

1/10

1800

96.4

80

1/10

1800

96.0

50

1/10

1800

40.5

60

1/10

1800

69.7

60

70

1/10

1800

81.1

160

60

90

1/10

1800

85.7

19

160

60

80

0

1800

32.6

20

160

60

80

1/20

1800

76.7

21

160

60

80

1/5

1800

94.1

22

160

60

80

80

0

58.6

23

160

60

80

80

600

70.3

24

160

60

80

80

1200

81.9

25

160

60

80

80

2400

94.0

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74.2

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1800

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Graphical abstract

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HIGHLIGHTS

Chromium-rich sulfate precipitated at high temperature and acid concentration or no agitation

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Cr% increased with temperature, duration, acid concentration, oxidant and agitation speed

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Leaching reaction rate increased with temperature, acid concentration and oxidant

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

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